MICROCHIP MICROCHIP SW006021-DGL Compiler Operating instructions

MPLAB® XC8 C Compiler
User’s Guide
 2012-2016 Microchip Technology Inc.
DS50002053G
Note the following details of the code protection feature on Microchip devices:
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Microchip products meet the specification contained in their particular Microchip Data Sheet.
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Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
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are for its PIC® MCUs and dsPIC® DSCs, KEELOQ® code hopping
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DS50002053G-page 2
Trademarks
The Microchip name and logo, the Microchip logo, AnyRate, AVR,
AVR logo, AVR Freaks, BeaconThings, BitCloud, CryptoMemory,
CryptoRF, dsPIC, FlashFlex, flexPWR, Heldo, JukeBlox, KEELOQ,
KEELOQ logo, Kleer, LANCheck, LINK MD, maXStylus,
maXTouch, MediaLB, megaAVR, MOST, MOST logo, MPLAB,
OptoLyzer, PIC, picoPower, PICSTART, PIC32 logo, Prochip
Designer, QTouch, RightTouch, SAM-BA, SpyNIC, SST, SST
Logo, SuperFlash, tinyAVR, UNI/O, and XMEGA are registered
trademarks of Microchip Technology Incorporated in the U.S.A.
and other countries.
ClockWorks, The Embedded Control Solutions Company,
EtherSynch, Hyper Speed Control, HyperLight Load, IntelliMOS,
mTouch, Precision Edge, and Quiet-Wire are registered
trademarks of Microchip Technology Incorporated in the U.S.A.
Adjacent Key Suppression, AKS, Analog-for-the-Digital Age, Any
Capacitor, AnyIn, AnyOut, BodyCom, chipKIT, chipKIT logo,
CodeGuard, CryptoAuthentication, CryptoCompanion,
CryptoController, dsPICDEM, dsPICDEM.net, Dynamic Average
Matching, DAM, ECAN, EtherGREEN, In-Circuit Serial
Programming, ICSP, Inter-Chip Connectivity, JitterBlocker,
KleerNet, KleerNet logo, Mindi, MiWi, motorBench, MPASM, MPF,
MPLAB Certified logo, MPLIB, MPLINK, MultiTRAK, NetDetach,
Omniscient Code Generation, PICDEM, PICDEM.net, PICkit,
PICtail, PureSilicon, QMatrix, RightTouch logo, REAL ICE, Ripple
Blocker, SAM-ICE, Serial Quad I/O, SMART-I.S., SQI,
SuperSwitcher, SuperSwitcher II, Total Endurance, TSHARC,
USBCheck, VariSense, ViewSpan, WiperLock, Wireless DNA, and
ZENA are trademarks of Microchip Technology Incorporated in the
U.S.A. and other countries.
SQTP is a service mark of Microchip Technology Incorporated in
the U.S.A.
Silicon Storage Technology is a registered trademark of Microchip
Technology Inc. in other countries.
GestIC is a registered trademark of Microchip Technology
Germany II GmbH & Co. KG, a subsidiary of Microchip Technology
Inc., in other countries.
All other trademarks mentioned herein are property of their
respective companies.
© 2012-2016, Microchip Technology Incorporated, All Rights
Reserved.
ISBN: 978-1-5224-1085-0
 2012-2016 Microchip Technology Inc.
MPLAB® XC8 C COMPILER
USER’S GUIDE
Table of Contents
Preface ........................................................................................................................... 7
Chapter 1. Compiler Overview
1.1 Introduction ................................................................................................... 13
1.2 Compiler Description and Documentation .................................................... 13
1.3 Device Description ....................................................................................... 14
Chapter 2. Common C Interface
2.1 Introduction ................................................................................................... 15
2.2 Background – The Desire for Portable Code ............................................... 15
2.3 Using the CCI ............................................................................................... 18
2.4 ANSI Standard Refinement .......................................................................... 19
2.5 ANSI Standard Extensions ........................................................................... 27
2.6 Compiler Features ........................................................................................ 41
Chapter 3. How To’s
3.1 Introduction ................................................................................................... 43
3.2 Installing and Activating the Compiler .......................................................... 43
3.3 Invoking the Compiler ................................................................................... 45
3.4 Writing Source Code .................................................................................... 48
3.5 Getting My Application to Do What I Want ................................................... 60
3.6 Understanding the Compilation Process ...................................................... 65
3.7 Fixing Code That Does Not Work ................................................................. 73
Chapter 4. XC8 Command-line Driver
4.1 Introduction ................................................................................................... 77
4.2 Invoking the Compiler ................................................................................... 78
4.3 The Compilation Sequence .......................................................................... 81
4.4 Runtime Files ............................................................................................... 87
4.5 Compiler Output ........................................................................................... 88
4.6 Compiler Messages ...................................................................................... 90
4.7 MPLAB XC8 Driver Options ......................................................................... 95
4.8 Option Descriptions ...................................................................................... 96
4.9 MPLAB X Option Equivalents ..................................................................... 126
Chapter 5. C Language Features
5.1 Introduction ................................................................................................. 135
5.2 ANSI C Standard Issues ............................................................................ 135
5.3 Device-Related Features ............................................................................ 137
5.4 Supported Data Types and Variables ........................................................ 149
5.5 Memory Allocation and Access .................................................................. 170
5.6 Operators and Statements ......................................................................... 187
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MPLAB® XC8 C Compiler User’s Guide
5.7 Register Usage ........................................................................................... 189
5.8 Functions .................................................................................................... 190
5.9 Interrupts .................................................................................................... 200
5.10 Main, Runtime Startup and Reset ............................................................ 210
5.11 Libraries .................................................................................................... 214
5.12 Mixing C and Assembly Code .................................................................. 216
5.13 Optimizations ............................................................................................ 228
5.14 Preprocessing .......................................................................................... 230
5.15 Linking Programs ..................................................................................... 241
Chapter 6. Macro Assembler
6.1 Introduction ................................................................................................. 265
6.2 MPLAB XC8 Assembly Language .............................................................. 266
6.3 Assembly-Level Optimizations ................................................................... 294
6.4 Assembly List Files ..................................................................................... 295
Chapter 7. Linker
7.1 Introduction ................................................................................................. 305
7.2 Operation .................................................................................................... 305
7.3 Relocation and Psects ................................................................................ 313
7.4 Map Files .................................................................................................... 314
Chapter 8. Utilities
8.1 Introduction ................................................................................................. 319
8.2 Librarian ..................................................................................................... 320
8.3 HEXMATE .................................................................................................. 323
8.4 Hash Functions .......................................................................................... 332
Appendix A. Library Functions
A.1 Introduction ................................................................................................ 339
Appendix B. Embedded Compiler Compatibility Mode
B.1 Introduction ................................................................................................ 431
B.2 Compiling in Compatibility Mode ................................................................ 431
B.3 Syntax Compatibility .................................................................................. 432
B.4 Data Type .................................................................................................. 433
B.5 Operator ..................................................................................................... 433
B.6 Extended Keywords ................................................................................... 434
B.7 Intrinsic Functions ...................................................................................... 435
B.8 Pragmas ..................................................................................................... 436
Appendix C. Error and Warning Messages
C.1 Introduction ................................................................................................ 437
Appendix D. Implementation-Defined Behavior
D.1 Introduction ................................................................................................ 565
D.2 Translation (G.3.1) ..................................................................................... 565
D.3 Environment (G.3.2) .................................................................................. 565
D.4 Identifiers (G.3.3) ....................................................................................... 566
D.5 Characters (G.3.4) ..................................................................................... 566
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Table of Contents
D.6 Integers (G.3.5) .......................................................................................... 567
D.7 Floating-Point (G.3.6) ................................................................................ 568
D.8 Arrays and Pointers (G.3.7) ....................................................................... 568
D.9 Registers (G.3.8) ....................................................................................... 568
D.10 Structures, Unions, Enumerations, and Bit-Fields (G.3.9) ....................... 569
D.11 Qualifiers (G.3.10) ................................................................................... 569
D.12 Declarators (G.3.11) ................................................................................ 569
D.13 Statements (G.3.12) ................................................................................ 569
D.14 Preprocessing Directives (G.3.13) ........................................................... 570
D.15 Library Functions (G.3.14) ....................................................................... 571
Glossary ..................................................................................................................... 573
Index ........................................................................................................................... 593
Worldwide Sales and Service .................................................................................. 606
 2012-2016 Microchip Technology Inc.
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MPLAB® XC8 C Compiler User’s Guide
NOTES:
DS50002053G-page 6
 2012-2016 Microchip Technology Inc.
MPLAB® XC8 C COMPILER
USER’S GUIDE
Preface
NOTICE TO CUSTOMERS
All documentation becomes dated, and this manual is no exception. Microchip tools and
documentation are constantly evolving to meet customer needs, so some actual dialogs
and/or tool descriptions can differ from those in this document. Please refer to our web site
(www.microchip.com) to obtain the latest documentation available.
Documents are identified with a “DS” number. This number is located on the bottom of each
page, in front of the page number. The numbering convention for the DS number is
“DSXXXXXA”, where “XXXXX” is the document number and “A” is the revision level of the
document.
For the most up-to-date information on development tools, see the MPLAB® IDE online help.
Select the Help menu, and then Topics to open a list of available online help files.
INTRODUCTION
This chapter contains general information that will be useful to know before using the
MPLAB® XC8 C Compiler User’s Guide. Items discussed in this chapter include:
• Document Layout
• Conventions Used in this Guide
• Recommended Reading
• Recommended Reading
• The Microchip Web Site
• Development Systems Customer Change Notification Service
• Customer Support
• Document Revision History
DOCUMENT LAYOUT
The MPLAB XC8 C Compiler User’s Guide is organized as follows:
• Chapter 1. Compiler Overview
• Chapter 2. Common C Interface
• Chapter 3. How To’s
• Chapter 4. XC8 Command-line Driver
• Chapter 5. C Language Features
• Chapter 6. Macro Assembler
• Chapter 7. Linker
• Chapter 8. Utilities
• Appendix A. Library Functions
• Appendix B. Embedded Compiler Compatibility Mode
• Appendix C. Error and Warning Messages
• Appendix D. Implementation-Defined Behavior
• Glossary
• Index
 2012-2016 Microchip Technology Inc.
DS50002053G-page 7
MPLAB® XC8 C Compiler User’s Guide
CONVENTIONS USED IN THIS GUIDE
This manual uses the following documentation conventions:
DOCUMENTATION CONVENTIONS
Description
Represents
Examples
Referenced books
Emphasized text
A window
A dialog
A menu selection
A field name in a window or
dialog
A menu path
MPLAB® IDE User’s Guide
...is the only compiler...
the Output window
the Settings dialog
select Enable Programmer
“Save project before build”
A dialog button
A tab
A number in verilog format,
where N is the total number of
digits, R is the radix and n is a
digit.
A key on the keyboard
Click OK
Click the Power tab
4‘b0010, 2‘hF1
Italic Courier New
Sample source code
Filenames
File paths
Keywords
Command-line options
Bit values
Constants
A variable argument
Square brackets [ ]
Optional arguments
Curly brackets and pipe
character: { | }
Ellipses...
Choice of mutually exclusive
arguments; an OR selection
Replaces repeated text
#define START
autoexec.bat
c:\mcc18\h
_asm, _endasm, static
-Opa+, -Opa0, 1
0xFF, ‘A’
file.o, where file can be
any valid filename
mcc18 [options] file
[options]
errorlevel {0|1}
Arial font:
Italic characters
Initial caps
Quotes
Underlined, italic text with
right angle bracket
Bold characters
N‘Rnnnn
Text in angle brackets < >
Courier New font:
Plain Courier New
Represents code supplied by
user
DS50002053G-page 8
File>Save
Press <Enter>, <F1>
var_name [,
var_name...]
void main (void)
{ ...
}
 2012-2016 Microchip Technology Inc.
Preface
RECOMMENDED READING
This user’s guide describes how to use MPLAB XC8 C Compiler. Other useful documents are listed below. The following Microchip documents are available and
recommended as supplemental reference resources.
Readme for MPLAB XC8 C Compiler
For the latest information on using MPLAB XC8 C Compiler, read MPLAB® XC8 C
Compiler Release Notes (an HTML file) in the Docs subdirectory of the compiler’s
installation directory. The release notes contain update information and known issues
that cannot be included in this user’s guide.
Readme Files
For the latest information on using other tools, read the tool-specific Readme files in
the Readmes subdirectory of the MPLAB IDE installation directory. The Readme files
contain update information and known issues that cannot be included in this user’s
guide.
THE MICROCHIP WEB SITE
Microchip provides online support via our web site at www.microchip.com. This web
site is used as a means to make files and information easily available to customers.
Accessible by using your favorite Internet browser, the web site contains the following
information:
• Product Support – Data sheets and errata, application notes and sample
programs, design resources, user’s guides and hardware support documents,
latest software releases and archived software
• General Technical Support – Frequently Asked Questions (FAQs), technical
support requests, online discussion groups, Microchip consultant program
member listing
• Business of Microchip – Product selector and ordering guides, latest Microchip
press releases, listing of seminars and events, listings of Microchip sales offices,
distributors and factory representatives
 2012-2016 Microchip Technology Inc.
DS50002053G-page 9
MPLAB® XC8 C Compiler User’s Guide
DEVELOPMENT SYSTEMS CUSTOMER CHANGE NOTIFICATION SERVICE
Microchip’s customer notification service helps keep customers current on Microchip
products. Subscribers will receive e-mail notification whenever there are changes,
updates, revisions or errata that are related to a specified product family or
development tool of interest.
To register, access the Microchip web site at www.microchip.com, click on Customer
Change Notification and follow the registration instructions.
The Development Systems product group categories are:
• Compilers – The latest information on Microchip C compilers, assemblers, linkers
and other language tools. These include all MPLAB C compilers; all MPLAB
assemblers (including MPASM™ assembler); all MPLAB linkers (including
MPLINK™ object linker); and all MPLAB librarians (including MPLIB™ object
librarian).
• Emulators – The latest information on Microchip in-circuit emulators.This
includes the MPLAB REAL ICE™ and MPLAB ICE 2000 in-circuit emulators.
• In-Circuit Debuggers – The latest information on the Microchip in-circuit
debuggers. This includes MPLAB ICD 3 in-circuit debuggers and PICkit™ 3
debug express.
• MPLAB® IDE – The latest information on Microchip MPLAB IDE, the Windows®
Integrated Development Environment for development systems tools. This list is
focused on the MPLAB IDE, MPLAB IDE Project Manager, MPLAB Editor and
MPLAB SIM simulator, as well as general editing and debugging features.
• Programmers – The latest information on Microchip programmers. These include
production programmers such as MPLAB REAL ICE in-circuit emulator, MPLAB
ICD 3 in-circuit debugger and MPLAB PM3 device programmers. Also included
are nonproduction development programmers such as PICSTART® Plus and
PICkit 2 and 3.
CUSTOMER SUPPORT
Users of Microchip products can receive assistance through several channels:
• Distributor or Representative
• Local Sales Office
• Field Application Engineer (FAE)
• Technical Support
Customers should contact their distributor, representative or field application engineer
(FAE) for support. Local sales offices are also available to help customers. A listing of
sales offices and locations is included in the back of this document.
Technical support is available through the web site at:
http://www.microchip.com/support
DS50002053G-page 10
 2012-2016 Microchip Technology Inc.
Preface
DOCUMENT REVISION HISTORY
Revision G (November 2016)
• Expanded sections on interrupts to deal with new devices with vector tables
• Added new driver option --UNDEFINTS and ivt suboption to --RUNTIME
• Added details on new assembler instructions and directives
• Clarified usage of EEPROM functions
• Added new errata workaround information
• Updated MPLAB X IDE optimizations project properties dialog
• Expanded section on HEXMATE hash algorithms
• Added new error and warning messages
Revision F (December 2015)
• Added new “How To’s”
• Added new driver option, --DEP, expanded --OPT, and updated -V option
• Updated predefined macros table
• Improved function allocation sections
• Added descriptions of new ‘relaxed’ 32-bit floating-point routines; new
__fpnormalize function
• Added EXTRN assembler directive
• Expanded assembly optimizations section
• Added new section on writing reentrant assembly routines with parameters
• Revised the sections relating to the main linker options used to link psects
• Added new section on HEXMATE algorithms; included new examples
• Added new error and warning messages
Revision E (January 2015)
• Added new “How To’s”
• Detailed the compiler’s use of hardware multiply instructions
• Updated information relating to psect definitions and their effect on optimizations
• Corrected information relating to maximum reentrant-function stack sizes
• Updated compiler warning and error messages; improved message descriptions
relating to fixup errors and malformed arrays
• Added further information relating to customizing user-defined psects
• Improved printf library function description and expanded code example
• Added new --MAXIPIC and --NOFALLBACK options
• Many general corrections and improvements
 2012-2016 Microchip Technology Inc.
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MPLAB® XC8 C Compiler User’s Guide
Revision D (Dec 2013)
• Added new information relating to the software stack and function reentrancy.
• Added information relating to code profiling features offered by the compiler.
• Removed information pertaining to MPLAB 8 IDE.
• Added new “How To’s”
• Removed sections on OBJTOHEX and CROMWELL.
• Added additional information relating to assembly code formats and operators.
• Corrected Fletcher algorithms used by HEXMATE.
• Added new driver options and updated existing option descriptions.
• Added and updated macros, built-ins and functions in Library Function chapter.
• Updated compiler warning and error messages.
Revision C (May 2013)
• Added Embedded Compiler Compatibility Mode chapter.
• Added information relating to new ELF/DWARF debugging files.
• Added new driver options and updated existing option descriptions.
• Updated MPLAB X IDE option dialog descriptions relating to compiler options.
• Expanded information relating to the available optimizations.
• Added code to illustrate algorithms used by HEXMATE.
• Updated compiler warning and error messages.
• Updated information relating to list and map file contents.
• Added information about multiplication routines.
• Expanded information about eeprom variables and bit objects.
• Expanded information relating to the configuration pragma.
• Added information and examples using the __section() specifier.
• Expanded and extended information relating to assembly code deviations and
assembler directives.
Revision B (July 2012)
• Added How To’s chapter.
• Expanded section relating to PIC18 erratas.
• Updated the section relating to compiler optimization settings.
• Updated MPLAB v8 and MPLAB X IDE project option dialogs.
• Added sections describing PIC18 far qualifier and in-line function qualifier.
• Expanded section describing the operation of the main() function
• Expanded information about equivalent assembly symbols for Baseline parts.
• Updated the table of predefined macro symbols.
• Added section on #pragma addrqual
• Added sections to do with in-lining functions
• Updated diagrams and text associated with call graphs in the list file
• Updated library function section to be consistent with packaged libraries
• Added new compiler warnings and errors.
• Added new chapter describing the Common C Interface Standard (CCI)
Revision A (February 2012)
Initial release of this document.
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 2012-2016 Microchip Technology Inc.
MPLAB® XC8 C COMPILER
USER’S GUIDE
Chapter 1. Compiler Overview
1.1
INTRODUCTION
This chapter is an overview of the MPLAB® XC8 C Compiler, including these topics.
• Compiler Description and Documentation
• Device Description
1.2
COMPILER DESCRIPTION AND DOCUMENTATION
The MPLAB XC8 C Compiler is a free-standing, optimizing ISO C90 (popularly known
as ANSI C) compiler. It supports all 8-bit PIC® microcontrollers: PIC10, PIC12, PIC16
and PIC18 series devices, as well as the PIC14000 device.
The compiler is available for several popular operating systems, including 32- and
64-bit Windows® and Mac OS® X 10.5. The compiler might also run on the various
Linux® distributions, such as Oracle Enterprise Linux 5, Ubuntu 8.x and 10.04, Red Hat
Enterprise Linux.
The compiler is available in three operating modes: Free, Standard or PRO. The Standard and PRO operating modes are licensed modes and require a serial number to
enable them. Free mode is available for unlicensed customers. The basic compiler
operation, supported devices and available memory are identical across all modes.
The modes only differ in the level of optimization employed by the compiler.
1.2.1
Conventions
Throughout this manual, the term “compiler” is used. It can refer to all, or a subset of,
the collection of applications that comprise the MPLAB XC8 C Compiler. When it is not
important to identify which application performed an action, it will be attributed to “the
compiler”.
In a similar manner, “compiler” is often used to refer to the command-line driver;
although specifically, the driver for the MPLAB XC8 C Compiler package is named xc8.
The driver and its options are discussed in Section 4.7 “MPLAB XC8 Driver Options”.
Accordingly, “compiler options” commonly refers to command-line driver options.
In a similar fashion, “compilation” refers to all or a selection of steps involved in
generating an executable binary image from source code.
 2012-2016 Microchip Technology Inc.
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MPLAB® XC8 C Compiler User’s Guide
1.3
DEVICE DESCRIPTION
This compiler supports 8-bit Microchip PIC devices with baseline, mid-range,
Enhanced mid-range, and PIC18 cores. The following descriptions indicate the
distinctions within those device cores:
The baseline core uses a 12-bit-wide instruction set and is available in PIC10, PIC12
and PIC16 part numbers.
The enhanced baseline core also uses a 12-bit instruction set, but this set includes
additional instructions. Some of the enhanced baseline chips support interrupts and the
additional instructions used by interrupts. These devices are available in PIC12 and
PIC16 part numbers.
The mid-range core uses a 14-bit-wide instruction set that includes more instructions
than the baseline core. It has larger data memory banks and program memory pages,
as well. It is available in PIC12, PIC14 and PIC16 part numbers.
The Enhanced mid-range core also uses a 14-bit-wide instruction set but incorporates
additional instructions and features. There are both PIC12 and PIC16 part numbers
that are based on the Enhanced mid-range core.
The PIC18 core instruction set is 16 bits wide and features additional instructions and
an expanded register set. PIC18 core devices have part numbers that begin with
PIC18.
The compiler takes advantage of the target device’s instruction set, addressing modes,
memory, and registers whenever possible.
See Section 4.8.19 “--CHIPINFO: Display List of Supported Devices” for information on
finding the full list of devices that are supported by the compiler.
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MPLAB® XC8 C COMPILER
USER’S GUIDE
Chapter 2. Common C Interface
2.1
INTRODUCTION
The Common C Interface (CCI) is available with all MPLAB XC C compilers and is
designed to enhance code portability between these compilers. For example,
CCI-conforming code would make it easier to port from a PIC18 MCU using the MPLAB
XC8 C compiler to a PIC24 MCU using the MPLAB XC16 C compiler.
The CCI assumes that your source code already conforms to the ANSI Standard. If you
intend to use the CCI, it is your responsibility to write code that conforms. Legacy projects will need to be migrated to achieve conformance. A compiler option must also be
set to ensure that the operation of the compiler is consistent with the interface when the
project is built.
The following topics are examined in this chapter of the MPLAB XC8 C Compiler User’s
Guide:
• Background – The Desire for Portable Code
• Using the CCI
• ANSI Standard Refinement
• ANSI Standard Extensions
• Compiler Features
2.2
BACKGROUND – THE DESIRE FOR PORTABLE CODE
All programmers want to write portable source code.
Portability means that the same source code can be compiled and run in a different
execution environment than that for which it was written. Rarely can code be one hundred percent portable, but the more tolerant it is to change, the less time and effort it
takes to have it running in a new environment.
Embedded engineers typically think of code portability as being across target devices,
but this is only part of the situation. The same code could be compiled for the same
target but with a different compiler. Differences between those compilers might lead to
the code failing at compile time or runtime, so this must be considered as well.
You can only write code for one target device and only use one brand of compiler, but
if there is no regulation of the compiler’s operation, simply updating your compiler
version can change your code’s behavior.
Code must be portable across targets, tools, and time to be truly flexible.
Clearly, this portability cannot be achieved by the programmer alone, since the compiler vendors can base their products on different technologies, implement different features and code syntax, or improve the way their product works. Many a great compiler
optimization has broken many an unsuspecting project.
Standards for the C language have been developed to ensure that change is managed
and code is more portable. The American National Standards Institute (ANSI) publishes standards for many disciplines, including programming languages. The ANSI C
Standard is a universally adopted standard for the C programming language.
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MPLAB® XC8 C Compiler User’s Guide
2.2.1
The ANSI Standard
The ANSI C Standard has to reconcile two opposing goals: freedom for compilers vendors to target new devices and improve code generation, with the known functional
operation of source code for programmers. If both goals can be met, source code can
be made portable.
The standard is implemented as a set of rules which detail not only the syntax that a
conforming C program must follow, but the semantic rules by which that program will
be interpreted. Thus, for a compiler to conform to the standard, it must ensure that a
conforming C program functions as described by the standard.
The standard describes implementation, the set of tools, and the runtime environment
on which the code will run. If any of these change, e.g., you build for, and run on, a different target device, or if you update the version of the compiler you use to build, then
you are using a different implementation.
The standard uses the term behavior to mean the external appearance or action of the
program. It has nothing to do with how a program is encoded.
Since the standard is trying to achieve goals that could be construed as conflicting,
some specifications appear somewhat vague. For example, the standard states that an
int type must be able to hold at least a 16-bit value, but it does not go as far as saying
what the size of an int actually is; and the action of right-shifting a signed integer can
produce different results on different implementations; yet, these different results are
still ANSI C compliant.
If the standard is too strict, device architectures cannot allow the compiler to conform.1
But, if it is too weak, programmers would see wildly differing results within different
compilers and architectures, and the standard would lose its effectiveness.
The standard organizes source code whose behavior is not fully defined into groups
that include the following behaviors:
Implementation-defined This is unspecified behavior in which each
behavior
implementation documents how the choice is made.
Unspecified behavior
The standard provides two or more possibilities and
imposes no further requirements on which possibility is
chosen in any particular instance.
Undefined behavior
This is behavior for which the standard imposes no
requirements.
Code that strictly conforms to the standard does not produce output that is dependent
on any unspecified, undefined, or implementation-defined behavior. The size of an
int, which was used as an example earlier, falls into the category of behavior that is
defined by implementation. That is to say, the size of an int is defined by which compiler is being used, how that compiler is being used, and the device that is being
targeted.
All the MPLAB XC compilers conform to the ANSI X3.159-1989 Standard for programming languages (with the exception of the MPLAB XC8 compiler’s inability to allow
recursion, as mentioned in the footnote). This is commonly called the C89 Standard.
Some features from the later standard, C99, are also supported.
1. For example, the mid-range PIC® microcontrollers do not have a data stack. Because a compiler
targeting this device cannot implement recursion, it (strictly speaking) cannot conform to the ANSI
C Standard. This example illustrates a situation in which the standard is too strict for mid-range
devices and tools.
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For freestanding implementations (or for what we typically call embedded applications),
the standard allows non-standard extensions to the language, but obviously does not
enforce how they are specified or how they work. When working so closely to the
device hardware, a programmer needs a means of specifying device setup and interrupts, as well as utilizing the often complex world of small-device memory
architectures. This cannot be offered by the standard in a consistent way.
While the ANSI C Standard provides a mutual understanding for programmers and
compiler vendors, programmers need to consider the implementation-defined behavior
of their tools and the probability that they may need to use extensions to the C language
that are non-standard. Both of these circumstances can have an impact on code
portability.
2.2.2
The Common C Interface
The Common C Interface (CCI) supplements the ANSI C Standard and makes it easier
for programmers to achieve consistent outcomes on all Microchip devices when using
any of the MPLAB XC C compilers.
It delivers the following improvements, all designed with portability in mind.
Refinement of the
ANSI C Standard
The CCI documents specific behavior for some code in which
actions are implementation-defined behavior under the ANSI
C Standard. For example, the result of right-shifting a signed
integer is fully defined by the CCI. Note that many
implementation-defined items that closely couple with device
characteristics, such as the size of an int, are not defined by
the CCI.
Consistent syntax The CCI non-standard extensions are mostly implemented
for non-standard
using keywords with a uniform syntax. They replace keywords,
extensions
macros and attributes that are the native compiler implementation. The interpretation of the keyword can differ across each
compiler, and any arguments to the keywords can be device
specific.
Coding guidelines The CCI can indicate advice on how code should be written so
that it can be ported to other devices or compilers. While you
may choose not to follow the advice, it will not conform to the
CCI.
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2.3
USING THE CCI
The CCI allows enhanced portability by refining implementation-defined behavior and
standardizing the syntax for extensions to the language.
The CCI is something you choose to follow and put into effect, thus it is relevant for new
projects, although you can choose to modify existing projects so they conform.
For your project to conform to the CCI, you must do the following things.
• Enable the CCI
Select the MPLAB X IDE widget Use CCI Syntax in your project, or use the
command-line option that is equivalent.
• Include <xc.h> in every module
Some CCI features are only enabled if this header is seen by the compiler.
• Ensure ANSI compliance
Code that does not conform to the ANSI C Standard does not confirm to the CCI.
• Observe refinements to ANSI by the CCI
Some ANSI implementation-defined behavior is defined explicitly by the CCI.
• Use the CCI extensions to the language
Use the CCI extensions rather than the native language extensions.
The next sections detail specific items associated with the CCI. These items are segregated into those that refine the standard, those that deal with the ANSI C Standard
extensions, and other miscellaneous compiler options and usage. Guidelines are
indicated with these items.
If any implementation-defined behavior or any non-standard extension is not discussed
in this document, then it is not part of the CCI. For example, GCC case ranges, label
addresses, and 24-bit short long types are not part of the CCI. Programs which use
these features do not conform to the CCI. The compiler may issue a warning or error
to indicate a non-CCI feature has been used and the CCI is enabled.
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2.4
ANSI STANDARD REFINEMENT
The following topics describe how the CCI refines the implementation-defined
behaviors outlined in the ANSI C Standard.
2.4.1
Source File Encoding
Under the CCI, a source file must be written using characters from the 7-bit ASCII set.
Lines can be terminated using a line feed ('\n') or carriage return ('\r') that is immediately
followed by a line feed. Escaped characters can be used in character constants or
string literals to represent extended characters that are not in the basic character set.
2.4.1.1
EXAMPLE
The following shows a string constant being defined that uses escaped characters.
const char myName[] = "Bj\370rk\n";
2.4.1.2
DIFFERENCES
All compilers have used this character set.
2.4.1.3
MIGRATION TO THE CCI
No action required.
2.4.2
The Prototype for main
The prototype for the main() function is:
int main(void);
2.4.2.1
EXAMPLE
The following shows an example of how main() might be defined:
int main(void)
{
while(1)
process();
}
2.4.2.2
DIFFERENCES
The 8-bit compilers used a void return type for this function.
2.4.2.3
MIGRATION TO THE CCI
Each program has one definition for the main() function. Confirm the return type for
main() in all projects previously compiled for 8-bit targets.
2.4.3
Header File Specification
Header file specifications that use directory separators do not conform to the CCI.
2.4.3.1
EXAMPLE
The following example shows two conforming include directives.
#include <usb_main.h>
#include "global.h"
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2.4.3.2
DIFFERENCES
Header file specifications that use directory separators have been allowed in previous
versions of all compilers. Compatibility problems arose when Windows-style separators “\” were used and the code was compiled under other host operating systems.
Under the CCI, no directory separators should be used.
2.4.3.3
MIGRATION TO THE CCI
Any #include directives that use directory separators in the header file specifications
should be changed. Remove all but the header file name in the directive. Add the directory path to the compiler’s include search path or MPLAB X IDE equivalent. This will
force the compiler to search the directories specified with this option.
For example, the following code:
#include <inc/lcd.h>
should be changed to:
#include <lcd.h>
and the path to the inc directory added to the compiler’s header search path in your
MPLAB X IDE project properties, or on the command-line as follows:
-Ilcd
2.4.4
Include Search Paths
When you include a header file under the CCI, the file should be discoverable in the
paths searched by the compiler that are detailed below.
Header files specified in angle bracket delimiters < > should be discoverable in the
search paths that are specified by -I options (or the equivalent MPLAB X IDE option),
or in the standard compiler include directories. The -I options are searched in the
order in which they are specified.
Header files specified in quote characters " " should be discoverable in the current
working directory or in the same directories that are searched when the header files are
specified in angle bracket delimiters (as above). In the case of an MPLAB X project, the
current working directory is the directory in which the C source file is located. If unsuccessful, the search paths should be to the same directories searched when the header
file is specified in angle bracket delimiters.
Any other options to specify search paths for header files do not conform to the CCI.
2.4.4.1
EXAMPLE
If including a header file, as in the following directive:
#include "myGlobals.h"
The header file should be locatable in the current working directory, or the paths specified by any -I options, or the standard compiler directories. A header file being located
elsewhere does not conform to the CCI.
2.4.4.2
DIFFERENCES
The compiler operation under the CCI is not changed. This is purely a coding guideline.
2.4.4.3
MIGRATION TO THE CCI
Remove any option that specifies header file search paths other than the -I option (or
the equivalent MPLAB X IDE option), and use the -I option in place of this. Ensure the
header file can be found in the directories specified in this section.
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2.4.5
The Number of Significant Initial Characters in an Identifier
At least the first 255 characters in an identifier (internal and external) are significant.
This extends upon the requirement of the ANSI C Standard that states a lower number
of significant characters are used to identify an object.
2.4.5.1
EXAMPLE
The following example shows two poorly named variables, but names which are
considered unique under the CCI.
int stateOfPortBWhenTheOperatorHasSelectedAutomaticModeAndMotorIsRunningFast;
int stateOfPortBWhenTheOperatorHasSelectedAutomaticModeAndMotorIsRunningSlow;
2.4.5.2
DIFFERENCES
Former 8-bit compilers used 31 significant characters by default, but an option allowed
this to be extended.
The 16- and 32-bit compilers did not impose a limit on the number of significant
characters.
2.4.5.3
MIGRATION TO THE CCI
No action required. You can take advantage of the less restrictive naming scheme.
2.4.6
Sizes of Types
The sizes of the basic C types, e.g., char, int and long, are not fully defined by the
CCI. These types, by design, reflect the size of registers and other architectural features in the target device. They allow the device to efficiently access objects of this type.
The ANSI C Standard does, however, indicate minimum requirements for these types,
as specified in <limits.h>.
If you need fixed-size types in your project, use the types defined in <stdint.h>, e.g.,
uint8_t or int16_t. These types are consistently defined across all XC compilers,
even outside of the CCI.
Essentially, the C language offers a choice of two groups of types: those that offer sizes
and formats that are tailored to the device you are using, or those that have a fixed size,
regardless of the target.
2.4.6.1
EXAMPLE
The following example shows the definition of a variable, native, whose size will allow
efficient access on the target device; and a variable, fixed, whose size is clearly indicated and remains fixed, even though it may not allow efficient access on every device.
int native;
int16_t fixed;
2.4.6.2
DIFFERENCES
This is consistent with previous types implemented by the compiler.
2.4.6.3
MIGRATION TO THE CCI
If you require a C type that has a fixed size, regardless of the target device, use one of
the types defined by <stdint.h>.
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2.4.7
Plain char Types
The type of a plain char is unsigned char. It is generally recommended that all
definitions for the char type explicitly state the signedness of the object.
2.4.7.1
EXAMPLE
The following example
char foobar;
defines an unsigned char object called foobar.
2.4.7.2
DIFFERENCES
The 8-bit compilers have always treated plain char as an unsigned type.
The 16- and 32-bit compilers used signed char as the default plain char type. The
-funsigned-char option on those compilers changed the default type to be
unsigned char.
2.4.7.3
MIGRATION TO THE CCI
Any definition of an object defined as a plain char and using the 16- or 32-bit compilers
needs review. Any plain char that was intended to be a signed quantity should be
replaced with an explicit definition, for example.
signed char foobar;
You can use the -funsigned-char option on MPLAB XC16 and XC32 to change the
type of plain char, but since this option is not supported on MPLAB XC8, the code is
not strictly conforming.
2.4.8
Signed Integer Representation
The value of a signed integer is determined by taking the two’s complement of the
integer.
2.4.8.1
EXAMPLE
The following shows a variable, test, that is assigned the value -28 decimal.
signed char test = 0xE4;
2.4.8.2
DIFFERENCES
All compilers have represented signed integers in the way described in this section.
2.4.8.3
MIGRATION TO THE CCI
No action required.
2.4.9
Integer Conversion
When converting an integer type to a signed integer of insufficient size, the original
value is truncated from the most-significant bit to accommodate the target size.
2.4.9.1
EXAMPLE
The following shows an assignment of a value that is truncated.
signed char destination;
unsigned int source = 0x12FE;
destination = source;
Under the CCI, the value of destination after the alignment is -2 (i.e., the bit pattern
0xFE).
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2.4.9.2
DIFFERENCES
All compilers have performed integer conversion in an identical fashion to that
described in this section.
2.4.9.3
MIGRATION TO THE CCI
No action required.
2.4.10
Bitwise Operations on Signed Values
Bitwise operations on signed values act on the two’s complement representation,
including the sign bit. See also Section 2.4.11 “Right-shifting Signed Values”.
2.4.10.1
EXAMPLE
The following shows an example of a negative quantity involved in a bitwise AND
operation.
signed char output, input = -13;
output = input & 0x7E;
Under the CCI, the value of output after the assignment is 0x72.
2.4.10.2
DIFFERENCES
All compilers have performed bitwise operations in an identical fashion to that
described in this section.
2.4.10.3
MIGRATION TO THE CCI
No action required.
2.4.11
Right-shifting Signed Values
Right-shifting a signed value will involve sign extension. This will preserve the sign of
the original value.
2.4.11.1
EXAMPLE
The following shows an example of a negative quantity involved in a right-shift
operation.
signed char output, input = -13;
output = input >> 3;
Under the CCI, the value of output after the assignment is -2 (i.e., the bit pattern
0xFE).
2.4.11.2
DIFFERENCES
All compilers have performed right-shifting as described in this section.
2.4.11.3
MIGRATION TO THE CCI
No action required.
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2.4.12
Conversion of Union Member Accessed Using Member With
Different Type
If a union defines several members of different types and you use one member identifier to try to access the contents of another (whether any conversion is applied to the
result) is implementation-defined behavior in the standard. In the CCI, no conversion is
applied and the bytes of the union object are interpreted as an object of the type of the
member being accessed, without regard for alignment or other possible invalid
conditions.
2.4.12.1
EXAMPLE
The following shows an example of a union defining several members.
union {
signed char code;
unsigned int data;
float offset;
} foobar;
Code that attempts to extract offset by reading data is not guaranteed to read the
correct value.
float result;
result = foobbar.data;
2.4.12.2
DIFFERENCES
All compilers have not converted union members accessed via other members.
2.4.12.3
MIGRATION TO THE CCI
No action required.
2.4.13
Default Bit-field int Type
The type of a bit-field specified as a plain int is identical to that of one defined using
unsigned int. This is quite different from other objects where the types int, signed
and signed int are synonymous. It is recommended that the signedness of the
bit-field be explicitly stated in all bit-field definitions.
2.4.13.1
EXAMPLE
The following shows an example of a structure tag containing bit-fields that are
unsigned integers and with the size specified.
struct OUTPUTS {
int direction :1;
int parity
:3;
int value
:4;
};
2.4.13.2
DIFFERENCES
The 8-bit compilers have previously issued a warning if type int was used for bit-fields,
but would implement the bit-field with an unsigned int type.
The 16- and 32-bit compilers have implemented bit-fields defined using int as having
a signed int type, unless the option -funsigned-bitfields was specified.
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2.4.13.3
MIGRATION TO THE CCI
Any code that defines a bit-field with the plain int type should be reviewed. If the intention was for these to be signed quantities, then the type of these should be changed to
signed int. For example, in the following example:
struct WAYPT {
int log
int direction
};
:3;
:4;
the bit-field type should be changed to signed int, as in:
struct WAYPT {
signed int log
:3;
signed int direction :4;
};
2.4.14
Bit-fields Straddling a Storage Unit Boundary
The standard indicates that implementations can determine whether bit-fields cross a
storage unit boundary. In the CCI, bit-fields do not straddle a storage unit boundary; a
new storage unit is allocated to the structure, and padding bits fill the gap.
Note that the size of a storage unit differs with each compiler, as this is based on the
size of the base data type (e.g., int) from which the bit-field type is derived. On 8-bit
compilers this unit is 8-bits in size; for 16-bit compilers, it is 16 bits; and for 32-bit
compilers, it is 32 bits in size.
2.4.14.1
EXAMPLE
The following shows a structure containing bit-fields being defined.
struct {
unsigned first : 6;
unsigned second :6;
} order;
Under the CCI and using MPLAB XC8, the storage allocation unit is byte sized. The
bit-field, second, is allocated a new storage unit since there are only 2 bits remaining
in the first storage unit in which first is allocated. The size of this structure, order,
is 2 bytes.
2.4.14.2
DIFFERENCES
This allocation is identical with that used by all previous compilers.
2.4.14.3
MIGRATION TO THE CCI
No action required.
2.4.15
The Allocation Order of Bit-fields
The memory ordering of bit-fields into their storage unit is not specified by the ANSI C
Standard. In the CCI, the first bit defined is the least significant bit of the storage unit in
which it is allocated.
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2.4.15.1
EXAMPLE
The following shows a structure containing bit-fields being defined.
struct {
unsigned lo : 1;
unsigned mid :6;
unsigned hi : 1;
} foo;
The bit-field lo is assigned the least significant bit of the storage unit assigned to the
structure foo. The bit-field mid is assigned the next 6 least significant bits; and hi, the
most significant bit of that same storage unit byte.
2.4.15.2
DIFFERENCES
This is identical with the previous operation of all compilers.
2.4.15.3
MIGRATION TO THE CCI
No action required.
2.4.16
The NULL Macro
The NULL macro is defined by <stddef.h>; however, its definition is
implementation-defined behavior. Under the CCI, the definition of NULL is the expression (0).
2.4.16.1
EXAMPLE
The following shows a pointer being assigned a null pointer constant via the NULL
macro.
int * ip = NULL;
The value of NULL, (0), is implicitly converted to the destination type.
2.4.16.2
DIFFERENCES
The 32-bit compilers previously assigned NULL the expression ((void *)0).
2.4.16.3
MIGRATION TO THE CCI
No action required.
2.4.17
Floating-point Sizes
Under the CCI, floating-point types must not be smaller than 32 bits in size.
2.4.17.1
EXAMPLE
The following shows the definition for outY, which is at least 32 bits in size.
float outY;
2.4.17.2
DIFFERENCES
The 8-bit compilers have allowed the use of 24-bit float and double types.
2.4.17.3
MIGRATION TO THE CCI
When using 8-bit compilers, the float and double type will automatically be made
32 bits in size once the CCI mode is enabled. Review any source code that may have
assumed a float or double type and may have been 24 bits in size.
No migration is required for other compilers.
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2.5
ANSI STANDARD EXTENSIONS
The following topics describe how the CCI provides device-specific extensions to the
standard.
2.5.1
Generic Header File
A single header file <xc.h> must be used to declare all compiler- and device-specific
types and SFRs. You must include this file into every module to conform with the CCI.
Some CCI definitions depend on this header being seen.
2.5.1.1
EXAMPLE
The following shows this header file being included, thus allowing conformance with the
CCI, as well as allowing access to SFRs.
#include <xc.h>
2.5.1.2
DIFFERENCES
Some 8-bit compilers used <htc.h> as the equivalent header. Previous versions of
the 16- and 32-bit compilers used a variety of headers to do the same job.
2.5.1.3
MIGRATION TO THE CCI
Change:
#include <htc.h>
previously used in 8-bit compiler code, or family-specific header files, e.g., from:
#include <p32xxxx.h>
#include <p30fxxxx.h>
#include <p33Fxxxx.h>
#include <p24Fxxxx.h>
#include "p30f6014.h"
to:
#include <xc.h>
2.5.2
Absolute Addressing
Variables and functions can be placed at an absolute address by using the __at()
construct. Stack-based (auto and parameter) variables cannot use the __at()
specifier.
2.5.2.1
EXAMPLE
The following shows two variables and a function being made absolute.
int scanMode __at(0x200);
const char keys[] __at(124) = { ’r’, ’s’, ’u’, ’d’};
__at(0x1000) int modify(int x) {
return x * 2 + 3;
}
2.5.2.2
DIFFERENCES
The 8-bit compilers have used an @ symbol to specify an absolute address.
The 16- and 32-bit compilers have used the address attribute to specify an object’s
address.
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2.5.2.3
MIGRATION TO THE CCI
Avoid making objects and functions absolute if possible.
In MPLAB XC8, change absolute object definitions, e.g., from:
int scanMode @ 0x200;
to:
int scanMode __at(0x200);
In MPLAB XC16 and XC32, change code, e.g., from:
int scanMode __attribute__((address(0x200)));
to:
int scanMode __at(0x200);
2.5.2.4
CAVEATS
If the __at() and __section() specifiers are both applied to an object when using
MPLAB XC8, the __section() specifier is currently ignored.
The __at() specifier must be placed at the beginning of function prototypes for the
16- and 32-bit compilers. If you prefer to use the specifier at the end of the prototype,
use the specifier with a declaration and leave it off the definition, for example:
int modify(int x) __at(0x1000);
int modify(int x)
{ ... }
2.5.3
Far Objects and Functions
The __far qualifier can be used to indicate that variables or functions are located in
‘far memory’. Exactly what constitutes far memory is dependent on the target device,
but it is typically memory that requires more complex code to access. Expressions
involving far-qualified objects usually generate slower and larger code.
Use the native keywords discussed in the Differences section to look up information on
the semantics of this qualifier.
Some devices may not have such memory implemented; in which case, use of this
qualifier is ignored. Stack-based (auto and parameter) variables cannot use the
__far specifier.
2.5.3.1
EXAMPLE
The following shows a variable and function qualified using __far.
__far int serialNo;
__far int ext_getCond(int selector);
2.5.3.2
DIFFERENCES
The 8-bit compilers have used the qualifier far to indicate this meaning. Functions
could not be qualified as far.
The 16-bit compilers have used the far attribute with both variables and functions.
The 32-bit compilers have used the far attribute with functions, only.
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2.5.3.3
MIGRATION TO THE CCI
For 8-bit compilers, change any occurrence of the far qualifier, e.g., from:
far char template[20];
to:
__far, i.e., __far char template[20];
In the 16- and 32-bit compilers, change any occurrence of the far attribute, e.g., from:
void bar(void) __attribute__ ((far));
int tblIdx __attribute__ ((far));
to:
void __far bar(void);
int __far tblIdx;
2.5.3.4
CAVEATS
None.
2.5.4
Near Objects
The __near qualifier can be used to indicate that variables or functions are located in
‘near memory’. Exactly what constitutes near memory is dependent on the target
device, but it is typically memory that can be accessed with less complex code. Expressions involving near-qualified objects generally are faster and result in smaller code.
Use the native keywords discussed in the Differences section to look up information on
the semantics of this qualifier.
Some devices may not have such memory implemented; in which case, use of this
qualifier is ignored. Stack-based (auto and parameter) variables cannot use the
__near specifier.
2.5.4.1
EXAMPLE
The following shows a variable and function qualified using __near.
__near int serialNo;
__near int ext_getCond(int selector);
2.5.4.2
DIFFERENCES
The 8-bit compilers have used the qualifier near to indicate this meaning. Functions
could not be qualified as near.
The 16-bit compilers have used the near attribute with both variables and functions.
The 32-bit compilers have used the near attribute for functions, only.
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2.5.4.3
MIGRATION TO THE CCI
For 8-bit compilers, change any occurrence of the near qualifier to __near, e.g., from:
near char template[20];
to:
__near char template[20];
In 16- and 32-bit compilers, change any occurrence of the near attribute to __near,
e.g., from:
void bar(void) __attribute__ ((near));
int tblIdx __attribute__ ((near));
to:
void __near bar(void);
int __near tblIdx;
2.5.4.4
CAVEATS
None.
2.5.5
Persistent Objects
The __persistent qualifier can be used to indicate that variables should not be
cleared by the runtime startup code.
Use the native keywords discussed in the Differences section to look up information on
the semantics of this qualifier.
2.5.5.1
EXAMPLE
The following shows a variable qualified using __persistent.
__persistent int serialNo;
2.5.5.2
DIFFERENCES
The 8-bit compilers have used the qualifier, persistent, to indicate this meaning.
The 16- and 32-bit compilers have used the persistent attribute with variables to
indicate they were not to be cleared.
2.5.5.3
MIGRATION TO THE CCI
With 8-bit compilers, change any occurrence of the persistent qualifier to
__persistent, e.g., from:
persistent char template[20];
to:
__persistent char template[20];
For the 16- and 32-bit compilers, change any occurrence of the persistent attribute
to __persistent, e.g., from:
int tblIdx __attribute__ ((persistent));
to:
int __persistent tblIdx;
2.5.5.4
CAVEATS
None.
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Common C Interface
2.5.6
X and Y Data Objects
The __xdata and __ydata qualifiers can be used to indicate that variables are
located in special memory regions. Exactly what constitutes X and Y memory is dependent on the target device, but it is typically memory that can be accessed independently
on separate buses. Such memory is often required for some DSP instructions.
Use the native keywords discussed in the "Differences" section to look up information
on the semantics of these qualifiers.
Some devices may not have such memory implemented; in which case, use of these
qualifiers is ignored.
2.5.6.1
EXAMPLE
The following shows a variable qualified using __xdata, as well as another variable
qualified with __ydata.
__xdata char data[16];
__ydata char coeffs[4];
2.5.6.2
DIFFERENCES
The 16-bit compilers have used the xmemory and ymemory space attribute with
variables.
Equivalent specifiers have never been defined for any other compiler.
2.5.6.3
MIGRATION TO THE CCI
For 16-bit compilers, change any occurrence of the space attributes xmemory or
ymemory to __xdata, or __ydata respectively, e.g., from:
char __attribute__((space(xmemory)))template[20];
to:
__xdata char template[20];
2.5.6.4
CAVEATS
None.
2.5.7
Banked Data Objects
The __bank(num) qualifier can be used to indicate that variables are located in a particular data memory bank. The number, num, represents the bank number. Exactly what
constitutes banked memory is dependent on the target device, but it is typically a subdivision of data memory to allow for assembly instructions with a limited address width
field.
Use the native keywords discussed in the Differences section to look up information on
the semantics of these qualifiers.
Some devices may not have banked data memory implemented; in which case, use of
this qualifier is ignored. The number of data banks implemented will vary from one
device to another.
2.5.7.1
EXAMPLE
The following shows a variable qualified using __bank().
__bank(0) char start;
__bank(5) char stop;
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2.5.7.2
DIFFERENCES
The 8-bit compilers have used the four qualifiers bank0, bank1, bank2 and bank3 to
indicate the same, albeit more limited, memory placement.
Equivalent specifiers have never been defined for any other compiler.
2.5.7.3
MIGRATION TO THE CCI
For 8-bit compilers, change any occurrence of the bankx qualifiers to __bank(), e.g.,
from:
bank2 int logEntry;
to:
__bank(2) int logEntry;
2.5.7.4
CAVEATS
This feature is not yet implemented in MPLAB XC8.
2.5.8
Alignment of Objects
The __align(alignment) specifier can be used to indicate that variables must be
aligned on a memory address that is a multiple of the alignment specified. The alignment term must be a power of 2. Positive values request that the object’s start address
be aligned; negative values imply the object’s end address be aligned.
Use the native keywords discussed in the Differences section to look up information on
the semantics of this specifier.
2.5.8.1
EXAMPLE
The following shows variables qualified using __align() to ensure they end on an
address that is a multiple of 8, and start on an address that is a multiple of 2,
respectively.
__align(-8) int spacer;
__align(2) char coeffs[6];
2.5.8.2
DIFFERENCES
An alignment feature has never been implemented on 8-bit compilers.
The 16- and 32-bit compilers used the aligned attribute with variables.
2.5.8.3
MIGRATION TO THE CCI
For 16- and 32-bit compilers, change any occurrence of the aligned attribute to
__align, e.g., from:
char __attribute__((aligned(4)))mode;
to:
__align(4) char mode;
2.5.8.4
CAVEATS
This feature is not yet implemented on MPLAB XC8.
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Common C Interface
2.5.9
EEPROM Objects
The __eeprom qualifier can be used to indicate that variables should be positioned in
EEPROM.
Use the native keywords discussed in the Differences section to look up information on
the semantics of this qualifier.
Some devices may not implement EEPROM. Use of this qualifier for such devices
generates a warning. Stack-based (auto and parameter) variables cannot use the
__eeprom specifier.
2.5.9.1
EXAMPLE
The following shows a variable qualified using __eeprom.
__eeprom int serialNos[4];
2.5.9.2
DIFFERENCES
The 8-bit compilers have used the qualifier, eeprom, to indicate this meaning for some
devices.
The 16-bit compilers have used the space attribute to allocate variables to the memory
space used for EEPROM.
2.5.9.3
MIGRATION TO THE CCI
For 8-bit compilers, change any occurrence of the eeprom qualifier to __eeprom, e.g.,
from:
eeprom char title[20];
to:
__eeprom char title[20];
For 16-bit compilers, change any occurrence of the eedata space attribute to
__eeprom, e.g., from:
int mainSw __attribute__ ((space(eedata)));
to:
int __eeprom mainSw;
2.5.9.4
CAVEATS
MPLAB XC8 does not implement the __eeprom qualifiers for any PIC18 devices; this
qualifier works as expected for other 8-bit devices.
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2.5.10
Interrupt Functions
The __interrupt(type) specifier can be used to indicate that a function is to act as
an interrupt service routine. The type is a comma-separated list of keywords that
indicate information about the interrupt function.
The current interrupt types are:
<empty>
Implement the default interrupt function.
low_priority
The interrupt function corresponds to the low priority interrupt
source.
(MPLAB XC8 - PIC18 only)
high_priority
The interrupt function corresponds to the high priority interrupt
source.
(MPLAB XC8)
save(symbol-list)
Save the listed symbols on entry, and restore on exit.
(MPLAB XC16)
irq(irqid)
Specify the interrupt vector associated with this interrupt.
(MPLAB XC16 and XC8)
altirq(altirqid)
Specify the alternate interrupt vector associated with this
interrupt.
(MPLAB XC16)
base(address)
Specify vector table address.
(MPLAB XC8)
preprologue(asm) Specify assembly code to be executed before any
compiler-generated interrupt code.
(MPLAB XC16)
shadow
Allow the ISR to utilize the shadow registers for context
switching.
(MPLAB XC16)
auto_psv
The ISR will set the PSVPAG register and restore it on exit.
(MPLAB XC16)
no_auto_psv
The ISR will not set the PSVPAG register.
(MPLAB XC16)
Use the native keywords discussed in the Differences section to look up information on
the semantics of this specifier.
Some devices may not implement interrupts. Use of this qualifier for such devices
generates a warning. If the argument to the __interrupt specifier does not make
sense for the target device, a warning or error is issued by the compiler.
2.5.10.1
EXAMPLE
The following shows a function qualified using __interrupt.
__interrupt(low_priority) void getData(void) {
if (TMR0IE && TMR0IF) {
TMR0IF=0;
++tick_count;
}
}
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Common C Interface
2.5.10.2
DIFFERENCES
The 8-bit compilers have used the interrupt and low_priority qualifiers to
indicate this meaning for some devices. Interrupt routines were, by default, high priority.
The __interrupt() specifier may now be used outside of the CCI.
The 16- and 32-bit compilers have used the interrupt attribute to define interrupt
functions.
2.5.10.3
MIGRATION TO THE CCI
For 8-bit compilers, change any occurrence of the interrupt qualifier, e.g., from:
void interrupt myIsr(void)
void interrupt low_priority myLoIsr(void)
to the following, respectively:
void __interrupt(high_priority) myIsr(void)
void __interrupt(low_priority) myLoIsr(void)
For 16-bit compilers, change any occurrence of the interrupt attribute, e.g., from:
void _attribute_((interrupt(auto_psv,irq(52))))
_T1Interrupt(void);
to:
void __interrupt(auto_psv,irq(52))) _T1Interrupt(void);
For 32-bit compilers, the __interrupt() keyword takes two parameters, the vector
number and the (optional) IPL value. Change code that uses the interrupt attribute,
similar to these examples:
void __attribute__((vector(0), interrupt(IPL7AUTO), nomips16))
myisr0_7A(void) {}
void __attribute__((vector(1), interrupt(IPL6SRS), nomips16))
myisr1_6SRS(void) {}
/* Determine IPL and context-saving mode at runtime */
void __attribute__((vector(2), interrupt(), nomips16))
myisr2_RUNTIME(void) {}
to:
void __interrupt(0,IPL7AUTO) myisr0_7A(void) {}
void __interrupt(1,IPL6SRS) myisr1_6SRS(void) {}
/* Determine IPL and context-saving mode at runtime */
void __interrupt(2) myisr2_RUNTIME(void) {}
2.5.10.4
CAVEATS
None.
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MPLAB® XC8 C Compiler User’s Guide
2.5.11
Packing Objects
The __pack specifier can be used to indicate that structures should not use memory
gaps to align structure members, or that individual structure members should not be
aligned.
Use the native keywords discussed in the Differences section to look up information on
the semantics of this specifier.
Some compilers cannot pad structures with alignment gaps for some devices, and use
of this specifier for such devices is ignored.
2.5.11.1
EXAMPLE
The following shows a structure qualified using __pack, as well as a structure where
one member has been explicitly packed.
__pack struct DATAPOINT {
unsigned char type;
int value;
} x-point;
struct LINETYPE {
unsigned char type;
__pack int start;
long total;
} line;
2.5.11.2
DIFFERENCES
The __pack specifier is a new CCI specifier that is available with MPLAB XC8. This
specifier has no apparent effect since the device memory is byte addressable for all
data objects.
The 16- and 32-bit compilers have used the packed attribute to indicate that a
structure member was not aligned with a memory gap.
2.5.11.3
MIGRATION TO THE CCI
No migration is required for MPLAB XC8.
For 16- and 32-bit compilers, change any occurrence of the packed attribute, e.g.,
from:
struct DOT
{
char a;
int x[2] __attribute__ ((packed));
};
to:
struct DOT
{
char a;
__pack int x[2];
};
Alternatively, you can pack the entire structure, if required.
2.5.11.4
CAVEATS
None.
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Common C Interface
2.5.12
Indicating Antiquated Objects
The __deprecate specifier can be used to indicate that an object has limited longevity and should not be used in new designs. It is commonly used by the compiler vendor
to indicate that compiler extensions or features can become obsolete, or that better
features have been developed and should be used in preference.
Use the native keywords discussed in the Differences section to look up information on
the semantics of this specifier.
2.5.12.1
EXAMPLE
The following shows a function that uses the __deprecate keyword.
void __deprecate getValue(int mode)
{
//...
}
2.5.12.2
DIFFERENCES
No deprecate feature was implemented on 8-bit compilers.
The 16- and 32-bit compilers have used the deprecated attribute (note the different
spelling) to indicate that objects should be avoided, if possible.
2.5.12.3
MIGRATION TO THE CCI
For 16- and 32-bit compilers, change any occurrence of the deprecated attribute to
__deprecate, e.g., from:
int __attribute__(deprecated) intMask;
to:
int __deprecate intMask;
2.5.12.4
CAVEATS
None.
2.5.13
Assigning Objects to Sections
The __section() specifier can be used to indicate that an object should be located
in the named section (or psect, using MPLAB XC8 terminology). This is typically used
when the object has special and unique linking requirements that cannot be addressed
by existing compiler features.
Use the native keywords discussed in the Differences section to look up information on
the semantics of this specifier.
2.5.13.1
EXAMPLE
The following shows a variable which uses the __section keyword.
int __section("comSec") commonFlag;
2.5.13.2
DIFFERENCES
The 8-bit compilers have previously used the #pragma psect directive to redirect
objects to a new section, or psect; however, the __section() specifier is the
preferred method to perform this task, even if you are not using the CCI.
The 16- and 32-bit compilers have used the section attribute to indicate a different
destination section name. The __section() specifier works in a similar way to the
attribute.
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2.5.13.3
MIGRATION TO THE CCI
For MPLAB XC8, change any occurrence of the #pragma psect directive, such as:
#pragma psect text%%u=myText
int getMode(int target) {
//...
}
to the __section() specifier, as in:
int __section ("myText") getMode(int target) {
//...
}
For 16- and 32-bit compilers, change any occurrence of the section attribute, e.g.,
from:
int __attribute__((section("myVars"))) intMask;
to:
int __section("myVars") intMask;
2.5.13.4
CAVEATS
None.
2.5.14
Specifying Configuration Bits
The #pragma config directive can be used to program the Configuration bits for a
device. The pragma has the form:
#pragma config setting = state|value
where setting is a configuration setting descriptor (e.g., WDT), state is a descriptive
value (e.g., ON) and value is a numerical value.
Use the native keywords discussed in the Differences section to look up information on
the semantics of this directive.
2.5.14.1
EXAMPLE
The following shows Configuration bits being specified using this pragma.
#pragma config WDT=ON, WDTPS = 0x1A
2.5.14.2
DIFFERENCES
The 8-bit compilers have used the __CONFIG() macro for some targets that did not
already have support for the #pragma config.
The 16-bit compilers have used a number of macros to specify the configuration
settings.
The 32-bit compilers supported the use of #pragma config.
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Common C Interface
2.5.14.3
MIGRATION TO THE CCI
For the 8-bit compilers, change any occurrence of the __CONFIG() macro, e.g.,
__CONFIG(WDTEN & XT & DPROT)
to the #pragma config directive, e.g.,
#pragma config WDTE=ON, FOSC=XT, CPD=ON
No migration is required if the #pragma config was already used.
For the 16-bit compilers, change any occurrence of the _FOSC() or _FBORPOR()
macros attribute, e.g., from:
_FOSC(CSW_FSCM_ON & EC_PLL16);
to:
#pragma config FCKSMEM = CSW_ON_FSCM_ON,
FPR = ECIO_PLL16
No migration is required for 32-bit code.
2.5.14.4
CAVEATS
None.
2.5.15
Manifest Macros
The CCI defines the general form for macros that manifest the compiler and target
device characteristics. These macros can be used to conditionally compile alternate
source code based on the compiler or the target device.
The macros and macro families are details in Table 2-1.
TABLE 2-1:
MANIFEST MACROS DEFINED BY THE CCI
Name
Meaning if defined
Example
__XC__
Compiled with an MPLAB XC compiler
__XC__
__CCI__
Compiler is CCI compliant and CCI enforcement is enabled
__CCI__
__XC##__
The specific XC compiler used (## can be 8,
16 or 32)
__XC8__
__DEVICEFAMILY__
The family of the selected target device
__dsPIC30F__
__DEVICENAME__
The selected target device name
__18F452__
2.5.15.1
EXAMPLE
The following shows code that is conditionally compiled dependent on the device
having EEPROM memory.
#ifdef __XC16__
void __interrupt(__auto_psv__) myIsr(void)
#else
void __interrupt(low_priority) myIsr(void)
#endif
2.5.15.2
DIFFERENCES
Some of these CCI macros are new (for example __CCI__), and others have different
names to previous symbols with identical meaning (e.g., __18F452 is now
__18F452__).
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2.5.15.3
MIGRATION TO THE CCI
Any code that uses compiler-defined macros needs review. Old macros will continue to
work as expected, but they are not compliant with the CCI.
2.5.15.4
CAVEATS
None.
2.5.16
In-line Assembly
The asm() statement can be used to insert assembly code in-line with C code. The
argument is a C string literal that represents a single assembly instruction. Obviously,
the instructions contained in the argument are device specific.
Use the native keywords discussed in the Differences section to look up information on
the semantics of this statement.
2.5.16.1
EXAMPLE
The following shows a MOVLW instruction being inserted in-line.
asm("MOVLW _foobar");
2.5.16.2
DIFFERENCES
The 8-bit compilers have used either the asm() or #asm ... #endasm constructs to
insert in-line assembly code.
This is the same syntax used by the 16- and 32-bit compilers.
2.5.16.3
MIGRATION TO THE CCI
For 8-bit compilers, change any instance of #asm ... #endasm, so that each instruction
in the #asm block is placed in its own asm()statement, e.g., from:
#asm
MOVLW 20
MOVWF _i
CLRF
Ii+1
#endasm
to:
asm("MOVLW20");
asm("MOVWF _i");
asm("CLRFIi+1");
No migration is required for the 16- or 32-bit compilers.
2.5.16.4
CAVEATS
None.
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Common C Interface
2.6
COMPILER FEATURES
The following item details the compiler options used to control the CCI.
2.6.1
Enabling the CCI
It is assumed that you are using the MPLAB X IDE to build projects that use the CCI.
The widget in the MPLAB X IDE Project Properties to enable CCI conformance is Use
CCI Syntax in the Compiler category.
If you are not using this IDE, then the command-line options are --EXT=cci for
MPLAB XC8 or -mcci for MPLAB XC16 and XC32.
2.6.1.1
DIFFERENCES
This option has never been implemented previously.
2.6.1.2
MIGRATION TO THE CCI
Enable the option.
2.6.1.3
CAVEATS
None.
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NOTES:
DS50002053G-page 42
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MPLAB® XC8 C COMPILER
USER’S GUIDE
Chapter 3. How To’s
3.1
INTRODUCTION
This section contains help and references for situations that are frequently encountered
when building projects for Microchip 8-bit devices. Click the links at the beginning of
each section to assist in finding the topic relevant to your question. Some topics are
indexed in multiple sections.
Start here:
• Installing and Activating the Compiler
• Invoking the Compiler
• Writing Source Code
• Getting My Application to Do What I Want
• Understanding the Compilation Process
• Fixing Code That Does Not Work
3.2
INSTALLING AND ACTIVATING THE COMPILER
This section details questions that might arise when installing or activating the compiler.
• How Do I Install and Activate My Compiler?
• How Can I Tell if the Compiler has Activated Successfully?
• Can I Install More Than One Version of the Same Compiler?
3.2.1
How Do I Install and Activate My Compiler?
Installation of the compiler and activation of the license are performed simultaneously
by the XC compiler installer. The guide Installing and Licensing MPLAB XC C Compilers (DS52059) is available on www.microchip.com/compilers, under the Documentation tab. It provides details on single-user and network licenses, as well as how to
activate a compiler for evaluation purposes.
3.2.2
How Can I Tell if the Compiler has Activated Successfully?
If you think the compiler cannot have installed correctly or is not working, it is best to
verify its operation outside of MPLAB X IDE to isolate possible problems.
The xclm application can be queried to determine the status of your compiler. From
your terminal or DOS-prompt, type the following line.
"C:\Program Files\Microchip\xc8\v1.00\bin\xclm" -status
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3.2.3
Can I Install More Than One Version of the Same Compiler?
Yes, the compilers and installation process has been designed to allow you to have
more than one version of the same compiler installed, and you can easily move
between the versions by changing options in MPLAB X IDE; see Section 3.3.4 “How
Can I Select Which Compiler I Want to Build With?”.
Compilers should be installed into a directory whose name is related to the compiler
version. This is reflected in the default directory specified by the installer. For example,
the 1.00 and 1.10 MPLAB XC8 compilers would typically be placed in separate
directories.
C:\Program Files\Microchip\xc8\v1.00\
C:\Program Files\Microchip\xc8\v1.10\
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How To’s
3.3
INVOKING THE COMPILER
This section discusses how the compiler is run, on the command-line or from the
MPLAB X IDE. It includes information about how to get the compiler to do what you
want it to do, in terms of options and the build process itself.
• How Do I Compile From Within MPLAB X IDE?
• How Do I Compile on the Command-line?
• How Do I Compile Using a Make Utility?
• How Can I Select Which Compiler I Want to Build With?
• How Can I Change the Compiler's Operating Mode?
• How Do I Build Libraries?
• How Do I Know What Compiler Options Are Available and What They Do?
• How Do I Know What the Build Options in MPLAB X IDE Do?
• What is Different About an MPLAB X IDE Debug Build?
See, also, the following linked information in other sections.
• What Do I Need to Do When Compiling to Use a Debugger?
• How Do I Use Library Files in My Project?
• How Do I Place a Function Into a Unique Section?
• What Optimizations Are Employed by the Compiler?
3.3.1
How Do I Compile From Within MPLAB X IDE?
MPLAB X IDE User’s Guide and online help provide directions for setting up a project
in the MPLAB X integrated development environment.
Alternatively, download the MPLAB® XC8 User’s Guide for Embedded Engineers
(DS50002400) or open the MPLAB® XC8 Getting Started Guide (DS50002173) from
the compiler’s docs directory.
3.3.2
How Do I Compile on the Command-line?
The compiler driver is called xc8 for all 8-bit PIC devices; e.g., in Windows, it is named
xc8.exe. This application should be invoked for all aspects of compilation. It is located
in the bin directory of the compiler distribution. Avoid running the individual compiler
applications (such as the assembler or linker) explicitly. You can compile and link in the
one command, even if your project is spread among multiple source files.
The driver is introduced in Section 4.2 “Invoking the Compiler”. See
Section 3.3.4 “How Can I Select Which Compiler I Want to Build With?”, to ensure you
are running the correct driver if you have more than one installed. The command-line
options to the driver are detailed in Section 4.7 “MPLAB XC8 Driver Options”. The files
that can be passed to the driver are listed and described in Section 4.2.3 “Input File
Types”.
3.3.3
How Do I Compile Using a Make Utility?
When compiling using a make utility (such as make), the compilation is usually performed as a two-step process: first generating the intermediate files, then the final compilation and link step to produce one binary output. This is described in
Section 4.3.3 “Multi-Step Compilation”.
The MPLAB XC8 compiler uses a unique technology called OCG that uses an intermediate file format that is different than traditional compilers (including XC16 and XC32).
The intermediate file format used by XC8 is a p-code file (.p1 extension), not an object
file. Generating object files as an intermediate file for multi-step compilation defeats
many of the advantages of this technology.
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3.3.4
How Can I Select Which Compiler I Want to Build With?
The compilation and installation process has been designed to allow you to have more
than one compiler installed at the same time. You can create a project in MPLAB X IDE
and then build this project with different compilers by simply changing a setting in the
project properties.
To select which compiler is actually used when building a project under MPLAB X IDE,
go to the Project Properties dialog. Select the Configuration category in the Project
Properties dialog (Conf: [default]). A list of MPLAB XC8 compilers is shown in the
Compiler Toolchain, on the far right. Select the compiler that you require.
Once selected, the controls for that compiler are then shown by selecting the MPLAB
XC8 global options, MPLAB XC8 Compiler and MPLAB XC8 Linker categories. These
reveal a pane of options on the right. Note that each category has several panes which
can be selected from a pull-down menu that is near the top of the pane.
3.3.5
How Can I Change the Compiler's Operating Mode?
The compiler’s operating mode (Free, Standard or PRO, see Section 1.2 “Compiler
Description and Documentation”) can be specified as a command line option when
building on the command line; see Section 4.8.39 “--MODE: Choose Compiler Operating Mode”. If you are building under MPLAB X IDE, there is a Project Properties selector in the XC8 Compiler category, under the Optimizations option selector; see
Section 4.9.2 “Compiler Category”.
You can only select modes that your license entitles you to use. The Free mode is
always available; Standard or PRO can be selected if you have purchased a license for
those modes.
3.3.6
How Do I Build Libraries?
Note that XC8 uses a different code generation framework (OCG) that uses additional
library files to those used by traditional compilers (including XC16 and XC32). See
Section 4.3.1 “The Compiler Applications”, for general information on the library types
available and how they fit into the compilation process.
When you have functions and data that are commonly used in applications, you can
either make all the C source and header files available so that other developers can
copy these into their projects. Alternatively you can bundle these source files up into a
library which, along with the accompanying header files, can be linked into a project.
Libraries are more convenient because there are fewer files to deal with. Compiling
code from a library can also be fractionally faster. However, libraries do need to be
maintained. XC8 must use LPP libraries for library routines written in C; the old-style
LIB libraries are used for library routines written in assembly source. It is recommended
that even these libraries be rebuilt if your project is moving to a new compiler version.
Using the compiler driver, libraries can be built by listing all of the files that are to be
included into the library on the command line. None of these files should contain a
main() function, nor settings for Configuration bits or any other such data. Use the
--OUTPUT=lpp option; see Section 4.8.47 “--OUTPUT= type: Specify Output File
Type”, to indicate that a library file is required. For example:
XC8 --chip=16f877a --output=lpp lcd.c utils.c io.c
creates a library file called lcd.lpp. You can specify another name using the -O
option; see Section 4.8.9 “-O: Specify Output File”, or just rename the file.
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To build a library in MPLAB X IDE, create a regular project.1 Add your source files in
the usual way. Add in the option --OUTPUT=lpp to the Additional Options field in the
MPLAB XC8 Linker category. Click Build. The IDE will issue a warning about the HEX
file being missing, but this can be ignored. The library output can be found in the
dist/default/production folder of the project directory.
Note that if you intend to step through your library code at a C level in MPLAB X IDE,
you will need to place the library source files so that the relative path between their
location and the project that is using them is the same as the relative path between
where the library build command was executed and where the source files were
located when they were built.
3.3.7
How Do I Know What Compiler Options Are Available and What
They Do?
A list of all compiler options can be obtained by using the --HELP option on the command line; see Section 4.8.34 “--HELP: Display Help”. If you give the --HELP option
an argument, being an option name, it will give specific information on that option, for
example --HELP=runtime.
Alternatively, all options are all listed in Section 4.8 “Option Descriptions” in this user’s
guide. If you are compiling in MPLAB X IDE, see Section 4.9 “MPLAB X Option
Equivalents”.
3.3.8
How Do I Know What the Build Options in MPLAB X IDE Do?
Each of the widgets and controls, in the MPLAB X IDE Project Properties, map directly
to one command-line driver option or suboption, in most instances. Section 4.8 “Option
Descriptions” in this user’s guide lists all command-line driver options and includes
cross references, where appropriate, to corresponding sections that relate to accessing those options from the IDE. (see Section 4.9 “MPLAB X Option Equivalents”).
3.3.9
What is Different About an MPLAB X IDE Debug Build?
In MPLAB X, there are distinct build buttons and menu items to build (production) a
project and to debug a project.
While there are many differences between the builds in the IDE – in the XC8 compiler,
there is very little that is different between the two types of build. The main difference
is the setting of a preprocessor macro called __DEBUG, which is assigned 1 when a
performing a debug build. This macro is not defined for production builds.
You can make code in your source conditional on this macro using #ifdef directives,
etc., (see Section 5.14.2 “Preprocessor Directives”); so that you can have your program behave differently when you are still in a development cycle. Some compiler
errors are easier to track down after performing a debug build.
In MPLAB X IDE, memory is reserved for your debugger (if selected) only when you
perform a debug build. See Section 3.5.4 “What Do I Need to Do When Compiling to
Use a Debugger?” for more information.
1. At present, the IDE library projects are incompatible with MPLAB XC8.
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3.4
WRITING SOURCE CODE
This section presents issues that pertain to the source code you write. It has been
subdivided into the sections listed below.
• C Language Specifics
• Device-Specific Features
• Memory Allocation
• Variables
• Functions
• Interrupts
• Assembly Code
3.4.1
C Language Specifics
This section discusses source code issues that directly relate to the C language itself,
but are commonly asked.
• When Should I Cast Expressions?
• Can Implicit Type Conversions Change the Expected Results of My Expressions?
• How Do I Enter Non-English Characters Into My Program?
• How Can I Use a Variable Defined in Another Source File?
3.4.1.1
WHEN SHOULD I CAST EXPRESSIONS?
Expressions can be explicitly case using the cast operator -- a type in round brackets,
e.g., (int). In all cases, conversion of one type to another must be done with caution
and only when absolutely necessary.
Consider the example:
unsigned long l;
unsigned int i;
i = l;
Here, a long type is being assigned to an int type, and the assignment will truncate
the value in l. The compiler will automatically perform a type conversion from the type
of the expression on the right of the assignment operator (long) to the type of the
lvalue on the left of the operator (int).This is called an implicit type conversion. The
compiler typically produces a warning concerning the potential loss of data by the truncation.
A cast to type int is not required and should not be used in the above example if a
long to int conversion was intended. The compiler knows the types of both operands
and performs the conversion accordingly. If you did use a cast, there is the potential for
mistakes if the code is later changed. For example, if you had:
i = (int)l;
the code works the same way; but if, in future, the type of i is changed to a long, for
example, then you must remember to adjust the cast, or remove it, otherwise the contents of l will continue to be truncated by the assignment, which cannot be correct.
Most importantly, the warning issued by the compiler will not be produced if the cast is
in place.
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How To’s
Only use a cast in situations where the types used by the compiler are not the types
that you require. For example, consider the result of a division assigned to a floating
point variable:
int i, j;
float fl;
fl = i/j;
In this case, integer division is performed, then the rounded integer result is converted
to a float format. So, if i contained 7 and j contained 2, the division yields 3 and this
is implicitly converted to a float type (3.0) and then assigned to fl. If you wanted the
division to be performed in a float format, then a cast is necessary:
fl = (float)i/j;
(Casting either i or j forces the compiler to encode a floating-point division.) The
result assigned to fl now is 3.5.
An explicit cast can suppress warnings that might otherwise have been produced. This
can also be the source of many problems. The more warnings the compiler produces,
the better chance you have of finding potential bugs in your code.
3.4.1.2
CAN IMPLICIT TYPE CONVERSIONS CHANGE THE EXPECTED
RESULTS OF MY EXPRESSIONS?
Yes! The compiler will always use integral promotion and there is no way to disable this;
see Section 5.6.1 “Integral Promotion”. In addition, the types of operands to binary
operators are usually changed so that they have a common type, as specified by the C
Standard. Changing the type of an operand can change the value of the final expression, so it is very important that you understand the type C Standard conversion rules
that apply when dealing with binary operators. You can manually change the type of an
operand by casting; see Section 3.4.1.1 “When Should I Cast Expressions?”.
3.4.1.3
HOW DO I ENTER NON-ENGLISH CHARACTERS INTO MY PROGRAM?
The ANSI standard (and accordingly, the MPLAB XC8 C compiler) does not support
extended characters set in character and string literals in the source character set. See
Section 5.4.6 “Constant Types and Formats”, to see how these characters can be
entered using escape sequences.
3.4.1.4
HOW CAN I USE A VARIABLE DEFINED IN ANOTHER SOURCE FILE?
Provided the variable defined in the other source file is not static (see
Section 5.5.2.1.1 “Static Variables”) or auto (see Section 5.5.2.2 “Auto Variable Allocation and access”), then adding a declaration for that variable into the current file will
allow you to access it. A declaration consists of the keyword extern in addition to the
type and the name of the variable, as specified in its definition, e.g.,
extern int systemStatus;
This is part of the C language. Your favorite C textbook will give you more information.
The position of the declaration in the current file determines the scope of the variable.
That is, if you place the declaration inside a function, it will limit the scope of the variable
to that function. If you place it outside of a function, it allows access to the variable in
all functions for the remainder of the current file.
Often, declarations are placed in header files and then they are #included into the C
source code; see Section 5.14.2 “Preprocessor Directives”.
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3.4.2
Device-Specific Features
This section discusses the code that needs to be written to set up or control a feature
that is specific to Microchip PIC devices.
• How Do I Set the Configuration Bits?
• How Do I Use the PIC Device’s ID Locations?
• How Do I Determine the Cause of Reset on Mid-range Parts?
• How Do I Access SFRs?
• How Do I Place a Function Into a Unique Section?
See, also, the following linked information in other sections.
• What Do I Need to Do When Compiling to Use a Debugger?
3.4.2.1
HOW DO I SET THE CONFIGURATION BITS?
These should be set in your code using either a macro or a pragma. MPLAB 8 IDE
allowed you to set these bits in a dialog, but MPLAB X IDE requires that they be specified in your source code. See Section 5.3.5 “Configuration Bit Access”, for details
about how these are set.
3.4.2.2
HOW DO I USE THE PIC DEVICE’S ID LOCATIONS?
There is a supplied macro or pragma that allows these values to be programmed; see
Section 5.3.7 “ID Locations”.
3.4.2.3
HOW DO I DETERMINE THE CAUSE OF RESET ON MID-RANGE
PARTS?
The TO and PD bits in the STATUS register allow you to determine the cause of a
Reset. However, these bits are quickly overwritten by the runtime startup code that is
executed before main is executed; see Section 5.10.1 “Runtime Startup Code”. You
can have the STATUS register saved into a location that is later accessible from C
code, so that the cause of Reset can be determined by the application after it is running
again; see Section 5.10.1.4 “STATUS Register Preservation”.
3.4.2.4
HOW DO I ACCESS SFRS?
The compiler ships with header files; see Section 5.3.3 “Device Header Files”, that
define the variables that are mapped over the top of memory-mapped SFRs. Since
these are C variables, they can be used like any other C variables and no new syntax
is required to access these registers.
Bits within SFRs can also be accessed. Individual bit-wide variables are defined that
are mapped over the bits in the SFR. Bit-fields are also available in structures that map
over the SFR as a whole. You can use either in your code; see Section 5.3.6 “Using
SFRs From C Code”.
The name assigned to the variable is usually the same as the name specified in the
device data sheet. See Section 3.4.2.5 “How Do I Find The Names Used to Represent
SFRs and Bits?”, if these names are not recognized.
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3.4.2.5
HOW DO I FIND THE NAMES USED TO REPRESENT SFRS AND BITS?
Special function registers and the bits within them are accessed via special variables
that map over the register; see Section 3.4.2.4 “How Do I Access SFRs?”. However,
the names of these variables sometimes differ from those indicated in the data sheet
for the device you are using.
If required, you can examine the <xc.h> header file to find the device-specific header
file that is relevant for your device. This file will define the variables that allow access
to these special variables. However, an easier way to find these variable names is to
look in any of the preprocessed files left behind from a previous compilation. Provided
the corresponding source file included <xc.h>, the preprocessed file will show the
definitions for all the SFR variables and bits for your target device.
If you are compiling under MPLAB X IDE, the preprocessed file(s) are left under the
build/default/production directory of your project for regular builds, or under
build/default/debug for debug builds. They are typically left in the source file
directory if you are compiling on the command line. These files have a .pre extension.
3.4.3
Memory Allocation
Here are questions relating to how your source code affects memory allocation.
• How Do I Position Variables at an Address I Nominate?
• How Do I Place a Variable Into a Unique Section?
• How Do I Position a Variable Into an Address Range?
• How Do I Position Functions at an Address I Nominate?
• How Do I Place Variables in Program Memory?
• How Do I Place a Function Into a Unique Section?
• How Do I Position a Function Into an Address Range?
• How Do I Place a Function Into a Unique Section?
See, also, the following linked information in other sections.
• Why Are Some Objects Positioned Into Memory That I Reserved?
• How Do I Use High-Endurance Flash for Data, Not Code?
3.4.3.1
HOW DO I POSITION VARIABLES AT AN ADDRESS I NOMINATE?
The easiest way to do this is to make the variable absolute by using the @ address
construct, see Section 5.5.4 “Absolute Variables”. This means that the address you
specify is used in preference to the variable’s symbol in generated code. Since you
nominate the address, you have full control over where objects are positioned. But, you
must also ensure that absolute variables do not overlap. Variables placed in the middle
of banks can cause havoc with the allocation of other variables and lead to “Can’t find
space” errors; see Section 3.7.6 “How Do I Fix a “Can’t find space...” Error?”. See also,
Section 5.5.2 “Variables in Data Space Memory” and Section 5.5.3 “Variables in
Program Space” for information on moving variables.
3.4.3.2
HOW DO I PLACE A VARIABLE INTO A UNIQUE SECTION?
Use the __section() specifier to have the variable positioned in a new section
(psect). After this has been done, the section can be linked to the desired address by
using the -L- driver option. See Section 5.15.4 “Changing and Linking the Allocated
Section” for examples of both these operations.
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3.4.3.3
HOW DO I POSITION A VARIABLE INTO AN ADDRESS RANGE?
You need to move the variable into a unique psect (section), define a memory range,
and then place the new section in that range.
Use the __section() specifier to have the variable positioned in a new section. Use
the -L- driver option to define a memory range and to place the new section in that
range. See Section 5.15.4 “Changing and Linking the Allocated Section” for examples
of all these operations.
3.4.3.4
HOW DO I POSITION FUNCTIONS AT AN ADDRESS I NOMINATE?
The easiest way to do this is to make the functions absolute by using the @ address
construct, see Section 5.8.4 “Changing the Default Function Allocation”. This means
that the address you specify is used in preference to the function’s symbol in generated
code. Since you nominate the address, you have full control over where functions are
positioned, but must also ensure that absolute functions do not overlap. Functions
placed in the middle of pages can cause havoc with the allocation of other functions
and lead to “Can’t find space” errors, see Section 3.7.6 “How Do I Fix a “Can’t find
space...” Error?”.
3.4.3.5
HOW DO I PLACE VARIABLES IN PROGRAM MEMORY?
The const qualifier implies that the qualified variable is read-only. As a consequence
of this, any variables (except for auto variables or function parameters) that are
qualified const are placed in program memory, thus freeing valuable data RAM. See
Section 5.5.3 “Variables in Program Space”, for more information. Variables that are
qualified const can also be made absolute, so that they can be positioned at an
address you nominate; see Section 5.5.4.2 “Absolute Objects in Program Memory”.
3.4.3.6
HOW DO I PLACE A FUNCTION INTO A UNIQUE SECTION?
Use the __section() specifier to have the function positioned into a new section
(psect). When this has been done, the section can be linked to the desired address by
using the -L- driver option. See Section 5.15.4 “Changing and Linking the Allocated
Section” for examples of both these operations.
3.4.3.7
HOW DO I POSITION A FUNCTION INTO AN ADDRESS RANGE?
Having one or more functions located in a special area of memory might mean that you
can ensure they are code protected, for example. To do this, you need to move the
function into a unique section (psect), define a memory range, and then place the new
section in that range.
Use the __section() specifier to have the function positioned into a new section.
Use the -L- driver option to define a memory range and to place the new section into
that range. See Section 5.15.4 “Changing and Linking the Allocated Section” for examples of all these operations.
3.4.3.8
HOW DO I STOP THE COMPILER FROM USING CERTAIN MEMORY
LOCATIONS?
Memory can be reserved when you build. The --RAM and --ROM options allow you to
adjust the ranges of data and program memory, respectively, when you build; see
Section 4.8.52 “--RAM: Adjust RAM Ranges”, and Section 4.8.53 “--ROM: Adjust ROM
Ranges”. By default, all the available on-chip memory is available for use. However,
these options allow you to reserve parts of this memory.
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3.4.4
Variables
This sections examines questions that relate to the definition and usage of variables
and types within a program.
• Why Are My Floating-point Results Not Quite What I Am Expecting?
• How Can I Access Individual Bits of a Variable?
• How Long Can I Make My Variable and Macro Names?
See, also, the following linked information in other sections.
• How Do I Share Data Between Interrupt and Main-line Code?
• How Do I Position Variables at an Address I Nominate?
• How Do I Place Variables in Program Memory?
• How Do I Place Variables in the PIC18 Device’s External Program Memory?
• How Can I Rotate a Variable?
• How Do I Utilize/Allocate the RAM Banks on My Device?
• How Do I Utilize the Linear Memory on Enhanced Mid-range PIC Devices?
• How Do I Find Out Where Variables and Functions Have Been Positioned?
3.4.4.1
WHY ARE MY FLOATING-POINT RESULTS NOT QUITE WHAT I AM
EXPECTING?
First, if you are watching floating-point variables in MPLAB X IDE, make sure that their
type and size agree with the way in which they are defined. For 24-bit floating point variables (whether they have type float or double), ensure that in MPLAB X IDE the
Display Column Value As popup menu to IEEE Float (24 bit). If the variable is a 32-bit
floating point object, set the types to IEEE Float.
The size of the floating point type can be adjusted for both float and double types;
see Section 4.8.32 “--FLOAT: Select Kind of Float Types”, and
Section 4.8.24 “--DOUBLE: Select Kind of Double Types”.
Since floating-point variables only have a finite number of bits to represent the values
they are assigned, they will hold an approximation of their assigned value; see
Section 5.4.3 “Floating-Point Data Types”. A floating-point variable can only hold one
of a set of discrete real number values. If you attempt to assign a value that is not in
this set, it is rounded to the nearest value. The more bits used by the mantissa in the
floating-point variable, the more values can be exactly represented in the set, and the
average error due to the rounding is reduced.
Whenever floating-point arithmetic is performed, rounding also occurs. This can also
lead to results that do not appear to be correct.
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3.4.4.2
HOW CAN I ACCESS INDIVIDUAL BITS OF A VARIABLE?
There are several ways of doing this. The simplest and most portable way is to define
an integer variable and use macros to read, set, or clear the bits within the variable
using a mask value and logical operations, such as the following.
#define
#define
#define
testbit(var, bit)
setbit(var, bit)
clrbit(var, bit)
((var) & (1 <<(bit)))
((var) |= (1 << (bit)))
((var) &= ~(1 << (bit)))
These, respectively, test to see if bit number, bit, in the integer, var, is set; set the
corresponding bit in var; and clear the corresponding bit in var. Alternatively, a
union of an integer variable and a structure with bit-fields (see
Section 5.4.4.2 “Bit-Fields in Structures”) can be defined, e.g.,
union both {
unsigned char byte;
struct {
unsigned b0:1, b1:1, b2:1, b3:1, b4:1, b5:1, b6:1, b7:1;
} bitv;
} var;
This allows you to access byte as a whole (using var.byte), or any bit within that
variable independently (using var.bitv.b0 through var.bitv.b7).
Note that the compiler does support bit variables (see Section 5.4.2.1 “Bit Data Types
and Variables”), as well as bit-fields in structures.
3.4.4.3
HOW LONG CAN I MAKE MY VARIABLE AND MACRO NAMES?
The C Standard indicates that a only a specific number of initial characters in an identifier are significant, but it does not actually state what this number is and it varies from
compiler to compiler. For XC8, the first 255 characters are significant, but this can be
reduced using the -N option; see Section 4.8.8 “-N: Identifier Length”. The fewer characters there are in your variable names, the more portable your code. Using the -N
option allows the compiler to check that your identifiers conform to a specific length.
This option affects variable and function names, as well as preprocessor macro names.
If two identifiers only differ in the non-significant part of the name, they are considered
to represent the same object, which will almost certainly lead to code failure.
3.4.5
Functions
This section examines questions that relate to functions.
• What is the Optimum Size For Functions?
• How Do I Stop An Unused Function Being Removed?
• How Do I Make a Function Inline?
See, also, the following linked information in other sections.
• How Can I Tell How Big a Function Is?
• How Do I Position Functions at an Address I Nominate?
• How Do I Know Which Resources Are Being Used by Each Function?
• How Do I Find Out Where Variables and Functions Have Been Positioned?
• How Do I Use Interrupts in C?
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3.4.5.1
WHAT IS THE OPTIMUM SIZE FOR FUNCTIONS?
Generally speaking, the source code for functions should be kept small, as this aids in
readability and debugging. It is much easier to describe and debug the operation of a
function that performs a small number of tasks. And, they typically have fewer side
effects, which can be the source of coding errors.
In the embedded programming world, a large number of small functions, and the calls
necessary to execute them, can result in excessive memory and stack usage, so a
compromise is often necessary.
The PIC10/12/16 devices employ pages in the program memory that are used to store
and execute function code. Although you are able to write C functions that will generate
more than one page of assembly code, functions of such a size should be avoided and
split into smaller routines where possible. The assembly call and jump sequences to
locations in other pages are much longer than those made to destinations in the same
page. If a function is so large as to cross a page boundary, then loops (or other code
constructs that require jumps within that function) can use the longer form of jump on
each iteration; see Section 5.8.3 “Allocation of Executable Code”.
PIC18 devices are less affected by internal memory paging and the instruction set
allows for calls and jumps to any destination with no penalty. But you should still
endeavor to keep functions as small as possible.
Interrupt functions must be written so that they do not exceed the size of a memory
page. They cannot be split to occupy more than one page.
With all devices, the smaller the function, the easier it is for the linker to allocate it to
memory without errors.
3.4.5.2
HOW DO I STOP AN UNUSED FUNCTION BEING REMOVED?
If a C function’s symbol is referenced in hand-written assembly code, the function will
never be removed, even if it is not called or never had its address taken in C code.
Create an assembly source file and add this file to your project. You only have to
reference the symbol in this file; so, the file can contain the following
GLOBAL _myFunc
where myFunc is the C name of the function in question (note the leading underscore
in the assembly name, see Section 5.12.3.1 “Equivalent Assembly Symbols”). This is
sufficient to prevent the function removal optimization from being performed.
3.4.5.3
HOW DO I MAKE A FUNCTION INLINE?
You can ask the compiler to inline a function by using the inline specifier (see
Section 5.8.1.2 “Inline Specifier”) or #pragma inline. This is only a suggestion to the
compiler and cannot always be obeyed. Do not confuse this specifier/pragma with the
intrinsic pragma1 (see Section 5.14.4.4 “The #pragma Intrinsic Directive”), which
is for functions that have no corresponding source code and which will be specifically
expanded by the code generator during compilation.
1. This specifier was originally named in-line but was changed to avoid confusion.
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3.4.6
Interrupts
Interrupt and interrupt service routine questions are discussed in this section.
How Do I Use Interrupts in C?
See, also, the following linked information in other sections.
• How Can I Make My Interrupt Routine Faster?
• How Do I Share Data Between Interrupt and Main-line Code?
3.4.6.1
HOW DO I USE INTERRUPTS IN C?
First, be aware of what sort of interrupt hardware is available on your target device.
Most baseline PIC devices do not implement interrupts at all; baseline devices with
interrupts and mid-range devices utilize a single interrupt vector, and PIC18 devices
implement two separate interrupt vector locations and use a simple priority scheme.
Some PIC18 devices use an interrupt controller macro module that can use a vector
table to invoke multiple interrupt functions.
In C source code, a function can be written to act as the interrupt service routine; see
Section 5.9.1 “Writing an Interrupt Service Routine”. Such functions save and/or
restore program context before and/or after executing the function body code and a
different return instruction is used, see Section 5.9.4 “Context Switching”.
Code inside the interrupt function can do anything you like, but see Section 3.6.7 “How
Can I Make My Interrupt Routine Faster?” for suggestions to enhance real-time
performance.
Prior to any interrupt occurring, your program must ensure that peripherals are
correctly configured and that interrupts are enabled; see Section 5.9.5 “Enabling
Interrupts”. On PIC18 devices, you must specify the priority of interrupt sources by
writing the appropriate SFRs.
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3.4.7
Assembly Code
This section examines questions that arise when writing assembly code as part of a C
project.
• How Should I Combine Assembly and C Code?
• What Do I Need Other than Instructions in an Assembly Source File?
• How Do I Access C Objects from Assembly Code?
• How Can I Access SFRs from Within Assembly Code?
• What Things Must I Manage When Writing Assembly Code?
3.4.7.1
HOW SHOULD I COMBINE ASSEMBLY AND C CODE?
Ideally, any hand-written assembly should be written as separate routines that can be
called. This offers some degree of protection from interaction between compiler-generated and hand-written assembly code. Such code can be placed into a separate
assembly module that can be added to your project; see Section 5.12.1 “Integrating
Assembly Language Modules”.
If necessary, assembly code can be added in-line with C code using either of two methods; see Section 5.12.2 “#asm, #endasm and asm()”. The code added in-line should
ideally be limited to instructions such as NOP, SLEEP or CLRWDT. Macros are already
provided which in-line all these instructions; see Appendix A. Library Functions. More
complex in-line assembly that changes register contents and the device state can
cause code failure if precautions are not taken and should be used with caution. See
Section 5.7 “Register Usage” for those registers used by the compiler.
3.4.7.2
WHAT DO I NEED OTHER THAN INSTRUCTIONS IN AN ASSEMBLY
SOURCE FILE?
Assembly code typically needs assembler directives as well as the instructions themselves. The operation of all the directives are described in the subsections of
Section 6.2.9 “Assembler Directives”. Common directives required are mentioned
below.
All assembly code must be placed in a psect so it can be manipulated as a whole by
the linker and placed in memory. See Section 5.15.1 “Program Sections” for general
information on psects; see Section 6.2.9.3 “PSECT” for information on the directive
used to create and specify psects.
The other commonly used directive is GLOBAL, defined in Section 6.2.9.1 “GLOBAL”
which is used to make symbols accessible across multiple source files.
3.4.7.3
HOW DO I ACCESS C OBJECTS FROM ASSEMBLY CODE?
Most C objects are accessible from assembly code. There is a mapping between the
symbols used in the C source and those used in the assembly code generated from
this source. Your assembly should access the assembly-equivalent symbols which are
detailed in Section 5.12.3 “Interaction between Assembly and C Code”.
Instruct the assembler that the symbol is defined elsewhere by using the GLOBAL
assembler directive; see Section 6.2.9.1 “GLOBAL”. This is the assembly equivalent of
a C declaration, although no type information is present. This directive is not needed
and should not be used if the symbol is defined in the same module as your assembly
code.
Any C variable accessed from assembly code will be treated as if it were qualified
volatile; see Section 5.4.7.2 “Volatile Type Qualifier”. Specifically specifying the
volatile qualifier in C code is preferred as it makes it clear that external code can
access the object.
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3.4.7.4
HOW CAN I ACCESS SFRS FROM WITHIN ASSEMBLY CODE?
The safest way to gain access to SFRs in assembly code is to have symbols defined
in your assembly code that equate to the corresponding SFR address. Header files are
provided with the compiler so that you do not need to define these yourselves, and they
are detailed in Section 5.12.3.2 “Accessing Registers from Assembly Code”.
There is no guarantee that you will be able to access symbols generated by the
compilation of C code, even the code that accesses the SFR that you require.
3.4.7.5
WHAT THINGS MUST I MANAGE WHEN WRITING ASSEMBLY CODE?
When writing assembly code by hand, you assume responsibility for managing certain
features of the device and formatting your assembly instructions and operands. The
following list describes some of the actions you must take.
• Whenever you access a RAM variable, you must ensure that the bank of the variable is selected before you read or write the location. This is done by one or more
assembly instructions. The exact code is based on the device you are using and
the location of the variable. Bank selection is not be required if the object is in
common memory, (which is called the access bank on PIC18 devices) or if you
are using an instruction that takes a full address (such as the MOVFF instruction on
PIC18 devices). Check your device data sheet to see the memory architecture of
your device, and the instructions and registers which control bank selection. Failure to select the correct bank will lead to code failure.
The BANKSEL pseudo instruction can be used to simplify this process; see
Section 6.2.1.2 “Bank and Page Selection”.
• You must ensure that the address of the RAM variable you are accessing has
been masked so that only the bank offset is being used as the instruction’s file
register operand. This should not be done if you are using an instruction that takes
a full address (such as the MOVFF instruction on PIC18 devices). Check your
device data sheet to see what address operand instructions requires. Failure to
mask an address can lead to a fixup error (see Section 3.7.8 “How Do I Fix a
Fixup Overflow Error?”) or code failure.
The BANKMASK macro can truncate the address for you; see
Section 5.12.3.2 “Accessing Registers from Assembly Code”.
• Before you call or jump to any routine, you must ensure that you have selected
the program memory page of this routine using the appropriate instructions. You
can either use the PAGESEL pseudo instruction; see Section 6.2.1.2 “Bank and
Page Selection”, or the FCALL or LJMP pseudo instructions (not required on
PIC18 devices); see Section 6.2.1.8 “Long Jumps and Calls” which will
automatically add page selection instructions, if required.
• You must ensure that any RAM used for storage has memory reserved. If you are
only accessing variables defined in C code, then reservation is already done by
the compiler. You must reserve memory for any variables you only use in the
assembly code using an appropriate directive such as DS or DABS; see
Section 6.2.9.11 “DS” or Section 6.2.9.12 “DABS”. It is often easier to define
objects in C code rather than in assembly code.
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• You must place any assembly code you write in a psect (see
Section 6.2.9.3 “PSECT” for the directive to do this, and Section 5.15.1 “Program
Sections” for general information about psects). A psect you define may need
flags (options) to be specified. Take particular notice of the delta, space, reloc
and class flags (see Section 6.2.9.3.4 “Delta”, and Section 6.2.9.3.17 “Space”,
Section 6.2.9.3.15 “Reloc” and Section 6.2.9.3.3 “Class”). If these are not set correctly, compile errors or code failure will almost certainly result. If the psect specifies a class and you are happy with it being placed anywhere in the memory range
defined by that class (see Section 7.2.1 “-Aclass =low-high,...”), it does not need
any additional options to be linked; otherwise, you will need to link the psect using
a linker option (see Section 7.2.18 “-Pspec” for the usual way to link psects and
Section 4.8.6 “-L-: Adjust Linker Options Directly” which indicates how you can
specify this option without running the linker directly).
Assembly code that is placed in-line with C code will be placed in the same psect
as the compiler-generated assembly and you should not place this into a separate
psect.
• You must ensure that any registers you write to in assembly code are not already
in used by compiler-generated code. If you write assembly in a separate module,
then this is less of an issue because the compiler will, by default, assume that all
registers are used by these routines (see Section 5.7 “Register Usage”). No
assumptions are made for in-line assembly (although the compiler will assume
that the selected bank was changed by the assembly, see Section 5.12.2 “#asm,
#endasm and asm()”) and you must be careful to save and restore any resources
that you use (modify) and which are already in use by the surrounding compiler-generated code.
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3.5
GETTING MY APPLICATION TO DO WHAT I WANT
This section provides programming techniques, applications and examples. It also
examines questions that relate to making an application perform a specific task.
• What Can Cause Glitches on Output Ports?
• Where Am I Allowed To Manually Link Psects?
• How Do I Link Bootloaders and Downloadable Applications?
• What Do I Need to Do When Compiling to Use a Debugger?
• How Can I Have Code Executed Straight After Reset?
• How Do I Share Data Between Interrupt and Main-line Code?
• How Can I Prevent Misuse of My Code?
• How Do I Use Printf to Send Text to a Peripheral?
• How Do I Setup the Oscillator in My Code?
• How Do I Place Variables in the PIC18 Device’s External Program Memory?
• How Can I Implement a Delay in My Code?
• How Can I Rotate a Variable?
• How Can I Stop Variables Being Cleared at Startup?
• How Do I Use High-Endurance Flash for Data, Not Code?
3.5.1
What Can Cause Glitches on Output Ports?
In most cases, this is caused by using ordinary variables to access port bits or the entire
port itself. These variables should be qualified volatile.
The value stored in a variable mapped over a port (hence the actual value written to
the port) directly translates to an electrical signal. It is vital that the values held by these
variables only change when the code intends them to, and that they change from their
current state to their new value in a single transition. See Section 5.4.7.2 “Volatile Type
Qualifier”. The compiler attempts to write to volatile variables in one operation.
3.5.2
Where Am I Allowed To Manually Link Psects?
It is recommended that the linker options for compiler-generated psects (sections) are
not modified. If these must be changed or if there are user-defined psects that need
special allocation, there might be device- or compiler-imposed restrictions on where
these can be placed in memory.
Try to link psects in a suitable compiler linker class, as shown in Section 5.15.3 “Default
Linker Classes”, as the definitions for these memory ranges take into consideration any
restrictions. Define your own linker class, if necessary. See for example
Section 3.4.3.3 “How Do I Position a Variable Into an Address Range?”. Check
Section 5.15.2 “Compiler-Generated Psects” to see the memory placement restrictions
that apply to compiler-generated psects which hold similar content to your psect.
Most limitations relate to psects straddling some memory boundary, such as a data
bank or program memory page. One typical limitation is that all psects holding executable code cannot straddle a device page boundary. Compiler-generated psects holding
variables must also be typically linked within the data bank for which they were created.
These boundaries, therefore, impose limits on the size to which the psect can grow.
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3.5.3
How Do I Link Bootloaders and Downloadable Applications?
Exactly how this is done depends on the device you are using and your project requirements, but the general approach when compiling applications that use a bootloader is
to allocate discrete program memory space to the bootloader and application so they
have their own dedicated memory. In this way the operation of one cannot affect the
other. This will require that either the bootloader or the application is offset in memory.
That is, the Reset and interrupt location are offset from address 0 and all program code
is offset by the same amount.
On PIC18 devices, typically the application code is offset, and the bootloader is linked
with no offset so that it populates the Reset and interrupt code locations. The bootloader Reset and interrupt code merely contains code which redirects control to the real
Reset and interrupt code defined by the application and which is offset.
On mid-range devices, this is not normally possible to perform when interrupts are
being used. Consider offsetting all of the bootloader with the exception of the code
associated with Reset, which must always be defined by the bootloader. The application code can define the code linked at the interrupt location. The bootloader will need
to remap any application code that attempts to overwrite the Reset code defined by the
bootloader.
The option --CODEOFFSET, (see Section 4.8.21 “--CODEOFFSET: Offset Program
Code to Address”), allows the program code (Reset and vectors included) to be moved
by a specified amount. The option also restricts the program from using any program
memory from address 0 (Reset vector) to the offset address. Always check the map
file; see Section 7.4.2 “Contents”, to ensure that nothing remains in reserved areas.
The contents of the HEX file for the bootloader can be merged with the code of the
application by adding the HEX file as a project file, either on the command line, or in
MPLAB X IDE. This results in a single HEX file that contains the bootloader and application code in the one image. HEX files are merged by the HEXMATE application; see
Section 8.3 “HEXMATE”. Check for warnings from this application about overlap, which
can indicate that memory is in use by both bootloader and the downloadable application.
3.5.4
What Do I Need to Do When Compiling to Use a Debugger?
You can use debuggers, such as ICD3 or REALICE, to debug code built with the
MPLAB XC8 compiler. These debuggers use some of the data and program memory
of the device for its own use, so it is important that your code does not also use these
resources.
There is a command-line option; see Section 4.8.22 “--DEBUGGER: Select Debugger
Type”, that can be used to tell the compiler which debugger is to be used. The compiler
can then reserve the memory used by the debugger so that your code will not be
located in these locations.
In the MPLAB X IDE, the appropriate debugger option is specified if you perform a
debug build. It will not be specified if you perform a regular Build Project or Clean and
Build.
Since some device memory is being used up by the debugger, there is less available
for your program and it is possible that your code or data might not fit in the device when
a debugger is selected.
Note that which specific memory locations used by the debuggers is an attribute of
MPLAB X IDE, not the device. If you move a project to a new version of the IDE, the
resources required can change. For this reason, you should not manually reserve
memory for the debugger, or make any assumptions in your code as to what memory
is used. A summary of the debugger requirements is available in the MPLAB X IDE help
files.
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To verify that the resources reserved by the compiler match those required by the
debugger, do the following. Compile your code with and without the debugger selected
and keep a copy of the map file produced for both builds. Compare the linker options
in the map files and look for changes in the -A options; see Section 7.2.1 “-Aclass
=low-high,...”. For example, the memory defined for the CODE class with no debugger
might be specified by this option:
-ACODE=00h-0FFh,0100h-07FFh,0800h-0FFFhx3
and with the ICD3 selected as the debugger, it becomes:
-ACODE=00h-0FFh,0100h-07FFh,0800h-0FFFhx2,01800h-01EFFh
This shows that a memory range from 1F00 to 1FFF has been removed by the compiler
and cannot be used by your program. See also Section 3.6.16 “Why Are Some Objects
Positioned Into Memory That I Reserved?”.
3.5.5
How Can I Have Code Executed Straight After Reset?
A special hook has been provided so you can easily add special “powerup” assembly
code that will be linked to the Reset vector; see Section 5.10.2 “The Powerup Routine”.
This code will be executed before the runtime startup code, which in turn is executed
before the main function; see Section 5.10 “Main, Runtime Startup and Reset”.
3.5.6
How Do I Share Data Between Interrupt and Main-line Code?
Variables accessed from both interrupt and main-line code can easily become corrupted or mis-read by the program. The volatile qualifier (see
Section 5.4.7.2 “Volatile Type Qualifier”) tells the compiler to avoid performing optimizations on such variables. This will fix some of the issues associated with this problem.
The other issues relates to whether the compiler/device can access the data atomically.
With 8-bit PIC devices, this is rarely the case. An atomic access is one where the entire
variable is accessed in only one instruction. Such access is uninterruptible. You can
determine if a variable is being accessed atomically by looking at the assembly code
the compiler produces in the assembly list file; see Section 6.4 “Assembly List Files”. If
the variable is accessed in one instruction, it is atomic. Since the way variables are
accessed can vary from statement to statement it is usually best to avoid these issues
entirely by disabling interrupts prior to the variable being accessed in main-line code,
then re-enable the interrupts afterwards; see Section 5.9.5 “Enabling Interrupts”.
3.5.7
How Can I Prevent Misuse of My Code?
First, many devices with flash program memory allow all or part of this memory to be
write protected. The device Configuration bits need to be set correctly for this to take
place; see Section 5.3.5 “Configuration Bit Access” and your device data sheet.
Second, you can prevent third-party code being programmed at unused locations in the
program memory by filling these locations with a value rather than leaving them in an
unprogrammed state. You can chose a fill value that corresponds to an instruction or
set all the bits so as the values cannot be further modified. (Consider what will happen
if your program somehow reaches and starts executing from these filled values.)
The compiler’s HEXMATE utility (see Section 8.3 “HEXMATE”) has the capability to fill
unused locations and this operation can be requested using a command-line driver
option; see Section 4.8.31 “--FILL: Fill Unused Program Memory”. As HEXMATE only
works with HEX files, this feature is only available when producing HEX/COF file outputs (as opposed to binary, for example), which is the default operation.
And last, if you wish to make your library files or intermediate p-code files available to
others but do not want the original source code to be viewable, then you can obfuscate
the files using the --SHROUD option; see Section 4.8.58 “--SHROUD: Obfuscate
P-code Files”
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3.5.8
How Do I Use Printf to Send Text to a Peripheral?
The printf function does two things: it formats text based on the format string and
placeholders you specify, and sends (prints) this formatted text to a destination (or
stream); see Appendix A. Library Functions. The printf function performs all the formatting; then it calls a helper function, called putch, to send each byte of the formatted
text. By customizing the putch function you can have printf send data to any peripheral or location; see Section 5.12 “Mixing C and Assembly Code”. You can choose the
printf output go to an LCD, SPI module or USART, for example.
A stub for the putch function can be found in the compiler’s sources directory. Copy
it into your project then modify it to send the single byte parameter passed to it to the
required destination. Before you can use printf, peripherals that you use will need to
be initialized in the usual way. Here is an example of putch for a USART on a mid-range
device.
void putch(char data) {
while( ! TXIF) // check buffer
continue;
// wait till ready
TXREG = data;
// send data
}
You can get printf to send to one of several destinations by using a global variable
to indicate your choice. Have the putch function send the byte to one of several destinations based on the contents of this variable.
3.5.9
How Do I Setup the Oscillator in My Code?
All PIC devices have several oscillator modes that must be selected by programming
the device’s configuration bits in your project. See your device data sheet for information on the modes and Section 5.3.5 “Configuration Bit Access” for assistance with programming the configuration bits.
Some devices have an OSCCON register, which further controls such runtime attributes as clock sources and internal clock frequencies. In C source, this register can be
written to in the usual way, based on information in your device data sheet.
Some devices allow the internal oscillator to be tuned at runtime via the OSCTUNE register. Other devices allow for calibration of their internal oscillators using values
pre-programmed into the device. The runtime startup code generated by the compiler,
(see Section 5.10.1 “Runtime Startup Code”), will by default provide code that performs
oscillator calibration. This can be disabled, if required, using an option; see
Section 4.8.54 “--RUNTIME: Specify Runtime Environment”.
If you intend to use some of the compiler’s built-in delay functions, you will need to set
the _XTAL_FREQ macro, which indicates the system frequency to the compiler. This
macro in no way affects the operating frequency of the device. See
Section “__DELAY_MS, __DELAY_US, __delaywdt_us, __delaywdt_Ms”.
3.5.10
How Do I Place Variables in the PIC18 Device’s External
Program Memory?
If all you mean to do is place read-only variables in program memory, qualify them as
const; see Section 5.5.3 “Variables in Program Space”. If you intend the variables to
be located in the external program memory then use the far qualifier and specify the
memory using the --RAM option; see Section 4.8.52 “--RAM: Adjust RAM Ranges”.
The compiler will allow far-qualified variables to be modified. Note that the time to
access these variables will be longer than for variables in the internal data memory. The
access mode to external memory can be specified with an option; see
Section 4.8.26 “--EMI: Select External Memory Interface Operating Mode”.
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3.5.11
How Can I Implement a Delay in My Code?
If an accurate delay is required, or if there are other tasks that can be performed during
the delay, then using a timer to generate an interrupt is the best way to proceed.
If these are not issues in your code, then you can use the compiler’s in-built delay
pseudo-functions: _delay, __delay_ms or __delay_us; see Appendix A. Library
Functions. These all expand into in-line assembly instructions or a (nested) loop of
instructions that will consume the specified number of cycles or time. The delay
argument must be a constant and less than 50,463,240.
Note that these code sequences will only use the NOP instruction and/or instructions
which form a loop. The alternate versions of these pseudo-functions, e.g., _delaywdt,
can use the CLRWDT instruction as well.
3.5.12
How Can I Rotate a Variable?
The C language does not have a rotate operator, but rotations can be performed using
the shift and bitwise OR operators. Since the PIC devices have a rotate instruction, the
compiler will look for code expressions that implement rotates (using shifts and ORs)
and use the rotate instruction in the generated output wherever possible; see
Section 5.6.2 “Rotation”.
3.5.13
How Can I Stop Variables Being Cleared at Startup?
Use the persistent qualifier (see Section 5.4.8.1 “Persistent Type Qualifier”), which
will place the variables in a different psect that is not cleared by the runtime startup
code.
3.5.14
How Do I Use High-Endurance Flash for Data, Not Code?
For devices that implement this Flash in the program memory space, it will need to be
reserved so that the compiler does not use it for executable code. See
Section 3.4.3.8 “How Do I Stop the Compiler From Using Certain Memory Locations?”
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3.6
UNDERSTANDING THE COMPILATION PROCESS
This section tells you how to find out what the compiler did during the build process,
how it encoded output code, where it placed objects, etc. It also discusses the features
that are supported by the compiler.
• What’s the Difference Between the Free, Standard and PRO Modes?
• How Can I Make My Code Smaller?
• How Can I Reduce RAM Usage?
• How Can I Make My Code Faster?
• How Can I Speed Up Programming Times
• How Does the Compiler Place Everything in Memory?
• How Can I Make My Interrupt Routine Faster?
• How Big Can C Variables Be?
• How Do I Utilize/Allocate the RAM Banks on My Device?
• How Do I Utilize the Linear Memory on Enhanced Mid-range PIC Devices?
• What Devices are Supported by the Compiler?
• How Do I Know What Code the Compiler Is Producing?
• How Can I Tell How Big a Function Is?
• How Do I Know Which Resources Are Being Used by Each Function?
• How Do I Find Out Where Variables and Functions Have Been Positioned?
• Why Are Some Objects Positioned Into Memory That I Reserved?
• How Do I Know How Much Memory Is Still Available?
• How Do I Use Library Files in My Project?
• What Optimizations Are Employed by the Compiler?
• Why Do I Get Out-of-memory Errors When I Select a Debugger?
• How Do I Know Which Stack Model the Compiler Has Assigned to a Function?
• How Do I Know What Value Has Been Programmed in the Configuration Bits or ID
Location?
See, also, the following linked information in other sections.
• How Do I Find Out What an Warning/Error Message Means?
• What is Different About an MPLAB X IDE Debug Build?
• How Do I Stop An Unused Function Being Removed?
• How Do I Build Libraries?
3.6.1
What’s the Difference Between the Free, Standard and PRO
Modes?
These modes (see Section 1.2 “Compiler Description and Documentation”) mainly differ in the optimizations that are performed when compiling. Compilers operating in Free
(formerly called Lite) and Standard mode can compile for all the same devices as supported by the Pro mode. The code compiled in Free and Standard mode can use all the
available memory for the selected device. What will be different is the size and speed
of the generated compiler output. Free mode output will be much less efficient when
compared to that produced in Standard mode, which in turn will be less efficient than
that produce when in Pro mode.
All these modes use the OCG compiler framework, so the entire C program is compiled
in one step and the source code does not need many non-standard extensions.
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There are a small number of command-line options disabled in Free mode, but these
do not relate to code features; merely how the compiler can be executed. Most
customers never need to use these options. The options are --GETOPTION (see
Section 4.8.33 “--GETOPTION: Get Command-line Options”) and --SETOPTION (see
Section 4.8.57 “--SETOPTION: Set the Command-line Options for Application”).
3.6.2
How Can I Make My Code Smaller?
There are a number of ways that this can be done, but results vary from one project to
the next. Use the assembly list file, (see Section 6.4 “Assembly List Files”), to observe
the assembly code produced by the compiler to verify that the following tips are relevant
to your code.
Use the smallest data types possible as less code is needed to access these. (This also
reduces RAM usage.) Note that a bit type and non-standard 24-bit integer type
(short long) exists for this compiler. Avoid multi-bit bit-fields whenever possible. The
code used to access these can be very large. See Section 5.4 “Supported Data Types
and Variables”, for all data types and sizes.
There are two sizes of floating-point type, as well, and these are discussed in the same
section. Avoid floating-point if at all possible. Consider writing fixed-point arithmetic
code.
Use unsigned types, if possible, instead of signed types; particularly if they are used in
expressions with a mix of types and sizes. Try to avoid an operator acting on operands
with mixed sizes whenever possible.
Whenever you have a loop or condition code, use a “strong” stop condition, i.e., the
following:
for(i=0; i!=10; i++)
is preferable to:
for(i=0; i<10; i++)
A check for equality (== or !=) is usually more efficient to implement than the weaker
< comparison.
In some situations, using a loop counter that decrements to zero is more efficient than
one that starts at zero and counts up by the same number of iterations. This is more
likely to be the case if the loop index is a byte-wide type. So you might be able to rewrite
the above as:
for(i=10; i!=0; i--)
There might be a small advantage in changing the order of function parameters so that
the first parameter is byte sized. A register is used if the first parameter is byte-sized.
For example consider:
char calc(char mode, int value);
over
char calc(int value, char mode);
Ensure that all optimizations are enabled; see Section 4.8.45 “--OPT: Invoke Compiler
Optimizations”. Be aware of what optimizations the compiler performs (see
Section 5.13 “Optimizations”, and Section 6.3 “Assembly-Level Optimizations”) so you
can take advantage of them and don’t waste your time manually performing optimizations in C code that the compiler already handles, e.g., don’t turn a multiply-by-4
operation into a shift-by-2 operation as this sort of optimization is already detected.
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3.6.3
How Can I Reduce RAM Usage?
Use the smallest data types possible. (This also reduces code size as less code is
needed to access these.) Note that a bit type and non-standard 24-bit integer type
(short long) exists for this compiler. See Section 5.4 “Supported Data Types and
Variables” for all data types and sizes. There are two sizes of floating-point type, as
well, and these are discussed in the same section.
Consider using auto variables over global or static variables as there is the potential that these can share memory allocated to other auto variables that are not active
at the same time. Memory allocation of auto variables is made on a compiled stack,
described in Section 5.5.2.2 “Auto Variable Allocation and access”.
Rather than pass large objects to, or from, functions, pass pointers which reference
these objects. This is particularly true when larger structures are being passed, but
there might be RAM savings to be made even when passing long variables.
Objects that do not need to change throughout the program can be located in program
memory using the const qualifier; see Section 5.4.7.1 “Const Type Qualifier”, and
Section 5.5.3 “Variables in Program Space”. This frees up precious RAM, but slows
execution.
Ensure that all optimizations are enabled; see Section 4.8.45 “--OPT: Invoke Compiler
Optimizations”. Be aware of which optimizations the compiler performs (see
Section 5.13 “Optimizations”), so that you can take advantage of them and don’t waste
your time manually performing optimizations in C code that the compiler already
handles.
3.6.4
How Can I Make My Code Faster?
To a large degree, smaller code is faster code, so efforts to reduce code size often
decrease execution time; see Section 3.6.2 “How Can I Make My Code Smaller?”. See
also, Section 3.6.7 “How Can I Make My Interrupt Routine Faster?”. However, there are
ways some sequences can be sped up at the expense of increased code size.
One of the compiler optimization settings is for speed (the alternate setting is for
space), so ensure this is selected; see Section 4.8.45 “--OPT: Invoke Compiler
Optimizations”. This will use alternate output in some instances that is faster, but larger.
Some library multiplication routines operate faster when one of their operands is a
smaller value. See Section 5.3.9 “Multiplication” for more information on how to take
advantage of this.
Generally, the biggest gains to be made in terms of speed of execution come from the
algorithm used in a project. Identify which sections of your program need to be fast.
Look for loops that might be linearly searching arrays and choose an alternate search
method such as a hash table and function. Where results are being recalculated,
consider if they can be cached.
3.6.5
How Can I Speed Up Programming Times
The linker can allocate sections to both ends of program memory: some sections initially placed at a low address and built up through memory; other sections assembled
at a high address and extended down. This does not affect code operation and makes
linking easier, but it can produce a HEX file covering the entire device memory space.
Programming this HEX file into the device may take a long time.
To reduce programming times in this situation, instruct the linker to not use all the
device’s program memory. Use the --ROM option to reserve the upper part of program
memory, see Section 4.8.53 “--ROM: Adjust ROM Ranges”.
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3.6.6
How Does the Compiler Place Everything in Memory?
In most situations, assembly instructions and directives associated with both code and
data are grouped into sections, called psects, and these are then positioned into containers that represent the device memory. An introductory explanation into this process
is given in Section 5.15.1 “Program Sections”. The exception is for absolute variables
(see Section 5.5.4 “Absolute Variables”), which are placed at a specific address when
they are defined and which are not placed in a psect.
3.6.7
How Can I Make My Interrupt Routine Faster?
Consider suggestions made in Section 3.6.2 “How Can I Make My Code Smaller?”
(code size) for any interrupt code. Smaller code is often faster code.
In addition to the code you write in the ISR there is the code the compiler produces to
switch context. This is executed immediately after an interrupt occurs and immediately
before the interrupt returns, so must be included in the time taken to process an interrupt; see Section 5.9.4 “Context Switching”. This code is optimal in that only registers
used in the ISR will be saved by this code. Thus, the less registers used in your ISR
will mean potentially less context switch code to be executed.
Mid-range devices have only a few registers that are used by the compiler, and there
is little context switch code. Even fewer registers are considered for saving when compiling for enhanced mid-range device. PIC18 devices will benefit most from the above
suggestion as they use a larger set of registers in generated code; see
Section 5.7 “Register Usage”.
Generally simpler code will require less resources than more complicated expressions.
Use the assembly list file to see which registers are being used by the compiler in the
interrupt code; see Section 6.4 “Assembly List Files”.
Consider having the ISR simply set a flag and return. The flag can then be checked in
main-line code to handle the interrupt. This has the advantage of moving the complicated interrupt-processing code out of the ISR so that it no longer contributes to its register usage. Always use the volatile qualifier (see Section 5.4.7.2 “Volatile Type
Qualifier”for variables shared by the interrupt and main-line code; see
Section 3.5.6 “How Do I Share Data Between Interrupt and Main-line Code?”.
3.6.8
How Big Can C Variables Be?
This question specifically relates to the size of individual C objects, such as arrays or
structures. The total size of all variables is another matter.
To answer this question you need to know in which memory space the variable will be
located. Objects qualified const will be located in program memory; other objects will
be placed in data memory. Program memory object sizes are discussed in
Section 5.5.3.1 “Size Limitations of Const Variables”. Objects in data memory are
broadly grouped into autos and non-autos and the size limitations of these objects,
respectively, are discussed in Section 5.5.2.2.3 “Size Limits of Auto Variables” and
Section 5.5.2.1.2 “Non-Auto Variable Size Limits”.
3.6.9
How Do I Utilize/Allocate the RAM Banks on My Device?
The compiler will automatically use all the available RAM banks on the device you are
programming. It is only if you wish to alter the default memory allocation that you need
take any action. Special bank qualifiers; see Section 5.4.8.4 “Bank0, Bank1, Bank2 and
Bank3 Type Qualifiers”, and an option (see Section 4.8.15 “--ADDRQUAL: Set Compiler Response to Memory Qualifiers”) to indicate how these qualifiers are interpreted
are used to manually allocate variables.
Note that there is no guarantee that all the memory on a device can be utilized as data
and code is packed in sections, or psects.
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3.6.10
How Do I Utilize the Linear Memory on Enhanced Mid-range PIC
Devices?
The linear addressing mode is a means of accessing the banked data memory as one
contiguous and linear block; see Section 5.5.1 “Address Spaces”. Use of the linear
memory is fully automatic. Objects that are larger than a data bank can be defined in
the usual way and will be accessed using the linear addressing mode; see
Section 5.5.2.2.2 “Software Stack Operation”, and Section 5.5.2.1.2 “Non-Auto Variable Size Limits”. If you define absolute objects at a particular location in memory, you
can use a linear address, if you prefer, or the regular banked address; see
Section 5.5.4.1 “Absolute Variables in Data Memory”.
3.6.11
What Devices are Supported by the Compiler?
Support for new devices usually takes place with each compiler release. To find
whether a device is supported by your compiler, you can do several things; see also,
Section 5.3.1 “Device Support”.
• HTML listings are provided in the compiler’s docs directory. Open these in your
favorite web browser. They are called pic_chipinfo.html and
pic18_chipinfo.html.
• Run the compiler driver on the command line (see Section 4.2 “Invoking the Compiler”) with the --CHIPINFO option; see Section 4.8.19 “--CHIPINFO: Display List
of Supported Devices”. A full list of all devices is printed to the screen.
3.6.12
How Do I Know What Code the Compiler Is Producing?
The assembly list file (see Section 6.4 “Assembly List Files”) shows the assembly output for almost the entire program, including library routines linked in to your program,
as well a large amount of the runtime startup code; see Section 5.10.1 “Runtime
Startup Code”. The list file is produced by default if you are using MPLAB X IDE. If you
are using the command-line, the option --ASMLIST will produce this file for you; see
Section 4.8.16 “--ASMLIST: Generate Assembler List Files”. The assembly list file will
have a .lst extension.
The list file shows assembly instructions, some assembly directives and information
about the program, such as the call graph (see Section 6.4.6 “Call Graph”), pointer
reference graph (see Section 6.4.5 “Pointer Reference Graph”), and information for
every function. Not all assembly directives are shown in the list file if the assembly
optimizers are enabled (they are produced in the intermediate assembly file).
Temporarily disable the assembly optimizers (see Section 4.8.45 “--OPT: Invoke
Compiler Optimizations”), if you wish to see all the assembly directives produced by the
compiler.
3.6.13
How Can I Tell How Big a Function Is?
Information that includes the size of functions is presented in the map file. Look for the
header “MODULE INFORMATION” near the bottom of the file. This information is
discussed in Section 7.4.2.8 “Module Information”.
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3.6.14
How Do I Know Which Resources Are Being Used by Each
Function?
In the assembly list file there is information printed for every C function, including library
functions; see Section 6.4 “Assembly List Files”. This information indicates what registers the function used, what functions it calls (this is also found in the call graph; see
Section 6.4.6 “Call Graph”), and how many bytes of data memory it requires. Note that
auto, parameter and temporary variables used by a function can overlap with those
from other functions as these are placed in a compiled stack by the compiler; see
Section 5.5.2.2.1 “Compiled Stack Operation”.
3.6.15
How Do I Find Out Where Variables and Functions Have Been
Positioned?
You can determine where variables and functions have been positioned from either the
assembly list file, see Section 6.4 “Assembly List Files”; or the map file, see
Section 7.4 “Map Files”. Only global symbols are shown in the map file; all symbols
(including locals) are listed in the assembly list file, but only for the code represented
by that list file. (Each assembly module has its own list file.)
There is a mapping between C identifiers and the symbols used in assembly code,
which are the symbols shown in both of these files; see Section 5.12.3.1 “Equivalent
Assembly Symbols”. The symbol associated with a variable is assigned the address of
the lowest byte of the variable; for functions it is the address of the first instruction
generated for that function.
3.6.16
Why Are Some Objects Positioned Into Memory That I
Reserved?
The memory reservation options (see Section 3.4.3.6 “How Do I Place a Function Into
a Unique Section?”) will adjust the range of addresses associated with classes used by
the linker. Most variables and function are placed into psects (see
Section 5.15.1 “Program Sections”) that are linked anywhere inside these class ranges
and so are affected by these reservation options.
Some psects are explicitly placed at an address rather than being linked anywhere in
an address range, e.g., the psect that holds the code to be executed at Reset is always
linked to address 0 because that is where the Reset location is defined to be for 8-bit
devices. Such a psect will not be affected by the --ROM option, even if you use it to
reserve memory address 0. Psects that hold code associated with Reset and interrupts
can be shifted using the --CODEOFFSET option; see Section 4.8.21 “--CODEOFFSET:
Offset Program Code to Address”.
Check the assembly list file (see Section 6.4 “Assembly List Files”) to determine the
names of psects that hold objects and code. Check the linker options in the map file;
see Section 7.4 “Map Files”, to see if psects have been linked explicitly or if they are
linked anywhere in a class. See also, the linker options -p (Section 7.2.18 “-Pspec”)
and -A (Section 7.2.1 “-Aclass =low-high,...”).
3.6.17
How Do I Know How Much Memory Is Still Available?
Although the memory summary printed by the compiler after compilation, (see
Section 4.8.61 “--SUMMARY: Select Memory Summary Output Type” options), or the
memory display available in MPLAB X IDE both indicate the amount of memory used
and the amount still available, neither of these features indicate whether this memory
is one contiguous block or broken into many small chunks. Small blocks of free memory
cannot be used for larger objects and so out-of-memory errors can be produced even
though the total amount of memory free is apparently sufficient for the objects to be
positioned. (See Section 3.7.6 “How Do I Fix a “Can’t find space...” Error?”.)
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The “UNUSED ADDRESS RANGES” section (see Section 7.4.2.5 “Unused Address
Ranges”) in the map file indicates exactly what memory is still available in each linker
class. It also indicated the largest contiguous block in that class if there are memory
bank or page divisions.
3.6.18
How Do I Use Library Files in My Project?
See Section 3.3.6 “How Do I Build Libraries?” for information on how you build your
own library files. The compiler will automatically include any applicable standard library
into the build process when you compile, so you never need to control these files.
To use one or more library files that were built by yourself or a colleague, include them
in the list of files being compiled on the command line. The library files can be specified
in any position in the file list relative to the source files, but if there is more than one
library file, they will be searched in the order specified in the command line. The LPP
libraries do not need to be specified if you are compiling to an intermediate file, i.e.,
using the --PASS1 option (see Section 4.8.49 “--PASS1: Compile to P-code”). For
example:
xc8 --chip=16f1937 main.c int.c lcd.lpp
If you are using MPLAB X IDE to build a project, add the library file(s) to the Libraries
folder that will shown in your project, in the order in which they should be searched. The
IDE will ensure that they are passed to the compiler at the appropriate point in the build
sequence.
3.6.19
What Optimizations Are Employed by the Compiler?
Optimizations are employed at both the C and assembly level of compilation. This is
described in Section 5.13 “Optimizations” and Section 6.3 “Assembly-Level Optimizations”, respectively. The options that control optimization are described in
Section 4.8.45 “--OPT: Invoke Compiler Optimizations”.
3.6.20
Why Do I Get Out-of-memory Errors When I Select a Debugger?
If you use a hardware tool debugger, such as the REAL ICE or ICD3, these require
memory for the on-board debug executive. When you select a debugger using the compiler’s --DEBUGGER option (Section 4.8.22 “--DEBUGGER: Select Debugger Type”),
or the IDE equivalent, the memory required for debugging is removed from that available to your project. See Section 3.5.4 “What Do I Need to Do When Compiling to Use
a Debugger?”
3.6.21
How Do I Know Which Stack Model the Compiler Has Assigned
to a Function?
Look in the function information section in the assembly list file, see
Section 6.4.3 “Function Information”. The last line of this block will indicate whether the
function uses a reentrant or non-reentrant model.
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3.6.22
How Do I Know What Value Has Been Programmed in the
Configuration Bits or ID Location?
Check the file startup.as (see Section 4.8.54 “--RUNTIME: Specify Runtime Environment”). This contains the output of the #pragma config directive. You will see the
numerical value programmed to the appropriate locations. In the following example, the
configuration value programmed is 0xFFBF. A breakdown of what this value means is
also printed.
; Config register CONFIG @ 0x2007
;
BOREN = OFF, BOR disabled
; ...
;
PWRTE = 0x1, unprogrammed default
psect
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config
org 0x0
dw 0xFFBF
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How To’s
3.7
FIXING CODE THAT DOES NOT WORK
This section examines issues relating to projects that do not build due to compiler
errors, or those that build, but do not work as expected.
• How Do I Find Out What an Warning/Error Message Means?
• How Do I Find the Code that Caused Compiler Errors or Warnings in My
Program?
• How Can I Stop Spurious Warnings From Being Produced?
• Why Can’t I Even Blink an LED?
• How Do I Know If the Hardware Stack Has Overflowed?
• How Do I Fix a “Can’t find space...” Error?
• How Do I Fix a “Can’t generate code...” Error?
• How Do I Fix a Fixup Overflow Error?
• What Can Cause Corrupted Variables and Code Failure When Using Interrupts?
3.7.1
How Do I Find Out What an Warning/Error Message Means?
Each warning or error message has a description, and possibly sample code that might
trigger such an error, listed in the messages chapter, see Appendix C. Error and Warning Messages. The compiler prints with each message a unique ID number in brackets.
Use this number to look up the message in the manual. This number also allows you
to control message behavior using options and pragmas, see Section 4.6.5 “Changing
Message Behavior”.
3.7.2
How Do I Find the Code that Caused Compiler Errors or
Warnings in My Program?
In most instances, where the error is a syntax error relating to the source code, the
message produced by the compiler indicates the offending line of code, see
Section 4.6 “Compiler Messages”. If you are compiling in MPLAB X IDE, then you can
double-click the message and have the editor take you to the offending line. But
identifying the offending code is not always so easy.
In some instances, the error is reported on the line of code following the line that needs
attention. This is because a C statement is allowed to extend over multiple lines of the
source file. It is possible that the compiler cannot be able to determine that there is an
error until it has started to scan to statement following. So in the following code
input = PORTB
if(input>6)
// ...
// oops - forgot the semicolon
The missing semicolon on the assignment statement will be flagged on the following
line that contains the if() statement.
In other cases, the error might come from the assembler, not the code generator. If the
assembly code was derived from a C source file then the compiler will try to indicate
the line in the C source file that corresponds to the assembly that is at fault. If the
source being compiled is an assembly module, the error directly indicates the line of
assembly that triggered the error. In either case, remember that the information in the
error relates to some problem is the assembly code, not the C code.
Finally, there are errors that do not relate to any particular line of code at all. An error
in a compiler option or a linker error are examples of these. If the program defines too
many variables, there is no one particular line of code that is at fault; the program as a
whole uses too much data. Note that the name and line number of the last processed
file and source can be printed in some situations even though that code is not the direct
source of the error.
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To determine the application that generated the error or warning, take a note of its
unique number printed in the message, see Section 4.6.1 “Messaging Overview”, and
check the message section of the manual, see Appendix C. Error and Warning Messages. At the top of each message description, on the right in brackets, is the name of
the application that produced this message. Knowing the application that produced the
error makes it easier to track down the problem. The compiler application names are
indicated in Section 4.3 “The Compilation Sequence”. If you need to see the assembly
code generated by the compiler, look in the assembly list file, see
Section 6.4 “Assembly List Files”. For information on where the linker attempted to
position objects, see the map file discussed in Section 7.4 “Map Files”.
3.7.3
How Can I Stop Spurious Warnings From Being Produced?
Warnings indicate situations that could possibly lead to code failure. In many situations
the code is valid and the warning is superfluous. Always check your code to confirm
that it is not a possible source of error and in cases where this is so, there are several
ways that warnings can be hidden.
• The warning level threshold can be adjusted so that only warnings of a certain
importance are printed, see Section 4.6.5.1 “Disabling Messages”
• All warnings with a specified ID can be inhibited
• In some situations, a pragma can be used to inhibit a warning with a specified ID
for certain lines of source code, see Section 5.14.4.11 “The #pragma warning
Directive”.
3.7.4
Why Can’t I Even Blink an LED?
Even if you have set up the TRIS register and written a value to the port, there are
several things that can prevent such a seemingly simple program from working.
• Make sure that the device’s Configuration registers are set up correctly, see
Section 5.3.5 “Configuration Bit Access”. Make sure that you explicitly specify
every bit in these registers and don’t just leave them in their default state. All the
configuration features are described in your device data sheet. If the Configuration bits that specify the oscillator source are wrong, for example, the device clock
cannot even be running.
• If the internal oscillator is being used, in addition to Configuration bits there can be
SFRs you need to initialize to set the oscillator frequency and modes, see
Section 5.3.6 “Using SFRs From C Code” and your device data sheet.
• Either turn off the Watch Dog Timer in the Configuration bits or clear the Watch
Dog Timer in your code (see Section Appendix A. “Library Functions”) so that the
device does not reset. If the device is resetting, it can never reach the lines of
code in your program that blink the LED. Turn off any other features that can
cause device Reset until your test program is working.
• The device pins used by the port bits are often multiplexed with other peripherals.
A pin might be connected to a bit in a port, or it might be an analog input, or it
might the output of a comparator, for example. If the pin connected to your LED is
not internally connected to the port you are using, then your LED will never operate as expected. The port function tables shown in your device data sheets will
show other uses for each pin that will help you identify peripherals to investigate.
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• Make sure you do not have a “read-modify-write” problem. If the device you are
using does not have a separate “latch” register (as is the case with mid-range PIC
devices) this problem can occur, particularly if the port outputs are driving large
loads, such as an LED. You can see that setting one bit turns off another or other
unusual events. Create your own latch by using a temporary variable. Rather than
read and write the port directly, make modifications to the latch variable. After
modifications are complete, copy the latch as a whole to the port. This means you
are never reading the port to modify it. Check the device literature for more
detailed information.
3.7.5
How Do I Know If the Hardware Stack Has Overflowed?
An 8-bit PIC device has a limited hardware stack that is used only for function (and
interrupt function) return addresses, see Section 5.3.4 “Stacks”. If the nesting of function calls and interrupts is too deep, the stack will overflow (wraps around and overwrites previous entries). Code will then fail at a later point – sometimes much later in
the call sequence – when it accesses the corrupted return address.
The compiler attempts to track stack depth and, when required, swap to a method of
calling that does not need the hardware stack (PIC10/12/16 devices only). You have
some degree of control over what happens when the stack depth has apparently overflowed, see Section 4.8.54 “--RUNTIME: Specify Runtime Environment” and the
stackcall suboption.
A call graph shows the call hierarchy and depth that the compiler has determined. This
graph is shown in the assembly list file. To understand the information in this graph, see
Section 6.4.6 “Call Graph”.
Since the runtime behavior of the program cannot be determined by the compiler, it can
only assume the worst case and can report that overflow is possible even though it is
not. However, no overflow should go undetected if the program is written entirely in C.
Assembly code that uses the stack is not considered by the compiler and this must be
taken into account.
3.7.6
How Do I Fix a “Can’t find space...” Error?
There are a number of different variants of this message, but all essentially imply a similar situation. They all relate to there being no free space large enough to place a block
of data or instructions. Due to memory paging, banking or other fragmentation, this
message can be issued when seemingly there is enough memory remaining. See
Appendix C. Error and Warning Messages for more information on your particular error
number.
3.7.7
How Do I Fix a “Can’t generate code...” Error?
This is a catch-all message which is generated if the compiler has exhausted all possible means of compiling a C expression, see Appendix C. Error and Warning Messages.
It does not usually indicate a fault in your code. The inability to compile the code can
be a deficiency in the compiler, or an expression that requires more registers or
resources than are available at that point in the code. This is more likely to occur on
baseline devices. In any case, simplifying the offending expression, or splitting a statement into several smaller statements, usually allows the compilation to continue. You
may need to use another variable to hold the intermediate results of complicated
expressions.
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3.7.8
How Do I Fix a Fixup Overflow Error?
Fixup – the linker action of replacing a symbolic reference with an actual address – can
overflow if the address assigned to the symbol is too large to fit in the address field of
an assembly instruction. Most 8-bit PIC assembly instructions specify a file address
that is an offset into the currently selected memory bank. If a full unmasked address is
specified with these instructions, the linker will be unable to encode the large address
value into the instruction and this error will be generated. For example, a mid-range
device instruction only allows for file addresses in the range of 0 to 0x7F. However, if
such a device has 4 data banks of RAM, the addresses of variables can range from 0
to 0x1FF.
For example, if the symbol of a variable that will be located at address 0x1D0 has been
specified with one of these instructions, then when the symbol is replaced with its final
value, this value will not fit in the address field of the instruction.
Many of the jump and call instructions also take a destination operand that is a truncated address. (The PIC18 CALL and GOTO instructions work with a full address, but
the branch and relative call instructions do not.) If the destination label to any of these
instructions is not masked, a fixup error can result.
The fixup process applies to the operands of assembler directives, as well as instructions; so if the operand to a directive overflows, a fixup error can also result. For
example, if the symbol error is resolved by the linker to be the value 0x238, the
directive:
DB error
which expects a byte value, will generate a fixup overflow error.
In most cases, fixup errors are caused by hand-written assembly code. When writing
assembly, it is the programmer’s responsibility to add instructions to select the destination bank or page, and then mask the address being used in the instruction (see
Section 3.4.7.5 “What Things Must I Manage When Writing Assembly Code?”).
In some situations assembly code generated from C code can produce a fixup overflow
message. Typically this will be related to jumps that are out of range. C switch statements that have become too large can trigger such a message. Changing how a compiler-generated psect is linked can also cause fixup overflow, as the new psect location
may break an assumption made by the compiler.
It is important to remember that this is an issue with an assembly instruction, and that
you need to find the instruction at fault before you can proceed. See the relevant error
number in Appendix C. Error and Warning Messages for specific details about how to
track down the offending instruction.
3.7.9
What Can Cause Corrupted Variables and Code Failure When
Using Interrupts?
This is usually caused by having variables used by both interrupt and main-line code.
If the compiler optimizes access to a variable or access is interrupted by an interrupt
routine, then corruption can occur. See Section 3.5.6 “How Do I Share Data Between
Interrupt and Main-line Code?” for more information.
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USER’S GUIDE
Chapter 4. XC8 Command-line Driver
4.1
INTRODUCTION
The name of the command-line driver is xc8. MPLAB XC8 can be invoked to perform
all aspects of compilation, including C code generation, assembly, and link steps. Even
if an IDE is used to assist with compilation, the IDE will ultimately call xc8.
Although the internal compiler applications can be called explicitly from the command
line, the xc8 driver is the recommended way to use the compiler as it hides the complexity of all the internal applications used and provides a consistent interface for all
compilation steps.
This chapter describes the steps that the driver takes during compilation, the files that
the driver can accept and produce, as well as the command-line options that control
the compiler’s operation. The relationship between these command-line options and
the controls in the MPLAB X IDE Build Options dialog is also described.
The following topics are examined in this chapter of the MPLAB XC8 C Compiler User’s
Guide:
• Invoking the Compiler
• The Compilation Sequence
• Runtime Files
• Compiler Output
• Compiler Messages
• MPLAB XC8 Driver Options
• MPLAB X Option Equivalents
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4.2
INVOKING THE COMPILER
This section explains how to invoke xc8 on the command line, as well as the files that
it can read.
4.2.1
Driver Command-line Format
The xc8 driver has the following basic command format:
xc8 [options] files [libraries]
Throughout this manual, it is assumed that the compiler applications are in the console’s search path or that the full path is specified when executing an application. The
compiler’s location can be added to the search path when installing the compiler by
selecting the Add to environment checkbox at the appropriate time during the
installation.
It is customary to declare options (identified by a leading dash “-” or double dash “–”)
before the files’ names. However, this is not mandatory.
The formats of the options are supplied in Section 4.7 “MPLAB XC8 Driver Options”,
along with corresponding descriptions of the options.
The files can be an assortment of C and assembler source files, and precompiled
intermediate files, such as relocatable object (.obj) files or p-code (.p1) files. While
the order in which the files are listed is not important, it can affect the order in which
code or data appears in memory, and can affect the name of some of the output files.
Libraries is a list of user-defined object code or p-code library files that will be
searched by the code generator (in the case of p-code libraries) or the linker (for object
code libraries), in addition to the standard C libraries. The order of these files will determine the order in which they are searched. It is customary to insert the Libraries list
after the list of source file names. However, this is not mandatory.
If you are building code using a make system, familiarity with the unique intermediate
p-code file format, as described in Section 4.3.3 “Multi-Step Compilation”, is
recommended. Object files are seldom used with the MPLAB XC8 C Compiler, unless
assembly source modules are in the project.
4.2.1.1
LONG COMMAND LINES
The xc8 driver is capable of processing command lines exceeding any operating system limitation if the driver is passed options via a command file. The command file is
specified by the @ symbol, which should be immediately followed (i.e., no intermediate
space character) by the name of the file containing the command-line arguments that
are intended for the driver. The same system of argument passing is used by all the
applications called by the compiler driver.
Each command-line argument must be separated by one or more spaces and can
extended to several lines by using a space and backslash character to separate lines.
The file can contain blank lines, which are simply skipped by the driver.
Compiler options and source code filenames can be permanently stored in command
files without the complexity of creating a make utility.
In the following example, a command file xyz.xc8 was constructed in a text editor to
contain both the options and the file names that are required to compile a project.
--chip=16F877A -m \
--opt=all -g \
main.c isr.c
After it is saved, the compiler can be invoked with the following command:
xc8 @xyz.xc8
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4.2.2
Environment Variables
When hosted on a Windows environment, the compiler uses the registry to store information relating to the compiler installation directory and activation details, along with
other configuration settings. That information is required whether the compiler is run on
the command line or from within an IDE.
Under Linux® and Mac OS® X environments, the registry is replaced by an XML file that
stores the same information.
On non-Windows hosts, the compiler searches for the XML file in the following ways:
1. The compiler looks for the presence of an environment variable called XC_XML.
If present, this variable should contain the full path to the XML file (including the
file’s name).
2. If this variable is not defined, the compiler then searches for an environment variable called HOME. This variable typically contains the path to the user’s home
directory. The compiler looks for the XML with a name .xc.xml in the directory
indicated by the HOME variable.
3. If the HOME environment variable is not defined, the compiler tries to open the file
/etc/xc.xml.
4. If none of these methods finds the XML file, an error is generated.
When running the compiler on the command line, you can wish to set the PATH environment variable. This allows you to run the compiler driver without specifying the full
compiler path with the driver name. Note that the directories specified by the PATH variable are only used to locate the compiler driver. Once the driver is running, it uses the
registry or XML file, described above, to locate the internal compiler applications, such
as the parser, assembler and linker, etc. The directories specified in the PATH variable
do not override the information contained in the registry or XML file. The MPLAB X IDE
allows the compiler to be selected via a dialog and execution of the compiler does not
depend on the PATH variable.
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4.2.3
Input File Types
xc8 distinguishes source files, intermediate files, and library files solely by the file type,
or extension. Recognized file types are listed in Table 4-1. Alphabetic case of the
extension is not important from the compiler’s point of view, but most operating system
shells are case sensitive.
TABLE 4-1:
xc8 INPUT FILE TYPES
File Type
Meaning
.c
C source file
.p1
p-code file
.lpp
p-code library file
.as or .asm
Assembler source file
.obj
Relocatable object code file
.lib
Relocatable object library file
.hex
Intel HEX file
This means, for example, that a C source file must have a .c extension. Assembler
files can use either .as or .asm extensions.
There are no compiler restrictions imposed on the names of source files, but be aware
of case, name-length, and other restrictions that are imposed by your operating system. Never use the same base name for assembly and C source files, even if they are
located in different directories; the name of the intermediate file produced from the C
source file might conflict with that of the assembly source file. If you are using an IDE,
avoid assembly source files whose base name is the same as the base name of any
project in which the file is used. This can result in the source file being overwritten by
a temporary file during the build process.
The terms “source file” and “module” are often used when talking about computer
programs. They are often used interchangeably, but they refer to the source code at
different points in the compilation sequence.
A source file is a file that contains all or part of a program. They can contain C code, as
well as preprocessor directives and commands. Source files are initially passed to the
preprocessor by the driver.
A module is the output of the preprocessor, for a given source file, after inclusion of any
header files (or other source files) which are specified by #include preprocessor
directives. All preprocessor directives and commands (with the exception of some commands for debugging) have been removed from these files. These modules are then
passed to the remainder of the compiler applications. Thus, a module can be the amalgamation of several source and header files. A module is also often referred to as a
translation unit. These terms can also be applied to assembly files, as they can include
other header and source files.
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4.3
THE COMPILATION SEQUENCE
When you compile a project, many internal applications are called to do the work. This
section looks at when these internal applications are executed, and how this relates to
the build process of multiple source files. This section should be of particular interest if
you are using a make system to build projects.
4.3.1
The Compiler Applications
The main internal compiler applications and files are illustrated in Figure 4-1.
You can consider the large underlying box to represent the whole compiler, which is
controlled by the command line driver, xc8. You can be satisfied just knowing that C
source files (shown on the far left) are passed to the compiler and the resulting output
files (shown here as a HEX and COFF debug file on the far right) are produced; however, internally there are many applications and temporary files being produced. An
understanding of the internal operation of the compiler, while not necessary, does
assist with using the tool.
To simplify the compiler design, some of the internal applications come in a PIC18 and
PIC10/12/16 variant. The appropriate application is executed based on the target
device. In fact, the xc8 driver delegates the build commands to one of two command-line drivers: PICC or PICC18. This operation is transparent and xc8 can be
considered as “the driver” which does all the work.
The driver will call the required compiler applications. These applications are shown as
the smaller boxed inside the large driver box. The temporary file produced by each
application can also be seen in this diagram.
FIGURE 4-1:
COMPILER APPLICATIONS AND FILES
p-code
libraries
Command-line driver
.p1
p-code
files
.as
assembly
source
files
processed
files (module)
p-code
files
assembly file
relocatable
object file
.pre
.p1
.as
.obj
C source
files
.c
.lpp
preprocessor
p
or
code
generator
parser
assembler
debug file
linker
relocatable
object files
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.obj
cromwell
.cof
.c
hexmate
.hhex
objtohex
.obj
.hex
absolute
object file
hex file
.lib
object
libraries
hex file
hex .hex
files
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Table 4-2 lists the compiler applications. The names shown are the names of the executables, which can be found in the bin directory under the compiler’s installation
directory.
TABLE 4-2:
COMPILER APPLICATION NAMES
Name
Description
xc8 (calls PICC or PICC18)
Command line driver; the interface to the compiler
CLIST
Text file formatter
CPP
The C preprocessor
P1
C code parser
CGPIC or CGPIC18
Code generator (based on the target device)
ASPIC or ASPIC18
Assembler (based on the target device)
HLINK
Linker
OBJTOHEX
Conversion utility to create HEX files
CROMWELL
Debug file converter
HEXMATE
HEX file utility
LIBR
Librarian
DUMP
Object file viewer
For example, C source files (.c files) are first passed to the C preprocessor, CPP. The
output of this application is .pre files. These files are then passed to the parser application, P1, which produces a p-code file output with extension .p1. The applications
are executed in the order specified and temporary files are used to pass the output of
one application to the next.
The compiler can accept more than just C source files. Table 4-1 lists all the possible
input file types, and these files can be seen in this diagram, on the top and bottom,
being passed to different compilation applications. They are processed by these
applications and then the application output joins the normal flow indicated in the
diagram.
For example, assembly source files are passed straight to the assembler application1
and are not processed at all by the code generator. The output of the assembler (an
object file with .obj extension) is passed to the linker in the usual way. You can see
that any p-code files (.p1 extension) or p-code libraries (.lpp extension) that are
supplied on the command line are initially passed to the code generator.
Other examples of input files include object files (.obj extension) and object libraries
(.lib extension), both of which are passed initially to the linker, and even HEX files
(.hex extension), which are passed to one of the utility applications, called HEXMATE,
which is run right at the end of the compilation sequence.
Some of the temporary files shown in this diagram are actually preserved and can be
inspected after compilation has concluded. There are also driver options to request that
the compilation sequence stop after a particular application and the output of that
application becomes the final output.
1. Assembly file will be preprocessed before being passed to the assembler if the -P option is
selected.
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FIGURE 4-2:
MULTI-FILE COMPILATION
Intermediate files
C file
preprocess
&
parse
pcode
C file
preprocess
&
parse
pcode
code
generation
assemble
link
library
files
First stage of compilation
4.3.2
Second stage of compilation
Single-Step Compilation
Figure 4-1 showed us the files that are generated by each application and the order in
which these applications are executed. However this does not indicate how these
applications are executed when there is more than one source file being compiled.
Consider the case when there are two C source files that form a complete project and
that are to be compiled, as is the case shown in Figure 4-2. If these files are called
main.c and io.c, these could be compiled with a single command, such as:
xc8 --chip=16F877A main.c io.c
This command will compile the two source files all the way to the final output, but
internally we can consider this compilation as consisting of two stages.
The first stage involves processing of each source file separately, and generating some
sort of intermediate file for each source file. The second stage involves combining all
these intermediate files and further processing to form the final output. An intermediate
file is a particular temporary file that is produced and marks the midpoint between the
first and second stage of compilation.
The intermediate file used by xc8 is the p-code (.p1 extension) file output by the
parser, so there will be one p-code file produced for each C source file. As indicated in
the diagram, CPP and then P1 are executed to form this intermediate file. (For clarity,
the CPP and P1 applications have been represented by the same block in the diagram.)
In the second stage, the code generator reads in all the intermediate p-code files and
produces a single assembly file output, which is then passed to the subsequent
applications that produce the final output.
The desirable attribute of this method of compilation is that the code generator, which
is the main application that transforms from the C to the assembly domain, sees the
entire project source code via the intermediate files.
Traditional compilers have always used intermediate files that are object files output by
the assembler. These intermediate object files are then combined by the linker and further processed to form the final output. This method of compilation is shown in
Figure 4-3. It shows that the code generator is executed once for each source file. So,
the code generator can only analyze that part of the project that is contained in the
source file that is currently being compiled. The MPLAB XC16 and XC32 compilers
work in this fashion.
Using object files as the intermediate file format with MPLAB XC8 C Compiler will
defeat many features the compiler uses to optimize code. Always use p-code files as
the intermediate file format if you are using a make system to build projects.
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FIGURE 4-3:
THE TRADITIONAL COMPILATION SEQUENCE
Intermediate files
C file
preprocess
&
parse
code
generation
assemble
.obj
files
C file
preprocess
&
parse
code
generation
assemble
.obj
files
link
library
files
Second stage
of compilation
First stage of compilation
When compiling files of mixed types, this can still be achieved with just one invocation
of the compiler driver. As discussed in Section 4.3 “The Compilation Sequence”, the
driver will pass each input file to the appropriate compiler application.
For example, the files, main.c, io.c, mdef.as and c_sb.lpp are to be compiled.
To perform this in a single step, the following command line could be used.
xc8 --chip=16F877A
main.c
io.c
mdef.as
c_sb.lpp
As shown in Figure 4-1 and Figure 4-2, the two C files (main.c and io.c) will be compiled to intermediate p-code files; these, along with the p-code library file (c_sb.lpp)
will be passed to the code generator. The output of the code generator, as well as the
assembly source file (mdef.as), will be passed to the assembler.
The driver will recompile all source files, regardless of whether they have changed
since the last build. IDEs (such as MPLAB® IDE) and make utilities must be employed
to achieve incremental builds. See also, Section 4.3.3 “Multi-Step Compilation”.
Unless otherwise specified, a HEX file and Microchip COFF file are produced as the
final output. All intermediate files remain after compilation has completed, but most
other temporary files are deleted, unless you use the --NODEL option (see
Section 4.8.42 “--NODEL: Do Not Remove Temporary Files”) which preserves all generated files except the run-time start-up file. Note that some generated files can be in
a different directory than your project source files. See Section 4.8.46 “--OUTDIR:
Specify a Directory for Output Files”, and Section 4.8.44 “--OBJDIR: Specify a Directory for Intermediate Files”, which can both control the destination for some output files.
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4.3.3
Multi-Step Compilation
Make utilities and IDEs, such as MPLAB X IDE, allow for an incremental build of projects that contain multiple source files. When building a project, they take note of which
source files have changed since the last build and use this information to speed up
compilation.
For example, if compiling two source files, but only one has changed since the last
build, the intermediate file corresponding to the unchanged source file need not be
regenerated.
MPLAB X IDE is aware of the different compilation sequence employed by xc8 and
takes care of this for you. From MPLAB X IDE you can select an incremental build
(Build Project icon), or fully rebuild a project (Clean and Build Project icon).
If the compiler is being invoked using a make utility, the make file will need to be configured to recognized the different intermediate file format and the options used to generate the intermediate files. Make utilities typically call the compiler multiple times: once
for each source file to generate an intermediate file, and once to perform the second
stage compilation.
You might also wish to generate intermediate files to construct your own library files.
However, xc8 is capable of constructing libraries in a single step, so this is typically not
necessary. See Section 4.8.47 “--OUTPUT= type: Specify Output File Type” for more
information on library creation.
The option --PASS1 (see Section 4.8.48 “--PARSER: Specify Parser Mode”) is used
to tell the compiler that compilation should stop after the parser has executed. This will
leave the p-code intermediate file behind on successful completion.
For example, the files main.c and io.c are to be compiled using a make utility. The
command lines that the make utility should use to compile these files might be
something like:
xc8 --chip=16F877A --pass1 main.c
xc8 --chip=16F877A --pass1 io.c
xc8 --chip=16F877A main.p1 io.p1
If is important to note that the code generator needs to compile all p-code or p-code
library files associated with the project in the one step. When using the --PASS1
option, the code generator is not being invoked; so the above command lines do not
violate this requirement.
Using object files as the intermediate file format with MPLAB XC8 C Compiler will
defeat many features the compiler uses to optimize code. Always use p-code files as
the intermediate file format if you are using a make system to build projects.
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4.3.4
Compilation of Assembly Source
Since the code generator performs many tasks that were traditionally performed by the
linker, there could be complications when assembly source is present in a project.
Assembly files are traditionally processed after C code, but it is necessary to have this
performed first so that specific information contained in the assembly code can be
conveyed to the code generator.
The specific information passed to the code generator is discussed in more detail in
Section 5.12.3 “Interaction between Assembly and C Code”.
When assembly source is present, the order of compilation is as shown in Figure 4-4.
FIGURE 4-4:
C file
C file
COMPILATION SEQUENCE WITH ASSEMBLY FILES
preprocess
&
parse
preprocess
&
parse
pcode
pcode
library
files
ASM
file
code
generation
assemble
link
assemble
driver
OBJ
file
First, any assembly source files are assembled to form object files. These files, along
with any other objects files that are part of the project, are scanned by the
command-line driver and the information is passed to the code generator; where it
subsequently builds the C files, as has been described earlier.
4.3.4.1
INTERMEDIATE FILES AND ASSEMBLY SOURCE
The intermediate file format associated with assembly source files is the same as that
used in traditional compilers; i.e., an object file (.obj extension). Assembly files are
never passed to the code generator and so the code generator technology does not
alter the way these files are compiled.
The -C option (see Section 4.8.1 “-C: Compile to Object File”) is used to generate
object files and to halt compilation after the assembly step.
4.3.5
Printf Check
An extra execution of the code generator is performed prior to the actual code generation phase. This pass is part of the process by which the printf library function is
customized, see Section 5.12 “Mixing C and Assembly Code”, for more details.
This pass is only associated with scanning the C source code for printf placeholder
usage and you will see the code generator being executed if you select the verbose
option when you build, see Section 4.8.14 “-V: Verbose Compile”.
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4.4
RUNTIME FILES
In addition to the C and assembly source files specified on the command line, there are
also compiler-generated source files and pre-compiled library files which might be
compiled into the project by the driver. These files are discussed in the following sections.
4.4.1
Library Files
The names of the C standard library files appropriate for the selected target device are
determined by the driver and passed to the code generator and linker. You do not need
to manually include library files into your project.
Most library routines are derived from p-code library files (.lpp libraries) which the
compiler will search for in the lib directory under the compiler installation directory.
The name format for the standard libraries is family-type-options.lpp, where
the following apply.
• family can be pic18 for PIC18 devices, or pic for all other 8-bit PIC devices
• type indicates the sort of library functionality provided and can be stdlib for the
standard library functions, or trace, etc.
• options indicates hyphen-separated names to indicate variants of the library to
accommodate different compiler options or modes, e.g., htc for the default flavor
of C used by MPLAB XC8, d32 for 32-bit doubles, etc.
For example, the standard library for baseline and midrange devices using 24-bit
double types is pic-stdlib-d24.lpp.
For more information on libraries, see Section 5.11 “Libraries”.
4.4.2
Startup and Initialization
A C program requires certain objects to be initialized and the device to be in a particular
state before it can begin execution of its function main. It is the job of the runtime
startup code to perform these tasks. Section 5.10.1 “Runtime Startup Code” details the
specific actions taken by this code and how it interacts with programs you write.
Rather than the traditional method of linking in a generic, precompiled routine, the
MPLAB XC8 C Compiler determines what runtime startup code is required from the
user’s program and then generates this code each time you build.
The file created by the driver can be deleted after compilation, but this operation can
be controlled with the keep suboption to the --RUNTIME option. The default operation
of the driver is to keep the startup module; however, if using MPLAB X IDE to build, the
file will be deleted unless you indicate otherwise in the Project Properties dialog.
If the startup module is kept, it will be called startup.as and will be located in the
current working directory. If you are using an IDE to perform the compilation, the
destination directory can be dictated by the IDE itself. MPLAB X IDE stores this file in
the dist/default/production(debug) directory in your project directory.
Generation of the runtime startup code is an automatic process that does not require
any user interaction; however, some aspects of the runtime code can be controlled, if
required, using the --RUNTIME option. Section 4.8.54 “--RUNTIME: Specify Runtime
Environment” describes the use of this option.
The runtime startup code is executed before main. However, if you require any special
initialization to be performed immediately after Reset, you should use the powerup
feature described later in Section 5.10.2 “The Powerup Routine”.
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4.5
COMPILER OUTPUT
There are many files created by the compiler during the compilation. A large number of
these are intermediate files. Some are deleted after compilation is complete, but many
remain and are used for programming the device, or for debugging purposes.
4.5.1
Output Files
The names of many output files use the same base name as the source file from which
they were derived. For example, the source file input.c will create a p-code file called
input.p1.
Some of the output files contain project-wide information and are not directly associated with any one particular input file, e.g., the map file. If the names of these output
files are not specified by a compiler option, their base name is derived from the first C
source file listed on the command line. If there are no files of this type specified, the
name is based on the first input file (regardless of type) on the command line.
If you are using an IDE, such as MPLAB X IDE, to specify options to the compiler, there
is typically a project file that is created for each application. The name of this project is
used as the base name for project-wide output files, unless otherwise specified by the
user. However, check the manual for the IDE you are using, for more details.
Note:
Throughout this manual, the term project name will refer to either the name
of the project created in the IDE, or the base name (file name without
extension) of the first C source file specified on the command line.
The compiler is directly able to produce a number of the output file formats that are
used by the 8-bit PIC development tools.
The default behavior of xc8 is to produce a Microchip format COFF and Intel HEX output. Unless changed by a driver option, the base names of these files will be the project
name. The default output file types can be controlled by compiler options, e.g., the
--OUTPUT option. The extensions used by these files are fixed and are listed together
with this option’s description in Section 4.8.47 “--OUTPUT= type: Specify Output File
Type”.
The COFF file is used by debuggers to obtain debugging information about the project.
The compiler can produce ELF/DWARF debugger files, although these are not compatible with MPLAB 8 IDE and early versions of MPLAB X IDE. You must specifically
select ELF output for these files to be produced. ELF/DWARF files allow for more accurate debugging. Use of these files correct several COFF-related issues that prevent
you from correctly viewing objects, in particular pointer variables, in the IDE. Ensure the
IDE version you are using supports ELF before selecting this option.
Table 4-17 shows all output format options available with xc8 using the --OUTPUT
option. The File Type column lists the filename extension that is used for the output file.
4.5.1.1
SYMBOL FILES
By default, xc8 creates symbol files that are used to generate the debug output files,
such as COFF and ELF files. These files include a SYM file (.sym extension) and a
CMF file (.cmf extension), and both are produced by the linker. In addition, there is a
SDB file (.sdb extension) produced by the code generator.
The SDB file contains type information, and the SYM and CMF files contain address
information. The SDB and SYM/CMF files, in addition to the HEX file, are combined by
the CROMWELL application to produce the output debug files, such as the COFF file.
The CMF file largely replaces the older SYM file format. They contain similar information, but CMF files are more detailed and enable more accurate debug files to be
generated.
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4.5.2
Diagnostic Files
Two valuable files produced by the compiler are the assembly list file, produced by the
assembler, and the map file, produced by the linker.
The compiler options --ASMLIST (see Section 4.8.15 “--ADDRQUAL: Set Compiler
Response to Memory Qualifiers”) generates a list file, and the -M option (see
Section 4.8.7 “-M: Generate Map File”) specifies generation of a map file.
The assembly list file contains the mapping between the original source code and the
generated assembly code. It is useful for information such as how C source was
encoded, or how assembly source can have been optimized. It is essential when confirming if compiler-produced code that accesses objects is atomic, and shows the
psects in which all objects and code are placed. For an introductory guide to psects,
see Section 5.15.1 “Program Sections”. Also, see Section 6.3 “Assembly-Level Optimizations”, for more information on the contents of this file.
There is one list file produced for the entire C program, including C library files. It is
assigned the project name and the extension .lst. One additional list file is produced
for each assembly source file compiled in the project.
The map file shows information relating to where objects were positioned in memory. It
is useful for confirming if user-defined linker options were correctly processed, and for
determining the exact placement of objects and functions. It also shows all the unused
memory areas in a device and memory fragmentation. See Section 7.4 “Map Files”, for
complete information on the contents of this file.
There is one map file produced when you build a project, assuming the linker was
executed and ran to completion. The file is assigned the project name and a .map
extension.
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4.6
COMPILER MESSAGES
All compiler applications, including the command-line driver, xc8, use textual messages to report feedback during the compilation process. A centralized messaging system is used to produce the messages, which allows consistency during all stages of the
compilation process. The messaging system is described in this section and a complete list of all warning and error messages can be found in Appendix C. Error and
Warning Messages.
4.6.1
Messaging Overview
A message is referenced by a unique number that is passed to the messaging system
by the compiler application that needs to convey the information. The message string
corresponding to this number is obtained from Message Description Files (MDF), which
are stored in the dat directory in the compiler’s installation directory.
When a message is requested by a compiler application, its number is looked up in the
MDF that corresponds to the currently selected language. The language of messages
can be altered as discussed in Section 4.6.2 “Message Language”.
Once found, the alert system can read the message type and the string to be displayed
from the MDF. Several different message types are described in
Section 4.6.3 “Message Type”; and the type can be overridden by the user, as
described in that same section.
The user is also able to set a threshold for warning message importance, so that only
those that the user considers significant will be displayed. In addition, messages with
a particular number can be disabled. A pragma can also be used to disable a particular
message number within specific lines of code. These methods are explained in
Section 4.6.5.1 “Disabling Messages”.
Provided the message is enabled and it is not a warning message whose level is below
the current warning threshold, the message string will be displayed.
In addition to the actual message string, there are several other pieces of information
that can be displayed, such as the message number, the name of the file for which the
message is applicable, the file’s line number and the application that issued the
message, etc.
If a message is an error, a counter is incremented. After a specific amount of errors has
been reached, compilation of the current module will cease. The default number of
errors that will cause this termination can be adjusted by using the --ERRORS option,
see Section 4.8.29 “--ERRORS: Maximum Number of Errors”. This counter is reset for
each internal compiler application, thus specifying a maximum of five errors will allow
up to five errors from the parser, five from the code generator, five from the linker, five
from the driver, etc.
Although the information in the MDF can be modified with any text editor, this is not recommended. Message behavior should only be altered using the options and pragmas
described in the following sections.
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4.6.2
Message Language
The xc8 driver supports more than one language for displayed messages. There is one
MDF for each language supported.
Under Windows® operating system, the default language can be specified when
installing the compiler.
The default language can be changed on the command line using the --LANG option,
see Section 4.8.36 “--LANG: Specify the Language for Messages”. Alternatively, it can
be changed permanently by using the --LANG option together with the --SETUP
option which will store the default language in either the registry, under Windows, or in
the XML configuration file on other systems. On subsequent builds, the default
language used will be that specified.
Table 4-3 shows the MDF applicable for the currently supported languages.
TABLE 4-3:
SUPPORTED LANGUAGES
Language
MDF name
English
en_msgs.txt
German
de_msgs.txt
French
fr_msgs.txt
If a language other than English is selected, and the message cannot be found in the
appropriate non-English MDF, the alert system tries to find the message in the English
MDF. If an English message string is not present, a message is displayed that is similar
to this one:
error/warning (*) generated, but no description available
where * indicates the message number that was generated that will be printed;
otherwise, the message in the requested language will be displayed.
4.6.3
Message Type
There are four types of messages. These are described below. The behavior of the
compiler when encountering a message of each type is also listed.
Advisory Messages
convey information regarding a situation the compiler has
encountered or some action the compiler is about to take.
The information is being displayed “for your interest”, and
typically requires no action to be taken. Compilation will
continue as normal after such a message is issued.
Warning Messages
indicate source code or some other situation that can be
compiled, but is unusual and can lead to a runtime failure of
the code. The code or situation that triggered the warning
should be investigated; however, compilation of the current
module will continue, as will compilation of any remaining
modules.
Error Messages
indicate source code that is illegal or that compilation of this
code cannot take place. Compilation will be attempted for
the remaining source code in the current module, but no
additional modules will be compiled and the compilation
process will then conclude.
Fatal Error Messages indicate a situation in which the compilation cannot proceed
and requires that the compilation process to stop
immediately.
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4.6.4
Message Format
By default, messages are printed in a human-readable format. This format can vary
from one compiler application to another, since each application reports information
about different file formats.
Some applications (for example, the parser) are typically able to pinpoint the area of
interest down to a position on a particular line of C source code, whereas other applications, such as the linker, can at best only indicate a module name and record number,
which is less directly associated with any particular line of code. Some messages relate
to issues in driver options that are in no way associated with any source code.
There are several ways of changing the format in which message are displayed. They
are discussed below.
The driver option -E (with or without a filename) alters the format of all displayed messages. See Section 4.8.3 “-E: Redirect Compiler Errors to a File”, for details. Using this
option produces messages that are better suited to machine parsing, and are less
user-friendly. Typically, each message is displayed on a single line. The general form
of messages produced when using the -E option is:
filename
line:
(message number)
message string
(type)
The -E option also has another effect. When used, the driver first checks to see if special environment variables have been set. If so, the format dictated by these variables
is used as a template for all messages that will be produced by all compiler
applications. The names of these environment variables are given in Table 4-4.
TABLE 4-4:
MESSAGING ENVIRONMENT VARIABLES
Variable
Effect
HTC_MSG_FORMAT
All advisory messages
HTC_WARN_FORMAT
All warning messages
HTC_ERR_FORMAT
All error and fatal error messages
The value of these environment variables are strings that are used as templates for the
message format. Printf-like placeholders can be placed within the string to allow the
message format to be customized. The placeholders, and what they represent, are
presented in Table 4-5.
TABLE 4-5:
MESSAGING PLACEHOLDERS
Placeholder
Replacement
%a
Application name
%c
Column number
%f
Filename
%l
Line number
%n
Message number
%s
Message string (from MDF)
If these options are used in a DOS batch file, two percent characters will need to be
used to specify the placeholders, as DOS interprets a single percent character as an
argument and will not pass this on to the compiler. For example:
SET HTC_ERR_FORMAT="file %%f: line %%l"
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Environment variables, in turn, can be overridden by the driver options: --MSGFORMAT,
--WARNFORMAT and --ERRFORMAT, see Section 4.8.28 “--ERRFORMAT: Define Format for Compiler Messages”. These options take a string as their argument. The option
strings are formatted, and can use the same placeholders as their variable
counterparts.
For example, a project is compiled, but, as shown, produces a warning from the parser
and an error from the linker (numbered 362 and 492, respectively).
main.c: main()
17: ip = &b;
^ (362) redundant "&" applied to array (warning)
(492) attempt to position absolute psect "text" is illegal
Notice that the parser message format identifies the particular line and position of the
offending source code.
If the -E option is now used and the compiler issues the same messages, the compiler
will output:
main.c: 12: (362) redundant "&" applied to array (warning)
(492) attempt to position absolute psect "text" is illegal (error)
The user now uses the --WARNFORMAT in the following fashion:
--WARNFORMAT="%a %n %l %f %s"
When recompiled, the following output will be displayed:
parser 362 12 main.c redundant "&" applied to array
(492) attempt to position absolute psect "text" is illegal (error)
Notice that the format of the warning was changed, but that of the error message was
not. The warning format now follows the specification of the environment variable. The
application name (parser) was substituted for the %a placeholder, the message
number (362) substituted the %n placeholder, etc.
4.6.5
Changing Message Behavior
Both the attributes of individual messages and general settings for the messaging system can be modified during compilation. There are both driver options and C pragmas
that can be used to achieve this.
4.6.5.1
DISABLING MESSAGES
Each warning message has a default number indicating a level of importance. This
number is specified in the MDF and ranges from -9 to 9. The higher the number, the
more important the warning.
Warning messages can be disabled by adjusting the warning level threshold using the
--WARN driver option, see Section 4.8.65 “--WARN: Set Warning Level”. Any warnings
whose level is below that of the current threshold are not displayed.
The default threshold is 0 which implies that only warnings with a warning level of 0 or
higher will be displayed by default. The information in this option is propagated to all
compiler applications, so its effect will be observed during all stages of the compilation
process.
Warnings can also be disabled by using the --MSGDISABLE option, see
Section 4.8.40 “--MSGDISABLE: Disable Warning Messages”. This option takes a
comma-separated list of warning numbers. The warnings corresponding to the numbers listed are disabled and will never be issued, regardless of the current warning level
threshold. If the special message number 0 is specified, then all warning messages are
disabled.
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Some warning messages can also be disabled by using the warning pragma. This
pragma will only affect warnings that are produced by either the parser or the code generator; i.e., errors directly associated with C code. See Section 5.14.4.11 “The
#pragma warning Directive” for more information on this pragma.
Error messages can also be disabled; however, a more verbose form of the above
command is required to confirm the action. To specify an error message number in the
--MSGDISABLE command, each error number must be followed by :off to ensure
that it is disabled. For example:
--MSGDISABLE=1257,195:off,194:off
will disable warning 1257, and errors 195 and 194.
Note:
4.6.5.2
Disabling error or warning messages in no way fixes the condition that
triggered the message. Always use extreme caution when exercising these
options.
CHANGING MESSAGE TYPES
It is also possible to change the type of some messages. This can only be done for
messages generated by the parser or code generator. See Section 5.14.4.11 “The
#pragma warning Directive”, for more information on this pragma.
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4.7
MPLAB XC8 DRIVER OPTIONS
This section looks at the general form of xc8 command-line options and what action
the compiler will perform if no option is specified for a certain feature.
4.7.1
General Option Formats
All single letter options are identified by a leading dash character, “-”, for example: -C.
Some single letter options specify an additional data field that follows the option name
immediately and without any whitespace, for example: -Ddebug. In this manual,
options are written in upper case and suboptions are written in lower case.
Multi-letter, or word, options have two leading dash characters, for example:
--ASMLIST. (Because of the double dash, the driver can determine that the option
--DOUBLE, for example, is not a -D option followed by the argument OUBLE.)
Some of these word options use suboptions which typically appear as a comma-separated list following an equal character, =, for example: --OUTPUT=hex,cof. The exact
formats of the options vary. The options and formats are described in detail in the
following sections.
Some commonly used suboptions include default, which represent the default specification that would be used if this option was absent altogether; all, which indicates
that all the available suboptions should be enabled as if they had each been listed; and
none, which indicates that all suboptions should be disabled. For example:
--OPT=none
will turn off all optimizers.
Some suboptions can be prefixed with a plus character, +, to indicate that they are in
addition to the other suboptions present; or a minus character “-”, to indicate that they
should be excluded. For example:
--OPT=default,-asm
indicates that the default optimization be used, but that the assembler optimizer should
be disabled. If the first character after the equal sign is + or -, then the default keyword
is implied. For example:
--OPT=-asm
is the same as the previous example.
See the –-HELP option, Section 4.8.34 “--HELP: Display Help”, for more information
about options and suboptions.
4.7.2
Default Options
If you run the compiler driver from the command line and do not specify the option for
a feature, it will default to a certain state. You can also specify the default suboption
(to double-dash options) that will also invoke the default behavior. You can check what
the default behavior is by using the --HELP=option on the command line, see
Section 4.8.34 “--HELP: Display Help”.
If you are compiling from within the MPLAB X IDE, it will, by default, issue explicit
options to the compiler (unless changed in the Project Properties dialog), and these
options can be different to those that are the default on the command line. For example,
unless you specify the --ASMLIST option on the command line, the default operation
of the compiler is not to produce an assembly list file. But, if you are compiling from
within the MPLAB X IDE, the default operation (this, in fact, cannot be disabled) is to
always produce an assembly list file.
If you are compiling the same project from the command line and from the MPLAB X
IDE, always check that all options are explicitly specified.
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4.8
OPTION DESCRIPTIONS
Most aspects of the compilation can be controlled using the command-line driver, xc8.
The driver will configure and execute all required applications, such as the code
generator, assembler and linker.
xc8 recognizes the compiler options which are tabled below and are explained in detail
in the sections following. The case of the options is not important; however, command
shells in most operating systems are case sensitive when it comes to the names of
files.
TABLE 4-6:
DRIVER OPTIONS
Option
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Meaning
-C
Compile to object file and stop
-Dmacro
Define preprocessor macro symbol
-Efilename
Redirect compile errors
-G[filename]
Generate symbolic debug information
-Ipath
Specify include path
-Largument
Set linker option
-M[filename]
Generate map file
-Nnumber
Specify identifier length
-Ofile
Specify output filename and type
-P
Preprocess assembly source
-Q
Quiet mode
-S
Compile to assembly file and stop
-Umacro
Undefine preprocessor macro symbol
-V
Verbose mode
--ADDRQUAL=qualifier
Specify address space qualifier handling
--ASMLIST
Generate assembly list file
--CHAR=type
Default character type (defunct)
--CHECKSUM=specification
Calculate a checksum or CRC and store the result in
program memory
--CHIP=device
Select target device
--CHIPINFO
Print device information
--CODEOFFSET=value
Specify ROM offset address
--DEBUGGER=type
Set debugger environment
--DEP=specifications
Generate dependency file
--DOUBLE=size
Size of double type
--ECHO
Echo command line
--EMI=mode
Select external memory interface operating mode
--ERRATA=type
Specify errata workarounds
--ERRFORMAT=format
Set error format
--ERRORS=number
Set maximum number of errors
--EXT=extensions
Specify C language extensions
--FILL=specification
Specify a ROM-fill value for unused memory
--FLOAT=size
Size of float type
--GETOPTION=argument
Get advanced options
--HELP=option
Help
--HTML=file
Generate HTML debug files
--LANG=language
Specify language
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TABLE 4-6:
DRIVER OPTIONS (CONTINUED)
Option
Meaning
--MAXIPIC
Maximize current device’s memory resources
--MEMMAP=mapfile
Display memory map
--MODE=mode
Choose operating mode
--MSGDISABLE=list
Disable warning messages
--MSGFORMAT=specification
Set advisory message format
--NODEL
Do not remove temporary files
--NOFALLBACK
Error if the request operating mode cannot be used
--OBJDIR=path
Set object files directory
--OPT=optimizations
Control optimization
--OUTDIR=path
Set output directory
--OUTPUT=path
Set output formats
--PARSER=mode
Specify parser mode
--PASS1
Produce intermediate p-code file and stop
--PRE
Produce preprocessed source files and stop
--PROTO
Generate function prototypes
--RAM=ranges
Adjust RAM ranges
--ROM=ranges
Adjust ROM ranges
--RUNTIME=options
Specify runtime options
--SCANDEP
Scan for dependencies
--SERIAL=specification
Insert a hexadecimal code or serial number
--SETOPTION=argument
Set advanced options
--SETUP=specification
Setup the compiler
--SHROUD
Shroud (obfuscate) generated p-code files
--STACK=type[:sizes]
Specify data stack type and sizes
--STRICT
Use strict ANSI keywords
--SUMMARY=type
Summary options
--TIME
Report compilation times
--UNDEFINTS=action
Program unused interrupt vectors
--VER
Show version information
--WARN=number
Set warning threshold level
--WARNFORMAT=specification
Set warning format
4.8.1
-C: Compile to Object File
The -C option is used to halt compilation after executing the assembler, leaving a relocatable object file as the output. It is frequently used when compiling assembly source
files using a make utility. It cannot be used unless all C source files are present on the
command line. Use --PASS1 to generate intermediate files from C source, see
Section 4.8.49 “--PASS1: Compile to P-code”.
See Section 4.3.3 “Multi-Step Compilation”, for more information on generating and
using intermediate files.
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4.8.2
-D: Define Macro
The -D option is used to define a preprocessor macro on the command line, exactly as
if it had been defined using a #define directive in the source code. This option can
take one of two forms, -Dmacro which is equivalent to:
#define macro
1
placed at the top of each module compiled using this option, or -Dmacro= text which
is equivalent to:
#define macro
text
where text is the textual substitution required. Thus, the command:
xc8 --CHIP=16F877AA -Ddebug -Dbuffers=10 test.c
will compile test.c with macros defined exactly as if the C source code had included
the directives:
#define
#define
debug
1
buffers 10
Defining macros as C string literals requires bypassing any interpretation issues in the
operating system that is being used. To pass the C string, "hello world", (including
the quote characters) in the Windows environment, use: "-DMY_STRING=\\\"hello
world\\\"" (you must include the quote characters around the entire option, as there
is a space character in the macro definition). Under Linux or Mac OS X, use:
-DMY_STRING=\"hello\ world\".
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
4.8.3
-E: Redirect Compiler Errors to a File
This option has two purposes. The first is to change the format of displayed messages.
The second is to optionally allow messages to be directed to a file, as some editors do
not allow the standard command line redirection facilities to be used when invoking the
compiler.
The general form of messages produced with the -E option in force is:
filename
line_number:
(message number)
message string
(type)
If a filename is specified immediately after -E, it is treated as the name of a file to which
all messages (errors, warnings, etc.) will be printed. For example, to compile x.c and
redirect all errors to x.err, use the command:
xc8 --CHIP=16F877AA -Ex.err x.c
The -E option also allows errors to be appended to an existing file by specifying an
addition character, +, at the start of the error filename, for example:
xc8 --CHIP=16F877AA -E+x.err y.c
If you wish to compile several files and combine all of the errors generated into a single
text file, use the -E option to create the file then use -E+ when compiling all the other
source files. For example, to compile a number of files with all errors combined into a
file called project.err, you could use the - E option as follows:
xc8 --CHIP=16F877AA -Eproject.err -O --PASS1 main.c
xc8 --CHIP=16F877AA -E+project.err -O --PASS1 part1.c
xc8 --CHIP=16F877AA -E+project.err -C asmcode.as
Section 4.6 “Compiler Messages” has more information regarding this option as well as
an overview of the messaging system and other related driver options.
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4.8.4
-I: Include Search Path
Use -I to specify an additional directory to search for header files which have been
included using the #include directive. The directory can either be an absolute or relative path. The -I option can be used more than once if multiple directories are to be
searched.
The compiler’s include directory containing all standard header files is always
searched, even if no -I option is present. If header filenames are specified using quote
characters rather than angle brackets, as in #include "lcd.h", then the current
working directory is searched in addition to the compiler’s include directory. Note that
if compiling within MPLAB X IDE, the search path is relative to the output directory, not
the project directory.
These default search paths are searched after any user-specified directories have
been searched. For example, the following code:
xc8 --CHIP=16F877AA -C -Ic:\include -Id:\myapp\include test.c
will search the directories c:\include and d:\myapp\include for any header files
included into the source code, then search the default include directory.
Under Windows OS, be aware that use of the directory backslash character may unintentionally form an escape sequence. For example, to specify an include file path that
ends with a directory separator character and which is quoted, use -I"E:\\" instead
of -I"E:\", to avoid the escape sequence \". Note that MPLAB X IDE will quote any
include file path you specify in the project properties.
This option has no effect for files that are included into assembly source using the
assembly INCLUDE directive. See Section 6.2.10.5 “INCLUDE”, for details.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
4.8.5
-L: Scan Library
The -L option is used to specify additional libraries that are to be scanned by the linker.
Libraries specified using the -L option are scanned before the standard C library, allowing additional versions of standard library functions to be accessed.
The argument to -L is a library keyword to which the prefix pic; numbers representing
the device range, number of ROM pages and the number of RAM banks; and the suffix
.lib are added.
In this way, the option -Ll, when compiling for a 16F877A, will, for example, scan the
library pic42c-l.lib and the option -Lxx will scan a library called
pic42c-xx.lib.
All libraries must be located in the lib directory of the compiler installation directory.
As indicated, the argument to the -L option is not a complete library filename. If you
wish the linker to scan libraries whose names do not follow the naming convention previously mentioned or whose locations are not in the lib subdirectory, simply include
the libraries’ names on the command line along with your source files, or add these to
your project.
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4.8.6
-L-: Adjust Linker Options Directly
The -L driver option can be used to specify an option that will be passed directly to the
linker. If -L is followed immediately by text starting with a dash character “-”, the text
will be passed directly to the linker without being interpreted by the xc8 command-line
driver. When not followed immediately by a dash character, the -L option is assumed
to be the library scan option; see Section 4.8.5 “-L: Scan Library” for more information.
For example, if the option -L-N is specified, the -N option will be passed on to the linker
without any subsequent interpretation by the driver.
Take care with command-line options. The linker cannot interpret command-line driver
options; similarly, the driver cannot interpret linker options. In most situations, it is
always the command-line driver, xc8, that is being executed. You are always invoking
the driver if you build from MPLAB X IDE.
If you are building from MPLAB X IDE, add any -L- driver options to the Extra linker
options field in the Additional options category of the Linker category in the Project
Properties dialog, see Section 4.9.3.4 “Additional”.
The -L option is especially useful when linking code that contains non-standard program sections (or psects), as can be the case if the program contains hand-written
assembly code that contains user-defined psects (see 6.2.9.3 PSECT, and
Section 5.15.1 “Program Sections”), or C code which uses the __section() specifier
(see Section 5.15.4 “Changing and Linking the Allocated Section”).
This option can also be used to replace default linker options. The default linker option
for a psect is removed if that psect’s name is specified in a -L- option. Any psects listed
after that psect’s name in the default options are also removed. For example, if a default
linker option was:
-preset_vec=00h,intentry,init,end_init
using the driver option -L-pinit=100h would result in the following options being
passed to the linker:
-pinit=100h -preset_vec=00h,intentry.
Note that the end_init linker option has been removed entirely.
If there are no characters following the first equal character in the -L option, then no
replacement will be made for the default linker options that will be deleted. For example, the driver option -L-pinit= was instead issued in the previous example, the following option would be passed to the linker: -preset_vec=00h,intentry.
No warning is generated if such a default linker option cannot be found. The default
option that you are deleting or replacing must contain an equal character.
4.8.7
-M: Generate Map File
The -M option is used to request the generation of a map file. The map file is generated
by the linker and includes detailed information about where objects are located in
memory. See Section 7.4 “Map Files” for information regarding the content of these
files.
If no filename is specified with the option, then the name of the map file will have the
project name (see Section 4.3 “The Compilation Sequence”), with the extension .map.
This option is on by default when compiling from within MPLAB X IDE and using the
Universal Toolsuite.
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4.8.8
-N: Identifier Length
This option allows the significant C identifier length (used by functions and variables)
to be decreased from the default value of 255. Valid sizes for this option are from 31 to
255. The option has no effect for all other values.
This option also controls the significant length of identifiers used by the preprocessor,
such as macro names. The default length is also 255, and can be adjusted to a
minimum of 31.
If the --STRICT option is used, the default significant identifier length is reduced to 31.
Code that uses a longer identifier length will be less portable.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
4.8.9
-O: Specify Output File
This option allows the base name of the output file(s) to be specified. If no -O option is
given, the base name of output file(s) will be the same as the project name, see
Section 4.3 “The Compilation Sequence”. The files whose names are affected by this
option are those files that are not directly associated with any particular source file,
such as the HEX file, MAP file and SYM file.
The -O option can also change the directory in which the output file is located by including the required path before the filename. This will then also specify the output directory
for any files produced by the linker or subsequently run applications. Any relative paths
specified are with respect to the current working directory.
For example, if the option -Oc:\project\output\first is used, the MAP and
HEX file, etc., will use the base name first, and will be placed in the directory
c:\project\output.
Any extension supplied with the filename will be ignored.
If a path is specified with the option that enables MAP file creation, -M, (see
Section 4.8.7 “-M: Generate Map File”), this overrides any name or path information
provided by -O.
To change the directory in which all output and intermediate files are written, use the
--OUTDIR option; see Section Section 4.8.46 “--OUTDIR: Specify a Directory for Output Files”. Note that if -O specifies a path that is inconsistent with the path specified in
the --OUTDIR option, it will result in an error.
4.8.10
-P: Preprocess Assembly Files
The -P option causes assembler source files to be preprocessed before they are
assembled, thus allowing the use of preprocessor directives, such as #include, and
C-style comments with assembler code.
By default, assembler files are not preprocessed.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
4.8.11
-Q: Quiet Mode
This option places the compiler in a quiet mode that suppresses the Microchip
Technology Incorporated copyright notice from being displayed.
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4.8.12
-S: Compile to Assembler Code
The -S option stops compilation after generating an assembly output file. One
assembly file will be generated for all the C source code, including p-code library code.
The command:
xc8 --CHIP=16F877A -S test.c
will produce an assembly file called test.as, which contains the assembly code generated from test.c. The generated file is valid assembly code that could be passed
to xc8 as a source file, however this should only be done for exploratory reasons. To
take advantage of the benefits of the compilation technology in the compiler, it must
compile and link all the C source code in a single step. See the --PASS1 option
(Section 4.8.49 “--PASS1: Compile to P-code”) to generate intermediate files if you
wish to compile code using a two-step process or use intermediate files.
This option is useful for checking assembly code output by the compiler. The file produced by this option differs to that produced by the --ASMLIST option (see
Section 4.8.15 “--ADDRQUAL: Set Compiler Response to Memory Qualifiers”) in that
it does not contain op-codes or addresses and it can be used as a source file in subsequent compilations. The assembly list file is more human readable, but is not a valid
assembly source file.
4.8.13
-U: Undefine a Macro
The -U option, the inverse of the -D option, is used to undefine predefined macros.
This option takes the form -Umacro, where macro is the name of the macro to be
undefined.
The option, -Udraft, for example, is equivalent to:
#undef
draft
placed at the top of each module compiled using this option.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
4.8.14
-V: Verbose Compile
The -V option specifies verbose compilation. When used, the compiler will display the
command lines used to invoke each of the compiler applications described in
Section 4.3 “The Compilation Sequence”.
When this option is used, the name and path of the compiler application being executed
will be displayed, followed by the name of the command file that holds the command-line arguments to this application, for example:
/Applications/dev/XC8/v1.35/bin/hexmate @/tmp/hexmated_xcLN1sWk6.cmd
If this option is used twice (-V -V), the display will additionally show the contents of
the command file in square brackets, for example.
/Applications/dev/XC8/v1.35/bin/hexmate @/tmp/hexmated_xcLN1sWk6.cmd [
--edf=/Applications/dev/XC8/v1.35/dat/en_msgs.txt main.hex -Omain.hex
-logfile=main.hxl -addressing=1 -break=300000 ]
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
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4.8.15
--ADDRQUAL: Set Compiler Response to Memory Qualifiers
The --ADDRQUAL option indicates the compiler’s response to non-standard memory
qualifiers in C source code.
By default, these qualifiers are ignored; i.e., they are accepted without error, but have
no effect. Using this option allows these qualifiers to be interpreted differently by the
compiler.
The near qualifier is affected by this option. On PIC18 devices, this option also affects
the far qualifier; and for other 8-bit devices, the bankx qualifiers (bank0, bank1,
bank2, etc.) are affected.
The suboptions are detailed in Table 4-7.
TABLE 4-7:
Selection
COMPILER RESPONSES TO MEMORY QUALIFIERS
Response
require
The qualifiers will be honored. If they cannot be met, an error will be
issued.
request
The qualifiers will be honored, if possible. No error will be generated if they
cannot be followed.
ignore
The qualifiers will be ignored and code compiled as if they were not used.
reject
If the qualifiers are encountered, an error will be immediately generated.
For example, when using the option --ADDRQUAL=request the compiler will try to
honor any non-standard qualifiers, but silently ignore them if they cannot be met.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
4.8.16
--ASMLIST: Generate Assembler List Files
The --ASMLIST option tells xc8 to generate assembler listing files for the C and
assembly source modules being compiled. One assembly list file is produced for the
entire C program, including code from the C library functions.
Additionally, one assembly list file is produced for each assembly source file in the
project, including the runtime startup code. For more information, see
Section 4.4.2 “Startup and Initialization”.
Assembly list files use a .lst extension and, due to the additional information placed
in these files, cannot be used as assembly source files.
In the case of listings for C source code, the list file shows both the original C code and
the corresponding assembly code generated by the compiler. See
Section 6.3 “Assembly-Level Optimizations”, for full information regarding the content
of these files.
The same information is shown in the list files for assembly source code.
This option is on by default when compiling under MPLAB X IDE.
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4.8.17
--CHECKSUM: Calculate a Checksum
This option will calculate a hash value (for example checksum or CRC) over the
address range specified and store the result at the destination address specified. The
general form of this option is as follows.
-CHECKSUM=start-end@destination[,specifications]
Additional specifications are appended as a comma-separated list to this option. Such
specifications are:
width=n
selects the width of the hash result in bytes for non-Fletcher
algorithms. A negative width will store the result in little-endian
byte order; positive widths in big-endian order. Result widths
from one to four bytes are permitted.
offset=nnnn
specifies an initial value or offset to be added to this checksum.
algorithm=n
selects one of the hash algorithms implemented in HEXMATE.
The selectable algorithms are described in Table 8-4.
polynomial=nnnn
selects the polynomial value when using CRC algorithms
code=nn
is a hexadecimal code that will trail each byte in the result.
This can allow each byte of the result to be embedded within
an instruction, for example code=34 will embed the result in a
RETLW instruction on mid-range devices.
The start, end and destination attributes are, by default, hexadecimal constants.
If an accompanying --FILL option has not been specified, unused locations within the
specified address range will be automatically filled with 0xFFF for baseline devices,
0x3FFF for mid-range devices, or 0xFFFF for PIC18 devices. This is to remove any
unknown values from the equation and ensure the accuracy of the result.
For example:
--checksum=800-fff@20,width=1,algorithm=2
will calculate a 1-byte checksum from address 0x800 to 0xfff and store this at address
0x20. A 16-bit addition algorithm will be used. See Table 4-8, for the available
algorithms and Section 8.4 “Hash Functions” for more information on calculating
checksum and hash values.
TABLE 4-8:
CHECKSUM ALGORITHM SELECTION
Selector
Algorithm description
-5
Reflected cyclic redundancy check (CRC)
-4
Subtraction of 32 bit values from initial value
-3
Subtraction of 24 bit values from initial value
-2
Subtraction of 16 bit values from initial value
-1
Subtraction of 8 bit values from initial value
1
Addition of 8 bit values from initial value
2
Addition of 16 bit values from initial value
3
Addition of 24 bit values from initial value
4
Addition of 32 bit values from initial value
5
Cyclic redundancy check (CRC)
7
Fletcher’s checksum (8 bit calculation, 2-byte result width)
8
Fletcher’s checksum (16 bit calculation, 4-byte result width)
The hash calculations are performed by the HEXMATE application. The information in
this driver option is passed to the HEXMATE application when it is executed.
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4.8.18
--CHIP: Define Device
This option must be used to specify the target device, or device, for the compilation.
This is the only compiler option that is mandatory when compiling code.
To see a list of supported devices that can be used with this option, use the
--CHIPINFO option described in the next section in this guide.
4.8.19
--CHIPINFO: Display List of Supported Devices
The --CHIPINFO option displays a list of devices the compiler supports. The names
listed are those chips that are defined in the chipinfo file and which can be used with
the --CHIP option.
Compiler execution will terminate after this list has been printed.
4.8.20
--CLIST: Generate C Listing File
Use of this option will generate a C listing file for each C source file specified on the
command line.
The listing files produced consist of the original C source code prepended with a line
number. Do not confuse these files with assembly list files, see
Section 4.8.16 “--ASMLIST: Generate Assembler List Files”.
4.8.21
--CODEOFFSET: Offset Program Code to Address
In some circumstances, such as bootloaders, it is necessary to shift the program image
to an alternative address. This option is used to specify a base address for the program
code image and to reserve memory from address 0 to that specified in the option.
When using this option, all code psects (including Reset and interrupt vectors and constant data) will be adjusted to the address specified. The address is assumed to be a
hexadecimal constant. A leading 0x, or a trailing h hexadecimal specifier can be used,
but is not necessary.
This option differs from the --ROM option in that it will move the code associated with
the Reset and interrupt vectors. That cannot be done using the --ROM option, see
Section 4.8.53 “--ROM: Adjust ROM Ranges”.
For example, if the option --CODEOFFSET=600 is specified, the Reset vector will be
moved from address 0 to address 0x600; the interrupt vector will be moved from
address 4 to 0x604, in the case of mid-range PIC devices, or to the addresses 0x608
and 0x618 for PIC18 devices. No code will be placed between address 0 and 0x600.
As the Reset and interrupt vector locations are fixed by the PIC device, it is the programmer’s responsibility to ensure code that can redirect control to the offset Reset
and interrupt routines is written and located at the original locations.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
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4.8.22
--DEBUGGER: Select Debugger Type
This option is intended for use for compatibility with development tools that can act as
a debugger. xc8 supports several debuggers and using this option will configure the
compiler to conform to the requirements of that selected. The possible selections for
this option are defined in Table 4-9.
TABLE 4-9:
SELECTABLE DEBUGGERS
Suboption
Debugger selected
none
No debugger (default)
icd2
MPLAB® ICD 2
icd3
MPLAB ICD 3
pickit2
PICkit™ 2
pickit3
PICkit 3
realice
MPLAB REAL ICE™ in-circuit emulator
For example:
xc8 --CHIP=16F877AA --DEBUGGER=icd2 main.c
Choosing the correct debugger is important as they can use memory resources that
could otherwise be used by the project if this option is omitted. Such a conflict would
lead to runtime failure.
If the debugging features of the development tool are not to be used (for example, if the
MPLAB ICD 3 is only being used as a programmer), then the debugger option can be
set to none, because memory resources are not being used by the tool.
MPLAB X IDE will automatically apply this option for debug builds once you have
indicated the hardware tool in the Project Preferences.
4.8.23
--DEP: Generate dependency file
This option controls the generation of dependency files. A dependency file lists those
files on which a source file’s content is dependent. Dependencies result when one file
is #included into another. Table 4-10 shows the suboptions which are allowed with
this option. See also Section 4.8.55 “--SCANDEP: Scan for Dependencies”.
TABLE 4-10:
SELECTABLE DEPENDENCY OPTIONS
Suboption
Dependency style
stop
Stop compilation after file generation
list
Format dependency file as a list (.dep file)
gcc
Create a GCC-style dependency file (.d file)
sys
Include system headers in dependency file
4.8.24
--DOUBLE: Select Kind of Double Types
This option allows the kind of double-precision floating-point types to be selected. By
default, the compiler will choose the truncated IEEE754 24-bit format for double
types. With this option, it can be changed to the full 32-bit IEEE754 format. See also
Section 4.8.32 “--FLOAT: Select Kind of Float Types” for specification of the float
type format.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
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4.8.25
--ECHO: Echo Command Line Before Processing
Use of this option will result in the driver command line being echoed to the stdout
stream before compilation commences. Each token of the command line will be printed
on a separate line and they will appear in the order in which they are placed on the
command line.
4.8.26
--EMI: Select External Memory Interface Operating Mode
The external memory interface available on some PIC18 devices can be operated in
several modes. The interface can operate in 16-bit modes; Word-write and Byte-select
modes or in an 8-bit mode: Byte-write mode. These modes are represented by those
specified in Table 4-11.
TABLE 4-11:
EXTERNAL MEMORY INTERFACE MODES
Mode
Operation
wordwrite
16-bit Word-write mode (default)
byteselect
16-bit Byte-select mode
bytewrite
8-bit Byte-write mode
The selected mode will affect the code generated when writing to the external data
interface. In word write mode, dummy reads and writes can be added to ensure that an
even number of bytes are always written. In Byte-select or Byte-write modes, dummy
reads and writes are not generated and can result in more efficient code.
Note that this option does not pre-configure the device for operation in the selected
mode. See your device data sheet for the registers and settings that are used to
configure the device’s external interface mode.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
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4.8.27
--ERRATA: Specify Errata Workarounds
This option allows specification of software workarounds to documented silicon errata
issues. A default set of errata issues apply to each device, but this set can be adjusted
by using this option and the arguments presented in Table 4-12.
TABLE 4-12:
Symbol
ERRATA WORKAROUNDS
Bit
pos.
Workaround
4000
0
Program memory accesses/jumps across 4000h address boundary
fastints
1
Fast interrupt shadow registers corruption
lfsr
2
Broken LFSR instruction
minus40
3
Program memory reads at -40 degrees
reset
4
GOTO instruction cannot exist at Reset vector
bsr15
5
Flag problems when BSR holds value 15
daw
6
Broken DAW instruction
eedatard
7
Read EEDAT in immediate instruction after RD set
eeadr
8
Don't set RD bit immediately after loading EEADR
ee_lvd
9
LVD must stabilize before writing EEPROM
fl_lvd
10
LVD must stabilize before writing Flash
tblwtint
11
Clear interrupt registers before tblwt instruction
fw4000
12
Flash write exe must act on opposite side of 4000h boundary
resetram
13
RAM contents can corrupt if async. Reset occurs during write access
fetch
14
Corruptible instruction fetch. – apply FFFFh (NOP) at required locations
clocksw
15
Code corruption if switching to external oscillator clock source – apply
switch to HFINTOSC high-power mode first
branch
16
The PC might become invalid when restoring from an interrupt during a
BRA or BRW instruction — avoid branch instructions
brknop2
17
Hardware breakpoints might be affected by BRA instruction — avoid
branching to the following location
nvmreg
18
The program will access data flash rather than program flash memory
after a reset — adjust the NVMCON register
At present, workarounds are mainly employed for PIC18 devices, but the clocksw and
branch errata are only applicable for some enhanced mid-range devices.
To disable all software workarounds, use the following.
--ERRATA=none
For example, to apply the default set of workarounds, but also to specifically disable
the jump across 4000 errata, use the following:
--ERRATA=default,-4000
A preprocessor macro _ERRATA_TYPES (see Section 5.14.3 “Predefined Macros”) is
set to a value to indicate the errata applied. Each errata listed in Table 4-12 represents
a bit position in the macro’s value, with the topmost errata in the table being the least
significant. The bit position is indicated in the table and is set if the corresponding errata
is applied. The header file <errata.h> contains definitions for each errata value, for
example ERRATA_4000 and ERRATA_FETCH, which can be compared with the compiler-defined _ERRATTA_TYPES macro.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
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4.8.28
--ERRFORMAT: Define Format for Compiler Messages
If the --ERRFORMAT option is not used, the default behavior of the compiler is to display any errors in a “human readable” form. This standard format is perfectly acceptable to a person reading the error output, but is not generally usable with environments
that support compiler error handling.
This option allows the exact format of printed error messages to be specified using special placeholders embedded within a message template. See Section 4.6 “Compiler
Messages” for full details of the messaging system employed by xc8, and the
placeholders which can be used with this option.
This section is also applicable to the --WARNFORMAT and --MSGFORMAT options,
which adjust the format of warning and advisory messages, respectively.
If you are compiling using MPLAB X IDE, the format of the compiler messages is automatically configured to what the IDE expects. It recommended that you do not adjust
the message formats if compiling using this IDE.
4.8.29
--ERRORS: Maximum Number of Errors
This option sets the maximum number of errors each compiler application, as well as
the driver, will display before execution is terminated. By default, up to 20 error
messages will be displayed by each application.
See Section 4.6 “Compiler Messages” for full details of the messaging system
employed by xc8.
4.8.30
--EXT: Specify C Language Extensions
The compiler can accept several different sets of non-standard C language extensions.
The suboption to --EXT indicates the set and these are shown in Table 4-13.
TABLE 4-13:
ACCEPTABLE C LANGUAGE EXTENSIONS
Suboption
Meaning
xc8
The native XC8 extensions (default)
cci
A common C interface acceptable by all MPLAB XC compilers
iar
Extensions defined by the IAR C/C++ Compiler for ARM
Enabling the cci suboption requests the compiler to check all source code and compiler options for compliance with the Common C Interface (CCI). Code that complies
with this interface is portable across all MPLAB XC compilers. Code or options that do
not conform to the CCI will be flagged by compiler warnings. See Chapter 2. Common
C Interface, for information on the features that are covered by this interface.
The iar suboption enables conformance with the non-standard extensions supported
by the IAR C/C++ Compiler™ for ARM. This is discussed further in
Appendix B. Embedded Compiler Compatibility Mode.
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4.8.31
--FILL: Fill Unused Program Memory
This option allows specification of a hexadecimal opcode that can be used to fill all
unused program memory locations. This option utilizes the features of the HEXMATE
application, so it is only available when producing a HEX output file, which is the default
operation.
This driver feature allows you to compile and fill unused locations in one step. If you
prefer not to use the driver option and you prefer to fill unused locations after compilation, then you need to use the HEXMATE application. Note that the corresponding option
in HEXMATE is -FILL (one leading dash) as opposed to the drivers --FILL option.
Note, also, that the unused tag that can be specified with the driver option does not
exist in the HEXMATE options.
The usage of the driver option is:
--FILL=[const_width:]fill_expr[@address[:end_address]]
where:
• const_width has the form wn and signifies the width (n bytes) of each constant
in fill_expr. If const_width is not specified, the default value is the native
width of the architecture. That is, --FILL=w1:1 with fill every byte with the value
0x01.
• fill_expr can use the syntax (where const and increment are n-byte
constants):
- const fill memory with a repeating constant; i.e., --FILL=0xBEEF becomes
0xBEEF, 0xBEEF, 0xBEEF, 0xBEEF
- const+=increment fill memory with an incrementing constant; i.e.,
--fill=0xBEEF+=1 becomes 0xBEEF, 0xBEF0, 0xBEF1, 0xBEF2
- const-=increment fill memory with a decrementing constant; i.e.,
--fill=0xBEEF-=0x10 becomes 0xBEEF, 0xBEDF, 0xBECF, 0xBEBF
- const,const,...,const fill memory with a list of repeating constants; i.e.,
--FILL=0xDEAD,0xBEEF becomes 0xDEAD,0xBEEF,0xDEAD,0xBEEF
• The options following fill_expr result in the following behavior:
- @unused (or nothing) fill all unused memory with fill_expr; i.e.,
--FILL=0xBEEF@unused fills all unused memory with 0xBEEF. The driver
will expand this to the appropriate ranges and pass these to HEXMATE.
- @address fill a specific address with fill_expr; i.e.,
--FILL=0xBEEF@0x1000 puts 0xBEEF at address 1000h
- @address:end_address fill a range of memory with fill_expr; i.e.,
--FILL=0xBEEF@0:0xFF puts 0xBEEF in unused addresses between 0 and
255
All constants can be expressed in (unsigned) binary, octal, decimal or hexadecimal, as
per normal C syntax. For example, 1234 is a decimal value, 0xFF00 is hexadecimal,
and FF00 is illegal.
See Section 4.9 “MPLAB X Option Equivalents”, or Section 4.9 “MPLAB X Option
Equivalents”, for information on using this option in MPLAB X IDE.
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4.8.32
--FLOAT: Select Kind of Float Types
This option allows the size of float types to be selected. The types available to be
selected are given in Table 4-14.
See also, the --DOUBLE option in Section 4.8.24 “--DOUBLE: Select Kind of Double
Types”.
TABLE 4-14:
FLOATING-POINT SELECTIONS
Suboption
double
Effect
Size of float matches size of double type
24
24-bit float (default)
32
32-bit float (IEEE754 format)
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
4.8.33
--GETOPTION: Get Command-line Options
This option is used to retrieve the command line options that are used for named compiler application. The options are then saved into the given file. This option is not
required for most projects, and is disabled when the compiler is operating in Free mode
(see Section 4.8.39 “--MODE: Choose Compiler Operating Mode”).
The options take an application name and a filename to store the options, for example:
--GETOPTION=hlink,options.txt
4.8.34
--HELP: Display Help
This option displays information on the xc8 compiler options. The option --HELP will
display all options available. To find out more about a particular option, use the option’s
name as a parameter. For example:
xc8 --help=warn
will display more detailed information about the --WARN option, the available
suboptions, and which suboptions are enabled by default.
4.8.35
--HTML: Generate HTML Diagnostic Files
This option will generate a series of HTML files that can be used to explore the compilation results of the latest build. The files are stored in a directory called html, located
in the output directory. The top-level file (which can be opened with your favorite web
browser) is called index.html.
Use this option at all stages of compilation to ensure files associated with all
compilation stages are generated.
The index page is a graphical representation of the compilation process. Each file icon
is clickable and will open to show the contents of that file. This is available for all intermediate files, and even binary files will open in a human-readable form. Each application icon can also be clicked to show a page that contains information about the
application’s options and build results.
The list of all preprocessor macros (preprocessor icon) and a graphical memory usage
map (Linker icon) provide information that is not otherwise readily accessible.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
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4.8.36
--LANG: Specify the Language for Messages
This option allows the compiler to be configured to produce error, warning and some
advisory messages in languages other than English.
English is the default language unless this has been changed at installation, or by the
use of the --SETUP option. Some messages are only ever printed in English regardless of the language specified with this option. For more information, see
Section 4.6.2 “Message Language”.
Table 4-15 shows those languages currently supported.
TABLE 4-15:
SUPPORTED LANGUAGES
Suboption
Language
en, english
English (default)
fr, french, francais
French
de, german, deutsch
German
4.8.37
--MAXIPIC: Maximize current device’s memory resources
The compiler will terminate compilation if the selected device runs out of program memory, data memory, or EEPROM. This option tells the compiler to generate code for a
hypothetical device with the same physical core and peripherals as the selected
device, but with the maximum allowable memory resources permitted by the device
family.
The program memory of PIC18 and mid-range devices will be maximized to either the
bottom of external memory or the maximum address permitted by the PC register,
whichever is lower. The program memory of baseline parts is maximized to the lower
address of the Configuration Words.
The number of data memory banks is expanded to the maximum number of selectable
banks as defined by the BSR register (for PIC18 devices), RP bits in the STATUS register (for mid-range devices), or the bank select bits in the FSR register (for baseline
devices). The amount of RAM in each additional bank is equal to the size of the largest
contiguous memory area within the physically implemented banks.
EEPROM is only maximized if the device implements this memory. If present,
EEPROM is maximized to a size dictated by the number of bits in the EEADR register.
If required, check the map file (see Section 7.4 “Map Files”) to see the size and
arrangement of the memory available when using this option with your device.
Note:
With the --MAXIPIC option enabled, you are not compiling for a real
device. The generated code may not load or execute in simulators or on
real silicon devices. This option cannot be used to fit additional code into a
device.
You might choose to use this option if your code does not fit in your intended target
device. This option will allow you to see the total memory requirements of your program
and give an indication of the code or data size reductions that need to be made to fit
the program to the device.
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4.8.38
--MEMMAP: Display Memory Map
This option will display a memory map for the map file specified with this option. The
information printed is controlled by the --SUMMARY option, see
Section 4.8.61 “--SUMMARY: Select Memory Summary Output Type”, for example:
xc8 --memmap=size.map --summary=psect,class,file
This option is seldom required, but would be useful if the linker is being driven explicitly;
i.e., instead of in the normal way through the command-line driver. This command
would display the memory summary that is normally produced at the end of compilation
by the driver.
4.8.39
--MODE: Choose Compiler Operating Mode
This option selects the basic operating mode of the compiler. The available types are
pro, std, and free. (For legacy projects, the mode “lite” is accepted to mean the Free
operating mode.)
• PRO mode uses full optimization and produces the smallest code size.
• Standard mode uses limited optimizations.
• Free mode uses a minimum optimization level and produces relatively large code.
See Section 5.13 “Optimizations” and Section 6.3 “Assembly-Level Optimizations”, for
more about which optimizations are available in each mode.
Only those modes permitted by the compiler license status are accepted. For example,
if you purchased a Standard compiler license, the compiler can be run in Standard or
Free mode, but not in PRO mode. If you attempt to run the compiler in a mode for which
it is not licensed, it will fall back to the highest-allowed mode.
However, the --NOFALLBACK option can be used to detect situations in which a compiler may have been activated incorrectly or not activated at all by preventing compilation in a lower operating mode than the one requested (see
Section 4.8.43 “--NOFALLBACK: Error if the Requested Operating Mode Cannot Be
Used”).
See Section 4.9 “MPLAB X Option Equivalents” for use of this option in MPLAB X IDE.
4.8.40
--MSGDISABLE: Disable Warning Messages
This option allows error, warning or advisory messages to be disabled during the
compilation of a project.
The option is passed a comma-separated list of message numbers that are to be disabled. Any error message numbers in this list are ignored unless they are followed by
an :off argument. If the message list is specified as 0, then all warnings are disabled.
For full information on the compiler’s messaging system and use of this option, see
Section 4.6 “Compiler Messages”. Also, see Section 4.6.5 “Changing Message
Behavior” for other ways to disable messages.
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4.8.41
--MSGFORMAT: Set Advisory Message Format
This option sets the format of advisory messages produced by the compiler. Warning
and error messages are controlled separately by other options. See
Section 4.8.28 “--ERRFORMAT: Define Format for Compiler Messages” and
Section 4.8.66 “--WARNFORMAT: Set Warning Message Format” for information on
changing the format of these sorts of messages.
See Section 4.6 “Compiler Messages” for full information on the compiler’s messaging
system.
If you are compiling using MPLAB X IDE, the format of the compiler messages is automatically configured to what the IDE expects. It recommended that you do not adjust
the message formats if compiling using this IDE.
4.8.42
--NODEL: Do Not Remove Temporary Files
Specifying --NODEL when building will instruct xc8 not to remove the intermediate and
temporary files that were created during the build process.
4.8.43
--NOFALLBACK: Error if the Requested Operating Mode Cannot
Be Used
This option can be used to ensure that the compiler is not executed with an operating
mode below that specified by the --MODE option (see Section 4.8.39 “--MODE:
Choose Compiler Operating Mode”). If the compiler has not been activated to run in the
requested mode, an error will be produced and compilation will terminate when this
option is used. Without this option, the compiler will fall back to either the Standard or
Free operating mode if it is not activated to run in the requested mode.
4.8.44
--OBJDIR: Specify a Directory for Intermediate Files
This option allows a directory to be nominated in xc8 to locate its intermediate files. If
this option is omitted, intermediate files will be created in the current working directory.
This option will not set the location of output files, instead use --OUTDIR. See
Section 4.8.46 “--OUTDIR: Specify a Directory for Output Files” and Section 4.8.9 “-O:
Specify Output File” for more information.
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4.8.45
--OPT: Invoke Compiler Optimizations
The --OPT option allows control of all the compiler optimizers. If this option is not specified, or it is specified as --OPT=all, the space and asm optimizations are enabled
(see below). Optimizations can be disabled by using --OPT=none, or individual optimizers can be controlled, for example: --OPT=asm will only enable some assembler
optimizations.
Table 4-16 lists the available optimization types.
TABLE 4-16:
OPTIMIZATION OPTIONS
Option name
Function
asm
Select optimizations of assembly code derived from C source (default)
asmfile
Select optimizations of assembly source files
debug
Favor accurate debugging over optimization
speed
Favor optimizations that result in faster code
space
Favor optimizations that result in smaller code (default)
all
Enable all compiler optimizations
none
Do not use any compiler optimizations
Note that different suboptions control assembler optimizations of assembly source files
and intermediate assembly files produced from C source code.
The speed and space suboptions are contradictory. Space optimizations are the
default. If speed and space suboptions are both specified, then speed optimizations
takes precedence. If all optimizations are requested, the space optimization is
enabled. These optimizations affect procedural abstraction, which is performed by the
assembler, and other optimizations at the code generation stage.
The asmfile selection optimizes assembly source files, which are otherwise not optimized by the compiler. By contrast, the asm control allows for optimization of assembly
code that was derived from C code, an optimization that is enabled by default.
Enabling the debug suboption disables inlining and procedural abstraction (reverse
inlining) assembler optimizations. These optimizations can adversely affect
source-level debugging.
See Section 4.9 “MPLAB X Option Equivalents” for use of this option in MPLAB X IDE.
4.8.46
--OUTDIR: Specify a Directory for Output Files
This option allows a directory to be nominated for xc8 to locate its output files. If this
option is omitted, output files will be created in the current working directory. See also
Section 4.8.44 “--OBJDIR: Specify a Directory for Intermediate Files” and
Section 4.8.9 “-O: Specify Output File” for more information.
4.8.47
--OUTPUT= type: Specify Output File Type
This option allows the type of the output file(s) to be specified. If no --OUTPUT option
is specified, the output file’s name will be the same as the project name (see
Section 4.3 “The Compilation Sequence”).
The available output file formats are shown in Table 4-17. More than one output format
can be specified by supplying a comma-separated list of tags. Not all formats are
supported by Microchip development tools.
For debugging, the ELF/DWARF format is preferred, but this format is not supported by
MPLAB 8 IDE or early versions of MPLAB X IDE. Before selecting the ELF file output,
ensure your IDE version has support for this format. Microchip COFF is the default
debugging file output.
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Output file types that specify library formats stop the compilation process before the
final stages of compilation are executed. So, specifying an output file format list that
contains, for example: lib or all, will prevent the other formats from being created.
TABLE 4-17:
OUTPUT FILE FORMATS
Type tag
File format
Object library file (for assembly source)
lib
lpp
P-code library file (for C source)
intel, inhx32
Intel HEX (default)
inhx032
Intel HEX, initialization of upper extended linear address to zero
tek
Tektronix Hex
aahex
American Automation symbolic HEX file
mot, motorola, s19
Motorola S19 HEX file
bin, binary
Binary file
mcof, mcoff, mpcoff
Microchip COFF (default)
elf
ELF/DWARF file
So, for example:
xc8 --CHIP=16F877AA --OUTPUT=lpp lcd_init.c lcd_data.c lcd_msgs.c
will compile the three names files into an LPP (p-code) library.
4.8.48
--PARSER: Specify Parser Mode
This option controls which symbols are stripped from intermediate (p-code) files produced by the parser. The default is to remove unused symbols, which can also be
specified using the --PARSER=lean mode. The rich mode will not remove any
symbols.
Note that the rich mode will generate larger intermediate files and will considerably
slow down the compilation, particularly if there are many SFRs defined for a device.
PIC18 devices typically have a large number of SFRs. Use the rich setting if you have
in-line assembly code that accesses symbols that are not referenced by C code otherwise undefined symbol errors can be produced by the assembler application.
4.8.49
--PASS1: Compile to P-code
The --PASS1 option is used to generate p-code intermediate files (.p1 files) from the
parser, and then stop compilation. Such files need to be generated if creating p-code
library files, however the compiler is able to generate library files in one step, if required.
See Section 4.8.47 “--OUTPUT= type: Specify Output File Type” for specifying a library
output file type.)
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4.8.50
--PRE: Produce Preprocessed Source Code
The --PRE option is used to generate preprocessed C source files (also called modules or translation units) with an extension .pre. This can be useful to ensure that preprocessor macros have expanded to what you think they should. Use of this option can
also create C source files that do not require any separate header files. If the .pre files
are renamed to .c files, they can be passed to the compiler for subsequent processing.
This is useful when sending files to a colleague or to obtain technical support without
sending all the header files, which can reside in many directories.
If you wish to see the preprocessed source for the printf() family of functions, do
not use this option. The source for this function is customized by the compiler, but only
after the code generator has scanned the project for printf() usage. Thus, as the
–-PRE option stops compilation after the preprocessor stage, the code generator will
not execute and no printf() code will be processed. If this option is omitted, the
preprocessed source for printf() will be automatically retained in the file
doprnt.pre.
4.8.51
--PROTO: Generate Prototypes
The --PROTO option is used to generate .pro files containing both ANSI C and K&R
style function declarations for all functions within the specified source files. Each .pro
file produced will have the same base name as the corresponding source file. Prototype files contain both ANSI C-style prototypes and old-style C function declarations
within conditional compilation blocks.
The extern declarations from each .pro file should be edited into a global header file,
which can then be included into all the C source files in the project. The .pro files can
also contain static declarations for functions that are local to a source file. These
static declarations should be edited into the start of the source file.
To demonstrate the operation of the --PROTO option, enter the following source code
as file test.c:
#include <stdio.h>
add(arg1, arg2)
int *
arg1;
int *
arg2;
{
return *arg1 + *arg2;
}
void printlist(int * list, int count)
while (count--)
printf("d " *list++);
putchar(’\n’);
}
{
If compiled with the command:
xc8 --CHIP=16F877AA --PROTO test.c
xc8 will produce test.pro containing the following declarations, which can then be
edited as necessary:
/* Prototypes from test.c */
/* extern functions - include these in a header file */
#if
PROTOTYPES
extern int add(int *, int *);
extern void printlist(int *, int);
#else
/* PROTOTYPES */
extern int add();
extern void printlist();
#endif
/* PROTOTYPES */
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4.8.52
--RAM: Adjust RAM Ranges
This option is used to adjust the default RAM, which is specified for the target device.
The default memory will include all the on-chip RAM specified for the target
PIC10/12/16 device, thus this option only needs be used if there are special memory
requirements. Typically this option is used to reserve memory (reduce the amount of
memory available). Specifying additional memory that is not in the target device will
typically result in a successful compilation, but can lead to code failures at runtime.
The default RAM memory for each target device is specified in the chipinfo file,
picc.ini.
Strictly speaking, this option specifies the areas of memory that can be used by writable
(RAM-based) objects; but, not necessarily those areas of memory that contain physical
RAM. The output that will be placed in the ranges specified by this option are typically
variables that a program defines.
For example, to specify an additional range of memory to that already present on-chip,
use:
--RAM=default,+100-1ff
This will add the range from 100h to 1ffh to the on-chip memory. To only use an external
range and ignore any on-chip memory, use:
--RAM=0-ff
This option can also be used to reserve memory ranges already defined as on-chip
memory in the chipinfo file. To do this, supply a range prefixed with a minus character,
-, for example:
--RAM=default,-100-103
will use all the defined on-chip memory, but not use the addresses in the range from
100h to 103h for allocation of RAM objects.
This option is also used to specify RAM for far objects on PIC18 devices. These
objects are stored in the PIC18 extended memory. Any additional memory specified
with this option whose address is above the on-chip program memory is assumed to
be extended memory implemented as RAM.
For example, to indicate that RAM has been implemented in the extended memory
space at addresses 0x20000 to 0x20fff, use the following option.
--RAM=default,+20000-20fff
This option will adjust the memory ranges used by linker classes (see
Section 7.2.1 “-Aclass =low-high,...”) so any object that is in a psect is placed in this
class. Any objects contained in a psect that are explicitly placed at a memory address
by the linker (see Section 7.2.18 “-Pspec”), i.e., are not placed into a memory class, are
not affected by the option. For an introductory guide to psects, see
Section 5.15.1 “Program Sections”.
See Section 4.9 “MPLAB X Option Equivalents” for use of this option in MPLAB X IDE.
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4.8.53
--ROM: Adjust ROM Ranges
This option is used to change the default ROM that is specified for the target device.
The default memory will include all the on-chip ROM specified for the target
PIC10/12/16 device, thus this option only needs to be used if there are special memory
requirements. Typically this option is used to reserve memory (reduce the amount of
memory available). Specifying additional memory that is not in the target device will
typically result in a successful compilation, but can lead to code failures at runtime.
The default ROM memory for each target device is specified in the chipinfo file,
picc.ini.
Strictly speaking, this option specifies the areas of memory that can be used by
read-only (ROM-based) objects; but, not necessarily those areas of memory that contain physical ROM. When producing code that can be downloaded into a system via a
bootloader, the destination memory can be some sort of (volatile) RAM. The output that
will be placed in the ranges specified by this option are typically executable code and
any data variables that are qualified as const.
For example, to specify an additional range of memory to that on-chip, use:
--ROM=default,+100-2ff
This will add the range from 100h to 2ffh to the on-chip memory. To only use an external
range and ignore any on-chip memory, use:
--ROM=100-2ff
This option can also be used to reserve memory ranges already defined as on-chip
memory in the chip configuration file. To do this supply a range prefixed with a minus
character, -, for example:
--ROM=default,-100-1ff
will use all the defined on-chip memory, but not use the addresses in the range from
100h to 1ffh for allocation of ROM objects.
This option will adjust the memory ranges used by linker classes (see
Section 7.2.1 “-Aclass =low-high,...”) so any object that is in a psect is placed in this
class. Any objects which are contained in a psect that are explicitly placed at a memory
address by the linker (see Section 7.2.18 “-Pspec”), i.e., are not placed into a memory
class, are not affected by the option. For an introductory guide to psects, see
Section 5.15.1 “Program Sections”.
Note that some psects must be linked above a threshold address, most notably some
psects that hold const data. Using this option to remove the upper memory ranges can
make it impossible to place these psects.
See Section 4.9 “MPLAB X Option Equivalents” for use of this option in MPLAB X IDE.
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4.8.54
--RUNTIME: Specify Runtime Environment
The --RUNTIME option is used to control what is included as part of the runtime environment. In this context, the runtime environment encapsulates any code that is present when the program is executing and that has not been defined by the user. Such
code is supplied by the compiler, typically in library files or compiler-generated source
files built alongside the user's code.
All required runtime features are enabled by default and this option is not required for
normal compilation.
Note that the code that clears or initializes variables, which is included by default, will
clobber the contents of the STATUS register. For mid-range and baseline devices, if
you need to check the cause of Reset using the TO or PD bits in this register, then you
can enable the resetbits suboption as well. See Section 5.10.1.4 “STATUS Register
Preservation” for how this feature is used.
The usable suboptions include those shown in Table 4-18.
TABLE 4-18:
RUNTIME ENVIRONMENT SUBOPTIONS
Suboption
Controls
On (+) Implies
Default
State
the code present in the main program
The ROM image is copied into RAM and
module that copies the ROM-image of initial initialized variables will contain their initial
values to RAM variables
value at main().
On
clear
the code present in the main program
module that clears uninitialized variables
On
config
programming the device with default config Configuration bits not specified will be
bytes
assigned a default value. (PIC18 only)
download
conditioning of the Intel HEX file for use
with bootloaders
init
flp
Uninitialized variables are cleared and will
contain 0 at main().
Data records in the Intel HEX file are padded
out to 16-byte lengths and will align on 16-byte
boundaries. Startup code will not assume
Reset values in certain registers.
additional code to provide function profiling Diagnostic code will be embedded into the
output to allow function profiling.
no_startup whether the startup module is linked in with Startup module will not be linked in.
user-defined code
ivt[:addr] enabling use and selection of the interrupt
vector table
Off
Off
Off
Off
A vector table will be selected by writing addr
to the IVTBASE register during startup
On
osccal
initialize the oscillator with the oscillator
constant
Oscillator will be calibrated (PIC10/12/16
only).
On
oscval:
value
set the internal clock oscillator calibration
value
Oscillator will be calibrated with value
supplied (PIC10/12/16 only).
n/a
keep
whether the startup module source file
(startup.as) is deleted after compilation
The startup module is not deleted.
plib
whether the peripheral library is linked in.
The peripheral library will be linked in to the
build (PIC18 only).
resetbits
preserve Power-down and Time-out
STATUS bits at start up
STATUS bits are preserved (PIC10/12/16
Off
allow function calls to use a table look-up
method after the hardware stack has filled
(Ignored if reentrant or hybrid function
model is used, or if function uses the
reentrant specifier.)
Functions called via CALL instruction while
the stack was not exhausted, then called via a
look-up table (PIC10/12/16 devices only).
Off
stackcall
only).
On
On
See Section 4.9 “MPLAB X Option Equivalents” for use of this option in MPLAB X IDE.
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4.8.55
--SCANDEP: Scan for Dependencies
When this option is used, .dep and .d dependency files are generated. The dependency file lists those files on which the source file is Dependant. Dependencies result
when one file is #included into another. The .d file format is used by GCC-based
compilers and it contains the same information as the .dep file.
Compilation will stop after the preprocessing stage if this option is used.
This option is equivalent to --DEP=list,gcc,sys,stop, see
Section 4.8.23 “--DEP: Generate dependency file”.
4.8.56
--SERIAL: Store a Value at this Program Memory Address
This option allows a hexadecimal code to be stored at a particular address in program
memory. A typical task for this option might be to position a serial number in program
memory.
The byte-width of data to store is determined by the byte-width of the hexcode parameter in the option. For example, to store a one-byte value, 0, at program memory
address 1000h, use --SERIAL=00@1000. To store the same value as a four byte
quantity use --SERIAL=00000000@1000.
This option is functionally identical to the corresponding HEXMATE option. For more
detailed information and advanced controls that can be used with this option, refer to
Section 8.3.1.15 “-SERIAL”.
The driver will also define a label at the location where the value was stored, and which
can be referenced from C code as _serial0. To enable access to this symbol,
remember to declare it, for example:
extern const int _serial0;
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
4.8.57
--SETOPTION: Set the Command-line Options for Application
This option is used to supply alternative command line options for the named
application when compiling. The general form of this option is shown below.
--SETOPTION=app,file
where the app component specifies the application that will receive the new options,
and the file component specifies the name of the file that contains the additional
options that will be passed to the application. This option is not required for most
projects.
If specifying more than one option to a component, each option must be entered on a
new line in the option file. This option can also be used to remove an application from
the build sequence. If the file parameter is specified as off, execution of the named
application will be skipped. In most cases, this is not desirable as almost all applications
are critical to the success of the build process. Disabling a critical application will result
in catastrophic failure. However, it is permissible to skip a non-critical application such
as CLIST or HEXMATE, if the final results are not relying on their function.
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4.8.58
--SHROUD: Obfuscate P-code Files
This option should be used in situations where either p-code files or p-code libraries
are to be distributed and are built from confidential source code.
C comments, which are normally included into these files, as well as line numbers and
variable names will be removed, or obfuscated, so that the original source code cannot
be reconstructed from the distributed files.
4.8.59
--STACK: Specify Data Stack Type For Entire Program
This option allows selection of the stack type to be used by a program’s stack-based
(auto and parameter) variables. The data stacks available are called a compiled stack
and a software stack, and they are described in Section 5.3.4.2 “Data Stacks”. The
stack types that can be used with this option are described in Table 4-19.
TABLE 4-19:
--STACK SUBOPTIONS
Stack types
Default Allocation for Stack-based Variables
compiled or nonreentrant
Use the compiled stack for all functions; functions are
non-reentrant (default).
software or reentrant
Use the software stack for eligible functions and devices;
such functions are reentrant.
hybrid
Use the compiled stack for functions not called reentrantly;
use the software stack for all other eligible functions and
devices; functions are only reentrant if required.
Suboptions that specify reentrancy only affect target devices that support a software
stack. Functions encoded for baseline and mid-range devices always use the compiled
stack. In addition, not all functions can use a software stack. Interrupt functions must
use the compiled stack, but functions they call may use the software stack.
The hybrid setting forces the compiler to consider both a compiled and software stack
for the program’s stack-based variables. The software stack will only be used if the
functions and device supports reentrancy. This mode allows for reentrancy, when
required, but takes advantage of the efficiency of the compiled stack for the majority of
the program’s functions. A function is compiled to use the software stack if it is called
reentrantly in the program; otherwise, it will use a compiled stack.
Any of these option settings can be overridden for individual functions by using function
specifiers, described in Section 5.8.1.3 “Reentrant and nonreentrant Specifiers”.
Note:
Use the software (reentrant) setting with caution. The maximum runtime size of the software stack is not accurately known at compile time, so
the compiler cannot warn of memory overwrites. The stack can overflow
and corrupt objects or data memory used by something outside the program (such as hardware or another independently-compiled applications).
When all functions are forced to use the software stack, the stack size will
increase substantially.
In addition to the stack type, this option can be used to specify the maximum size of
memory reserved by the compiler for the software stack. This option configuration only
affects the software stack; there are no controls for the size of the compiled stack.
Distinct memory areas are allocated for the software stack that is used by main-line
code and each interrupt function. Basically, there are separate stacks for each interrupt
and main-line code, but this is transparent at the program level. The compiler automatically manages the allocation of memory to each stack. If your program does not define
any interrupt functions, all the available memory is made available to the software stack
used by main-line code; but, you can explicitly allocate memory.
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You can manually specify the maximum space allocated for each stack by following the
stack type with a colon-separated list of decimal values, each value being the maximum
size, in bytes, of the memory to be reserved. The sizes specified correspond to the
main-line code, the lowest priority interrupt through the highest priority interrupt. (PIC18
devices have two separate interrupts; other devices have only one.) Alternatively, you
can explicitly state that you have no size preference by using a size of auto. For PIC18
devices, the following example:
--STACK=reentrant:auto:30:50
will arrange the stack starting locations so that the low-priority interrupt stack can grow
to, at most, 30 bytes (before overflow); the high-priority interrupt stack can grow to, at
most, 50 bytes (before overflow); and the main-line code stack can consume the
remainder of the free memory that can be allocated to the stack (before overflow). If
you are compiling for a PIC18 device and only one interrupt is used, it is recommended
that you explicitly set the unused interrupt stack size to zero using this option.
If you do specify the stack sizes using this option, each size must be specified numerically or you can use the auto token. Do not leave a size field empty. If you try to use
this option to allocate more stack memory than is available, a warning is issued and
only the available memory will be utilized.
4.8.60
--STRICT: Strict ANSI Conformance
The --STRICT option is used to enable strict ANSI C conformance of all special,
non-standard keywords.
The MPLAB XC8 C compiler supports various special keywords (for example the
persistent type qualifier). If the --STRICT option is used, these keywords are
changed to include two underscore characters at the beginning of the keyword (for
example, __persistent) so as to strictly conform to the ANSI standard. Thus if you
use this option, you will need to use the qualifier __persistent in your code, not
persistent.
Be warned that use of this option can cause problems with some standard header files
(e.g., <xc.h>) as they contain special keywords.
See Section 4.9 “MPLAB X Option Equivalents”, for use of this option in MPLAB X IDE.
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4.8.61
--SUMMARY: Select Memory Summary Output Type
Use this option to select the type of memory summary that is displayed after compilation. By default, or if the mem suboption is selected, a memory summary is shown. This
shows the total memory usage for all memory spaces.
A psect summary can be shown by enabling the psect suboption. This shows individual psects, after they have been grouped by the linker, and the memory ranges they
cover. Table 4-20 shows what summary types are available. The output printed when
compiling normally corresponds to the mem setting.
TABLE 4-20:
Suboption
MEMORY SUMMARY SUBOPTIONS
Controls
psect
A summary of psect names and the addresses where they were linked
will be shown.
mem
A concise summary of memory used will be shown. (default)
class
A summary of all classes in each memory space will be shown.
hex
A summary of addresses and HEX files that make up the final output file
will be shown.
file
Summary information will be shown on screen and saved to a file.
xml
Summary information will be shown on the screen, and usage information for the main memory spaces will be saved in an XML file
xmlfull
Summary information will be shown on the screen, and usage information for all memory spaces will be saved in an XML file
If produced, the XML files contain information about memory spaces on the selected
device – consisting of the space’s name, addressable unit, size, amount used and
amount free.
See Section 4.9 “MPLAB X Option Equivalents” for use of this option in MPLAB X IDE.
4.8.62
--TIME: Report Time Taken For Each Phase of Build Process
Adding the --TIME option when building generates a summary that shows how much
time elapsed during each stage of the build process.
4.8.63
--UNDEFINTS: Program Unused Interrupt Vectors
This option allows you to control how the compiler responds to uninitialized interrupt
vectors, including undefined legacy low- and high-priority vectors, and entries in the
interrupt vector table. A warning is generated if any uninitialized vectors are detected,
except if the ignore suboption is specified.
The suboptions are shown in Table 4-21 as well as the corresponding actions taken for
projects that are using the Interrupt Controller Macro (ICM) module and for all other
projects (which are those projects using devices that do not have the ICM, or those
projects in which the vector tables are disabled and the device is running in legacy
mode).
For example, to have a software breakpoint executed by any vector location that is not
linked to an interrupt function, use the option --UNDEFINTS:swbp.
The default action for projects using the ICM is to program the address of a RESET
instruction (which will be located immediately after the vector table) into each unassigned vector location; for all other devices, it is to leave the locations unprogrammed
and available for other use.
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If the target device does not implement a RESET instruction or software breakpoint
instruction and execution of these instructions have been requested for unused vectors, an instruction that can jump to itself will be programmed instead.
TABLE 4-21:
Suboption
UNUSED INTERRUPT SUBOPTIONS
Action for devices using the ICM
Action for all other devices
ignore
No action; vector location available for
program code.
No action; vector location available for
program code (default).
reset
Program each unassigned vector with
the address of a RESET instruction
(default).
Program a RESET instruction at each
unassigned vector.
swbp
Program each unassigned vector with
the address of a software breakpoint
instruction.
Program a software breakpoint
instruction at each unassigned vector.
An interrupt function can be assigned to any otherwise unassigned vector location by
using the default interrupt source when defining that function, see
Section 5.9.1 “Writing an Interrupt Service Routine”.
The --UNDEFINTS option is ignored if the target device does not support interrupts.
4.8.64
--VER: Display the Compiler’s Version Information
The --VER option will display which version of the compiler is running, and then exit
the compiler.
4.8.65
--WARN: Set Warning Level
The --WARN option is used to set the compiler warning level threshold. Allowable warning levels range from -9 to 9. The warning level determines how pedantic the compiler
is about dubious type conversions and constructs. Each compiler warning has a designated warning level; the higher the warning level, the more important the warning
message. If the warning message’s warning level exceeds the set threshold, the warning is printed by the compiler. The default warning level threshold is 0 and will allow all
normal warning messages.
Use this option with care as some warning messages indicate code that is likely to fail
during execution, or compromise portability.
Warning message can be individually disabled with the --MSGDISABLE option, see
Section 4.8.40 “--MSGDISABLE: Disable Warning Messages”. See also
Section 4.6 “Compiler Messages” for full information on the compiler’s messaging system.
See Section 4.9 “MPLAB X Option Equivalents” for use of this option in MPLAB X IDE.
4.8.66
--WARNFORMAT: Set Warning Message Format
This option sets the format of warning messages produced by the compiler. See
Section 4.8.28 “--ERRFORMAT: Define Format for Compiler Messages” for more information on this option. For full information on the compiler’s messaging system, see
Section 4.6 “Compiler Messages”.
If you are compiling using MPLAB X IDE, the format of the compiler messages is automatically configured to what the IDE expects. It recommended that you do not adjust
the message formats if compiling using this IDE.
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4.9
MPLAB X OPTION EQUIVALENTS
Even when compiling under the MPLAB X IDE, it is still the compiler’s command-line
driver that is being executed and compiling the program. The MPLAB XC8 compiler
plugins control the MPLAB X IDE Properties dialog that is used to access the compiler
options. However, these graphical controls ultimately adjust the command-line options
passed to the command-line driver when compiling. You can see the command-line
options being used when building in MPLAB X IDE in the Output window.
The following dialogs and descriptions identify the mapping between the MPLAB X IDE
dialog controls and command-line options. Click any option to see online help and
examples shown in the Option Description field in the lower part of the Project
Properties dialog.
4.9.1
Global Category
The options in the panel of this category control the final output of the compiler.
4.9.1.1
GLOBAL OPTIONS
See Figure 4-5 in conjunction with the following command-line option equivalent.
FIGURE 4-5:
GLOBAL OPTIONS
1
Output file format
This selector specifies the output source-level debug format that will be used by
debuggers, see Section 4.8.47 “--OUTPUT= type: Specify Output File Type”.
4.9.1.2
STACK OPTIONS
See Figure 4-6 in conjunction with the following command-line option equivalent.
FIGURE 4-6:
STACK OPTIONS
All the widgets in Figure 4-6 correspond to suboptions of the --STACK option, see
Section 4.8.59 “--STACK: Specify Data Stack Type For Entire Program”.
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4.9.2
Compiler Category
The panels in this category control aspects of compilation of C source.
4.9.2.1
PREPROCESSING AND MESSAGES
These options relate to the C preprocessor and messages produced by the compiler,
see Section 4.6 “Compiler Messages” for more information.
See Figure 4-7 in conjunction with the following command-line option equivalents.
FIGURE 4-7:
PREPROCESSING AND MESSAGES OPTIONS
1
2
3
4
5
6
7
8
9
10
11
1. Define macros
The button and field on this line can be used to define preprocessor macros, see
Section 4.8.2 “-D: Define Macro”.
2. Undefine macros
The button and field on this line can be used to undefine preprocessor macros,
see Section 4.8.13 “-U: Undefine a Macro”.
3. Preprocess assembly files
This checkbox controls whether assembly source files are scanned by the
preprocessor, see Section 4.8.10 “-P: Preprocess Assembly Files”.
4. Identifier length
Not implemented, see Section 4.8.8 “-N: Identifier Length”.
5. Include directories
This selection uses the buttons and fields grouped in the bracket to specify
include (header) file search directories, see Section 4.8.4 “-I: Include Search
Path”.
6. Strict ANSI Conformance
This forces the compiler to reject any non-standard keywords, see
Section 4.8.60 “--STRICT: Strict ANSI Conformance”.
7. Verbose
This checkbox controls whether the full command lines for the compiler applications are displayed when building, see Section 4.8.14 “-V: Verbose Compile”.
8. Warning level
This selector allows the warning level print threshold to be set, see
Section 4.8.65 “--WARN: Set Warning Level”.
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9. Use CCI Syntax
This option indicates that the compiler should use the Common C Interface
compiler extensions, see Section 4.8.30 “--EXT: Specify C Language
Extensions”, and Chapter 2. Common C Interface, for more information.
10. Use IAR Syntax
This option indicates that the compiler should use the IAR compiler extensions,
see Section 4.8.30 “--EXT: Specify C Language Extensions”.
11. Generate the ASM listing file
This option indicates that the compiler should generate an assembly listing file.
This file should be used to examine the assembly code produced by the compiler,
see Section 4.8.16 “--ASMLIST: Generate Assembler List Files”.
4.9.2.2
OPTIMIZATIONS
These options, shown in Figure 4-8, relate to optimizations performed by the compiler.
1. Disable optimizations
This control acts like a master disable for all optimizations. Selecting this control
turns off all C- and assembler-level optimizations.
2. Individual optimization controls
Provided the above master control is enabled, these controls allow control over
which and how optimizations are performed. They control optimization of assembly code produced from C source (asm), optimization of hand-written assembly
source modules (asmfile), whether optimizations should aim to produce the
smallest or fastest output (space/speed), and whether optimizations should be
limited to only those that do not significantly affect the debugability of source
code (debug). See Section 4.8.45 “--OPT: Invoke Compiler Optimizations” for
more information on the --OPT suboptions bracketed above.
3. Address qualifiers
This selector allows the user to select the behavior of the address qualifiers, see
Section 4.8.15 “--ADDRQUAL: Set Compiler Response to Memory Qualifiers”.
4. Operation mode
This selector allows the user to force another available operating mode (free,
std, or pro) other than the default, see Section 4.8.39 “--MODE: Choose Compiler Operating Mode”. The operating mode affects OCG-optimizations.
FIGURE 4-8:
OPTIMIZATIONS OPTIONS
2
{
1
3
4
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XC8 Command-line Driver
4.9.3
Linker Category
The options in this dialog control the aspects of the second stage of compilation
including code generation and linking.
4.9.3.1
RUNTIME
All the widgets in Figure 4-9 correspond to suboptions of the --RUNTIME option, see
Section 4.8.54 “--RUNTIME: Specify Runtime Environment”. Respectively, these map
to the clear, init, keep, no_startup, osccal, oscval, resetbits, download,
stackcall, config, clib (now defunct) and plib suboptions of the --RUNTIME
option.
FIGURE 4-9:
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RUNTIME OPTIONS
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4.9.3.2
MEMORY MODEL
The panel in this category, shown in Figure 4-10, controls settings that apply to the
entire project.
FIGURE 4-10:
MEMORY MODEL OPTIONS
1
2
3
4
5
1. Size of Double
This selector allows the size of the double type to be selected, see
Section 4.8.24 “--DOUBLE: Select Kind of Double Types”.
2. Size of Float
This selector allows the size of the float type to be selected, see
Section 4.8.32 “--FLOAT: Select Kind of Float Types”.
3. External memory
This option allows specification of how external memory access is performed.
This only affects those devices that can access external memory, see
Section 4.8.26 “--EMI: Select External Memory Interface Operating Mode”.
4. RAM ranges
This field allows the default RAM (data space) memory used to be adjusted, see
Section 4.8.52 “--RAM: Adjust RAM Ranges”.
5. ROM ranges
This field allows the default ROM (program memory space) memory used to be
adjusted, see Section 4.8.53 “--ROM: Adjust ROM Ranges”.
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4.9.3.3
FILL FLASH MEMORY
All the controls shown in Figure 4-11 relate to options associated with filling unused
program memory. See Section 4.8.31 “--FILL: Fill Unused Program Memory”, for more
information on the different fields.
FIGURE 4-11:
FILL FLASH MEMORY OPTIONS
1
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4.9.3.4
ADDITIONAL
The options shown in Figure 4-12 relate to miscellaneous options.
1. Extra linker options
This field allows you to enter additional options to the linker. Enter the entire -Loption in this field, for example -L-pmytest=300h. See Section 4.8.6 “-L-:
Adjust Linker Options Directly”.
2. Serial
This option allows you to specify a string that can be inserted into your output
HEX file. See Section 4.8.56 “--SERIAL: Store a Value at this Program Memory
Address”, for details.
3. Codeoffset
This field allows an offset for the program to be specified, see
Section 4.8.20 “--CLIST: Generate C Listing File”.
4. Checksum
This field allows the checksum specification to be specified, see
Section 4.8.17 “--CHECKSUM: Calculate a Checksum”.
5. Errata
This allows customization of the errata workarounds applied by the compiler, see
Section 4.8.27 “--ERRATA: Specify Errata Workarounds”.
6. Trace type
Not implemented. Native trace supported.
7. Extend address 0 in HEX file
This option specifies that the Intel HEX file should have initialization to zero of
the upper address, see Section 4.8.47 “--OUTPUT= type: Specify Output File
Type”.
8. Use response file to link
This option allows a file name to be specified. The file must contain
command-line options which are then used by MPLAB XC8 during the link step
and in preference to the other link-step settings in the project properties, see
Section 4.2.1.1 “Long Command Lines”. This option is only relevant when running MPLAB X IDE under Windows.
FIGURE 4-12:
ADDITIONAL OPTIONS
1
3
5
7
2
4
6
8
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4.9.3.5
REPORTING
These options, shown in Figure 4-13 relate to information produced during and after
compilation.
1. Display memory usage after compilation
These checkboxes allow you to specify what information is displayed after
compilation. The correspond to the psect, class, mem and hex suboptions to
the --SUMMARY option, see Section 4.8.61 “--SUMMARY: Select Memory
Summary Output Type”.
2. Create summary file
Selecting this checkbox will send the information you have selected above to a
file, as well as to the standard output. This corresponds to the file suboption to
the --SUMMARY option, see Section 4.8.61 “--SUMMARY: Select Memory Summary Output Type”.
3. Create html files
This will create HTML files summarizing the previous build, see
Section 4.8.35 “--HTML: Generate HTML Diagnostic Files”.
FIGURE 4-13:
REPORTING OPTIONS
1
{
2
3
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NOTES:
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MPLAB® XC8 C COMPILER
USER’S GUIDE
Chapter 5. C Language Features
5.1
INTRODUCTION
MPLAB XC8 C Compiler supports a number of special features and extensions to the
C language which are designed to ease the task of producing ROM-based applications
for 8-bit PIC devices. This chapter documents the special language features which are
specific to these devices.
• ANSI C Standard Issues
• Device-Related Features
• Supported Data Types and Variables
• Memory Allocation and Access
• Operators and Statements
• Register Usage
• Functions
• Interrupts
• Main, Runtime Startup and Reset
• Libraries
• Mixing C and Assembly Code
• Optimizations
• Preprocessing
• Linking Programs
5.2
ANSI C STANDARD ISSUES
This compiler conforms to the ISO/IEC 9899:1990 Standard for programming languages. This is commonly called the C90 Standard. It is referred to as the ANSI C
Standard in this manual.
Some violations to the ANSI C Standard are discussed in this section, as well as some
features from the later standard C99 that are supported.
5.2.1
Divergence from the ANSI C Standard
The C language implemented on MPLAB XC8 C Compiler can diverge from the ANSI
C Standard in several areas.
One divergence is due to limited device memory and no hardware implementation of a
data stack. For this reason, recursion is not supported and functions are not reentrant
on baseline and some mid-range devices. Functions can be encoded reentrantly for
enhanced mid-range and PIC18 devices. See Section 5.3.4 “Stacks” for more
information on the stack models used by the compiler for each device family.
For those devices that do not support reentrancy, the compiler can make functions
called from main-line and interrupt code appear to be reentrant via a duplication
feature. See Section 5.9.7 “Function Duplication”for more about duplication.
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Another divergence from the Standard is that you cannot reliably use the C sizeof
operator with pointer types; however, this operator may be used with pointer variable
identifiers. This is a result of the dynamic size of pointers assigned by the compiler.
So, for the following code:
char * cp;
size_t size;
size = sizeof(char *);
size = sizeof(cp);
size in the first example will be assigned the maximum size a pointer can be for the
particular target device you have chosen. In the second example, size will be
assigned the actual size of the pointer variable, cp. The sizeof operator using a
pointer variable operand cannot be used as the number-of-elements expression in an
array declaration. For example, the size of the following array is unpredictable:
unsigned buffer[sizeof(cp) * 10];
5.2.2
Implementation-Defined behavior
Certain features of the ANSI C standard have implementation-defined behavior. This
means that the exact behavior of some C code can vary from compiler to compiler. The
exact behavior of the compiler is detailed throughout this manual, and is fully
summarized in Appendix D. Implementation-Defined Behavior.
5.2.3
Common C Interface Standard
This compiler conforms to the Microchip XC compiler Common C Interface standard
(CCI). This is a further refinement of the ANSI standard that attempts to standardize
implementation-defined behavior and non-standard extensions across the entire
MPLAB XC compiler family. It is described in Chapter 2. Common C Interface.
MPLAB XC8 accepts all CCI extensions in all language extension modes except for
IAR compatibility mode. However, if you choose to write code that must conform to this
standard, the compiler option (see Section 4.8.30 “--EXT: Specify C Language Extensions”) should be enabled. This option indicates that the compiler should enforce conformance. Alternatively, you can continue to write code using the non-standard ANSI
extensions provided by the compiler.
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C Language Features
5.3
DEVICE-RELATED FEATURES
MPLAB XC8 has several features which relate directly to the 8-bit PIC architectures
and instruction sets. These are detailed in the following sections.
5.3.1
Device Support
MPLAB XC8 C Compiler aims to support all 8-bit PIC devices. However, new devices
in these families are frequently released. There are several ways you can check
whether the compiler you are using supports a particular device.
From the command line, run the compiler you wish to use and pass it the option
--CHIPINFO (See Section 4.8.19 “--CHIPINFO: Display List of Supported Devices”).
A list of all devices will be printed.
If you use the -V option in addition to the --CHIPINFO option, more detailed
information my be shown about each device.
You can also see the supported devices in your favorite web browser. Open the files
pic_chipinfo.html for a list of all supported baseline or mid-range device, or
pic18_chipinfo.html for all PIC18 devices. Both these files are located in the
DOCS directory under your compiler’s installation directory.
5.3.2
Instruction Set Support
The compiler support all instruction sets for PIC10/12/16 devices as well as the standard (legacy) PIC18 instruction set. The extended instruction mode available on some
PIC18 devices is not currently supported. Ensure you set the Configuration bits to use
the PIC18 legacy instruction mode when appropriate.
5.3.3
Device Header Files
There is one header file that is typically included into each C source file you write. The
file is <xc.h> and is a generic header file that will include other device- and
architecture-specific header files when you build your project.
Inclusion of this file will allow access to SFRs via special variables, as well as macros
which allow special memory access or inclusion of special instructions, like CLRWDT.
Legacy projects can continue to use the <htc.h> header file.
Do not include chip-specific header files in your code as this will reduce portability, and
these headers do not contain all the required definitions for successful compilation of
your code.
The header files shipped with the compiler are specific to that compiler version. Future
compiler versions can ship with modified header files. If you copy compiler header files
into your project, particularly if you modify these files, be aware that they might not be
compatible with future versions of the compiler.
For information about assembly include files (.inc), see Section 5.12.3.2 “Accessing
Registers from Assembly Code”.
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5.3.4
Stacks
Stacks are used for two different purposes by programs running on 8-bit devices: one
stack is for storing function return addresses, and one or two other stacks are used for
data allocation.
5.3.4.1
FUNCTION RETURN ADDRESS STACK
The 8-bit PIC devices use what is referred to in this user’s guide as a hardware stack.
This stack is limited in depth and cannot be manipulated directly. It is only used for
function return addresses and cannot be used for program data.
You must ensure that the maximum hardware stack depth is not exceeded; otherwise,
code can fail. Nesting function calls too deeply will overflow the stack. It is important to
take into account implicitly called library functions and interrupts, which also use levels
of the stack. The compiler can be made to manage stack usage for some devices using
the stackcall suboption to the --RUNTIME compiler option, see
Section 4.8.54 “--RUNTIME: Specify Runtime Environment”. This enables an alternate
means of calling functions to prevent stack overflow.
A call graph is provided by the code generator in the assembler list file, see
Section 6.4.6 “Call Graph”. This will indicate the stack levels at each function call and
can be used as a guide to stack depth. The code generator can also produce warnings
if the maximum stack depth is exceeded.
The warnings and call graphs are guides to stack usage. Optimizations and the use of
interrupts can decrease or increase the program’s stack depth over that determined by
the compiler.
5.3.4.2
DATA STACKS
The compiler can implement two types of data stack: a compiled stack and a software
stack. Both these stacks are for storing stack-based variables, such as a function’s
auto, parameter, and temporary variables.
Either one or both of these types of stacks may be used by a program. Compiler
options, specifiers, and how the functions are called will dictate which stacks are used.
See Section 5.5.2.2 “Auto Variable Allocation and access”, for more information on
how the compiler allocates a function’s stack-based objects.
A compiled stack is a static allocation of memory for stack-based objects that can be
built up in multiple data banks. See Section 5.5.2.2.1 “Compiled Stack Operation” for
information about how objects are allocated to this stack. Objects in the stack are in
fixed locations and can be accessed using an identifier (hence it is a static allocation).
Thus, there is no stack pointer. The size of the compiled stack is known at compile time,
and so available space can be confirmed by the compiler. The compiled stack is allocated to psects that use the base name cstack; for example, cstackCOMMON,
cstackBANK0. See Section 5.15.2 “Compiler-Generated Psects” for more information
on the naming convention for compiler-generated psects.
By contrast, the software stack has a size that is dynamic and varies as the program is
executed. The maximum size of the stack is not exactly known at compile time and the
compiler typically reserves as much space as possible for the stack to grow during program execution. The stack is always allocated a single memory range, which may cross
bank boundaries, but within this range it may be segregated into one area for main-line
code and an area for each interrupt routine, if required. A stack pointer is used to
indicate the current position in the stack. This pointer is permanently allocated to FSR1.
A psect is used as a placeholder to reserve the memory used by the stack. This psect
is called stack.
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C Language Features
5.3.5
Configuration Bit Access
The PIC devices have several locations which contain the Configuration bits or fuses.
These bits specify fundamental device operation, such as the oscillator mode, watchdog timer, programming mode and code protection. Failure to correctly set these bits
can result in code failure, or a non-running device.
These bits can be set using a configuration pragma. The pragma has the following
forms.
#pragma config setting = state|value
#pragma config register = value
where setting is a configuration setting descriptor, e.g., WDT, and state is a textual
description of the desired state, e.g., OFF. The value field is a numerical value that can
be used in preference to a descriptor. Numerical values can only be specified in decimal or in hexadecimal, the latter radix indicated by the usual 0x prefix. Values must
never be specified in binary (i.e., using the 0b prefix).
Consider the following examples.
#pragma config WDT = ON
#pragma config WDTPS = 0x1A
// turn on watchdog timer
// specify the timer postscale value
One pragma can be used to program several settings by separating each setting-value
pair with a comma. For example, the above could be specified with one pragma, as in
the following.
#pragma config WDT=ON, WDTPS = 0x1A
The setting-value pairs can also be quoted to ensure that the preprocessor does not
perform substitution of these tokens, for example:
#pragma config "BOREN=OFF"
Without the quotes and with the preprocessor macro OFF defined, for example, substitution within the pragma would take place. You should never assume that the OFF and
ON tokens used in configuration macros equate to 0 and 1, respectively, as that is often
not the case.
Rather than specify individual settings, the entire register can be programmed with one
numerical value, for example:
#pragma config CONFIG1L = 0x8F
The upper and lower half of each register must be programmed separately.
The settings and values associated with each device can be determined from an HTML
guide. Open the pic_chipinfo.html file (or the pic18_chipinfo.html file) that
is located in the DOCS directory of your compiler installation. Click the link to your target device, and the page will show you the settings and values that are appropriate with
this pragma. Review your device data sheet for more information.
If you are using MPLAB X IDE, take advantage of its built-in tools to generate the
required pragmas, so that you can copy and paste them into your source code.
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5.3.5.1
CONFIGURATION BIT LEGACY SUPPORT
You can continue to use the configuration macros for legacy projects, but you should
use the pragma for new projects.
The compiler supports the __CONFIG and __PROG_CONFIG macros, which allow configuration bit symbols or a Configuration Word value, respectively, to be specified, for
example:
#include <xc.h>
__CONFIG(WDTDIS & HS & UNPROTECT);
For PIC10/12/16 devices that have more than one Configuration Word, each subsequent invocation of __CONFIG() will modify the next Configuration Word in sequence.
When using the legacy macros for these devices, the order of the macros must match
the order of the Configuration Words. Typically this might look like:
#include <xc.h>
__CONFIG(WDTDIS & XT & UNPROTECT);
// Program config. word 1
__CONFIG(FCMEN);
The __CONFIG macro used for PIC18 devices takes an additional argument being the
number of the Configuration Word location. For example:
__CONFIG(2, BW8 & PWRTDIS & WDTPS1 & WDTEN);
// specify symbols
If you want to use a literal value to program the entire Configuration Word, you must
use the __PROG_CONFIG macro. For PIC10/12/16 devices, that might appear as
follows:
__PROG_CONFIG(0xFFFA);
and with PIC18 devices, you must again specify the word being programmed, as in the
following:
__PROG_CONFIG(1, 0xFE57);
// specify a literal constant value
You cannot use the setting symbols in the __PROG_CONFIG macro, nor can you use a
literal value in the __CONFIG macro.
The configuration locations do not need to be programmed in order, except as noted
above for multi-word PIC10/12/16 devices using the legacy macros.
To use the legacy macros, ensure you include <xc.h> in your source file. Symbols for
the macros can be found in the .cfgmap files contained in the dat/cfgmap directory
of your compiler installation.
5.3.5.2
CONFIGURATION CONSIDERATIONS
Neither the config pragma nor the __CONFIG macro produce executable code. They
should both be placed outside function definitions so as not to interfere with the
operation of the function’s code.
MPLAB X IDE does not allow the Configuration bits to be adjusted. They must be
specified in your source code using the pragma (or legacy macro).
All the bits in the Configuration Words should be programmed to prevent erratic program behavior. Do not leave them in their default/unprogrammed state. Not all Configuration bits have a default state of logic high; some have a logic low default state.
Consult your device data sheet for more information.
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C Language Features
5.3.6
Using SFRs From C Code
The Special Function Registers (SFRs) are registers which control aspects of the MCU
operation or that of peripheral modules on the device. Most of these registers are memory mapped, which means that they appear at, and can be accessed using, specific
addresses in the device’s data memory space. Individual bits within some registers
control independent features. Some registers are read-only; some are write-only. See
your device data sheet for more information.
Memory-mapped SFRs are accessed by special C variables that are placed at the
address of the register. (Variables that are placed at specific addresses are called
absolute variables and are described in Section 5.5.4 “Absolute Variables”.) These
variables can be accessed like any ordinary C variable so that no special syntax is
required to access SFRs.
The SFR variables are predefined in header files and are accessible once you have
included the <xc.h> header file (see Section 5.3.3 “Device Header Files”) into your
source code. Both bit variables and structures with bit-fields are defined, so you can
use either of them in your source code to access bits within a register.
The names given to the C variables that map over registers and bits within those registers are based on the names specified in the device data sheet. However, as there
can be duplication of some bit names within registers, there can be differences in the
nomenclature.
The names of the structures that hold the bit-fields will typically be those of the corresponding register followed by bits. For example, the following shows code that
includes the generic header file, clears PORTA as a whole, sets bit 0 of PORTA using
a bit variable and sets bit 2 of PORTA using the structure/bit-field definitions.
#include <xc.h>
void main(void)
{
PORTA = 0x00;
RA0 = 1;
PORTAbits.RA2 = 1;
}
To confirm the names that are relevant for the device you are using, check the
device-specific header file that <xc.h> will include for the definitions of each variable.
These files will be located in the include directory of the compiler and will have a
name that represents the device. There is a one-to-one correlation between device and
header file name that will be included by <xc.h>, e.g., when compiling for a
PIC16LF1826 device, <xc.h> will include the header file <pic16lf1826.h>.
Remember that you do not need to include this chip-specific file into your source code;
it is automatically included by <xc.h>.
Care should be taken when accessing some SFRs from C code or from assembly
in-line with C code. Some registers are used by the compiler to hold intermediate values of calculations, and writing to these registers directly can result in code failure. The
compiler does not detect when SFRs have changed as a result of C or assembly code
that writes to them directly. The list of registers used by the compiler and further
information can be found in Section 5.7 “Register Usage”.
SFRs associated with peripherals are not used by the compiler to hold intermediate
results and can be changed as you require. Always ensure that you confirm the
operation of peripheral modules from the device data sheet.
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5.3.6.1
SPECIAL BASELINE/MID-RANGE REGISTER ISSUES
Some SFRs are not memory mapped, do not have a corresponding variable defined in
the device specific header file, and cannot be directly accessed from C code.
For example, the W register is not memory mapped on baseline devices. Some devices
use OPTION and TRIS registers, that are only accessible via special instructions and
that are also not memory mapped. See Section 5.3.10 “Baseline PIC MCU Special
Instructions” on how these registers are accessed by the compiler.
5.3.6.2
SPECIAL PIC18 REGISTER ISSUES
Some of the SFRs associated with the PIC18 can be grouped to form multi-byte values,
e.g., the TMRxH and TMRxL register combined form a 16-bit timer count value.
Depending on the device and mode of operation, there can be hardware requirements
to read these registers in certain ways, e.g., often the TMRxL register must be read
before trying to read the TMRxH register to obtain a valid 16-bit result.
Although it is possible to define a word-sized C variable to map over such registers,
e.g., an int variable TMRx that maps over both TMRxL and TMRxH, the order in which
the compiler would read the bytes of such an object will vary from expression to expression. Some expressions require that the Most Significant Byte (MSB) is read first;
others start with the Least Significant Byte (LSB) first.
It is recommended that the existing SFR definitions in the chip header files be used.
Each byte of the SFR should be accessed directly, and in the required order, as dictated
by the device data sheet. This results in a much higher degree of portability.
The following code copies the two timer registers into a C unsigned variable count
for subsequent use.
count = TMR0L;
count += TMR0H << 8;
Macros are also provided to perform reading and writing of the more common timer registers. See the macros READTIMERx and WRITETIMERx in Appendix A. Library Functions. These guarantee the correct byte order is used.
5.3.7
ID Locations
The 8-bit PIC devices have locations outside the addressable memory area that can be
used for storing program information, such as an ID number. The config pragma is
also used to place data into these locations by using a special register name. The
pragma is used as follows:
#pragma config IDLOCx = value
where x is the number (position) of the ID location, and value is the nibble or byte that
is to be positioned into that ID location. The value can only be specified in decimal or
in hexadecimal, the latter radix indicated by the usual 0x prefix. Values must never be
specified in binary (i.e., using the 0b prefix). If value is larger than the maximum value
allowable for each location on the target device, the value will be truncated and a warning message is issued. The size of each ID location varies from device to device. See
your device data sheet for more information. For example:
#pragma config IDLOC0 = 1
#pragma config IDLOC1 = 4
will attempt fill the first two ID locations with 1 and 4. One pragma can be used to program several locations by separating each register-value pair with a comma. For
example, the above could also be specified as shown below.
#pragma config IDLOC0 = 1, IDLOC1 = 4
The config pragma does not produce executable code and so should ideally be placed
outside function definitions.
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C Language Features
5.3.7.1
ID LOCATION LEGACY SUPPORT
The compiler also has legacy support for the __IDLOC macro. The macro is used in a
manner similar to:
#include <xc.h>
__IDLOC(x);
where x is a list of hexadecimal digits, which are positioned into the ID locations. Do
not use the usual 0x hexadecimal radix specifier with these values. If an invalid character is encountered, the value 0 will be programmed into the corresponding location.
Only the lower four bits of each ID location are programmed, so the following:
__IDLOC(15F0);
will attempt to fill ID locations with the hexadecimal values: 1, 5, F and 0.
To use this macro, ensure you include <xc.h> in your source file.
The __IDLOC macro does not produce executable code and so should ideally be
placed outside function definitions.
Some devices permit programming up to seven bits within each ID location. The
__IDLOC() macro is not suitable for such devices and the __IDLOC7(a,b,c,d)
macro should be used instead. The parameters a to d must be constants which represent the values to be programmed. The values can be entered in either decimal or
hexadecimal format, such as:
__IDLOC7(0x7f,1,70,0x5a);
It is not appropriate to use the __IDLOC7() macro on a device that does not permit
seven-bit programming of ID locations. The __IDLOC7 macro does not produce
executable code and so should ideally be placed outside function definitions.
5.3.8
Bit Instructions
Wherever possible, the MPLAB XC8 C Compiler will attempt to use bit instructions,
even on non-bit integer values. For example, when using a bitwise operator and a mask
to alter a bit within an integral type, the compiler will check the mask value to determine
if a bit instruction can achieve the same functionality.
unsigned int foo;
foo |= 0x40;
will produce the instruction:
BSF _foo,6
To set or clear individual bits within integral type, the following macros could be used:
#define bitset(var, bitno)
#define bitclr(var, bitno)
((var) |= 1UL << (bitno))
((var) &= ~(1UL << (bitno)))
To perform the same operation on foo as above, the bitset macro could be
employed as follows:
bitset(foo,6);
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5.3.9
Multiplication
The PIC18 instruction set includes several 8-bit by 8-bit hardware multiple instructions,
and these are used by the compiler in many situations. Non-PIC18 targets always use
a library routine for multiplication operations.
There are three ways that 8x8-bit integer multiplication can be implemented by the
compiler:
Hardware Multiply These assembly instructions are the most efficient method of
Instructions
multiplication, but they are only available on PIC18 devices.
(HMI)
A bitwise iteration Where dedicated multiplication instructions are not available,
(8loop)
this implementation produces the smallest amount of code – a
loop cycles through the bit pattern in the operands and
constructs the result bit-by-bit.
The speed of this implementation varies and is dependent on
the operand values; however, this is typically the slowest
method of performing multiplication.
This implementation performs a sequence of instructions that is
An unrolled
bitwise sequence identical to the bitwise iteration (above), but the loop is
unrolled.
(8seq)
The generated code is larger, but execution is faster than the
loop version.
Multiplication of operands larger than 8 bits can be performed one of the following two
ways:
A bitwise iteration This is the same algorithm used by 8-bit multiplication (above)
(xloop)
but the loop runs over all (x) bits of the operands.
Like its 8-bit counterpart, this implementation produces the
smallest amount of code but is typically the slowest method of
performing multiplication.
A bytewise
decomposition
(bytdec)
This is a decomposition of the multiplication into a summation
of many 8-bit multiplications. The 8-bit multiplications can then
be performed using any of the methods described above.
This decomposition is highly advantageous for PIC18 devices,
which can then use their hardware multiply instruction to
perform such operations efficiently.
For other devices, this method is still fast, but the code size can
become impractical.
Multiplication of floating-point operands operates in a similar way – the integer mantissas can be multiplied using either a bitwise loop (xfploop) or by a bytewise
decomposition.
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C Language Features
The following tables indicate which of the multiplication methods are chosen by the
compiler when performing multiplication of both integer and floating point operands.
The method is dependent on the size of the operands, the type of optimizations
enabled, and the target device.
Table 5-1 shows the methods chosen when speed optimizations are enabled, see
Section 4.8.45 “--OPT: Invoke Compiler Optimizations”.
TABLE 5-1:
Device
MULTIPLICATION WITH SPEED OPTIMIZATIONS
8-bit
16-bit
24-bit
32-bit
24-bit FP
32-bit FP
PIC18
HMI
bytdec+HMI
bytdec+HMI
bytdec+HMI
bytdec+HMI
bytdec+HMI
Enhanced
mid-range
8seq bytdec+8seq bytdec+8seq bytdec+8seq bytdec+8seq bytdec+8seq
Mid-range/
baseline
8seq
16loop
24loop
32loop
24fploop
32fploop
Table 5-2 shows the method chosen when space optimizations are enabled or when no
C-level optimizations are enabled.
TABLE 5-2:
MULTIPLICATION WITH NO OR SPACE OPTIMIZATIONS
Device
8-bit
16-bit
24-bit
32-bit
24-bit FP
32-bit FP
PIC18
HMI
bytdec+HMI
24loop
32loop
24fploop
32fploop
Enhanced mid-range
8loop
bytdec+8loop
24loop
32loop
24fploop
32fploop
Mid-range/baseline
8loop
16loop
24loop
32loop
24fploop
32fploop
The source code for the multiplication routines (documented with the algorithms
employed) is available in the SOURCES directory, located in the compiler’s installation
directory. Look for files whose name has the form Umulx.c. where x is the size of the
operation in bits.
If your device and optimization settings dictate the use of a bitwise multiplication loop
you can sometimes arrange the multiplication operands in your C code to improve the
operation’s speed. Where possible, ensure that the left operand to the multiplication is
the smallest of the operands. For example, for the code:
x = 10;
y = 200;
result = x * y;
result = y * x;
// first multiply
// second multiply
the variable result will be assigned the same value in both statements, but the first
multiplication expression will be performed faster than the second.
5.3.10
Baseline PIC MCU Special Instructions
The Baseline devices have some registers which are not in the normal SFR space and
cannot be accessed using an ordinary file instruction. These are the OPTION and TRIS
registers.
Both registers are write-only and cannot be used in expression that read their value.
They can only be accessed using special instructions which the compiler will use
automatically.
The definition of the variables that map to these registers make use of the control
qualifier. This qualifier informs the compiler that the registers are outside of the normal
address space and that a different access method is required. You should not use this
qualifiers for any other registers.
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When you write to either of these SFR variables, the compiler will use the appropriate
instruction to load the value. So, for example, to load the TRIS register, the following
code:
TRIS = 0xFF;
can be encoded by the compiler as:
MOVLW 0ffh
TRIS
Those PIC devices which have more than one output port can have definitions for
objects: TRISA, TRISB and TRISC, depending on the exact number of ports available.
These objects are used in the same manner as described above.
Any register that uses the control qualifier must be accessed as a full byte. Do not
attempt to access bits within the register. Copy the register to a temporary variable if
required.
5.3.11
Oscillator Calibration Constants
Some Baseline and Mid-range devices come with an oscillator calibration constant
which is pre-programmed into the device’s program memory. This constant can be read
from program memory and written to the OSCCAL register to calibrate the internal RC
oscillator.
On some Baseline PIC devices, the calibration constant is stored as a MOVLW instruction at the top of program memory, e.g., the PIC10F509 device. On Reset, the program
counter is made to point to this instruction and it is executed first before the program
counter wraps around to 0x0000, which is the effective Reset vector for the device. The
default runtime startup routine (see Section 5.10.1 “Runtime Startup Code”) will automatically include code to load the OSCCAL register with the value contained in the W
register after Reset on such devices. No other code is required.
For other chips, such as PIC12F629 device, the oscillator constant is also stored at the
top of program memory, but as a RETLW instruction. The compiler’s startup code will
automatically generate code to retrieve this value and perform the configuration.
At runtime, the calibration value stored as a RETLW instruction can be read using the
‘function’ __osccal_val(),as a label is assigned the RETLW instruction address. A
prototype for the function is provided in <xc.h>. For example:
calVal = __osccal_val();
Loading of the calibration value can be turned off via the osccal suboption to the
--RUNTIME option (see Section 4.8.54 “--RUNTIME: Specify Runtime Environment”).
At runtime, this calibration value can be read using the macro
_READ_OSCCAL_DATA(). To be able to use this macro, make sure that <xc.h> is
included into the relevant modules of your program. This macro returns the calibration
constant which can then be stored into the OSCCAL register, as follows:
OSCCAL = _READ_OSCCAL_DATA();
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C Language Features
Note:
5.3.12
The location that stores the calibration constant is never code protected
and will be lost if you reprogram the device. Thus, if you are using a windowed or Flash device, the calibration constant must be saved from the
last ROM location before it is erased. The constant must then be reprogrammed at the same location along with the new program and data.
If you are using an in-circuit emulator (ICE), the location used by the calibration RETLW instruction cannot be programmed. Calling the
_READ_OSCCAL_DATA() macro will not work and will almost certainly not
return correctly. If you wish to test code that includes this macro on an ICE,
you will have to program a RETLW instruction at the appropriate location in
program memory. Remember to remove this instruction when
programming the actual part so you do not destroy the calibration value.
MPLAB REAL ICE In-Circuit Emulator Support
The compiler supports log and trace functions (instrumented trace) when using a
Microchip MPLAB REAL ICE In-Circuit Emulator. See the emulator’s documentation for
more information on the instrumented trace features.
Only native trace is currently supported by the compiler. Not all devices support
instrumented trace.
The log and trace macro calls need to be either added by hand to your source code or
inserted by right-clicking on the appropriate location in MPLAB X IDE editor, as
described by the emulator documentation. The <xc.h> header must be included in
any modules that will use these macros. The macros have the following form.
__TRACE(id);
__LOG(id, expression);
MPLAB X IDE will automatically substitute an appropriate value for id when you compile; however, you can specify these by hand if required. The trace id should be a constant in the range of 0x40 to 0x7F, and the log id is a constant in the range of 0x0 to
0x7F. Each macro should be given a unique number so that it can be properly identified.
The same valid number can be used for both trace and log macros.
Macros should be inserted in the C source code at the desired locations, taking care
that braces will ensure the correct program flow. They will trigger information to be sent
to the debugger and IDE when they are executed, recording that execution reached
that location in the program. The log expression can be any integer or 32-bit floating-point expression whose value will also be recorded. Typically, this expression is
simply a variable name so the variable’s contents are logged.
Adding trace and log macros will increase the size of your code as they contribute to
the program image that is downloaded to the device.
Here is an example of these macros that you might add.
#include <xc.h>
inpStatus = readUser();
if(inpStatus == 0) {
__TRACE(id);
recovery();
}
__LOG(id, inpStatus);
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5.3.13
Function profiling
The compiler can generate function registration code for the MPLAB REAL ICE
In-Circuit Emulator to provide function profiling. The flp suboption to the --RUNTIME
option (see Section 4.8.54 “--RUNTIME: Specify Runtime Environment”) enables this
feature. To obtain profiling results, you must also use a Power Monitor Board and
MPLAB X IDE and power monitor plugin that support code profiling for the MPLAB XC8
C Compiler.
When enabled, the compiler inserts assembly code into the prologue and epilogue of
each function. This code communicates runtime information to the debugger to signal
when a function is being entered and when it exits. This information, along with further
measurements made by a Microchip Power Monitor Board, can determine how much
energy each function is using. This feature is transparent, but note the following points
when profiling is enabled:
• The program will increase in size and run slower due to the profiling code
• One extra level of hardware stack is used
• Some additional RAM memory is consumed
• Inlining of functions will not take place for any profiled function
If a function cannot be profiled (due to hardware stack constraints) but is qualified
inline, the compiler might inline the function. See Section 5.8.1.2 “Inline Specifier”
for more information on inlining functions.
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C Language Features
5.4
SUPPORTED DATA TYPES AND VARIABLES
5.4.1
Identifiers
A C variable identifier (the following is also true for function identifiers) is a sequence
of letters and digits, where the underscore character “_” counts as a letter. Identifiers
cannot start with a digit. Although they can start with an underscore, such identifiers are
reserved for the compiler’s use and should not be defined by your programs. Such is
not the case for assembly domain identifiers, which often begin with an underscore,
see Section 5.12.3.1 “Equivalent Assembly Symbols”.
Identifiers are case sensitive, so main is different to Main.
Not every character is significant in an identifier. The maximum number of significant
characters can be set using an option, see Section 4.8.8 “-N: Identifier Length”. If two
identifiers differ only after the maximum number of significant characters, then the
compiler will consider them to be the same symbol.
5.4.2
Integer Data Types
The MPLAB XC8 compiler supports integer data types with 1, 2, 3 and 4 byte sizes as
well as a single bit type. Table 5-3 shows the data types and their corresponding size
and arithmetic type. The default type for each type is underlined.
TABLE 5-3:
INTEGER DATA TYPES
Type
Size (bits)
Arithmetic Type
bit
1
Unsigned integer
signed char
8
Signed integer
unsigned char
8
Unsigned integer
signed short
16
Signed integer
unsigned short
16
Unsigned integer
signed int
16
Signed integer
unsigned int
16
Unsigned integer
signed short long
24
Signed integer
unsigned short long
24
Unsigned integer
signed long
32
Signed integer
unsigned long
32
Unsigned integer
signed long long
32
Signed integer
unsigned long long
32
Unsigned integer
The bit and short long types are non-standard types available in this implementation. The long long types are C99 Standard types, but this implementation limits their
size to only 32 bits.
All integer values are represented in little endian format with the Least Significant Byte
(LSB) at the lower address.
If no signedness is specified in the type, then the type will be signed except for the
char types which are always unsigned. The bit type is always unsigned and the
concept of a signed bit is meaningless.
Signed values are stored as a two’s complement integer value.
The range of values capable of being held by these types is summarized in Table 5-4
The symbols in this table are preprocessor macros which are available after including
<limits.h> in your source code.
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As the size of data types are not fully specified by the ANSI Standard, these macros
allow for more portable code which can check the limits of the range of values held by
the type on this implementation.
The macros associated with the short long type are non-standard macros available
in this implementation; those associated with the long long types are defined by the
C99 Standard.
TABLE 5-4:
RANGES OF INTEGER TYPE VALUES
Symbol
Meaning
Value
CHAR_BIT
bits per char
CHAR_MAX
max. value of a char
127
CHAR_MIN
min. value of a char
-128
SCHAR_MAX
max. value of a signed char
127
SCHAR_MIN
min. value of a signed char
-128
UCHAR_MAX
max. value of an unsigned char
SHRT_MAX
max. value of a short
32767
SHRT_MIN
min. value of a short
-32768
USHRT_MAX
max. value of an unsigned short
65535
INT_MAX
max. value of an int
32767
INT_MIN
min. value of a int
-32768
UINT_MAX
max. value of an unsigned int
SHRTLONG_MAX
max. value of a short long
8388607
SHRTLONG_MIN
min. value of a short long
-8388608
USHRTLONG_MAX
max. value of an unsigned short long
LONG_MAX
max. value of a long
2147483647
LONG_MIN
min. value of a long
-2147483648
ULONG_MAX
max. value of an unsigned long
4294967295
LLONG_MAX
max. value of a long long
2147483647
LLONG_MIN
min. value of a long long
-2147483648
ULLONG_MAX
max. value of an unsigned long long
4294967295
8
255
65535
16777215
Macros are also available in <stdint.h> which define values associated with
fixed-width types.
When specifying a signed or unsigned short int, short long int, long int
or long long int type, the keyword int can be omitted. Thus a variable declared
as short will contain a signed short int and a variable declared as unsigned
short will contain an unsigned short int.
It is a common misconception that the C char types are intended purely for ASCII character manipulation. However, the C language makes no guarantee that the default
character representation is even ASCII. (This implementation does use ASCII as the
character representation.)
The char types are the smallest of the multi-bit integer sizes, and behave in all
respects like integers. The reason for the name “char” is historical and does not mean
that char can only be used to represent characters. It is possible to freely mix char
values with values of other types in C expressions. With the MPLAB XC8 C Compiler,
the char types are used for a number of purposes – as 8-bit integers, as storage for
ASCII characters, and for access to I/O locations.
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C Language Features
5.4.2.1
BIT DATA TYPES AND VARIABLES
The MPLAB XC8 C Compiler supports bit integral types which can hold the values 0
or 1. Single bit variables can be declared using the keyword bit (or __bit), for
example:
bit init_flag;
These variables cannot be auto or parameters to a function, but can be qualified
static, allowing them to be defined locally within a function. For example:
int func(void) {
static bit flame_on;
// ...
}
A function can return a bit object by using the bit keyword in the function’s prototype
in the usual way. The 1 or 0 value will be returned in the carry flag in the STATUS register.
The bit variables behave in most respects like normal unsigned char variables, but
they can only contain the values 0 and 1, and therefore provide a convenient and efficient method of storing flags. Eight bit objects are packed into each byte of memory
storage, so they don’t consume large amounts of internal RAM.
Operations on bit objects are performed using the single bit instructions (bsf and
bcf) wherever possible, thus the generated code to access bit objects is very
efficient.
It is not possible to declare a pointer to bit types or assign the address of a bit object
to any pointer. Nor is it possible to statically initialize bit variables so they must be
assigned any non-zero starting value (i.e., 1) in the code itself. Bit objects will be
cleared on startup, unless the bit is qualified persistent.
When assigning a larger integral type to a bit variable, only the LSb is used. For
example, if the bit variable bitvar was assigned as in the following:
int data = 0x54;
bit bitvar;
bitvar = data;
it will be cleared by the assignment since the LSb of data is zero. This sets the bit
type apart from the C99 Standard __Bool, which is a boolean type, not a 1-bit wide
integer. The __Bool type is not supported on the MPLAB XC8 compiler. If you want to
set a bit variable to be 0 or 1 depending on whether the larger integral type is zero
(false) or non-zero (true), use the form:
bitvar = (data != 0);
The psects in which bit objects are allocated storage are declared using the bit
PSECT directive flag, see Section 6.2.9.3 “PSECT”. All addresses assigned to bit
objects and psects will be bit addresses. For absolute bit variables (see
Section 5.5.4 “Absolute Variables”), the address specified in code must be a bit
address. Take care when comparing these addresses to byte addresses used by all
other variables.
If the xc8 flag --STRICT is used, the bit keyword becomes unavailable, but you can
use the __bit keyword.
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5.4.3
Floating-Point Data Types
The MPLAB XC8 compiler supports 24- and 32-bit floating-point types. Floating point
is implemented using either a IEEE 754 32-bit format, or a truncated, 24-bit form of this.
Table 5-5 shows the data types and their corresponding size and arithmetic type.
TABLE 5-5:
FLOATING-POINT DATA TYPES
Type
Size (bits)
Arithmetic Type
float
24 or 32
Real
double
24 or 32
Real
same as double
Real
long double
For both float and double values, the 24-bit format is the default. The options
--FLOAT=24 and --DOUBLE=24 can also be used to specify this explicitly. The 32-bit
format is used for double values if the --DOUBLE=32 option is used and for float
values if --FLOAT=32 is used.
Variables can be declared using the float and double keywords, respectively, to
hold values of these types. Floating-point types are always signed and the unsigned
keyword is illegal when specifying a floating-point type. Types declared as long
double will use the same format as types declared as double. All floating-point
values are represented in little endian format with the LSB at the lower address.
The 32-bit floating-point type supports "relaxed" floating-point semantics when
compared to the full IEEE implementation. This means the following rules are
observed.
Tiny (sub-normal) arguments to floating-point routines are interpreted as zeros. There
are no representable floating-point values possible between -1.17549435E-38 and
1.17549435E-38, except for 0.0. This range is called the denormal range. Sub-normal
results of routines are flushed to zero. There are no negative 0 results produced.
Not-a-number (NaN) arguments to routines are interpreted as infinities. NaN results are
never created in addition, subtraction, multiplication, or division routines where a NaN
would be normally expected—an infinity of the proper sign is created instead. The
square root of a negative number will return the "distinguished" NaN (default NaN used
for error return).
Infinities are legal arguments for all operations and behave as the largest representable
number with that sign. For example, +inf + -inf yields the value 0.
The format for both floating-point types is described in Table 5-6, where:
• Sign is the sign bit, which indicates if the number is positive or negative
• The Biased Exponent is 8 bits wide and is stored as excess 127 (i.e., an exponent
of 0 is stored as 127).
• Mantissa is the mantissa, which is to the right of the radix point. There is an
implied bit to the left of the radix point which is always 1 except for a zero value,
where the implied bit is zero. A zero value is indicated by a zero exponent.
The value of this number is (-1)sign x 2(exponent-127) x 1. mantissa.
TABLE 5-6:
FLOATING-POINT FORMATS
Format
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Sign
Biased Exponent
Mantissa
IEEE 754 32-bit
x
xxxx xxxx
xxx xxxx xxxx xxxx xxxx xxxx
modified IEEE 754
24-bit
x
xxxx xxxx
xxx xxxx xxxx xxxx
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C Language Features
Here are some examples of the IEEE 754 32-bit formats shown in Table 5-7. Note that
the Most Significant Bit (MSb) of the mantissa column (i.e., the bit to the left of the radix
point) is the implied bit, which is assumed to be 1 unless the exponent is zero.
TABLE 5-7:
FLOATING-POINT FORMAT EXAMPLE IEEE 754
Format
Value
32-bit
7DA6B69Bh
24-bit
Biased Exponent
42123Ah
1.mantissa
Decimal
11111011b
1.0100110101101101
0011011b
2.77000e+37
(251)
(1.302447676659)
—
10000100b
1.001001000111010b
36.557
(132)
(1.142395019531)
—
Use the following process to manually calculate the 32-bit example in Table 5-7.
The sign bit is zero; the biased exponent is 251, so the exponent is 251-127=124. Take
the binary number to the right of the decimal point in the mantissa. Convert this to decimal and divide it by 223 where 23 is the size of the mantissa, to give 0.302447676659.
Add 1 to this fraction. The floating-point number is then given by:
-1021241.302447676659
which is approximately equal to:
2.77000e+37
Binary floating-point values are sometimes misunderstood. It is important to remember
that not every floating-point value can be represented by a finite sized floating-point
number. The size of the exponent in the number dictates the range of values that the
number can hold, and the size of the mantissa relates to the spacing of each value that
can be represented exactly. Thus the 24-bit format allows for values with approximately
the same range of values representable by the 32-bit format, but the values that can be
exactly represented by this format are more widely spaced.
So, for example, if you are using a 24-bit wide floating-point type, it can exactly store
the value 95000.0. However, the next highest number it can represent is 95002.0 and
it is impossible to represent any value in between these two in such a type as it will be
rounded. This implies that C code which compares floating-point values might not
behave as expected. For example:
volatile float myFloat;
myFloat = 95002.0;
if(myFloat == 95001.0)
PORTA++;
// value will be rounded
// this line will be executed!
in which the result of the if() expression will be true, even though it appears the two
values being compared are different.
Compare this to a 32-bit floating-point type, which has a higher precision. It also can
exactly store 95000.0 as a value. The next highest value which can be represented is
(approximately) 95000.00781.
The characteristics of the floating-point formats are summarized in Table 5-8. The symbols in this table are preprocessor macros that are available after including <float.h>
in your source code.
Two sets of macros are available for float and double types, where XXX represents
FLT and DBL, respectively. So, for example, FLT_MAX represents the maximum floating-point value of the float type. It can have two values depending on whether float
is a 24 or 32 bit wide format. DBL_MAX represents the same values for the double
type.
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As the size and format of floating-point data types are not fully specified by the ANSI
Standard, these macros allow for more portable code which can check the limits of the
range of values held by the type on this implementation.
TABLE 5-8:
RANGES OF FLOATING-POINT TYPE VALUES
Symbol
24-bit
Value
Meaning
32-bit
Value
XXX_RADIX
Radix of exponent representation
2
2
XXX_ROUNDS
Rounding mode for addition
0
1
XXX_MIN_EXP
Min. n such that FLT_RADIXn-1 is a
normalized float value
-125
-125
-37
-37
128
128
value
38
38
XXX_MANT_DIG
Number of FLT_RADIX mantissa digits
16
24
XXX_EPSILON
The smallest number which added to 1.0
does not yield 1.0
n
XXX_MIN_10_EXP Min. n such that 10 is a normalized float
value
n-1
XXX_MAX_EXP
Max. n such that FLT_RADIX
normalized float value
XXX_MAX_10_EXP
Max. n such that 10n is a normalized float
5.4.4
is a
3.05176e-05 1.19209e-07
Structures and Unions
MPLAB XC8 C Compiler supports struct and union types. Structures and unions
only differ in the memory offset applied to each member.
These types will be at least 1 byte wide. The members of structures and unions cannot
be objects of type bit, but bit-fields are fully supported.
Structures and unions can be passed freely as function arguments and function return
values. Pointers to structures and unions are fully supported.
5.4.4.1
STRUCTURE AND UNION QUALIFIERS
The compiler supports the use of type qualifiers on structures. When a qualifier is
applied to a structure, all of its members will inherit this qualification. In the following
example the structure is qualified const.
const struct {
int number;
int *ptr;
} record = { 0x55, &i };
In this case, the entire structure will be placed into the program space and each member will be read-only. Remember that all members are usually initialized if a structure
is const as they cannot be initialized at runtime.
If the members of the structure were individually qualified const, but the structure was
not, then the structure would be positioned into RAM, but each member would be
read-only. Compare the following structure with the above.
struct {
const int number;
int * const ptr;
} record = { 0x55, &i };
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5.4.4.2
BIT-FIELDS IN STRUCTURES
MPLAB XC8 C Compiler fully supports bit-fields in structures.
Bit-fields are always allocated within 8-bit words, even though it is usual to use the type
unsigned int in the definition.
The first bit defined will be the LSb of the word in which it will be stored. When a bit-field
is declared, it is allocated within the current 8-bit unit if it will fit; otherwise, a new byte
is allocated within the structure. Bit-fields can never cross the boundary between 8-bit
allocation units. For example, the declaration:
struct {
unsigned
unsigned
unsigned
} foo;
lo : 1;
dummy : 6;
hi : 1;
will produce a structure occupying 1 byte. If foo was ultimately linked at address 0x10,
the field lo will be bit 0 of address 0x10 and field hi will be bit 7 of address 0x10. The
LSb of dummy will be bit 1 of address 0x10, and the MSb of dummy will be bit 6 of
address 0x10.
Note:
Accessing bit-fields larger than a single bit can be very inefficient. If code
size and execution speed are critical, consider using a char type or a char
structure member, instead. Be aware that some SFRs are defined as
bit-fields. Most are single bits, but some can be multi-bit objects.
Unnamed bit-fields can be declared to pad out unused space between active bits in
control registers. For example, if dummy is never referenced, the structure above could
have been declared as:
struct {
unsigned
unsigned
unsigned
} foo;
lo : 1;
: 6;
hi : 1;
A structure with bit-fields can be initialized by supplying a comma-separated list of initial
values for each field. For example:
struct {
unsigned
unsigned
unsigned
} foo = {1, 8, 0};
lo : 1;
mid : 6;
hi : 1;
Structures with unnamed bit-fields can be initialized. No initial value should be supplied
for the unnamed members, for example:
struct {
unsigned
unsigned
unsigned
} foo = {1, 0};
lo
hi
: 1;
: 6;
: 1;
will initialize the members lo and hi correctly.
A bit-field that has a size of 0 is a special case. The Standard indicates that no further
bit-field is to be packed into the allocation unit in which the previous bit-field, if any, was
placed.
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5.4.4.3
ANONYMOUS STRUCTURES AND UNIONS
The MPLAB XC8 compiler supports anonymous structures and unions. These are constructs with no identifier and whose members can be accessed without referencing the
identifier of the construct. Anonymous structures and unions must be placed inside
other structures or unions. For example:
struct {
union {
int x;
double y;
};
} aaa;
void main(void)
{
aaa.x = 99;
// ...}
Here, the union is not named and its members accessed as if they are part of the
structure.
Objects defined with anonymous structures or unions cannot be initialized.
Note that anonymous structures and unions are not part of the ISO C90 C Standard.
Their use limits the portability of any code, and they are not recommended
5.4.5
Pointer Types
There are two basic pointer types supported by the MPLAB XC8 C Compiler: data
pointers and function pointers. Data pointers hold the addresses of variables which can
be indirectly read, and possible indirectly written, by the program. Function pointers
hold the address of an executable function which can be called indirectly via the pointer.
To conserve memory requirements and reduce execution time, pointers are made different sizes and formats. The MPLAB XC8 C Compiler uses sophisticated algorithms
to track the assignment of addresses to all pointers, and, as a result, non-standard
qualifiers are not required when defining pointer variables. The standard qualifiers
const and volatile can still be used and have their usual meaning. Despite this, the
size of each pointer is optimal for its intended usage in the program.
5.4.5.1
COMBINING TYPE QUALIFIERS AND POINTERS
It is helpful to first review the ANSI C standard conventions for definitions of pointer
types.
Pointers can be qualified like any other C object, but care must be taken when doing
so as there are two quantities associated with pointers. The first is the actual pointer
itself, which is treated like any ordinary C variable and has memory reserved for it. The
second is the target, or targets, that the pointer references, or to which the pointer
points. The general form of a pointer definition looks like the following:
target_type_&_qualifiers * pointer’s_qualifiers pointer’s_name;
Any qualifiers to the right of the * (i.e., next to the pointer’s name) relate to the pointer
variable itself. The type and any qualifiers to the left of the * relate to the pointer’s targets. This makes sense since it is also the * operator that dereferences a pointer, which
allows you to get from the pointer variable to its current target.
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C Language Features
Here are three examples of pointer definitions using the volatile qualifier. The fields
in the definitions have been highlighted with spacing:
volatile int *
int
* volatile
volatile int * volatile
vip ;
ivp ;
vivp ;
The first example is a pointer called vip. It contains the address of int objects that
are qualified volatile. The pointer itself – the variable that holds the address – is not
volatile; however, the objects that are accessed when the pointer is dereferenced
are treated as being volatile. In other words, the target objects accessible via the
pointer can be externally modified.
The second example is a pointer called ivp which also contains the address of int
objects. In this example, the pointer itself is volatile, that is, the address the pointer
contains can be externally modified; however, the objects that can be accessed when
dereferencing the pointer are not volatile.
The last example is of a pointer called vivp which is itself qualified volatile, and
which also holds the address of volatile objects.
Bear in mind that one pointer can be assigned the addresses of many objects; for
example, a pointer that is a parameter to a function is assigned a new object address
every time the function is called. The definition of the pointer must be valid for every
target address assigned.
Note:
Care must be taken when describing pointers. Is a “const pointer” a pointer
that points to const objects, or a pointer that is const itself? You can talk
about “pointers to const” and “const pointers” to help clarify the definition,
but such terms cannot be universally understood.
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5.4.5.2
DATA POINTERS
The MPLAB XC8 compiler monitors and records all assignments of addresses to each
data pointer the program contains. This includes assignment of the addresses of
objects to pointers; assignment of one pointer to another; initialization of pointers when
they are defined; and takes into account when pointers are ordinary variables and function parameters, and when pointers are used to access basic objects, or structures or
arrays.
The size and format of the address held by each pointer is based on this information.
When more than one address is assigned to a pointer at different places in the code, a
set of all possible targets the pointer can address is maintained. This information is specific to each pointer defined in the program, thus two pointers with the same C type can
hold addresses of different sizes and formats due to the way the pointers were used in
the program.
The compiler tracks the memory location of all targets, as well as the size of all targets
to determine the size and scope of a pointer. The size of a target is important as well,
particularly with arrays or structures. It must be possible to increment a pointer so it can
access all the elements of an array, for example.
There are several pointer classifications used with the MPLAB XC8 C Compiler, such
as those indicated below.
For baseline and mid-range devices:
• 8-bit pointer capable of accessing common memory and two consecutive
(even-odd) banks, e.g., banks 0 and 1, or banks 6 and 7, etc.
• 16-bit pointer capable of accessing the entire data memory space
• 8-bit pointer capable of accessing up to 256 bytes of program space data
• 16-bit pointer capable of accessing up to 64 Kbytes of program space data
• 16-bit mixed target space pointer capable of accessing the entire data space
memory and up to 64 Kbytes of program space data
For PIC18 devices:
• 8-bit pointer capable of accessing the access bank
• 16-bit pointer capable of accessing the entire data memory space
• 8-bit pointer capable of accessing up to 256 bytes of program space data
• 16-bit pointer capable of accessing up to 64 Kbytes of program space data
• 24-bit pointer capable of accessing the entire program space
• 16-bit mixed target space pointer capable of accessing the entire data space
memory and up to 64 Kbytes of program space data
• 24-bit mixed target space pointer capable of accessing the entire data space
memory and the entire program space
Each data pointer will be allocated one of the available classifications after preliminary
scans of the source code. There is no mechanism by which the programmer can specify the style of pointer required (other than by the assignments to the pointer). The C
code must convey the required information to the compiler.
Information about the pointers and their targets are shown in the pointer reference
graph, which is described in Section 6.4.5 “Pointer Reference Graph”. This graph is
printed in the assembly list file, which is controlled by the option described in
Section 4.8.16 “--ASMLIST: Generate Assembler List Files”.
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Consider the following mid-range device program in the early stages of development.
It consists of the following code:
int i, j;
int getValue(const int * ip) {
return *ip;
}
void main(void) {
j = getValue(&i);
// ... code that uses j
}
A pointer, ip, is a parameter to the function getValue(). The pointer target type uses
the qualifier const because we do not want the pointer to be used to write to any
objects whose addresses are passed to the function. The const qualification serves
no other purpose and does not alter the format of the pointer variable.
If the compiler allocates the variable i (defined in main()) to bank 0 data memory, it
will also be noted that the pointer ip (parameter to getValue()) only points to one
object that resides in bank 0 of the data memory. In this case, the pointer, ip, is made
an 8-bit wide data pointer. The generated code that dereferences ip in getValue()
will be generated assuming that the address can only be to an object in bank 0.
As the program is developed, another variable, x, is defined and (unknown to the programmer) is allocated space in bank 2 data memory. The main() function now looks
like:
int i, j;
int x;
// allocated to bank 0 in this example
// allocated to bank 2 in this example
int getValue(const int * ip) {
return *ip;
}
void main(void) {
j = getValue(&i);
// ... code that uses j
j = getValue(&x);
// ... code that uses j
}
The pointer, ip, now has targets that are in bank 0 and in bank 2.To be able to accommodate this situation, the pointer is made 16 bits wide, and the code used to dereference the pointer will change accordingly. This takes place without any modification to
the source code.
One positive aspect of tracking pointer targets is less of a dependence on pointer qualifiers. The standard qualifiers const and volatile must still be used in pointer definitions to indicate a read-only or externally-modifiable target object, respectively.
However, this is in strict accordance with the ANSI C standard. Non-standard qualifiers,
like near and bank2, are not required to indicate pointer targets, have no effect, and
should be avoided. Omitting these qualifiers will result in more portable and readable
code, and reduce the chance of extraneous warnings being issued by the compiler.
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5.4.5.2.1
Pointers to Both Memory Spaces
When a pointer is assigned the address of one or more objects that have been allocated memory in the data space, and also assigned the address of one or more const
objects, the pointer is said to have targets with mixed memory spaces. Such pointers
fall into one of the mixed target space pointer classifications, which are listed in
Section 5.4.5.2 “Data Pointers”, and the address will be encoded so that the target
memory space can be determined at runtime. The encoding of these pointer types are
as follows.
For the Baseline/Mid-range 16-bit mixed target space pointer, the MSb of the address
(i.e., bit number 15) indicates the memory space that the address references. If this bit
is set, it indicates that the address is of something in program memory; clear indicates
an object in the data memory. The remainder of this address represents the full address
in the indicated memory space.
For the PIC18 24-bit mixed target space pointer, bit number 21 indicates the memory
space that the address references. If this bit is set, it indicates that the address is of an
object residing in data memory; if it is clear, it indicates an object in the program memory. The remainder of this address represents the full address in the indicated memory
space. Note that for efficiency reasons, the meaning of the memory space bit is the
opposite to that for baseline and mid-range devices.
To extend the mid-range device example given in Section 5.4.5.2 “Data Pointers”, the
code is now developed further. The function getValue() is now called with the
address of an object that resides in the program memory, as shown.
int i, j; // allocated to bank 0 in this example
int x;
// allocated to bank 2 in this example
const int type = 0x3456;
int getValue(const int * ip) {
return *ip;
}
void main(void) {
j = getValue(&i);
// ... code that uses j
j = getValue(&x);
// ... code that uses j
j = getValue(&type);
// ... code that uses j
}
Again, the targets to the pointer, ip, are determined, and now the pointer is made of
the class that can access both data and program memory. The generated code to
dereference the pointer will be such that it can determine the required memory space
from the address, and access either space accordingly. Again, this takes place without
any change in the definition of the pointer.
If assembly code references a C pointer, the compiler will force that pointer to become
a 16-bit mixed target space pointer, in the case of baseline or mid-range programs, or
a 24-bit mixed target space pointer, for PIC18 programs. These pointer types have
unrestricted access to all memory areas and will operate correctly, even if assignments
(of a correctly formatted address) are made to the pointer in the assembly code.
5.4.5.3
FUNCTION POINTERS
The MPLAB XC8 compiler fully supports pointers to functions, which allows functions
to be called indirectly. These are often used to call one of several function addresses
stored in a user-defined C array, which acts like a lookup table.
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For baseline and mid-range devices, function pointers are always one byte in size and
hold an offset into a jump table that is output by the compiler. This jump table contains
jumps to the destination functions.
For enhanced mid-range devices, function pointers are always 16-bits wide and can
hold the full address of any function.
For PIC18 devices, function pointers are either 16 or 24 bits wide. The pointer size is
purely based on the amount of program memory available on the target device.
As with data pointers, the target assigned to function pointers is tracked. This is an easier process to undertake compared to that associated with data pointers as all function
instructions must reside in program memory. The pointer reference graph (described in
Section 6.4.5 “Pointer Reference Graph”) will show function pointers, in addition to
data pointers, as well as all their targets. The targets will be names of functions that
could possibly be called via the pointer.
One notable runtime feature for baseline and mid-range devices is that a function
pointer which contains null (the value 0) and is used to call a function indirectly will
cause the code to become stuck in a loop which branches to itself. This endless loop
can be used to detect this erroneous situation. Typically calling a function via a null
function would result in the code crashing or some other unexpected behavior. The
label to which the endless loop will jump is called fpbase.
5.4.5.4
SPECIAL POINTER TARGETS
Pointers and integers are not interchangeable. Assigning an integer constant to a
pointer will generate a warning to this effect. For example:
const char * cp = 0x123;
// the compiler will flag this as bad code
There is no information in the integer constant, 0x123, relating to the type, size or memory location of the destination. There is a very good chance of code failure if pointers
are assigned integer addresses and dereferenced, particularly for PIC devices that
have more than one memory space. Is 0x123 an address in data memory or program
memory? How big is the object found at address 0x123?
Always take the address of a C object when assigning an address to a pointer. If there
is no C object defined at the destination address, then define or declare an object at
this address which can be used for this purpose. Make sure the size of the object
matches the range of the memory locations that are to be accessed by the pointer.
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For example, a checksum for 1000 memory locations starting at address 0x900 in program memory is to be generated. A pointer is used to read this data. You can be
tempted to write code such as:
const char * cp;
cp = 0x900; // what resides at 0x900???
and increment the pointer over the data.
However, a much better solution is this:
const char * cp;
const char inputData[1000] @ 0x900;
cp = &inputData;
// cp is incremented over inputData and used to read values there
In this case, the compiler can determine the size of the target and the memory space.
The array size and type indicates the size of the pointer target, the const qualifier on
the object (not the pointer) indicates the target is located in program memory space.
Note that the const array does not need initial values to be specified in this instance,
see Section 5.4.7.1 “Const Type Qualifier” and can reside over the top of other objects
at these addresses.
If the pointer has to access objects in data memory, you need to define a different object
to act as a dummy target. For example, if the checksum was to be calculated over 10
bytes starting at address 0x90 in data memory, the following code could be used.
const char * cp;
extern char inputData[10] @ 0x90;
cp = &inputData;
// cp is incremented over inputData and used to read values there
No memory is consumed by the extern declaration, and this can be mapped over the
top of existing objects.
User-defined absolute objects will not be cleared by the runtime startup code and can
be placed over the top of other absolute variables.
Take care when comparing (subtracting) pointers. For example:
if(cp1 == cp2)
; // take appropriate action
The ANSI C standard only allows pointer comparisons when the two pointer targets are
the same object. One exception is that the address can extend to one element past the
end of an array.
Comparisons of pointers to integer constants are even more risky, for example:
if(cp1 == 0x246)
; // take appropriate action
Never compare pointers with integer constants.
A null pointer is the one instance where a constant value can be assigned to a pointer
and this is handled correctly by the compiler. A null pointer is numerically equal to 0
(zero), but this is a special case imposed by the ANSI C standard. Comparisons with
the macro NULL are also allowed.
If null is the only value assigned to a pointer, the pointer will be made as small as
possible.
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5.4.6
Constant Types and Formats
A constant is used to represent an immediate value in the source code, as opposed to
a variable that could hold the same value. For example 123 is a constant.
Like any value, a constant must have a C type. In addition to a constant’s type, the
actual value can be specified in one of several formats.
5.4.6.1
INTEGRAL CONSTANTS
The format of integral constants specifies their radix. MPLAB XC8 supports the ANSI
standard radix specifiers, as well as ones which enables binary constants to be
specified in C code.
The formats used to specify the radices are given in Table 5-9. The letters used to specify binary or hexadecimal radices are case insensitive, as are the letters used to specify
the hexadecimal digits.
TABLE 5-9:
RADIX FORMATS
Radix
Format
Example
binary
0b number or 0B number
octal
0 number
0763
decimal
number
129
hexadecimal
0x number or 0X number
0x2F
0b10011010
Any integral constant will have a type of int, long int or long long int, so that
the type can hold the value without overflow. Constants specified in octal or hexadecimal can also be assigned a type of unsigned int, unsigned long int or
unsigned long long int if the signed counterparts are too small to hold the value.
The default types of constants can be changed by the addition of a suffix after the digits;
e.g., 23U, where U is the suffix. Table 5-10 shows the possible combination of suffixes
and the types that are considered when assigning a type. So, for example, if the suffix
l is specified and the value is a decimal constant, the compiler will assign the type
long int, if that type will hold the constant; otherwise, it will assigned long long
int. If the constant was specified as an octal or hexadecimal constant, then unsigned
types are also considered.
TABLE 5-10:
SUFFIXES AND ASSIGNED TYPES
Suffix
Decimal
Octal or Hexadecimal
u or U
unsigned int
unsigned long int
unsigned long long int
unsigned int
unsigned long int
unsigned long long int
l or L
long int
long long int
long int
unsigned long int
long long int
unsigned long long int
u or U, and l or L
unsigned long int
unsigned long long int
unsigned long int
unsigned long long int
ll or LL
long long int
long long int
unsigned long long int
u or U, and ll or LL
unsigned long long int
unsigned long long int
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Here is an example of code that can fail because the default type assigned to a
constant is not appropriate:
unsigned long int result;
unsigned char shifter;
void main(void)
{
shifter = 20;
result = 1 << shifter;
// code that uses result
}
The constant 1 (one) will be assigned an int type, hence the result of the shift operation will be an int. Even though this result is assigned to the long variable, result,
it can never become larger than the size of an int, regardless of how much the constant is shifted. In this case, the value 1 shifted left 20 bits will yield the result 0, not
0x100000.
The following uses a suffix to change the type of the constant, hence ensure the shift
result has an unsigned long type.
result = 1UL << shifter;
5.4.6.2
FLOATING-POINT CONSTANT
Floating-point constants have double type unless suffixed by f or F, in which case it
is a float constant. The suffixes l or L specify a long double type which is
considered an identical type to double by MPLAB XC8.
5.4.6.3
CHARACTER AND STRING CONSTANTS
Character constants are enclosed by single quote characters, ’, for example ’a’. A
character constant has int type, although this can be later optimized to a char type
by the compiler.
To comply with the ANSI C standard, the compiler does not support the extended character set in characters or character arrays. Instead, they need to be escaped using the
backslash character, as in the following example.
const char name[] = "Bj\370rk";
printf("%s's Resum\351", name);
\\ prints "Bjørk's Resumé"
Multi-byte character constants are not supported by this implementation.
String constants, or string literals, are enclosed by double quote characters “, for example “hello world”. The type of string constants is const char * and the character
that make up the string are stored in the program memory, as are all objects qualified
const.
A common warning relates to assigning a string literal to a pointer that does not specify
a const target, for example:
char * cp = "hello world\n";
The string characters cannot be modified, but this type of pointer allows writes to take
place, hence the warning. To prevent yourself from trying to overwrite the string,
qualifier the pointer target as follows. See also Section 5.4.5.1 “Combining Type
Qualifiers and Pointers”.
const char * cp = "hello world\n";
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C Language Features
Defining and initializing an array (i.e., not a pointer) with a string is an exception. For
example:
char ca[]= "hello world\n";
will actually copy the string characters into the RAM array, rather than assign the
address of the characters to a pointer, as in the previous examples. The string literal
remains read-only, but the array is both readable and writable.
The MPLAB XC8 compiler will use the same storage location and label for strings that
have identical character sequences, except where the strings are used to initialize an
array residing in the data space. For example, in the code snippet
if(strncmp(scp, "hello", 6) == 0)
fred = 0;
if(strcmp(scp, "world") == 0)
fred--;
if(strcmp(scp, "hello world") == 0)
fred++;
the characters in the string “world” and the last 6 characters of the string “hello
world” (the last character is the null terminator character) would be represented by
the same characters in memory. The string “hello” would not overlap with the same
characters in the string “hello world” as they differ in terms of the placement of the
null character.
Two adjacent string constants (i.e., two strings separated only by white space) are
concatenated by the compiler. Thus:
const char * cp = "hello"
"world";
will assign the pointer with the address of the string “hello world “.
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5.4.7
Standard Type Qualifiers
Type qualifiers provide information regarding how an object can be used. The MPLAB
XC8 compiler supports the ANSI C qualifiers const and volatile and additional
qualifiers which allow programs take advantage of the 8-bit PIC MCU architecture.
5.4.7.1
CONST TYPE QUALIFIER
The const type qualifier is used to tell the compiler that an object is read only and will
not be modified. If any attempt is made to modify an object declared const, the
compiler will issue a warning or error.
User-defined objects declared const are placed in a special psect linked into the program space. Objects qualified const can be absolute. The @ address construct is
used to place the object at the specified address in program memory, as in the following
example which places the object tableDef at address 0x100.
const int tableDef[] @ 0x100 = { 0, 1, 2, 3, 4};
Usually a const object must be initialized when it is declared, as it cannot be assigned
a value at any point at runtime. For example:
const int
version = 3;
will define version as being a read-only int variable, holding the value 3. However,
uninitialized const objects can be defined and are useful if you need to place an object
in program memory over the top of other objects at a particular location. Usually uninitialized const objects will be defined as absolute, as in the following example.
const char checksumRange[0x100] @ 0x800;
will define checksumRange as an array of 0x100 characters located at address 0x800
in program memory. This definition will not place any data in the HEX file.
5.4.7.2
VOLATILE TYPE QUALIFIER
The volatile type qualifier is used to tell the compiler that an object cannot be guaranteed to retain its value between successive accesses. This prevents the optimizer
from eliminating apparently redundant references to objects declared volatile
because it can alter the behavior of the program to do so.
Any SFR which can be modified by hardware or which drives hardware is qualified as
volatile, and any variables which can be modified by interrupt routines should use
this qualifier as well. For example:
volatile static unsigned int
TACTL @ 0x160;
The volatile qualifier does not guarantee that any access will be atomic, which is
often not the case since the 8-bit PIC MCU architecture can only access a maximum
of 1 byte of data per instruction.
The code produced by the compiler to access volatile objects can be different to
that to access ordinary variables, and typically the code will be longer and slower for
volatile objects, so only use this qualifier if it is necessary. However, failure to use
this qualifier when it is required can lead to code failure.
Another use of the volatile keyword is to prevent unused global variables being
removed. If a non-volatile variable is never used, or used in a way that has no
effect, then it can be removed before code is generated by the compiler.
A C statement that consists only of a volatile variable’s name will produce code that
reads the variable’s memory location and discards the result. For example, the entire
statement, PORTB; will produce assembly code the reads PORTB. This is useful for
some peripheral registers that require reading to reset the state of interrupt flags.
Some variables are treated as being volatile even though they are not qualified.
See Section 5.12.3.5 “Undefined Symbols” if you have assembly code in your project.
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5.4.8
Special Type Qualifiers
The MPLAB XC8 C Compiler supports special type qualifiers to allow the user to control
placement of static and extern class variables into particular address spaces.
5.4.8.1
PERSISTENT TYPE QUALIFIER
By default, any C variables that are not explicitly initialized are cleared on startup. This
is consistent with the definition of the C language. However, there are occasions where
it is desired for some data to be preserved across a Reset.
The persistent type qualifier (or __persistent) is used to qualify variables that
should not be cleared by the runtime startup code.
In addition, any persistent variables will be stored in a different area of memory to
other variables. Different psects are used to hold these objects. See
5.15.2 Compiler-Generated Psects for more information.
This type qualifier cannot be used on variables of class auto; however, statically
defined local variables can be qualified persistent. For example, you should write:
void test(void)
{
static persistent int intvar;
// ...
}
/* must be static */
If the xc8 option, --STRICT is used, you cannot use the persistent qualifier, but
you can continue to use __persistent.
5.4.8.2
NEAR TYPE QUALIFIER
Some of the 8-bit PIC architectures implement data memory which can be always
accessed regardless of the currently selected bank. This common memory can be
used to reduce code size and execution times as the bank selection instructions that
are normally required to access data in banked memory are not required when accessing the common memory. PIC18 devices refer to this memory as the access bank memory. Mid-range and baseline devices have very small amounts of this memory, if it is
present at all. PIC18 devices have substantially more common memory, but the amount
differs between devices. See your device data sheet for more information.
The near type qualifier (or __near) can be used to place global variables in common
memory. This qualifier cannot be used with auto or static local objects.
The compiler automatically uses the common memory for frequently accessed
user-defined variables so this qualifier would only be needed for special memory placement of objects, for example if C variables are accessed in hand-written assembly code
that assumes that they are located in this memory.
This qualifier is controlled by the compiler option --ADDRQUAL, which determines its
effect, see Section 4.8.15 “--ADDRQUAL: Set Compiler Response to Memory Qualifiers”. Based on this option’s settings, this qualifier can be binding or ignored (which is
the default operation). Qualifiers which are ignored will not produce an error or warning,
but will have no effect.
Here is an example of an unsigned char object qualified as near:
near unsigned char fred;
Note that the compiler can store some temporary objects in the common memory, so
not all of this space can be available for user-defined variables.
If the xc8 option, --STRICT is used, the near qualifier is no longer available, but you
can continue to use __near.
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5.4.8.3
FAR TYPE QUALIFIER
The far type qualifier (or __far) is used to place global variables into the program
memory space for those PIC18 devices that can support external memory. It will be
ignored when compiling for PIC10/12/16 targets. This qualifier cannot be used with
auto or static local objects.
The compiler assumes that far variables will be located in RAM that is implemented
in the external memory space.
Access of far variables are less efficient than that of internal variables and will result
in larger, slower code.
This qualifier is controlled by the compiler option --ADDRQUAL, which determines its
effect on PIC18 devices, see Section 4.8.15 “--ADDRQUAL: Set Compiler Response
to Memory Qualifiers”. Based on this option’s settings, this qualifier can be binding or
ignored (which is the default operation). Qualifiers which are ignored will not produce
an error or warning, but will have no effect.
Here is an example of an unsigned int object placed into the device’s external
program memory space:
far unsigned int farvar;
If the --STRICT is used, you can only use the __far form of the qualifier.
Note that not all PIC18 devices support external memory in their program memory
space and, thus, the far qualifier is not applicable to all PIC18 devices. On supported
devices, the address range where the additional memory will be mapped must first be
specified with the --RAM option, Section 4.8.52 “--RAM: Adjust RAM Ranges”. For
example, to map additional data memory from 20000h to 2FFFFh use
--RAM=default,+20000-2FFFF.
5.4.8.4
BANK0, BANK1, BANK2 AND BANK3 TYPE QUALIFIERS
The bank0, bank1, bank2 and bank3 type qualifiers are recognized by the compiler
and allow some degree of control of the placement of objects in the device’s data memory banks. When compiling for PIC18 targets, these qualifiers are only accepted for
portability and have no effect on variable placement; on other devices they can be used
to define C objects that are assumed to be located in certain memory banks by
hand-written assembly code. The compiler automatically allocates variables to all data
banks, so these qualifiers are not normally needed.
Although a few devices implement more than 4 banks of data RAM, bank qualifiers to
allow placement into these upper banks are not currently available.
These qualifiers are controlled by the compiler option --ADDRQUAL, which determines
their effect, see Section 4.8.15 “--ADDRQUAL: Set Compiler Response to Memory
Qualifiers”. Based on this option’s settings, these qualifiers can be binding or ignored
(which is the default operation). Qualifiers which are ignored will not produce an error
or warning, but will have no effect.
Objects qualified with any of these qualifiers cannot be auto or parameters to a function, but can be qualified static, allowing them to be defined locally within a function,
as in:
void myFunc(void) {
static bank1 unsigned char play_mode;
If the xc8 option, --STRICT is used, these qualifiers are changed to __bank0,
__bank1, __bank2 and __bank3.
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5.4.8.5
EEPROM TYPE QUALIFIER
The eeprom type qualifier (or __eeprom) is recognized by the compiler for baseline
and mid-range devices only and indicates that objects should be placed in the
EEPROM memory. Not all devices implement EEPROM memory. Check your device
data sheet for more information. A warning is produced if the qualifier is not supported
for the selected device.
Objects qualified with this qualifier cannot be auto or parameters to a function, but can
be qualified static, allowing them to be defined locally within a function, as in:
void myFunc(void) {
static eeprom unsigned char inputData[3];
See Section 5.5.5 “Variables in EEPROM” for more information on these variables and
other ways of accessing the EEPROM.
If the --STRICT option is used, only the __eeprom form of this qualifier is available.
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5.5
MEMORY ALLOCATION AND ACCESS
There are two broad groups of RAM-based variables: auto/parameter variables, which
are allocated to some form of stack, and global/static variables, which are positioned
freely throughout the data memory space at static locations. The memory allocation of
these two groups is discussed separately in the following sections.
5.5.1
Address Spaces
All 8-bit PIC devices have a Harvard architecture, which has a separate data memory
(RAM) and program memory space (often flash). Some devices also implement
EEPROM.
The data memory uses banking to increase the amount of available memory (referred
to in the data sheets as the general purpose register file) without having to increase the
assembly instruction width. One bank is “selected” by setting one or more bits in an
SFR. (Consult your device data sheet for the exact operation of the device you are
using.) Most instructions which access a data address use only the offset into the currently selected bank to access data. The exceptions are the PIC18 instructions MOVFF
and MOVFFL, which take a full banked address and operates independently of the
selected bank. Some devices only have one bank but many have more than one.
Both the general purpose RAM and SFRs both share the same data space and can
appear in all available memory banks. PIC18 devices have all SFRs in the one data
bank, but mid-range and baseline devices have SFRs at the lower addresses of each
bank. Due to the location of SFRs in these devices, the general purpose memory
becomes fragmented and this limits the size of most C objects.
The Enhanced mid-range devices overcome this fragmentation by allowing a linear
addressing mode, which allows the general purpose memory to be accessed as one
contiguous chunk. Thus, when compiling for these devices, the maximum allowable
size of objects typically increases. Objects defined when using PIC18 devices can also
typically use the entire data memory. See Section 5.5.2.2.3 “Size Limits of Auto
Variables” and Section 5.5.2.1.2 “Non-Auto Variable Size Limits”.
Many devices have several bytes which can be accessed regardless of which bank is
currently selected. This memory is called common memory. The PIC18 data sheets
refer to the bank in which this memory is stored as the access bank, and hence it is
often referred to as the access bank memory. Since no code is required to select a bank
before accessing these locations, access to objects in this memory is typically faster
and produces smaller code. The compiler always tries to use this memory if possible.
The program memory space is primarily for executable code, but data can also be
located here. There are several ways the different device families locate and read data
from this memory, but all objects located here will be read-only.
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5.5.2
Variables in Data Space Memory
Most variables are ultimately positioned into the data space memory. The exceptions
are non-auto variables which are qualified as const, which are placed in the program
memory space, or eeprom qualified variables.
Due to the fundamentally different way in which auto variables and non-auto variables are allocated memory, they are discussed separately. To use the C language terminology, these two groups of variables are those with automatic storage duration and
those with permanent storage duration, respectively.
Note:
5.5.2.1
The terms “local” and “global” are commonly used to describe variables,
but are not ones defined by the language Standard. The term “local variable” is often taken to mean a variable which has scope inside a function,
and “global variable” is one which has scope throughout the entire program. However, the C language has three common scopes: block, file (i.e.,
internal linkage) and program (i.e., external linkage), so using only two
terms to describe these can be confusing. For example, a static variable
defined outside a function has scope only in that file, so it is not globally
accessible, but it can be accessed by more than one function inside that
file, so it is not local to any one function, either.
NON-AUTO VARIABLE ALLOCATION
Non-auto variables (those with permanent storage duration) are located by the compiler into any of the available data banks. This is done in a two-stage process: placing
each variable into an appropriate psect and later linking that psect into a predetermined
bank. See Section 5.15.1 “Program Sections” for an introductory guide to psects. Thus,
during compilation, the code generator can determine which bank will hold each variable and encode the output accordingly, but it will not know the exact location within
that bank.
The compiler will attempt to locate all variables in one bank (i.e., place all variables in
the psect destined for this bank), but if this fills (i.e., if the compiler detects that the psect
has become too large for the free space in a bank), variables will be located in other
banks via different psects. Qualifiers are not required to have these variables placed in
banks other than bank 0 but can be used if you want to force a variable to a particular
bank. See Section 5.4.8.4 “Bank0, Bank1, Bank2 and Bank3 Type Qualifiers” and
Section 4.8.15 “--ADDRQUAL: Set Compiler Response to Memory Qualifiers” for more
information on how to do this. If common memory is available on the target device, this
will also be considered for variables. This memory can be limited in size and can be
reserved for special use, so only a few variables can be allocated to it.
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The compiler considers three categories of non-auto variables, which all relate to the
value the variable should contain by the time the program begins. Each variable category has a corresponding psect which is used to hold the output code which reserves
memory for each variable. The base name of each psect category is tabulated below.
A full list of all psect names are in Section 5.15.2 “Compiler-Generated Psects”.
nv
These psects are used to store variables qualified persistent, whose
values should not be altered by the runtime startup code. They are not
cleared or otherwise modified at startup.
bss
These psects contain any uninitialized variables, which are not assigned a value when they are defined, or variables which should be
cleared by the runtime startup code.
data
These psects contain the RAM image of any initialized variables, which
are assigned a non-zero initial value when they are defined and which
must have a value copied to them by the runtime startup code.
As described in Section 5.10 “Main, Runtime Startup and Reset”, the base name of
data space psects is always used in conjunction with a linker class name to indicate the
RAM bank in which the psect will be positioned. This section also lists other variants of
these psects and indicates where these psect must be linked. See also
Section 5.15.2 “Compiler-Generated Psects” for more information on how initial values
are assigned to the variables.
Note that the data psect used to hold initialized variables is the psect that holds the
RAM variables themselves. There is a corresponding psect (called idata) that is
placed into program memory (so it is non-volatile) and which is used to hold the initial
values that are copied to the RAM variables by the runtime startup code.
All non-auto variables, except for static variables, discussed in
Section 5.5.2.1.1 “Static Variables”, always use their lexical name with a leading underscore character as the assembly identifier used for this object. See
Section 5.12.3.1 “Equivalent Assembly Symbols” for more information on the mapping
between C- and assembly-domain symbols.
5.5.2.1.1
Static Variables
All static variables have permanent storage duration, even those defined inside a
function which are “local static” variables. Local static variables only have scope in
the function or block in which they are defined, but unlike auto variables, their memory
is reserved for the entire duration of the program. Thus they are allocated memory like
other non-auto variables.
All static variables can be accessed by other functions via pointers since they have
permanent duration.
Local static variables are guaranteed to retain their value between calls to the
function in which they are defined, unless explicitly modified via a pointer.
Variables which are static and which are initialized only have their initial value
assigned once during the program’s execution. Thus, they can be preferable over initialized auto objects which are assigned a value every time the block in they are
defined begins execution. Any initialized static variables are initialized in the same
way as other non-auto initialized objects by the runtime startup code, see
Section 4.4.2 “Startup and Initialization”.
All static variables which are also specified as const will be stored in program
memory, like any other non-auto variable.
The assembly symbols used to access static objects in assembly code are discussed
in Section 5.12.3.1 “Equivalent Assembly Symbols”.
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5.5.2.1.2
Non-Auto Variable Size Limits
Arrays of any type (including arrays of aggregate types) are fully supported by the compiler. So too are the structure and union aggregate types, see 5.4.4 Structures and
Unions. These objects can often become large in size and can affect memory
allocation.
When compiling for enhanced mid-range PIC devices, the size of an object (array or
aggregate object) is typically limited only by the total available data memory. Single
objects that will not fit into any of the available general purpose RAM ranges will be
allocated memory in several RAM banks and accessed using the device’s linear GPR
(general purpose RAM).
Note that the special function registers (which reside in the data memory space) or
memory reservations in general purpose RAM can prevent objects from being allocated contiguous memory in the one bank. In this case objects that are smaller than
the size of a RAM bank can also be allocated across multi-banks. The generated code
to access multi-bank objects will always be slower and the associated code size will be
larger than for objects fully contained within a single RAM bank.
When compiling for PIC18 devices, the size of an object is also typically limited only by
the data memory available. Objects can span several data banks.
On baseline and other mid-range devices, arrays and structures are limited to the maximum size of the available GPR memory in each RAM bank, not the total amount of
memory remaining. An error will result if an array is defined which is larger than this
size.
With any device, reserving memory in general purpose RAM (see
Section 4.8.52 “--RAM: Adjust RAM Ranges”), or defining absolute variables in the
middle of data banks (see Section 5.5.4 “Absolute Variables”), further restricts the contiguous memory in the data banks and can reduce the maximum size of objects you
can define.
5.5.2.1.3
Changing the Default Non-Auto Variable Allocation
There are several ways in which non-auto variables can be located in locations other
than those chosen by the compiler.
Variables can be placed in other memory spaces by the use of qualifiers. For example
if you wish to place variables in the program memory space, then the const specifier
should be used (see Section 5.4.7.1 “Const Type Qualifier”). The eeprom qualifier (see
5.4.8.5 Eeprom Type Qualifier) can be used to allocate variables to the EEPROM, if
such memory exists on your target device.
If you wish to prevent variables from using one or more data memory locations so that
these locations can be used for some other purpose, you are best reserving the memory using the memory adjust options. See Section 4.8.52 “--RAM: Adjust RAM Ranges”
for information on how to do this.
If only a few non-auto variables are to be located at specific addresses in data space
memory, then the variables can be made absolute. This allows individual variables to
be explicitly positioned in memory at an absolute address. Absolute variables are
described in Section 5.5.4 “Absolute Variables”. Once variables are made absolute,
their address is hard coded in generated output code, they are no longer placed in a
psect and do not follow the normal memory allocation procedure.
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The psects in which the different categories of non-auto variables (the nv, bss and
data psects described in Section 5.5.2.1 “Non-Auto Variable Allocation”) can be
shifted as a whole by changing the default linker options. So, for example, you could
move all the persistent variables. However, typically these psects can only be moved
within the data bank in which they were allocated by default. See Section 5.10 “Main,
Runtime Startup and Reset”, for more information on changing the default linker
options for psects. The code generator makes assumptions as to the location of these
psects and if you move them to a location that breaks these assumptions, code can fail.
Non-auto can also be placed at specific positions by using the __section() specifier,
see Section 5.15.4 “Changing and Linking the Allocated Section”. The decision
whether variables should be positioned this way or using absolute variables should be
based on the location requirements.
5.5.2.2
AUTO VARIABLE ALLOCATION AND ACCESS
This section discusses allocation of auto variables (those with automatic storage duration) to a data stack. This also includes function parameter variables, which behave like
auto variables, in terms of their storage duration and scope. Temporary variables
defined by the compiler also fall into this group. They are identical to auto variables,
except they are defined by the compiler and, hence, have no C name. Together, these
objects are often called stack-based objects.
The auto (short for automatic) variables are the default type of local variable. Unless
explicitly declared to be static, a local variable will be made auto. The auto keyword can be used if desired.
The auto variables, as their name suggests, automatically come into existence when
a function is executed, then disappear once the function returns. Since they are not in
existence for the entire duration of the program, there is the possibility to reclaim memory they use when the variables are not in existence and allocate it to other variables
in the program.
Typically such variables are stored on some sort of a dynamic data stack where memory can be easily allocated and deallocated by each function. This is not possible on all
8-bit devices supported by MPLAB XC8. Nor is it the most efficient means of storing
objects.
MPLAB XC8 has two methods of implementing data stacks for stack-based variables:
a compiled stack and a software stack1. Section 5.3.4 “Stacks” describes all the stacks
used by MPLAB XC8 and the 8-bit PIC devices.
Each C function is compiled to use exactly one of these stacks. The stack used affects
whether a function allows reentrancy. If a function is encoded to place its stack-based
objects on the software stack, it is said to be using a reentrant function model. A
function uses a non-reentrant function model if it places its stack-based objects on the
compiled stack. This information is summarized in Table 5-11 along with the devices
that support each model. The function model directly implies the stack used by a
function. See subsections below for specific details on how the compiled stack and
software stack operate.
TABLE 5-11:
FUNCTION MODELS IMPLEMENTATION
Function Model
Data Stack Used
Supported Device Families
Non-reentrant
Compiled stack
All devices
Reentrant
Software stack
Enhanced mid-range and PIC18 devices
1. What is referred to as a software stack in this user’s guide is the typical dynamic stack arrangement
employed by most computers. It is ordinary data memory accessed by some sort of push and pop
instructions, and a stack pointer register.
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C Language Features
When compiling for those devices that do not support the reentrant function model, all
functions are encoded to use the compiled stack, and these functions are
non-reentrant.
For the enhanced mid-range and PIC18 devices, by default the compiler will use the
non-reentrant model for all functions. Alternatively the user can dictate which functions
are to be compiled reentrantly (and those which are not) by using compiler options or
function specifiers. There is also a hybrid stack mode which allows the compiler to
choose which functions need to be compiled using a reentrant model and which can
use the non-reentrant model. The hybrid mode allows the program to use recursion but
still take advantage of the more efficient compiled stack.
The --STACK option (see Section 4.8.59 “--STACK: Specify Data Stack Type For
Entire Program”) can be used to change the compiler’s default behavior when assigning function models. Set the --STACK option to software so the compiler will always
choose the reentrant model (software stack) for each function. If the --STACK option
is set to compiled or this option is omitted, all functions are encoded to use the
non-reentrant (compiled stack) function model. Set this option to hybrid for hybrid
stack mode and to allow the compiler to decide how each function should be encoded.
In hybrid mode the compiler will choose a function model based on how the function is
called in the program. If the function is not reentrantly called, then it will be encoded to
use the non-reentrant model and the compiled stack. If the function appears in more
than one call graph (i.e., it is called from main-line and interrupt code), or it appears in
a loop in a call graph (i.e., it is called recursively), then the compiler will use the
reentrant model.
The --STACK option’s software and compiled settings changes the function model
for all functions. You can change the function model for individual functions by using
function specifiers when you define the function.
Use either the compiled or nonreentrant specifier (identical meanings) to indicate
that the specified function must use the compiled stack, without affecting any other
function. Alternatively, use either the software or reentrant specifier to indicate a
function must be encoded to use the software stack.
The function specifiers have precedence over the --STACK option setting. If, for example, the option --STACK=compiled has been used, but one function uses the
software (or reentrant) specifier, then the specified function will use the software
stack and all the remaining functions will use the compiled stack. These functions
specifiers also override any choice made by the compiler in hybrid mode.
If a function has been specified as compiled (or nonreentrant), or the
--STACK=compiled option has been issued, and that function appears in more than
one call graph in the program, then the usual function duplication feature automatically
comes into effect. See Section 5.9.7 “Function Duplication”, for more information on
how this is performed. Duplicating a non-reentrant function allows it to be called from
multiple call graphs, but cannot be used if the function is called recursively.
The auto variables defined in a function will not necessarily be allocated memory in
the order declared, in contrast to parameters which are always allocated memory
based on their lexical order. In fact, auto variables for one function can be allocated in
many RAM banks.
The standard qualifiers: const and volatile can both be used with auto variables
and these do not affect how they are positioned in memory. This implies that a local
const-qualified object is still an auto object and, as such, will be allocated memory in
the stack in the data space memory, not in the program memory as with non-auto
const objects. If you want to define a local-scope object that is placed in program
memory, specify it as static const.
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The compiler will try to locate the stack in one data bank, but if this fills (i.e., if the compiler detects that the stack psect has become too large), it can build up the stack into
several components (each with their own psect) and link each in a different bank.
Each auto object is referenced in assembly code using a special symbol defined by
the code generator. If accessing auto variables defined in C source code, you must use
these symbols, which are discussed in Section 5.12.3 “Interaction between Assembly
and C Code”.
5.5.2.2.1
Compiled Stack Operation
A compiled stack consists of fixed memory areas that are usable by each function’s
stack-based variables. When a compiled stack is used, functions are not re-entrant
since stack-based variables in each function will use the same fixed area of memory
every time the function is invoked.
Fundamental to the generation of the compiled stack is the call graph, which defines a
tree-like hierarchy of function calls, i.e it shows what functions can be called by each
function.
There will be one graph produced for each root function. A root function is typically not
called, but which is executed via other means and contains a program entry point. The
function main() is an example of a root function that will be in every project. Interrupt
functions which are executed when a hardware interrupt occurs, are another example.
FIGURE 5-1:
FORMATION OF CALL GRAPH
main {
F1(…);
F2(…);
F3(…);
}
Call graph
main
F1
F4
F1 {
F4(…);
}
isr {
F5(…);
F6(…);
}
code
generator
F2
F3
isr
F5
F6
Analysis of program
Figure 5-1 shows sections of a program being analyzed by the code generator to form
a call graph. In the original source code, the function main() calls F1(), F2() and
F3(). F1() calls F4(), but the other two functions make no calls. The call graph for
main() indicates these calls. The symbols F1, F2 and F3 are all indented one level
under main. F4 is indented one level under F1.
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C Language Features
This is a static call graph which shows all possible calls. If the exact code for function
F1() looked like:
int F1(void) {
if(PORTA == 44)
return F4();
return 55;
}
the function F4() will always appear in the call graph, even though it is conditionally
executed in the actual source code. Thus, the call graph indicates all functions that
might be called.
In the diagram, there is also an interrupt function, isr(), and it too has a separate
graph generated.
The term main-line code is often used, and refers to any code that is executed as a
result of the main() function being executed. In the above figure, F1(), F2(), F3()
and F4() are only ever called by main-line code.
The term interrupt code refers to any code that is executed as a result of an interrupt
being generated, in the above figure, F5() and F6() are called by interrupt code.
Figure 5-2 graphically shows an example of how the compiled stack is formed.
FIGURE 5-2:
FORMATION OF THE COMPILED STACK
1
Formation of auto-parameter block (APB)
for function F2
int aa , int
int bb ) {
F2(int
;
char c
a
b
c
F2
2 Analysis of call graph
}
main
F1
compiled
stack
main
F4
F3
F
3
1
F1
F2
F
2
F4
4
isr
F5
5
3
F2
F3
isr
F5
F6
F6
F
6
Overlap of non-concurrently active APBs
to form compiled stack
Each function in the program is allocated a block of memory for its parameter, auto
and temporary variables. Each block is referred to as an auto-parameter block (APB).
The figure shows the APB being formed for function F2(), which has two parameters,
a and b, and one auto variable, c.
The parameters to the function are first grouped in an order strictly determined by the
lexical order in which they appear in the source code. These are then followed by any
auto objects; however, the auto objects can be placed in any order. So we see
memory for a is followed by that for b and lastly c.
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Once these variables have been grouped, the exact location of each object is not
important at this point and we can represent this memory by one block — the APB for
this function.
The APBs are formed for all functions in the program. Then, by analyzing the call graph,
these blocks are assigned positions, or base values, in the compiled stack.
Memory can be saved if the following point is observed: If two functions are never
active at the same time, then their APBs can be overlapped.
In the example shown in the figure, F4() and F1() are active at the same time, in fact
F1() calls F4(). However, F2(), F3() and F1() are never active at the same time;
F1() must return before F2() or F3() can be called by main(). The function main()
will always be active and so its APB can never overlap with that of another function.
In the compiled stack, you can see that the APB for main() is allocated unique memory. The blocks for F1(), F2() and F3() are all placed on top of each other and the
same base value in the compiled stack; however, the memory taken up by the APBs
for F1() and F4() are unique and do not overlap.
Our example also has an interrupt function, isr(), and its call graph is used to assemble the APBs for any interrupt code in the same way. Being the root of a graph, isr()
will always be allocated unique memory, and the APBs for interrupt functions will be
allocated memory following.
The end result is a block of memory which forms the compiled stack. This block can
then be placed into the device’s memory by the linker.
Once auto variables have been allocated a relative position in the compiled stack, the
stack itself is then allocated memory in the data space. This is done is a similar fashion
to the way other variables are assigned memory: a psect is used to hold the stack and
this psect is placed into the available data memory by the linker. The psect base name
used to hold the compiled stack is called cstack, and, like with other psects, the base
name is always used in conjunction with a linker class name to indicate the RAM bank
in which the psect will be positioned. See Section 5.15.2 “Compiler-Generated Psects”
for the limitations associated with where this psect can be linked.
For devices with more than one bank of data memory, the compiled stack can be built
up into components, each located in a different memory bank. The compiler will try to
allocate the compiled stack in one bank, but if this fills, it will consider other banks. The
process of building these components of the stack is the same, but each function can
have more than one APB and these will be allocated to one of the stack components
based on the remaining memory in the component’s destination bank.
Human readable symbols are defined by the code generator which can be used to
access auto and parameter variables in the compiled stack from assembly code, if
required. See Section 5.12.3 “Interaction between Assembly and C Code” for full
information between C domain and assembly domain symbols.
5.5.2.2.2
Software Stack Operation
Functions using a software stack (reentrant model) dynamically allocate memory for
their stack-based variables in a region of memory specifically reserved for this software
stack.
Allocation starts at one end of this reserved area, and the stack memory grows as new
function instances come into existence. When a function using the reentrant model
exits, any stack memory it used is freed and made available for other functions. The
stack grows up in memory, toward larger addresses.
Main-line code and each interrupt routine use unique areas in the stack space. The
maximum size allocated to each area can be specified using the --STACK option, see
Section 4.8.59 “--STACK: Specify Data Stack Type For Entire Program”.
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MPLAB XC8 designates a register, known as the stack pointer, which always holds the
address of the next free location in the software stack. The register used by the stack
pointer is FSR1 for both enhanced mid-range and PIC18 devices. The address held by
the stack pointer is increased when variables are allocated (pushed) to the stack; it is
decreased when a function returns and variables are removed (popped) from the stack.
Note that if there are any functions in the program that are reentrantly encoded, the
FSR1 register is reserved for the stack pointer for the entire program’s duration, even
when executing code associated with non-reentrant functions. With this register
unavailable for use with general statements, the code generated may be less efficient
or “Can’t generate code” errors may result.
The stack pointer is reloaded when an interrupt occurs so it accesses the interrupt function’s unique stack area. It is restored by the interrupt context switch code when the
interrupt routine is complete.
There is no register assigned to hold a frame pointer. All access of stack-based objects
must use an address that is an offset from the stack pointer.
When a function is called, any arguments to that function are pushed onto the stack by
the calling function, in a reverse order to that in which the corresponding parameters
appear in the function’s prototype. If required, the called function will increase the value
stored in the stack pointer to allocate storage for any auto or temporary variables it
needs to allocate.
If the reentrant function returns a value on the stack (this might happen for return values
larger than 4 bytes in size), the calling function will adjust the stack to remove the return
value.
Recall that a function’s return address is not stored on this stack. It is automatically
stored on the hardware stack by the device, see Section 5.3.4.1 “Function Return
Address Stack”.
The compiler can detect if the software stack memory requirements for each function
will exceed set limits. These limits are 127 bytes for PIC18 devices and typically 31
bytes for enhanced mid-range devices. Note that the compiler cannot detect for overflow of the memory reserved for the stack as a whole. There is no runtime check made
for software stack overflows. If the software stack overflows, data corruption and code
failure might result.
When reentrant functions call other reentrant functions, the stack pointer is incremented as any parameters to the called function are loaded. This increases the offset
from the new top-of-stack position to the stack-based objects defined by the calling
function. If this offset becomes too large, a warning (1488) or error might result when
trying to access stack-based objects in the calling function. A similar situation exists if
the called reentrant function returns a value, as this might also be located on the stack.
For these reasons, the entire stack depth might not be usable for every function.
5.5.2.2.3
Size Limits of Auto Variables
The compiled stack is built up as one contiguous block which can be placed into one
of the available data banks. However, if the stack becomes too large for this space, it
can be assembled into several blocks, with each block being positioned in a different
bank of memory. Thus the total size of the stack is roughly limited only by the available
memory on the device.
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Unlike with non-auto variables, it is not efficient to access auto variables within the
compiled stack using the linear memory of enhanced mid-range devices. For all
devices, including PIC18 and Enhanced mid-range PIC MCUs, each component of the
compiled stack must fit entirely within one bank of data memory on the target device
(however, you can have more than one component, each allocated to a different bank).
This limits the size of objects within the stack to the maximum free space of the bank
in which it is allocated. The more auto variables in the stack; the more restrictive the
space is to large objects. Recall that SFRs on mid-range devices are usually present
in each data bank, so the maximum amount of GPR available in each bank is typically
less than the bank size for these devices.
The software stack is always allocated one block of memory. This memory may cross
bank boundaries. The size is typically limited by the amount of free data space
remaining. An auto object placed on the software stack may be any size, providing it
fits in the allocated stack space. It may be allocated memory that crosses a bank
boundary and will always be accessed via the stack pointer, FSR1.
If a program requires large objects that should not be accessible to the entire program,
consider leaving them as local objects, but using the static specifier. Such variables
are still local to a function, but are no longer auto and have fewer size limitations. They
are allocated memory as described in Section 5.5.2.1 “Non-Auto Variable Allocation”.
5.5.2.2.4
Changing the Default Auto Variable Allocation
As auto variables are stack based, there is no means to move them. They cannot be
made absolute, nor can they be moved using the __section() specifier.
The psects in which the auto variables reside can be shifted as a whole by changing
the default linker options. However, these psects can only be moved within the data
bank in which they were allocated by default. See Section 5.10 “Main, Runtime Startup
and Reset” for more information on changing the default linker options for psects. The
code generate makes assumptions as to the location of these psects and if you move
them to a location that breaks these assumptions, code can fail.
5.5.3
Variables in Program Space
The only variables that are placed into program memory are those that are not auto
and which have been qualified const. Any auto variables qualified const are placed
in the compiled stack along with other auto variables, and all components of the compiled stack will only ever be located in the data space memory.
Any const-qualified (auto or non-auto) variable will always be read-only and any
attempt to write to these in your source code will result in an error being issued by the
compiler.
On some 8-bit PIC devices, the program space is not directly readable by the device.
For these devices, the compiler stores data in the program memory by means of RETLW
instructions which can be called, and which will return a byte of data in the W register.
The compiler will generate the code necessary to make it appear that program memory
is being read directly.
Enhanced mid-range PIC devices can directly read their program memory, although
the compiler will still usually store data as RETLW instructions. This way the compiler
can either produce code that can call these instructions to obtain the program memory
data as with the ordinary mid-range devices, or directly read the operand to the instruction (the LSB of the RETLW instruction). The most efficient access method can be
selected by the compiler when the data needs to be read.
Data can be stored as individual bytes in the program memory of PIC18 devices. This
can be read using table read instructions.
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C Language Features
On all devices, accessing data located in program memory is much slower than
accessing objects in the data memory. The code associated with the access is also
larger.
A const object is usually defined with initial values, as the program cannot write to
these objects at runtime. However, this is not a requirement. An uninitialized const
object can be defined to define a symbol, or label, but not make a contribution to the
output file. Uninitialized const objects are often made absolute, see
Section 5.5.4 “Absolute Variables”. Here are examples of const object definitions.
const char IOtype = ’A’;
const char buffer[10];
// initialized const object
// I just define a label
The data held by non-auto const variables is placed in one of several psects, based
on the target device. See Section 5.15.2 “Compiler-Generated Psects” for the
limitations associated with where these psects can be linked.
See Section 5.12.3 “Interaction between Assembly and C Code” for the equivalent
assembly symbols that are used to represent const-qualified variables in program
memory.
5.5.3.1
SIZE LIMITATIONS OF CONST VARIABLES
Arrays of any type (including arrays of aggregate types) can be qualified const and
placed in the program memory. So too can structure and union aggregate types, see
5.4.4 Structures and Unions. These objects can often become large in size and can
affect memory allocation.
For baseline PIC devices, the maximum size of a single const object is 255 bytes.
However, you can define as many const objects as required provided the total size
does not exceed the available program memory size of the device. Note that as well as
other program code, there is also code required to be able to access const-qualified
data in the program memory space. Thus, you can need additional program memory
space over the size of the object itself. This additional code to access the const data
is only included once, regardless of the amount or number of const-qualified objects.
For all other 8-bit devices, the maximum size of a const-qualified object is limited only
by the available program memory. These devices also use additional code that
accesses the const data. PIC18 devices need additional code each time an object is
accessed, but this is typically small. The mid-range devices include a larger routine, but
this code is also only included once, regardless of the amount or number of
const-qualified objects.
5.5.3.2
CHANGING THE DEFAULT ALLOCATION
If you only intend to prevent all variables from using one or more program memory locations so that you can use those locations for some other purpose, you are best reserving the memory using the memory adjust options. See Section 4.8.53 “--ROM: Adjust
ROM Ranges” for information on how to do this.
If only a few non-auto const variables are to be located at specific addresses in program space memory, then the variables can be made absolute. This allows individual
variables to be explicitly positioned in memory at an absolute address. Absolute variables are described in Section 5.5.4 “Absolute Variables”. Once variables are made
absolute, their address is hard coded in generated output code, they are no longer
placed in a psect and do not follow the normal memory allocation procedure.
The psects in which the different categories of non-auto const variables can be
shifted as a whole by changing the default linker options. However, there are limitations
in where these psects can be moved to. See Section 5.10 “Main, Runtime Startup and
Reset” for more information on changing the default linker options for these psects.
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Variables in program memory can also be placed at specific positions by using the
__section() specifier, see Section 5.15.4 “Changing and Linking the Allocated Section”. The decision whether variables should be positioned this way or using absolute
variables should be based on the location requirements.
5.5.4
Absolute Variables
Most variables can be located at an absolute address by following its declaration with
the construct @ address, where address is the location in memory where the
variable is to be positioned. Such a variable is known as an absolute variable.
The compiler does not make any checks for overlap of absolute variables with other
absolute variables, so this must be considered when choosing the variable locations.
No warning will be issued if the address of an absolute object lies outside the memory
of the device or outside the memory defined by the linker classes. There is no harm in
defining more than one absolute variable to live at the same address if this is what you
require. The compiler will not locate ordinary variables over the top of absolutes.
5.5.4.1
ABSOLUTE VARIABLES IN DATA MEMORY
Absolute variables are primarily intended for equating the address of a C identifier with
a special function register, but can be used to place ordinary variables at an absolute
address in data memory. The auto variables cannot be made absolute as they are
located in a stack. Nor can you make static local objects absolute.
For example:
volatile unsigned char Portvar @ 0x06;
will declare a variable called Portvar located at 06h in the data memory. The compiler
will reserve storage for this object (if the address falls into general-purpose RAM) and
will equate the variable’s identifier to that address.
Objects should not be made absolute to force them into common (unbanked) memory.
Always use the near qualifier for this purpose (see Section 5.4.8.2 “Near Type Qualifier”. Objects defined by the compiler have first priority for common memory. Common
memory remaining after the compiler has allocated special variables in this area is then
available for objects qualified near.
Note:
Defining absolute objects can fragment memory and can make it impossible for the linker to position other objects. Avoid absolute objects if at all
possible. If absolute objects must be defined, try to place them at either end
of a memory bank or page so that the remaining free memory is not
fragmented into smaller chunks.
When defining absolute bit variables (see Section 5.4.2.1 “Bit Data Types and Variables”), the address specified must be a bit address. A bit address is obtained by multiplying the desired byte address by 8, then adding the bit offset within that bit. So, for
example, to place a bit variable called mode at bit position #2 at byte address 0x50,
use the following:
bit mode @ 0x282;
If you wish to place a bit variable over an existing object (typically this will be an SFR
variable or another absolute variable) then you can use the symbol of that object, as in
the following example which places flag at bit position #3 in the char variable
MOT_STATUS:
bit flag @ ((unsigned) &MOT_STATUS)*8 + 3;
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C Language Features
When compiling for an enhanced mid-range PIC device, the memory allocated for
some objects can be spread over multiple RAM banks. Such objects will only ever be
accessed indirectly in assembly code, and will use the linear GPR memory implemented on these devices. A linear address (which can be mapped back to the ordinary
banked address) will be used with these objects internally by the compiler.
The address specified for absolute objects on these devices can either be the traditional banked memory address or the linear address. As the linear addresses start
above the largest banked address, it is clear which address is intended. In the following
example:
int inputBuffer[100] @ 0x2000;
it is clear that inputBuffer should placed at address 0x2000 in the linear address
space, which is address 0x20 in bank 0 RAM in the traditional banked address space.
See the device data sheet for exact details regarding your selected device.
Absolute variables in RAM cannot be initialized when they are defined. Define the
absolute variables, then assign them a value at a suitable point in your main-line code.
5.5.4.2
ABSOLUTE OBJECTS IN PROGRAM MEMORY
Non-auto objects qualified const can also be made absolute in the same way,
however, the address will indicate an address in program memory. For example:
const int settings[] @ 0x200 = { 1, 5, 10, 50, 100 };
will place the array settings at address 0x200 in the program memory.
Both initialized and uninitialized const objects can be made absolute. That latter is
useful when you only need to define a label in program memory without making a
contribution to the output file.
Variables can also be placed at specific positions by using the __section() specifier,
see Section 5.15.4 “Changing and Linking the Allocated Section”. The decision
whether variables should be positioned this way or using absolute variables should be
based on the location requirements. Using absolute variables is the easiest method,
but only allows placement at an address which must be known prior to compilation. The
__section() specifier is more complex, but offers all the flexibility of the linker to
position the new psect into memory. You can, for example, specify that variables reside
at a fixed address, or that they be placed after other psects, or that they be placed
anywhere in a compiler-defined or user-defined range of address.
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5.5.5
Variables in EEPROM
For devices with on-chip EEPROM, the compiler offers several methods of accessing
this memory. You can defined named variables in this memory space, or use
block-access routines to read or write EEPROM. The EEPROM access methods are
described in the following sections.
5.5.5.1
EEPROM VARIABLES
When compiling for baseline and mid-range parts, the eeprom qualifier allows you to
create named C variables that reside in the EEPROM space. See
Section 5.4.8.5 “Eeprom Type Qualifier”.
Variables qualified eeprom are cleared or initialized, just like ordinary RAM-based variables; however, the initialization process is not carried out by the runtime startup code.
Initial values are placed into the HEX file and are burnt into the EEPROM when you
program the device. Thus, if you modify the EEPROM during program execution and
then reset the device, these variables will not contain the initial values specified in your
code at startup up.
The following example defines two arrays in EEPROM.
eeprom char regNumber[10] = "A93213";
eeprom int lastValues[3];
For both these objects, initial values will be placed into psects and will appear in the
HEX file. Zeros will be used as the initial values for lastValues.
The generated code to access eeprom-qualified variables will be much longer and
slower than code to access RAM-based variables. You should avoid using
eeprom-qualified variables in complicated expressions. Consider copying values from
the EEPROM to regular RAM-based variables and using these in your code.
5.5.5.2
EEPROM INITIALIZATION
For those devices that support external programming of their EEPROM data area, the
__EEPROM_DATA() macro can be used to place initial values into the HEX file ready
for programming. The macro is used as follows.
#include <xc.h>
__EEPROM_DATA(0, 1, 2, 3, 4, 5, 6, 7);
The macro has eight parameters, representing eight data values. Each value should be
a byte in size. Unused values should be specified with zero.
The __EEPROM_DATA() macro expands into in-line assembly code. If expressions are
used to evaluate the macro arguments, ensure that any operators or tokens in these
expressions are written in assembly code (see Section 6.2 “MPLAB XC8 Assembly
Language”).
The macro can be called multiple times to define the required amount of EEPROM
data. It is recommended that the macro be placed outside any function definition.
This macro cannot used to write to EEPROM locations during runtime; it is used for
pre-loading EEPROM contents at program time only.
The values defined by this macro share the EEPROM space with any eeprom-qualified
variables. The macro cannot be used to initialize eeprom-qualified variables. The psect
used by this macro to hold the data values is different to those used by eeprom-qualified variables. The link order of these psects can be adjusted, if required (see
Section 4.8.6 “-L-: Adjust Linker Options Directly”).
For convenience, the macro _EEPROMSIZE represents the number of bytes of
EEPROM available on the target device.
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C Language Features
5.5.5.3
EEPROM ACCESS FUNCTIONS
The library functions eeprom_read() and eeprom_write(), can be called to read
from, and write to, the EEPROM during program execution. They are available for all
midrange devices which implement EEPROM. For PIC18 devices, calls to these routines will instead attempt to call the equivalent functions in the PIC18 peripheral library
It is recommended that for PIC18 devices you call the equivalent peripheral library
functions directly or use MPLAB MCC to generate EEPROM access code.
The prototypes for these functions are as below.
#include <xc.h>
unsigned char eeprom_read(unsigned char address);
void eeprom_write(unsigned char address, unsigned char value);
These functions test and wait for any concurrent writes to EEPROM to conclude before
performing the required operation. The eeprom_write() function will initiate the process of writing to EEPROM and this process will not have completed by the time that
eeprom_write() returns. The new data written to EEPROM will become valid at a
later time. See your device data sheet for exact information about EEPROM on your
target device.
It can also be convenient to use the preprocessor symbol, _EEPROMSIZE, in conjunction with some of these access methods. This symbol defines the number of EEPROM
bytes available for the selected chip.
5.5.5.4
EEPROM ACCESS MACROS
Macro versions of the EEPROM functions are also provided for midrange devices. For
PIC18 devices, it is recommended that you call the equivalent PIC18 peripheral library
functions directly or use MPLAB MCC to generate EEPROM access code. These macros perform similar operations to their function counterparts, with the exception of some
timing issues described below. Use the macro forms of these routines for faster execution and to save a level of stack, but note that their repeated use will increase code size.
The usage of these macros for all devices is as follows.
EEPROM_READ(address)
EEPROM_WRITE(address, value)
The EEPROM_READ macro returns the byte read.
In the case of the baseline and mid-range macro EEPROM_READ(), there is another
very important difference from the function version to note. Unlike eeprom_read(),
this macro does not wait for any concurrent EEPROM writes to complete before proceeding to select and read EEPROM. If it cannot be guaranteed that all writes to
EEPROM have completed at the time of calling EEPROM_READ(), the appropriate flag
should be polled prior to executing EEPROM_READ().
For example:
#include <xc.h>
// wait for end-of-write before EEPROM_READ
while(WR)
continue;
// read from EEPROM at address
value = EEPROM_READ(address);
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5.5.6
Variables in Registers
Allocating variables to registers, rather than to a memory location, can make code more
efficient. With MPLAB XC8, there is no direct control of placement of variables in registers. The register keyword (which can only be used with auto variables) is silently
ignored and has no effect on memory allocation of variables.
There are very few registers available for caching of variables on PIC baseline and
mid-range devices, and as these registers must be frequently used by generated code
for other purposes, there is little advantage in using them. The cost involved in loading
variables into registers would far outweigh any advantage of accessing the register. At
present, code compiled for PIC18 devices also does not utilize registers other than that
described below.
Some arguments are passed to functions in the W register rather than in a memory
location; however, these values will typically be stored back to memory by code inside
the function so that W can be used by code associated with that function. See
Section 5.8.5 “Function Size Limits” for more information as to which parameter variables can use registers.
5.5.7
Dynamic Memory Allocation
Dynamic memory allocation, (heap-based allocation using malloc, etc.) is not supported on any 8-bit device. This is due to the limited amount of data memory, and that
this memory is banked. The wasteful nature of dynamic memory allocation does not
suit itself to the 8-bit PIC device architectures.
5.5.8
Memory Models
MPLAB XC8 C Compiler does not use fixed memory models to alter allocation of variables to memory. Memory allocation is fully automatic and there are no memory model
controls.
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C Language Features
5.6
OPERATORS AND STATEMENTS
The MPLAB XC8 C Compiler supports all the ANSI operators. The exact results of
some of these are implementation defined. Implementation-defined behavior is fully
documented in Appendix D. Implementation-Defined Behavior. The following sections
illustrate code operations that are often misunderstood as well as additional operations
that the compiler is capable of performing.
5.6.1
Integral Promotion
When there is more than one operand to an operator, they typically must be of exactly
the same type. The compiler will automatically convert the operands, if necessary, so
they do have the same type. The conversion is to a “larger” type so there is no loss of
information; however, the change in type can cause different code behavior to what is
sometimes expected. These form the standard type conversions.
Prior to these type conversions, some operands are unconditionally converted to a
larger type, even if both operands to an operator have the same type. This conversion
is called integral promotion and is part of Standard C behavior. The compiler performs
these integral promotions where required, and there are no options that can control or
disable this operation. If you are not aware that the type has changed, the results of
some expressions are not what would normally be expected.
Integral promotion is the implicit conversion of enumerated types, signed or
unsigned varieties of char, short int or bit-field types to either signed int or
unsigned int. If the result of the conversion can be represented by an signed int,
then that is the destination type, otherwise the conversion is to unsigned int.
Consider the following example.
unsigned char count, a=0, b=50;
if(a - b < 10)
count++;
The unsigned char result of a - b is 206 (which is not less than 10), but both a and
b are converted to signed int via integral promotion before the subtraction takes
place. The result of the subtraction with these data types is -50 (which is less than 10)
and hence the body of the if() statement is executed.
If the result of the subtraction is to be an unsigned quantity, then apply a cast. For
example:
if((unsigned int)(a - b) < 10)
count++;
The comparison is then done using unsigned int, in this case, and the body of the
if() would not be executed.
Another problem that frequently occurs is with the bitwise complement operator, ~. This
operator toggles each bit within a value. Consider the following code.
unsigned char count, c;
c = 0x55;
if( ~c == 0xAA)
count++;
If c contains the value 0x55, it often assumed that ~c will produce 0xAA; however, the
result is 0xFFAA and so the comparison in the above example would fail. The compiler
can be able to issue a mismatched comparison error to this effect in some
circumstances. Again, a cast could be used to change this behavior.
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The consequence of integral promotion as illustrated above is that operations are not
performed with char -type operands, but with int -type operands. However, there are
circumstances when the result of an operation is identical regardless of whether the
operands are of type char or int. In these cases, the compiler will not perform the
integral promotion so as to increase the code efficiency. Consider this example.
unsigned char a, b, c;
a = b + c;
Strictly speaking, this statement requires that the values of b and c should be promoted
to unsigned int, the addition performed, the result of the addition cast to the type of
a, and then the assignment can take place. Even if the result of the unsigned int
addition of the promoted values of b and c was different to the result of the unsigned
char addition of these values without promotion, after the unsigned int result was
converted back to unsigned char, the final result would be the same. If an 8-bit
addition is more efficient than a 16-bit addition, the compiler will encode the former.
If, in the above example, the type of a was unsigned int, then integral promotion
would have to be performed to comply with the ANSI C standard.
5.6.2
Rotation
The C language does not specify a rotate operator; however, it does allow shifts. The
compiler will detect expressions that implement rotate operations using shift and logical
operators and compile them efficiently.
For the following code:
c = (c << 1) | (c >> 7);
if c is unsigned and non-volatile, the compiler will detect that the intended
operation is a rotate left of 1 bit and will encode the output using the PIC MCU rotate
instructions. A rotate left of 2 bits would be implemented with code like:
c = (c << 2) | (c >> 6);
This code optimization will also work for integral types larger than a char. If the optimization cannot be applied, or this code is ported to another compiler, the rotate will be
implemented, but typically with shifts and a bitwise OR operation.
5.6.3
Switch Statements
The compiler can encode switch statements using one of several strategies. By
default, the compiler chooses a strategy based on the case values that are used inside
the switch statement. Each switch statement is assigned its strategy independently.
The type of strategy can be indicated by using the #pragma switch directive. See
Section 5.14.4.10 “The #pragma switch Directive”, which also lists the available strategy types. There can be more than one strategy associated with each type.
There is information printed in the assembly list file for each switch statement detailing the value being switched and the case values listed. See Section 6.4.4 “Switch
Statement Information”.
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C Language Features
5.7
REGISTER USAGE
The assembly generated from C source code by the compiler will use certain registers
in the PIC MCU register set. Most importantly, the compiler assumes that nothing other
than code it generates can alter the contents of these registers.
So, if compiler-generated assembly code loads a register with a value and no subsequent code requires this register, the compiler will assume that the contents of the
register are still valid later in the output sequence.
If any of the applicable registers listed are used by interrupt code, they will be saved
and restored when an interrupt occurs, either in hardware or software. See
Section 5.9.4 “Context Switching”.
The registers that are special and which are used by the compiler are listed in
Table 5-12.
TABLE 5-12:
REGISTERS USED BY THE COMPILER
Applicable devices
Register name
All 8-bit devices
W
All 8-bit devices
STATUS
All mid-range devices
PCLATH
All PIC18 devices
PCLATH, PCLATU
Enhanced mid-range and PIC18 devices
BSR
Non-enhanced mid-range devices
FSR
Enhanced mid-range and PIC18 devices
FSR0L, FSR0H, FSR1L, FSR1H
All PIC18 devices
FSR2L, FSR2H
All PIC18 devices
TBLPTRL, TBLPTRH, TBLPTRU, TABLAT
All PIC18 devices
PRODL, PRODH
Enhanced mid-range and PIC18 devices
btemp, wtemp, ttemp, ltemp
The xtemp registers are variables that the compiler treats as registers, These are
saved like any other register if they are used in interrupt code.
The state of these registers must never be changed directly by C code, or by any
assembly code in-line with C code. The following example shows a C statement and
in-line assembly that violates these rules and changes the ZERO bit in the STATUS
register.
#include <xc.h>
void getInput(void)
{
ZERO = 0x1; // do not write using C code
c = read();
#asm
bcf ZERO ; do not write using inline assembly code
#endasm
process(c);
}
MPLAB XC8 is unable to interpret the register usage of in-line assembly code that is
encountered in C code. Nor does it associate a variable mapped over an SFR to the
actual register itself. Writing to an SFR register using either of these two methods will
not flag the register as having changed and can lead to code failure.
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5.8
FUNCTIONS
Functions are written in the usual way, in accordance with C language. Implementation
and special features associated with functions are discussed in following sections.
5.8.1
Function Specifiers
Functions can, in the usual way, use the standard specifier static. A function defined
using the static specifier only affects the scope of the function; i.e., limits the places
in the source code where the function can be called. Functions that are static can
only be directly called from code in the file in which the function is defined. The equivalent symbol used in assembly code to represent the function can change if the function is static, see 5.12.3 Interaction between Assembly and C Code. This specifier
does not change the way the function is encoded. Non-standard qualifiers are
discussed below.
5.8.1.1
INTERRUPT SPECIFIER
The interrupt specifier indicates that the function is an interrupt service routine and
that it is to be encoded specially to suit this task. Interrupt functions are described in
detail in 5.9.1 Writing an Interrupt Service Routine.
5.8.1.2
INLINE SPECIFIER
The inline function specifier is a recommendation that the compiler replace calls to
the specified function with the function’s body, if possible.
The following is an example of a function which has been made a candidate for inlining.
inline int combine(int x, int y) {
return 2*x-y;
}
All function calls to any function that were inlined by the compiler will be encoded as if
the call was replaced with the body of the called function. This is performed at the
assembly code level. Inlining will only take place if the assembly optimizers are enabled
and the compiler is not operating in Free mode. The function itself might still be
encoded normally by the compiler even if it is inlined.
If inlining takes place, this will increase the program’s execution speed, since the call
and return sequences associated with the call will be eliminated. It will also reduce the
hardware stack usage as no call instruction is actually executed; however, this reduction is not reflected in the call graphs as this is generated before inlining takes place.
Code size can be reduced if the assembly code associated with the body of the inlined
function is very small, but code size can increase if the body of the inlined function is
larger than the call/return sequence it replaces. You should only consider this specifier
for functions which generate small amounts of assembly code. Note that the amount of
C code in the body of a function is not a good indicator of the size of the assembly code
that it generates (see Section 3.6.13 “How Can I Tell How Big a Function Is?”).
A function containing in-line assembly will not be inlined. Some generated assembly
code sequences will also prevent inlining of a function. A warning will be generated if
the inline function references static objects (to comply with the ANSI Standard) or
is not inlined successfully. Your code should not make any assumption about whether
inlining was successful.
This specifier performs the same task as the #pragma inline directive, see
Section 5.14.4.4 “The #pragma Intrinsic Directive”.
Note that the optimizers can also implicitly inline small called-only-once routines, see
Section 6.3 “Assembly-Level Optimizations”.
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C Language Features
5.8.1.3
REENTRANT AND NONREENTRANT SPECIFIERS
The reentrant and nonreentrant function specifiers indicate the function model
(stack) that should be used for that function’s stack-based variables (auto, parameter,
and temporary variables), as shown in Table 5-13. The aliases software and
compiled, respectively, can also be used. If the --STRICT option has been enabled
(see Section 4.8.60 “--STRICT: Strict ANSI Conformance”) these specifiers must use
two leading underscore characters, (e.g., __reentrant).
TABLE 5-13:
STACK RELATED FUNCTION SPECIFIERS
Specifier
Allocation for Stack-based variables
compiled, nonreentrant
Always use the compiled stack; functions are non-reentrant.
software, reentrant
Use the software stack, if available; functions are reentrant.
You would only use these specifiers if the default allocation of a function’s stack-based
variables is unacceptable. These specifiers override any setting indicated using the
--STACK option, see Section 4.8.59 “--STACK: Specify Data Stack Type For Entire
Program”. If no specifier or --STACK option has been used, all functions are encoded
as non-reentrant and use the compiled stack.
The following shows an example of a function that will always be encoded as reentrant.
reentrant int setWriteMode(int mode)
{
if(mode != 3)
mode = 0;
return mode;
}
The reentrant specifier only has an effect if the target device supports a software
stack. In addition, not all functions allow reentrancy. Interrupt functions must always
use the compiled stack, but functions they call may use the software stack. Functions
encoded for baseline and mid-range devices always use the non-reentrant model and
the compiled stack.
Repeated use of the software (reentrant) specifier will increase substantially the
size of the software stack leading to possible overflow. The size of the software stack
is not accurately known at compile time, so the compiler cannot issue a warning if it is
likely to overwrite memory used for some other purpose. The stack may overwrite other
sections of the program in data memory, or memory used by something outside the
program, such as hardware or another independently-compiled application.
See Section 5.3.4.2 “Data Stacks” for device specific information relating to the data
stacks available on each device.
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5.8.2
External Functions
If a call is made to a function that is defined outside the program’s C source code (e.g.,
a function defined in a separate bootloader project), you will need to provide a declaration of the function so that the compiler knows how to encode the call. If this function
takes arguments or returns a value, you might need to define a symbol to represent the
memory locations used to store these values. See Section 5.8.6 “Function Parameters” and Section 5.8.7 “Function Return Values” to determine if a register or memory
locations are used in this transfer. If this symbol is not defined, the compiler will issue
an undefined symbol error. This error can be used to verify the name being used by the
compiler to encode the call, if required.
The required value can be determined from the map file of the external build. Look for
the symbol ?_funcName, where funcName is the name of the function defined externally. Define this symbol in your code that makes the call via a simple EQU directive in
assembler. For example, the following snippet of code could be placed in your C source
to allow you to call the function extReadFn() defined in another project:
#asm
GLOBAL ?_extReadFn
?_extReadFn EQU 0x20
#endasm
This defines the base address of the parameter area for extReadFn() to be 0x20.
It is not recommended to call the function indirectly by casting an integer to a function
pointer, but in such a circumstance, the compiler will use the value of the constant in
the symbol name; for example, calling a function at address 0x200 will require the definition of the symbol ?0x200 to be the location of the parameter/return value location for
the function. For example:
#asm
GLOBAL ?0x200
?0x200 EQU 0x55
#endasm
Note that the return value of a function (if used) shares the same locations assigned to
any parameters to that function and both use the same symbol.
If an external function uses the reentrant model, it will never use the W register for
parameter passing. All arguments are stored on the stack.
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C Language Features
5.8.3
Allocation of Executable Code
Code associated with functions is always placed in the program memory of the target
device.
On baseline and mid-range devices, the program memory is paged (compare: banking
used in the data memory space). This memory is still sequential (addresses are contiguous across a page boundary), but the paging means that any call or jump from code
in one page to a label in another must use a longer sequence of instructions to accomplish this. See your device data sheet for more information on the program memory and
instruction set.
PIC18 devices do not implement any program memory paging. The CALL and GOTO
instruction are two-word instructions and their destinations are not limited. The relative
branch instructions have a limited range, but this is not based on any paging boundaries.
The generated code associated with each function is initially placed in its own psect by
the compiler, see Section 5.15.1 “Program Sections”. These psects have names such
as textn, where n is a number, e.g., text98. However, psects can be merged later
in the compilation process so that more than one function can contribute to a psect.
When the program memory is paged, functions within the same psect can use a shorter
form of call and jump to labels so it is advantageous to merge the code for as many
functions into the same psect. These text psects are linked anywhere in the program
memory (see 5.10 Main, Runtime Startup and Reset).
If the size of a psect that holds the code associated with a function exceeds the size of
a page, it can be split by the assembler optimizer. A split psect will have a name of the
form textn_split_s. So, for example, if the text102 psect exceeds the size of a
page, it can be split into a text102_split_1 and a text102_split_2 psect. This
process is fully automatic, but you should be aware that if the code associated with a
function does become larger than one page in size, the efficiency of that code can drop
fractionally due to any longer jump and call instruction sequences being used to transfer control to code in other pages.
The base name of each psect category is tabulated below. A full list of all program-memory psects psect names are listed in Section 5.15.2.1 “Program Space
Psects”.
maintext
The generated code associated with the special function, main, is
placed in this psect. Some optimizations and features are not applied to
this psect.
textn
These psects (where n is a decimal number) contain all other
executable code that does not require a special link location.
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5.8.4
Changing the Default Function Allocation
You can change the default memory allocation of functions by either:
• Reserving memory locations
• Making functions absolute; or
• Placing functions in their own section and linking that section
If you don’t want functions (or any other program memory object) to be placed in certain
memory ranges, reserve those ranges using the --ROM memory option, see
Section 4.8.53 “--ROM: Adjust ROM Ranges”.
The easiest method to explicitly place individual functions at a known address is to
make them absolute functions by using the @ address construct in a similar fashion
to that used with absolute variables.
The compiler will issue a warning if code associated with an absolute function overlaps
with code from other absolute functions. No warning will be issued if the address of an
absolute object lies outside the memory of the device or outside the memory defined
by the linker classes. The compiler will not locate code associated with ordinary functions over the top of absolute functions.
The following example of an absolute function will place the function at address 400h:
int mach_status(int mode) @ 0x400
{
/* function body */
}
If this construct is used with interrupt functions, it will only affect the position of the code
associated with the interrupt function body. The interrupt context switch code associated with the interrupt vector will not be relocated. See also
Section 4.8.21 “--CODEOFFSET: Offset Program Code to Address”, for information on
how to move Reset and interrupt vector locations (which can be useful for designing
applications such as bootloaders).
The code generated for absolute functions is placed in a psect dedicated only to that
function. The psect name has the form shown below. A full list of all psect names are
listed in Section 5.15.2 “Compiler-Generated Psects”.
xxx_text
Defines the psect for a function that has been made absolute. xxx will
be the assembly symbol associated with the function, e.g., the absolute
function rv() would appear in the psect called _rv_text.
Functions can be allocated to a user-defined psect using the __section() specifier
(see Section 5.15.4 “Changing and Linking the Allocated Section”) so that this new
psect can then be linked at the required location. This method is the most flexible and
allows functions to be placed at a fixed address, after other psects, or anywhere in an
address range. As with absolute functions, when used with interrupt functions, it will
only affect the position of the interrupt function body. Never place functions into a psect
that is also used to hold non-executable objects, such as const objects, as this might
affect the ability to debug the functions.
Regardless of how a function is located, take care choosing its address. If possible,
place functions at either end of a program memory page to avoid fragmenting memory
and increasing the possibility of linker errors. Note that the compiler imposes pseudo
page boundaries on some PIC18 devices to work around published errata. Place functions in the first page, which contains the reset and interrupt code, rather than in pages
higher in memory, as this will assist the optimizations that merge psects.
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C Language Features
5.8.5
Function Size Limits
For all devices, the code generated for a regular function is limited only by the available
program memory. Functions can become larger than one page in size on paged
devices, but the longer call sequences to other pages decrease efficiency. See
5.8.3 Allocation of Executable Code for more details.
Interrupt functions (see Section 5.9.1 “Writing an Interrupt Service Routine”) however,
are limited to one page in size and cannot be split over multiple pages.
5.8.6
Function Parameters
MPLAB XC8 uses a fixed convention to pass arguments to a function. The method
used to pass the arguments depends on the size and number of arguments involved,
and on which stack model is used with the function.
Note:
5.8.6.1
The names “argument” and “parameter” are often used interchangeably,
but typically an argument is the actual value that is passed to the function
and a parameter is the variable defined by the function to store the
argument.
COMPILED STACK PARAMETERS
For functions using the non-reentrant model, the compiler will either pass arguments in
the W register, or in the called function’s parameter memory. If the first parameter is one
byte in size, it is passed in the W register. All other parameters are passed in the
parameter memory. This applies to basic types and to aggregate types, like structures.
The parameters are grouped along with the function’s auto variables in the parameter
memory, and are placed in the compiled stack. See Section 5.5.2.2.1 “Compiled Stack
Operation”, for detailed information on the compiled stack. The parameter variables will
be referenced as an offset from the symbol ?_function, where function is the
name of the function in which the parameter is defined (i.e., the function that is to be
called).
Unlike auto variables, parameter variables are allocated memory strictly in the order
in which they appear in the function’s prototype. This means that the parameters will
always be placed in the same memory bank. The auto variables for a function can be
allocated across multiple banks and in any order.
The parameters for variadic functions that take a variable argument list (defined using
an ellipsis in the prototype and which are called non-prototyped parameters) are placed
in the parameter memory, along with named parameters.
Take, for example, the following ANSI-style function.
void test(char a, int b);
The function test() will receive the parameter b in its function auto-parameter block
and a in the W register. A call to this function:
test(xyz, 8);
would generate code similar to:
MOVLW
MOVWF
CLRF
MOVF
CALL
08h
?_test
?_test+1
_xyz,w
(_test)
; move literal 0x8 into...
; the auto-parameter memory
; locations for the 16-bit parameter
; move xyz into the W register
In this example, the parameter b is held in the memory locations ?_test (LSB) and
?_test+1 (MSB).
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The compiler needs to take special action if more than one function can be indirectly
called via the same function pointer. Such would be the case in the following example,
where any of sp_ad, sp_sub or sp_null could be called via the pointer, fp.
int (*funcs[])(int, int) = {sp_add, sp_sub, sp_null};
int (*fp)(int, int);
fp = funcs[getOperation()];
result = fp(37, input);
In such a case, the compiler treats all three functions referenced in the array as being
“buddies”.
The parameter variable(s) to all buddy functions will be aligned in memory, i.e., they will
all reside at the same address(es). This way the compiler does not need to know
exactly which function is being called, which is often the case. The implication of this is
that a function cannot call (either directly or indirectly) any of its buddies. To do so would
corrupt the caller function’s parameter variables. An error will be issued if such a call is
attempted. This restriction does not apply to similar functions that use the software
stack (see Section 5.8.6.2 “Software Stack Parameters”).
The exact code used to call a function, or the code used to access parameters from
within a function, can always be examined in the assembly list file. See
Section 4.8.16 “--ASMLIST: Generate Assembler List Files” for the option that
generates this file. This is useful if you are writing an assembly routine that must call a
function with parameters, or accept arguments when it is called. The above example
does not consider data memory banking or program memory paging, which can require
additional instructions.
5.8.6.2
SOFTWARE STACK PARAMETERS
When a function uses the reentrant model, most arguments to that function will be
passed on the software stack. Parameters placed on the software stack are pushed in
the reverse order to which they were defined in the called function’s prototype. This is
unlike auto variables, which may be allocated memory in any order.
The W register is sometimes used for the first function argument if it is byte-sized and
the function uses the reentrant model. This will only take place for enhanced mid-range
devices and provided the function is not variadic and returns a value in btemp registers
(see Section 5.8.7.2 “Software Stack Return Values”Section 5.8.7.2 “Software Stack
Return Values”). If a reentrant function is external (see Section 5.8.2 “External Functions”), the W register will never be used to store any function arguments. The W register is never used by reentrant function arguments when compiling for PIC18 devices.
For variadic functions, which take a variable argument list (defined using an ellipsis in
the prototype), the unprototyped parameters are placed on the software stack, before
the named parameters. After all the function’s arguments have been pushed, the total
size of the non-prototyped parameters is pushed on to the stack (except if this function
has a return value which is returned on the stack). A maximum of 256 bytes of
non-prototyped parameters are permitted per function.
Recall that the function return address is not stored on this data stack. It is automatically stored on the hardware stack by the device, see Section 5.3.4.1 “Function Return
Address Stack”.
As there is no frame pointer, accessing function parameters, or other stack-based
objects, is not recommended in hand-written assembly code.
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C Language Features
5.8.7
Function Return Values
Values returned from functions are loaded into a register or placed on the stack used
by that function. The mechanism will depend on the function model used by the
function.
5.8.7.1
COMPILED STACK RETURN VALUES
For functions that use the non-reentrant model, return values are passed to the calling
function using the W register, or the function’s parameter memory. Re-using the memory
used by the parameters (which is no longer needed when the function is ready to
return) can reduce the code and data requirements for functions.
Eight-bit values are returned from a function in the W register. Values larger than a byte
are returned in the function’s parameter memory area, with the least significant word
(lsw) in the lowest memory location.
For example, the function:
int return_16(void)
{
return 0x1234;
}
will exit with the code similar to:
MOVLW 34h
MOVWF (?_return_16)
MOVLW 12h
MOVWF (?_return_16)+1
RETURN
For PIC18 targets returning values greater than 4 bytes in size, the address of the
parameter area is also placed in the FSR0 register.
Functions that return a bit do so using the carry bit of the STATUS register.
5.8.7.2
SOFTWARE STACK RETURN VALUES
Functions that use the reentrant model will pass values back to the calling function via
btemp variables, provided the value is 4 bytes or less in size. The W register will be
used to return byte-sized values for enhanced mid-range device functions that are not
variadic. For objects larger than 4 bytes in size, they are returned on the stack.
Reentrant PIC18 functions that return a bit do so using bit #0 in btemp0; other
devices use the carry bit in the STATUS register.
As there is no frame pointer, accessing the return value location, or other stack-based
objects, is not recommended in hand-written assembly code.
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5.8.8
Calling Functions
All 8-bit devices use a hardware stack for function return addresses. The depth of this
stack varies from device to device.
Typically, CALL assembly instructions are used to transfer control to a C function when
it is called. Each function call uses one level of stack. This stack level is freed after the
called routine executes a RETURN instruction. The stack usage grows if a called function calls another before returning. If the hardware stack overflows, function return
addresses will be destroyed and the code will eventually fail.
The stackcall suboption to the --RUNTIME option controls how the compiler
behaves when the compiler detects that the hardware stack is about to overflow due to
too many nested calls. See Section 4.8.54 “--RUNTIME: Specify Runtime
Environment” for details on this option. If this suboption is disabled (the default state),
where the depth of the stack will be exceeded by a call, the compiler will issue a
warning to indicate that this is the case. For PIC18 devices, this is the only way in which
calls are made, but for other 8-bit devices, the compiler can swap to an alternate way
of making calls, as detailed below.
If the stackcall suboption is enabled, the compiler will, instead of issuing a warning,
automatically swap to using a method that involves the use of a lookup table and which
does not require use of the hardware stack. This managed stack feature is not available
for PIC18 devices.
When the lookup method is being employed, a function is reached by a jump (not a call)
directly to its address. Before this is done the address of a special “return” instruction
(implemented as a jump instruction) is stored in a temporary location inside the called
function. This return instruction will be able to return control back to the calling function.
This means of calling functions allows functions to be nested deeply without overflowing the limited stack available on baseline and mid-range devices; however, it does
come at the expense of memory and program speed.
5.8.8.1
INDIRECT CALLS
When functions are called indirectly using a pointer, the compiler employs a variety of
techniques to call the intended function.
The PIC18 and enhanced mid-range devices all use the value in the function pointer to
load the program counter with the appropriate address. For PIC18 devices, the code
loads the TOS registers and executes a RETURN to perform the call. For enhanced
mid-range devices, the CALLW instruction is used. The number of functions that can be
called indirectly is limited only by the available memory of the device.
The baseline and mid-range devices all use a lookup table which is loaded with jump
instructions. The lookup table code is called and an offset is used to execute the appropriate jump in the table. The table increases in size as more functions are called indirectly, but cannot grow beyond 0xFF bytes in size. This places a limit on the number of
functions that can be called indirectly, and typically this limit is approximately 120 functions. Note that this limit does not affect the number of function pointers a program can
define, which are subject to the normal limitations of available memory on the device.
Indirect calls are not affected by the stackcall suboption to the --RUNTIME option
and the depth of indirect calls on baseline and mid-range devices are limited by the
hardware stack depth.
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5.8.8.2
BANK SELECTION WITHIN FUNCTIONS
A function can return with any RAM bank selected. See Section 5.5.1 “Address
Spaces” for more information on RAM banks.
The compiler tracks the bank selections made in the generated code associated with
each function, even across function calls to other functions. If the bank that is selected
when a function returns can be determined, the compiler will use this information to try
to remove redundant bank selection instructions which might otherwise be inserted into
the generated code.
The compiler will not be able to track the bank selected by routines written in assembly,
even if they are called from C code. The compiler will make no assumptions about the
selected bank when such routines return.
The “Tracked objects” section associated with each function, and which is shown in the
assembly list file, relates to this bank tracking mechanism. See 6.3 Assembly-Level
Optimizations for more information of the content of these files.
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5.9
INTERRUPTS
The MPLAB XC8 compiler incorporates features allowing interrupts to be fully handled
from C code. Interrupt functions are often called Interrupt Service Routines, or ISRs.
The operation of interrupts is handled differently by the different device families. Most
baseline devices do not implement interrupts at all; mid-range devices have one vector
location which is linked to all interrupt sources; some PIC18 devices have two independent interrupt vectors, one assigned to low-priority interrupt sources, the other to
high-priority sources; and, finally, some PIC18 devices implement an interrupt controller macro (ICM) module with support for one or more interrupt vector tables (IVTs),
which can be populated with the addresses of high- or low-priority interrupt functions.
The operation of the IVT on devices with an ICM can be disabled by clearing the
MVECEN configuration bit. The device is then said to be operating in legacy mode, and
will operate with dual priorities and dual vector locations. Although the vector table is
disabled in this mode, the vector locations are still relocatable. By default the vector
location will be 0x8 and 0x18, the same for regular PIC18 devices without the ICM.
The priority scheme implemented by PIC18 devices can also be disabled by clearing
the IPEN SFR bit. Such devices are then said to be operating in mid-range compatibility
mode and utilize only one interrupt vector, located at address 0x8.
The following are the general steps you need to follow to use interrupts. More detail
about these steps is provided in the sections that follow.
For enhanced baseline devices with interrupts, mid-range devices, or PIC18 devices
operating in mid-range compatibility mode:
• Write one interrupt function to process all interrupt sources.
• At the appropriate point in your main-line code, unmask the interrupt sources
required by setting their interrupt enable bit in the corresponding SFR.
• At the appropriate point in your code, enable the global interrupt enable to allow
interrupts to be generated.
For PIC18 devices without the ICM module, or PIC18 devices operating in legacy
mode:
• Plan the priorities to be assigned to each interrupt source. If the device is operating in legacy mode, determine the number of interrupt vector tables you require.
• Write one interrupt function to process each priority being used. You can define at
most two interrupt functions, or two interrupt functions per vector table for devices
operating in legacy mode. Consider implementing both interrupt functions to handle accidental triggering of unused interrupts, or use the --UNDEFINTS option to
provide a default action, see Section 4.8.63 “--UNDEFINTS: Program Unused
Interrupt Vectors”.
• Write code to assign the required priority to each interrupt source by writing the
appropriate bits in the SFRs.
• If the device is operating in legacy mode and if required, at the appropriate points
in your code, select the required IVT by writing to the IVTBASE registers. Never
write the IVTBASE registers if interrupts are enabled. The initial IVT can also be
selected by using the ivt sub-option to the --RUNTIME option, see
Section 4.8.54 “--RUNTIME: Specify Runtime Environment”.
• At the appropriate point in your code, enable the interrupt sources required.
• At the appropriate point in your code, enable the global interrupt enable.
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For devices using the Interrupt Controller Macro module:
• Plan the priorities associated with each interrupt source, and determine the number of interrupt vector tables you require.
• Write as many interrupt functions as required. For fast interrupt response times,
write a dedicated function for each interrupt source, although multiple sources can
be processed by one function, if desired. Consider one or more additional functions to handle accidental triggering of unused interrupt sources, or use the
--UNDEFINTS option to provide a default action, see
Section 4.8.63 “--UNDEFINTS: Program Unused Interrupt Vectors”.
• Write code to assign the required priority to each interrupt source by writing the
appropriate bits in the SFRs.
• If you are using more than one interrupt vector table, at the appropriate points in
your code, select the required IVT by writing to the IVTBASE registers. Never
write the IVTBASE registers if interrupts are enabled. The initial IVT can also be
selected by using the ivt sub-option to the --RUNTIME option, see
Section 4.8.54 “--RUNTIME: Specify Runtime Environment”.
• At the appropriate point in your code, enable the interrupt sources required.
• At the appropriate point in your code, enable the global interrupt enable.
Interrupt functions must not be called directly from C code (due to the different return
instruction that is used), but interrupt functions can call other functions, both
user-defined and library functions.
Interrupt code is the name given to any code that executes as a result of an interrupt
occurring, including functions called from the ISR and library code. Interrupt code completes at the point where the corresponding return from interrupt instruction is executed. This contrasts with main-line code, which, for a freestanding application, is
usually the main part of the program that executes after Reset.
5.9.1
Writing an Interrupt Service Routine
The prototype and content of an ISR will vary based on the target device and the project
being compiled. Observe the following guidelines when writing an ISR.
For devices that do not have the ICM module:
• Write each ISR prototype using either the interrupt or
__interrupt()specifiers.
• Use void as the return type and for the parameter specification.
• If your device supports interrupt priorities, with each function use the
low_priority or high_priority arguments to __interrupt(), or use
these as stand-alone specifiers with interrupt.
• Inside the ISR body, determine the source of the interrupt by checking the interrupt flag and the interrupt enable for each source that is to be processed, and
make the relevant interrupt code conditional on those being set.
For devices operating in legacy mode:
• Write each ISR prototype using either the interrupt or __interrupt() specifiers.
• Use void as the return type, and specify a parameter list of either void or one
char argument if you need to identify the interrupt source.1
• As arguments to the __interrupt() specifier in the ISR prototype, specify the
interrupt priority assigned to the function’s source, using low_priority or
high_priority; and optionally, specify the base address of the IVT in which to
1.It is recommended that the parameter list be set to void if you want to ensure portability with devices
that do not have the ICM module.
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place the function’s address, using base().1
• If the ISR processes more than one source, determine the source of the interrupt
from the function’s parameter, if specified, or by checking the interrupt flag and the
interrupt enable for each source that is to be processed.
For devices which are using the ICM module:
• Write each ISR prototype using only the __interrupt() specifier.
• Use void as the return type, and specify a parameter list of either void or one
char argument if you need to identify the interrupt source.
• As arguments to the __interrupt() specifier in the ISR prototype, specify
which sources each interrupt function should handle, using irq(); specify the
interrupt priority assigned to the function’s source, using low_priority or
high_priority; and optionally, specify the base address of the IVT in which to
place the function’s address, using base().
• If the ISR processes more than one source, determine the source of the interrupt
from the function’s parameter, if specified, or by checking the interrupt flag and the
interrupt enable for each source that is to be processed.
For all devices:
• Inside the ISR body, clear the relevant interrupt flag once the source has been
processed.
• Do not re-enable interrupts inside the ISR body. This is performed automatically
when the ISR returns.
• Keep the ISR as short and as simple as possible. Complex code will typically use
more registers that will increase the size of the context switch code.
When the xc8 option --STRICT is enabled, the interrupt specifier should be written as __interrupt. (Note the difference between this and the __interrupt()
specifier which uses brackets.)
If interrupt priorities are being used but an ISR does not specify a priority, it will default
to being high priority. It is recommended that you always specify the ISR priority to
ensure your code is readable.
If you use the __interrupt() specifier with a device that does not have the ICM,
supplying an irq() or base() argument will result in an error from the compiler. If you
use this specifier with a device that is configured for legacy mode, supplying an irq()
argument will result in an error from the compiler; however, you may continue to use
the base() argument if required.
Devices that have the ICM module identify each interrupt with a number. This number
can be specified with the irq() argument to __interrupt() if the vector table is
enabled, or you can use a compiler-defined symbol that equates to that number. You
can see a list of all interrupt numbers, symbols and descriptions by opening the files
pic_chipinfo.html or pic18_chipinfo.html in your favorite web browser, and
selecting your target device. Both these files are located in the DOCS directory under
your compiler’s installation directory.
Interrupt functions always use the non-reentrant function model. These functions
ignore any option or function specifier that might otherwise specify reentrancy.
The compiler will process interrupt functions differently to other functions, generating
code to save and restore any registers used by the function and using a special instruction to return.
1.It is recommended that the base address be left as the default if you want to ensure portability with
devices that do not have the ICM module.
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An example of an interrupt function written for code not using the IVT is shown below.
Notice that the interrupt function checks for the source of the interrupt by looking at the
interrupt enable bit (e.g., TMR0IE) and the interrupt flag bit (e.g., TMR0IF). Checking
the interrupt enable flag is required since interrupt flags associated with a peripheral
can be asserted even if the peripheral is not configured to generate an interrupt.
int tick_count;
void high_priority interrupt tcInt(void)
{
if (TMR0IE && TMR0IF) { // any timer 0 interrupts?
TMR0IF=0;
++tick_count;
}
if (TMR1IE && TMR1IF) { // any timer 1 interrupts?
TMR1IF=0;
tick_count += 100;
}
// process other interrupt sources here, if required
return;
}
Here is the same function code, split and modified for a device using vector tables. Note
that since only one source is associated each ISR, the interrupt code does not need to
determine the source of the interrupt and is therefor faster.
void __interrupt(irq(TMR0),high_priority) tc0Int(void)
{
TMR0IF=0;
++tick_count;
return;
}
void __interrupt(irq(TMR1),high_priority) tc1Int(void)
{
TMR1IF=0;
tick_count += 100;
return;
}
If you prefer to process multiple interrupt sources in one function, that can be done by
specifying more than one interrupt source in the irq() argument and using a function
parameter to hold the source number, such as in the following example.
void __interrupt(irq(TMR0,TMR1),high_priority) tInt(unsigned char src)
{
switch(src) {
case IRQ_TMR0:
TMR0IF=0;
++tick_count;
break;
case IRQ_TMR1:
TMR1IF=0;
tick_count += 100;
break;
}
return;
}
The ICM module will load the parameter, in this example, src, with the interrupt source
number when the interrupt occurs.
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The special interrupt source symbol, default, can be used to indicate that the ISR will
be linked to any interrupt vector not already explicitly specified using irq(). You can
also populate unused vector locations by using the --UNDEFINTS option, see
Section 4.8.63 “--UNDEFINTS: Program Unused Interrupt Vectors”.
By default, the interrupt vector table will be located at an address equal to the reset
value of the IVTBASE register, which is the legacy address of 0x8. The base() argument to __interrupt() can be used to specify a different table base address for that
function. This argument can take one or more comma-separated addresses. The base
address cannot be set to an address lower than the reset value of the IVTBASE register.
By default and if required, the compiler will initialize the IVTBASE register in the runtime
startup code. You can disable this functionality by turning off the ivt sub-option to the
--RUNTIME option, see Section 4.8.54 “--RUNTIME: Specify Runtime Environment”.
This sub-option also allows you to specify an initial address for this register, hence for
the initial vector table that will be used. If vectored interrupts are enabled but you do
not specify an address using this option, the vector table location will be set to the lowest table address used in the program, as specified by the base() arguments to
__interrupt().
If you use the base() argument to implement more than one table of interrupt vectors,
you must ensure that you allocate sufficient memory for each table. The compiler will
emit an error message if it detects an overlap of any interrupt vectors.
The following examples show the interrupt function prototypes for two ISRs which handle the timer 0 and 1 interrupt sources. These are configured to reside in independent
vector tables whose base addresses are 0x100 and 0x200. All other interrupt sources
are handled by a low-priority ISR, defIsr(), which appears in both vector tables. For
these ISRs to become active, the IVTBASE register must first be loaded either 0x100
or 0x200. Changing the address in this register allows you to select which vector table
is active.
void __interrupt(irq(TMR0,TMR1),base(0x100)) timerIsr(void)
{...}
void __interrupt(irq(TMR0,TMR1),base(0x200)) altTimerIsr(void)
{...}
void __interrupt(irq(default),base(0x100,0x200),low_priority)
defIsr(void)
{...}
5.9.2
Changing the Default Interrupt Function Allocation
Moving the code associated with interrupt functions is more difficult than that for
ordinary functions, as interrupt routines have entry points strictly defined by the device.
You can use the __section() specifier (see Section 5.15.4 “Changing and Linking
the Allocated Section”) if you want to move the interrupt function, but leave the interrupt
entry point at the default vector location.
To move the vector location, see Section 5.9.3 “Specifying the Interrupt Vector”.
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5.9.3
Specifying the Interrupt Vector
For devices that do not have the ICM module, the process of populating the interrupt
vector locations is fully automatic. The compiler links the interrupt code entry point to
the fixed vector locations. Typically the entry point code will be all or part of the code
that performs the interrupt context switch, and the body of the interrupt function will be
located elsewhere.
The location of these interrupt vectors cannot be changed at runtime, nor can you
change the code linked to the vector. That is, you cannot have alternate interrupt functions and select which will be active during program execution. An error will result if
there are more interrupt functions than interrupt vectors in a program.
For devices that have the ICM module, you have more freedom in how interrupt functions can be executed at runtime. When the IVT is enabled, these devices employ a
table of interrupt vectors. Each table entry can hold an address, which is read when the
corresponding interrupt is triggered, and the device will jump to that address. The vector table entry corresponding to an interrupt function is automatically completed by the
compiler, based on the information in the irq() and base() arguments to
__interrupt(), see Section 5.9.1 “Writing an Interrupt Service Routine”.
Although the addresses in the vector table cannot be changed at runtime, it is possible
to construct more than one table and have the device swap from one table to another.
Changing the active vector table is performed by changing the vector table base
address, which is stored in the IVTBASE registers. Since these registers cannot be
modified atomically, you must disable all interrupts before changing their content. The
following example shows how this might be performed in C code.
di();
// disable all interrupts
IVTBASEU = 0x0;
IVTBASEH = 0x2;
IVTBASEL = 0x0;
ei();
// re-enable interrupts
If a device with the ICM module is operating in legacy mode, the vector table is disabled
and the dual priority vectors employed by regular PIC18 devices are used. These vector locations will then hold an instruction, not an address, but unlike regular PIC18
devices, the device can still use the IVTBASE register to remap the vector locations to
any address and you can define an additional two interrupt functions for each table.
Do not confuse the operation of the IVTBASE register with the --CODEOFFSET option,
see Section 4.8.21 “--CODEOFFSET: Offset Program Code to Address”. This option
moves the entry point of the code associated with each interrupt but does not move the
vector location. When using this option, your program will not execute correctly until
you provide code which remaps the vector locations to the shifted interrupt entry points.
By comparison, adjusting the IVTBASE registers does not move the location of the
interrupt functions, but changes the vector locations. Your project code will continue to
operate normally after adjusting this register.
Interrupt vectors that have not been specified explicitly in the project can be assigned
a default function address by defining an interrupt function that uses default as its
irq() interrupt source, or assigned a default instruction by using the --UNDEFINTS
option, see Section 4.8.63 “--UNDEFINTS: Program Unused Interrupt Vectors”.
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5.9.4
Context Switching
5.9.4.1
CONTEXT SAVING ON INTERRUPTS
Some registers are automatically saved by the hardware when an interrupt occurs. Any
registers or compiler temporary objects used by the interrupt function, other than those
saved by the hardware, must be saved in code generated by the compiler. This is the
context save or context switch code.
See Section 5.7 “Register Usage” for the registers that must be saved and restored
either by hardware or software when an interrupt occurs.
Enhanced mid-range PIC devices save the W, STATUS, BSR and FSRx registers in
hardware (using special shadow registers) and hence these registers do not need to
be saved by software. The registers that might need to be saved by software are the
BTEMP registers1, compiler temporary locations that acts like registers.
Other mid-range PIC processors only save the entire PC (excluding the PCLATH register) when an interrupt occurs. The W, STATUS, FSR and PCLATH registers and any
BTEMP registers must be saved by code produced by the compiler, if required.
By default, the PIC18 high-priority interrupt function will utilize its internal shadow register to save the W, STATUS and BSR registers. For the low priority PIC18 interrupts,
or when the shadow registers cannot be used, all registers that has been used by the
interrupt code will be saved by software.
If the PIC18 device has the Interrupt Controller Macro module, it additionally saves the
FSRx, PCLATHx, and PRODx registers to shadow registers. All other used registers
are saved in software. Separate shadow registers are available for low- and
high-priority interrupts.
Note that for some older devices, the compiler will not use the shadow registers if compiling for the MPLAB ICD debugger, as the debugger itself utilizes these shadow registers. Some errata workarounds also prevent the use of the shadow registers see
Section 4.8.27 “--ERRATA: Specify Errata Workarounds”.
The compiler determines exactly which registers and objects are used by an interrupt
function, or any of the functions that it calls (based on the call graph generated by the
compiler), and saves these appropriately.
Assembly code placed in-line within the interrupt function is not scanned for register
usage. Thus, if you include in-line assembly code into an interrupt function, you can
have to add extra assembly code to save and restore any registers or locations used.
The same is true for any assembly routines called by the interrupt code.
If the W register is to be saved by the compiler, it can be stored to memory reserved in
the common RAM. If the device for which the code is written does not have common
memory, a byte is reserved in all RAM banks for the storage location for W register.
Most registers to be saved are allocated memory in the interrupt function’s auto area.
They can be treated like any other auto variable and use the same assembly symbols.
On mid-range devices, the W register is stored in BTEMP0, a pseudo register, see
Section 5.7 “Register Usage”.
If the software stack is in use, the context switch code will also initialize the stack
pointer register so it is accessing the area of the stack reserved for the interrupt. See
Section 5.5.2.2.2 “Software Stack Operation”, for more information on the software
stack.
1. These registers are memory locations allocated by the compiler, but are treated like registers for code
generation purposes. They are typically used when generating reentrant code.
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5.9.4.2
CONTEXT RESTORATION
Any objects saved by software are automatically restored by software before the interrupt function returns. The order of restoration is the reverse of that used when context
is saved.
If the software stack is in use, the context restoration code will also restore the stack
pointer register so that it is accessing the area of the stack used before the interrupt
occurred. See Section 5.5.2.2.2 “Software Stack Operation” for more information on
the software stack.
5.9.5
Enabling Interrupts
Two macros are available, once you have included <xc.h>, that control the masking
of all available interrupts. These macros are ei(), which enable or unmask all
interrupts, and di(), which disable or mask all interrupts.
On all devices, they affect the GIE bit in the INTCON register. These macros should be
used once the appropriate interrupt enable bits for the interrupts that are required in a
program have been enabled.
For example:
ADIE = 1;
PEIE = 1;
ei();
// ...
di();
Note:
5.9.6
// A/D interrupts will be used
// all peripheral interrupts are enabled
// enable all interrupts
// disable all interrupts
Never re-enable interrupts inside the interrupt function itself. Interrupts are
automatically re-enabled by hardware on execution of the RETFIE
instruction. Re-enabling interrupts inside an interrupt function can result in
code failure.
Accessing Objects From Interrupt Routines
Reading or writing globally accessible objects from interrupt routines can be unsafe.
The compiler will automatically mark as volatile any globally accessible variables
that are referenced in an interrupt routine; however, it is recommended that you explicitly mark these variables using the volatile specifier to ensure your code is portable,
see Section 5.4.7.2 “Volatile Type Qualifier”. The compiler will restrict the optimizations
performed on volatile objects, see Section 5.13 “Optimizations”.
Even when objects are marked as volatile, the compiler cannot guarantee that they
will be accessed atomically. This is particularly true of operations on multi-byte objects,
but, indeed, many operations on single-byte or bit objects cannot be performed in one
instruction.
Interrupts should be disabled around any code that modifies an object that is used by
interrupt functions, unless you can guarantee that the access is atomic. Check the
assembler list file to see the code generated for a statement, but remember that the
instructions can change as the program is developed, particularly if the optimizers are
enabled.
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5.9.7
Function Duplication
It is assumed by the compiler that an interrupt can occur at any time. Functions
encoded to use the compiled stack are not reentrant (see Section 5.5.2.2.1 “Compiled
Stack Operation”). So, if such a function is called by an interrupt function and by
main-line code, this could lead to code failure.
MPLAB XC8 compiler has a feature which will duplicate the generated code associated
with any function that uses the non-reentrant function model and which is called from
more than one call graph. There is one call graph associated with main-line code, and
one for each interrupt function, if defined. This allows reentrancy, but recursion is
still not possible, even if the function is duplicated.
Although the compiler can compile functions using a reentrant model, this feature is not
available with all devices; it can also be disabled using the --STACK option or the
nonreentrant specifier. See Section 5.5.2.2 “Auto Variable Allocation and access”,
for information on which function model is chosen for a function.
Main-line code will call the generated code for the original function, and the interrupt
will call that for the duplicated function. The duplication takes place only in the called
function’s generated code; there is no duplication of the C source code itself. The duplicated code and data uses different symbols and are allocated different memory, so are
fully independent.
This is similar to the process you would need to undertake if this feature was not implemented in the compiler: the C function could be duplicated by hand, given different
names and one called from main-line code; the other from the interrupt function. However, you would have to maintain both functions, and the code would need to be
reverted if it was ported to a compiler that did support reentrancy.
The compiler-generated duplicate will have unique identifiers for the assembly symbols
used within it. A duplicate identifier is identical to that used by the original code, but is
prefixed with i1. Duplicated PIC18 functions use the prefixes i1 and i2 for the lowand high-priority interrupts, respectively.
The generated code of the function called from main-line code will not use any prefixes
and the assembly names will be those normally used.
To illustrate, in a program the function main calls a function called input. This function
is also called by an interrupt function.
Examination of the assembly list file will show generated assembly code for both the
original and duplicate function. The assembly code corresponding to the C function
input() will use the assembly label _input. The corresponding label used by the
duplicate function will be i1_input. If the original function makes reference to a temporary variable, the generated code will use the symbol ??_input, compared to
??i1_input for the duplicate. Even local labels within the function’s generated code
will be duplicated in the same way. The call graph, in the assembly list file, will show
the calls made to both of these functions as if they were independently written. These
symbols will also be seen in the map file symbol table.
This feature allows the programmer to use the same source code with compilers that
use either reentrant or non-reentrant models. It does not handle cases where functions
are called recursively.
Code associated with library functions are duplicated in the same way. This also
applies to implicitly-called library routines, such as those that perform division or
floating-point operations associated with C operators.
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5.9.7.1
DISABLING DUPLICATION
The automatic duplication of the function can be inhibited by the use of a special
pragma.
This should only be done if the source code guarantees that an interrupt cannot occur
while the function is being called from any main-line code. Typically this would be
achieved by disabling interrupts before calling the function. It is not sufficient to disable
the interrupts inside the function after it has been called; if an interrupt occurs when
executing the function, the code can fail. See Section 5.9.5 “Enabling Interrupts”, for
more information on how interrupts can be disabled.
The pragma is:
#pragma interrupt_level 1
The pragma should be placed before the definition of the function that is not to be duplicated. The pragma will only affect the first function whose definition follows.
For example, if the function read is only ever called from main-line code when the
interrupts are disabled, then duplication of the function can be prevented if it is also
called from an interrupt function as follows.
#pragma interrupt_level 1
int read(char device)
{
// ...
}
In main-line code, this function would typically be called as follows:
di(); // turn off interrupts
read(IN_CH1);
ei(); // re-enable interrupts
The level value specified indicates for which interrupt the function will not be duplicated.
For mid-range devices, the level should always be 1; for PIC18 devices it can be 1 or
2 for the low- or high-priority interrupt functions, respectively. To disable duplication for
both interrupt priorities, use the pragma twice to specify both levels 1 and 2. The following function will not be duplicated if it is also called from the low- and high-priority
interrupt functions.
#pragma interrupt_level 1
#pragma interrupt_level 2
int timestwo(int a) {
return a * 2;
}
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5.10
MAIN, RUNTIME STARTUP AND RESET
The identifier main is special. You must always have one, and only one, function called
main() in your programs. This is the first function to execute in your program.
Code associated with main(); however, is not the first code to execute after Reset.
Additional code provided by the compiler, and known as the runtime startup code, is
executed first and is responsible for transferring control to the main() function. The
actions and control of this code is described in the following sections.
The compiler inserts special code at the end of main() which is executed if this function ends, i.e., a return statement inside main() is executed, or code execution
reaches the main()’s terminating right brace. This special code causes execution to
jump to address 0, the Reset vector for all 8-bit PIC devices. This essentially performs
a software Reset. Note that the state of registers after a software Reset can be different
to that after a hardware Reset.
It is recommended that the main() function does not end. Add a loop construct (such
as a while(1)) that will never terminate either around your code in main() or at the
end of your code, so that execution of the function will never terminate. For example,
void main(void)
{
// your code goes here
// finished that, now just wait for interrupts
while(1)
continue;
}
5.10.1
Runtime Startup Code
A C program requires certain objects to be initialized and the device to be in a particular
state before it can begin execution of its function main(). It is the job of the runtime
startup code to perform these tasks, specifically (and in no particular order):
• Initialization of global variables assigned a value when defined
• Clearing of non-initialized global variables
• General set up of registers or device state
Rather than the traditional method of linking in a generic, precompiled routine, MPLAB
XC8 uses a more efficient method which actually determines what runtime startup code
is required from the user’s program. Details of the files used and how the process can
be controlled are described in Section 4.4.2 “Startup and Initialization”. The following
sections detail exactly what the runtime startup code actually does.
The runtime startup code is executed before main(), but If you require any special initialization to be performed immediately after Reset, you should use the powerup
feature described later in Section 5.10.2 “The Powerup Routine”.
The runtime startup code assumes that the device has just come out of Reset and that
registers will be holding their power-on-reset value. If your program is an application
invoked by a bootloader that will have already executed, you might need to ensure that
data bank 0 is selected so that the runtime startup code executes correctly. This can
be achieved by placing the appropriate code sequence towards the end of the bootloader as in-line assembly.
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The following table lists the significant assembly labels used by the startup and
powerup code.
TABLE 5-14:
SIGNIFICANT ASSEMBLY LABELS
Label
Location
reset_vec
at the Reset vector location (0x0)
powerup
the beginning of the powerup routine, if used
start
the beginning of the runtime startup code, in startup.as
start_initialization
the beginning of the C initialization startup code, in the C
output code
5.10.1.1
INITIALIZATION OF OBJECTS
One task of the runtime startup code is to ensure that any initialized variables contain
their initial value before the program begins execution. Initialized variables are those
which are not auto objects and which are assigned an initial value in their definition. A
case in point would be input in the following example.
int input = 88;
void main(void) { ...
Such initialized objects have two components: their initial value (0x0088 in the above
example) stored in program memory (i.e., placed in the HEX file), and space for the
variable reserved in RAM it will reside and be accessed during program execution
(runtime).
The psects used for storing these components are described in
Section 5.15.2 “Compiler-Generated Psects”.
The runtime startup code will copy all the blocks of initial values from program memory
to RAM so that the variables will contain the correct values before main() is executed.
This action can be omitted by disabling the init suboption of --RUNTIME.
For example:
--RUNTIME=default,-init
With this part of the runtime startup code absent, the contents of initialized variables
will be unpredictable when the program begins execution. Code relying on variables
containing their initial value will fail.
Since auto objects are dynamically created, they require code to be positioned in the
function in which they are defined to perform their initialization. As a result, initialized
auto objects are not considered by the runtime startup code but are instead initialized
by assembly code in each function output.
Note:
Initialized auto variables can impact on code performance, particularly if
the objects are large in size. Consider using static local objects instead.
Variables with contents that should be preserved over a Reset, or even power off,
should be qualified with the persistent qualifier, see Section 5.4.8.1 “Persistent
Type Qualifier”. Such variables are linked at a different area of memory and are not
altered by the runtime startup code in any way.
If objects are initialized, the runtime startup code which performs this will destroy the
contents of the STATUS register. With some devices, the TO and PD bits in this register
are required to determine the cause of Reset. You can choose to have a copy of this
register taken so that it can later be examined. See Section 5.10.1.4 “STATUS Register
Preservation”, for more information.
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5.10.1.2
CLEARING OBJECTS
Those non-auto objects which are not initialized must be cleared before execution of
the program begins. This task is also performed by the runtime startup code.
Uninitialized variables are those which are not auto objects and which are not
assigned a value in their definition, for example output in the following example.
int output;
void main(void) {...
Such uninitialized objects will only require space to be reserved in RAM where they will
reside and be accessed during program execution (runtime).
The psects used for storing these components are described in
Section 5.15.2 “Compiler-Generated Psects” and typically have a name based on the
initialism “bss” (Block Started by Symbol).
The runtime startup code will clear all the memory location occupied by uninitialized
variables so they will contain zero before main() is executed.
Variables whose contents should be preserved over a Reset should be qualified with
persistent. See Section 5.4.8.1 “Persistent Type Qualifier” for more information.
Such variables are linked at a different area of memory and are not altered by the
runtime startup code in any way.
If objects are initialized, the runtime startup code that performs this will destroy the contents of the STATUS register. With some devices, the TO and PD bits in this register
are required to determine the cause of Reset. You can choose to have a copy of this
register taken so that it can later be examined. See Section 5.10.1.4 “STATUS Register
Preservation” for more information.
5.10.1.3
SETUP OF DEVICE STATE
Some PIC devices come with an oscillator calibration constant which is pre-programmed into the device’s program memory. This constant can be written to the
OSCCAL register to calibrate the internal RC oscillator, if required.
Code is automatically placed in the runtime startup code to load this calibration value,
see Section 5.3.11 “Oscillator Calibration Constants”.
If the software stack is being used by the program, the stack pointer (FSR1) is also
initialized by the runtime startup code. See Section 5.5.2.2.2 “Software Stack
Operation”.
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5.10.1.4
STATUS REGISTER PRESERVATION
The resetbits suboption of the --RUNTIME option (see 4.8.54 --RUNTIME: Specify
Runtime Environment) preserves some of the bits in the STATUS register before they
are clobbered by the remainder of the runtime startup code. The state of these bits can
be examined after recovering from a Reset condition to determine the cause of the
Reset. This option is not available when compiling for PIC18 devices.
The entire STATUS register is saved to an assembly variable ___resetbits. This
variable can be accessed from C code using the declaration:
extern unsigned char __resetbits;
The compiler defines the assembly symbols ___powerdown and ___timeout to represent the bit address of the Power-down and Time-out bits within the STATUS register
and can be used if required. These can be accessed from C code using the
declarations:
extern bit __powerdown;
extern bit __timeout;
In the above symbols, note that the C variables use two leading underscore characters,
and the assembly equivalent symbols use three. See Section 5.12.3.1 “Equivalent
Assembly Symbols” for more details of the mapping.
The compiler will detect the usage of the above symbols in your code and automatically
enable the resetbits suboption to the --RUNTIME option, if they are present. You
may choose to enable this feature manually, if desired.
See Section 4.9 “MPLAB X Option Equivalents” for use of this option in MPLAB X IDE.
5.10.2
The Powerup Routine
Some hardware configurations require special initialization, often within the first few
instruction cycles after Reset. To achieve this there is a hook to the Reset vector provided via the powerup routine.
This routine can be supplied in a user-defined assembler module that will be executed
immediately after Reset. A template powerup routine is provided in the file
powerup.as which is located in the sources directory of your compiler distribution.
Refer to comments in this file for more details.
The file should be copied to your working directory, modified and included into your
project as a source file. No special linker options or other code is required. The compiler
will detect if you have defined a powerup routine and will automatically use it, provided
the code in this routine is contained in a psect called powerup.
For correct operation (when using the default compiler-generated runtime startup
code), the code must end with a GOTO instruction to the label called start. As with all
user-defined assembly code, any code inside this file must take into consideration program memory paging and/or data memory banking, as well as any applicable errata
issues for the device you are using. The program’s entry point is already defined by the
runtime startup code, so this should not be specified in the powerup routine with the
END directive (if used). See Section 6.2.9.2 “END” for more information on this
assembler directive.
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5.11
LIBRARIES
5.11.1
Standard Libraries
The C standard libraries contain a standardized collection of functions, such as string,
math, and input/output routines. These functions are described in Appendix A. Library
Functions.
The libraries also contain C routines that are implicitly called by programs to perform
tasks such as floating-point operations, integer division, and type conversions. These
routines will not directly correspond to a function call in the source code.
5.11.1.1
THE PRINTF ROUTINE
The code associated with the printf function is not precompiled into the library files.
The printf() function is generated from a special C template file that is customized
after analysis of the user’s C code. See “PRINTF” for more information on using the
printf library function.
The template file is found in the lib directory of the compiler distribution and is called
doprnt.c. It contains a minimal implementation of the printf()function, but with the
more advanced features included as conditional code that can be utilized via
preprocessor macros that are defined when it (along with your code) is compiled.
The parser and code generator analyze the C source code, searching for calls to the
printf function. For all calls, the placeholders that were specified in the printf()
format strings are collated to produce a list of the desired functionality of the final function. The doprnt.c file is then preprocessed with the those macros specified by the
preliminary analysis, thus creating a custom printf() function for the project being
compiled. After parsing, the p-code output derived from doprnt.c is then combined
with the remainder of the C program in the final code generation step.
For example, if a program contains one call to printf(), which looks like:
printf(”input is: %d”);
The compiler will note that only the %d placeholder is used and the doprnt.c module
that is linked into the program will only handle printing of decimal integers.
Consider now that the code is changed and the following call to printf() is added.
printf(”output is %6d”);
Now the compiler will detect that additional code to handle printing decimal integers to
a specific width must be enabled as well. As more features of printf() are detected,
the size of the code generated for the printf() function will increase.
If the format string in a call to printf() is not a string literal as above, but is rather a
pointer to a string, then the compiler will not be able to reliably predict the printf()
usage, and so it forces a more complete version of printf() to be generated.
However, even without being able to scan printf() placeholders, the compiler can
still make certain assumptions regarding the usage of the function. In particular, the
compiler can look at the number and type of the additional arguments to printf()
(those following the format string expression) to determine which placeholders could
be valid. This enables the size and complexity of the generated printf() routine to
be kept to a minimum even in this case.
For example, if printf() was called as follows:
printf(myFormatString, 4, 6);
the compiler could determine that, for example, no floating-point placeholders are
required and omit these from being included in the printf() function output. As the
arguments after the format string are non-prototyped parameters, their type must
match that of the placeholders.
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No aspect of this operation is user-controllable (other than by adjusting the calls to
printf() ); however, the final printf() code can be observed by opening the
pre-processed output of doprnt.c, which will be called doprnt.pre in your working
directory, will show the C code that will actually be contained in the printf routine. As
this code has been pre-processed, indentation and comments will have been stripped
out as part of the normal actions taken by the C pre-processor.
5.11.2
User-Defined Libraries
User-defined libraries can be created and linked in with programs as required. Library
files are easier to manage and can result in faster compilation times, but must be compatible with the target device and options for a particular project. Several versions of a
library might need to be created to allow it to be used for different projects.
Although libraries can be created manually using the librarian, LIBR, (see
Section 8.2 “Librarian”) it is recommended that the be created directly from the
compiler using the --OUTPUT=lpp option, see Section 4.8.47 “--OUTPUT= type:
Specify Output File Type”.
Library files should be built with the --ASMLIST option disabled (see
Section 4.8.16 “--ASMLIST: Generate Assembler List Files”), and libraries should
never contain a main() function or interrupt functions, although any function called
from these may be included.
Once built, user-created libraries can be used on the command line along with the
source files or added to the Libraries folder in your MPLAB X IDE project.
As with Standard C library functions, any functions contained in user-defined libraries
should have a declaration added to a header file. It is common practice to create one
or more header files that are packaged with the library file. These header files are then
included into source code when required.
Library files specified on the command line are initially scanned for unresolved symbols; so, these files can redefine anything that is defined in the C standard libraries. See
also, Section 5.15.5 “Replacing Library Modules”.
5.11.3
Using Library Routines
Standard library functions (and any associated variables) will be automatically linked
into a program once they have been referenced in your source code. The use of a function from one library file will not include any other functions from that library. Only used
library functions will be linked into the program output and consume memory.
Your program will require declarations for any functions or symbols used from libraries.
These are contained in the standard C header (.h) files. Header files are not library
files and the two files types should not be confused. Library files contain precompiled
code, typically functions and variable definitions; the header files provide declarations
(as opposed to definitions) for functions, variables and types in the library files, as well
as other preprocessor macros.
In the following example, the definition for sqrt is not contained in source code, so the
compiler searches the libraries to find a definition there. Once found, it links in the
function for sqrt into your program.
#include <math.h>
// declare function prototype for sqrt
void main(void)
{
double i;
// sqrt referenced; sqrt will be linked in from library file
i = sqrt(23.5);
}
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5.12
MIXING C AND ASSEMBLY CODE
Assembly language code can be mixed with C code using two different techniques:
writing assembly code and placing it into a separate assembler module, or including it
as in-line assembly in a C module.
Note:
5.12.1
The more assembly code a project contains, the more difficult and time consuming will be its maintenance. As the project is developed, the compiler
can perform different optimizations as these are based on the entire program. Assembly code might need revision if the compiler is updated due to
differences in the way the updated compiler may work. These factors do not
affect code written in C.
If assembly must be added, it is preferable to write this as a self-contained
routine in a separate assembly module, rather than in-lining it in C code.
Integrating Assembly Language Modules
Entire functions can be coded in assembly language as separate .as or .asm source
files included into your project. They will be assembled and combined into the output
image using the linker.
By default, such modules are not optimized by the assembler optimizer. Optimization
can be enabled by using the --OPT option, see Section 4.8.45 “--OPT: Invoke
Compiler Optimizations”.
The following are guidelines that must be adhered to when writing a C-callable
assembly routine.
• Include the <xc.inc> assembly header file if you need to use SFRs in your code.
(If this is included using #include, enable the assembly source preprocessing
option, see Section 4.8.10 “-P: Preprocess Assembly Files”.)
• Select, or define, a suitable psect for the executable assembly code (See
Section 5.15.1 “Program Sections” for an introductory guide to these.)
• Select a name (label) for the routine using a leading underscore character
• Ensure that the routine’s label is globally accessible from other modules
• Select an appropriate C-equivalent prototype for the routine on which argument
passing can be modeled
• Limit arguments and return values to single byte-sized objects (Assembly routines
cannot define variables that reside in the compiled stack. Use global variables for
additional arguments.)
• Optionally, use a signature value to enable type checking when the function is
called
• Use bank selection instructions and mask addresses of any variable symbols
The following example goes through these steps for a mid-range device. The process
is the same for other devices. A mapping is performed on the names of all C functions
and non-static global variables. See Section 5.12.3 “Interaction between Assembly
and C Code” for a complete description of mappings between C and assembly
identifiers.
An assembly routine is required which can add an 8-bit quantity passed to the routine
with the contents of PORTB and return this as an 8-bit quantity.
Most compiler-generated executable code is placed in psects called textn, where n is
a number. (see Section 5.15.2 “Compiler-Generated Psects”). We will create our own
text psect based on the psect the compiler uses. Check the assembly list file to see how
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the text psects normally appear for assembly generated from C code. You can see a
psect, such as the following, generated by the code generator when compiling for baseline or mid-range devices.
PSECT text0,local,class=CODE,delta=2
See Section 6.2.9.3 “PSECT” for detailed information on the flags used with the PSECT
assembler directive. This psect is called text0. It is flagged local, which means that
it is distinct from other psects with the same name. This flag is not important in this
example and can be omitted, if required. It lives in the CODE class. This flag is important
as it means it will be automatically placed in the area of memory set aside for code.
With this flag in place, you do not need to adjust the default linker options to have the
psect correctly placed in memory. The last option, the delta value, is also very important. This indicates that the memory space in which the psect will be placed is word
addressable (value of 2). The PIC10/12/16 program memory space is word
addressable; the data space is byte addressable.
For PIC18 devices, program memory is byte addressable, but instructions must be
word-aligned, so you will see code such as the following.
PSECT text0,local,class=CODE,reloc=2
In this case, the delta value is 1 (which is the default setting), but the reloc (alignment) flag is set to 2, to ensure that the section starts on a word-aligned address.
We simply need to choose a different name, so we might choose the name mytext,
as the psect name in which we will place out routine, so we have for our mid-range
example:
PSECT mytext,local,class=CODE,delta=2
Let’s assume we would like to call this routine add in the C domain. In assembly
domain we must choose the name _add as this then maps to the C identifier add. If we
had chosen add as the assembly routine, then it could never be called from C code.
The name of the assembly routine is the label that we will place at the beginning of the
assembly code. The label we would use would look like this.
_add:
We need to be able to call this from other modules, so make this label globally
accessible, by using the GLOBAL assembler directive (Section 6.2.9.1 “GLOBAL”).
GLOBAL _add
By compiling a dummy C function with a similar prototype to this assembly routine, we
can determine the signature value. The C-equivalent prototype to this routine would
look like:
unsigned char add(unsigned char);
Check the assembly list file for the signature value of such a function. You will need to
turn the assembler optimizer off for this step, as the optimizer removes these values
from the assembly list file. Signature values are not mandatory, but allow for additional
type checking to be made by the linker. We determine that the following SIGNAT
directive (Section 6.2.9.22 “SIGNAT”) can be used.
SIGNAT _add,4217
The W register will be used for passing in the argument. See Section 5.8.6 “Function
Parameters”, for the convention used to pass parameters.
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Here is an example of the complete routine for a mid-range device which could be
placed into an assembly file and added to your project. The GLOBAL and SIGNAT directives do not generator code, and hence do not need to be inside the mytext psect,
although you can place them there if you prefer. The BANKSEL directive and BANKMASK
macro have been used to ensure that the correct bank was selected and that all
addresses are masked to the appropriate size.
#include <xc.inc>
GLOBAL _add
SIGNAT _add,4217
; make _add globally accessible
; tell the linker how it should be called
; everything following will be placed into the mytext psect
PSECT mytext,local,class=CODE,delta=2
; our routine to add to ints and return the result
_add:
; W is loaded by the calling function;
BANKSEL
(PORTB)
; select the bank of this object
ADDWF
BANKMASK(PORTB),w
; add parameter to port
; the result is already in the required location (W)so we can
; just return immediately
RETURN
To compile this, the assembly file must be preprocessed (as we have used the C preprocessor #include directive). See Section 4.8.10 “-P: Preprocess Assembly Files”.
To call an assembly routine from C code, a declaration for the routine must be provided.
This ensures that the compiler knows how to encode the function call in terms of
parameters and return values.
Here is a C code snippet that declares the operation of the assembler routine, then calls
the routine.
// declare the assembly routine so it can be correctly called
extern unsigned char add(unsigned char a);
void main(void) {
volatile unsigned char result;
result = add(5);
// call the assembly routine
}
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5.12.2
#asm, #endasm and asm()
Assembly instructions can also be directly embedded in-line into C code using the
directives #asm, #endasm or the statement asm();.
The #asm and #endasm directives are used to start and end a block of assembly
instructions which are to be embedded into the assembly output of the code generator.
The #asm block is not syntactically part of the C program, and thus it does not obey
normal C flow-of-control rules. This means that you should not use this form of in-line
assembly inside or near C constructs like if(), while(), and for() statements.
However, this is the easiest means of adding multiple assembly instructions. The #asm
and #endasm directives should appear on lines separate from the assembly code and
each other.
The asm(); statement is used to embed assembler instructions in-line with C code.
This form looks and behaves like a C statement. The instructions are placed in a string
inside what look like function call brackets, although no call takes place. Typically one
instruction is placed in the string, but you can specify more than one assembly instruction by separating the instructions with a \n character, e.g., asm(“MOVLW 55\nMOVWF
_x”);, Code will be more readable if you one place one instruction in each statement
and use multiple statements.
You can use the asm() form of in-line assembly at any point in the C source code as
it will correctly interact with all C flow-of-control structures. It is recommended, where
possible, to use the asm() form of in-line assembly because this can result in better
debugging.
The following example shows both methods used:
unsigned int var;
void main(void)
{
var = 1;
#asm
// like this...
BCF 0,3
BANKSEL(_var)
RLF (_var)&07fh
RLF (_var+1)&07fh
#endasm
// do it again the other way...
asm(“BCF 0,3”);
asm(“BANKSEL _var”);
asm(“RLF (_var)&07fh”);
asm(“RLF (_var+1)&07fh”);
}
In-line assembly code is never optimized by the assembler optimizer.
When using in-line assembler code, it is extremely important that you do not interact
with compiler-generated code. The code generator cannot scan the assembler code for
register usage; so, it will remain unaware if registers are clobbered or used by the
assembly code. However, the compiler will reset all bank tracking once it encounters
in-line assembly, so any SFRs or bits within SFRs that specify the current bank do not
need to be preserved by in-line assembly.
The registers used by the compiler are explained in Section 5.7 “Register Usage”. If
you are in doubt as to which registers are being used in surrounding code, compile your
program with the --ASMLIST option (see Section 4.8.15 “--ADDRQUAL: Set Compiler
Response to Memory Qualifiers”) and examine the assembler code generated by the
compiler. Remember that as the rest of the program changes, the registers and code
strategy used by the compiler will change as well.
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If a C function is called from main-line and interrupt code, it can be duplicated, see
Section 5.9.7 “Function Duplication”. Although a special prefix is used to ensure that
labels generated by the compiler are not duplicated, this does not apply to labels
defined in hand-written, in-line assembly code in C functions. Thus, you should not
define assembly labels in in-line assembly if the containing function might be
duplicated.
5.12.3
Interaction between Assembly and C Code
MPLAB XC8 C Compiler incorporates several features designed to allow C code to
obey requirements of user-defined assembly code. There are also precautions that
must be followed to ensure that assembly code does not interfere with the assembly
generated from C code.
The command-line driver ensures that all user-defined assembly files have been
processed first, before compilation of C source files begin. The driver is able to read
and analyze certain information in the relocatable object files and pass this information
to the code generator. This information is used to ensure the code generator takes into
account requirement of the assembly code. See Section 4.3.4 “Compilation of
Assembly Source” for further information on the compile sequence.
5.12.3.1
EQUIVALENT ASSEMBLY SYMBOLS
Most C symbols map to an corresponding assembly equivalent.
This mapping is such that an “ordinary” symbol defined in the assembly domain cannot
interfere with an “ordinary” symbol in the C domain. So for example, if the symbol main
is defined in the assembly domain, it is quite distinct to the main symbol used in C code
and they refer to different locations.
The name of a C function maps to an assembly label that will have the same name, but
with an underscore prepended. So the function main() will define an assembly label
_main.
Baseline PIC devices can use alternate assembly domain symbols for functions. The
destinations of call instructions on these devices are limited to the first half of a program
memory page. The compiler, thus, encodes functions in two parts, as illustrated in the
following example of a C function, add(), compiled for a baseline device.
entry__add:
LJMP
_add
The label entry__add is the function’s entry point and will always be located in a special psect linked in the first half of a program memory page. The code associated with
this label is simply a long jump (see Section 6.2.1.8 “Long Jumps and Calls”) to the
actual function body located elsewhere and identified by the label _add.
If you plan to call routines from assembly code, you must be aware of this limitation in
the device and the way the compiler works around it for C functions. Hand-written
assembly code should always call the entry__funcName label rather than the usual
assembly-equivalent function label.
If a C function is qualified static, and there is more than one static function in the
program with exactly the same name, the name of the first function will map to the usual
assembly symbol and the subsequent functions will map to a special symbol with the
form: fileName@functionName, where fileName is the name of the file that
contains the function, and functionName is the name of the function.
For example, a program contains the definition for two static functions, both called
add. One lives in the file main.c and the other in lcd.c. The first function will
generate an assembly label _add. The second will generate the label lcd@add.
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The name of a non-auto C variable also maps to an assembler label that will have the
same name, but with an underscore prepended. So the variable result will define an
assembly label: _result.
If the C variable is qualified static, there, again, is a chance that there could be more
than one variable in the program with exactly the same C name. The same rules apply
to non-local static variables as to static functions. The name of the first variable
will map to a symbol prepended with an underscore; the subsequent symbols will have
the form: fileName@variableName, where fileName is the name of the file that
contains the variable, and variableName is the name of the variable.
For example a program contains the definition for two static variables, both called
result. One lives in the file main.c and the other in lcd.c. The first function will
generate an assembly label _result. The second will generate the label
lcd@result.
If there is more than one local static variable (i.e., it is defined inside a function definition) then all the variables will have an assembly name of the form:
functionName@variableName.
So, if there is a static variable called output in the function read, and another
static variable with the same name defined in the function update, then in assembly
the symbols can be accessed using the symbols read@output and
update@output, respectively.
If there is more than one static function with the same name, and they contain definitions for static variables of the same name, then the assembly symbol used for
these variables will be of the form: fileName@functionName@variableName.
Having two static variables or functions with the same name is legal, but not
recommended as is easy to write code that accesses the wrong variable or calls the
wrong function.
Functions that use the reentrant model do not define any symbols that allow you to
access auto and parameter variables. You should not attempt to access these in
assembly code. Special symbols for auto and parameter variables are defined,
however, by functions that use the non-reentrant model. These symbols are described
in the following paragraphs.
To allow easy access to parameter and auto variables on the compiled stack, special
equates are defined which map a unique symbol to each variable. The symbol has the
form: functionName@variableName. Thus, if the function main defines an auto
variable called foobar, the symbol main@foobar can be used in assembly code to
access this C variable.
Function parameters use the same symbol mapping as auto variables. If a function
called read has a parameter called channel, then the assembly symbol for that
parameter is read@channel.
Function return values have no C identifier associated with them. The return value for
a function shares the same memory as that function’s parameter variables, if they are
present. The assembly symbol used for return values has the form ?_funcName,
where funcName is the name of the function returning the value. Thus, if a function,
getPort returns a value, it will be located the address held by the assembly symbol
?_getPort. If this return value is more than one byte in size, then an offset is added
to the symbol to access each byte, e.g., ?_getPort+1.
If the compiler creates temporary variables to hold intermediate results, these will
behave like auto variables. As there is no corresponding C variable, the assembly
symbol is based on the symbol that represents the auto block for the function plus an
offset. That symbol is ??_funcName, where funcName is the function in which the
symbol is being used. So for example, if the function main uses temporary variables,
they will be accessed as an offset from the symbol ??_main.
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5.12.3.2
ACCESSING REGISTERS FROM ASSEMBLY CODE
If writing separate assembly modules, SFR definitions will not automatically be accessible. The assembly header file <xc.inc> can be used to gain access to these register
definitions. Do not use this file for assembly in-line with C code as it will clash with
definitions in <xc.h>.
Include the file using the assembler’s INCLUDE directive, (see
Section 6.2.10.5 “INCLUDE”) or use the C preprocessor’s #include directive. If you
are using the latter method, make sure you compile with the -P driver option to
preprocess assembly files, see Section 4.8.10 “-P: Preprocess Assembly Files”.
The symbols for registers in this header file look similar to the identifiers used in the C
domain when including <xc.h>, e.g., PORTA, EECON1, etc. They are different symbols
in different domains, but will map to the same memory location.
Bits within registers are defined as the registerName,bitNumber. So, for example,
RA0 is defined as PORTA,0.
Here is an example of a mid-range assembly module that uses SFRs.
#include <xc.inc>
GLOBAL _setports
PSECT text,class=CODE,local,delta=2
_setports:
MOVLW
0xAA
BANKSEL
(PORTA)
MOVWF
BANKMASK(PORTA)
BANKSEL
(PORTB)
BSF
RB1
If you wish to access register definitions from assembly that is in-line with C code,
ensure that the <xc.h> header is included into the C module. Information included by
this header will define in-line assembly symbols as well as the usual symbols
accessible from C code.
The symbols used for register names will be the same as those defined by <xc.inc>.
So for example, the example given previously could be rewritten as in-line assembly as
follows.
#asm
MOVLW
BANKSEL
MOVWF
BANKSEL
BSF
#endasm
0xAA
(PORTA)
BANKMASK(PORTA)
(PORTB)
RB1
It is extremely important to ensure that you do not destroy the contents of registers that
are holding intermediate values of calculations. Some registers are used by the compiler and writing to these registers directly can result in code failure. The code generator does not detect when SFRs have changed as a result of assembly code that writes
to them. The list of registers used by the compiler and further information can be found
in Section 5.7 “Register Usage”.
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5.12.3.3
WRITING REENTRANT ASSEMBLY ROUTINES WITH PARAMETERS
Hand-written assembly routines for Enhanced midrange and PIC18 devices can be
written to use the software stack and be reentrantly called from C code. Such routines
can take parameters, return values, and define their own local objects, if required.
The following are the steps that need to be followed to create such routines.
• Declare the C prototype for the routine in C source code, choosing appropriate
parameter and return value types.
• In the assembly code, include the header <xc.inc>. Use the -p option if you use
#include to do this, see Section 4.8.10 “-P: Preprocess Assembly Files”.
• If required, define each auto-like variable using the stack_auto name,size
macro, where name can be any valid assembler identifier and size is the
variable’s size in bytes.
• If required, define each parameter using the macro stack_param name,size,
where name can be any valid assembly identifier and size is the variable’s size in
bytes. Parameters must be defined after autos and their order must match the
order in which they appear in the C prototype.
• Initialize the stack once using the macro alloc_stack before any instructions in
the routine.
• Immediately before each return instruction, restore the stack using the macro
restore_stack.
Write the routine in assembly in the usual way, remembering to place the code in a
psect with appropriate flags, and make the routine’s label globally accessible. Ensure
that the routine’s label can be referenced by C code by having it begin with an underscore character. See Section 3.4.7.2 “What Do I Need Other than Instructions in an
Assembly Source File?”, for more assistance. You can place more than one assembly
routine in the same module.
Each auto and parameter variable will be located at a unique offset to the stack pointer
(FSR1). If you follow the above guidelines, you can use the symbol name_offset,
which will be assigned the stack-pointer offset for the variable with name. These macros will exist for both auto and parameter variables. The examples that follow show how
you might use this offset to access stack-based objects.
If the routine returns a value, this must be placed into the location expected by the code
that calls the routine. For full details of C-callable routines, see
Section 5.8.7.2 “Software Stack Return Values”. But, to summarize, for objects 1 to 4
bytes in size, these must be loaded to temporary variables referenced as btemp, plus
an offset. This symbol is automatically linked into your routine if you use the macros
described above.
It is recommended that you do not arbitrarily adjust the stack pointer during the routine.
The symbols that define the offset for each auto and parameter variable assume that
the stack pointer has not been modified. However, if your assembly routine calls other
reentrant routines (regardless of whether they are defined in C or assembly code), you
must write the assembly code that pushes the arguments onto the stack, calls the
function, and then removes any return value from the stack.
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The following is an example of a reentrant assembly routine, _inc, written for a
PIC16F1xxx device. Its arguments and return value are described by the C prototype:
extern reentrant int inc(int foo);
This routine returns an int value that is one higher than the int argument that is
passed to it. It uses an auto variable, x, strictly for illustrative purposes.
#include <xc.inc>
PSECT
text2,local,class=CODE,delta=2
GLOBAL _inc
_inc:
stack_auto x,2
stack_param foo,2
alloc_stack
;an auto called 'x'; 2 bytes wide
;a parameter called 'foo'; 2 bytes wide
;x = foo + 1;
moviw [foo_offset+0]FSR1
addlw low(01h)
movwf btemp+0
moviw [foo_offset+1]FSR1
movwf btemp+1
movlw high(01h)
addwfc btemp+1,f
movf
btemp+0,w
movwi [x_offset+0]FSR1
movf
btemp+1,w
movwi [x_offset+1]FSR1
;return x;
moviw [x_offset+0]FSR1
movwf btemp+0
moviw [x_offset+1]FSR1
movwf btemp+1
restore_stack
return
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C Language Features
The following is an example of a reentrant assembly routine, _add, written for a PIC18
device. Its arguments and return value are described by the C prototype:
extern reentrant int add(int base, int index);
This routine returns an int value that is one higher than the int sum of the base and
index arguments that are passed to it. It uses the auto variables, tmp and result,
strictly for illustrative purposes.
#include <xc.inc>
psect
text1,class=CODE,space=0,reloc=2
GLOBAL _add
_add:
stack_auto tmp,2
stack_auto result,2
stack_param base,2
stack_param index,2
alloc_stack
;an auto called 'tmp'; 2 bytes wide
;an auto called 'result'; 2 bytes wide
;a parameter called 'base'; 2 bytes wide
;a parameter called 'index'; 2 bytes wide
;tmp = base + index;
movlw base_offset
movff PLUSW1,btemp+0
movlw base_offset+1
movff PLUSW1,btemp+1
movlw index_offset
movf
PLUSW1,w,c
addwf btemp+0,f,c
movlw index_offset+1
movf
PLUSW1,w,c
addwfc btemp+1,f,c
movlw tmp_offset
movff btemp+0,PLUSW1
movlw tmp_offset+1
movff btemp+1,PLUSW1
;result = tmp + 1;
movlw tmp_offset
movf
PLUSW1,w,c
addlw 1
movwf btemp+0,c
movlw tmp_offset+1
movff PLUSW1,btemp+1
movlw 0
addwfc btemp+1,f,c
movlw result_offset
movff btemp+0,PLUSW1
movlw result_offset+1
movff btemp+1,PLUSW1
;return result;
movlw result_offset
movff PLUSW1,btemp+0
movlw result_offset+1
movff PLUSW1,btemp+1
restore_stack
return
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MPLAB® XC8 C Compiler User’s Guide
5.12.3.4
ABSOLUTE PSECTS
Some of the information that is extracted from the initial compilation of assembly code
(see Section 4.3.4 “Compilation of Assembly Source”) relates to absolute psects, specifically psects defined using the abs and ovrld, PSECT flags. See
Section 6.2.9.3 “PSECT”, for information on this directive.
MPLAB XC8 is able to determine the address bounds of absolute psects and uses this
information to ensure that the code produced from C source by the code generator
does not use memory that is required by the assembly code. The code generator will
reserve any memory used by the assembly code prior to compiling C source.
Here is an example of how this works. Code contained in an assembly code file defines
a table that must be located at address 0x110 in the data space.
The source file contains:
PSECT lkuptbl,class=RAM,space=1,abs,ovrld
ORG 110h
lookup:
DS 20h
An absolute psect always starts at address 0. For such psects, you can specify a
non-zero starting address by using the ORG directive. See Section 6.2.9.4 “ORG”, for
important information on this directive.
When the project is compiled, this file is assembled and the resulting relocatable object
file is scanned for absolute psects. As this psect is flagged as being abs and ovrld,
the bounds and space of the psect will be noted – in this case, a memory range from
address 0x110 to 0x12F in memory space 1 is noted as being used. This information is
passed to the code generator to ensure that this address range is not used by the
assembly generated from the C code.
The linker handles all of the allocation into program memory, and so for hand-written
assembly, only the psects located in data memory need be defined in this way.
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C Language Features
5.12.3.5
UNDEFINED SYMBOLS
If a variable needs to be accessible from both assembly and C source code, it can be
defined in assembly code, if required, but it is easier to do so in C source code.
A problem could occur if there is a variable defined in C source, but is only ever referenced in the assembly code. In this case, the code generator would remove the variable believing it is unused. The linker would be unable to resolve the symbol referenced
by the assembly code and an error will result.
To work around this issue, MPLAB XC8 also searches assembly-derived object files for
symbols which are undefined. see Section 4.3.4 “Compilation of Assembly Source”.
These will be symbols that are used, but not defined, in assembly code. The code generator is informed of these symbols, and if they are encountered in the C code, the variable is automatically marked as being volatile. This action has the same effect as
qualifying the variable volatile in the source code, see Section 5.4.7.2 “Volatile
Type Qualifier”.
Global variables qualified as volatile will never be removed by the code generator,
even if they appear to be unused throughout the program.
For example, if a C program defines a global variable as follows:
int input;
but this variable is only ever used in assembly code. The assembly module(s) can
simply declare this symbol using the GLOBAL assembler directive, and then use it. The
following PIC18 example illustrates the assembly code accessing this variable.
GLOBAL _input, _raster
PSECT text,local,class=CODE,reloc=2
_raster:
MOVF
_input,w
The compiler knows of the mapping between the C symbol input, and the corresponding assembly symbol _input (see Section 5.12.3 “Interaction between Assembly and C Code”). In this instance the C variable input will not be removed and be
treated as if it was qualified volatile.
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5.13
OPTIMIZATIONS
The optimizations in the MPLAB XC8 compiler can be broadly grouped into C-level
optimizations performed on the source code before conversion into assembly, and
assembly-level optimizations performed on the assembly code generated by the
compiler.
The C-level optimizations are performed early during the code generation phase and
so have flow-on benefits: performing one optimization might mean that another can
then be applied. As these optimizations are applied before the debug information has
been produced, there is less impact on source-level debugging of programs.
Some of these optimizations are integral to the code generation process and so cannot
be disabled via an option. Suggestions as to how specific optimizations can be
defeated are given in the sections below.
In Standard mode, and particularly Free mode, some of these optimizations are disabled. (Hence if you want to disable as many optimizations as possible, run the compiler in the Free operating mode.) Some of the following optimizations are disabled or
modified by the selection of alternate optimization settings use the --OPT option, see
Section 4.8.45 “--OPT: Invoke Compiler Optimizations”.Even if they are enabled, optimizations can only be applied if very specific conditions are met. As a result, you might
see that some lines of code are optimized, but other similar lines are not.
The compiler operating mode determines the available optimizations, which are listed
in Table 5-15.
TABLE 5-15:
Mode
OPERATING MODE OPTIMIZATION SETS
Optimization sets available
Free
• Basic code generator and assembler optimizations
STD
• Basic code generator and assembler optimizations
• Whole program assembly optimizations
PRO
• Basic code generator and assembler optimizations
• Whole program assembly optimizations
• Procedural abstraction (assembly optimization)
• OCG C-level optimizations
Assembly-level optimizations are described in Section 6.3 “Assembly-Level
Optimizations”.
The basic code generator optimizations consist of the following.
• Whole-program analysis for object allocation into data banks without having to
use non-standard keywords or compiler directives.
• Simplification and folding of constant expressions to simplify expressions.
• Expression tree optimizations to ensure efficient assembly generation.
The following is a list of main OCG C-level optimizations, which simplify C expressions
or code produced from C expressions. These are applied across the entire program,
not just on a module-by-module basis.
• Tracking of the current data bank is performed by the compiler as it generates
assembly code. This allows the compiler to reduce the number of bank-selection
instructions generated.
• Strength reductions and expression transformations are applied to all expression trees before code is generated. This involves replacing expressions with
equivalent, but less costly, operations.
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C Language Features
• Unused variables in a program are removed. This applies to local as well as
global variables. Variables removed will not have memory reserved for them, will
not appear in any list or map file, and will not be present in debug information, and
so will not be observable in the debugger. A warning is produced if an unused
variable is encountered. Global objects qualified volatile will never be
removed, see Section 5.4.7.2 “Volatile Type Qualifier”. Taking the address of a
variable or referencing its assembly-domain symbol in hand-written assembly
code also constitutes use of the variable.
• Redundant assignments to variables not subsequently used are removed,
unless the variable is volatile. The assignment statement is completely
removed, as if it was never present in the original source code. No code will be
produced for it, and you will not be able to set a breakpoint on that line in the
debugger.
• Unused functions in a program are removed. A function is considered unused
if it is not called, directly or indirectly, nor has had its address taken. The entire
function is removed, as if it was never present in the original source code. No
code will be produced for it and you will not be able to set a breakpoint on any line
in the function in the debugger. Referencing a function’s assembly-domain symbol
in a separate hand-written assembly module will prevent it being removed. The
assembly code need only use the symbol in the GLOBAL directive.
• Unused return expressions in a function are removed. The return value is
considered unused if the result of all calls to that function discard the return value.
The code associated with calculation of the return value will be removed, and the
function will be encoded as if its return type was void.
• Propagation of constants is performed where the numerical contents of a variable can be determined. Variables which are not volatile and whose value can be
exactly determined are replaced with the numerical value. Uninitialized global
variables are assumed to contain zero prior to any assignment to them.
• Variables assigned a value before being read are not cleared or initialized by
the runtime startup code. Only non-auto variables are considered and if they are
assigned a value before other code can read their value, they are treated as being
persistent, see Section 5.4.8.1 “Persistent Type Qualifier”. All persistent
objects are not cleared by the runtime startup code, so this optimization will speed
execution of the program startup.
• Pointer sizes are optimized to suit the target objects they can access. The compiler tracks all assignments to pointer variables and keeps a list of targets each
pointer can access. As the memory space of each target is known, the size and
dereference method used can be customized for each pointer.
• Dereferencing pointers with only target can be replaced with direct access of
the target object. This applies to data and function pointers.
• Unreachable code is removed. C Statements that cannot be reached are
removed before they generate assembly code. This allows subsequent optimizations to be applied at the C level.
MPLAB X IDE or other IDEs can indicate incorrect values when watching variables if
optimizations hold a variable in a register. Try to use the ELF/DWARF debug file format
to minimize such occurrences. Check the assembly list file to see if registers are used
in the routine that is being debugged.
The assembly-level optimizations are described in Section 6.3 “Assembly-Level
Optimizations”.
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MPLAB® XC8 C Compiler User’s Guide
5.14
PREPROCESSING
All C source files are preprocessed before compilation. The preprocessed file is not
deleted after compilation. It will have a .pre extension and the same base name as
the source file from which it is derived.
The --PRE option can be used to preprocess and then stop the compilation. See
Section 4.8.50 “--PRE: Produce Preprocessed Source Code”.
Assembler files can also be preprocessed if the -P driver option is issued. See
Section 4.8.10 “-P: Preprocess Assembly Files”.
5.14.1
C Language Comments
The MPLAB XC8 C compiler supports standard ANSI C comments, as well as C++/C99
comments. Both types are illustrated in the following table. Extended characters are not
supported in comments and can cause splicing of source lines or other errors
Comment Syntax
Description
/* */
Standard ANSI C code comment.
Used for one or more lines.
/* This is line 1
This is line 2 */
//
C++/C99 code comment. Used for
one line only.
// This is line 1
// This is line 2
5.14.2
Example
Preprocessor Directives
MPLAB XC8 accepts several specialized preprocessor directives, in addition to the
standard directives. All of these are listed in Table 5-16 on the next page.
Macro expansion using arguments can use the # character to convert an argument to
a string, and the ## sequence to concatenate arguments. If two expressions are being
concatenated, consider using two macros in case either expression requires
substitution itself; so, for example
#define __paste1(a,b)
#define __paste(a,b)
a##b
__paste1(a,b)
lets you use the paste macro to concatenate two expressions that themselves can
require further expansion. Remember, also, that once a macro identifier has been
expanded, it will not be expanded again if it appears after concatenation.
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C Language Features
TABLE 5-16:
PREPROCESSOR DIRECTIVES
Directive
Meaning
Example
#
preprocessor null directive, do nothing
#
#advisory
generate an advisory message
#advisory TODO: I need to
finish this
#assert
generate error if condition false
#assert SIZE > 10
#asm
signifies the beginning of in-line
assembly
#asm MOVLW FFh
#endasm
#define
define preprocessor macro
#define SIZE 5
#define FLAG
#define add(a,b) ((a)+(b))
#elif
short for #else #if
see #ifdef
#else
conditionally include source lines
see #if
#endasm
terminate in-line assembly
see #asm
#endif
terminate conditional source inclusion
see #if
#error
generate an error message
#error Size too big
#if
include source lines if constant
expression true
#if SIZE < 10
c = process(10)
#else
skip();
#endif
#ifdef
include source lines if preprocessor
symbol defined
#ifdef FLAG
do_loop();
#elif SIZE == 5
skip_loop();
#endif
#ifndef
include source lines if preprocessor
symbol not defined
#ifndef FLAG
jump();
#endif
#include
include text file into source
#include <stdio.h>
#include “project.h”
#info
alias for #advisory (see above)
#info I wrote this bit
#line
specify line number and filename for
listing
#line 3 final
#nn
(where nn is a number) short for
#line nn
#20
#pragma
compiler specific options
Refer to Section 5.14.4 “Pragma
Directives”.
#undef
undefines preprocessor symbol
#undef FLAG
#warning
generate a warning message
#warning Length not set
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Preprocessor macro replacement expressions are textual and do not utilize types.
Unless part of the controlling expression to the inclusion directives (discussed below),
macros are not evaluated by the preprocessor. Once macros have been expanded and
preprocessing is complete, the expansion forms a C expression which is evaluated by
the code generator along with other C code. Tokens within the expanded C expression
inherit a type, and values are then subject to integral promotion and type conversion in
the usual way.
If a macro is part of the controlling expression to a conditional inclusion directive (#if
or #elif), the macro must be evaluated by the preprocessor. The result of this evaluation is often different to the C-domain result for the same sequence. The preprocessor
assigns sizes to literal values in the controlling expression that are equal to the largest
integer size accepted by the compiler. For the MPLAB XC8 C compiler, this size is 32
bits.
The following code does not work as you might expect it to work. The preprocessor will
evaluate MAX to be the result of a 32-bit multiplication, 0xF4240. However, the definition of the long int variable, max, will be assigned the value 0x4240 (since the constant 1000 has a signed int type, and, therefore, the C-domain multiplication will
also be performed using a 16-bit signed int type).
#define MAX 1000*1000
...
#if MAX > INT16_MAX
// evaluation of MAX by preprocessor
long int max = MAX;
// evaluation of MAX by C compiler
#else
int max = MAX;
// evaluation of MAX by C compiler
#endif
Overflow in the C domain can be avoided by using a constant suffix in the macro (see
Section 5.4.6 “Constant Types and Formats”). For example, an L after a number in a
macro expansion indicates it should be interpreted by the C compiler as a long, but
this suffix does not affect how the preprocessor interprets the value, if it needs to
evaluate it.
So, for example:
#define MAX 1000*1000L
will evaluate to 0xF4240 in C expressions.
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C Language Features
5.14.3
Predefined Macros
The compiler drivers define certain symbols to the preprocessor, allowing conditional
compilation based on chip type, etc. The symbols listed in Table 5-17 show the more
common symbols defined by the drivers.
Each symbol, if defined, is equated to 1 (unless otherwise stated).
TABLE 5-17:
PREDEFINED MACROS
Symbol
When set
__CHIPNAME and
__CHIPNAME__
when chip selected
to indicate the specific chip type selected, e.g.,
__16F877
__DATABANK
if eeprom or flash memory
implemented
identifies which bank the EEDATA/PMDATA register is
found
__DATE__
always
to indicate the current date, e.g., May 21 2004
__EXTMEM
if device has external memory to indicate the size of external memory, if applicable
__FILE__
always
to indicate this source file being preprocessed
__FLASHTYPE
if non-PIC18 flash memory is
implemented
to indicate the type of flash memory employed by the
target device, see _PROGMEM below
__LINE__
always
to indicate this source line number
if PIC
__J_PART
Usage
® 18 J device
indicates device is a ‘J’ series part
®
(where X is 2, or 3)
assigned 1 to indicate that the code is generated for
use with the Microchip MPLAB ICD 2, or ICD 3
__MPLAB_ICDX__
if compiling for MPLAB
ICD2/3 debugger
__MPLAB_PICKITX__
if compiling for MPLAB
PICkit™ 2/3
assigned 1 to indicate that the code is generated for
use with the Microchip MPLAB PICkit 2 or PICkit 3
__MPLAB_REALICE__
if compiling for MPLAB REAL
ICE™ In-Circuit Emulator
assigned 1 to indicate that the code is generated for
use with the Microchip MPLAB REAL ICE
__OPTIMIZE_SPEED__
if --OPT set to speed
to indicate speed optimizations in effect
__OPTIMIZE_SPACE__
and __OPTIMIZE_SIZE__
if --OPT set to space
to indicate space optimizations in effect
__OPTIMIZE_NONE__
if --OPT set to none
to indicate no optimizations in effect
__OPTIM_FLAGS
always
to indicate the optimizations in effect (see text following this table)
__PICCPRO__ and
__PICC__
if any non-PIC18 device
to indicate the target device is any PIC10/12/14/16
__PICC18__
if not in C18 compatibility
mode
to indicate non-C18 compatibility mode operation
__RESETBITS_ADDR
if --RUNTIME option request
STATUS register save
indicates the address at which the STATUS register
will be saved
__STACK
always
assigned with __STACK_COMPILED(1),
__STACK_HYBRID(2) or __STACK_REENTRANT(4) to
indicate the global stack setting: compiled, hybrid or
software, respectively
__STRICT
if the --STRICT option is
enabled
to indicate that strict ANSI compliance of keywords is
in force
__TIME__
always
to indicate the current time, e.g., 08:06:31
Note 1:
2:
To determine the family macro relevant to your device, look for the FAMILY field in the picc-18.ini
file in the compiler's DAT directory.
These macros relate only to Flash program memory. They do not convey any information regarding Flash
data memory.
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TABLE 5-17:
PREDEFINED MACROS (CONTINUED)
Symbol
When set
Usage
__TRADITIONAL18__
if PIC18 device
to indicate the non-extended instruction set is
selected
__XC
always
indicates MPLAB XC compiler for Microchip is in use
__XC8
always
indicates MPLAB XC compiler for Microchip 8-bit
devices is in use
__XC8_VERSION
always
to indicate the compiler’s version number multiplied
by 1000, e.g., v1.00 will be represented by 1000
_CHIPNAME
when chip selected
to indicate the specific chip type selected, e.g.,
_16F877
_BANKBITS_
if non-PIC18 device
assigned 0, 1 or 2 to indicate 1, 2 or 4 available banks
or RAM
_BANKCOUNT_
if non-PIC18 device
to indicate the number of banks of data memory
implemented
_COMMON_
if common RAM present
to indicate whether device has common RAM area
_EEPROMSIZE
if non-PIC18 device
to indicate how many bytes of EEPROM are available
_EEPROM_INT
any non-PIC18 device
value is one of 2 (_NVMREG_INT), 1 (_EEREG_INT),
or 0 (_NOREG_INT) to indicate the device uses the
RVMREG, EEREG, or no register interface to access
EEPROM.
_ERRATA_TYPES
always
indicates the errata workarounds being applied, see
--ERRATA option, Section 4.8.27 “--ERRATA:
Specify Errata Workarounds”
(1)
if PIC18 device
indicates PIC18 family
_FLASH_ERASE_SIZE (2)
always
size of flash erase block
(2)
always
size of flash write block
_GPRBITS_
if non-PIC18 device
assigned 0, 1 or 2 to indicate 1, 2 or 4 available banks
or general purpose RAM.
_GPRCOUNT_
if non-PIC18 device
assigned a value which indicates the number of banks
that contain general-purpose RAM
_HAS_OSCVAL_
if the target device has an
oscillator calibration register
to indicate target device can require oscillator
calibration
_HTC_EDITION_
always
indicates which compiler, PRO, Standard or Free, is in
use
Values of 2, 1 or 0 are assigned, respectively.
_HTC_VER_MAJOR_
_HTC_VER_MINOR_
always
to indicate the whole or decimal component,
respectively, of the compiler’s version number
_HTC_VER_PATCH_
_HTC_VER_PLVL_
always
to indicate the patch level of the compiler’s version
number
_MPC_
always
indicates compiling for Microchip PIC MCU family
_OMNI_CODE_
always
indicates compiling using an OCG compiler
_PIC12
if baseline (12-bit instruction)
device
to indicate selected device is a baseline PIC device
_PIC12E
if enhanced baseline (12-bit
instruction) device
to indicate selected device is an enhanced baseline
PIC device
_FAMILY_FAMILY_
_FLASH_WRITE_SIZE
Note 1:
2:
To determine the family macro relevant to your device, look for the FAMILY field in the picc-18.ini
file in the compiler's DAT directory.
These macros relate only to Flash program memory. They do not convey any information regarding Flash
data memory.
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C Language Features
TABLE 5-17:
PREDEFINED MACROS (CONTINUED)
Symbol
When set
Usage
_PIC14
if mid-range (14-bit instruction) device
to indicate selected device is a mid-range PIC device
_PIC14E
if Enhanced mid-range (14-bit to indicate selected device is an Enhanced mid-range
instruction) device
PIC device
_PIC14EX
if extended-bank Enhanced
mid-range (14-bit instruction)
device
to indicate selected device is an extended-bank
Enhanced mid-range PIC device
_PIC18
if PIC18 (16-bit instruction)
device
to indicate selected device is an PIC18 device
_PROGMEM_
if compiling for mid-range
device with flash memory
to indicate the type of flash memory employed by the
target device:
values 0xFF (unknown)
0xF0 (none)
0 (read-only)
1 (word write with auto erase)
2 (block write with auto erase)
3 (block write with manual erase)
_RAMSIZE
if PIC18 device
to indicate how many bytes of data memory are
available
_ROMSIZE
always
to indicate how much program memory is available
(byte units for PIC18 devices; words for other devices)
ERRATA_4000_BOUNDARY
if the ERRATA_4000 applies
to indicate that the 4000 word boundary errata is
applied
HI_TECH_C
always
to indicate that the C language variety is HI-TECH C
compatible
MPLAB_ICD
if compiling for MPLAB ICD
2/3 debugger
assigned 2 to indicate that the code is generated for
use with the Microchip MPLAB ICD 2
assigned 3 for the MPLAB ICD 3
Note 1:
2:
To determine the family macro relevant to your device, look for the FAMILY field in the picc-18.ini
file in the compiler's DAT directory.
These macros relate only to Flash program memory. They do not convey any information regarding Flash
data memory.
The compiler-defined macros shown in Table 5-18 can be used as bitwise AND masks
to determine the value held by __OPTIM_FLAGS, hence the optimizations used.
TABLE 5-18:
OPTIMIZATION FLAGS
Macro
Value
Meaning
__OPTIM_NONE
0x0
no optimizations applied (on equality)
__OPTIM_ASM
0x1
assembler optimizations on C code
__OPTIM_ASMFILE
0x2
assembler optimizations on assembly source code
__OPTIM_SPEED
0x20000
optimized for speed
__OPTIM_SPACE
0x40000
optimized for size
__OPTIM_SIZE
0x40000
optimized for size
__OPTIM_DEBUG
0x80000
optimized for accurate debug
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5.14.4
Pragma Directives
There are certain compile-time directives that can be used to modify the behavior of the
compiler. These are implemented through the use of the ANSI standard #pragma
facility. The format of a pragma is:
#pragma keyword options
where keyword is one of a set of keywords, some of which are followed by certain
options. A list of the keywords is given in Table 5-19. Those keywords not discussed
elsewhere are detailed below.
TABLE 5-19:
PRAGMA DIRECTIVES
Directive
Meaning
Example
addrqual
specify action of qualifiers
#pragma addrqual require
config
specify configuration bits
#pragma config WDT=ON
inline
inline function if possible
#pragma inline(getPort)
intrinsic
specify function is inline
#pragma intrinsic(_delay)
interrupt_level
allow call from interrupt and #pragma interrupt_level 1
main-line code
pack
specify structure packing
#pragma pack 1
printf_check
enable printf-style format
string checking
#pragma
printf_check(printf) const
psect
rename compiler-generated psect
#pragma psect
nvBANK0=my_nvram
regsused
specify registers used by
function
#pragma regsused myFunc
wreg,fsr
switch
specify code generation for #pragma switch direct
switch statements
warning
control messaging
parameters
5.14.4.1
#pragma warning disable
299,407
THE #PRAGMA ADDRQUAL DIRECTIVE
This directive allows you to control the compiler’s response to non-standard memory
qualifiers. This pragma is an in-code equivalent to the --ADDRQUAL option and both
use the same arguments, see Section 4.8.15 “--ADDRQUAL: Set Compiler Response
to Memory Qualifiers”.
The pragma has an effect over the entire C program and should be issued once, if
required. If the pragma is issued more than once, the last pragma determines the
compiler’s response.
For example:
#pragma addrqual require
bank2 int foobar;
5.14.4.2
THE #PRAGMA CONFIG DIRECTIVE
This directive allows the device Configuration bits to be specified for PIC18 target
devices. See Section 5.3.5 “Configuration Bit Access” for full details.
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5.14.4.3
THE #PRAGMA INLINE DIRECTIVE
The #pragma inline directive indicates to the compiler that calls to the specified
function should be as fast as possible. This pragma has the same effect as using the
inline function specifier.
5.14.4.4
THE #PRAGMA INTRINSIC DIRECTIVE
The #pragma intrinsic directive is used to indicate to the compiler that a function
will be inlined intrinsically by the compiler. The directive is only usable with special functions that the code generator will expand internally, e.g., the _delay function. Such
functions do not have corresponding source code and are handled specially by the
compiler.
Note:
Use of this pragma with a user-defined function does not mean that the
function will be inlined, and an error will result. See the inline function
specifier for that operation, in Section 5.8.1.2 “Inline Specifier”.
You should not attempt to redefine an existing library function that uses the intrinsic
pragma. If you need to develop your own version of such a routine, it must not use the
same name as the intrinsic function. For example, if you need to develop your own version of memcpy(), give this a unique name, such as sp_memcpy(). Check the
standard header files to determine which library functions use this pragma.
5.14.4.5
THE #PRAGMA INTERRUPT_LEVEL DIRECTIVE
The #pragma interrupt_level directive can be used to prevent duplication of
functions called from main-line and interrupt code. For more information, see
Section 5.9.7.1 “Disabling Duplication”.
5.14.4.6
THE #PRAGMA PACK DIRECTIVE
All 8-bit PIC devices can only perform byte accesses to memory and so do not require
any alignment of memory objects within structures. This pragma will have no effect
when used.
5.14.4.7
THE #PRAGMA PRINTF_CHECK DIRECTIVE
Certain library functions accept a format string followed by a variable number of arguments in the manner of printf(). Although the format string is interpreted at runtime,
it can be compile-time checked for consistency with the remaining arguments.
This directive enables this checking for the named function, for example the system
header file <stdio.h> includes the directive:
#pragma printf_check(printf) const
to enable this checking for printf(). You can also use this for any user-defined
function that accepts printf -style format strings.
The qualifier following the function name is to allow automatic conversion of pointers in
variable argument lists. The above example would cast any pointers to strings in RAM
to be pointers of the type (const char *).
Note that the warning level must be set to -1 or below for this option to have any visible
effect. See Section 4.8.65 “--WARN: Set Warning Level”.
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5.14.4.8
THE #PRAGMA PSECT DIRECTIVE
The #pragma psect was used to redirect objects and functions to a new psect (section). It has been replaced by the __section() specifier (see
Section 5.15.4 “Changing and Linking the Allocated Section”), which not only performs
the same task, but is easier to use, and allows a greater flexibility because the new
psects can be linked. It is recommended you always use the __section() specifier
to location variables and function in unique psects.
The general form of this pragma looks like:
#pragma psect standardPsect=newPsect
and instructs the code generator that anything that would normally appear in the standard psect standardPsect, will now appear in a new psect called newPsect. This
psect will be identical to standardPsect in terms of its flags and attributes; however,
it will have a unique name. Thus, you can explicitly position this new psect without
affecting the placement of anything in the original psect.
If the name of the standard psect that is being redirected contains a counter (e.g.,
text0, text1, text2, etc.), the placeholder %%u should be used in the name of the
psect at the position of the counter, e.g., text%%u.
5.14.4.9
THE #PRAGMA REGSUSED DIRECTIVE
The #pragma regsused directive allows the programmer to indicate register usage
for functions that will not be “seen” by the code generator; for example, if they were written in assembly code. It has no effect when used with functions defined in C code, but
in those cases the register usage of these functions can be accurately determined by
the compiler, and the pragma is not required.
The compiler can determine those registers and objects that need to be saved by an
interrupt function. This pragma could be used, for example, to allow the compiler to
ensure that interrupt functions also save any registers used by assembly routines
called by that function.
The general form of the pragma is:
#pragma regsused routineName registerList
where routineName is the C-equivalent name of the function or routine whose register usage is being defined, and registerList is a space-separated list of registers’
names, as shown in Table 5-12.
Those registers that are not listed are assumed to be unused by the function or routine.
The code generator can use any of these registers to hold values across a function call.
So, if the routine does in fact use these registers, unreliable program execution can
happen.
The register names are not case sensitive and a warning will be produced if the register
name is not recognized. A blank list indicates that the specified function or routine uses
no registers. If this pragma is not used, the compiler will assume that the external
function uses all registers.
For example, a routine called _search is written in PIC18 assembly code. In the C
source, we can write:
extern void search(void);
#pragma regsused search wreg status fsr0
to indicate that this routine used the W register, STATUS and FSR0. Here, FSR0
expands to both FSR0L and FSR0H. These could be listed individually, if required.
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C Language Features
5.14.4.10 THE #PRAGMA SWITCH DIRECTIVE
Normally, the compiler chooses how switch statements will be encoded to produce
the smallest possible code size. The #pragma switch directive can be used to force
the compiler to use a different coding strategy.
The general form of the switch pragma is:
#pragma switch switchType
where switchType is one of the available switch types (the only switch type currently
implemented for PIC18 devices is space) that are listed in Table 5-20.
TABLE 5-20:
SWITCH TYPES
Switch Type
Description
speed
use the fastest switch method
space
use the smallest code size method
time
use a fixed delay switch method
auto
use smallest code size method (default)
direct (deprecated)
use a fixed delay switch method
simple (deprecated)
sequential xor method
Specifying the time option to the #pragma switch directive forces the compiler to
generate a table look-up style switch method. The time taken to execute each case
is the same, so this is useful where timing is an issue, e.g., state machines.
This pragma affects all subsequent code.
The auto option can be used to revert to the default behavior.
Information is printed in the assembly list file for each switch statement, showing the
chosen strategy – see Section 6.4.4 “Switch Statement Information”.
5.14.4.11 THE #PRAGMA WARNING DIRECTIVE
This pragma allows control over some of the compiler’s messages, such as warnings
and errors. For full information on the messaging system employed by the compiler,
see Section 4.6 “Compiler Messages”.
5.14.4.11.1 The Warning Disable Pragma
Some warning messages can be disabled by using the warning disable pragma.
This pragma will only affect warnings that are produced by the parser or the code generator; i.e., errors directly associated with C code. The position of the pragma is only
significant for the parser; i.e., a parser warning number can be disabled for one section
of the code to target specific instances of the warning. Specific instances of a warning
produced by the code generator cannot be individually controlled and the pragma will
remain in force during compilation of the entire module.
The state of those warnings which have been disabled can preserved and recalled
using the warning push and warning pop pragmas. Pushes and pops can be
nested to allow a large degree of control over the message behavior.
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The following example shows the warning associated with assigning the address of a
const object to a pointer to non-const objects. Such code normally produces warning
number 359.
int readp(int * ip) {
return *ip;
}
const int i = 'd';
void main(void) {
unsigned char c;
#pragma warning disable 359
readp(&i);
#pragma warning enable 359
}
This same effect would be observed using the following code.
#pragma warning push
#pragma warning disable 359
readp(&i);
#pragma warning pop
Here, the state of the messaging system is saved by the warning push pragma.
Warning 359 is disabled, then after the source code which triggers the warning, the
state of the messaging system is retrieved by using the warning pop pragma.
5.14.4.11.2 The Warning Error/Warning Pragma
It is also possible to change the types of some messages.
This is only possible by the use of the warning pragma and only affects messages
generated by the parser or code generator. The position of the pragma is only significant for the parser; i.e., a parser message number can have its type changed for one
section of the code to target specific instances of the message. Specific instances of a
message produced by the code generator cannot be individually controlled and the
pragma will remain in force during compilation of the entire module.
The following example shows the warning produced in the previous example being
converted to an error for the instance in the function main().
void main(void) {
unsigned char c;
#pragma warning error 359
readp(&i);
}
Compilation of this code would result in an error, not the usual warning. The error will
force compilation to cease after the current module has concluded, or immediately if
the maximum error count has been reached.
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C Language Features
5.15
LINKING PROGRAMS
The compiler will automatically invoke the linker unless the compiler has been
requested to stop after producing an intermediate file.
The linker will run with options that are obtained from the command-line driver. These
options specify the memory of the device and how the psects should be placed in the
memory. No linker scripts are used.
The linker options passed to the linker can be adjusted by the user, but this is only
required in special circumstances. See Section 4.8.6 “-L-: Adjust Linker Options
Directly” for more information.)
The linker creates a map file which details the memory assigned to psects and some
objects within the code. The map file is the best place to look for memory information.
See Section 7.4 “Map Files” for a more comprehensive explanation of the detailed
information in this file.
5.15.1
Program Sections
There is a lot of confusion as to what psects (program sections) actually are and even
more confusion as to how they are placed in memory. The following aside takes the
form of an analogy and examples, and serves as an introduction to how compilers must
generate code and have it allocated into memory. Such an understanding is vital for
assembly programmers and understanding “Can’t find space” error messages issued
by the linker. Like all analogies, it can be misleading and can only be taken so far, but
it relates the main principles of code generation, the linker and sections back to
something that you should understand.
By the end of this section, you should have a better understanding of:
• Why assembly code has to be packed and manipulated in sections
• Why the linker packs sections into classes rather than the device memory
• Why a “Can’t find space” error message can be issued even though there is plenty
of space left in a device’s memory
5.15.1.1
AN ANALOGY
Our analogy is based around a company which sells components. Customers throughout the world place orders for these components. The consignments are sent from a
central warehouse in shipping containers to a regional office and then delivered to the
customer.
In the warehouse, a robot assembles each order. The required components are collected and placed in a box of sufficient size. The box is labeled, then placed on a conveyor belt. The label on the box indicates the destination city and country, as well as
any special packing instructions.
At the other end of the conveyor belt, a packing machine reads the labels on the boxes
and sorts them by destination city. Thus, all the boxes destined for the same city are
collated into one holding bay.
Once the day’s order are all processed, the collated boxes in each holding bay are first
wrapped in plastic to keep them together. These bundles are then placed into a shipping container bound for that country. As there might be more than one destination city
in the same country, there could be many bundles placed in the same container.
And so ends another productive day in the warehouse.
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5.15.1.2
THE COMPILER EQUIVALENT
Let’s now look at the similarities and differences between this situation and the
compilation process. Both these processes are compared in Figure 5-3.
FIGURE 5-3:
THE WAREHOUSE AND COMPILER SIDE BY SIDE
ORDER 1
3 x TSC235ORDER 2
2 x RC123a
1 x TSD87
2 x XAM99
10 x TD66
P
R
‧‧‧
if(in > 4)
scale = 0;
‧‧‧
int result;
float scale;
PSECT cstackBANK0,class=BANK0,space=1
global __pcstackBANK0
__pcstackBANK0:
global ?___ftge
?___ftge: ;@ 0x0
global ?___ftmul
?___ftmul: ;@ 0x0
global ?_frexp
?_frexp: ;@ 0x0
global ___ftmul@f1
___ftmul@f1:
; 3 bytes @ 0x0
PSECT text,class=CODE
fcall
_srv
movlw
(?_barrr2)&0ffh
movwf
fsr0
movlw
(?_srv)&0ffh
movwf
(??_main+0+0)
movf
fsr0,w
movwf
(??_main+0+0+1)
movlw
12
movwf
(??_main+0+0+2)
u60:
movf
(??_main+0+0),w
movwf
fsr0
PSECT bssBANK0,class=BANK0,space=1
global __pbssCOMMON
__pbssCOMMON:
_c:
ds
1
_more:
ds
8
code
generator
PSECT text,class=CODE
movlw
(?_barrr2)&0ffh
movwf
fsr0
movlw
(?_srv)&0ffh
movwf
(??_main+0+0)
movf
fsr0,w
movwf
(??_main+0+0+1)
movlw
12
movwf
(??_main+0+0+2)
u60:
movf
(??_main+0+0),w
int
getDat(void) {
...
linker
In this analogy, the warehouse is likened to the compiler. The robot is akin to the compiler’s code generator application, which turns the C code into assembly code; and the
packing machine, the linker, which is responsible for arranging how everything will
appear in memory.
The packing machine is not actually delivering the bundles of boxes, it is just putting
them into containers in the warehouse. In the same way, the linker is not actually placing sections in the device’s memory; it is arranging them in conceptual containers. This
analogy is not concerned with what happens outside the warehouse, nor after the
compilation process – that is another story.
The following sections detail points relevant at different stages of the process.
5.15.1.2.1 Orders and Source Code
Both the warehouse and compiler take descriptions of something and use this to produce the final product: The warehouse receives orders, and assembles the components to fulfill that order; the compiler reads (variable and function) definitions in a C
program and generates the assembly code to implement each definition.
An order from a customer can be for any number of components. In the same way, each
C definition can require few or many assembly instructions or directives to be
produced.
Orders arrive at the warehouse randomly, but the components are always shipped to
one of several countries. Source code (variables and functions) can also be written in
any order, but the compiled code is always allocated to one of several memory areas.
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5.15.1.2.2 Boxes, Labels and Sections
In our analogy, the components for each order are placed in a box. In the same way,
the assembly output generated is also placed into boxes, called program sections (or
psects, for short).
There are several reasons why code is placed in a section.
• The generated assembly code is more manageable.
• The ordering of the code sequence within the section is preserved.
• The ordering of sections with the same name are preserved.
• Code is easily sorted based on where it needs to reside in memory.
• Only one command is required to locate an entire section into memory.
Any code sequence that must be contiguous is placed in the one section. The output
of the compiler will typically appear in many different sections, but all sections with the
same name will collate in the order in which they are produced.
The compiler has a list of section names and chooses a section based on the code it
is generating, see 5.15.2 Compiler-Generated Psects.
A section is not a physical box, but rather a special directive is used in the assembly
code to define the start of each section. The directive signifies the end of the previous
box and the start of a new box. A section can hold any amount of assembly code and
can even be empty.
Both the warehouse boxes and compiler sections are labeled; and, in both instances,
the label indicates a destination rather than the contents. In Figure 5-3 color is used to
identify the destination city of a box, or the name of a section.
Figure 5-4 shows what a typical box label might look like. The packing machine in the
warehouse is only concerned with packing boxes in containers and so, other than the
city and country, the customer’s actual address is not important here.
FIGURE 5-4:
A TYPICAL BOX LABEL
BOX paris,country=FRANCE,fragile
Figure 5-5 shows an example of the assembly directive that is used to start a new
section. The city name is now a section name and the destination country a linker class.
FIGURE 5-5:
THE SECTION DIRECTIVE AS A BOX LABEL
the name of this box
special packing instructions
for this box
PSECT bss0,class=BANK0,space=1,reloc=4
close the previous box,
start a new box
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the container this box
must be packed in
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5.15.1.2.3 Down the Conveyor Belt and Object files
Once the robot has assembled an order and placed the components in a box, the contents of the box are no longer relevant, and the remaining activities in the warehouse
only deal with boxes.
It is a similar situation in the compiler: once assembly code has been placed into a section, the instructions are no longer relevant during the link process. The linker only
handles sections and is oblivious to each section’s contents.1
5.15.1.2.4 Sorting Boxes and Sections
In the warehouse, all the boxes are sorted by destination city. After the day’s orders are
processed, all these similar boxes are wrapped so they can be easily kept together.
The concept is the same in the compiler domain: Sections are sorted based on the section’s name, and are merged into one larger section by the linker. The order in which
the sections come together strictly follows the order in which they are produced.
5.15.1.2.5 Loading the Containers and Classes
In the warehouse, the bundled boxes are loaded into the shipping containers. One or
more shipping containers will be provided for each country. If there are orders for more
than one city in the same country, then the bundled boxes for those cities can share the
same container (as long as there is room).
The linker does a similar thing: It arranges the collated sections in the classes. Typically, several sections are placed in each class. The classes represent the device memory, but might take into account addressing modes or banking/paging restrictions
present in the target device. Classes are often smaller than the memory space in which
they are theoretically located.
Think of a large cargo ship. Although the ship can be very long, the biggest object it can
hold is determined by the size of the shipping containers it transports. The shipping
company cannot be able to carry an item due to size constraints even though the item
is a fraction the size of the vessel.
1. Inside the compiler, there are actually a few steps between the code generator and linker. Assembly
code is passed first to the assembler; however, the object files produced, which are passed to the linker,
preserve all the sections and can be ignored in this exercise.
The assembler optimizer can alter assembly instructions, but this is only after reading and interpreting
the code – something the linker cannot do. Modified code is still functionally identical to the original.
Other optimizations can further merge or split sections, but this is not important to a basic understanding
of the concepts.
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C Language Features
5.15.1.3
SECTIONS AT WORK
Now that we have a basic understanding of the concepts, let’s work through a simple
example to help see how these work when compiling.
An engineer has written a program. Exactly what it does or how it is written is not
important. The target device has only one bank of RAM and one page of flash.
In our very simplified example, the compiler will choose from the sections listed in
Table 5-21. This table also shows the linker class that is associated with each section,
and the memory location in the device that the class represents.
TABLE 5-21:
SECTION NAMES FOR OUR SIMPLE EXAMPLE
Section
Name
Contents
Linker
Class
Memory
Location
text
Executable code
CODE
Flash
bss0
Variables that need to be cleared
BANK0
RAM
data0
Variables that need to be initialized
BANK0
RAM
idata
Initialized variable’s values
CODE
Flash
The code generator starts to read the program description in C and produces the
assembly code required to implement this on the target device.
The first thing the code generator encounters is the C definition for a variable with no
initial value. All variables like this will be kept together so that clearing them at startup
will be easier and more efficient. The code generator outputs the directive that starts a
new section. Its internal table of section names indicates that bss0 is the appropriate
choice and that this section is placed in the BANK0 class. The directive looks like this.
PSECT bss0,class=BANK0
Code follows this directive to define the variable: it merely consists of a label and
another directive to reserve the required amount of memory. Altogether, this might look
like the following. (Note that examples given in this section are generic and may not be
relevant for your particular device)
PSECT bss0,class=BANK0
myVariable:
DS 2
; reserve 2 bytes for this object
The code generator continues and sees yet another uninitialized variable. As this variable will also use the same section, the code generator can keep adding to the current
section and so immediately outputs another label and directive to reserve memory for
the second variable.
Now a function definition is encountered in the C source. The code generator sees that
output now needs to be placed in the text section and outputs the following directive.
PSECT text,class=CODE
This directive ends the bss0 section and starts the text section. Any code following
will be contained in the text section. Code associated with the function is generated
and placed into this section.
Moving on, the code generator encounters a variable which is initialized with a value.
These variables will have their initial values stored in flash and copied as a block into
the RAM space allocated to them. By using one section for the RAM image and another
for the initial values, the compiler can ensure that all the initial values will align after
linking, as the sections will be collated in order.
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The compiler first outputs the directive for the section with the name data0. It then outputs a label and directive to reserve memory for the variable in RAM. It next changes
to the idata section that will hold all the initial values. Code is output that will place the
initial value in this section. Notice that the code associated with initialized variables is
output into two separate sections.
PSECT data0,class=BANK0
_StartCount:
DS 2
; space in RAM for the variable
PSECT idata,class=CODE
DB 012h
;initial values places as bytes
DB 034h
; which will be copied to RAM later
The code generator reads another initialized variable. It selects the data0 section, outputs the label and memory reservation directive, then selects the idata section and
stores the initial value.
This process continues until all the program has been generated. The output is passed
to the linker (via the assembler), which then looks for the sections it contains.
The linker sorts each section as it is encountered. All text sections are collated, as
well as the bss0, data0 and idata sections. The order in which they will be assembled will match the order in which they are passed to the linker, which in turn will be the
order in which they were produced by the code generator. There is now one collated
text, bss0, data0 and idata section.
These sections are now located in memory. The linker is passed options (from the compiler driver) which define the linker classes and the memory ranges they represent, see
Section 7.2.1 “-Aclass =low-high,...”1. For our device, the linker options might look like
this.
-ACODE=0-7ffh
-ABANK0=20h-6fh
So for example, the BANK0 class covers memory from 20h to 6fh. You can see all the
options passed to the linker in the map file, see Section 7.4 “Map Files”.
The linker “fits” each section into the memory associated with the class it is associated
with. It might, for example, place the text section at address 0 in the CODE class, then
immediately follow it with the idata section at address 374h, for example. The bss0
and data0 sections will similarly be placed in the BANK0 class.
All the sections are now allocated an address within a class. The addresses of symbols
can now be determined, and ultimately a hex file is produced. The compiler’s job is
over.
1. Remember these are linker options and you cannot pass these straight to the compiler driver. You
can, however, encapsulate these options in the driver’s -L- options, see Section 4.8.6 “-L-: Adjust
Linker Options Directly”.
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5.15.1.4
MORE ADVANCED TOPICS
Let’s look at more complex issues with linking, now that we understand the basics.
5.15.1.4.1 Allocation at Specific Locations
We have seen in the preceding analogy that the linker places sections in their corresponding class when it comes time to determine where they will live in memory. This is
how most sections are linked, but there are some exceptions. Code to be executed on
Reset or an interrupt are examples. They cannot just be placed anywhere in a class;
they must be located at specific addresses. So how are these positioned by the linker?
Any code that is to be linked at a particular address is placed in a section in the usual
way. These sections will even have a class associated with them, but allocation anywhere in this class can be overridden by a special linker option which tells the linker to
place the section at a specific address. In terms of our previous analogy, think of the
special linker options as being explicit instructions that are given to the packing
machine as to where to place the box in a container. We will see in the next example
the linker options to place a section explicitly.
-preset_vec=0h
-pint_text=04h
Note that if a section is linked in this way, the linker will follow these instructions strictly.
It will warn if it is asked to place a section over the top of another, but since there is no
container, which essentially represents a memory range, the linker cannot check for
sections extending past the device’s memory limits.
5.15.1.4.2 Where Classes and Containers Differ
Containers and linker classes differ in one important aspect: Linker classes are
conceptual and merely represent the memory of a device; they are not physical
storage.
The compiler can, and often does, use more than one class to represent the same
memory range. This is illustrated in Section 5.15.1.5 “More Advanced Sections at
Work”, where the example uses CODE and CONST classes for flash memory. Although
classes can cover the same range, typically the sizes of the containers vary. This allows
code with different restrictions and requirements to be accommodated.
When the memory ranges of classes overlap, allocating to one class will result in the
same memory addresses becoming unavailable in all of those classes. In fact, when
any memory is allocated by the linker, by whatever means, it checks each class to see
if it covers this memory and marks it as being used. This is quite a different concept
from physical containers.
5.15.1.4.3 Multi-bin containers
Linker classes usually define one memory range, but there are instances where a class
defines multiple memory ranges. You can think of this as several separate containers,
but all with identical shipping destinations. Memory ranges in the class do not need to
be contiguous.
The compiler typically uses a multi-range class to represent program memory that is
paged. The boundaries in the memory ranges coincide with the page boundaries. This
prevents sections from crossing a page boundary.
The compiler could use a similar class for banked RAM, but code can be considerably
reduced in size if the destination bank of each variable is known by the code generator.
You will usually see a separate class defined for each bank, and dedicated sections
that are associated with these classes. The code generator will allocate a bank for each
variable and choose a section destined for the class that represents that bank.
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5.15.1.5
MORE ADVANCED SECTIONS AT WORK
Let’s build on the previous example. Our target device now has two banks of RAM and
two pages of flash. Table 5-22 shows the extended list of sections the compiler now
uses. These sections reference new classes, also shown in the table.
TABLE 5-22:
SECTION NAMES FOR OUR EXTENDED EXAMPLE
Section Name
Contents
Linker
Class
Memory
Location
textn
Executable code
CODE
Flash
bss0
Variables that need to be cleared
BANK0
RAM
bss1
Variables that need to be cleared
BANK1
RAM
data0
Variables that need to be initialized
BANK0
RAM
idata
Initialized variable’s values
CODE
Flash
reset_vec
Code associated with Reset
CODE
Flash
const
Read-only variables
CONST
Flash
init
Runtime startup code
CODE
Flash
int_text
Interrupt function code
CODE
Flash
The compiler operates as it did in the previous example, selecting and producing a
section directive prior to generating the assembly code associated with the C source
that is currently being compiled.
The assembly code associated with ordinary functions is still placed in a “text” section,
but as there are now two pages of flash, we have to ensure that both pages can be
used. If each function was placed into the same section, they will be merged by the
linker and that section would grow until it is too large to fit into either page. To ensure
that all the “text” sections do not merge, each function is placed in its own unique numbered section: text0, text1, text2, etc. As these sections do not have the same
name, they will not be merged by the linker.
The linker option to define the CODE class might now look like:
-ACODE=0-7ffhx2
which tells the linker that CODE represents two pages of memory: one from 0 to 7ffh and
another from 800h to fffh.
This specification indicates that there is some sort of memory boundary that occurs at
address 800h (the devices internal memory page) and is very different from a class
definition that reads -ACODE=0-fffh, which covers the same memory range, but
does not have the boundary. The linker will try allocating each textx section to one
page (class memory range); if it does not fit, it will try the other.
If an interrupt function is encountered, the int_text section is selected for this code.
As this is separate from the sections used to hold ordinary functions, it can be linked
explicitly at the interrupt vector location. Assuming that the interrupt vector is at address
4, the linker option to locate this section might look like the following, see
Section 7.2.18 “-Pspec”.
-pint_text=4h
For simplicity in this example, initialized variables are treated as they were in the previous example, even though there are now two RAM banks; i.e., they are always
allocated in the first bank of RAM.
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C Language Features
In the previous example we ignored the code that would have copied the initial values
from flash into RAM. This code is executed after Reset and before the function main,
and forms part of the runtime startup code. It reads each initial value from flash and
writes this to the corresponding variable in the RAM space. Provided the order in which
the variables are allocated memory in RAM matches the order their initial values are
allocated in flash, a single loop can be used to perform the copy. Even though the variables might be defined at many places in the source code, the order in memory of each
variable and value will be preserved since the compiler uses sections to hold the code
associated with each.
This runtime startup code is output into a section called init. Code which jumps to the
runtime startup codes is placed in the reset_vec section, which is linked to the Reset
location. By linking these sections in the same page, the jump from one section to
another will be shorter and faster. The linker options to make this happen might look
like:
-preset_vec=0
-pinit=int_text
which says that the Reset vector code is linked to address 0 (which is the Reset vector
location) and that the init section, which contains the runtime startup code, should be
linked immediately after the interrupt code in the int_text section. If the int_text
section is empty; i.e., there is no interrupt code defined in the program, then init will
be placed at address 4.
Previously all uninitialized variables were placed in the bss0 section. Now the code
generator first checks that there will actually be enough room in bank 0 memory before
doing so. If not, it chooses the bss1 section that will ultimately be linked into bank 1
memory. The code generator keeps track of any object allocated to RAM so it can maintain the amount of free memory in each RAM bank. For these variables, the linker allocates the sections to memory, but it is the code generator that decides which section
will be used by each variable. Thus, both applications play a part in the memory
allocation process.
In this example, we also consider const variables which are stored in flash, not RAM.
Each byte of data is encapsulated in a RETLW instruction that return the byte in the W
register. Code is needed to access each byte of a variable or array. One way of doing
this is a “computed goto” which involves loading the W register with an offset into the
block of data and adding this to the PC.(The Microchip application note AN556 has
examples of how this can be done for several devices.) A computed goto requires that
the destination address (the result of adding W and PC) must not cross over a 256 word
boundary (i.e., the addresses 100h, 200h, 300h, etc.). This requirement can be met
using sections and a class.
In this example a new class, called CONST, is created and defined as follows
-ACONST=0-0ffhx16
which is to say that CONST is a container 100h long, but there are 16 of them one after
the other in memory, so 0-ffh is one container, 100-1ffh is another, etc. We have the
compiler place all the RETLW instructions and the computed goto code into the const
section, which are linked into this class. The section can be located in any of the 16
containers, but must fit entirely within one.
In this example, the compiler only allows one block of const data. It could be made to
allow many by having each block of const data in a unique numbered section as we
did for the text sections (e.g., const1, const2, etc.). Thus each sections could remain
independent and be allocated to any memory bin of the CONST class.
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5.15.1.6
EXPLAINING COMMON LINKER ERRORS AND PROBLEMS
We can also use our knowledge to help explain some common linker problems and
error messages.
5.15.1.6.1 Not Finding Space
A common linker error is, “can’t find x words for psect ‘abc’ in class ‘XYZ’,” which we
can now think of as, “can’t find 3 cubic feet for the boxes ‘paris’, in container ‘FRANCE’.”
The most obvious reason for this error is that the containers have been steadily filling
and have finally run out of free space, i.e., the total amount of code or data you need
to store exceeds the amount of memory on the device.
Another reason is that a box is larger than the container(s) in which it has to be placed.
If this is the case, the section will never fit, even if the entire memory space is empty.
This situation might occur when defining a very large C function, RAM or const array
resulting in an abnormally large section. Other possible sources include large switch
statements or even a switch with too many case labels.
The hold of a ship or aircraft might be a very large space, but freight is always packed
into shipping containers and it is the size of the shipping container that dictates the
maximum size of a object that can be transported. In the same way, the total amount
of memory on a target device is irrelevant if sections must first be allocated to a class.
Classes can seem restrictive, but without them, code will typically be less efficient or
can simply fail to work altogether. The computed goto is a good example. If the table of
instructions in a computed goto crosses a 100h boundary, it will fail due to the limited
jump range of the instruction set.
This space error can also occur if there are many free spaces remaining in containers,
but none are large enough to hold the section. This can be confusing since the total
amount of free space can be larger than the section to be placed. In the same way that
boxes cannot be removed from a bundle or unpacked, if a section does not fit into any
remaining space, it cannot be split by the linker and an error will result. The error
message indicates that largest free space that is still available.
In the same way that the label on a box can indicate special packing instructions, e.g.,
“fragile – pack at the top of the container”, or “this way up”, etc, a section can also
indicate special memory allocation instructions. One of the most common requirements
is that a section must start on an address boundary, see the reloc PSECT flag in
Section 6.2.9.3.15 “Reloc”. If a section has to fit into a class, but also has to be aligned
on an address, this makes it much more difficult to locate and increases the chance that
this error is produced. This is also the case if other sections or objects have been
placed in the middle of a container, as we saw in Section 5.15.1.4.1 “Allocation at
Specific Locations”.
5.15.1.6.2 Not Being Shipped in the Right Container
Clearly, boxes will not be delivered correctly if they are placed in the wrong container.
So too, code cannot run if it is placed in the wrong class or address. The compiler will
always ensure that code and data is allocated correctly, but it is possible to manually
change linker options.
The code generator assumes that sections will be located in certain memory areas and
the code it generates can rely on this being the case. Typically, sections placed in RAM
must remain in the bank in which they were originally destined. Sections containing
executable code possibly more forgiving, but some have very specific requirements.
Remember, the linker will allow any allocation you indicate; it has no way of checking
if what you tell it is correct. Always take care if you override linker options and reposition
a section. Section 5.15.2 “Compiler-Generated Psects” lists the common sections used
by the compiler. it also indicates restrictions on how these sections can be linked.
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5.15.1.6.3 Doing Things by Hand
So far we have discussed assembly code produced by the code generator and how it
is linked into memory; now, we consider hand-written assembly.
Imagine that in the warehouse an order is not processed by the robot, but by a human
worker. The components ordered are assembled by hand, packed in a box by hand and
then placed on the conveyor belt along with those boxes packed by the robot. This is
quite similar to when there is hand-written assembly code in a project: the linker is
passed a number of sections, some created by the code generator and some by a
human.
Mistakes can be made by the warehouse worker or the assembly programmer. In the
warehouse, the worker might not use a box and place the components loose on the
conveyor belt, or a box might be used but it is not labeled, or it might be labeled incorrectly. In the compiler’s domain, assembly code can be written without a section, or it
is in a section but with the wrong (or no) class, or the section can have incorrect
allocation instructions.
If assembly code is not in a section, the compiler will actually place it into a default section. But since there are no specific instructions as to what to do with this section, it
could be linked anywhere. Such sections are like boxes labeled “ship to anywhere you
want”. As a rule of thumb, put all assembly code inside a section, but some directives
(e.g., GLOBAL or EQU) do not generate code and can be placed anywhere.
The easiest way to write and locate hand-written assembly code is to associate the
section you create with an existing linker class that represents a suitable memory area.
This means the default linker options do not need to be altered.
The association is made using the class flag of the PSECT directive, see
Section 6.2.9.3.3 “Class”. If a section is to be placed at an explicit address rather than
having it placed anywhere in a class, the class flag should still be used. A list of linker
classes and the memory they represent is given in Section 5.15.3 “Default Linker
Classes”.
Even if you place your code into a section and use an appropriate class, other flags can
be necessary to correctly link the code. The most important section flags are the
delta, see Section 6.2.9.3.4 “Delta”, reloc, see Section 6.2.9.3.15 “Reloc” and
space, see 6.2.9.3.17 Space flags. If these are incorrectly specified, the code not be
positioned correctly and will almost certainly fail.
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5.15.2
Compiler-Generated Psects
The code generator places code and data into psects (program sections) with standard
names, which are subsequent positioned by the default linker options. The linker does
not treat these compiler-generated psects any differently to a psect that has been
defined by yourself. A psect can be created in assembly code by using the PSECT
assembler directive (see Section 6.2.9.3 “PSECT”). The code generator uses this
directive to direct assembly code it produces into the appropriate psect. For an
introductory guide to psects, see Section 5.15.1 “Program Sections”.
Some psects, in particular the data memory psects, use special naming conventions.
For example, take the bss psect. The name bss is historical. It holds uninitialized variables. However, there can be some uninitialized variables that will need to be located
in bank 0 data memory; others can need to be located in bank 1 memory. As these two
groups of variables will need to be placed into different memory banks, they will need
to be in separate psects so they can be independently controlled by the linker. In addition, the uninitialized variables that are bit variables need to be treated specially so
they need their own psect. So there are a number of different psects that all use the
same base name, but which have prefixes and suffixes to make them unique.
The general form of these psect names is:
[bit]psectBaseNameCLASS[div]
where psectBaseName is the base name of the psect, such as bss or data. The
CLASS is a name derived from the linker class (see Section 7.2.1 “-Aclass
=low-high,...”) in which the psect will be linked, e.g., BANK0. The prefix bit is used if
the psect holds bit variables. So there can be psects like: bssBANK0, bssBANK1 and
bitbssBANK0 defined by the compiler to hold the uninitialized variables.
Note that eeprom-qualified variables can define psects called bssEEDATA or
dataEEDATA, for example, in the same way. Any psect using the class suffix EEDATA
is placed in the HEX file and is burnt into the EEPROM space when you program the
device.
If locations in a bank are reserved or are taken up by absolute objects for example, a
psect cannot be formed over the entire bank. Instead, a separate psect will be used to
represent the free memory on either side of the used memory. The letters l (elle) and
h are used as the div field in the psect name to indicate if it is the lower or higher
division. Thus you might see bssBANK0l and bssBANK0h psects if a split took place.
If you are unsure which psect holds an object or code in your project, check the
assembly list file (see Section 6.4.1 “General Format”)
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C Language Features
The contents of these psects are described below, listed by psect base name.
5.15.2.1
PROGRAM SPACE PSECTS
checksum –
this is a psect that is used to mark the position of a checksum that has
been requested using the --CHECKSUM option.
See Section 4.8.16 “--ASMLIST: Generate Assembler List Files” for
more information.
The checksum value is added after the linker has executed so you will
not see the contents of this psect in the assembly list file, nor specific
information in the map file. Linking this psect at a non-default location
will have no effect on where the checksum is stored, although the map
file will indicate it located at the new address.
Do not change the default linker options relating to this psect.
cinit –
used by the C initialization runtime startup code.
Code in this psect is output by the code generator along with the generated code for the C program and does not appear in the runtime
startup assembly module.
This psect can be linked anywhere within a program memory page,
provided it does not interfere with the requirements of other psects.
config –
used to store the Configuration Words.
This psect must be stored in a special location in the HEX file.
Do not change the default linker options relating to this psect.
const –
these PIC18-only psects hold objects that are declared const and
string literals which are not modifiable.
Used when the total amount of const data in a program exceeds 64k.
This psect can be linked anywhere within a program memory page,
provided it does not interfere with the requirements of other psects.
eeprom (PIC18: eeprom_data) – used to store initial values in the EEPROM
memory.
Do not change the default linker options relating to this psect.
idata –
these psects contain the ROM image of any initialized variables.
These psects are copied into the data psects at startup. In this case, the
class name is used to describe the class of the corresponding
RAM-based data psect. These psects will be stored in program memory, not the data memory space.
These psects are implicitly linked to a location that is anywhere within
the CODE linker class.
The linker options can be changed allowing this psect to be placed at
any address within a program memory page, provided that it does not
interfere with the requirements of other psects.
idloc –
used to store the ID location words.
This psect must be stored in a special location in the HEX file.
Do not change the default linker options relating to this psect.
init –
used by assembly code in the runtime startup assembly module.
The code in this and the cinit define the runtime startup code.
If no interrupt code is defined code from the Reset vector can “fall
through” into this psect.
It is recommended that the default linker options relating to this psect
are not changed in case this situation is in effect.
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intcode, intcodelo – are the psects which contains the executable code for the
high-priority (default) and low-priority interrupt service routines, respectively. These psects are linked to interrupt vector at address 0x8 and
0x18, respectively.
Do not change the default linker options relating to these psects. See
Section 4.8.20 “--CLIST: Generate C Listing File” if you want to move
code when using a bootloader.
intentry –
contains the entry code for the interrupt service routine which is linked
to the interrupt vector.
This code saves the necessary registers and jumps to the main interrupt
code in the case of mid-range devices; for enhanced mid-range devices
this psect will contain the interrupt function body. (PIC18 devices use
the intcode psects.)
This psect must be linked at the interrupt vector. Do not change the default linker options relating to this psect. See the --CODEOFFSET option
Section 4.8.20 “--CLIST: Generate C Listing File” if you want to move
code when using a bootloader.
ivt0xn –
contains the vector table located at address n for devices which use interrupt vector tables or which are operating in legacy mode, see
Section 5.9.1 “Writing an Interrupt Service Routine”.
jmp_tab –
only used for the baseline processors, this is a psect used to store jump
addresses and function return values.
Do not change the default linker options relating to this psect.
maintext –
this psect will contain the assembly code for the main() function.
The code for main() is segregated as it contains the program entry
point.
Do not change the default linker options relating to this psect as the runtime startup code can “fall through” into this psect which requires that it
be linked immediately after this code.
mediumconst – these PIC18-only psects hold objects that are declared const and
string literals which are not modifiable. Used when the total amount of
const data in a program exceeds 255 bytes, but does not exceed 64k.
This psect can be linked anywhere in the lower 64k of program memory,
provided it does not interfere with the requirements of other psects.
powerup –
contains executable code for a user-supplied powerup routine.
Do not change the default linker options relating to this psect.
reset_vec – this psect contains code associated with the Reset vector.
Do not change the default linker options relating to this psect as it must
be linked to the Reset vector location of the target device. See the
--CODEOFFSET option Section 4.8.20 “--CLIST: Generate C Listing
File”, if you want to move code when using a bootloader.
reset_wrap – for baseline PIC devices, this psect contains code which is executed
after the device PC has wrapped around to address 0x0 from the oscillator calibration location at the top of program memory.
Do not change the default linker options relating to this psect as it must
be linked to the Reset vector location of the target device.
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C Language Features
smallconst – these psects hold objects that are declared const and string literals
which are not modifiable.
Used when the total amount of const data in a program is less than
255 bytes.
This psect can be linked anywhere in the program memory, provided it
does not cross a 0x100 boundary and it does not interfere with the requirements of other psects.
strings –
the strings psect is used for const objects.
It also includes all unnamed string literals. This psect is linked into
ROM, since the contents do not need to be modified.
This psect can be linked anywhere in the program memory, provided it
does not cross a 0x100 boundary or interfere with the requirements of
other psects.
stringtext – the stringtext psect is used for const objects when compiling for
baseline devices.
This psect is linked into ROM, since the contents do not need to be
modified.
This psect must be linked within the first half of each program memory
page.
textn –
these psects (where n is a decimal number) contain all other executable
code that does not require a special link location.
These psects can be linked anywhere in the program memory, provided
they do not straddle a page boundary and do not interfere with the requirements of other psects. Note that the compiler imposes pseudo
page boundaries on some PIC18 devices to work around published errata. Check the default CODE linker class for the presence of pages, and
their size, in the executable memory.
temp –
this psect contains compiler-defined temporary variables.<return>
This psect must be linked in common memory, but can be placed at any
address in that memory, provided it does not interfere with other psects.
xxx_text –
defines the psect for a function that has been made absolute; i.e.,
placed at an address. xxx will be the assembly symbol associated with
the function.
For example if the function rv() is made absolute, code associated
with it will appear in the psect called _rv_text.
As these psects are already placed at the address indicated in the C
source code, the linker options that position them should not be
changed.
xxx_const – defines the psect for const object that has been made absolute; i.e.,
placed at an address. xxx will be the assembly symbol associated with
the object.
For example, if the array nba is made absolute, values stored in this
array will appear in the psect called _nba_const.
As these psects are already placed at the address indicated in the C
source code, the linker options that position them should not be
changed.
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5.15.2.2
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DATA SPACE PSECTS
nv –
these psects are used to store variables qualified persistent.
They are not cleared or otherwise modified at startup.
These psects can be linked anywhere in their targeted memory bank
and should not overlap any common (unbanked memory) that the
device supports if it is a banked psect.
bss –
these psects contain any uninitialized variables.
These psects can be linked anywhere in their targeted memory bank
and should not overlap any common (unbanked memory) that the
device supports if it is a banked psect.
data –
these psects contain the RAM image of any initialized variables.
These psects can be linked anywhere in their targeted memory bank
and should not overlap any common (unbanked memory) that the
device supports if it is a banked psect.
cstack –
these psects contain the compiled stack.
On the stack are auto and parameter variables for the entire program.
See Section 5.5.2.2.1 “Compiled Stack Operation”, for information on
the compiled stack.
These psects can be linked anywhere in their targeted memory bank
and should not overlap any common (unbanked memory) that the
device supports if it is a banked psect.
stack –
this psect is used as a placeholder for the software stack.
This stack is dynamic and its size is not known by the compiler. As described in 5.3.4.2 Data Stacks this psect is typically allocated the remainder of the free data space so that the stack may grow as large as
possible.
This psect may be linked anywhere in the data memory, but adjusting
the default linker options for this psect may limit the size of the software
stack. Any overflow of the software stack may cause code failure.
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C Language Features
5.15.3
Default Linker Classes
The linker uses classes to represent memory ranges. For an introductory guide to
psects and linker classes, see Section 5.15.1 “Program Sections”.
The classes are defined by linker options, see Section 7.2.1 “-Aclass =low-high,...”
passed to the linker by the compiler driver. Psects are typically allocated space in the
class they are associated with. The association is made using the class flag of the
PSECT directive, see Section 6.2.9.3.3 “Class”. Alternatively, a psect can be explicitly
placed into a class using a linker option, see Section 7.2.18 “-Pspec”.
Classes can represent a single memory range, or multiple ranges. Even if two ranges
are contiguous, the address where one range ends and the other begins forms a
boundary and psects placed in the class can never cross such boundaries. You will see
classes that cover the same addresses, but will be divided into different ranges and
have different boundaries. This is to accommodate psects whose contents were compiled under assumptions about where they would be located in memory.
Memory allocated from one class will also be reserved from other classes that specify
the same memory. To the linker, there is no significance to a class name or the memory
it defines.
Memory will be subtracted from these classes if using the --ROM or --RAM options, see
Section 4.8.53 “--ROM: Adjust ROM Ranges” and Section 4.8.52 “--RAM: Adjust RAM
Ranges”, to reserve memory. When specifying a debugger, such as an ICD, see
Section 4.8.22 “--DEBUGGER: Select Debugger Type”, memory can also be removed
from the ranges associated with some classes so that this memory is not used by your
program.
Although you can manually adjust the ranges associated with a class, this is not recommended. Never change or remove address boundaries specified by a class
definition option.
The following are the linker classes that can be defined by the compiler. Not all classes
can be present for each device.
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5.15.3.1
PROGRAM MEMORY CLASSES
CODE —
consists of ranges that map to the pages on the target device.
Thus, it is typically used for psects containing executable code.
On baseline devices, it can only be used by code that is accessed via a
jump table.
ENTRY —
is used mainly by baseline devices for psects containing executable
code that is accessed via a CALL instruction (calls can only be to the
first half of a page).
The class is defined in such a way that it is the size of a page, but psects
it holds will be positioned so that they start in the first half of the page.
This class is also used in mid-range devices and will consist of many
ranges, each 0x100 words long and aligned on a 0x100 boundary.
Thus, it is useful for psects whose contents cannot span a 0x100 word
boundary.
STRING —
consists of ranges that are 0x100 words long and aligned on a 0x100
boundary. Thus, it is useful for psects whose contents cannot span a
0x100 word boundary.
STRCODE —
defines a single memory range that covers the entire program memory.
It is useful for psects whose content can appear in any page and can
cross page boundaries.
CONST —
consists of ranges that are 0x100 words long and aligned on a 0x100
boundary. Thus, it is useful for psects whose contents cannot span a
0x100 word boundary.
5.15.3.2
DATA MEMORY CLASSES
RAM —
consist of ranges that cover all the general purpose RAM memory of the
target device, but excluding any common (unbanked) memory.
Thus, it is useful for psects that must be placed in general-purpose
banked RAM, but can be placed in any bank.
BIGRAM —
consists of a single memory range that is designed to cover the linear
data memory of enhanced mid-range devices, or the entire available
memory space of PIC18 devices.
It is suitable for any psect whose contents are accessed using linear
addressing or which does not need to be contained in a single data
bank.
ABS1 —
consist of ranges that cover all the general purpose RAM memory of the
target device, including any common (unbanked) memory.
Thus, it is useful for psects that must be placed in general purpose
RAM, but can be placed in any bank or the common memory,
BANKx (where x is a bank number) — each consist of a single range that covers the
general purpose RAM in that bank, but excluding any common
(unbanked) memory.
COMMON —
consists of a single memory range that covers the common (unbanked)
RAM, if present, for all mid-range devices.
COMRAM —
consists of a single memory range that covers the common (unbanked)
RAM, if present, for all PIC18 devices.
SFRx (where x is a bank number) — each consists of a single range that covers the
SFR memory in that bank.
These classes would not typically be used by programmers as they do
not represent general purpose RAM.
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5.15.3.3
MISCELLANEOUS CLASSES
CONFIG —
consists of a single range that covers the memory reserved for
configuration bit data in the hex file.
This class would not typically be used by programmers as it does not
represent general purpose RAM.
IDLOC —
consists of a single range that covers the memory reserved for ID
location data in the hex file.
This class would not typically be used by programmers as it does not
represent general purpose RAM.
EEDATA —
consists of a single range that covers the EEPROM memory of the
target device, if present.
This class would typically be used for psects that contain data that is to
be programmed into the EEPROM.
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5.15.4
Changing and Linking the Allocated Section
Section 5.15.2 “Compiler-Generated Psects” lists the default sections the compiler
uses to hold objects and code. You can change the default section of a function or variable if the object has unique linking requirements that cannot be addressed by existing
compiler features.
The __section() specifier allows you to have a variable or function redirected into
a user-define psect (section).
New psects created by the specifier for variables will have no linker class associated
with them. In addition, the compiler will not make assumptions about the final location
of the new psect (hence, variable). Thus, you can link variables specified with
__section() into any data bank.
Note that the compiler will not know where the new psect will be placed until the linker
is executed and any additional linker commands (see below) are processed. This
means that the code used to access the relocated variable can be less efficient than
the code used to access the variable without the specifier, and that other variables the
compiler positions might not fit in the same bank used by the new psect. If you receive
memory errors when using the __section() specifier, you might need to reserve
memory for those variables in the new psect, see Section 4.8.52 “--RAM: Adjust RAM
Ranges”.
New psects created by the specifier for functions will inherit the same psect flags. However, there are fewer linking restrictions relating to functions and this has minimal
impact on the generated code.
The name of the new psect you specify must be a valid identifier in the assembler’s
name space. The name must contain only alphabetic or numeric characters, or the
underscore character, _. It cannot have a name which is the same as that of an assembler directive, control, or directive flag. If the new psect will contain executable code and
you wish this code to be optimized by the assembler, ensure that the psect name contains the substring “text”, e.g., usb_text. Psects named otherwise will not be modified
by the assembler optimizer.
Variables that use the __section() specifier will be cleared or initialized (based on
how they are defined) in the usual way by the runtime startup code (see
Section 4.4.2 “Startup and Initialization”). For the case of initialized variables, the compiler will automatically allocate an additional new psect (whose name will be the same
as the psect specified, prefixed with the letter i), which will contain the initial values.
This psect must be stored in program memory, and you might need to locate this psect
explicitly with a linker option.
The following are examples of a variable and function allocated to a non-default
section.
int __section("myData") foobar;
int __section("myCode") helper(int mode) {
/* ... */ }
You must reserve memory, and locate via an explicit linker option, any new psect created with a __section() specifier. So, for example, if you wanted to place the
sections created in the above example, you could use the following driver options:
-L-PmyData=0200h
-L-AMYCODE=50h-3ffh
-L-PmyCode=MYCODE
which will place the section myData at address 0x200, and the section myCode anywhere in the range 0x50 to 0x3ff represented by the linker class, MYCODE. See
Section 7.2 “Operation” for linker options that can be passed using the -L- driver
option (Section 4.8.6 “-L-: Adjust Linker Options Directly”).
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C Language Features
If you are creating a new class for a memory range in program memory and your target
is a baseline or mid-range PIC device, then you will need to inform the linker that this
class defines memory that is word addressable. Do this by using the linker’s -D option,
which indicates a delta value for a class. For example:
-L-DMYCODE=2
The PIC18 devices have byte-addressable program memory and can use the default
delta value associated with a class.
If you would like to set special psect flags with the new section that is created with the
__section(), you can do this by providing a definition of the new section in your
source code. For example, if you wanted the mycode section to be placed at an
address that is a multiple of 100h, then you can place the following in your source file:
asm("PSECT mycode,reloc=100h");
int __section("myCode") helper(int mode) {
/* ... */ }
The reloc, size and limit psect flags can all be redefined in this way. Redefinitions
might trigger assembler warning messages; however, these can be ignored in this
circumstance.
5.15.5
Replacing Library Modules
The MPLAB XC8 C compiler comes with a librarian, LIBR, which allows you to unpack
a library file and replace modules with your own modified versions. See
Section 8.2 “Librarian”. However, you can easily replace a library module that is linked
into your program without having to do this.
If you add a source file to your project and it contains the definition for a routine with
the same name as a library routine, then the library routine will be replaced by your
routine. This is due to the way the compiler scans source and library files.
When trying to resolve a symbol (a function name, or variable name, for example) the
compiler first scans all the source modules for the definition. Only if it cannot resolve
the symbol in these files does it then search the library files.
If the symbol is defined in a source file, the compiler will never actually search the libraries for this symbol. No error will result if the symbol was present in both your source
code and the library files. This cannot be true if a symbol is defined twice in source files
and an error can result if there is a conflict in the definitions.
All library source code is written in C, and the p-code library files that contain these
library routines are actually passed to the code generator, not the linker, but both these
applications work in the way described above in resolving library symbols.
You cannot replace a C library function with an equivalent written in assembly code
using the above method. If this is required, you will need to use the librarian to edit or
create a new library file.
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5.15.6
Signature Checking
The compiler automatically produces signatures for all functions. A signature is a 16-bit
value computed from a combination of the function’s return data type, the number of its
parameters and other information affecting the calling sequence for the function. This
signature is generated and placed the object code whenever a function is referenced
or defined.
At link time, the linker will report any mismatch of signatures, which will indicate a discrepancy between how the function is defined. MPLAB XC8 is only likely to issue a mismatch error from the linker when the routine is either a precompiled object file or an
assembly routine. Other function mismatches are reported by the code generator.
It is sometimes necessary to write assembly language routines which are called from
C using an extern declaration. Such assembly language functions should include a
signature which is compatible with the C prototype used to call them. The simplest
method of determining the correct signature for a function is to write a dummy C
function with the same prototype and check the assembly list file using the --ASMLIST
option (see Section 4.8.15 “--ADDRQUAL: Set Compiler Response to Memory
Qualifiers”).
For example, suppose you have an assembly language routine called _widget which
takes a char argument and returns a char. The prototype used to call this function
from C would be:
extern char widget(char);
Where a call to _widget is made in the C code, the signature for a function with one
char argument and a char return value would be generated. In order to match the
correct signature, the source code for widget needs to contain an assembler SIGNAT
directive which defines the same signature value. To determine the correct value, you
would write the following code into a dummy file:
char widget(char arg1)
{
}
The resultant assembler code seen in the assembly list file includes the following line:
SIGNAT
_widget,4217
The SIGNAT directive tells the assembler to include a record in the .obj file which
associates the signature value 4217 with symbol _widget. The value 4217 is the
correct signature for a function with one char argument and a char return value.
If this directive is copied into the assembly source file which contains the _widget
code, it will associate the correct signature with the function and the linker will be able
to check for correct argument passing.
If a C source file contains the declaration:
extern char widget(long);
then a different signature will be generated and the linker will report a signature
mis-match which will alert you to the possible existence of incompatible calling
conventions.
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C Language Features
5.15.7
Linker-Defined Symbols
The linker defines some special symbols that can be used to determine where some
psects where linked in memory. These symbols can be used in code, if required.
The link address of a psect can be obtained from the value of a global symbol with
name __Lname (two leading underscores) where name is the name of the psect. For
example, __LbssBANK0 is the low bound of the bssBANK0 psect.
The highest address of a psect (i.e., the link address plus the size) is represented by
the symbol __Hname.
If the psect has different load and link addresses, the load start address is represented
by the symbol __Bname.
Not all psects are assigned these symbols, in particular those that are not placed in
memory by a -P linker option. See Section 7.2.18 “-Pspec”. Psect names can change
from one device to another.
Assembly code can use these symbol by globally declaring them, for example:
GLOBAL __Lidata
and C code could use them by declaring a symbol such as the following.
extern char * _Lidata;
Note that there is only one leading underscore in the C domain, see
Section 5.12.3.1 “Equivalent Assembly Symbols”. As the symbol represents an
address, a pointer is the typical type choice.
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USER’S GUIDE
Chapter 6. Macro Assembler
6.1
INTRODUCTION
Two macro assemblers are included with the MPLAB XC8 C Compiler to assemble
source files for all 8-bit PIC devices. The operation and assembler directives are almost
identical for both assemblers. The appropriate assembler application is invoked when
you use the compiler driver to build projects.
The assembler is called ASPIC18 for PIC18 devices and ASPIC for all other 8-bit
devices. It is available to run on Windows®, Linux® and Mac OS® X systems. Note that
the assembler will not produce any messages unless there are errors or warnings –
there are no “assembly completed” messages.
The command-line driver, xc8, should be used to invoke the assembler.
This chapter describes the directives (assembler pseudo-ops and controls) accepted
by the assembler in the assembly source files or assembly inline with C code.
Although the term “assembler” is almost universally used to describe the tool that converts human-readable mnemonics into machine code, both “assembler” and “assembly” are used to describe the source code which such a tool reads. The latter is more
common and is used in this manual to describe the language. Thus you will see the
terms assembly language (or just assembly), assembly listing and other assembly
terms, but also, assembler options, assembler directive and assembler optimizer.
The following topics are examined in this chapter of the user’s guide:
• MPLAB XC8 Assembly Language
• Assembly-Level Optimizations
• Assembly List Files
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6.2
MPLAB XC8 ASSEMBLY LANGUAGE
Information about the source language accepted by the macro assemblers is described
in this section.
All opcode mnemonics and operand syntax are specific to the target device, and you
should consult your device data sheet. Additional mnemonics, deviations from the
instruction set, and assembler directives and controls are documented in this section.
The same assembler application is used for compiler-generated intermediate
assembly and hand-written assembly source code, and for hand-written assembly
modules and assembly inline with C code.
6.2.1
Assembly Instruction Deviations
The MPLAB XC8 assembler uses a slightly modified form of assembly language to that
specified by the Microchip data sheets. The following information details changes to the
instruction format, and pseudo instructions that can be used in addition to the device
instruction set.
These deviations can be used in assembly code in-line with C code or in hand-written
assembly modules.
6.2.1.1
DESTINATION LOCATION
The PIC device data sheets indicate that some instructions use the operands “,0” or
“,1” to specify the destination for the result of that operation. The XC8 assemblers
instead use the more-readable operands “,w” and “,f” to specify the destination.
The W register is selected as the destination when using the “,w” operand, and the file
register is selected when using the “,f” operand. The case of the letter in the destination operand in not important. For example (ignoring bank selection and address
masking for this example):
MOVF
ADDWF
ADDWF
_foo,w
_foo,f
_foo,w
;move _foo into wreg
;add wreg to _foo, updating the content of _foo
;add wreg to _foo, leaving the result in wreg
It is highly recommended that the destination is always specified with each instruction
that requires this operand. If the destination is omitted, it is assumed to be the file
register. Never use the numeric destination operands.
In the same way, the PIC18 assembler also uses the RAM access operand “,b”
(instead of “,1”) to indicate that PIC18 instructions should use the bank select register
(BSR) when accessing the specified file register address. The “,c” operand (instead of
“,0”) indicates that the address is in the common memory, which is known as the
access bank on PIC18 devices. Alternatively, an instruction operand can be preceded
by the characters “c:” to indicate that the address resides in common memory. These
operands and prefix affect the RAM access bit in the instruction. For example:
ADDWF
BTFSC
_bar,f,c
c:_bar,3
;add wreg to _bar in common memory
;test bit three in the common memory symbol _bar
These operands and prefix are not applicable with operands to the PIC18 MOVFF
instruction, which takes two untruncated addresses, and which always works
independently of the BSR.
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Macro Assembler
For example, the following instructions show the W register being moved to first, an
absolute location; and, then, to an address represented by an identifier. Bank selection
and masking has been used in this example. The PIC18 op codes for these instructions, assuming that the address assigned to _foo is 0x516 and to _bar is 0x55, are
shown below.
6EE5
6E55
0105
6F16
6F16
MOVWF 0FE5h
MOVWF _bar,c
BANKSEL(_foo)
MOVWF BANKMASK(_foo),b
MOVWF BANKMASK(_foo)
;write to access bank location 0xFE5
;write to access bank location 0x55
;set up BSR to access _foo
;write to _foo (banked)
;defaults to banked access
Notice that the first two instruction opcodes have the RAM access bit (bit 8 of the
op-code) cleared, but that the bit is set in the last two instructions.
It is recommended that you always specify the RAM access operand or the common
memory prefix. If these are not present, the instruction address is absolute, and the
address is within the upper half of the access bank (which dictates that the address
must not masked), the instruction will use the access bank RAM. In all other situations,
the instruction will access banked memory.
The destination operand and the RAM access operand can be listed in any order for
PIC18 instructions. For example, the following two instructions are identical:
ADDWF
ADDWF
6.2.1.2
_foo,f,c
_foo,c,f
BANK AND PAGE SELECTION
The BANKSEL pseudo instruction can be used to generate instructions to select the
bank of the operand specified. The operand should be the symbol or address of an
object that resides in the data memory.
Depending on the target device, the generated code will either contain one or more bit
instructions to set/clear bits in the appropriate register, or use a MOVLB instruction (in
the case of enhanced mid-range or PIC18 devices). As this pseudo instruction can
expand to more than one instruction on mid-range or baseline parts, it should not
immediately follow a BTFSX instruction on those devices.
For example:
MOVLW 20
BANKSEL(_foobar)
;select bank for next file instruction
MOVWF BANKMASK(_foobar) ;write data and mask address
In the same way, the PAGESEL pseudo instruction can be used to generate code to
select the page of the address operand. For the current page, you can use the location
counter, $, as the operand.
Depending on the target device, the generated code will either contain one or more
instructions to set/clear bits in the appropriate register, or use a MOVLP instruction in
the case of enhanced mid-range PIC devices. As the directive could expand to more
than one instruction, it should not immediately follow a BTFSX instruction.
For example:
FCALL _getInput
PAGESEL $
;select this page
This directive is accepted when compiling for PIC18 targets but has no effect and does
not generate any code. Support is purely to allow easy migration across the 8-bit
devices.
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6.2.1.3
ADDRESS MASKING
A macro, BANKMASK(), can be used with an identifier; so, it is usable as an operand
to instructions that expect a file register address. The macro does this by ANDing out
the bank information using a suitable mask. It is available once you include the
<xc.inc> file. An example of this macro is given in Section 6.2.1.2 “Bank and Page
Selection”.
All MPLAB XC8 assembly identifiers represent a full address. This address includes
the bank information for the object it represents. Virtually all instructions in the 8-bit PIC
instruction sets that take a file register operand expect this operand value to be an offset into the currently selected bank. As the device families have different bank sizes,
the width of this offset is different for each family. Use of this macro increases assembly
code portability across Microchip devices, since it adjusts the mask to suit the bank size
of the target device.
Do not use this macro with either operand to the PIC18’s MOVFF instruction, which
requires two full, banked addresses to be specified, or with any other instruction that
expects a full address.
6.2.1.4
MOVFW PSEUDO INSTRUCTION
The MOVFW pseudo instruction implemented by MPLAB C18 is not implemented in
MPLAB XC8. You will need to use the standard PIC instruction that performs an identical function. Note that the MPLAB C18 instruction:
MOVFW foobar
maps directly to the standard PIC instruction:
MOVF foobar,w
6.2.1.5
MOVIW/MOVWI INSTRUCTIONS
Both the MOVIW and MOVWI instructions have operands which differ in syntax to that
indicated in the data sheet. These instructions are only available with enhanced
mid-range devices.
The indexed Indirect operands to these instructions have the FSR offset specified first
in square brackets, followed by the FSR name, for example:
MOVIW [6]FSR0
MOVWI [0x10]FSR1
The pre/post increment/decrement form of these instructions use the name of the FSR
register, not the indirection register (INDF), for example:
MOVIW ++FSR0
MOVWI FSR1++
MOVWI FSR0--
6.2.1.6
MOVFF/MOVFFL INSTRUCTIONS
The MOVFF instruction is a physical device instruction, but for PIC18 devices that have
extended data memory, it also serves as a placeholder for the MOVFFL instruction.
For these devices, when generating output for the MOVFF instruction, the assembler
checks the psects that hold the operand symbols. If the psect containing the source
operand is the same psect that contains the destination operand, then the instruction
is encoded as a MOVFF instruction. If the psects of both source and destination operands have the lowdata psect flag set, the instruction is also encoded as a MOVFF
instruction. In all other situations, the instruction is encoded as a MOVFFL instruction.
Note that assembly list files will always show the MOVFF placeholder regardless of how
it is encoded.
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6.2.1.7
INTERRUPT RETURN MODE
The RETFIE PIC18 instruction can be followed by “f” (no comma) to indicate that the
shadow registers should be retrieved and copied to their corresponding registers on
execution. Without this modifier, the registers are not updated from the shadow
registers. This replaces the “0” and “1” operands indicated in the device data sheet.
The following examples show both forms and the opcodes they generate.
0011
0010
RETFIE f
RETFIE
;shadow registers copied
;return without copy
The baseline and mid-range devices do not allow such a syntax.
6.2.1.8
LONG JUMPS AND CALLS
The assembler recognizes several mnemonics that expand into regular PIC MCU
assembly instructions. The mnemonics are FCALL and LJMP. On baseline and
mid-range parts, these instructions expand into regular CALL and GOTO instructions
respectively, but also ensure the instructions necessary to set the bits in PCLATH (for
mid-range devices) or STATUS (for baseline devices) will be generated when the destination is in another page of program memory. Whether the page selection instructions
are generated, and exactly where they will be located, is dependent on the surrounding
source code. Page selection instructions can appear immediately before the call or
jump, or be generated as part of, and immediately after, a previous FCALL/LJMP
instruction.
On PIC18 devices, these mnemonics are present purely for compatibility with smaller
8-bit devices and are always expanded as regular PIC18 CALL and GOTO instructions.
These additional mnemonics should be used where possible as they make assembly
code independent of the final position of the routines that are to be executed. If the call
or jump is determined to be within the current page, the additional code to set the
PCLATH bits can be optimized away. Note that assembly code that is added in-line with
C code is never optimized and assembly modules require a specific option to enable
optimization, see Section 4.8.45 “--OPT: Invoke Compiler Optimizations”. Unoptimized
FCALL and LJMP instruction will always generate page selection code.
The following mid-range PIC example shows an FCALL instruction in the assembly list
file. You can see that the FCALL instruction has expanded to five instructions. In this
example, there are two bit instructions that set/clear bits in the PCLATH register. Bits
are also set/cleared in this register after the call to reselect the page that was selected
before the FCALL.
13
14
0079
007A
15
007F
3021
120A
120A
3400
158A
118A
2000
movlw
fcall
33
_phantom
retlw
0
Since FCALL and LJMP instructions can expand into more than one instruction, they
should never be preceded by an instruction that can skip, e.g., a BTFSC instruction.
The FCALL and LJMP instructions assume that the psect that contains them is smaller
than a page. Do not use these instructions to transfer control to a label in the current
psect if it is larger than a page. The default linker options will not permit code psects to
be larger than a page.
On PIC18 devices, the regular CALL instruction can be followed by a “,f” to indicate
that the W, STATUS and BSR registers should be pushed to their respective shadow
registers. This replaces the “,1” syntax indicated on the device data sheet.
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6.2.1.9
RELATIVE BRANCHES
The PIC18 devices implement conditional relative branch instructions, e.g., BZ, BNZ.
These instructions have a limited jump range compared to the GOTO instruction.
Note that in some instance, the assembler can change a relative branch instruction to
be a relative branch with the opposite condition over a GOTO instruction. For example:
BZ error
;next
can become:
BNZ l18
GOTO error
l18:
;next
This is functionally identical and is performed so that the conditional branch can use
the same destination range as the GOTO instruction.
6.2.2
Statement Formats
Legal statement formats are shown in Table 6-1: "ASPIC Statement Formats".
The label field is optional and, if present, should contain one identifier. A label can
appear on a line of its own, or precede a mnemonic as shown in the second format.
The third format is only legal with certain assembler directives, such as MACRO, SET
and EQU. The name field is mandatory and should contain one identifier.
If the assembly file is first processed by the C preprocessor, see Section 4.8.10 “-P:
Preprocess Assembly Files”, then it can also contain lines that form valid preprocessor
directives. See Section 5.14.1 “C Language Comments”, for more information on the
format for these directives.
There is no limitation on what column or part of the line in which any part of the
statement should appear.
TABLE 6-1:
ASPIC STATEMENT FORMATS
Format #
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Field1
Field2
Field3
Field4
Format 1
label:
Format 2
label:
mnemonic
operands
; comment
Format 3
name
pseudo-op
operands
; comment
Format 4
; comment only
Format 5
empty line
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Macro Assembler
6.2.3
Characters
The character set used is standard 7 bit ASCII. Alphabetic case is significant for
identifiers, but not mnemonics and reserved words. Tabs are treated as equivalent to
spaces.
6.2.3.1
DELIMITERS
All numbers and identifiers must be delimited by white space, non-alphanumeric
characters or the end of a line.
6.2.3.2
SPECIAL CHARACTERS
There are a few characters that are special in certain contexts. Within a macro body,
the character & is used for token concatenation. To use the bitwise & operator within a
macro body, escape it by using && instead. In a macro argument list, the angle brackets
< and > are used to quote macro arguments.
6.2.4
Comments
An assembly comment is initiated with a semicolon that is not part of a string or
character constant.
If the assembly file is first processed by the C preprocessor, see Section 4.8.10 “-P:
Preprocess Assembly Files”, then the file can also contain C or C++ style comments
using the standard /* ... */ and // syntax.
6.2.4.1
SPECIAL COMMENT STRINGS
Several comment strings are appended to compiler-generated assembly instructions
by the code generator. These comments are typically used by the assembler optimizer.
The comment string ;volatile is used to indicate that the memory location being
accessed in the instruction is associated with a variable that was declared as
volatile in the C source code. Accesses to this location which appear to be
redundant will not be removed by the assembler optimizer if this string is present.
This comment string can also be used in hand-written assembly source to achieve the
same effect for locations defined and accessed in assembly code.
The comment string ;wreg free is placed on some CALL instructions. The string indicates that the W register was not loaded with a function parameter; i.e., it is not in use.
If this string is present, optimizations can be made to assembler instructions before the
function call, which loads the W register redundantly.
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6.2.5
Constants
6.2.5.1
NUMERIC CONSTANTS
The assembler performs all arithmetic with signed 32-bit precision.
The default radix for all numbers is 10. Other radices can be specified by a trailing base
specifier, as given in Table 6-2.
TABLE 6-2:
ASPIC NUMBERS AND BASES
Radix
Format
Binary
Digits 0 and 1 followed by B
Octal
Digits 0 to 7 followed by O, Q, o or q
Decimal
Digits 0 to 9 followed by D, d or nothing
Hexadecimal
Digits 0 to 9, A to F preceded by 0x or followed by H or h
Hexadecimal numbers must have a leading digit (e.g., 0ffffh) to differentiate them from
identifiers. Hexadecimal digits are accepted in either upper or lower case.
Note that a binary constant must have an upper case B following it, as a lower case b
is used for temporary (numeric) label backward references.
In expressions, real numbers are accepted in the usual format, and are interpreted as
IEEE 32-bit format.
6.2.5.2
CHARACTER CONSTANTS AND STRINGS
A character constant is a single character enclosed in single quotes ’.
Multi-character constants, or strings, are a sequence of characters, not including carriage return or newline characters, enclosed within matching quotes. Either single
quotes ’ or double quotes " can be used, but the opening and closing quotes must be
the same.
6.2.6
Identifiers
Assembly identifiers are user-defined symbols representing memory locations or numbers. A symbol can contain any number of characters drawn from the alphabetics,
numerics, and the special characters: dollar, $; question mark, ?; and underscore, _.
The first character of an identifier cannot be numeric. The case of alphabetics is significant, e.g., Fred is not the same symbol as fred. Some examples of identifiers are
shown here:
An_identifier
an_identifier
an_identifier1
$
?$_12345
An identifier cannot be one of the assembler directives, keywords, or psect flags.
An identifier that begins with at least one underscore character can be accessed from
C code. Care must be taken with such symbols that they do not interact with C code
identifiers. Identifiers that do not begin with an underscore can only be accessed from
the assembly domain. See Section 5.12.3.1 “Equivalent Assembly Symbols” for the
mapping between the C and assembly domains.
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Macro Assembler
6.2.6.1
SIGNIFICANCE OF IDENTIFIERS
Users of other assemblers that attempt to implement forms of data typing for identifiers
should note that this assembler attaches no significance to any symbol, and places no
restrictions or expectations on the usage of a symbol.
The names of psects (program sections) and ordinary symbols occupy separate,
overlapping name spaces, but other than this, the assembler does not care whether a
symbol is used to represent bytes, words or sports cars. No special syntax is needed
or provided to define the addresses of bits or any other data type, nor will the assembler
issue any warnings if a symbol is used in more than one context. The instruction and
addressing mode syntax provide all the information necessary for the assembler to
generate correct code.
6.2.6.2
ASSEMBLER-GENERATED IDENTIFIERS
Where a LOCAL directive is used in a macro block, the assembler will generate a
unique symbol to replace each specified identifier in each expansion of that macro.
These unique symbols will have the form ??nnnn where nnnn is a 4-digit number. The
user should avoid defining symbols with the same form.
6.2.6.3
LOCATION COUNTER
The current location within the active program section is accessible via the symbol $.
This symbol expands to the address of the currently executing instruction (which is different than the address contained in the program counter (PC) register when executing
this instruction). Thus:
GOTO $
;endless loop
will represent code that will jump to itself and form an endless loop. By using this
symbol and an offset, a relative jump destination can be specified.
Any address offset added to $ has the native addressability of the target device. So, for
baseline and mid-range devices, the offset is the number of instructions away from the
current location, as these devices have word-addressable program memory. For PIC18
instructions, which use byte addressable program memory, the offset to this symbol
represents the number of bytes from the current location. As PIC18 instructions must
be word aligned, the offset to the location counter should be a multiple of 2. All offsets
are rounded down to the nearest multiple of 2. For example:
GOTO
MOVLW
MOVWF
$+2
8
_foo
;skip...
;to here for PIC18 devices, or
;to here for baseline and mid-range devices
will skip the MOVLW instruction on baseline or mid-range devices. On PIC18 devices,
GOTO $+2 will jump to the following instruction; i.e., act like a NOP instruction.
6.2.6.4
REGISTER SYMBOLS
Code in assembly modules can gain access to the special function registers by including pre-defined assembly header files. The appropriate file can be included by add the
line:
#include <xc.inc>
to the assembler source file. Note that the file must be included using a C pre-processor
directive and hence the option to preprocess assembly files must be enabled when
compiling, see Section 4.8.10 “-P: Preprocess Assembly Files”. This header file contains appropriate commands to ensure that the header file specific for the target device
is included into the source file.
These header files contain EQU declarations for all byte or multi-byte sized registers
and #define macros for named bits within byte registers.
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6.2.6.5
SYMBOLIC LABELS
A label is a symbolic alias that is assigned a value that is equal to the current address
within the current psect. Labels are not assigned a value until link time.
A label definition consists of any valid assembly identifier followed by a colon, :. The
definition can appear on a line by itself or it can be positioned before a statement. Here
are two examples of legitimate labels interspersed with assembly code.
frank:
MOVLW
1
GOTO
fin
simon44: CLRF _input
Here, the label frank will ultimately be assigned the address of the MOVLW instruction,
and simon44 the address of the CLRF instruction. Regardless of how they are defined,
the assembler list file produced by the assembler will always show labels on a line by
themselves.
Note that the colon following the label is mandatory for PIC18 assembly, but is recommended in assembly for all other devices. Symbols that are not interpreted as instructions are assumed to be labels. Mistyped assembly instructions can sometimes be
treated as labels without an error message being issued. Thus the code:
mistake:
MOVLW 23h
MOVWF 37h
REUTRN
; oops
defines a symbol called REUTRN, which was intended to be the RETURN instruction.
This cannot occur with PIC18 assembly code, as the colon following a label is
mandatory; the compiler would report an error when reached the line containing
REUTRN.
Labels can be used (and are preferred) in assembly code, rather than using an absolute address with other instructions. In this way, they can be used as the target location
for jump-type instructions or to load an address into a register.
Like variables, labels have scope. By default, they can be used anywhere in the module
in which they are defined. They can be used by code located before their definition. To
make a label accessible in other modules, use the GLOBAL directive. See
Section 6.2.9.1 “GLOBAL” for more information.
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Macro Assembler
6.2.7
Expressions
The operands to instructions and directives are comprised of expressions. Expressions
can be made up of numbers, identifiers, strings and operators.
Operators can be unary (one operand, e.g., not) or binary (two operands, e.g., +). The
operators allowable in expressions are listed in Table 6-3.
TABLE 6-3:
ASPIC OPERATORS
Operator
Purpose
Example
*
multiplication
MOVLW 4*33,w
+
addition
BRA $+1
-
subtraction
DB 5-2
/
division
MOVLW 100/4
= or eq
equality
IF inp eq 66
> or gt
signed greater than
IF inp > 40
>= or ge
signed greater than or equal to
IF inp ge 66
< or lt
signed less than
IF inp < 40
<= or le
signed less than or equal to
IF inp le 66
<> or ne
signed not equal to
IF inp <> 40
low
low byte of operand
MOVLW low(inp)
high
high byte of operand
MOVLW high(1008h)
highword
high 16 bits of operand
DW highword(inp)
mod
modulus
MOVLW 77mod4
& or and
bitwise AND
CLRF inp&0ffh
^
bitwise XOR (exclusive or)
MOVF inp^80,w
|
bitwise OR
MOVF inp|1,w
not
bitwise complement
MOVLW not 055h,w
<< or shl
shift left
DB inp>>8
>> or shr
shift right
MOVLW inp shr 2,w
rol
rotate left
DB inp rol 1
ror
rotate right
DB inp ror 1
float24
24-bit version of real operand
DW float24(3.3)
nul
tests if macro argument is null
The usual rules governing the syntax of expressions apply.
The operators listed can all be freely combined in both constant and relocatable
expressions. The linker permits relocation of complex expressions, so the results of
expressions involving relocatable identifiers cannot be resolved until link time.
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6.2.8
Program Sections
Program sections, or psects, are simply a section of code or data. They are a way of
grouping together parts of a program (via the psect’s name) even though the source
code cannot be physically adjacent in the source file, or even where spread over
several modules. For an introductory guide to psects, see Section 5.15.1 “Program
Sections”.
A psect is identified by a name and has several attributes. The PSECT assembler directive is used to define a psect. It takes as arguments a name and an optional
comma-separated list of flags. See Section 5.15.2 “Compiler-Generated Psects” for a
list of all psects that the code generator defines. Chapter 7. Linker has more information on the operation of the linker and on options that can be used to control psect
placement in memory.
The assembler associates no significance to the name of a psect. The linker, also, is
not aware of which psects are compiler-generated or which are user-defined. Unless
defined as abs (absolute), psects are relocatable.
Code or data that is not explicitly placed into a psect will become part of the default
(unnamed) psect.
When writing assembly code, you can use the existing compiler-generated psects,
described in Section 5.15.2 “Compiler-Generated Psects”, or create your own. You will
not need to adjust the linker options if you are using compiler-generated psects. If you
create your own psects, try to associate them with an existing linker class (see
Section 5.15.3 “Default Linker Classes” and Section 6.2.9.3.3 “Class”) otherwise you
can need to specify linker options for them to be allocated correctly.
Note, that the length and placement of psects is important. It is easier to write code if
all executable code is located in psects that do not cross any device pages boundaries;
so, too, if data psects do not cross bank boundaries. The location of psects (where they
are linked) must match the assembly code that accesses the psect contents.
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Macro Assembler
6.2.9
Assembler Directives
Assembler directives, or pseudo-ops, are used in a similar way to instruction mnemonics. With the exception of PAGESEL and BANKSEL, these directives do not generate
instructions. The DB, DW and DDW directives place data bytes into the current psect. The
directives are listed in Table 6-4, and are detailed below in the following sections.
TABLE 6-4:
ASPIC ASSEMBLER DIRECTIVES
Directive
Purpose
GLOBAL
make symbols accessible to other modules or allow reference to
other global symbols defined in other modules
END
end assembly
PSECT
declare or resume program section
ORG
set location counter within current psect
EQU
define symbol value
EXTRN
link with global symbols defined in other modules
SET
define or re-define symbol value
DB
define constant byte(s)
DW
define constant word(s)
DDW
define double-width constant word(s) (PIC18 devices only)
DS
reserve storage
DABS
define absolute storage
IF
conditional assembly
ELSIF
alternate conditional assembly
ELSE
alternate conditional assembly
ENDIF
end conditional assembly
FNCALL
inform the linker that one function calls another
FNROOT
inform the linker that a function is the “root” of a call graph
MACRO
macro definition
ENDM
end macro definition
LOCAL
define local tabs
ALIGN
align output to the specified boundary
BANKSEL
generate code to select bank of operand
PAGESEL
generate set/clear instruction to set PCLATH bits for this page
PROCESSOR
define the particular chip for which this file is to be assembled.
REPT
repeat a block of code n times
IRP
repeat a block of code with a list
IRPC
repeat a block of code with a character list
SIGNAT
define function signature
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6.2.9.1
GLOBAL
The GLOBAL directive declares a list of comma-separated symbols. If the symbols are
defined within the current module, they are made public. If the symbols are not defined
in the current module, they are made references to public symbols defined in external
modules. Thus to use the same symbol in two modules the GLOBAL directive must be
used at least twice: once in the module that defines the symbol to make that symbol
public, and again in the module that uses the symbol to link in with the external
definition.
For example:
GLOBAL
lab1,lab2,lab3
6.2.9.2
END
The END directive is optional, but if present should be at the very end of the program.
It will terminate the assembly and not even blank lines should follow this directive.
If an expression is supplied as an argument, that expression will be used to define the
entry point of the program. This is stored in a start record in the object file produced by
the assembler. Whether this is of any use will depend on the linker.
The default runtime startup code that is defined by the compiler will contain an END
directive with a start address. As only one start address can be specified for each project, you generally do not need to define this address – you can use the END directive
with no entry point in any file.
For example:
END
start_label
;defines the entry point
or
END
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Macro Assembler
6.2.9.3
PSECT
The PSECT directive declares or resumes a program section. For an introductory guide
to psects, see Section 5.15.1 “Program Sections”.
The directive takes as argument a name and, optionally, a comma-separated list of
flags. The allowed flags specify attributes of the psect. They are listed in Table 6-5.
The psect name is in a separate name space to ordinary assembly symbols, so a psect
can use the same identifier as an ordinary assembly identifier. However, a psect name
cannot be one of the assembler directives, keywords, or psect flags.
Once a psect has been declared, it can be resumed later by another PSECT directive;
however, the flags need not be repeated and will be propagated from the earlier declaration. If two PSECT directives are encountered with contradicting flags, then an error
is generated.
TABLE 6-5:
PSECT FLAGS
Flag
Meaning
abs
psect is absolute
bit
psect holds bit objects
class=name
specify class name for psect
delta=size
size of an addressing unit
global
psect is global (default)
inline
psect contents (function) can be inlined when called
keep
psect will not be deleted after inlining
limit=address
upper address limit of psect
local
psect is unique and will not link with others having the
same name
lowdata
psect will be entirely located below the 0x1000 address
merge=allow
allow or prevent merging of this psect
noexec
for debugging purposes, this psect contains no executable
code
optim=optimizations specify optimizations allowable with this psect
ovrld
psect will overlap same psect in other modules
pure
psect is to be read-only
reloc=boundary
start psect on specified boundary
size=max
maximum size of psect
space=area
represents area in which psect will reside
split=allow
allow or prevent splitting of this psect
with=psect
place psect in the same page as specified psect
Some examples of the use of the PSECT directive follow:
PSECT fred
PSECT bill,size=100h,global
PSECT joh,abs,ovrld,class=CODE,delta=2
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6.2.9.3.1
Abs
The abs flag defines the current psect as being absolute; i.e., it is to start at location 0.
This does not mean that this module’s contribution to the psect will start at 0, since
other modules can contribute to the same psect. See also Section 6.2.9.3.13 “Ovrld”.
An abs-flagged psect is not relocatable and an error will result if a linker option is
issued that attempts to place such a psect at any location.
6.2.9.3.2
Bit
The bit flag specifies that a psect holds objects that are 1 bit long. Such psects will
have a scale value of 8 to indicate that there are 8 addressable units to each byte of
storage and all addresses associated with this psect will be bit address, not byte
addresses. The scale value is indicated in the map file; see Section 7.4 “Map Files”.
6.2.9.3.3
Class
The class flag specifies a corresponding linker class name for this psect. A class is a
range of addresses in which psects can be placed.
Class names are used to allow local psects to be located at link time, since they cannot
always be referred to by their own name in a -P linker option (as would be the case if
there are more than one local psect with the same name).
Class names are also useful where psects need only be positioned anywhere within a
range of addresses rather than at a specific address. The association of a class with a
psect that you have defined typically means that you do not need to supply a custom
linker option to place it in memory.
See Section 7.2.1 “-Aclass =low-high,...” for information on how linker classes are
defined.
6.2.9.3.4
Delta
The delta flag defines the size of the addressable unit. In other words, the number of
data bytes that are associated with each address.
With PIC mid-range and baseline devices, the program memory space is word
addressable; so, psects in this space must use a delta of 2. That is to say, each address
in program memory requires 2 bytes of data in the HEX file to define their contents. So,
addresses in the HEX file will not match addresses in the program memory.
The data memory space on these devices is byte addressable; so, psects in this space
must use a delta of 1. This is the default delta value.
All memory spaces on PIC18 devices are byte addressable; so, a delta of 1 (the
default) should be used for all psects on these devices.
The redefinition of a psect with conflicting delta values can lead to phase errors being
issued by the assembler.
6.2.9.3.5
Global
A psect defined as global will be combined with other global psects with the same
name at link time. Psects are grouped from all modules being linked.
Psects are considered global by default, unless the local flag is used.
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6.2.9.3.6
Inline
This flag is deprecated. Consider, instead, using the optim psect flag.
The inline flag is used by the code generator to tell the assembler that the contents
of a psect can be inlined. If this operation is performed, the contents of the inline
psect will be copied and used to replace calls to the function defined in the psect.
6.2.9.3.7
Keep
This flag is deprecated. Consider, instead, using the optim psect flag.
Psects that are candidates for inlining (see Section 6.2.9.3.6 “Inline”) can be deleted
after the inlining takes place. This flag ensures that the psect is not deleted after any
inlining by the assembler optimizer.
6.2.9.3.8
Limit
The limit flag specifies a limit on the highest address to which a psect can extend. If
this limit is exceeded when it is positioned in memory, an error will be generated.
6.2.9.3.9
Local
A psect defined as local will not be combined with other local psects from other
modules at link time, even if there are others with the same name. Where there are two
local psects in the one module, they reference the same psect. A local psect
cannot have the same name as any global psect, even one in another module.
Psects which are local and which are not associated with a linker class (see
Section 6.2.9.3.3 “Class”) cannot be linked to an address using the -P linker option,
since there could be more than one psect with this name. Typically a class is specified
with these psects and they are placed anywhere in the memory range associated with
that class.
6.2.9.3.10 Merge
This flag is deprecated. Consider, instead, using the optim psect flag.
This flag can be assigned 0, 1, or not specified. When assigned 0, the psect will never
be merged by the assembly optimizer during optimizations. If assigned the value 1, the
psect can be merged if other psect attributes allow it and the optimizer can see an
advantage in doing so. If this flag is not specified, then merging will not take place.
Typically, merging is only performed on code-based psects (text psects).
6.2.9.3.11
Noexec
The noexec flag is used to indicate that the psect contains no executable code. This
information is only relevant for debugging purposes and does not affect the assembler
output.
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6.2.9.3.12 Optim
The optim psect flag can be used to indicate the optimizations that can be performed
on the psect. The optimizations are indicated by a colon-separated list of names. An
empty list implies that no optimizations can be performed on the psect contents. The
allowable optimizations will be performed on the psect if that optimization is available
in the compiler’s operating mode, and the assembler optimizer is enabled (see
Section 4.8.45 “--OPT: Invoke Compiler Optimizations”). The available optimizations
are shown in Table 6-6.
TABLE 6-6:
OPTIM FLAG NAMES
Name
Optimization
inline
allow the psect content to be inlined
jump
perform jump-based optimizations
merge
allow the psect’s content to be merged with that of other similar
psects (PIC10/12/16 devices only)
pa
perform proceedural abstraction
peep
perform peephole optimizations
remove
allow the psect to be removed entirely if it is completely inlined
split
allow the psect to be split into smaller psects if it surpasses size
restrictions (PIC10/12/16 devices only)
empty
perform no optimization on this psect
So, for example, the psect definition:
PSECT myText,class=CODE,reloc=2,optim=inline:jump:split
allows the assembler optimizer to perform inlining, splitting and jump-type optimizations of the myText psect content if those optimizations are enabled. The definition:
PSECT myText,class=CODE,reloc=2,optim=
disables all optimizations associated with this psect regardless of the optimizer setting.
The optim psect flag replaces the use of the separate psect flags: merge, split,
inline, and keep.
6.2.9.3.13 Ovrld
A psect defined as ovrld will have the contribution from each module overlaid, rather
than concatenated at link time. This flag in combination with the abs flag (see
Section 6.2.9.3.1 “Abs”) defines a truly absolute psect; i.e., a psect within which any
symbols defined are absolute.
6.2.9.3.14 Pure
The pure flag instructs the linker that this psect will not be modified at runtime. So, for
example, be placed in ROM. This flag is of limited usefulness since it depends on the
linker and target system enforcing it.
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6.2.9.3.15 Reloc
The reloc flag allows the specification of a requirement for alignment of the psect on
a particular boundary. The boundary specification must be a power of two, for example
2, 8 or 0x40. For example, the flag reloc=100h would specify that this psect must
start on an address that is a multiple of 0x100 (e.g. 0x100, 0x400, or 0x500).
PIC18 instructions must be word aligned, so a reloc value of 2 must be used for any
PIC18 psect that contains executable code. All other sections, and all sections for all
other devices, can typically use the default reloc value of 1.
6.2.9.3.16 Size
The size flag allows a maximum size to be specified for the psect, e.g., size=100h.
This will be checked by the linker after psects have been combined from all modules.
6.2.9.3.17 Space
The space flag is used to differentiate areas of memory that have overlapping
addresses, but are distinct. Psects that are positioned in program memory and data
memory have a different space value to indicate that the program space address 0, for
example, is a different location to the data memory address 0.
The memory spaces associated with the space flag numbers are shown in Table 6-7.
TABLE 6-7:
SPACE FLAG NUMBERS
Space Flag Number
Memory Space
0
Program memory
1
Data memory
2
Reserved
3
EEPROM
Devices that have a banked data space do not use different space values to identify
each bank. A full address that includes the bank number is used for objects in this
space. So, each location can be uniquely identified. For example, a device with a bank
size of 0x80 bytes will use address 0 to 0x7F to represent objects in bank 0, and then
addresses 0x80 to 0xFF to represent objects in bank 1, etc.
6.2.9.3.18 Split
This flag is deprecated. Consider, instead, using the optim psect flag.
This flag can be assigned 0, 1, or not specified. When assigned 0, the psect will never
be split by the assembly optimizer during optimizations. If assigned the value 1, the
psect can be split if other psect attributes allow it and the psect is too large to fit in available memory. If this flag is not specified, then the splitability of this psect is based on
whether the psect can be merged, see Section 6.2.9.3.10 “Merge”.
6.2.9.3.19 With
The with flag allows a psect to be placed in the same page with another psect. For
example the flag with=text will specify that this psect should be placed in the same
page as the text psect.
The term withtotal refers to the sum of the size of each psect that is placed “with” other
psects.
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6.2.9.4
ORG
The ORG directive changes the value of the location counter within the current psect.
This means that the addresses set with ORG are relative to the base address of the
psect, which is not determined until link time.
Note:
The much-abused ORG directive does not move the location counter to the
absolute address you specify. Only if the psect in which this directive is
placed is absolute and overlaid will the location counter be moved to the
address specified. To place objects at a particular address, place them in a
psect of their own and link this at the required address using the linkers -P
option, see Section 7.2.18 “-Pspec”. The ORG directive is not commonly
required in programs.
The argument to ORG must be either an absolute value, or a value referencing the current psect. In either case, the current location counter is set to the value determined by
the argument. It is not possible to move the location counter backward. For example:
ORG 100h
will move the location counter to the beginning of the current psect plus 100h. The
actual location will not be known until link time.
In order to use the ORG directive to set the location counter to an absolute value, the
directive must be used from within an absolute, overlaid psect. For example:
PSECT absdata,abs,ovrld
ORG 50h
;this is guaranteed to reside at address 50h
6.2.9.5
EQU
This pseudo-op defines a symbol and equates its value to an expression. For example
thomas EQU 123h
The identifier thomas will be given the value 123h. EQU is legal only when the symbol
has not previously been defined. See also, Section 6.2.9.7 “SET”, which allows for
redefinition of values.
This directive performs a similar function to the preprocessor’s #define directive, see
Section 5.14.1 “C Language Comments”.
6.2.9.6
EXTRN
This pseudo-op is similar to GLOBAL (see Section 6.2.9.1 “GLOBAL”), but can only be
used to link in with global symbols defined in other modules. An error will be triggered
if you use EXTRN with a symbol that is defined in the same module.
6.2.9.7
SET
This pseudo-op is equivalent to EQU (Section 6.2.9.5 “EQU”), except that allows a symbol to be re-defined without error. For example:
thomas SET 0h
This directive performs a similar function to the preprocessor’s #define directive, see
Section 5.14.1 “C Language Comments”.
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6.2.9.8
DB
The DB directive is used to initialize storage as bytes. The argument is a comma-separated list of expressions, each of which will be assembled into one byte and
assembled into consecutive memory locations.
Examples:
alabel: DB
’X’,1,2,3,4,
If the size of an address unit in the program memory is 2 bytes, as it will be for baseline
and mid-range devices (see Section 6.2.9.3.4 “Delta”), the DB pseudo-op will initialize
a word with the upper byte set to zero. So, the above example will define bytes padded
to the following words.
0058 0001 0002 0003 0004
However, on PIC18 devices (PSECT directive’s delta flag should be 1), no padding
will occur and the following data will appear in the HEX file.
58 01 02 03 04
6.2.9.9
DW
The DW directive operates in a similar fashion to DB, except that it assembles
expressions into 16-bit words. Example:
DW -1, 3664h, ’A’
6.2.9.10
DDW
The DDW directive operates in a similar fashion to DW, except that it assembles
expressions into double-width (32-bit) words. Example:
DDW ’d’, 12345678h, 0
6.2.9.11
DS
This directive reserves, but does not initialize, memory locations. The single argument
is the number of bytes to be reserved.
This directive is typically used to reserve memory location for RAM-based objects in
the data memory. If used in a psect linked into the program memory, it will move the
location counter, but not place anything in the HEX file output. Note that because the
size of an address unit in the program memory is 2 bytes (see
Section 6.2.9.3.4 “Delta”), the DS pseudo-op will actually reserve an entire word.
A variable is typically defined by using a label and then the DS directive to reserve
locations at the label location.
Examples:
alabel: DS 23
xlabel: DS 2+3
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;Reserve 23 bytes of memory
;Reserve 5 bytes of memory
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6.2.9.12
DABS
This directive allows one or more bytes of memory to be reserved at the specified
address. The general form of the directive is:
DABS memorySpace, address, bytes[ ,symbol]
where memorySpace is a number representing the memory space in which the reservation will take place, address is the address at which the reservation will take place,
and bytes is the number of bytes that is to be reserved. The symbol is optional and
refers to the name of the object at the address.
Use of symbol in the directive will aid debugging. The symbol is automatically made
globally accessible and is equated to the address specified in the directive. So, for
example, the following directive uses a symbol:
DABS 1,0x100,4,foo
that is identical to the following directives:
GLOBAL foo
foo EQU 0x100
DABS 1,0x100,4
This directive differs to the DS directive in that it can be used to reserve memory at any
location, not just within the current psect. Indeed, these directives can be placed anywhere in the assembly code and do not contribute to the currently selected psect in any
way.
The memory space number is the same as the number specified with the space flag
option to psects (see Section 6.2.9.3.17 “Space”).
The code generator issues a DABS directive for every user-defined absolute C variable,
or for any variables that have been allocated an address by the code generator.
The linker reads this DABS-related information from object files and ensures that the
reserved addresses are not used for other memory placement.
6.2.9.13
IF, ELSIF, ELSE AND ENDIF
These directives implement conditional assembly. The argument to IF and ELSIF
should be an absolute expression. If it is non-zero, then the code following it up to the
next matching ELSE, ELSIF or ENDIF will be assembled. If the expression is zero, then
the code up to the next matching ELSE or ENDIF will not be output.
At an ELSE, the sense of the conditional compilation will be inverted, while an ENDIF
will terminate the conditional assembly block.
These directives do not implement a runtime conditional statement in the same way
that the C statement if() does; they are only evaluated at compile time. In addition,
assembly code in both true and false cases is always scanned and interpreted, but the
machine code corresponding to instructions is output only if the condition matches.
This implies that assembler directives (e.g., EQU) will be processed regardless of the
state of the condition expression, and so, should not be used inside an IF construct.
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For example:
IF ABC
GOTO aardvark
ELSIF DEF
GOTO denver
ELSE
GOTO grapes
ENDIF
ENDIF
In this example, if ABC is non-zero, the first GOTO instruction will be assembled but not
the second or third. If ABC is zero and DEF is non-zero, the second GOTO instruction will
be assembled but the first and third will not. If both ABC and DEF are zero, the third
GOTO instruction will be assembled. Note in the above example, only one GOTO instruction will appear in the output; which one will be determined by the values assigned to
ABC and DEF.
Conditional assembly blocks can be nested.
6.2.9.14
MACRO AND ENDM
These directives provide for the definition of assembly macros, optionally with arguments. See Section 6.2.9.5 “EQU” for simple association of a value with an identifier,
or Section 5.14.1 “C Language Comments” for the preprocessor’s #define macro
directive, which can also work with arguments.
The MACRO directive should be preceded by the macro name and optionally followed
by a comma-separated list of formal arguments. When the macro is used, the macro
name should be used in the same manner as a machine opcode, followed by a list of
arguments to be substituted for the formal parameters.
For example:
;macro: movlf
;args: arg1 - the literal value to load
;
arg2 - the NAME of the source variable
;descr: Move a literal value into a nominated file register
movlf
MACRO
arg1,arg2
MOVLW arg1
MOVWF arg2 mod 080h
ENDM
When used, this macro will expand to the 2 instructions in the body of the macro, with
the formal parameters substituted by the arguments. Thus:
movlf 2,tempvar
expands to:
MOVLW 2
MOVWF tempvar mod 080h
The & character can be used to permit the concatenation of macro arguments with
other text, but is removed in the actual expansion. For example:
loadPort MACRO port, value
MOVLW value
MOVWF PORT&port
ENDM
will load PORTA if port is A when called, etc. The special meaning of the & token in
macros implies that you can not use the bitwise AND operator, (also represented by
&), in assembly macros; use the and form of this operator instead.
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A comment can be suppressed within the expansion of a macro (thus saving space in
the macro storage) by opening the comment with a double semicolon, ;;.
When invoking a macro, the argument list must be comma-separated. If it is desired to
include a comma (or other delimiter such as a space) in an argument then angle
brackets < and > can be used to quote
If an argument is preceded by a percent sign, %, that argument will be evaluated as an
expression and passed as a decimal number, rather than as a string. This is useful if
evaluation of the argument inside the macro body would yield a different result.
The nul operator can be used within a macro to test a macro argument, for example:
IF nul
...
ELSE
...
ENDIF
arg3
; argument was not supplied.
; argument was supplied
See Section 6.2.9.15 “LOCAL” for use of unique local labels within macros.
By default, the assembly list file will show macro in an unexpanded format; i.e., as the
macro was invoked. Expansion of the macro in the listing file can be shown by using
the EXPAND assembler control; see Section 6.2.10.4 “EXPAND”.
6.2.9.15
LOCAL
The LOCAL directive allows unique labels to be defined for each expansion of a given
macro. Any symbols listed after the LOCAL directive will have a unique assembler
generated symbol substituted for them when the macro is expanded. For example:
down MACRO count
LOCAL more
more: DECFSZ count
GOTO more
ENDM
when expanded, will include a unique assembler generated label in place of more. For
example:
down foobar
expands to:
??0001 DECFSZ foobar
GOTO ??0001
If invoked a second time, the label more would expand to ??0002, and multiply defined
symbol errors will be averted.
6.2.9.16
ALIGN
The ALIGN directive aligns whatever is following, data storage or code etc., to the specified offset boundary within the current psect. The boundary is specified as a number
of bytes following the directive.
For example, to align output to a 2-byte (even) address within a psect, the following
could be used.
ALIGN 2
Note that what follows will only begin on an even absolute address if the psect begins
on an even address; i.e., alignment is done within the current psect. See
Section 6.2.9.3.15 “Reloc” for psect alignment.
The ALIGN directive can also be used to ensure that a psect’s length is a multiple of a
certain number. For example, if the above ALIGN directive was placed at the end of a
psect, the psect would have a length that was always an even number of bytes long.
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6.2.9.17
REPT
The REPT directive temporarily defines an unnamed macro, then expands it a number
of times as determined by its argument.
For example:
REPT 3
ADDWF fred,w
ENDM
will expand to:
ADDWF fred,w
ADDWF fred,w
ADDWF fred,w
See also, Section 6.2.9.18 “IRP and IRPC”.
6.2.9.18
IRP AND IRPC
The IRP and IRPC directives operate in a similar way to REPT; however, instead of
repeating the block a fixed number of times, it is repeated once for each member of an
argument list.
In the case of IRP, the list is a conventional macro argument list. In the case or IRPC,
it is each character in one argument. For each repetition, the argument is substituted
for one formal parameter.
For example:
IRP number,4865h,6C6Ch,6F00h
DW number
ENDM
would expand to:
DW 4865h
DW 6C6Ch
DW 6F00h
Note that you can use local labels and angle brackets in the same manner as with
conventional macros.
The IRPC directive is similar, except it substitutes one character at a time from a string
of non-space characters.
For example:
IRPC char,ABC
DB ’char’
ENDM
will expand to:
DB ’A’
DB ’B’
DB ’C’
6.2.9.19
BANKSEL
This directive can be used to generate code to select the bank of the operand. The
operand should be the symbol or address of an object that resides in the data memory.
See Section 6.2.1.2 “Bank and Page Selection”.
6.2.9.20
PAGESEL
This directive can be used to generate code to select the page of the address operand.
See Section 6.2.1.2 “Bank and Page Selection”.
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6.2.9.21
PROCESSOR
The output of the assembler can vary, depending on the target device. The device
name is typically set using the --CHIP option to the command-line driver xc8, see
Section 4.8.18 “--CHIP: Define Device”. However, it can also be set with this directive,
for example:
PROCESSOR 16F877
This directive will override any device selected by any command-line option.
6.2.9.22
SIGNAT
This directive is used to associate a 16-bit signature value with a label. At link time, the
linker checks that all signatures defined for a particular label are the same. The linker
will produce an error if they are not. The SIGNAT directive is used by MPLAB XC8 to
enforce link time checking of C function prototypes and calling conventions.
Use the SIGNAT directive if you want to write assembly language routines that are
called from C. For example:
SIGNAT _fred,8192
associates the signature value 8192 with the symbol _fred. If a different signature
value for _fred is present in any object file, the linker will report an error.
The easiest way to determine the correct signature value for a routine is to write a C
routine with the same prototype as the assembly routine and check the signature value
determined by the code generator. This will be shown in the assembly list file; see
Section 4.8.15 “--ADDRQUAL: Set Compiler Response to Memory Qualifiers”, and
Section 6.3 “Assembly-Level Optimizations”.
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6.2.10
Assembler Controls
Assembler controls can be included in the assembler source to control assembler operation. These keywords have no significance anywhere else in the program. The control
is invoked by the directive OPT, followed by the control name. Some keywords are
followed by one or more arguments. For example:
OPT EXPAND
A list of keywords is given in Table 6-8, and each is described in the text that follows
the table.
TABLE 6-8:
ASPIC ASSEMBLER CONTROLS(1)
Control
Meaning
ASMOPT_ON, ASMOPT_OFF
Format
start and stop assembly optimizations OPT ASMOPT_OFF
;protected code
OPT ASMOPT_ON
ASMOPT_PUSH,
ASMOPT_POP
save and restore the state of the
assembly optimizations
OPT ASMOPT_PUSH ;save state
OPT ASMOPT_OFF
;protected code
OPT ASMOPT_POP ;restore state
COND*, NOCOND
include/do not include conditional
code in the listing
OPT COND
EXPAND, NOEXPAND
expand/do not expand macros in the
listing output
OPT EXPAND
INCLUDE
textually include another source file
OPT INCLUDE < pathname >
LIST*, NOLIST
define options for listing output/disable OPT LIST [< listopt >, ..., <
listopt >]
listing output
PAGE
start a new page in the listing output
OPT PAGE
SPACE
add blank lines to listing
OPT SPACE 3
SUBTITLE
specify the subtitle of the program
OPT SUBTITLE “< subtitle >”
TITLE
specify the title of the program
OPT TITLE “< title >”
Note 1:
The default options are listed with an asterisk (*)
6.2.10.1
ASMOPT_OFF AND ASMOPT_ON
These controls allow the assembler optimizer to be selectively disabled for sections of
assembly code. No code is modified after an ASMOPT_OFF control until a subsequent
ASMOPT_ON control is encountered.
6.2.10.2
ASMOPT_PUSH AND ASMOPT_POP
These controls allow the state of the assembler optimizer to be saved onto a stack of
states and then restored at a later time. They are useful when you need to ensure the
optimizers are disabled for a small section of code, but you do not know if the optimizers have previously been disabled. See Table 6-8 for an example of how these might
be used.
6.2.10.3
COND
Any conditional code is included in the listing output. See also, the NOCOND control in
Section 6.2.10.7 “NOCOND”.
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6.2.10.4
EXPAND
When EXPAND is in effect, the code generated by macro expansions appears in the listing output. See also, the NOEXPAND control in Section 6.2.10.8 “NOEXPAND”.
6.2.10.5
INCLUDE
This control causes the file specified by pathname to be textually included at that point
in the assembly file. The INCLUDE control must be the last control keyword on the line,
for example:
OPT INCLUDE "options.h"
The driver does not pass any search paths to the assembler, so if the include file is not
located in the working directory, the pathname must specify the exact location.
See also, the driver option -P in (Section 4.8.10 “-P: Preprocess Assembly Files”) that
forces the C preprocessor to preprocess the assembly file, thus allowing use of
preprocessor directives, such as #include (see Section 5.14.1 “C Language
Comments”).
6.2.10.6
LIST
If, previously, the listing was turned off using the NOLIST control, the LIST control
automatically turns listing on.
Alternatively, the LIST control can include options to control the assembly and the
listing. The options are listed in Table 6-9.
TABLE 6-9:
LIST CONTROL OPTIONS
List Option
Default
Description
c= nnn
80
Set the page (i.e., column) width.
n= nnn
59
Set the page length.
t= ON|OFF
OFF
Truncate listing output lines. The default wraps lines.
p=< device >
n/a
Set the device type.
r=< radix >
HEX
Set the default radix to HEX, dec or oct.
x= ON|OFF
OFF
Turn macro expansion on or off.
See also, the NOLIST control in Section 6.2.10.9 “NOLIST”.
6.2.10.7
NOCOND
Using this control will prevent conditional code from being included in the assembly list
file output. See also, the COND control in Section 6.2.10.3 “COND”.
6.2.10.8
NOEXPAND
The NOEXPAND control disables macro expansion in the assembly list file. The macro
call will be listed instead. See the EXPAND control in Section 6.2.10.4 “EXPAND”.
Assembly macros are discussed in Section 6.2.9.14 “MACRO and ENDM”.
6.2.10.9
NOLIST
This control turns the listing output off from a precise point forward. See also, the LIST
control in Section 6.2.10.6 “LIST”.
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6.2.10.10 PAGE
The PAGE control causes a new page to be started in the listing output. A Control-L
(form feed) character will also cause a new page when it is encountered in the source.
6.2.10.11 SPACE
The SPACE control places a number of blank lines in the listing output, as specified by
its parameter.
6.2.10.12 SUBTITLE
The SUBTITLE control defines a subtitle to appear at the top of every listing page, but
under the title. The string should be enclosed in single or double quotes. See also, the
TITLE control in Section 6.2.10.13 “TITLE”.
6.2.10.13 TITLE
This control keyword defines a title to appear at the top of every listing page. The string
should be enclosed in single or double quotes. See also, the SUBTITLE control in
Section 6.2.10.12 “SUBTITLE”.
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6.3
ASSEMBLY-LEVEL OPTIMIZATIONS
The assembler performs optimizations on assembly code, in addition to those
optimizations performed by the code generator directly on the C code; see
Section 5.13 “Optimizations”.
The assembler only optimizes hand-written assembly source modules if the asmfile
optimization setting is enabled, see Section 4.8.45 “--OPT: Invoke Compiler Optimizations”. Assembly added in-line (see Section 5.12.2 “#asm, #endasm and asm()”) with
C code is never optimized.
The optimizations that can be performed by the assembler include the following. Note,
however, that these optimizations are skipped if the compiler is operating in Free mode
(unless indicated below). The compiler operating mode selection is made by an option,
see Section 4.8.39 “--MODE: Choose Compiler Operating Mode”.
Assembly-level optimizations include:
• In-lining of small routines is done so that a call to the routine is not required. Only
very small routines (typically a few instructions) that are called only once will be
changed so that code size is not adversely impacted. This speeds code execution
without a significant increase in code size.
• Explicit inlining of functions that use the inline specifier, see
Section 5.8.1.2 “Inline Specifier”.
• Procedural abstraction is performed on assembly code sequences that appear
more than once. This is essentially a reverse in-lining process. The code sequences
are abstracted into callable routines that use a label, PLx, where x is a number. A
call to this routine will replace every instance of the original code sequence. This
optimization reduces code size considerably, with a small impact on code speed. It
can, however, adversely impact debugging. Procedural abstraction is only employed
by compilers operating in PRO mode.
• Jump-to-jump type optimizations are made primarily to tidy the output related to
conditional code sequences that follow a generic template. Jump-to-jump optimizations can remove jump instructions whose destinations are also jump instructions.
This optimization is enabled in all modes, including Free mode.
• Unreachable code is removed. Code can become orphaned by other optimizations and cannot be reached during normal execution, e.g., instructions after a
return instruction. The presence of any label is considered a possible entry point,
and code following a label is always considered reachable.
• Peephole optimizations are performed on every instruction. These optimizations
consider the state of execution at, and immediately around, each instruction –
hence the name. They either alter or delete one or more instructions at each step.
For example, if W is known to contain the value 0, and an instruction moves W to an
address (MOVWF), this might be replaceable with a CLRF instruction.
• Psect merging can be performed to allow other optimizations to take place. Code
within the same psect is guaranteed to be located in the same program memory
page. So, calls and jumps within the psect do not need to have the page selection
bits set before executing. Code using the LJMP and FCALL instructions will benefit
from this optimization, see Section 6.2.1 “Assembly Instruction Deviations”.
Assembly optimizations can often interfere with debugging in some tools, such as
MPLAB X IDE. It can be necessary to disable them when debugging code, if that is possible. See Section 4.8.45 “--OPT: Invoke Compiler Optimizations”, for more details. The
assembler optimizations can drastically reduce code size. However, they typically have
little effect on RAM usage.
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6.4
ASSEMBLY LIST FILES
The assembler will produce an assembly list file if instructed. The xc8 driver option
--ASMLIST is typically used to request generation of such a file, see
Section 4.8.16 “--ASMLIST: Generate Assembler List Files”.
The assembly list file shows the assembly output produced by the compiler for both C
and assembly source code. If the assembler optimizers are enabled, the assembly
output can be different than assembly source code. So, it is still useful for assembly
programming.
The list file is in a human-readable form and cannot be go any farther in the compilation
sequence. It differs from an assembly output file in that it contains address and op-code
data. In addition, the assembler optimizer simplifies some expressions and removes
some assembler directives from the listing file for clarity, although these directives are
included in the true assembly output files. If you are using the assembly list file to look
at the code produced by the compiler, you might wish to turn off the assembler optimizer so that all the compiler-generated directives are shown in the list file. Re-enable
the optimizer when continuing development. Section 4.8.45 “--OPT: Invoke Compiler
Optimizations” gives more information on controlling the optimizers.
Provided that the link stage has successfully concluded, the listing file is updated by
the linker so that it contains absolute addresses and symbol values. Thus, you can use
the assembler list file to determine the position and exact op codes of instructions.
Tick marks “'” in the assembly listing, next to addresses or opcodes, indicate that the
linker did not update the list file, most likely due to a compiler error, or a compiler option
that stopped compilation before the link stage. For example, in the following listing:
85
86
87
000A' 027F
000B' 1D03
000C' 2800'
subwf
skipz
goto
127,w
u15
These marks indicate that addresses are just address offsets into their enclosing psect,
and that opcodes have not been fixed up. Any address field in the opcode that has not
been fixed up is shown with a value of 0.
There is a single assembly list file produced by the assembler for each assembly file
passed to it. So, there is a single file produced for all the C source code in a project,
including p-code based library code. The file also contains some of the C initialization
that forms part of the runtime startup code. There is also a single file produced for each
assembly source file. Typically, there is at least one assembly file in each project. It
contains some of the runtime startup file and is typically named startup.as.
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6.4.1
General Format
The format of the main listing is in the form shown in Figure 6-1.
The line numbers purely relate to the assembly list file and are not associated with the
lines numbers in the C or assembly source files. Any assembly that begins with a semicolon indicates it is a comment added by the code generator. Such comments contain
either the original source code, which corresponds to the generated assembly, or is a
comment inserted by the code generator to explain some action taken.
Before the output for each function, there is detailed information regarding that function
summarized by the code generator. This information relates to register usage, local
variable information, functions called, and the calling function.
FIGURE 6-1:
1
768
769
770
771
772
773
774
775
776
777
778
6.4.2
GENERAL FORM OF ASSEMBLY LISTING FILE
4
2
0243
3
0243
00A3
0244
0245
3007
05A3
0252
0008
;sp2_inpADC.c: 119: void ADC_start(unsigned char chan)
;sp2_inpADC.c: 120: {
_ADC_start:
; Regs used in _ADC_start: [reg0,reg3]
3]
]
movwf
ADC_start@chan
1 line number
;sp2_inpADC.c: 121: chan &= 0x07;
2 address
movlw
7
andwf
ADC_start@chan
3 op code
5
;sp2_inpADC.c:
128: }
instruction
4 source comment
; ========= function _ADC_start ends ========
5
assembly
Psect Information
The assembly list file can be used to determine the name of the psect in which a data
object or section of code has been placed.
For global symbols, you can check the symbol table in the map file which lists the psect
name with each symbol. For symbols local to a module, find the definition of the symbol
in the list file. For labels, it is the symbol’s name followed by a colon, ‘:’. Look for the
first PSECT assembler directive above this code. The name associated with this directive is the psect in which the code is placed, see Section 6.2.9.3 “PSECT”.
6.4.3
Function Information
For each C function, printed before the function’s assembly label (search for the function’s name that is immediately followed by a colon :), is general information relating
to the resources used by that function. A typical printout is shown in Figure
6-2: Function Information. Most of the information is self-explanatory, but special comments follow.
The locations shown use the format offset[space]. For example, a location of
42[BANK0] means that the variable was located in the bank 0 memory space and that
it appears at an offset of 42 bytes into the compiled stack component in this space, see
Section 5.5.2.2.1 “Compiled Stack Operation”.
Whenever pointer variables are shown, they are often accompanied by the targets that
the pointer can reference, these targets appear after the arrow ->. See, also,
Section 6.4.5 “Pointer Reference Graph”. The auto and parameter section of this information is especially useful because the size of pointers is dynamic; see
Section 5.4.5 “Pointer Types”. This information shows the actual number of bytes
assigned to each pointer variable.
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The tracked objects are generally not used. It indicates the known state of the currently
selected RAM bank on entry to the function and at its exit points. It also indicates the
bank selection bits that did, or did not, change in the function.
The hardware stack information shows how many stack levels were taken up by this
function alone, and the total levels used by this function and any functions it calls. Note
that this is only valid for functions that are have not been inlined.
Functions that use a non-reentrant model are those that allocate auto and parameter
variables to a compiled stack and which are, as a result, not reentrant. If a function is
marked as being reentrant, it allocates stack-based variables to the software stack and
can be reentrantly called.
FIGURE 6-2:
FUNCTION INFORMATION
4064 ;;
1 *************** function _render *****************
4065 ;; Defined at:
4066 ;;
line 29 in file "draw.c" 2
3 Parameters:
4067 ;;
Size Location
Type
4068 ;;
None
4069 ;; Auto vars:
Size Location
Type
4070 ;; lll
4
42[BANK0 ] long
4071 ;; x
2
46[BANK0 ] volatile int
4
4072 ;; cp
1
41[BANK0 ] PTR unsigned char
4073 ;;
-> inputData(2),
4074 ;;
Type
5 Return value: Size Location
4075 ;;
None
void
4076 ;;
6 Registers used:
4077 ;;
wreg, fsr0l, fsr0h, status,2, status,0, pclath, cstack
4078 ;;
7 Tracked objects:
4079 ;;
On entry : 17F/0
4080 ;;
On exit : 0/0
4081 ;;
Unchanged: FFE00/0
4082 ;; Data sizes:
COMMON
BANK0
BANK1
BANK2
4083 ;;
Params:
0
0
0
0
4084 ;;
Locals:
0
7
0
0
8
4085 ;;
Temps:
0
5
0
0
4086 ;;
Totals:
0
12
0
0
1 function's name
4087 ;;Total ram usage:
12 bytes
4088 ;; Hardware stack levels used:
1
2 file name and line number of definition
4089 ;;
4
9 Hardware stack levels required when called:
4090 ;; This function calls:
3 size, location and type of parameters
4091 ;;
_lrv
4 size, location and type of auto variables
4092 ;;
___altofl
10
4093 ;;
___awdiv
5 size, location and type of return value
4094 ;;
___awmod
11
6 registers that the function code used
4095 ;; This function is called by:
4096 ;;
_main
7 selected GPR bank on entry and exit
4097 12
;; This function uses a non-reentrant
rant model
r
8 RAM memory summary for entire function
9 hardware stack requirements
10 functions called by this function
11 which functions call this function
12 how the function was encoded
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6.4.4
Switch Statement Information
Along with the generated code for each switch statement is information about how
that statement was encoded. There are several strategies the compiler can use for
switch statements. The compiler determines the appropriate strategy, see
Section 5.6.3 “Switch Statements”, or you can indicate a preference for a particular
type of strategy using a pragma, see Section 5.14.4.10 “The #pragma switch
Directive”. The information printed will look similar to that shown in Figure 6-3.
FIGURE 6-3:
206
207
208
209
210
211
212
213
214
215
SWITCH STATEMENT INFORMATION
1 ; Switch size 1, requested type "space"
3
4
; Number of cases is 4, Range of values is 0 to 254
; switch strategies available:
; Name
Instructions Cycles
; simple_byte
13
7 (average)
; jumptable
260
6 (fixed)
; rangetable
259
6 (fixed)
; spacedrange
516
9 (fixed)
; locatedrange
255
3 (fixed)
;
Chosen strategy
egy
e
gy is simple_byte
2
1 size of the switched value
2 number and range of the case values
3 all switch strategies and their attributes
4 the strategy choosen for this switch
statement
6.4.5
Pointer Reference Graph
Other important information contained in the assembly list file is the pointer reference
graph (look for pointer list with targets: in the list file). This is a list of each pointer contained in the program and each target the pointer can reference through the program.
The size and type of each target is indicated, as well as the size and type of the pointer
variable itself.
For example, the following shows a pointer called task_tmr in the C code. It is local
to the function timer_intr(). It is also a pointer to an unsigned int, and it is one
byte wide. There is only one target to this pointer and it is the member timer_count
in the structure called task. This target variable resides in the BANK0 class and is two
bytes wide.
timer_intr@task_tmr
PTR unsigned int size(1); Largest target is 2
-> task.timer_count(BANK0[2]),
The pointer reference graph shows both pointers to data objects and pointers to
functions.
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Macro Assembler
6.4.6
Call Graph
The other important information in the assembly list file is the call graph. This is produced for all 8-bit devices, which can use a compiled stack to facilitate stack-based
variables (function parameters, auto and temporary variables). See
Section 5.5.2.2.1 “Compiled Stack Operation”, for more detailed information on
compiled stack operation.
Call graph tables, showing call information on a function-by-function basis, are presented in the map file, followed by more traditional call graphs for the entire program.
The call graphs are built by the code generator, and are used to allow overlapping of
functions’ auto-parameter blocks (APBs) in the compiled stack. The call graphs are not
used when functions use the software stack (see Section 5.5.2.2.2 “Software Stack
Operation”). You can obtain the following information from studying the call graph.
• The functions in the program that are “root” nodes marking the top of a call tree,
and that are called spontaneously
• The functions that the linker deemed were called, or can have been called, during
program execution (and those which were called indirectly via a pointer)
• The program’s hierarchy of function calls
• The size of the auto and parameter areas within each function’s APB
• The offset of each function’s APB within the compiled stack
• The estimated call tree depth.
These features are discussed in sections that follow.
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6.4.6.1
CALL GRAPH TABLES
A typical call graph table can look like the extract shown in Figure 6-4. Look for Call
Graph Tables: in the list file.
FIGURE 6-4:
CALL GRAPH FORM
Call Graph Tables:
--------------------------------------------------------------------------------(Depth) Function
Calls
Base Space
Used Autos Params
Refs
--------------------------------------------------------------------------------(0) _main
12
12
0
34134
43 BANK0
5
5
0
0 BANK1
7
7
0
_aOut
_initSPI
--------------------------------------------------------------------------------(1) _aOut
2
0
2
68
2 BANK0
2
0
2
_SPI
_GetDACValue (ARG)
--------------------------------------------------------------------------------(1) _initSPI
0
0
0
0
--------------------------------------------------------------------------------(2) _SPI
2
2
0
23
0 BANK0
2
2
0
...
Estimated maximum stack depth 6
---------------------------------------------------------------------------------
The graph table starts with the function main(). Note that the function name will
always be shown in the assembly form, thus the function main() appears as the symbol _main. main() is always a root of a call tree. Interrupt functions will form separate
trees.
All the functions that main() calls, or can call, are shown below the function name in
the Calls column. So in this example, main() calls aOut() and initSPI(). These
have been grouped in the orange box in the figure. If a star (*) appears next to the function’s name, this implies the function has been called indirectly via a pointer. A function’s inclusion into the call graph does not imply the function was actually called, but
there is a possibility that the function was called. For example, code such as:
int test(int a) {
if(a)
foo();
else
bar();
}
will list foo() and bar() under test(), as either can be called. If a is always true,
then the function bar() will never be called, even though it appears in the call graph.
In addition to the called functions, information relating to the memory allocated in the
compiled stack for main() is shown. This memory will be used for the stack-based
variables that are defined in main(), as well as a temporary location for the function’s
return value, if appropriate.
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Macro Assembler
In the orange box for main() you can see that it defines 12 auto and temporary variable (under the Autos column). It defines no parameters – main() never has parameters – under the Params column. There is a total of 34134 references in the assembly
code to local objects in main(), shown under the Refs column. The Used column indicates the total number of bytes of local storage, i.e., the sum of the Autos and Params
columns.
Rather than the compiled stack being one block of memory in one memory space, it
can be broken up into multiple blocks placed in different memory spaces to utilize all of
the available memory on the target device. This breakdown is shown under the memory
summary line for each function. In this example, it shows that 5 bytes of auto objects
for main() are placed in the bank 0 component of the compiled stack (Space column),
at an offset of 43 (Base column) into this stack. It also shows that 7 bytes of auto
objects were placed in the bank 1 data component of the compiled stack at an offset of
0. The name listed under the Space column, is the same name as the linker class which
will hold this section of the stack.
Below the information for main() (outside the orange box) you will see the same information repeated for the functions that main() called, i.e., aOut()and initSPI().
For clarity, only the first few functions of this program are shown in the figure.
Before the name of each function, and in brackets, is the call stack depth for that particular function. A function can be called from many places in a program, and it can
have a different stack depth in the call graph at each call. The maximum call depth is
always shown for a function, regardless of its position in the call table. The main()
function will always have a depth of 0. The starting call depth for interrupt functions
assumes a worst case and will start at the start depth of the previous tree plus one.
After each tree in the call graph, there is an indication of the maximum stack depth that
might be realized by that tree. This stack depth is not printed if any functions in the
graph use the software stack. (In that case, a single stack depth estimate is printed for
the entire program at the end of the graphs.) In the example shown, the estimated maximum stack depth is 6. Check the associated data sheet for the depth of your device’s
hardware stack (see Section 5.3.4 “Stacks”). The stack depth indicated can be used as
a guide to the stack usage of the program. No definitive value can be given for the
program’s total stack usage for several reasons:
• Certain parts of the call tree may never be reached, reducing that tree’s stack
usage.
• The exact contribution of interrupt (or other) trees to the main() tree cannot be
determined as the point in main’s call tree at which the interrupt (or other function
invocation) will occur cannot be known. (The compiler assumes the worst case
situation of interrupts occurring at the maximum main() depth.)
• The assembler optimizer may have replaced function calls with jumps to
functions, reducing that tree’s stack usage.
• The assembler’s procedural abstraction optimizations can have added in calls to
abstracted routines, increasing the stack depth. (Checks are made to ensure this
does not exceed the maximum stack depth.)
• Functions which are inlined are not called, reducing the stack usage.
The compiler can be configured to manage the hardware stack for PIC10/12/16
devices only, see Section 4.8.54 “--RUNTIME: Specify Runtime Environment”. When
this mode is selected, the compiler will convert calls to jumps if it thinks the maximum
stack depth of the device is being exceeded. The stack depth estimate listed in the call
table will reflect the stack savings made by this feature. Thus, the stack depth and call
depth cannot be the same. Note that main() is jumped to by the runtime startup, not
called; so, main() itself does not consume a level of stack. See also
Section 5.10.1 “Runtime Startup Code”.
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The code generator produces a warning if the maximum stack depth appears to have
been exceeded and the stack is not being managed by the compiler. For the above
reasons, this warning, too, is intended to be a only a guide to potential stack problems.
6.4.6.2
CALL GRAPH CRITICAL PATHS
Immediately prior to the call graph tables in the list file are the critical paths for memory
usage identified in the call graphs. A critical path is printed for each memory space and
for each call graph. Look for a line similar to Critical Paths under _main in BANK0,
which, for this example, indicates the critical path for the main function (the root of one
call graph) in bank 0 memory. There will be one call graph for the function main and
another for each interrupt function. Each of these will appear for every memory space
the device defines.
A critical path here represents the biggest range of APBs stacked together in a contiguous block. Essentially, it identifies those functions whose APBs are contributing to the
program’s memory usage in that particular memory space. If you can reduce the memory usage of these functions in the corresponding memory space, then you will affect
the program’s total memory usage in that memory space.
This information can be presented as follows.
3793 ;; Critical Paths under _main in BANK0
3794 ;;
3795 ;;
_main->_foobar
3796 ;;
_foobar->___flsub
3797 ;;
___flsub->___fladd
In this example, it shows that of all the call graph paths starting from the function main,
the path in which main calls foobar, which calls flsub, which calls fladd, is using
the largest block of memory in bank 0 RAM. The exact memory usage of each function
is shown in the call graph tables.
The memory used by functions that are not in the critical path will overlap entirely with
that in the critical path. Reducing the memory usage of these will have no impact on
the memory usage of the entire program.
6.4.6.3
CALL GRAPH GRAPHS
Following the call tables are the call graphs, which show the full call tree for main()
and any interrupt functions. This is a subset of the information presented in the call
tables, and it is shown in a different form. The call graphs will look similar to the one
shown in Figure 6-5.
FIGURE 6-5:
CALL GRAPH GRAPHS
Call Graph Graphs:
_main (ROOT)
_initSPI
_aOut
_SPI
_GetDACValue
___ftadd
___ftpack
___ftmul (ARG)
...
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Macro Assembler
Indentation is used to indicate the call depth. In the diagram, you can see that main()
calls aOut(), which in turn calls GetDACValue(), which in turn calls the library
function __ftadd(), etc. If a star (*) appears next to the function’s name, this implies
that the function has been called indirectly via a pointer.
6.4.6.4
ARG NODES
In both the call trees and the call graph itself, you can see functions listed with the annotation (ARG) after its name. This implies that the call to that function at that point in the
call graph is made to obtain an argument to another function. For example, in the following code snippet, the function input() is called to obtain an argument value to the
function process().
result = process(input(0x7));
For such code, if it were to appear inside the main() function, the call graph would
contain the following.
_main (ROOT)
_input
_process
_input (ARG)
This indicates that main() calls input() and main() also calls process(), but
input() is also called as an argument expression to process().
These argument nodes in the graph do not contribute to the overall stack depth usage
of the program, but they are important for the creation of the compiled stack. The call
depth stack usage of the tree indicated above would only be 1, not 2, even though the
argument node function is at an indicated depth of 2. This is because there is no actual
reentrancy in terms of an actual call and a return address being stored on the hardware
stack.
The compiler must ensure that the parameter area for a function and any of its ‘argument functions’ must be at unique addresses in the compiled stack to avoid data corruption. Note that a function’s return value is also stored in its parameter area; so, that
needs to be considered by the compiler even if there are no parameters. A function’s
parameters become ‘active’ before the function is actually called (when the arguments
are passed) and its return value location remains ‘active’ after the function has returned
(while that return value is being processed).
In terms of data allocation, the compiler assumes a function has been ‘called’ the
moment that any of its parameters have been loaded and is still considered ‘called’ up
until its return value is no longer required. Thus, the definition for ‘reentrancy’ is much
broader when considering data allocation than it is when considering stack call depth.
6.4.7
Symbol Table
At the bottom of each assembly list file is a symbol table. This differs from the symbol
table presented in the map file (see Section 7.4.2.6 “Symbol Table”) in two ways:
• It lists only those symbols associated with the assembly module from which the
list file is produced (as opposed to the entire program); and
• It lists local as well as global symbols associated with that module.
Each symbol is listed along with the address it has been assigned.
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MPLAB® XC8 C COMPILER
USER’S GUIDE
Chapter 7. Linker
7.1
INTRODUCTION
This chapter describes the theory behind, and the usage of, the linker.
The application name of the linker is HLINK. In most instances it will not be necessary
to invoke the linker directly, as the compiler driver, xc8, will automatically execute the
linker with all the necessary arguments. Using the linker directly is not simple, and
should be attempted only by those with a sound knowledge of the compiler, and linking
in general. The compiler often makes assumptions about the way in which the program
will be linked. If the psects are not linked correctly, code failure can result.
If it is absolutely necessary to use the linker directly, the best way to start is to copy the
linker arguments constructed by the compiler driver, and modify them as is appropriate.
This ensures that the necessary startup module and arguments are present.
The following topics are examined in this chapter of the MPLAB XC8 C Compiler User’s
Guide:
• Operation
• Relocation and Psects
• Map Files
7.2
OPERATION
A command to the linker takes the following form:
hlink [options] files
The options are zero or more linker options, each of which modifies the behavior of
the linker in some way. The files is one or more object files, and zero or more object
code library names (.lib extension).
Note that P-code libraries (.lpp extension) are always passed to the code generator
application. They cannot be passed to the linker.
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The options recognized by the linker are listed in Table 7-1 and discussed in the
following paragraphs.
TABLE 7-1:
LINKER COMMAND-LINE OPTIONS
Option
-8
Effect
use 8086 style segment:offset address form
-Aclass=low-high ,... specify address ranges for a class
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-Cpsect=class
specify a class name for a global psect
-Cbaseaddr
produce binary output file based at baseaddr
-Dclass=delta
specify a class delta value
-Dsymfile
produce old-style symbol file
-Eerrfile
write error messages to errfile
-F
produce .obj file with only symbol records
-G spec
specify calculation for segment selectors
-H symfile
generate symbol file
-H+ symfile
generate enhanced symbol file
-I
ignore undefined symbols
-J num
set maximum number of errors before aborting
-K
prevent overlaying function parameter and auto areas
-L
preserve relocation items in .obj file
-LM
preserve segment relocation items in .obj file
-N
sort symbol table in map file by address order
-Nc
sort symbol table in map file by class address order
-Ns
sort symbol table in map file by space address order
-Mmapfile
generate a link map in the named file
-Ooutfile
specify name of output file
-Pspec
specify psect addresses and ordering
-Qprocessor
specify the device type (for cosmetic reasons only)
-S
inhibit listing of symbols in symbol file
-Sclass=limit[,bound]
specify address limit, and start boundary for a class of psects
-Usymbol
pre-enter symbol in table as undefined
-Vavmap
use file avmap to generate an Avocet format symbol file
-Wwarnlev
set warning level (-9 to 9)
-Wwidth
set map file width (>=10)
-X
remove any local symbols from the symbol file
-Z
remove trivial local symbols from the symbol file
--DISL=list
specify disabled messages
--EDF=path
specify message file location
--EMAX=number
specify maximum number of errors
--NORLF
do not relocate list file
--VER
print version number and stop
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Linker
If the standard input is a file, then this file is assumed to contain the command-line
argument. Lines can be broken by leaving a backslash \ at the end of the preceding
line. In this fashion, HLINK commands of almost unlimited length can be issued. For
example, a link command file called x.lnk and containing the following text:
-Z -OX.OBJ -MX.MAP \
-Ptext=0,data=0/,bss,nvram=bss/. \
X.OBJ Y.OBJ Z.OBJ
can be passed to the linker by one of the following:
hlink @x.lnk
hlink < x.lnk
Several linker options require memory addresses or sizes to be specified. The syntax
for all of these is similar. By default, the number is interpreted as a decimal value. To
force interpretation as a HEX number, a trailing H, or h, should be added. For example,
765FH will be treated as a HEX number.
7.2.1
-Aclass =low-high,...
-A option allows one or more of the address ranges to be assigned a linker class, so
that psects can be placed anywhere in this class. Ranges do not need to be contiguous.
For example:
-ACODE=1020h-7FFEh,8000h-BFFEh
specifies that the class called CODE represents the two distinct address ranges shown.
Psect can be placed anywhere in these ranges by using the -P option and the class
name as the address (see Section 7.2.18 “-Pspec”), for example:
-PmyText=CODE
Alternatively, any psect that is made part of the CODE class, when it is defined (see
Section 6.2.9.3.3 “Class”), will automatically be linked into this range, unless they are
explicitly located by another option.
Where there are a number of identical, contiguous address ranges, they can be
specified with a repeat count following an x character. For example:
-ACODE=0-0FFFFhx16
specifies that there are 16 contiguous ranges, each 64k bytes in size, starting from
address zero. Even though the ranges are contiguous, no psect will straddle a 64k
boundary, thus this can result in different psect placement to the case where the option
-ACODE=0-0FFFFFh
had been specified, which does not include boundaries on 64k multiples.
The -A option does not specify the memory space associated with the address. Once
a psect is allocated to a class, the space value of the psect is then assigned to the
class, see Section 6.2.9.3.17 “Space”.
7.2.2
-Cpsect=class
This option allows a psect to be associated with a specific class. Normally, this is not
required on the command line because psect classes are specified in object files. See
Section 6.2.9.3.3 “Class”.
7.2.3
-Dclass=delta
This option allows the delta value for psects that are members of the specified class to
be defined. The delta value should be a number. It represents the number of bytes per
addressable unit of objects within the psects. Most psects do not need this option as
they are defined with a delta value. See Section 6.2.9.3.4 “Delta”.
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7.2.4
-Dsymfile
Use this option to produce an old-style symbol file. An old-style symbol file is an ASCII
file, where each line has the link address of the symbol followed by the symbol name.
7.2.5
-Eerrfile
Error messages from the linker are written to the standard error stream. Under DOS,
there is no convenient way to redirect this to a file (the compiler drivers will redirect
standard errors, if standard output is redirected). This option makes the linker write all
error messages to the specified file instead of the screen, which is the default standard
error destination.
7.2.6
-F
Normally the linker will produce an object file that contains both program code and data
bytes, and symbol information. Sometimes you want to produce a symbol-only object
file that can be used again in a subsequent linker run to supply symbol values. The -F
option suppresses data and code bytes from the output file, leaving only the symbol
records.
This option can be used when part of one project (i.e., a separate build) is to be shared
with another, as might be the case with a bootloader and application. The files for one
project are compiled using this linker option to produce a symbol-only object file. That
file is then linked with the files for the other project.
7.2.7
-Gspec
When linking programs using segmented, or bank-switched psects, there are two ways
the linker can assign segment addresses, or selectors, to each segment. A segment is
defined as a contiguous group of psects where each psect in sequence has both its link
and load addresses concatenated with the previous psect in the group. The segment
address or selector for the segment is the value derived when a segment type
relocation is processed by the linker.
By default the segment selector is generated by dividing the base load address of the
segment by the relocation quantum of the segment, which is based on the reloc= flag
value given to psects at the assembler level, see Section 6.2.9.3.15 “Reloc”. The -G
option allows an alternate method for calculating the segment selector. The argument
to -G is a string similar to:
A /10h-4h
where A represents the load address of the segment and / represents division. This
means “Take the load address of the psect, divide by 10 HEX, then subtract 4”. This
form can be modified by substituting N for A, * for / (to represent multiplication), and
adding, rather than subtracting, a constant. The token N is replaced by the ordinal
number of the segment, which is allocated by the linker. For example:
N*8+4
means “take the segment number, multiply by 8, then add 4”. The result is the segment
selector. This particular example would allocate segment selectors in the sequence 4,
12, 20, ... for the number of segments defined.
The selector of each psect is shown in the map file. See Section 7.4.2.2 “Psect
Information Listed by Module”.
7.2.8
-Hsymfile
This option instructs the linker to generate a symbol file. The optional argument
symfile specifies the name of the file to receive the data. The default file name is
l.sym.
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Linker
7.2.9
-H+symfile
This option will instruct the linker to generate an enhanced symbol file, which provides,
in addition to the standard symbol file, class names associated with each symbol and
a segments section which lists each class name and the range of memory it occupies.
This format is recommended if the code is to be run in conjunction with a debugger. The
optional argument symfile specifies a file to receive the symbol file. The default file
name is l.sym.
7.2.10
-I
Usually, failure to resolve a reference to an undefined symbol is a fatal error. Using this
option causes undefined symbols to be treated as warnings, instead.
7.2.11
-Jerrcount
The linker will stop processing object files after a certain number of errors (other than
warnings). The default number is 10, but the -J option allows this to be altered.
7.2.12
-K
This option should not be used. It is for older compilers that use a compiled stack. In
those cases, the linker tries to overlay function auto and parameter blocks to reduce
the total amount of RAM required. For debugging purposes, that feature can be disabled with this option. However, doing so will increase the data memory requirements.
This option has no effect when compiled stack allocation is performed by the code generator. This is the case for OCG (PRO-Standard-Free mode) compilers, and this option
should not be used.
7.2.13
-L
When the linker produces an output file it does not usually preserve any relocation
information, since the file is now absolute. In some circumstances a further “relocation”
of the program is done at load time. The -L option generates, in the output file, one null
relocation record for each relocation record in the input.
7.2.14
-LM
Similar to the above option, this preserves relocation records in the output file, but only
segment relocations.
7.2.15
-Mmapfile
This option causes the linker to generate a link map in the named file, or on the standard output, if the file name is omitted. The format of the map file is illustrated in
Section 7.4 “Map Files”.
7.2.16
-N, -Ns and-Nc
By default the symbol table in the map file is sorted by name. The -N option causes it
to be sorted numerically, based on the value of the symbol. The -Ns and -Nc options
work similarly except that the symbols are grouped by either their space value, or class.
7.2.17
-Ooutfile
This option allows specification of an output file name for the linker. The default output
file name is l.obj. Use of this option overrides that default.
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7.2.18
-Pspec
Psects are linked together and assigned addresses based on information supplied to
the linker via -P options. The argument to the -P option consists of comma-separated
sequences with the form:
-Ppsect=linkaddr+min/loadaddr+min,psect=linkaddr/loadaddr,...
All values can be omitted, in which case a default will apply, depending on previous values. The link address of a psect is the address at which it can be accessed at runtime.
The load address is the address at which the psect starts within the output file (HEX or
binary file etc.), but it is rarely used by 8-bit PIC devices. The addresses specified can
be numerical addresses, the names of other psects, classes, or special tokens.
Examples of the basic and most common forms of this option are:
-Ptext10=02000h
which places (links) the starting address of psect text10 at address 0x2000;
-PmyData=AUXRAM
which places the psect myData anywhere in the range of addresses specified by the
linker class AUXRAM (which would need to be defined using the -A option, see
Section 7.2.1 “-Aclass =low-high,...”), and
-PstartCode=0200h,endCode
which places endCode immediately after the end of startCode, which will start at
address 0x200.
The additional variants of this option are rarely needed; but, are described below.
If a link or load address cannot be allowed to fall below a minimum value, the +min,
suffix indicates the minimum address.
If the link address is a negative number, the psect is linked in reverse order with the top
of the psect appearing at the specified address minus one. Psects following a negative
address will be placed before the first psect in memory.
If the load address is omitted entirely, it defaults to the link address. If the slash / character is supplied with no address following, the load address will concatenate with the
load address of the previous psect. For example, after processing the option:
-Ptext=0,data=0/,bss
the text psect will have a link and load address of 0; data will have a link address of
0 and a load address following that of text. The bss psect will concatenate with data
in terms of both link and load addresses.
A load address specified as a dot character, “.” tells the linker to set the load address
to be the same as the link address.
The final link and load address of psects are shown in the map file. See
Section 7.4.2.2 “Psect Information Listed by Module”.
7.2.19
-Qprocessor
This option allows a device type to be specified. This is purely for information placed in
the map file. The argument to this option is a string describing the device. There are no
behavioral changes attributable to the device type.
7.2.20
-S
This option prevents symbol information relating from being included in the symbol file
produced by the linker. Segment information is still included.
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7.2.21
-Sclass =limit[,bound]
A class of psects can have an upper address limit associated with it. The following
example places a limit on the maximum address of the CODE class of psects to one less
than 400h.
-SCODE=400h
Note that to set an upper limit to a psect, this must be set in assembler code using the
psect limit flag, see Section 6.2.9.3.8 “Limit”).
If the bound (boundary) argument is used, the class of psects will start on a multiple of
the bound address. This example below places the FARCODE class of psects at a
multiple of 1000h, but with an upper address limit of 6000h.
-SFARCODE=6000h,1000h
7.2.22
-Usymbol
This option will enter the specified symbol into the linker’s symbol table as an undefined
symbol. This is useful for linking entirely from libraries, or for linking a module from a
library where the ordering has been arranged so that by default a later module will be
linked.
7.2.23
-Vavmap
To produce an Avocet format symbol file, the linker needs to be given a map file to allow
it to map psect names to Avocet memory identifiers. The avmap file will normally be
supplied with the compiler, or created automatically by the compiler driver as required.
7.2.24
-Wnum
The -W option can be used to set the warning level, in the range -9 to 9, or the width of
the map file, for values of num >= 10.
-W9 will suppress all warning messages. -W0 is the default. Setting the warning level
to -9 (-W-9) will give the most comprehensive warning messages.
7.2.25
-X
Local symbols can be suppressed from a symbol file with this option. Global symbols
will always appear in the symbol file.
7.2.26
-Z
Some local symbols are compiler generated and not of interest in debugging. This
option will suppress from the symbol file all local symbols that have the form of a single
alphabetic character, followed by a digit string. The set of letters that can start a trivial
symbol is currently “klfLSu“. The -Z option will strip any local symbols starting with
one of these letters, and followed by a digit string.
7.2.27
--DISL=message numbers Disable Messages
This option is mainly used by the command-line driver, xc8, to disable particular
message numbers. It takes a comma-separate list of message numbers that will be
disabled during compilation.
This option is applied if compiling using xc8, the command-line driver and the
--MSGDISABLE driver option, see Section 4.8.40 “--MSGDISABLE: Disable Warning
Messages”.
See Section 4.6 “Compiler Messages” for full information about the compiler’s
messaging system.
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7.2.28
--EDF=message file: Set Message File Path
This option is mainly used by the command-line driver, xc8, to specify the path of the
message description file. The default file is located in the dat directory in the compiler’s
installation directory.
See Section 4.6 “Compiler Messages” for full information about the compiler’s
messaging system.
7.2.29
--EMAX=number: Specify Maximum Number of Errors
This option is mainly used by the command-line driver, xc8, to specify the maximum
number of errors that can be encountered before the assembler terminates. The default
number is 10 errors.
This option is applied if compiling using xc8, the command-line driver and the
--ERRORS driver option, see Section 4.8.29 “--ERRORS: Maximum Number of
Errors”.
See Section 4.6 “Compiler Messages” for full information about the compiler’s
messaging system.
7.2.30
--NORLF: Do Not Relocate List File
Use of this option prevents the linker applying fixups to the assembly list file produced
by the assembler. This option is normally using by the command line driver, xc8, when
performing pre-link stages, but is omitted when performing the final link step so that the
list file shows the final absolute addresses.
If you are attempting to resolve fixup errors, this option should be disabled so as to fix
up the assembly list file and allow absolute addresses to be calculated for this file. If
the compiler driver detects the presence of a preprocessor macro __DEBUG, which is
equated to 1, then this option will be disabled when building. This macro is set when
choosing a Debug build in MPLAB X IDE. So, always have this option selected if you
encounter such errors.
7.2.31
--VER: Print Version Number
This option prints information stating the version and build of the linker. The linker will
terminate after processing this option, even if other options and files are present on the
command line.
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7.3
RELOCATION AND PSECTS
This section looks at the input files that the linker has to work with.
The linker can read both relocatable object files and object-file libraries (.lib extension). The library files are a collection of object files packaged into a single unit. So,
essentially, we only need consider the format of object files.
Each object file consists of a number of records. Each record has a type that indicates
what sort of information it holds. Some record types hold general information about the
target device and its configuration, other records types can hold data; and others,
program debugging information, for example.
A lot of the information in object files relates to psects. Psects are an assembly domain
construct and are essentially a block of something, either instructions or data. Everything that contributes to the program is located in a psect. See Section 6.2.8 “Program
Sections”, for an introductory guide. There is a particular record type that is used to hold
the data in psects. The bulk of each object file consists of psect records containing the
executable code and variables etc.
We are now in a position to look at the fundamental tasks the linker performs, which
are:
• combining all the relocatable object files into one
• relocation of psects contained in the object files into memory
• fixup of symbolic references in the psects
There are at least two object files that are passed to the linker. One is produced from
all the C code in the project, including C library code. There is only one of these files
since the code generator compiles and combines all the C code of the program and
produces just the one assembly output. The other file passed to the linker will be the
object code produced from the runtime startup code, see Section 4.4.2 “Startup and
Initialization”.
If there are assembly source files in the project, then there will also be one object file
produced for each source file, and these will be passed to the linker. Existing object
files, or object file libraries can also be specified in a project; and if present, these will
also be passed to the linker.
The output of the linker is also an object file, but there is only a single file produced.
The file is absolute, since relocation will have been performed by the linker. The output
file consists of the information from all input object files, merged together.
Relocation consists of placing the psect data into the memory of the target device.
The target device memory specification is passed to the linker by the way of linker
options. These options are generated by the command-line driver, xc8. There are no
linker scripts or means of specifying options in any source file. The default linker
options rarely need adjusting. But, they can be changed, if required, with caution, using
the driver option -L-, see Section 4.8.6 “-L-: Adjust Linker Options Directly”.
When the psects are placed at actual memory locations, symbolic references made in
the psects data can be replaced with absolute values. This is a process called fixup.
For each psect record in the object file, there is a corresponding relocation record that
indicates which bytes (or bits) in the psect record need to be adjusted once relocation
is complete. The relocation records also specify how the values are to be determined.
A linker fixup overflow error can occur if the value determined by the linker is too large
to fit in the “hole” reserved for the value in the psect. See (477) fixup overflow in
expression (location 0x* (0x*+*), size *, value 0x*) (Linker) for information on finding
the cause of these errors.
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7.4
MAP FILES
The map file contains information relating to the relocation of psects and the addresses
assigned to symbols within those psects.
7.4.1
Generation
If compilation is being performed via MPLAB X IDE, a map file is generated by default.
If you are using the driver from the command line, use the -M option to request that the
map file be produced, see Section 7.2.15 “-Mmapfile”. Map files have the extension
.map.
Map files are produced by the linker. If the compilation process is stopped before the
linker is executed, then no map file is produced. The linker produces a map file, even
if it encounters errors. The file, then, helps you track down the cause of the errors. However, if the linker ultimately reports too many errors, the linker did not run to completion, the map file was not created. You can use the --ERRORS option (see
Section 4.8.29 “--ERRORS: Maximum Number of Errors”) on the command line to
increase the number of errors encountered before the linker exits.
7.4.2
Contents
The sections in the map file, in order of appearance, are as follows.
• the compiler name and version number
• a copy of the command line used to invoke the linker
• the version number of the object code in the first file linked
• the machine type
• a psect summary sorted by the psect’s parent object file
• a psect summary sorted by the psect’s CLASS
• a segment summary
• unused address ranges summary
• the symbol table
• information summary for each function
• information summary for each module
Portions of an example map file, along with explanatory text, are shown in the following
sections.
7.4.2.1
GENERAL INFORMATION
At the top of the map file is general information relating to the execution of the linker.
When analyzing a program, always confirm the compiler version number shown in the
map file if you have more than one compiler version installed to ensure the desired
compiler is being executed.
The device selected with the --CHIP option (Section 4.8.18 “--CHIP: Define Device”),
or the one selected in your IDE, should appear after the Machine type entry.
The object code version relates to the file format used by relocatable object files produced by the assembler. Unless either the assembler or linker have been updated
independently, this should not be of concern.
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A typical map file can begin something like the following. This example has been cut
down for clarity.
--edf=/home/jeff/Microchip/XC8/1.00/dat/en_msgs.txt -cs -h+main.sym -z \
-Q16F946 -ol.obj -Mmain.map -ver=XC8 -ACONST=00h-0FFhx32 \
-ACODE=00h-07FFhx4 -ASTRCODE=00h-01FFFh -AENTRY=00h-0FFhx32 \
-ASTRING=00h-0FFhx32 -ACOMMON=070h-07Fh -ABANK0=020h-06Fh \
-ABANK1=0A0h-0EFh -ABANK2=0120h-016Fh -ABANK3=01A0h-01EFh \
-ARAM=020h-06Fh,0A0h-0EFh,0120h-016Fh,01A0h-01EFh \
-AABS1=020h-07Fh,0A0h-0EFh,0120h-016Fh,01A0h-01EFh -ASFR0=00h-01Fh \
-ASFR1=080h-09Fh -ASFR2=0100h-011Fh -ASFR3=0180h-019Fh \
-preset_vec=00h,intentry,init,end_init -ppowerup=CODE -pfunctab=CODE \
-ACONFIG=02007h-02007h -pconfig=CONFIG -DCONFIG=2 -AIDLOC=02000h-02003h \
-pidloc=IDLOC -DIDLOC=2 -AEEDATA=00h-0FFh/02100h -peeprom_data=EEDATA \
-DEEDATA=2 -DCODE=2 -DSTRCODE=2 -DSTRING=2 -DCONST=2 -DENTRY=2 -k \
startup.obj main.obj
Object code version is 3.10
Machine type is 16F946
The Linker command line shows all the command-line options and files that were
passed to the linker for the last build. Remember, these are linker options, not
command-line driver options.
The linker options are necessarily complex. Fortunately, they rarely need adjusting
from their default settings. They are formed by the command-line driver, xc8, based on
the selected target device and the specified driver options. You can often confirm that
driver options were valid by looking at the linker options in the map file. For example, if
you ask the driver to reserve an area of memory, you should see a change in the linker
options used.
If the default linker options must be changed, this can be done indirectly through the
driver using the driver -L- option, see Section 4.8.6 “-L-: Adjust Linker Options
Directly”. If you use this option, always confirm the change appears correctly in the map
file.
7.4.2.2
PSECT INFORMATION LISTED BY MODULE
The next section in the map file lists those modules that have made a contribution to
the output, and information regarding the psects that these modules have defined. See
Section 5.15.1 “Program Sections” for an introductory explanation of psects.
This section is heralded by the line that contains the headings:
Name
Link
Load
Length
Selector
Space
Scale
Under this on the far left is a list of object files. These object files include both files generated from source modules and those that were extracted from object library files
(.lib extension). In the latter case, the name of the library file is printed before the
object file list. Note that since the code generator combines all C source files (and
p-code libraries), there is only one object file representing the entire C part of the program. The object file corresponding to the runtime startup code is normally present in
this list.
The information in this section of the map file can be used to confirm that a module is
making a contribution to the output file and to determine the exact psects that each
module defines.
Shown are all the psects (under the Name column) that were linked into the program
from each object file, and information about that psect.
The linker deals with two kinds of addresses: link and load. Generally speaking, the link
address of a psect is the address by which it is accessed at runtime.
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The load address, which is often the same as the link address, is the address at which
the psect starts within the output file (HEX or binary file etc.). If a psect is used to hold
bits, the load address is irrelevant and is, instead, used to hold the link address (in bit
units) converted into a byte address.
The Length of the psect is shown in the units that are used by that psect.
The Selector is less commonly used and is of no concern when compiling for PIC
devices.
The Space field is important as it indicates the memory space in which the psect was
placed. For Harvard architecture machines, with separate memory spaces (such as the
PIC10/12/16 devices), this field must be used in conjunction with the address to specify
an exact storage location. A space of 0 indicates the program memory, and a space of
1 indicates the data memory. See Section 6.2.9.3.17 “Space”.
The Scale of a psect indicates the number of address units per byte. This remains blank
if the scale is 1, and shows 8 for psects that hold bit objects. The load address of psects
that hold bits is used to display the link address converted into units of bytes, rather
than the load address. See Section 6.2.9.3.2 “Bit”.
For example, the following appears in a map file.
ext.obj
Name
text
bss
rbit
Link
3A
4B
50
Load
3A
4B
A
Length Selector
22
30
10
4B
2
0
Space
0
1
1
Scale
8
This indicates that one of the files that the linker processed was called ext.obj. (This
can have been derived from C code or a source file called ext.as.)
This object file contained a text psect, as well as psects called bss and rbit.
The psect text was linked at address 3A and bss at address 4B. At first glance, this
seems to be a problem, given that text is 22 words long. However, they are in different
memory areas, as indicated by the space flag (0 for text and 1 for bss), and so they
do not occupy the same memory.
The psect rbit contains bit objects, and this can be confirmed by looking at the scale
value, which is 8. Again, at first glance it seems that there could be an issue with rbit
linked over the top of bss. Their space flags are the same, but since rbit contains bit
objects, its link address is in units of bits. The load address field of rbit psect displays
the link address converted to byte units, i.e., 50h/8 => Ah.
Underneath the object file list there can be a label COMMON. This shows the contribution to the program from program-wide psects, in particular that used by the compiled
stack.
7.4.2.3
PSECT INFORMATION LISTED BY CLASS
The next section in the map file shows the same psect information but grouped by the
psects’ class.
This section is heralded by the line that contains the headings:
TOTAL
Name
Link
Load
Length
Under this are the class names followed by those psects which belong to this class, see
Section 6.2.9.3.3 “Class”. These psects are the same as those listed by module in the
above section; there is no new information contained in this section, just a different
presentation.
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7.4.2.4
SEGMENT LISTING
The class listing in the map file is followed by a listing of segments. Typically this section of the map file can be ignored by the user.
A segment is a conceptual grouping of contiguous psects in the same memory space,
and is used by the linker as an aid in psect placement. There is no segment assembler
directive and segments cannot be controlled in any way.
This section is heralded by the line that contains the headings:
SEGMENTS
Name
Load
Length
Top
Selector
Space
Class
The name of a segment is derived from the psect in the contiguous group with the lowest link address. This can lead to confusion with the psect with the same name. Do not
read psect information from this section of the map file.
Again, this section of the map file can be ignored.
7.4.2.5
UNUSED ADDRESS RANGES
The last of the memory summaries show the memory that has not been allocated, and
is unused. The linker is aware of any memory allocated by the code generator (for
absolute variables), and so this free space is accurate.
This section follows the heading:
UNUSED ADDRESS RANGES
and is followed by a list of classes and the memory that is still available in each class.
If there is more than one memory range available in a class, each range is printed on
a separate line. Any paging boundaries located within a class are not displayed. But,
the column Largest block shows the largest contiguous free space (which takes into
account any paging in the memory range). If you are looking to see why psects cannot
be placed into memory (e.g., cant-find-space type errors) then this is important information to study.
Note that the memory associated with a class can overlap that in others, thus the total
free space is not simply the addition of all the unused ranges.
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7.4.2.6
SYMBOL TABLE
The next section in the map file alphabetically lists the global symbols that the program
defines. This section has the heading:
Symbol Table
As always with the linker, any C-derived symbol is shown with its assembler-equivalent
symbol name. See Section 5.12.3 “Interaction between Assembly and C Code”.
The symbols listed in this table are:
• Global assembly labels
• Global EQU /SET assembler directive labels
• Linker-defined symbols
Assembly symbols are made global via the GLOBAL assembler directive, see
Section 6.2.9.1 “GLOBAL” for more information.
Linker-defined symbols act like EQU directives. However, they are defined by the linker
during the link process, and no definition for them appears in any source or
intermediate file. See Section 5.15.7 “Linker-Defined Symbols”.
Each symbol is shown with the psect in which it is defined, and the value (usually an
address) it has been assigned. There is not any information encoded into a symbol to
indicate whether it represents code or data – nor in which memory space it resides.
If the psect of a symbol is shown as (abs), this implies that the symbol is not directly
associated with a psect. Such is the case for absolute C variables, or any symbols that
are defined using an EQU directive in assembly.
Note that a symbol table is also shown in each assembler list file. (See
Section 4.8.15 “--ADDRQUAL: Set Compiler Response to Memory Qualifiers” for information on generating these files.) These differ to that shown in the map file as they also
list local symbols, and they only show symbols defined in the corresponding module.
7.4.2.7
FUNCTION INFORMATION
Following the symbol table is information relating to each function in the program. This
information is identical to the function information displayed in the assembly list file.
However, the information from all functions is collated in the one location. See
Section 6.4.3 “Function Information” for detailed descriptions of this information.
7.4.2.8
MODULE INFORMATION
The final section in the map file shows code usage summaries for each module. Each
module in the program will show information similar to the following.
Module
main.c
Function
Class
Link
Load
Size
init
main
getInput
CODE
CODE
CODE
07D8
07E5
07D9
0000
0000
0000
1
13
4
main.c estimated size: 18
The module name is listed (main.c in the above example). The special module name
shared is used for data objects allocated to program memory and to code that is not
specific to any particular module.
Next, the user-defined and library functions defined by each module are listed along
with the class in which that psect is located (see Section 5.15.3 “Default Linker
Classes”), the psect’s link and load address, and its size (shown as bytes for PIC18
devices and words for other 8-bit devices).
After the function list is an estimated size of the program memory used by that module.
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USER’S GUIDE
Chapter 8. Utilities
8.1
INTRODUCTION
This chapter discusses some of the utility applications that are bundled with the
compiler.
The applications discussed in this chapter are those more commonly used, but you do
not typically need to execute them directly. Most of their features are invoked indirectly
by the command line driver that is based on the command-line arguments or MPLAB
X IDE project property selections.
The following applications are described in this chapter of the MPLAB XC8 C Compiler
User’s Guide:
• Librarian
• HEXMATE
• Hash Functions
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8.2
LIBRARIAN
The librarian program, LIBR, has the function of combining several files into a single
file known as a library. The reasons you might want to use a library in a project are:
• there will be fewer files to link
• the file content will be accessed faster
• libraries uses less disk space
The librarian can build p-code libraries (.lpp extension) from p-code files (.p1 extension), or object code libraries (.lib extension) from object files (.obj extension).
P-code libraries should be only created if all the library source code is written in C.
Object code libraries should be used for assembly code that is to be built into a library.
With both library types, only those modules required by a program will be extracted and
included in the program output.
8.2.1
The Library Format
The modules in a library are simply concatenated, but a directory of the modules and
symbols in the library is maintained at the beginning of a library file. Since this directory
is smaller than the sum of the modules, on the first pass the linker can perform faster
searches by just reading the directory, and not all the modules. On the second pass, it
needs to read only those modules which are required, seeking them over the others.
This all minimizes disk I/O when linking.
It should be noted that the library format is not a general purpose archiving mechanism
as is used by some other compiler systems. This has the advantage that the format can
be optimized toward speeding up the linkage process.
8.2.2
Using the Librarian
Library files can be built directly using the command-line driver; see
Section 4.8.47 “--OUTPUT= type: Specify Output File Type”. In this case, the driver will
invoke LIBR with the appropriate options saving you from having to use the librarian
directly. You might wish to perform this step manually, or you might need to look at the
contents of library files, for example. This section shows how the librarian can be executed from the command-line. The librarian cannot be called from IDEs, such as
MPLAB X IDE.
The librarian program is called LIBR, and the formats of commands to it are as follows:
LIBR [options]
LIBR [options]
k
k
file.lpp [file1.p1 file2.p1...]
file.lib [file1.obj file2.obj ...]
options is zero or more librarian options that affect the output of the program. These
are listed in Table 8-1.
TABLE 8-1:
LIBRARIAN COMMAND-LINE OPTIONS
Option
-P width
-W
Effect
Specify page width
Suppress non-fatal errors
A key letter, k, denotes the command requested of the librarian (replacing, extracting,
or deleting modules, listing modules or symbols). These commands are listed in
Table 8-2.
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TABLE 8-2:
LIBRARIAN KEY LETTER COMMANDS
Key
Meaning
r
Replace modules
d
Delete modules
x
Extract modules
m
List modules
s
List modules with symbols
o
Re-order modules
The first file name listed after the key is the name of the library file to be used. The
following files, if required, are the modules of the library that is required by the
command specified.
If you are building a p-code library, the modules listed must be p-code files. If you are
building an object file library, the modules listed must be object files.
When replacing or extracting modules, the names of the modules to be replaced or
extracted must be specified. If no names are supplied, all the modules in the library will
be replaced or extracted respectively.
Adding a file to a library is performed by requesting the librarian to replace it in the
library. Since it is not present, the module will be appended to the library. If the r key is
used and the library does not exist, it will be created.
When using the d key letter, the named modules will be deleted from the library. In this
instance, it is an error not to give any module names.
The m and s key letters will list the named modules and, in the case of the s key letter,
the global symbols defined or referenced within. A D or U letter is used to indicate
whether each symbol is defined in the module, or referenced but undefined. As with the
r and x key letters, an empty list of modules means all the modules in the library.
The o key takes a list of module names and re-orders the matching modules in the
library file so that they have the same order as the one listed on the command line.
Modules that are not listed are left in their existing order, and will appear after the
re-ordered modules.
8.2.2.1
EXAMPLES
Here are some examples of usage of the librarian. The following command:
LIBR s pic-stdlib-d24.lpp ctime.p1
lists the global symbols in the modules ctime.p1, as shown here:
ctime.p1
D _moninit
D _localtime
D _gmtime
D _asctime
D _ctime
The D letter before each symbol indicates that these symbols are defined by the
module.
Using the command above without specifying the module name will list all the symbols
defined (or undefined) in the library.
The following command deletes the object modules a.obj, b.obj and c.obj from
the library lcd.lib:
LIBR d lcd.lib a.obj b.obj c.obj
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8.2.3
Supplying Arguments
Since it is often necessary to supply many object file arguments to LIBR, arguments
will be read from standard input if no command-line arguments are given. If the
standard input is attached to the console, LIBR will prompt for input.
Multiple line input can be given by using a backslash as a continuation character on the
end of a line. If standard input is redirected from a file, LIBR will take input from the file,
without prompting. For example:
libr
libr> r file.lib 1.obj 2.obj 3.obj \
libr> 4.obj 5.obj 6.obj
will perform much the same as if the object files had been typed on the command line.
The libr> prompts were printed by LIBR itself, the remainder of the text was typed
as input.
libr <lib.cmd
LIBR will read input from lib.cmd, and execute the command found therein. This
allows a virtually unlimited length command to be given to LIBR.
8.2.4
Ordering of Libraries
The librarian creates libraries with the modules in the order in which they were given
on the command line. When updating a library the order of the modules is preserved.
Any new modules added to a library after it has been created will be appended to the
end.
The ordering of the modules in a library is significant to the linker. If a library contains
a module that references a symbol defined in another module in the same library, the
module defining the symbol should come after the module referencing the symbol.
8.2.5
Error Messages
LIBR issues various error messages, most of which represent a fatal error. However,
some of those messages represent harmless occurrences, which will, nonetheless, be
reported. That is, unless the -W option was used. In that case, all warning messages
are suppressed.
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Utilities
8.3
HEXMATE
The HEXMATE utility is a program designed to manipulate Intel HEX files. HEXMATE is
a post-link stage utility that is automatically invoked by the compiler driver, and that
provides the facility to:
• Calculate and store variable-length hash values
• Fill unused memory locations with known data sequences
• Merge multiple Intel HEX files into one output file
• Convert INHX32 files to other INHX formats (e.g., INHX8M)
• Detect specific or partial opcode sequences within a HEX file
• Find/replace specific or partial opcode sequences
• Provide a map of addresses used in a HEX file
• Change or fix the length of data records in a HEX file
• Validate checksums within Intel HEX files.
Typical applications for HEXMATE might include:
• Merging a bootloader or debug module into a main application at build time
• Calculating a checksum or CRC value over a range of program memory and
storing its value in program memory or EEPROM
• Filling unused memory locations with an instruction to send the PC to a known
location if it gets lost
• Storage of a serial number at a fixed address
• Storage of a string (e.g., time stamp) at a fixed address
• Store initial values at a particular memory address (e.g., initialize EEPROM)
• Detecting usage of a buggy/restricted instruction
• Adjusting HEX file to meet requirements of particular bootloaders
8.3.1
HEXMATE Command Line Options
HEXMATE is automatically called by the command line driver, xc8. This is primarily to
merge HEX files in with the output generated by the source files. However, there are
some xc8 options which map directly to HEXMATE options. So, other functionality can
be requested without running HEXMATE on the command line explicitly. For other functionality, the following sections detail the options that are available when running this
application.
If HEXMATE is to be run directly, its usage is:
HEXMATE [specs,]file1.HEX [[specs,]file2.HEX ...
[specs,]fileN.HEX] [options]
where file1.HEX through to fileN.HEX form a list of input Intel HEX files to merge
using HEXMATE.
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If only one HEX file is specified, no merging takes place, but other functionality is
specified by additional options. Table 8-3 lists the command line options that HEXMATE
accepts.
TABLE 8-3:
HEXMATE COMMAND-LINE OPTIONS
Option
Effect
-ADDRESSING
Set address fields in all HEXMATE options to use word addressing
or other
-BREAK
Break continuous data so that a new record begins at a set address.
-CK
Calculate and store a value.
-FILL
Program unused locations with a known value.
-FIND
Search and notify if a particular code sequence is detected.
-FIND...,DELETE
Remove the code sequence if it is detected (use with caution).
-FIND...,REPLACE
Replace the code sequence with a new code sequence.
-FORMAT
Specify maximum data record length or select INHX variant.
-HELP
Show all options or display help message for specific option.
-LOGFILE
Save HEXMATE analysis of output and various results to a file.
-MASK
Logically AND a memory range with a bitmask
-Ofile
Specify the name of the output file.
-SERIAL
Store a serial number or code sequence at a fixed address.
-SIZE
Report the number of bytes of data contained in the resultant HEX
image.
-STRING
Store an ASCII string at a fixed address.
-STRPACK
Store an ASCII string at a fixed address using string packing.
-W
Adjust warning sensitivity.
+
Prefix to any option to overwrite other data in its address range, if
necessary.
If you are using the driver, xc8, to compile your project (or the IDE), a log file is
produced by default. It will have the project’s name and the extension .hxl.
The input parameters to HEXMATE are now discussed in detail. Note that any integral
values supplied to the HEXMATE options should be entered as hexadecimal values
without leading 0x or trailing h characters. Note also, that any address fields specified
in these options are to be entered as byte addresses, unless specified otherwise in the
-ADDRESSING option.
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8.3.1.1
SPECIFICATIONS,FILENAME.HEX
HEXMATE can process Intel HEX files that use either INHX32 or INHX8M format. Additional specifications can be applied to each HEX file to place restrictions or conditions
on how this file should be processed.
If any specifications are used, they must precede the filename. The list of specifications
will then be separated from the filename by a comma.
A range restriction can be applied with the specification rStart-End, where Start
and End are both assumed to be hexadecimal values. A range restriction will cause
only the address data falling within this range to be used. For example:
r100-1FF,myfile.hex
will use myfile.hex as input, but only process data which is addressed within the
range 100h-1FFh (inclusive) from that file.
An address shift can be applied with the specification sOffset. If an address shift is
used, data read from this HEX file will be shifted (by the offset specified) to a new
address when generating the output. The offset can be either positive or negative. For
example:
r100-1FFs2000,myfile.HEX
will shift the block of data from 100h-1FFh to the new address range 2100h-21FFh.
Be careful when shifting sections of executable code. Program code should only be
shifted if it is position independent.
8.3.1.2
+ PREFIX
When the + operator precedes an argument or input file, the data obtained from that
source will be forced into the output file and will overwrite another other data existing
at that address range. For example:
+input.HEX +-STRING@1000=”My string”
Ordinarily, HEXMATE will issue an error if two sources try to store differing data at the
same location. Using the + operator informs HEXMATE that if more than one data
source tries to store data to the same address, the one specified with a + prefix will take
priority.
8.3.1.3
-ADDRESSING
By default, all address arguments in HEXMATE options expect that values will be
entered as byte addresses. In some device architectures, the native addressing format
can be something other than byte addressing. In these cases, it would be much simpler
to be able to enter address-components in the device’s native format. To facilitate this,
the -ADDRESSING option is used.
This option takes one parameter that configures the number of bytes contained per
address location. If, for example, a device’s program memory naturally used a 16-bit (2
byte) word-addressing format, the option -ADDRESSING=2 will configure HEXMATE to
interpret all command line address fields as word addresses. The affect of this setting
is global and all HEXMATE options will now interpret addresses according to this setting.
This option will allow specification of addressing modes from one byte per address to
four bytes per address.
8.3.1.4
-BREAK
This option takes a comma-separated list of addresses. If any of these addresses are
encountered in the HEX file, the current data record will conclude and a new data
record will recommence from the nominated address. This can be useful to use new
data records to force a distinction between functionally different areas of program
space. Some HEX file readers depend on this.
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8.3.1.5
-CK
The -CK option is for calculating a hash value. The usage of this option is:
-CK=start-end@destination [+offset][wWidth][tCode][gAlgorithm][pPolynomial]
where:
• start and end specify the address range over which the hash will be calculated.
• destination is the address where the hash result will be stored. This value
cannot be within the range of calculation.
• offset is an optional initial value to be used in the calculations.
• Width is optional and specifies the byte-width of the result. Results can be calculated for byte-widths of 1 to 4 bytes. If a positive width is requested, the result will
be stored in big-endian byte order. A negative width will cause the result to be
stored in little-endian byte order. If the width is left unspecified, the result will be 2
bytes wide and stored in little-endian byte order. This width argument is not used if
you have selected any Fletcher algorithm.
• Code is a hexadecimal code that will trail each byte in the result. This can allow
each byte of the hash result to be embedded within an instruction.
• Algorithm is an integer to select which HEXMATE hash algorithm to use to calculate the result. A list of selectable algorithms is provided in Table 8-4. If
unspecified, the default algorithm used is 8-bit checksum addition (1).
• Polynomial is a hexadecimal value which is the polynomial to be used if you
have selected a CRC algorithm.
All numerical arguments are assumed to be hexadecimal values, except for the
algorithm selector and result width, which are assumed to be decimal values.
A typical example of the use of the checksum option is:
-CK=0-1FFF@2FFE+2100w2g2
This will calculate a checksum over the range 0 to 0x1FFF and program the checksum
result at address 0x2FFE. The checksum value will be offset by 0x2100. The result will
be two bytes wide.
TABLE 8-4:
HEXMATE HASH ALGORITHM SELECTION
Selector
Algorithm Description
-5
Reflected cyclic redundancy check (CRC)
-4
Subtraction of 32 bit values from initial value
-3
Subtraction of 24 bit values from initial value
-2
Subtraction of 16 bit values from initial value
-1
Subtraction of 8 bit values from initial value
1
Addition of 8 bit values from initial value
2
Addition of 16 bit values from initial value
3
Addition of 24 bit values from initial value
4
Addition of 32 bit values from initial value
5
Cyclic redundancy check (CRC)
7
Fletcher’s checksum (8 bit calculation, 2-byte result width)
8
Fletcher’s checksum (16 bit calculation, 4-byte result width)
For more details about the algorithms that are used to calculate checksums, see
Section 8.4 “Hash Functions”.
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8.3.1.6
-FILL
The -FILL option is used for filling unused memory locations with a known value. The
usage of this option is:
-FILL=[const_width:]fill_expr[@address[:end_address]]
where:
• const_width has the form wn and signifies the width (n bytes) of each constant
in fill_expr. If const_width is not specified, the default value is the native
width of the architecture. That is, -FILL=w1:1 with fill every byte with the value
0x01.
• fill_expr can use the syntax (where const and increment are n-byte
constants):
- const fill memory with a repeating constant; i.e., -FILL=0xBEEF becomes
0xBEEF, 0xBEEF, 0xBEEF, 0xBEEF
- const+=increment fill memory with an incrementing constant; i.e.,
-FILL=0xBEEF+=1 becomes 0xBEEF, 0xBEF0, 0xBEF1, 0xBEF2
- const-=increment fill memory with a decrementing constant; i.e.,
-FILL=0xBEEF-=0x10 becomes 0xBEEF, 0xBEDF, 0xBECF, 0xBEBF
- const,const,...,const fill memory with a list of repeating constants; i.e.,
-FILL=0xDEAD,0xBEEF becomes 0xDEAD,0xBEEF,0xDEAD,0xBEEF
• The options following fill_expr result in the following behavior:
- @address fill a specific address with fill_expr; i.e.,
-FILL=0xBEEF@0x1000 puts 0xBEEF at address 1000h
- @address:end_address fill a range of memory with fill_expr; i.e.,
-FILL=0xBEEF@0:0xFF puts 0xBEEF in unused addresses between 0 and
255
All constants can be expressed in (unsigned) binary, octal, decimal or hexadecimal, as
per normal C syntax, for example, 1234 is a decimal value, 0xFF00 is hexadecimal and
FF00 is illegal.
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8.3.1.7
-FIND
This option is used to detect and log occurrences of an opcode or partial code
sequence. The usage of this option is:
-FIND=Findcode [mMask]@Start-End [/Align][w][t”Title”]
where:
• Findcode is the hexadecimal code sequence to search for and is entered in little
endian byte order.
• Mask is optional. It specifies a bit mask applied over the Findcode value to allow
a less restrictive search. It is entered in little endian byte order.
• Start and End limit the address range to search.
• Align is optional. It specifies that a code sequence can only match if it begins on
an address that is a multiple of this value.
• w, if present, will cause HEXMATE to issue a warning whenever the code sequence
is detected.
• Title is optional. It allows a title to be given to this code sequence. Defining a
title will make log-reports and messages more descriptive and more readable. A
title will not affect the actual search results.
All numerical arguments are assumed to be hexadecimal values.
Here are some examples.
The option -FIND=3412@0-7FFF/2w will detect the code sequence 1234h when
aligned on a 2 (two) byte address boundary, between 0h and 7FFFh. w indicates that
a warning will be issued each time this sequence is found.
In this next example, -FIND=3412M0F00@0-7FFF/2wt”ADDXY”, the option is the
same as in last example but the code sequence being matched is masked with 000Fh,
so HEXMATE will search for any of the opcodes 123xh, where x is any digit. If a
byte-mask is used, is must be of equal byte-width to the opcode it is applied to. Any
messaging or reports generated by HEXMATE will refer to this opcode by the name,
ADDXY, as this was the title defined for this search.
If HEXMATE is generating a log file, it will contain the results of all searches. -FIND
accepts whole bytes of HEX data from 1 to 8 bytes in length. Optionally, -FIND can be
used in conjunction with REPLACE or DELETE (as described below).
8.3.1.8
-FIND...,DELETE
If the DELETE form of the -FIND option is used, any matching sequences will be
removed. This function should be used with extreme caution and is not normally
recommended for removal of executable code.
8.3.1.9
-FIND...,REPLACE
If the REPLACE form of the -FIND option is used, any matching sequences will be
replaced, or partially replaced, with new codes. The usage for this sub-option is:
-FIND...,REPLACE=Code [mMask]
where:
• Code is a little endian hexadecimal code to replace the sequences that match the
-FIND criteria.
• Mask is an optional bit mask to specify which bits within Code will replace the
code sequence that has been matched. This can be useful if, for example, it is
only necessary to modify 4 bits within a 16-bit instruction. The remaining 12 bits
can masked and left unchanged.
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8.3.1.10
-FORMAT
The -FORMAT option can be used to specify a particular variant of INHX format or
adjust maximum record length. The usage of this option is:
-FORMAT=Type [,Length]
where:
• Type specifies a particular INHX format to generate.
• Length is optional and sets the maximum number of bytes per data record. A
valid length is between 1 and 16 decimal, with 16 being the default.
Consider the case of a bootloader trying to download an INHX32 file, which fails
because it cannot process the extended address records that are part of the INHX32
standard. You know that this bootloader can only program data addressed within the
range 0 to 64k, and that any data in the HEX file outside of this range can be safely
disregarded. In this case, by generating the HEX file in INHX8M format the operation
might succeed. The HEXMATE option to do this would be -FORMAT=INHX8M.
Now, consider if the same bootloader also required every data record to contain exactly
8 bytes of data. This is possible by combining the -FORMAT with -FILL options. Appropriate use of -FILL can ensure that there are no gaps in the data for the address range
being programmed. This will satisfy the minimum data length requirement. To set the
maximum length of data records to 8 bytes, just modify the previous option to become
-FORMAT=INHX8M,8.
The possible types that are supported by this option are listed in Table 8-5. Note that
INHX032 is not an actual INHX format. Selection of this type generates an INHX32 file,
but will also initialize the upper address information to zero. This is a requirement of
some device programmers.
TABLE 8-5:
INHX TYPES USED IN -FORMAT OPTION
Type
Description
INHX8M
cannot program addresses beyond 64K
INHX32
can program addresses beyond 64K with extended linear address records
INHX032 INHX32 with initialization of upper address to zero
8.3.1.11
-HELP
Using -HELP will list all HEXMATE options. Entering another HEXMATE option as a
parameter of -HELP will show a detailed help message for the given option. For
example:
-HELP=string
will show additional help for the -STRING HEXMATE option.
8.3.1.12
-LOGFILE
The -LOGFILE option saves HEX file statistics to the named file. For example:
-LOGFILE=output.hxl
will analyze the HEX file that HEXMATE is generating, and save a report to a file named
output.hxl.
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8.3.1.13
-MASK
Use this option to logically AND a memory range with a particular bitmask. This is used
to ensure that the unimplemented bits in program words (if any) are left blank. The
usage of this option is as follows:
-MASK=hexcode@start-end
where hexcode is a value that will be ANDed with data within the start to end
address range. All values are assumed to be hexadecimal. Multibyte mask values can
be entered in little endian byte order.
8.3.1.14
-OFILE
The generated Intel HEX output will be created in this file. For example:
-Oprogram.hex
will save the resultant output to program.hex. The output file can take the same name
as one of its input files; but, by doing so, it will replace the input file entirely.
8.3.1.15
-SERIAL
This option will store a particular HEX value at a fixed address. The usage of this option
is:
-SERIAL=Code [+/-Increment]@Address [+/-Interval][rRepetitions]
where:
• Code is a hexadecimal value to store and is entered in little-endian byte order.
• Increment is optional and allows the value of Code to change by this value with
each repetition (if requested).
• Address is the location to store this code, or the first repetition thereof.
• Interval is optional and specifies the address shift per repetition of this code.
• Repetitions is optional and specifies the number of times to repeat this code.
All numerical arguments are assumed to be hexadecimal values, except for the
Repetitions argument, which is assumed to be a decimal value.
For example:
-SERIAL=000001@EFFE
will store HEX code 00001h to address EFFEh.
Another example:
-SERIAL=0000+2@1000+10r5
will store 5 codes, beginning with value 0000 at address 1000h. Subsequent codes
will appear at address intervals of +10h and the code value will change in increments
of +2h.
8.3.1.16
-SIZE
Using the -SIZE option will report the number of bytes of data within the resultant HEX
image to standard output. The size will also be recorded in the log file if one has been
requested.
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8.3.1.17
-STRING
The -STRING option will embed an ASCII string at a fixed address. The usage of this
option is:
-STRING@Address [tCode]=”Text”
where:
• Address is assumed to be a hexadecimal value representing the address at
which the string will be stored.
• Code is optional and allows a byte sequence to trail each byte in the string. This
can allow the bytes of the string to be encoded within an instruction.
• Text is the string to convert to ASCII and embed.
For example:
-STRING@1000=”My favorite string”
will store the ASCII data for the string, My favorite string (including the null
character terminator), at address 1000h.
And again:
-STRING@1000t34=”My favorite string”
will store the same string, trailing every byte in the string with the HEX code 34h.
8.3.1.18
-STRPACK
This option performs the same function as -STRING, but with two important differences. First, only the lower seven bits from each character are stored. Pairs of 7-bit
characters are then concatenated and stored as a 14-bit word rather than in separate
bytes. This is known as string packing. This is usually only useful for devices where program space is addressed as 14-bit words (PIC10/12/16 devices). The second
difference is that -STRING’s t specifier is not applicable with the -STRPACK option.
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8.4
HASH FUNCTIONS
A hash value is a small fixed-size value that is calculated from, and used to represent,
all the values in an arbitrary-sized block of data. If that data block is copied, a hash
recalculated from the new block can be compared to the original hash. Agreement
between the two hashes provides a high level of certainty that the copy is valid. There
are many hash algorithms. More complex algorithms provide a more robust verification,
but could use too many resources when used in an embedded environment.
HEXMATE can be used to calculate the hash of a program image that is contained in
a HEX file built by the MPLAB XC8 compiler. This hash can be embedded into that HEX
file and burned into the target device along with the program image. At runtime, the target device might be able to run a similar hash algorithm over the program image, now
stored in its memory. If the stored and calculated hashes are the same, the embedded
program can assume that it has a valid program image to execute.
HEXMATE implements several checksum and cyclic redundancy check algorithms to
calculate the hash. If you are using the XC8 C Compiler driver or MPLAB X IDE to perform project builds, the driver’s --CHECKSUM option (see
Section 4.8.17 “--CHECKSUM: Calculate a Checksum”) will instruct the driver to
invoke HEXMATE and pass it the appropriate HEXMATE options. If you are driving
HEXMATE explicitly, the option to select the algorithm is described in
Section 8.3.1.5 “-CK”. In the discussion of the algorithms below, it is assumed you are
using the compiler driver to request a checksum or CRC.
Some consideration is required when program images contain unused memory locations. The driver’s --CHECKSUM option automatically requests that HEXMATE fill
unused memory locations to match unprogrammed device memory. You might need to
mimic this action if invoking HEXMATE explicitly.
Although HEXMATE will work with any PIC device, not all devices can read the entire
width of their program memory. Thus some devices cannot calculate a hash at runtime
from memory containing instructions, and the code examples in the following sections
are not usable with all devices.
The following sections provide examples of the algorithms that can be used to calculate
the hash at runtime.
8.4.1
Addition Algorithms
HEXMATE has several simple checksum algorithms that sum data values over a range
in the program image. These algorithms correspond to the selector values 1, 2, 3, and
4 in the algorithm suboption and read the data in the program image as 1, 2, 3 or 4
byte quantities, respectively. This summation is added to an initial value (offset) that is
supplied to the algorithm via the same option. The width to which the final checksum is
truncated is also specified by this option and can be 1, 2, 3, or 4 bytes. HEXMATE will
automatically store the checksum in the HEX file at the address specified in the
checksum option.
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Utilities
The function shown below can be customized to work with any combination of data size
(readType) and checksum width (resultType).
typedef unsigned char readType; // size of data values read and summed
typedef unsigned int resultType; // size of checksum result
// add to offset n additions of values starting at address data,
// truncating and returning the result
// data: the address of the first value to sum
// n:
the number of sums to perform
// offset: the intial value to which the sum is added
resultType ck_add(const readType *data, unsigned n, resultType offset)
{
resultType chksum;
chksum = offset;
while(n--) {
chksum += *data;
data++;
}
return chksum;
}
The readType and resultType type definitions should be adjusted to suit the data
read/sum width and checksum result width, respectively. When using MPLAB XC8 and
for a size of 1, use a char type; for a size of 3, use a short long type, etc., or consider using the exact-width types provided by <stdint.h>. If you never use an offset,
that parameter can be removed and chksum assigned 0 before the loop.
Here is how this function might be used when, for example, a 2-byte-wide checksum is
to be calculated from the addition of 1-byte-wide values over the address range 0x100
to 0x7fd, starting with an offset of 0x20. The checksum is to be stored at 0x7fe and 0x7ff
in little endian format. The following option is specified when building the project. (In
MPLAB X IDE, only enter the information to the right of the first = in the Checksum field
in the Additional options Option category in the XC8 Linker category.)
--CHECKSUM=100-7fd@7fe,offset=20,algorithm=1,width=-2
Into your project, add the following code snippet, which calls ck_add, above, and
compare the runtime checksum with that stored by HEXMATE at compile time.
extern const readType ck_range[0x6fe/sizeof(readType)] @ 0x100;
extern const resultType hexmate @ 0x7fe;
resultType result;
result = ck_add(ck_range, sizeof(ck_range)/sizeof(readType), 0x20);
if(result != hexmate)
ck_failure(); // take appropriate action
This code uses the placeholder array, ck_range, to represent the memory over which
the checksum is calculated, and the variable hexmate is mapped over the locations
where HEXMATE will have stored its checksum result. Being extern and absolute,
neither of these objects consume additional device memory. Adjust the addresses and
sizes of these objects to match the option you pass to HEXMATE.
HEXMATE can calculate a checksum over any address range; however, the test function, ck_add, assumes that the start and end address of the range being summed are
a multiple of the readType width. (Clearly this is a non-issue if the size of readType
is 1.) It is recommended that your checksum specification adheres to this assumption,
otherwise you will need to modify the test code to perform partial reads of the starting
and/or ending data values. This will significantly increase the code complexity.
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8.4.2
Subtraction Algorithms
HEXMATE has several checksum algorithms that subtract data values over a range in
the program image. These algorithms correspond to the selector values -1, -2, -3, and
-4 in the algorithm suboption and read the data in the program image as 1-, 2-, 3- or
4-byte quantities, respectively. In other respects, these algorithms are identical to the
addition algorithms described in Section 8.4.1 “Addition Algorithms”.
The function shown below can be customized to work with any combination of data size
(readType) and checksum width (resultType).
typedef unsigned char readType; // size of data values read and summed
typedef unsigned int resultType; // size of checksum result
// add to offset n subtractions of values starting at address data,
// truncating and returning the result
// data: the address of the first value to subtract
// n:
the number of subtractions to perform
// offset: the intial value to which the subtraction is added
resultType ck_sub(const readType *data, unsigned n, resultType offset)
{
resultType chksum;
chksum = offset;
while(n--) {
chksum -= *data;
data++;
}
return chksum;
}
Here is how this function might be used when, for example, a 4-byte-wide checksum is
to be calculated from the addition of 2-byte-wide values over the address range 0x0 to
0x7fd, starting with an offset of 0x0. The checksum is to be stored at 0x7fe and 0x7ff in
little endian format. The following option is specified when building the project. (In
MPLAB X IDE, only enter the information to the right of the first = in the Checksum field
in the Additional options Option category in the XC8 Linker category.)
--CHECKSUM=0-7fd@7fe,offset=0,algorithm=-2,width=-4
Into your project add the following code snippet, which calls ck_sub, above, and
compare the runtime checksum with that stored by HEXMATE at compile time.
extern const readType ck_range[0x7fe/sizeof(readType)] @ 0x0;
extern const resultType hexmate @ 0x7fe;
resultType result;
result = ck_sub(ck_range, sizeof(ck_range)/sizeof(readType), 0x0);
if(result != hexmate)
ck_failure(); // take appropriate action
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8.4.3
Fletcher Algorithms
HEXMATE has several algorithms that implement Fletcher’s checksum. These algorithms are more complex, providing a robustness approaching that of a cyclic redundancy check, but with less computational effort. There are two forms of this algorithm,
which correspond to the selector values 7 and 8 in the algorithm suboption, and
which implement a 1-byte calculation and 2-byte result, and a 2-byte calculation and
4-byte result, respectively. HEXMATE will automatically store the checksum in the HEX
file at the address specified in the checksum option.
The function shown below performs a 1-byte-wide addition and produces a 2-byte
result.
unsigned int
fletcher8(const unsigned char * data, unsigned int n )
{
unsigned int sum = 0xff, sumB = 0xff;
unsigned char tlen;
while (n) {
tlen = n > 20 ? 20 : n;
n -= tlen;
do {
sumB += sum += *data++;
} while (--tlen);
sum = (sum & 0xff) + (sum >> 8);
sumB = (sumB & 0xff) + (sumB >> 8);
}
sum = (sum & 0xff) + (sum >> 8);
sumB = (sumB & 0xff) + (sumB >> 8);
return sumB << 8 | sum;
}
This code can be called in a manner similar to that shown for the addition algorithms,
see Section 8.4.1 “Addition Algorithms”.
The code for the 2-byte-addition Fletcher algorithm, producing a 4-byte result is shown
below.
unsigned long
fletcher16(const unsigned int * data, unsigned n)
{
unsigned long sum = 0xffff, sumB = 0xffff;
unsigned tlen;
while (n) {
tlen = n > 359 ? 359 : n;
n -= tlen;
do {
sumB += sum += *data++;
} while (--tlen);
sum = (sum & 0xffff) + (sum >> 16);
sumB = (sumB & 0xffff) + (sumB >> 16);
}
sum = (sum & 0xffff) + (sum >> 16);
sumB = (sumB & 0xffff) + (sumB >> 16);
return sumB << 16 | sum;
}
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8.4.4
CRC Algorithms
HEXMATE has several algorithms that implement the robust cyclic redundancy checks
(CRC). There is a choice of two algorithms that correspond to the selector values 5 and
-5 in the algorithm suboption to --CHECKSUM, and that implement a CRC calculation
and reflected CRC calculation, respectively. The reflected algorithm works on the least
significant bit of the data first. The polynomial to be used and the initial value can be
specified in the option. HEXMATE will automatically store the CRC result in the HEX
file at the address specified in the checksum option.
The function shown below can be customized to work with any result width
(resultType). It calculates a CRC hash value using the polynomial specified by the
POLYNOMIAL macro.
typedef unsigned int resultType;
#define POLYNOMIAL
0x1021
#define WIDTH
(8 * sizeof(resultType))
#define MSb
((resultType)1 << (WIDTH - 1))
resultType
crc(const unsigned char * data, unsigned n, resultType remainder) {
unsigned pos;
unsigned char bitp;
for (pos = 0; pos != n; pos++) {
remainder ^= ((resultType)data[pos] << (WIDTH - 8));
for (bitp = 8; bitp > 0; bitp--) {
if (remainder & MSb) {
remainder = (remainder << 1) ^ POLYNOMIAL;
} else {
remainder <<= 1;
}
}
}
return remainder;
}
The resultType type definition should be adjusted to suit the result width. When
using MPLAB XC8 and for a size of 1, use a char type; for a size of 3, use a short
long type, etc., or consider using the exact-width types provided by <stdint.h>.
Here is how this function might be used when, for example, a 2-byte-wide CRC hash
value is to be calculated values over the address range 0x0 to 0xFF, starting with an
initial value of 0xFFFF. The result is to be stored at 0x100 and 0x101 in little endian
format. The following option is specified when building the project. (In MPLAB X IDE,
only enter the information to the right of the first = in the Checksum field in the Additional options Option category in the XC8 Linker category.)
--CHECKSUM=0-FF@100,offset=0xFFFF,algorithm=5,width=-2,polynomial=0x1021
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Utilities
Into your project, add the following code snippet, which calls crc(), above, and
compares the runtime hash result with that stored by HEXMATE at compile time.
extern const unsigned char ck_range[0x100] @ 0x0;
extern const resultType hexmate @ 0x100;
resultType
result;
result = crc(ck_range, sizeof(ck_range), 0xFFFF));
if(result != hexmate){
// something’s not right, take appropriate action
ck_failure();
}
// data verifies okay, continue with the program
The reflected CRC result can be calculated by reflecting the input data and final result,
or by reflecting the polynomial. The functions shown below can be customized to work
with any result width (resultType). The crc_reflected_IO() function calculates
a reflected CRC hash value by reflecting the data stream bit positions. The
crc_reflected_poly() function does not adjust the data stream but reflects
instead the polynomial, which in both functions is specified by the POLYNOMIAL macro.
Both functions use the reflect() function to perform bit reflection.
typedef unsigned int resultType;
typedef unsigned char readType;
typedef unsigned int reflectWidth;
// This is the polynomial used by the CRC-16 algorithm we are using.
#define POLYNOMIAL
0x1021
#define WIDTH
#define MSb
#define LSb
(8 * sizeof(resultType))
((resultType)1 << (WIDTH - 1))
(1)
#define REFLECT_DATA(X)
#define REFLECT_REMAINDER(X)
((readType) reflect((X), 8))
(reflect((X), WIDTH))
reflectWidth
reflect(reflectWidth data, unsigned char nBits)
{
reflectWidth reflection = 0;
reflectWidth reflectMask = (reflectWidth)1 << nBits - 1;
unsigned char bitp;
for (bitp = 0; bitp != nBits; bitp++) {
if (data & 0x01) {
reflection |= reflectMask;
}
data >>= 1;
reflectMask >>= 1;
}
return reflection;
}
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resultType
crc_reflected_IO(const unsigned char * data, unsigned n, resultType
remainder) {
unsigned pos;
unsigned char reflected;
unsigned char bitp;
for (pos = 0; pos != n; pos++) {
reflected = REFLECT_DATA(data[pos]);
remainder ^= ((resultType)reflected << (WIDTH - 8));
for (bitp = 8; bitp > 0; bitp--) {
if (remainder & MSb) {
remainder = (remainder << 1) ^ POLYNOMIAL;
} else {
remainder <<= 1;
}
}
}
remainder = REFLECT_REMAINDER(remainder);
return remainder;
}
resultType
crc_reflected_poly(const unsigned char * data, unsigned n, resultType
remainder) {
unsigned pos;
unsigned char bitp;
resultType rpoly;
rpoly = reflect(POLYNOMIAL, WIDTH);
for (pos = 0; pos != n; pos++) {
remainder ^= data[pos];
for (bitp = 8; bitp > 0; bitp--) {
if (remainder & LSb) {
remainder = (remainder >> 1) ^ rpoly;
} else {
remainder >>= 1;
}
}
}
return remainder;
}
Here is how this function might be used when, for example, a 2-byte-wide reflected
CRC result is to be calculated over the address range 0x0 to 0xFF, starting with an initial value of 0xFFFF. The result is to be stored at 0x100 and 0x101 in little endian format. The following option is specified when building the project. (Note the algorithm
selected is negative 5 in this case.)
--CHECKSUM=0-FF@100,offset=0xFFFF,algorithm=-5,width=-2,polynomial=0x1021
Into your project, call crc(), as shown previously.
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USER’S GUIDE
Appendix A. Library Functions
A.1
INTRODUCTION
The functions and preprocessor macros within the standard compiler library are
alphabetically listed in this chapter.
The synopsis indicates the header file in which a declaration or definition for function
or macro is found. It also shows the function prototype for functions, or the equivalent
prototype for macros.
Note that where printf() is shown in example code, this assumes that the putch()
function has been defined to suit the peripheral that will act as the stdout stream.
Initialization of that peripheral must also be performed before you attempt to print.
__BUILTIN_SOFTWARE_BREAKPOINT
Synopsis
#include <xc.h>
void __builtin_software_breakpoint(void);
Description
This builtin unconditionally inserts code into the program output which triggers a
software breakpoint when the code is executed using a debugger.
The software breakpoint code is only generated for mid-range and PIC18 devices.
Baseline devices do not support software breakpoints in this way, and the builtin will be
ignored if used with these devices.
Example
#include <xc.h>
int
main (void)
{
__builtin_software_breakpoint();
...
// stop here to begin
See also
__debug_break()
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__CONDITIONAL_SOFTWARE_BREAKPOINT
Synopsis
#include <assert.h>
__conditional_software_breakpoint(expression)
Description
This macro implements a light-weight embedded version of the standard C assert()
macro, and is used in the same way.
When executed, the expression argument is evaluated. If the argument is false the
macro attempts to halt program execution; the macro performs no action if the argument is true.
The macro is removed from the program output if the manifest constant NDEBUG is
defined. In addition, it is included only for debug builds (i.e., when the __DEBUG macro
is defined). Thus, it does not consume device resources for production builds.
If the target device does not support the ability to halt via a software breakpoint, use of
this macro will trigger a compiler error.
Example
#include <assert.h>
void getValue(int * ip) {
__conditional_software_breakpoint(ip != NULL);
...
}
See also
ASSERT(), __builtin_software_breakpoint()
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Library Functions
__CONFIG (BASELINE & MID-RANGE DEVICES)
Synopsis
#include <xc.h>
__CONFIG(data)
Description
This macro is provided for legacy support only. Use the #pragma config for new
projects.
This macro is used to program the Configuration fuses that set the device’s operating
modes.
The macro assumes the argument is a16-bit value, which will be used to program the
Configuration bits.
16-bit masks have been defined to describe each programmable attribute available on
each device. These masks can be found in the chip-specific header files included via
<xc.h>.
Multiple attributes can be selected by ANDing them together.
Example
#include <xc.h>
__CONFIG(RC & UNPROTECT)
void
main (void)
{
}
See also
__EEPROM_DATA(), __IDLOC(), __IDLOC7(), CONFIG() (PIC18),
#pragma config
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__CONFIG (PIC18)
Synopsis
#include <xc.h>
__CONFIG(num, data)
Description
This macro is provided for legacy support only. Use the #pragma config for new
projects.
This macro is used to program the Configuration fuses that set the device’s operating
modes.
The macro accepts the number corresponding to the Configuration register it is to
program, then the 16-bit value it is to update it with.
16-bit masks have been defined to describe each programmable attribute available on
each device. These masks can be found in the chip-specific header files included via
<xc.h>.
Multiple attributes can be selected by ANDing them together.
Example
#include <xc.h>
__CONFIG(1,RC & OSCEN)
__CONFIG(2,WDTPS16 & BORV45)
__CONFIG(4, DEBUGEN)
void
main (void)
{
}
See also
__EEPROM_DATA(), __IDLOC(), __IDLOC7(), CONFIG()
(baseline & mid-range devices), #pragma config
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Library Functions
__DEBUG_BREAK
Synopsis
#include <xc.h>
void __debug_break(void);
Description
This macro conditionally inserts code into the program output which triggers a software
breakpoint when the code is executed using a debugger. The code is only generated
for debug builds (see Section 3.3.9 “What is Different About an MPLAB X IDE Debug
Build?”) and is omitted for production builds (i.e., when the __DEBUG macro is defined).
The software breakpoint code is only generated for mid-range and PIC18 devices.
Baseline devices do not support software breakpoints in this way, and the macro will
be ignored if used with these devices.
Example
#include <xc.h>
int
main (void)
{
__debug_break();
...
// stop here to begin
See also
__builtin_software_breakpoint()
__DELAY_MS, __DELAY_US, __DELAYWDT_US, __DELAYWDT_MS
Synopsis
__delay_ms(x) // request a delay in milliseconds
__delay_us(x) // request a delay in microseconds
__delaywdt_ms(x) // request a delay in milliseconds
__delaywdt_us(x) // request a delay in microseconds
Description
It is often more convenient to request a delay in time-based terms, rather than in cycle
counts. The macros __delay_ms(x) and __delay_us(x) are provided to meet this
need. These macros convert the time-based request into instruction cycles that can be
used with _delay(n). In order to achieve this, these macros require the prior definition of preprocessor macro _XTAL_FREQ, which indicates the system frequency. This
macro should equate to the oscillator frequency (in hertz) used by the system. Note that
this macro only controls the behavior of these delays and does not affect the device
execution speed.
On PIC18 devices only, you can use the alternate WDT-form of these functions, which
uses the CLRWDT instruction as part of the delay code. See the _delaywdt function.
The macro argument must be a constant expression. An error will result if these macros
are used without defining the oscillator frequency symbol, the delay period requested
is too large, or the delay period is not a constant.
See also
_delay(), _delaywdt()
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__EEPROM_DATA
Synopsis
#include <xc.h>
__EEPROM_DATA(a,b,c,d,e,f,g,h)
Description
This macro is used to store initial values in the device’s EEPROM registers at the time
of programming.
The macro must be given blocks of 8 bytes to write each time it is called, and can be
called repeatedly to store multiple blocks.
__EEPROM_DATA() will begin writing to EEPROM address zero, and auto-increments
the address written to by 8 each time it is used.
Example
#include <xc.h>
__EEPROM_DATA(0x00,0x01,0x02,0x03,0x04,0x05,0x06,0x07)
__EEPROM_DATA(0x08,0x09,0x0A,0x0B,0x0C,0x0D,0x0E,0x0F)
void
main (void)
{
}
See also
__CONFIG()
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Library Functions
__FPNORMALIZE
Synopsis
#include <xc.h>
double __fpnormalize(double fnum)
Description
This function can be used to ensure that an arbitrary 32-bit floating-point value (which
is not the result of a calculation performed by the compiler) conforms to the "relaxed"
floating-point rules, as described in Section 5.4.3 “Floating-Point Data Types”.
This function returns the value passed to it, but ensures that any subnormal argument
is flushed to zero, and converts any negative zero argument to a positive zero result.
Example
#include <xc.h>
void
main (void)
{
double input_fp;
// read in a floating-point value from an external source
input_fp = getFP();
// ensure it is formatted using the relaxed rules
input_fp = __fpnormalize(input_fp);
...
}
__IDLOC
Synopsis
#include <xc.h>
__IDLOC(x)
Description
This macro is provided for legacy support only. Use the #pragma config for new
projects.
This macro places data into the device’s special locations, outside of addressable
memory, that is reserved for ID. This would be useful for storage of serial numbers, etc.
The macro will attempt to write 4 nibbles of data to the 4 ID locations.
Example
#include <xc.h>
__IDLOC(15F0);
/* stores 1, 5, F and 0 in the ID registers */
void
main (void)
{
}
See also
__IDLOC7(), __CONFIG()
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__IDLOC7
Synopsis
#include <xc.h>
__IDLOC7(a,b,c,d)
Description
This macro is provided for legacy support only. Use the #pragma config for new
projects.
This macro places data into the device’s special locations, outside of addressable
memory, that is reserved for ID. This would be useful for storage of serial numbers, etc.
The macro will attempt to write 7 bits of data to each of the 4 locations reserved for ID
purposes.
Example
#include <xc.h>
/* will store 7Fh, 70, 1 and 5Ah in the ID registers */
__IDLOC(0x7F,70,1,0x5A);
void
main (void)
{
}
Note
Not all devices permit 7-bit programming of the ID locations. Refer to the device data
sheet to see whether this macro can be used on your particular device.
See also
__IDLOC(), __CONFIG()
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Library Functions
__OSCCAL_VAL
Synopsis
#include <xc.h>
unsigned char __osccal_val(void);
Description
This is a pseudo-function that is defined by the code generator to be a label only. The
label’s value is equated to the address of the RETLW instruction, which encapsulates
the oscillator configuration value. This function is only available for those devices that
are shipped with such a value stored in program memory.
Calls to the function will return the device’s oscillator configuration value, which can
then be used in any expression, if required.
Note that this function is automatically called by the runtime start-up code (unless you
have explicitly disabled this option, see Section 4.8.54 “--RUNTIME: Specify Runtime
Environment”) and you do not need to call it to calibrate the internal oscillator.
Example
#include <xc.h>
void
main (void)
{
unsigned char c;
c = __osccal_val();
}
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_DELAY() , _DELAYWDT
Synopsis
#include <xc.h>
void _delay(unsigned long cycles);
void _delaywdt(unsigned long cycles);
Description
This is an in-line function that is expanded by the code generator. When called, this routine expands to an in-line assembly delay sequence. The sequence will consist of code
that delays for the number of instruction cycles that is specified as the argument. The
argument must be a constant expression.
The _delay in-line function can use loops and the NOP instruction to implement the
delay. The _delaywdt in-line function performs the same task, but will use the
CLRWDT instruction, as well as loops, to achieve the specified delay.
An error will result if the delay period requested is not a constant expression or is too
large (greater than 50,463,240 instructions). For even larger delays, call this function
multiple times.
Example
#include <xc.h>
void
main (void)
{
control |= 0x80;
_delay(10);
// delay for 10 cycles
control &= 0x7F;
}
See Also
_delay3(), __delay_us(), __delay_ms()
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Library Functions
_DELAY3()
Synopsis
#include <xc.h>
void _delay3(unsigned char cycles);
Description
This is an in-line function that is expanded by the code generator. It is only available on
PIC18 and enhanced mid-range devices. When called, this routine expands to an
in-line assembly delay sequence.The sequence will consist of code that delays for 3
times the number of cycles that is specified as argument, assuming that the argument
can be loaded to WREG in one instruction, and that there are no errata-workaround
NOPs present in the loop. If this is not the case, the delay will be longer. The argument
can be a byte-sized constant or variable.
Example
#include <xc.h>
void
main (void)
{
control |= 0x80;
_delay3(10);
// delay for 30 cycles
control &= 0x7F;
}
See Also
_delay
ABS
Synopsis
#include <stdlib.h>
int abs (int j)
Description
The abs() function returns the absolute value of j.
Example
#include <stdio.h>
#include <stdlib.h>
void
main (void)
{
int a = -5;
printf(“The absolute value of %d is %d\n”, a, abs(a));
}
See Also
labs(), fabs()
Return Value
The absolute value of j.
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ACOS
Synopsis
#include <math.h>
double acos (double f)
Description
The acos() function implements the inverse of cos(); i.e., it is passed a value in the
range -1 to +1, and returns an angle in radians thats cosine is equal to that value.
Example
#include <math.h>
#include <stdio.h>
/* Print acos() values for -1 to 1 in degrees. */
void
main (void)
{
float i, a;
for(i = -1.0; i < 1.0 ; i += 0.1) {
a = acos(i)*180.0/3.141592;
printf(“acos(%f) = %f degrees\n”, i, a);
}
}
See Also
sin(), cos(), tan(), asin(), atan(), atan2()
Return Value
An angle in radians, in the range 0 to 
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Library Functions
ASCTIME
Synopsis
#include <time.h>
char * asctime (struct tm * t)
Description
The asctime() function takes the time broken down into the struct tm structure,
pointed to by its argument, and returns a 26 character string describing the current date
and time in the format:
Sun Sep 16 01:03:52 1973\n\0
Note the newline at the end of the string. The width of each field in the string is fixed.
The example gets the current time, converts it to a struct tm with localtime(), it
then converts this to ASCII and prints it. The time() function will need to be provided
by the user (see time() for details).
Example
#include <stdio.h>
#include <time.h>
void
main (void)
{
time_t clock;
struct tm * tp;
time(&clock);
tp = localtime(&clock);
printf(“%s”, asctime(tp));
}
See Also
ctime(), gmtime(), localtime(), time()
Return Value
A pointer to the string.
Note
The example will require the user to provide the time() routine as it cannot be
supplied with the compiler. See time() for more details.
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ASIN
Synopsis
#include <math.h>
double asin (double f)
Description
The asin() function implements the converse of sin(); i.e., it is passed a value in the
range -1 to +1, and returns an angle in radians whose sine is equal to that value.
Example
#include <math.h>
#include <stdio.h>
void
main (void)
{
float i, a;
for(i = -1.0; i < 1.0 ; i += 0.1) {
a = asin(i)*180.0/3.141592;
printf(“asin(%f) = %f degrees\n”, i, a);
}
}
See Also
sin(), cos(), tan(), acos(), atan(), atan2()
Return Value
An angle in radians, in the range -/2 – /2.
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Library Functions
ASSERT
Synopsis
#include <assert.h>
void assert (int e)
Description
This macro is used for debugging purposes; the basic method of usage is to place
assertions liberally throughout your code at points where correct operation of the code
depends upon certain conditions being true initially. An assert() routine can be used
to ensure at runtime that an assumption holds true. For example, the following
statement asserts that mode is larger than zero:
assert(mode > 0);
If the expression passed to assert() evaluates to false at runtime, the macro
attempts to print diagnostic information and abort the program. A fuller discussion of
the uses of assert() is impossible in limited space, but it is closely linked to methods
of proving program correctness.
The assert() macro depends on the implementation of the function _fassert().
The default _fassert() function, built into the library files, first calls the printf()
function, which prints a message identifying the source file and line number of the
assertion. Next, _fassert() attempts to terminate program execution by calling
abort(). The exact behaviour of abort() is dependent on the selected device and
whether the executable is a debug or production build. For debug builds, abort() will
consist of a software breakpoint instruction followed by a Reset instruction, if possible.
For production builds, abort() will consist only of a Reset instruction, if possible. In
both cases, if a Reset instruction is not available, a goto instruction that jumps to itself
in an endless loop is output.
The _fassert() routine can be adjusted to ensure it meets your application needs.
Include the source file defining this function into your project, if you modify it.
Example
#include <assert.h>
void
ptrfunc (struct xyz * tp)
{
assert(tp != 0);
}
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ATAN
Synopsis
#include <math.h>
double atan (double x)
Description
This function returns the arc tangent of its argument; i.e., it returns an angle ‘e’ in the
range -/2 – /2.
Example
#include <stdio.h>
#include <math.h>
void
main (void)
{
printf(“atan(%f) is %f\n”, 1.5, atan(1.5));
}
See Also
sin(), cos(), tan(), asin(), acos(), atan2()
Return Value
The arc tangent of its argument.
ATAN2
Synopsis
#include <math.h>
double atan2 (double x, double x)
Description
This function returns the arc tangent of y/x.
Example
#include <stdio.h>
#include <math.h>
void
main (void)
{
printf(“atan2(%f, %f) is %f\n”, 10.0, -10.0, atan2(10.0,
-10.0));
}
See Also
sin(), cos(), tan(), asin(), acos(), atan()
Return Value
The arc tangent of y/x.
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Library Functions
ATOF
Synopsis
#include <stdlib.h>
double atof (const char * s)
Description
The atof() function scans the character string passed to it, skipping leading blanks. It
then converts an ASCII representation of a number to a double. The number can be in
decimal, normal floating point or scientific notation.
Example
#include <stdlib.h>
#include <stdio.h>
void
main (void)
{
char buf[80];
double i;
gets(buf);
i = atof(buf);
printf(“Read %s: converted to %f\n”, buf, i);
}
See Also
atoi(), atol(), strtod()
Return Value
A double precision floating-point number. If no number is found in the string, 0.0 will be
returned.
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ATOI
Synopsis
#include <stdlib.h>
int atoi (const char * s)
Description
The atoi() function scans the character string passed to it, skipping leading blanks
and reading an optional sign. It then converts an ASCII representation of a decimal
number to an integer.
Example
#include <stdlib.h>
#include <stdio.h>
void
main (void)
{
char buf[80];
int i;
gets(buf);
i = atoi(buf);
printf(“Read %s: converted to %d\n”, buf, i);
}
See Also
xtoi(), atof(), atol()
Return Value
A signed integer. If no number is found in the string, 0 will be returned.
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Library Functions
ATOL
Synopsis
#include <stdlib.h>
long atol (const char * s)
Description
The atol() function scans the character string passed to it, skipping leading blanks. It
then converts an ASCII representation of a decimal number to a long integer.
Example
#include <stdlib.h>
#include <stdio.h>
void
main (void)
{
char buf[80];
long i;
gets(buf);
i = atol(buf);
printf(“Read %s: converted to %ld\n”, buf, i);
}
See Also
atoi(), atof()
Return Value
A long integer. If no number is found in the string, 0 will be returned.
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BSEARCH
Synopsis
#include <stdlib.h>
void * bsearch (const void * key, void * base, size_t n_memb,
size_t size, int (*compar)(const void *, const void *))
Description
The bsearch() function searches a sorted array for an element matching a particular
key. It uses a binary search algorithm, calling the function pointed to by compar to
compare elements in the array.
Example
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
struct value {
char name[10];
int value;
} values[] = {
{ "foobar", 66 };
{ "casbar", 87 };
{ "crossbar", 105 };
};
int
val_cmp (const void * p1, const void * p2) {
return strcmp(((const struct value *)p1)->name,
((const struct value *)p2)->name);
}
void
main (void) {
int i = sizeof(values)/sizeof(struct value);
struct value * vp;
qsort(values, i, sizeof values[0], val_cmp);
vp = bsearch(“fred”, values, i, sizeof values[0], val_cmp);
if(!vp)
printf(“Item ’fred’ was not found\n”);
else
printf(“Item ’fred’ has value %d\n”, vp->value);
}
See Also
qsort()
Return Value
A pointer to the matched array element (if there is more than one matching element,
any of these can be returned). If no match is found, a null is returned.
Note
The comparison function must have the correct prototype.
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Library Functions
CEIL
Synopsis
#include <math.h>
double ceil (double f)
Description
This routine returns the smallest whole number not less than f.
Example
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
void
main (void)
{
double j;
j = 2.345 * rand()
printf(“The ceiling of %f is %f\n”, j, ceil(j));
}
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CGETS
Synopsis
#include <conio.h>
char * cgets (char * s)
Description
The cgets() function will read one line of input from the console into the buffer passed
as an argument. It does so by repeated calls to getche(). As characters are read,
they are buffered, with backspace deleting the previously typed character, and ctrl-U
deleting the entire line typed so far. Other characters are placed in the buffer, with a
carriage return or line feed (newline) terminating the function. The collected string is
null terminated.
Example
#include <conio.h>
#include <string.h>
char buffer[80];
void
main (void)
{
for(;;) {
cgets(buffer);
if(strcmp(buffer, “exit” == 0)
break;
cputs(“Type ’exit’ to finish\n”);
}
}
See Also
getch(), getche(), putch(), cputs()
Return Value
The return value is the character passed as the sole argument.
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Library Functions
CLRWDT
Synopsis
#include <xc.h>
CLRWDT();
Description
This macro is used to clear the device’s internal watchdog timer.
Example
#include <xc.h>
void
main (void)
{
WDTCON=1;
/* enable the WDT */
CLRWDT();
}
COS
Synopsis
#include <math.h>
double cos (double f)
Description
This function yields the cosine of its argument, which is an angle in radians. The cosine
is calculated by expansion of a polynomial series approximation.
Example
#include <math.h>
#include <stdio.h>
#define C 3.141592/180.0
void
main (void)
{
double i;
for(i = 0 ; i <= 180.0 ; i += 10)
printf(“sin(%3.0f) = %f, cos = %f\n”, i, sin(i*C),
cos(i*C));
}
See Also
sin(), tan(), asin(), acos(), atan(), atan2()
Return Value
A double in the range -1 to +1.
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COSH, SINH, TANH
Synopsis
#include <math.h>
double cosh (double f)
double sinh (double f)
double tanh (double f)
Description
These functions are the implement hyperbolic equivalents of the trigonometric
functions; cos(), sin() and tan().
Example
#include <stdio.h>
#include <math.h>
void
main (void)
{
printf(“%f\n”, cosh(1.5));
printf(“%f\n”, sinh(1.5));
printf(“%f\n”, tanh(1.5));
}
Return Value
The function cosh() returns the hyperbolic cosine value.
The function sinh() returns the hyperbolic sine value.
The function tanh() returns the hyperbolic tangent value.
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Library Functions
CPUTS
Synopsis
#include <conio.h>
void cputs (const char * s)
Description
The cputs() function writes its argument string to the console, outputting carriage
returns before each newline in the string. It calls putch() repeatedly. On a hosted system cputs() differs from puts() in that it writes to the console directly, rather than
using file I/O. In an embedded system cputs() and puts() are equivalent.
Example
#include <conio.h>
#include <string.h>
char buffer[80];
void
main (void)
{
for(;;) {
cgets(buffer);
if(strcmp(buffer, “exit” == 0)
break;
cputs(“Type ’exit’ to finish\n”;
}
}
See Also
cputs(), puts(), putch()
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CTIME
Synopsis
#include <time.h>
char * ctime (time_t * t)
Description
The ctime() function converts the time in seconds pointed to by its argument to a
string of the same form as described for asctime(). Thus the example program prints
the current time and date.
Example
#include <stdio.h>
#include <time.h>
void
main (void)
{
time_t clock;
time(&clock);
printf(“%s”, ctime(&clock));
}
See Also
gmtime(), localtime(), asctime(), time()
Return Value
A pointer to the string.
Note
The example will require the user to provide the time() routine as one cannot be
supplied with the compiler. See time() for more detail.
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Library Functions
DI, EI
Synopsis
#include <xc.h>
void ei (void)
void di (void)
Description
The di() and ei() routines disable and re-enable interrupts respectively. These are
implemented as macros. The example shows the use of ei() and di() around access
to a long variable that is modified during an interrupt. If this was not done, it would be
possible to return an incorrect value, if the interrupt occurred between accesses to
successive words of the count value.
The ei() macro should never be called in an interrupt function, and there is no need
to call di() in an interrupt function.
Example
#include <xc.h>
long count;
void
interrupt tick (void)
{
count++;
}
long
getticks (void)
{
long val;
/* Disable interrupts around access
to count, to ensure consistency.*/
di();
val = count;
ei();
return val;
}
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DIV
Synopsis
#include <stdlib.h>
div_t div (int numer, int denom)
Description
The div() function computes the quotient and remainder of the numerator divided by
the denominator.
Example
#include <stdlib.h>
#include <stdio.h>
void
main (void)
{
div_t x;
x = div(12345, 66);
printf(“quotient = %d, remainder = %d\n”, x.quot, x.rem);
}
See Also
udiv(), ldiv(), uldiv()
Return Value
Returns the quotient and remainder into the div_t structure.
EEPROM ROUTINES
Description
These functions can now be generated by MPLAB Code Configurator.
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Library Functions
EVAL_POLY
Synopsis
#include <math.h>
double eval_poly (double x, const double * d, int n)
Description
The eval_poly() function evaluates a polynomial, whose coefficients are contained in
the array d, at x, for example:
y = x*x*d2 + x*d1 + d0.
The order of the polynomial is passed in n.
Example
#include <stdio.h>
#include <math.h>
void
main (void)
{
double x, y;
double d[3] = {1.1, 3.5, 2.7};
x = 2.2;
y = eval_poly(x, d, 2);
printf(“The polynomial evaluated at %f is %f\n”, x, y);
}
Return Value
A double value, being the polynomial evaluated at x.
EXP
Synopsis
#include <math.h>
double exp (double f)
Description
This function returns the exponential function of its argument; i.e., e to the power of f.
Example
#include <math.h>
#include <stdio.h>
void
main (void)
{
printf(“e to 2.0 = %f\n”, exp(2.0));
}
See Also
log(), log10(), pow()
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FABS
Synopsis
#include <math.h>
double fabs (double f)
Description
This routine returns the absolute value of its double argument.
Example
#include <stdio.h>
#include <math.h>
void
main (void)
{
printf("%f %f\n", fabs(1.5), fabs(-1.5));
}
See Also
abs(), labs()
FLASH ROUTINES
Description
These functions can now be generated by MPLAB® Code Configurator.
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Library Functions
FLOOR
Synopsis
#include <math.h>
double floor (double f)
Description
This routine returns the largest whole number not greater than f.
Example
#include <stdio.h>
#include <math.h>
void
main (void)
{
printf("%f\n", floor( 1.5 ));
printf("%f\n", floor( -1.5));
}
FMOD
Synopsis
#include <math.h>
double fmod (double x, double y)
Description
The function fmod returns the remainder of x/y as a floating-point quantity.
Example
#include <math.h>
void
main (void)
{
double rem, x;
x = 12.34;
rem = fmod(x, 2.1);
}
Return Value
The floating-point remainder of x/y.
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FREXP
Synopsis
#include <math.h>
double frexp (double f, int * p)
Description
The frexp() function breaks a floating-point number into a normalized fraction and an
integral power of 2. The integer is stored into the int object pointed to by p. Its return
value x is in the interval (0.5, 1.0) or zero, and f equals x times 2 raised to the power
stored in *p. If f is zero, both parts of the result are zero.
Example
#include <math.h>
#include <stdio.h>
void
main (void)
{
double f;
int i;
f = frexp(23456.34, &i);
printf("23456.34 = %f * 2^%d\n", f, i);
}
See Also
ldexp()
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Library Functions
FTOA
Synopsis
#include <stdlib.h>
char * ftoa (float f, int * status)
Description
The function ftoa converts the contents of f into a string which is stored into a buffer
which is then return.
Example
#include <stdlib.h>
#include <stdio.h>
void
main (void)
{
char * buf;
float input = 12.34;
int status;
buf = ftoa(input, &status);
printf("The buffer holds %s\n", buf);
}
See Also
strtol(), itoa(), utoa(), ultoa()
Return Value
This routine returns a reference to the buffer into which the result is written.
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GETCH
Synopsis
#include <conio.h>
char getch (void)
Description
The getch() function is provided as an empty stub which can be completed as each
project requires. Typically this function will read one byte of data from a peripheral that
is associated with stdin, and return this value.
Example
#include <conio.h>
char result;
void
main (void)
{
result = getch();
}
See Also
getche(), getchar()
GETCHE
Synopsis
#include <conio.h>
char getche (void)
Description
The getche() function is provided as an empty stub which can be completed as each
project requires. Typically this function will read one byte of data from a peripheral that
is associated with stdin, and return this value. Unlike getch(), it should echo the
received character. This function is independent of getch(), unless you should
choose to implement it otherwise.
Example
#include <conio.h>
char result;
void
main (void)
{
result = getche();
}
See Also
getch(), getchar()
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Library Functions
GETCHAR
Synopsis
#include <stdio.h>
int getchar (void)
Description
The getchar() routine usually reads from stdin, but is implemented as a call to
getche().
Example
#include <stdio.h>
void
main (void)
{
int c;
while((c = getchar()) != EOF)
putchar(c);
}
See Also
getc(), getche()
Note
This routine calls getche().
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GETS
Synopsis
#include <stdio.h>
char * gets (char * s)
Description
The gets() function reads a line from standard input into the buffer at s, deleting the
newline (compare: fgets()). The buffer is null terminated. In an embedded system,
gets() is equivalent to cgets(), and results in getche() being called repeatedly to
get characters. Editing (with backspace) is available.
Example
#include <stdio.h>
void
main (void)
{
char buf[80];
printf("Type a line: ");
if(gets(buf))
puts(buf);
}
See Also
fgets(), freopen(), puts()
Return Value
It returns its argument, or NULL on end-of-file.
Note
As you cannot specify a maximum number of characters to read with this function, it is
unsafe to use.
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Library Functions
GET_CAL_DATA
Synopsis
#include <xc.h>
double get_cal_data (const unsigned char * code_ptr)
Description
This function returns the 32-bit floating-point calibration data from the PIC MCU 14000
calibration space. Only use this function to access KREF, KBG, VHTHERM and KTC (that
is, the 32-bit floating-point parameters). FOSC and TWDT can be accessed directly as
they are bytes.
Example
#include <xc.h>
void
main (void)
{
double x;
unsigned char y;
/* Get the slope reference ratio. */
x = get_cal_data(KREF);
/* Get the WDT time-out. */
y = TWDT;
}
Return Value
The value of the calibration parameter
Note
This function can only be used on the PIC14000.
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GMTIME
Synopsis
#include <time.h>
struct tm * gmtime (time_t * t)
Description
This function converts the time pointed to by t which is in seconds since 00:00:00 on
Jan 1, 1970, into a broken down time stored in a structure as defined in time.h. The
structure is defined in the ‘Data Types’ section.
Example
#include <stdio.h>
#include <time.h>
void
main (void)
{
time_t clock;
struct tm * tp;
time(&clock);
tp = gmtime(&clock);
printf("It’s %d in London\n", tp->tm_year+1900);
}
See Also
ctime(), asctime(), time(), localtime()
Return Value
Returns a structure of type tm.
Note
The example will require the user to provide the time() routine as one cannot be
supplied with the compiler. See time() for more detail.
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Library Functions
ISALNUM, ISALPHA, ISDIGIT, ISLOWER, ET. AL.
Synopsis
#include <ctype.h>
int isalnum (char c)
int isalpha (char c)
int isascii (char c)
int iscntrl (char c)
int isdigit (char c)
int islower (char c)
int isprint (char c)
int isgraph (char c)
int ispunct (char c)
int isspace (char c)
int isupper (char c)
int isxdigit(char c)
Description
These macros, defined in ctype.h, test the supplied character for membership in one
of several overlapping groups of characters. Note that all except isascii() are
defined for c, if isascii(c) is true or if c = EOF.
isalnum(c)
isalpha(c)
isascii(c)
iscntrl(c)
isdigit(c)
islower(c)
isprint(c)
isgraph(c)
ispunct(c)
isspace(c)
isupper(c)
isxdigit(c)
c is in 0-9 or a-z or A-Z
c is in A-Z or a-z
c is a 7 bit ASCII character
c is a control character
c is a decimal digit
c is in a-z
c is a printing char
c is a non-space printable character
c is not alphanumeric
c is a space, tab or newline
c is in A-Z
c is in 0-9 or a-f or A-F
Example
#include <ctype.h>
#include <stdio.h>
void
main (void)
{
char buf[80];
int i;
gets(buf);
i = 0;
while(isalnum(buf[i]))
i++;
buf[i] = 0;
printf("’%s’ is the word\n", buf);
}
See Also
toupper(), tolower(), toascii()
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ISDIG
Synopsis
#include <ctype.h>
int isdig (int c)
Description
The isdig() function tests the input character c to see if is a decimal digit (0 – 9) and
returns true is this is the case; false otherwise.
Example
#include <ctype.h>
void
main (void)
{
char buf[] = "1998a";
if(isdig(buf[0]))
printf(" type detected\n");
}
See Also
isdigit() (listed under isalnum())
Return Value
Zero if the character is a decimal digit; a non-zero value otherwise.
ITOA
Synopsis
#include <stdlib.h>
char * itoa (char * buf, int val, int base)
Description
The function itoa converts the contents of val into a string which is stored into buf.
The conversion is performed according to the radix specified in base. buf is assumed
to reference a buffer which has sufficient space allocated to it.
Example
#include <stdlib.h>
#include <stdio.h>
void
main (void) {
char buf[10];
itoa(buf, 1234, 16);
printf("The buffer holds %s\n", buf);
}
See Also
strtol(), utoa(), ltoa(), ultoa()
Return Value
This routine returns a copy of the buffer into which the result is written.
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Library Functions
LABS
Synopsis
#include <stdlib.h>
int labs (long int j)
Description
The labs() function returns the absolute value of long value j.
Example
#include <stdio.h>
#include <stdlib.h>
void
main (void)
{
long int a = -5;
printf("The absolute value of %ld is %ld\n", a, labs(a));
}
See Also
abs()
Return Value
The absolute value of j.
LDEXP
Synopsis
#include <math.h>
double ldexp (double f, int i)
Description
The ldexp() function performs the inverse of frexp() operation; the integer i is
added to the exponent of the floating-point f and the resultant returned.
Example
#include <math.h>
#include <stdio.h>
void
main (void) {
double f;
f = ldexp(1.0, 10);
printf("1.0 * 2^10 = %f\n", f);
}
See Also
frexp()
Return Value
The return value is the integer i added to the exponent of the floating-point value f.
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LDIV
Synopsis
#include <stdlib.h>
ldiv_t ldiv (long number, long denom)
Description
The ldiv() routine divides the numerator by the denominator, computing the quotient
and the remainder. The sign of the quotient is the same as that of the mathematical
quotient. Its absolute value is the largest integer which is less than the absolute value
of the mathematical quotient.
The ldiv() function is similar to the div() function, the difference being that the
arguments and the members of the returned structure are all of type long int.
Example
#include <stdlib.h>
#include <stdio.h>
void
main (void)
{
ldiv_t lt;
lt = ldiv(1234567, 12345);
printf("Quotient = %ld, remainder = %ld\n", lt.quot,
lt.rem);
}
See Also
div(), uldiv(), udiv()
Return Value
Returns a structure of type ldiv_t
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Library Functions
LOCALTIME
Synopsis
#include <time.h>
struct tm * localtime (time_t * t)
Description
The localtime() function converts the time pointed to by t which is in seconds since
00:00:00 on Jan 1, 1970, into a broken down time stored in a structure as defined in
time.h. The routine localtime() takes into account the contents of the global integer
time_zone. This should contain the number of minutes that the local time zone is
westward of Greenwich. On systems where it is not possible to predetermine this value,
localtime() will return the same result as gmtime().
Example
#include <stdio.h>
#include <time.h>
char * wday[] = {
"Sunday", "Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday"
};
void
main (void)
{
time_t clock;
struct tm * tp;
time(&clock);
tp = localtime(&clock);
printf("Today is %s\n", wday[tp->tm_wday]);
}
See Also
ctime(), asctime(), time()
Return Value
Returns a structure of type tm.
Note
The example will require the user to provide the time() routine as one cannot be
supplied with the compiler. See time() for more detail.
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LOG, LOG10
Synopsis
#include <math.h>
double log (double f)
double log10 (double f)
Description
The log() function returns the natural logarithm of f. The function log10() returns the
logarithm to base 10 of f.
Example
#include <math.h>
#include <stdio.h>
void
main (void)
{
double f;
for(f = 1.0 ; f <= 10.0 ; f += 1.0)
printf("log(%1.0f) = %f\n", f, log(f));
}
See Also
exp(), pow()
Return Value
Zero if the argument is negative.
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Library Functions
LONGJMP
Synopsis
#include <setjmp.h>
void longjmp (jmp_buf buf, int val)
Description
The longjmp() function, in conjunction with setjmp(), provides a mechanism for
non-local goto’s. To use this facility, setjmp() should be called with a jmp_buf
argument in some outer level function. The call from setjmp() will return 0.
To return to this level of execution, longjmp() can be called with the same jmp_buf
argument from an inner level of execution. However, the function that called setjmp()
must still be active when longjmp() is called. Breach of this rule will cause errors, due
to the use of a stack containing invalid data. The val argument to longjmp() will be
the value apparently returned from the setjmp(). This should normally be non-zero,
to distinguish it from the genuine setjmp() call.
Example
#include <stdio.h>
#include <setjmp.h>
#include <stdlib.h>
jmp_buf jb;
void
inner (void)
{
longjmp(jb, 5);
}
void
main (void)
{
int i;
if(i = setjmp(jb)) {
printf("setjmp returned %d\n" i);
exit(0);
}
printf("setjmp returned 0 - good\n");
printf("calling inner...\n");
inner();
printf("inner returned - bad!\n");
}
See Also
setjmp()
Return Value
The longjmp() routine never returns.
Note
The function which called setjmp() must still be active when longjmp() is called.
Breach of this rule will cause disaster, due to the use of a stack containing invalid data.
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LTOA
Synopsis
#include <stdlib.h>
char * ltoa (char * buf, long val, int base)
Description
The function ltoa converts the contents of val into a string which is stored into buf.
The conversion is performed according to the radix specified in base. buf is assumed
to reference a buffer which has sufficient space allocated to it.
Example
#include <stdlib.h>
#include <stdio.h>
void
main (void)
{
char buf[10];
ltoa(buf, 12345678L, 16);
printf("The buffer holds %s\n", buf);
}
See Also
strtol(), itoa(), utoa(), ultoa()
Return Value
This routine returns a copy of the buffer into which the result is written.
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Library Functions
MEMCHR
Synopsis
#include <string.h>
void * memchr (const void * block, int val, size_t length)
Description
The memchr() function is similar to strchr() except that instead of searching null-terminated strings, it searches a block of memory specified by length for a particular byte.
Its arguments are a pointer to the memory to be searched, the value of the byte to be
searched for, and the length of the block. A pointer to the first occurrence of that byte
in the block is returned.
Example
#include <string.h>
#include <stdio.h>
unsigned int ary[] = {1, 5, 0x6789, 0x23};
void
main (void)
{
char * cp;
cp = memchr(ary, 0x89, sizeof ary);
if(!cp)
printf("Not found\n");
else
printf("Found at offset %u\n", cp - (char *)ary);
}
See Also
strchr()
Return Value
A pointer to the first byte matching the argument if one exists; NULL otherwise.
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MEMCMP
Synopsis
#include <string.h>
int memcmp (const void * s1, const void * s2, size_t n)
Description
The memcmp() function compares two blocks of memory, of length n, and returns a
signed value similar to strncmp(). Unlike strncmp() the comparison does not stop
on a null character.
Example
#include <stdio.h>
#include <string.h>
void
main (void)
{
int buf[10], cow[10], i;
buf[0] = 1;
buf[2] = 4;
cow[0] = 1;
cow[2] = 5;
buf[1] = 3;
cow[1] = 3;
i = memcmp(buf, cow, 3*sizeof(int));
if(i < 0)
printf("Less than\n");
else if(i > 0)
printf("Greater than\n");
else
printf("Equal\n");
}
See Also
strncmp(), strchr(), memset(), memchr()
Return Value
Returns negative one, zero or one, depending on whether s1 points to string which is
less than, equal to or greater than the string pointed to by s2 in the collating sequence.
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Library Functions
MEMCPY
Synopsis
#include <string.h>
void * memcpy (void * d, const void * s, size_t n)
Description
The memcpy() function copies n bytes of memory starting from the location pointed to
by s to the block of memory pointed to by d. The result of copying overlapping blocks
is undefined. The memcpy() function differs from strcpy() in that it copies a specified
number of bytes, rather than all bytes up to a null terminator.
Example
#include <string.h>
#include <stdio.h>
void
main (void)
{
char buf[80];
memset(buf, 0, sizeof buf);
memcpy(buf, "A partial string", 10);
printf("buf = ’%s’\n", buf);
}
See Also
strncpy(), strncmp(), strchr(), memset()
Return Value
The memcpy() routine returns its first argument.
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MEMMOVE
Synopsis
#include <string.h>
void * memmove (void * s1, const void * s2, size_t n)
Description
The memmove() function is similar to the function memcpy() except copying of
overlapping blocks is handled correctly. That is, it will copy forwards or backwards as
appropriate to correctly copy one block to another that overlaps it.
See Also
strncpy(), strncmp(), strchr(), memcpy()
Return Value
The function memmove() returns its first argument.
MEMSET
Synopsis
#include <string.h>
void * memset (void * s, int c, size_t n)
Description
The memset() function fills n bytes of memory starting at the location pointed to by s
with the byte c.
Example
#include <string.h>
#include <stdio.h>
void
main (void)
{
char abuf[20];
strcpy(abuf, "This is a string";
memset(abuf, ’x’, 5);
printf("buf = ’%s’\n", abuf);
}
See Also
strncpy(), strncmp(), strchr(), memcpy(), memchr()
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Library Functions
MKTIME
Synopsis
#include <time.h>
time_t mktime (struct tm * tmptr)
Description
The mktime() function converts and returns the local calendar time referenced by the
tm structure tmptr into a time being the number of seconds passed since Jan 1, 1970,
or returns -1 if the time cannot be represented.
Example
#include <time.h>
#include <stdio.h>
void
main (void)
{
struct tm birthday;
birthday.tm_year = 83;
// the 5th of May 1983
birthday.tm_mon = 5;
birthday.tm_mday = 5;
birthday.tm_hour = birthday.tm_min = birthday.tm_sec = 0;
printf("you were born approximately %ld seconds after the
unix epoch\n",
mktime(&birthday));
}
See Also
ctime(), asctime()
Return Value
The time contained in the tm structure represented as the number of seconds since the
1970 Epoch, or -1 if this time cannot be represented.
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MODF
Synopsis
#include <math.h>
double modf (double value, double * iptr)
Description
The modf() function splits the argument value into integral and fractional parts, each
having the same sign as value. For example, -3.17 would be split into the integral part
(-3) and the fractional part (-0.17).
The integral part is stored as a double in the object pointed to by iptr.
Example
#include <math.h>
#include <stdio.h>
void
main (void)
{
double i_val, f_val;
f_val = modf( -3.17, &i_val);
}
Return Value
The signed fractional part of value.
NOP
Synopsis
#include <xc.h>
NOP();
Description
Execute NOP instruction here. This is often useful to fine tune delays or create a handle
for breakpoints. The NOP instruction is sometimes required during some sensitive
sequences in hardware.
Example
#include <xc.h>
void
crude_delay(unsigned char x) {
while(x--){
NOP(); /* Do nothing for 3 cycles */
NOP();
NOP();
}
}
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Library Functions
POW
Synopsis
#include <math.h>
double pow (double f, double p)
Description
The pow() function raises its first argument, f, to the power p.
Example
#include <math.h>
#include <stdio.h>
void
main (void)
{
double f;
for(f = 1.0 ; f <= 10.0 ; f += 1.0)
printf("pow(2, %1.0f) = %f\n", f, pow(2, f));
}
See Also
log(), log10(), exp()
Return Value
f to the power of p.
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PRINTF
Synopsis
#include <stdio.h>
int printf (const char * fmt, ...)
Description
The printf() function is a formatted output routine, operating on stdout. It relies on
the putch() function to determine the destination of the standard output stream.
A putch() function must be written for each project that uses printf(), and the project must include code that initializes any peripherals used by this routine. A stub for
putch() can be found in the sources directory of the compiler, and one possible
implementation is shown in the example of this entry. Ensure the source code for your
putch() added to your project once it is complete.
The printf() routine is passed a format string, followed by a list of zero or more arguments. In the format string are conversion specifications, each of which is used to print
out one of the argument list values.
Each conversion specification is of the form %fm.nc, where the percent symbol % introduces a conversion, followed by zero or more flags or modifiers, f, (in any order), and
followed by an optional width specification m. The n specification is an optional precision specification (introduced by the dot (‘.’)) and c is a letter specifying the type of the
conversion, which must appear last in the specification.
Note that the standard default argument promotions are applied to all unprototyped
arguments (those appearing after the format string argument, in the case of
printf()). These promotions consist of the standard integral promotions (see
Section 5.6.1 “Integral Promotion”) and conversion of all float values to double.
Thus, it is not possible to pass any sort of char, short, or float value, for example,
to the printf() function.
The flags and modifiers may consist of the following.
• A minus sign (‘-’), which indicates left rather than right adjustment of the converted value in the field. Where the field width is larger than required for the
conversion, blank padding is performed at the left or right as specified.
• A plus sign (‘+’), which indicates that the sign of the converted value will always
be printed, even if the value is positive.
• A space (‘ ’), which will prefix a space to the converted result if it does not contain
a sign (hence this has no effect if the + flag is specified) or if a signed conversion
results in no characters.
• The digit zero (‘0’), which indicates that any padding will be performed with zeros
rather than blanks. This flag is ignored if the ‘-’ flag has been specified.
• A hash character (‘#’), which indicates that an alternate format is to be used. The
nature of the alternate format is discussed below. Not all formats have alternates.
In those cases, the presence of the hash character has no effect.
• An elle character (‘l’), which indicates that the argument corresponding to a
following d, i, o, u, x or X is a long or unsigned long.
If the character * is used in place of a decimal constant for the width or precision, e.g.,
in the format %*d, then one integer argument will be taken from the list to provide that
value.
Note that the h and L modifiers are not currently supported. Manually cast the argument to a short type instead of using the h modifier. The MPLAB XC8 long double
type is equivalent to the double type, so the L modifier will never have an effect.
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Library Functions
The types of conversion are:
f Floating point - m is the total width and n is the number of digits after the decimal
point. If n is omitted it defaults to 6. If the precision is zero, the decimal point will be
omitted unless the alternate format is specified.
e Print the corresponding argument in scientific notation. Otherwise similar to f.
g Use e or f format, whichever gives maximum precision in minimum width. If the
alternate format is not specified, any trailing zeros after the decimal point will be
removed, and if no digits remain after the decimal point, it will also be removed.
o x X u d Integer conversion - in radices 8, 16, 16, 10 and 10 respectively. The conversion is signed in the case of d, unsigned otherwise. The precision value is the total
number of digits to print, and can be used to force leading zeros. For example, %8.4x
will print at least 4 HEX digits in an 8 wide field. Preceding the key letter with an l indicates that the value argument is a long integer. The letter X prints out hexadecimal
numbers using the upper case letters A-F rather than a- f as would be printed when
using x. When the alternate format is specified, a leading zero will be supplied for the
octal format, and a leading 0x or 0X for the HEX format.
s Print a string - the value argument is assumed to be a character. At most n
characters from the string will be printed, in a field m characters wide.
c The argument is assumed to be a single character and is printed literally.
p The argument is assumed to a pointer to an object. The pointer is promoted to a
large pointer type, so this placeholder can be used with any pointer size (1, 2 or 3
bytes).
Any other characters used as conversion specifications will be printed. Thus %% will
produce a single percent sign.
The example shown below can be used to print to the simulator console or a file from
within MPLAB X IDE. The code might need modification to suit your target device.
Ensure that in your project properties you have turned on the Enable Uart1 IO check
box in the Uart1 IO Options option category, in the Simulator category.
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Example
#include <xc.h>
#include <stdio.h>
#include <stdarg.h>
long size;
void putch(char data)
{
while( ! TXIF)
continue;
TXREG = data;
}
void init_uart(void)
{
SPBRG = 0x19;
TXEN = 1;
BRGH = 1;
SPEN = 1;
CREN = 1;
}
// 9600 baud @ 4 MHz
// enable transmitter
// select high baud rate
// enable serial port
// enable continuous operation
void main(void) {
init_uart();
size = 0x12345678;
printf("Total = %4d%%\n", 23);
printf("Size is %lx\n", size);
printf("Name = %.8s\n", "a1234567890");
printf("xx%*d\n", 3, 4);
NOP();
return;
}
will print:
Total =
23%
Size is 12345678
Name = a1234567
xx 4
See Also
sprintf()
Return Value
The printf() function returns the number of characters written to stdout.
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Library Functions
PUTCH
Synopsis
#include <conio.h>
void putch (char c)
Description
The putch() function is provided as an empty stub which can be completed as each
project requires. It must be defined if you intend to use the printf() function. Typically this function will accept one byte of data and send this to a peripheral which is
associated with stdout.
Example
#include <conio.h>
char * x = "This is a string";
void
main (void)
{
char * cp;
cp = x;
while(*x)
putch(*x++);
putch(’\n’);
}
See Also
printf(), putchar()
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PUTCHAR
Synopsis
#include <stdio.h>
int putchar (int c)
Description
The putchar() function calls putch() to print one character to stdout, and is defined
in stdio.h.
Example
#include <stdio.h>
char * x = "This is a string";
void
main (void)
{
char * cp;
cp = x;
while(*x)
putchar(*x++);
putchar(’\n’);
}
See Also
putc(), getc(), freopen(), fclose()
Return Value
The character passed as argument, or EOF if an error occurred.
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Library Functions
PUTS
Synopsis
#include <stdio.h>
int puts (const char * s)
Description
The puts() function writes the string s to the stdout stream, appending a newline. The
null character terminating the string is not copied.
Example
#include <stdio.h>
void
main (void)
{
puts("Hello, world!");
}
See Also
fputs(), gets(), freopen(), fclose()
Return Value
EOF is returned on error; zero otherwise.
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QSORT
Synopsis
#include <stdlib.h>
void qsort (void * base, size_t nel, size_t width,
int (*func)(const void *, const void *))
Description
The qsort() function is an implementation of the quicksort algorithm. It sorts an array
of nel items, each of length width bytes, located contiguously in memory at base. The
argument func is a pointer to a function used by qsort() to compare items. It calls
func with s to two items to be compared. If the first item is considered to be greater
than, equal to or less than the second then func should return a value greater than
zero, equal to zero or less than zero respectively.
Example
#include <stdio.h>
#include <stdlib.h>
int array[] = {
567, 23, 456, 1024, 17, 567, 66
};
int
sortem (const void * p1, const void * p2)
{
return *(int *)p1 - *(int *)p2;
}
void
main (void)
{
register int i;
qsort(aray, sizeof array/sizeof array[0],
sizeof array[0], sortem);
for(i = 0 ; i != sizeof array/sizeof array[0] ; i++)
printf("%d\t", array[i]);
putchar(’\n’);
}
Note
The function parameter must be a pointer to a function of type similar to:
int func (const void *, const void *)
For example, it must accept two const void * parameters, and must be prototyped.
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Library Functions
RAND
Synopsis
#include <stdlib.h>
int rand (void)
Description
The rand() function is a pseudo-random number generator. It returns an integer in the
range 0 to 32767, which changes in a pseudo-random fashion on each call. The algorithm will produce a deterministic sequence if started from the same point. The starting
point is set using the srand() call. The example shows use of the time() function to
generate a different starting point for the sequence each time.
Example
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
void
main (void)
{
time_t toc;
int i;
time(&toc);
srand((int)toc);
for(i = 0 ; i != 10 ; i++)
printf("%d\t", rand());
putchar(’\n’);
}
See Also
srand()
Note
The example will require the user to provide the time() routine as one cannot be
supplied with the compiler. See time() for more detail.
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READTIMERx
Synopsis
#include <xc.h>
unsigned short READTIMERx (void);
Description
The READTIMERx() macro returns the value held by the TMRx register, where x is one
of the digits 0, 1 or 3.
Example
#include <xc>
void
main (void)
{
while(READTIMER0() != 0xFF)
continue;
SLEEP();
}
See Also
WRITETIMERx()
Return Value
The value held by the TMRx register.
Note
This macro can only be used with PIC18 devices.
RESET
Synopsis
#include <xc.h>
RESET();
Description
Execute a RESET instruction here. This will trigger a software device Reset.
Example
#include <xc.h>
void
main(void)
{
init();
while( ! (fail_code = getStatus())) {
process();
}
if(fail_code > 2) // something’s serious wrong
RESET(); // reset the whole device
// otherwise try restart code from main()
}
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Library Functions
ROUND
Synopsis
#include <math.h>
double round (double x)
Description
The round function round the argument to the nearest integer value, but in
floating-point format. Values midway between integer values are rounded up.
Example
#include <math.h>
void
main (void)
{
double input, rounded;
input = 1234.5678;
rounded = round(input);
}
See Also
trunc()
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SETJMP
Synopsis
#include <setjmp.h>
int setjmp (jmp_buf buf)
Description
The setjmp() function is used with longjmp() for non-local goto’s. See longjmp()
for further information.
Example
#include <stdio.h>
#include <setjmp.h>
#include <stdlib.h>
jmp_buf jb;
void
inner (void)
{
longjmp(jb, 5);
}
void
main (void)
{
int i;
if(i = setjmp(jb)) {
printf("setjmp returned %d\n", i);
exit(0);
}
printf("setjmp returned 0 - good\n");
printf("calling inner...\n");
inner();
printf("inner returned - bad!\n");
}
See Also
longjmp()
Return Value
The setjmp() function returns zero after the real call, and non-zero if it apparently
returns after a call to longjmp().
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Library Functions
SIN
Synopsis
#include <math.h>
double sin (double f)
Description
This function returns the sine function of its argument.
Example
#include <math.h>
#include <stdio.h>
#define C 3.141592/180.0
void
main (void)
{
double i;
for(i = 0 ; i <= 180.0 ; i += 10)
printf("sin(%3.0f) = %f\n", i, sin(i*C));
printf("cos(%3.0f) = %f\n", i, cos(i*C));
}
See Also
cos(), tan(), asin(), acos(), atan(), atan2()
Return Value
Sine vale of f.
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SLEEP
Synopsis
#include <xc.h>
SLEEP();
Description
This macro is used to put the device into a low-power standby mode.
Example
#include <xc.h>
extern void init(void);
void
main (void)
{
init(); /* enable peripherals/interrupts */
while(1)
SLEEP();
/* save power while nothing happening */
}
SPRINTF
Synopsis
#include <stdio.h>
int sprintf (char * buf, const char * fmt, ...)
Description
The sprintf() function operates in a similar fashion to printf(), except that instead
of placing the converted output on the stdout stream, the characters are placed in the
buffer at buf. The resultant string will be null terminated, and the number of characters
in the buffer will be returned.
See Also
printf()
Return Value
This routine return the number of characters placed into the buffer.
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Library Functions
SQRT
Synopsis
#include <math.h>
double sqrt (double f)
Description
The function sqrt(), implements a square root routine using Newton’s approximation.
Example
#include <math.h>
#include <stdio.h>
void
main (void)
{
double i;
for(i = 0 ; i <= 20.0 ; i += 1.0)
printf("square root of %.1f = %f\n", i, sqrt(i));
}
See Also
exp()
Return Value
Returns the value of the square root.
Note
A domain error occurs if the argument is negative and errno will be set to EDOM.
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SRAND
Synopsis
#include <stdlib.h>
void srand (unsigned int seed)
Description
The srand() function initializes the random number generator accessed by rand()
with the given seed. This provides a mechanism for varying the starting point of the
pseudo-random sequence yielded by rand().
Example
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
void
main (void)
{
time_t toc;
int i;
time(&toc);
srand((int)toc);
for(i = 0 ; i != 10 ; i++)
printf("%d\t", rand());
putchar(’\n’);
}
See Also
rand()
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Library Functions
STRCAT
Synopsis
#include <string.h>
char * strcat (char * s1, const char * s2)
Description
This function appends (concatenates) string s2 to the end of string s1. The result will
be null terminated. The argument s1 must point to a character array big enough to hold
the resultant string.
Example
#include <string.h>
#include <stdio.h>
void
main (void)
{
char buffer[256];
char * s1, * s2;
strcpy(buffer, "Start of line");
s1 = buffer;
s2 = "... end of line";
strcat(s1, s2);
printf("Length = %d\n", strlen(buffer));
printf("string = \"%s\"\n", buffer);
}
See Also
strcpy(), strcmp(), strncat(), strlen()
Return Value
The value of s1 is returned.
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STRCHR, STRICHR
Synopsis
#include <string.h>
char * strchr (const char * s, int c)
char * strichr (const char * s, int c)
Description
The strchr() function searches the string s for an occurrence of the character c. If
one is found, a pointer to that character is returned, otherwise null is returned.
The strichr() function is the case-insensitive version of this function.
Example
#include <strings.h>
#include <stdio.h>
void
main (void)
{
static char temp[] = "Here it is...";
char c = ’s’;
if(strchr(temp, c))
printf("Character %c was found in string\n", c);
else
printf("No character was found in string");
}
See Also
strrchr(), strlen(), strcmp()
Return Value
A pointer to the first match found, or NULL if the character does not exist in the string.
Note
Although the function takes an integer argument for the character, only the lower 8 bits
of the value are used.
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Library Functions
STRCMP, STRICMP
Synopsis
#include <string.h>
int strcmp (const char * s1, const char * s2)
int stricmp (const char * s1, const char * s2)
Description
The strcmp() function compares its two, null terminated, string arguments and returns
a signed integer to indicate whether s1 is less than, equal to or greater than s2. The
comparison is done with the standard collating sequence, which is that of the ASCII
character set.
The stricmp() function is the case-insensitive version of this function.
Example
#include <string.h>
#include <stdio.h>
void
main (void)
{
int i;
if((i = strcmp("ABC", "ABc")) < 0)
printf("ABC is less than ABc\n");
else if(i > 0)
printf("ABC is greater than ABc\n");
else
printf("ABC is equal to ABc\n");
}
See Also
strlen(), strncmp(), strcpy(), strcat()
Return Value
A signed integer less than, equal to or greater than zero.
Note
Other C implementations can use a different collating sequence; the return value is
negative, zero, or positive; i.e., do not test explicitly for negative one (-1) or one (1).
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STRCPY
Synopsis
#include <string.h>
char * strcpy (char * s1, const char * s2)
Description
This function copies a null terminated string s2 to a character array pointed to by s1.
The destination array must be large enough to hold the entire string, including the null
terminator.
Example
#include <string.h>
#include <stdio.h>
void
main (void)
{
char buffer[256];
char * s1, * s2;
strcpy(buffer, "Start of line");
s1 = buffer;
s2 = "... end of line";
strcat(s1, s2);
printf("Length = %d\n", strlen(buffer));
printf("string = \"%s\"\n", buffer);
}
See Also
strncpy(), strlen(), strcat(), strlen()
Return Value
The destination buffer s1 is returned.
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Library Functions
STRCSPN
Synopsis
#include <string.h>
size_t strcspn (const char * s1, const char * s2)
Description
The strcspn() function returns the length of the initial segment of the string pointed to
by s1 which consists of characters NOT from the string pointed to by s2.
Example
#include <stdio.h>
#include <string.h>
void
main (void)
{
static char set[] = "xyz";
printf("%d\n", strcspn("abcdevwxyz", set));
printf("%d\n", strcspn("xxxbcadefs", set));
printf("%d\n", strcspn("1234567890", set));
}
See Also
strspn()
Return Value
Returns the length of the segment.
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STRLEN
Synopsis
#include <string.h>
size_t strlen (const char * s)
Description
The strlen() function returns the number of characters in the string s, not including
the null terminator.
Example
#include <string.h>
#include <stdio.h>
void
main (void)
{
char buffer[256];
char * s1, * s2;
strcpy(buffer, "Start of line");
s1 = buffer;
s2 = "... end of line";
strcat(s1, s2);
printf("Length = %d\n", strlen(buffer));
printf("string = \"%s\"\n", buffer);
}
Return Value
The number of characters preceding the null terminator.
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Library Functions
STRNCAT
Synopsis
#include <string.h>
char * strncat (char * s1, const char * s2, size_t n)
Description
This function appends (concatenates) string s2 to the end of string s1. At most n characters will be copied, and the result will be null terminated. s1 must point to a character
array big enough to hold the resultant string.
Example
#include <string.h>
#include <stdio.h>
void
main (void)
{
char buffer[256];
char * s1, * s2;
strcpy(buffer, "Start of line");
s1 = buffer;
s2 = "... end of line";
strncat(s1, s2, 5);
printf("Length = %d\n", strlen(buffer));
printf("string = \"%s\"\n", buffer);
}
See Also
strcpy(), strcmp(), strcat(), strlen()
Return Value
The value of s1 is returned.
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STRNCMP, STRNICMP
Synopsis
#include <string.h>
int strncmp (const char * s1, const char * s2, size_t n)
int strnicmp (const char * s1, const char * s2, size_t n)
Description
The strncmp() function compares its two, null terminated, string arguments, up to a
maximum of n characters, and returns a signed integer to indicate whether s1 is less
than, equal to or greater than s2. The comparison is done with the standard collating
sequence, which is that of the ASCII character set.
The strnicmp() function is the case-insensitive version of this function.
Example
#include <stdio.h>
#include <string.h>
void
main (void)
{
int i;
i = strncmp("abcxyz", "abcxyz", 6);
if(i == 0)
printf("The strings are equal\n");
else if(i > 0)
printf("String 2 less than string 1\n");
else
printf("String 2 is greater than string 1\n");
}
See Also
strlen(), strcmp(), strcpy(), strcat()
Return Value
A signed integer less than, equal to or greater than zero.
Note
Other C implementations can use a different collating sequence; the return value is
negative, zero, or positive; i.e., do not test explicitly for negative one (-1) or one (1).
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Library Functions
STRNCPY
Synopsis
#include <string.h>
char * strncpy (char * s1, const char * s2, size_t n)
Description
This function copies a null terminated string s2 to a character array pointed to by s1.
At most, n characters are copied, but n characters are always written. If string s2 is
longer than n, then the destination string will not be null terminated. If string s2 is
shorter than n, then the remaining bytes of the destination array are filled with ’\0’. It
is up to the programmer to ensure that the destination array is large enough to hold the
entire string, including the null terminator.
Example
#include <string.h>
#include <stdio.h>
void
main (void)
{
char buffer[256];
char * s1, * s2;
strncpy(buffer, "Start of line", 6);
s1 = buffer;
s2 = "... end of line";
strcat(s1, s2);
printf("Length = %d\n", strlen(buffer));
printf("string = \"%s\"\n", buffer);
}
See Also
strcpy(), strcat(), strlen(), strcmp()
Return Value
The destination buffer s1 is returned.
Note
This function always writes n characters to the destination array. Ensure that this operation is what you require in your program. The copied string will not be null terminated
if the destination is not large enough to hold the source string.
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STRPBRK
Synopsis
#include <string.h>
char * strpbrk (const char * s1, const char * s2)
Description
The strpbrk() function returns a pointer to the first occurrence in string s1 of any
character from string s2, or a null if no character from s2 exists in s1.
Example
#include <stdio.h>
#include <string.h>
void
main (void)
{
char * str = "This is a string.";
while(str != NULL) {
printf("%s\n", str);
str = strpbrk(str+1, "aeiou");
}
}
Return Value
to the first matching character, or NULL if no character found.
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Library Functions
STRRCHR, STRRICHR
Synopsis
#include <string.h>
char * strrchr (char * s, int c)
char * strrichr (char * s, int c)
Description
The strrchr() function is similar to the strchr() function, but searches from the end
of the string rather than the beginning; i.e., it locates the last occurrence of the character c in the null terminated string s. If successful it returns a pointer to that occurrence,
otherwise it returns NULL.
The strrichr() function is the case-insensitive version of this function.
Example
#include <stdio.h>
#include <string.h>
void
main (void)
{
char * str = "This is a string.";
while(str != NULL) {
printf("%s\n", str);
str = strrchr(str+1, ’s’);
}
}
See Also
strchr(), strlen(), strcmp(), strcpy(), strcat()
Return Value
A pointer to the character, or NULL if none is found.
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STRSPN
Synopsis
#include <string.h>
size_t strspn (const char * s1, const char * s2)
Description
The strspn() function returns the length of the initial segment of the string pointed to
by s1 which consists entirely of characters from the string pointed to by s2.
Example
#include <stdio.h>
#include <string.h>
void
main (void)
{
printf("%d\n", strspn("This is a string", "This"));
printf("%d\n", strspn("This is a string", "this"));
}
See Also
strcspn()
Return Value
The length of the segment.
STRSTR, STRISTR
Synopsis
#include <string.h>
char * strstr (const char * s1, const char * s2)
char * stristr (const char * s1, const char * s2)
Description
The strstr() function locates the first occurrence of the sequence of characters in the
string pointed to by s2 in the string pointed to by s1.
The stristr() routine is the case-insensitive version of this function.
Example
#include <stdio.h>
#include <string.h>
void
main (void)
{
printf("%d\n", strstr("This is a string", "str"));
}
Return Value
to the located string or a null if the string was not found.
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Library Functions
STRTOD
Synopsis
#include <stdlib.h>
double strtod (const char * s, const char ** res)
Description
Parse the string s converting it to a double floating-point type. This function converts
the first occurrence of a substring of the input that is made up of characters of the
expected form after skipping leading white-space characters. If res is not NULL, it will
be made to point to the first character after the converted sub-string.
Example
#include <stdio.h>
#include <strlib.h>
void
main (void)
{
char buf[] = "35.7
char * end;
double in1, in2;
23.27";
in1 = strtod(buf, &end);
in2 = strtod(end, NULL);
printf("in comps: %f, %f\n", in1, in2);
}
See Also
atof()
Return Value
Returns a double representing the floating-point value of the converted input string.
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STRTOL
Synopsis
#include <stdlib.h>
double strtol (const char * s, const char ** res, int base)
Description
Parse the string s converting it to a long integer type. This function converts the first
occurrence of a substring of the input that is made up of characters of the expected
form after skipping leading white-space characters. The radix of the input is determined
from base. If this is zero, then the radix defaults to base 10. If res is not NULL, it will be
made to point to the first character after the converted sub-string.
Example
#include <stdio.h>
#include <strlib.h>
void
main (void)
{
char buf[] = "0X299 0x792";
char * end;
long in1, in2;
in1 = strtol(buf, &end, 16);
in2 = strtol(end, NULL, 16);
printf("in (decimal): %ld, %ld\n", in1, in2);
}
See Also
strtod()
Return Value
Returns a long int representing the value of the converted input string using the
specified base.
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Library Functions
STRTOK
Synopsis
#include <string.h>
char * strtok (char * s1, const char * s2)
Description
A number of calls to strtok() breaks the string s1 (which consists of a sequence of
zero or more text tokens separated by one or more characters from the separator string
s2) into its separate tokens.
The first call must have the string s1. This call returns a pointer to the first character of
the first token, or NULL if no tokens were found. The inter-token separator character is
overwritten by a null character, which terminates the current token.
For subsequent calls to strtok(), s1 should be set to a NULL. These calls start searching from the end of the last token found, and again return a pointer to the first character
of the next token, or NULL if no further tokens were found.
Example
#include <stdio.h>
#include <string.h>
void
main (void)
{
char * ptr;
char buf[] = "This is a string of words.";
char * sep_tok = ",?! "
ptr = strtok(buf, sep_tok);
while(ptr != NULL) {
printf("%s\n", ptr);
ptr = strtok(NULL, sep_tok);
}
}
Return Value
Returns a pointer to the first character of a token, or a null if no token was found.
Note
The separator string s2 can be different from call to call.
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TAN
Synopsis
#include <math.h>
double tan (double f)
Description
The tan() function calculates the tangent of f.
Example
#include <math.h>
#include <stdio.h>
#define C 3.141592/180.0
void
main (void)
{
double i;
for(i = 0 ; i <= 180.0 ; i += 10)
printf("tan(%3.0f) = %f\n", i, tan(i*C));
}
See Also
sin(), cos(), asin(), acos(), atan(), atan2()
Return Value
The tangent of f.
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Library Functions
TIME
Synopsis
#include <time.h>
time_t time (time_t * t)
Description
This function is not provided as it is dependent on the target system supplying the current time. This function will be user implemented. When implemented, this function
should return the current time in seconds since 00:00:00 on Jan 1, 1970. If the argument t is not equal to NULL, the same value is stored into the object pointed to by t.
Example
#include <stdio.h>
#include <time.h>
void
main (void)
{
time_t clock;
time(&clock);
printf("%s", ctime(&clock));
}
See Also
ctime(), gmtime(), localtime(), asctime()
Return Value
This routine, when implemented, will return the current time (in seconds) since
00:00:00 on Jan 1, 1970.
Note
The time() routine is not supplied, if required the user will have to implement this
routine to the specifications outlined above.
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TOLOWER, TOUPPER, TOASCII
Synopsis
#include <ctype.h>
char toupper (int c)
char tolower (int c)
char toascii (int c)
Description
The toupper() function converts its lower case alphabetic argument to upper case, the
tolower() routine performs the reverse conversion and the toascii() macro returns
a result that is guaranteed in the range 0-0177. The functions toupper() and
tolower() return their arguments if it is not an alphabetic character.
Example
#include <stdio.h>
#include <ctype.h>
#include <string.h>
void
main (void)
{
char * array1 = "aBcDE";
int i;
for(i=0;i < strlen(array1); ++i) {
printf("%c", tolower(array1[i]));
}
printf("n");
}
See Also
islower(), isupper(), isascii(), et. al.
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Library Functions
TRUNC
Synopsis
#include <math.h>
double trunc (double x)
Description
The trunc function rounds the argument to the nearest integer value, in floating-point
format, that is not larger in magnitude than the argument.
Example
#include <math.h>
void
main (void)
{
double input, rounded;
input = 1234.5678;
rounded = trunc(input);
}
See Also
round()
UDIV
Synopsis
#include <stdlib.h>
int udiv (unsigned num, unsigned denom)
Description
The udiv() function calculate the quotient and remainder of the division of number
and denom, storing the results into a udiv_t structure which is returned.
Example
#include <stdlib.h>
void
main (void)
{
udiv_t result;
unsigned num = 1234, den = 7;
result = udiv(num, den);
}
See Also
uldiv(), div(), ldiv()
Return Value
Returns the quotient and remainder as a udiv_t structure.
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ULDIV
Synopsis
#include <stdlib.h>
int uldiv (unsigned long num, unsigned long denom)
Description
The uldiv() function calculate the quotient and remainder of the division of number
and denom, storing the results into a uldiv_t structure which is returned.
Example
#include <stdlib.h>
void
main (void)
{
uldiv_t result;
unsigned long num = 1234, den = 7;
result = uldiv(num, den);
}
See Also
ldiv(), udiv(), div()
Return Value
Returns the quotient and remainder as a uldiv_t structure.
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Library Functions
UTOA
Synopsis
#include <stdlib.h>
char * utoa (char * buf, unsigned val, int base)
Description
The function utoa() converts the unsigned contents of val into a string which is stored
into buf. The conversion is performed according to the radix specified in base. buf is
assumed to reference a buffer which has sufficient space allocated to it.
Example
#include <stdlib.h>
#include <stdio.h>
void
main (void)
{
char buf[10];
utoa(buf, 1234, 16);
printf("The buffer holds %s\n", buf);
}
See Also
strtol(), itoa(), ltoa(), ultoa()
Return Value
This routine returns a copy of the buffer into which the result is written.
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VA_START, VA_ARG, VA_END
Synopsis
#include <stdarg.h>
void va_start (va_list ap, parmN)
type va_arg (ap, type)
void va_end (va_list ap)
Description
These macros are provided to give access in a portable way to parameters to a function
represented in a prototype by the ellipsis symbol (...), where type and number of
arguments supplied to the function are not known at compile time.
The right most parameter to the function (shown as parmN) plays an important role in
these macros, as it is the starting point for access to further parameters. In a function
taking variable numbers of arguments, a variable of type va_list should be declared,
then the macro va_start() invoked with that variable and the name of parmN. This will
initialize the variable to allow subsequent calls of the macro va_arg() to access successive parameters.
Each call to va_arg() requires two arguments; the variable previously defined and a
type name which is the type that the next parameter is expected to be. Note that any
arguments thus accessed will have been widened by the default conventions to int,
unsigned int or double. For example, if a character argument has been passed, it
should be accessed by va_arg(ap, int) since the char will have been widened to
int.
An example is given below of a function taking one integer parameter, followed by a
number of other parameters. In this example the function expects the subsequent
parameters to be s to char, but note that the compiler is not aware of this, and it is the
programmers responsibility to ensure that correct arguments are supplied.
Example
#include <stdio.h>
#include <stdarg.h>
void
pf (int a, ...)
{
va_list ap;
va_start(ap, a);
while(a--)
puts(va_arg(ap, char *));
va_end(ap);
}
void
main (void)
{
pf(3, "Line 1", "Line 2", "Line 3");
}
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Library Functions
WRITETIMERx
Synopsis
#include <xc.h>
void WRITETIMERx (int n);
Description
The WRITETIMERx() macro writes the 16-bit argument, n, to both bytes of the TMRx
register, where x is one of the digits 0, 1 or 3.
Example
#include <xc.h>
void
main (void)
{
WRITETIMER1(0x4A);
while(1)
continue;
}
See Also
READTIMERx()
Note
This macro can only be used with PIC18 devices.
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XTOI
Synopsis
#include <stdlib.h>
unsigned xtoi (const char * s)
Description
The xtoi() function scans the character string passed to it, skipping leading blanks
reading an optional sign, and converts an ASCII representation of a hexadecimal
number to an integer.
Example
#include <stdlib.h>
#include <stdio.h>
void
main (void)
{
char buf[80];
int i;
gets(buf);
i = xtoi(buf);
printf("Read %s: converted to %x\n", buf, i);
}
See Also
atoi()
Return Value
An unsigned integer. If no number is found in the string, zero will be returned.
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MPLAB® XC8 C COMPILER
USER’S GUIDE
Appendix B. Embedded Compiler Compatibility Mode
B.1
INTRODUCTION
All three MPLAB XC C compilers can be placed into a compatibility mode. In this mode,
they are syntactically compatible with the non-standard C language extensions used by
other non-Microchip embedded compiler vendors. This compatibility allows C source
code written for other compilers to be compiled with minimum modification when using
the MPLAB XC compilers.
Since very different device architectures can be targeted by other compilers, the
semantics of the non-standard extensions can be different to that in the MPLAB XC
compilers. This document indicates when the original C code can need to be reviewed.
The compatibility features offered by the MPLAB C compilers are discussed in the
following topics:
• Compiling in Compatibility Mode
• Syntax Compatibility
• Data Type
• Operator
• Extended Keywords
• All assembly code specified by this construct is device-specific and will need
review when porting to any Microchip device.
• Pragmas
B.2
COMPILING IN COMPATIBILITY MODE
An option is used to enable vendor-specific syntax compatibility. When using MPLAB
XC8, this option is --ext=vendor; when using MPLAB XC16 or MPLAB XC32, the
option is -mext=vendor. The argument vendor is a key that is used to represent the
syntax. See Table B-1 for a list of all keys usable with the MPLAB XC compilers.
TABLE B-1:
Vendor
key
cci
iar
VENDOR KEYS
Syntax
Common C Interface
IAR C/C++ Compiler
TM
for ARM
XC8
Support
XC16
Support
XC32
Support
Yes
Yes
Yes
Yes
Yes
Yes
The Common C Interface (CCI) is a language standard that is common to all Microchip
MPLAB XC compilers. The non-standard extensions associated with this syntax are
already described in Chapter 2. Common C Interface and are not repeated here.
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B.3
SYNTAX COMPATIBILITY
The goal of this syntax compatibility feature is to ease the migration process when
porting source code from other C compilers to the native MPLAB XC compiler syntax.
Many non-standard extensions are not required when compiling for Microchip devices
and, for these, there are no equivalent extensions offered by MPLAB XC compilers.
These extensions are then simply ignored by the MPLAB XC compilers, although a
warning message is usually produced to ensure you are aware of the different compiler
behavior. You should confirm that your project will still operate correctly with these features disabled.
Other non-standard extensions are not compatible with Microchip devices. Errors will
be generated by the MPLAB XC compiler if these extensions are not removed from the
source code. You should review the ramifications of removing the extension and decide
whether changes are required to other source code in your project.
Table B-2 indicates the various levels of compatibility used in the tables that are presented throughout this guide.
TABLE B-2:
Level
support
LEVEL OF SUPPORT INDICATORS
Explanation
The syntax is accepted in the specified compatibility mode, and its
meaning will mimic its meaning when it is used with the original compiler.
support (no args) In the case of pragmas, the base pragma is supported in the specified
compatibility mode, but the arguments are ignored.
native support
The syntax is equivalent to that which is already accepted by the MPLAB
XC compiler, and the semantics are compatible. You can use this feature
without a vendor compatibility mode having been enabled.
ignore
The syntax is accepted in the specified compatibility mode, but the implied
action is not required or performed. The extension is ignored and a warning
will be issued by the compiler.
error
The syntax is not accepted in the specified compatibility mode. An error will
be issued and compilation will be terminated.
Note that even if a C feature is supported by an MPLAB XC compiler, addresses, register names, assembly instructions, or any other device-specific argument is unlikely to
be valid when compiling for a Microchip device. Always review code which uses these
items in conjunction with the data sheet of your target Microchip device.
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Embedded Compiler Compatibility Mode
B.4
DATA TYPE
Some compilers allow use of the boolean type, bool, as well as associated values
true and false, as specified by the C99 ANSI Standard. This type and these values
can be used by all MPLAB XC compilers when in compatibility mode1, as shown in
Table B-3.
As indicated by the ANSI Standard, the <stdbool.h> header must be included for this
feature to work as expected when it is used with MPLAB XC compilers.
TABLE B-3:
SUPPORT FOR C99 BOOL TYPE
IAR Compatibility Mode
Type
bool
MPLAB XC8
support
MPLAB XC16
support
MPLAB XC32
support
Do not confuse the boolean type, bool, and the integer type, bit, implemented by
MPLAB XC8.
B.5
OPERATOR
The @ operator can be used with other compilers to indicate the desired memory location of an object. As Table B-4 indicates, support for this syntax in MPLAB C is limited
to MPLAB XC8 only.
Any address specified with another device is unlikely to be correct on a new architecture. Review the address in conjunction with the data sheet for your target Microchip
device.
Using @ in a compatibility mode with MPLAB XC8 will work correctly, but will generate
a warning. To prevent this warning from appearing again, use the reviewed address
with the MPLAB C __at() specifier instead.
For MPLAB XC16 and XC32, consider using the address attribute.
TABLE B-4:
SUPPORT FOR NON-STANDARD OPERATOR
IAR Compatibility Mode
Operator
MPLAB XC8
MPLAB XC16
MPLAB XC32
@
native support
error
error
1. Not all C99 features have been adopted by all Microchip MPLAB XC compilers.
 2012-2016 Microchip Technology Inc.
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MPLAB® XC8 C Compiler User’s Guide
B.6
EXTENDED KEYWORDS
Non-standard extensions often specify how objects are defined or accessed. Keywords
are usually used to indicate the feature. The non-standard C keywords corresponding
to other compilers are listed in Table B-5, as well as the level of compatibility offered by
MPLAB XC compilers. The table notes offer more information about some extensions.
TABLE B-5:
SUPPORT FOR NON-STANDARD KEYWORDS
IAR Compatibility Mode
Keyword
MPLAB XC16
MPLAB XC32
__section_begin
ignore
support
support
__section_end
ignore
support
support
__section_size
ignore
support
support
__segment_begin
ignore
support
support
__segment_end
ignore
support
support
__segment_size
ignore
support
support
__sfb
ignore
support
support
__sfe
ignore
support
support
__sfs
ignore
support
support
__asm or asm(1)
support(2)
native support
native support
__arm
ignore
ignore
ignore
__big_endian
error
error
error
__fiq
support
error
error
__intrinsic
ignore
ignore
ignore
__interwork
ignore
ignore
ignore
__irq
support
error
error
__little_endian(3)
ignore
ignore
ignore
__nested
ignore
ignore
ignore
__no_init
support
support
support
__noreturn
ignore
support
support
__ramfunc
ignore
ignore
support(4)
__packed
ignore(5)
support
support
__root
ignore
support
support
__swi
ignore
ignore
ignore
__task
ignore
support
support
__weak
ignore
support
support
__thumb
ignore
ignore
ignore
__farfunc
ignore
ignore
ignore
__huge
ignore
ignore
ignore
__nearfunc
ignore
ignore
ignore
__inline
support
native support
native support
Note 1:
2:
3:
4:
5:
DS50002053G-page 434
MPLAB XC8
All assembly code specified by this construct is device-specific and will need review
when porting to any Microchip device.
The keyword, asm, is supported natively by MPLAB XC8, but this compiler only supports the __asm keyword in IAR compatibility mode.
This is the default (and only) endianism used by all MPLAB XC compilers.
When used with MPLAB XC32, this must be used with the __longcall__ macro
for full compatibility.
Although this keyword is ignored, by default, all structures are packed when using
MPLAB XC8, so there is no loss of functionality.
 2012-2016 Microchip Technology Inc.
Embedded Compiler Compatibility Mode
B.7
INTRINSIC FUNCTIONS
Intrinsic functions can be used to perform common tasks in the source code. The
MPLAB XC compilers’ support for the intrinsic functions offered by other compilers is
shown in Table B-6.
TABLE B-6:
SUPPORT FOR NON-STANDARD INTRINSIC FUNCTIONS
IAR Compatibility Mode
Function
__disable_fiq
1
MPLAB XC8
MPLAB XC16
MPLAB XC32
support
ignore
ignore
__disable_interrupt
support
support
support
__disable_irq1
support
ignore
ignore
__enable_fiq1
support
ignore
ignore
__enable_interrupt
support
support
support
__enable_irq1
support
ignore
ignore
__get_interrupt_state
ignore
support
support
__set_interrupt_state
ignore
support
support
Note 1:
These intrinsic functions map to macros which disable or enable the global interrupt
enable bit on 8-bit PIC® devices.
The header file <xc.h> must be included for supported functions to operate correctly.
 2012-2016 Microchip Technology Inc.
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MPLAB® XC8 C Compiler User’s Guide
B.8
PRAGMAS
Pragmas can be used by a compiler to control code generation. Any compiler will
ignore an unknown pragma, but many pragmas implemented by another compiler have
also been implemented by the MPLAB XC compilers in compatibility mode. Table B-7
shows the pragmas and the level of support when using each of the MPLAB XC
compilers.
Many of these pragmas take arguments. Even if a pragma is supported by an MPLAB
XC compiler, this support cannot apply to all of the pragma’s arguments. This is
indicated in the table.
TABLE B-7:
SUPPORT FOR NON-STANDARD PRAGMAS
IAR Compatibility Mode
Pragma
DS50002053G-page 436
MPLAB XC8
MPLAB XC16
MPLAB XC32
bitfields
ignore
ignore
ignore
data_alignment
ignore
support
support
diag_default
ignore
ignore
ignore
diag_error
ignore
ignore
ignore
diag_remark
ignore
ignore
ignore
diag_suppress
ignore
ignore
ignore
diag_warning
ignore
ignore
ignore
include_alias
ignore
ignore
ignore
inline
support (no args)
support (no args)
support (no args)
language
ignore
ignore
ignore
location
ignore
support
support
message
support
native support
native support
object_attribute
ignore
ignore
ignore
optimize
ignore
native support
native support
pack
ignore
native support
native support
__printf_args
support
support
support
required
ignore
support
support
rtmodel
ignore
ignore
ignore
__scanf__args
ignore
support
support
section
ignore
support
support
segment
ignore
support
support
swi_number
ignore
ignore
ignore
type_attribute
ignore
ignore
ignore
weak
ignore
native support
native support
 2012-2016 Microchip Technology Inc.
MPLAB® XC8 C COMPILER
USER’S GUIDE
Appendix C. Error and Warning Messages
C.1
INTRODUCTION
This chapter lists the MPLAB XC8 C Compiler error, warning, and advisory messages
with an explanation of each message. This is the complete and historical message set
covering all former HI-TECH C compilers and all compiler versions. Not all messages
shown here will be relevant for the compiler version you are using.
Most messages have been assigned a unique number that appears in brackets before
each message description. It is also printed by the compiler when the message is
issued. The messages shown here are sorted by their number. Unnumbered messages
appear toward the end and are sorted alphabetically.
The name of the application(s) that could have produced the messages are listed in
brackets opposite the error message. In some cases examples of code or options that
could trigger the error are given. The use of * in the error message is used to represent
a string that the compiler will substitute that is specific to that particular error.
Note that one problem in your C or assembler source code can trigger more than one
error message. You should attempt to resolve errors or warnings in the order in which
they are produced.
MESSAGES 1-249
(1) too many errors (*)
(all applications)
The executing compiler application has encountered too many errors and will exit
immediately. Other uncompiled source files will be processed, but the compiler applications that would normally be executed in due course will not be run. The number of
errors that can be accepted is controlled using the --ERRORS option, See
Section 4.8.29 “--ERRORS: Maximum Number of Errors”.
(2) error/warning (*) generated but no description available
(all applications)
The executing compiler application has emitted a message (advisory/warning/error),
but there is no description available in the message description file (MDF) to print. This
could be because the MDF is out-of-date, or the message issue has not been
translated into the selected language.
(3) malformed error information on line * in file *
(all applications)
The compiler has attempted to load the messages for the selected language, but the
message description file (MDF) was corrupted and could not be read correctly.
(100) unterminated #if[n][def] block from line *
(Preprocessor)
A #if or similar block was not terminated with a matching #endif, for example:
#if INPUT
void main(void)
{
run();
}
 2012-2016 Microchip Technology Inc.
/* error flagged here */
/* no #endif was found in this module */
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(101) #* cannot follow #else
(Preprocessor)
A #else or #elif has been used in the same conditional block as a #else. These
can only follow a #if, for example:
#ifdef FOO
result = foo;
#else
result = bar;
#elif defined(NEXT)
result = next(0);
#endif
/* the #else above terminated the #if */
(102) #* must be in an #if
(Preprocessor)
The #elif, #else or #endif directive must be preceded by a matching #if line. If
there is an apparently corresponding #if line, check for things like extra #endifs, or
improperly terminated comments, for example:
#ifdef FOO
result = foo;
#endif
result = bar;
#elif defined(NEXT)
result = next(0);
#endif
/* the #endif above terminated the #if */
(103) #error: *
(Preprocessor)
This is a programmer generated error; there is a directive causing a deliberate error.
This is normally used to check compile time defines, etc. Remove the directive to
remove the error, but first determine why the directive is there.
(104) preprocessor #assert failure
(Preprocessor)
The argument to a preprocessor #assert directive has evaluated to zero. This is a
programmer induced error.
#assert SIZE == 4
/* size should never be 4 */
(105) no #asm before #endasm
(Preprocessor)
A #endasm operator has been encountered, but there was no previous matching
#asm, for example:
void cleardog(void)
{
clrwdt
#endasm /* in-line assembler ends here,
only where did it begin? */
}
(106) nested #asm directives
(Preprocessor)
It is not legal to nest #asm directives. Check for a missing or misspelled #endasm
directive, for example:
#asm
MOVE
r0, #0aah
#asm
; previous #asm must be closed before opening another
SLEEP
#endasm
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Error and Warning Messages
(107) illegal # directive "*"
(Preprocessor, Parser)
The compiler does not understand the # directive. It is probably a misspelling of a
directive token, for example:
#indef DEBUG
/* oops -- that should be #undef DEBUG */
(108) #if[n][def] without an argument
(Preprocessor)
The preprocessor directives #if, #ifdef, and #ifndef must have an argument. The
argument to #if should be an expression, while the argument to #ifdef or #ifndef
should be a single name, for example:
#if
/* oops -- no argument to check */
output = 10;
#else
output = 20;
#endif
(109) #include syntax error
(Preprocessor)
The syntax of the filename argument to #include is invalid. The argument to
#include must be a valid file name, either enclosed in double quotes "" or angle
brackets < >. Spaces should not be included, and the closing quote or bracket must
be present. There should be nothing else on the line other than comments, for example:
#include stdio.h
/* oops -- should be: #include <stdio.h> */
(110) too many file arguments; usage: cpp [input [output]]
(Preprocessor)
CPP should be invoked with at most two file arguments. Contact Microchip Technical
Support if the preprocessor is being executed by a compiler driver.
(111) redefining preprocessor macro "*"
(Preprocessor)
The macro specified is being redefined to something different than the original definition. If you want to deliberately redefine a macro, use #undef first to remove the original definition, for example:
#define ONE 1
/* elsewhere: */
/* Is this correct? It will overwrite the first definition. */
#define ONE one
(112) #define syntax error
(Preprocessor)
A macro definition has a syntax error. This could be due to a macro or formal parameter
name that does not start with a letter or a missing closing parenthesis, ), for example:
#define FOO(a, 2b)
bar(a, 2b)
/* 2b is not to be! */
(113) unterminated string in preprocessor macro body
(Preprocessor, Assembler)
A macro definition contains a string that lacks a closing quote.
(114) illegal #undef argument
(Preprocessor)
The argument to #undef must be a valid name. It must start with a letter, for example:
#undef 6YYY
 2012-2016 Microchip Technology Inc.
/* this isn’t a valid symbol name */
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MPLAB® XC8 C Compiler User’s Guide
(115) recursive preprocessor macro definition of "*" defined by "*"
(Preprocessor)
The named macro has been defined in such a manner that expanding it causes a
recursive expansion of itself.
(116) end of file within preprocessor macro argument from line *
(Preprocessor)
A macro argument has not been terminated. This probably means the closing parenthesis has been omitted from a macro invocation. The line number given is the line
where the macro argument started, for example:
#define FUNC(a, b) func(a+b)
FUNC(5, 6;
/* oops -- where is the closing bracket? */
(117) misplaced constant in #if
(Preprocessor)
A constant in a #if expression should only occur in syntactically correct places. This
error is probably caused by omission of an operator, for example:
#if FOO BAR
/* oops -- did you mean: #if FOO == BAR ? */
(118) stack overflow processing #if expression
(Preprocessor)
The preprocessor filled up its expression evaluation stack in a #if expression. Simplify
the expression – it probably contains too many parenthesized subexpressions.
(119) invalid expression in #if line
(Preprocessor)
This is an internal compiler error. Contact Microchip Technical Support with details.
(120) operator "*" in incorrect context
(Preprocessor)
An operator has been encountered in a #if expression that is incorrectly placed (two
binary operators are not separated by a value), for example:
#if FOO * % BAR == 4
#define BIG
#endif
/* what is "* %" ? */
(121) expression stack overflow at operator "*"
(Preprocessor)
Expressions in #if lines are evaluated using a stack with a size of 128. It is possible
for very complex expressions to overflow this. Simplify the expression.
(122) unbalanced parenthesis at operator "*"
(Preprocessor)
The evaluation of a #if expression found mismatched parentheses. Check the expression for correct parenthesizing, for example:
#if ((A) + (B) /* oops -- a missing ), I think */
#define ADDED
#endif
(123) misplaced "?" or ":"; previous operator is "*"
(Preprocessor)
A colon operator has been encountered in a #if expression that does not match up
with a corresponding ? operator, for example:
#if XXX : YYY
DS50002053G-page 440
/* did you mean:
#if COND ? XXX : YYY */
 2012-2016 Microchip Technology Inc.
Error and Warning Messages
(124) illegal character "*" in #if
(Preprocessor)
There is a character in a #if expression that should not be there. Valid characters are
the letters, digits, and those comprising the acceptable operators, for example:
#if YYY /* what are these characters doing here? */
int m;
#endif
(125) illegal character (* decimal) in #if
(Preprocessor)
There is a non-printable character in a #if expression that should not be there. Valid
characters are the letters, digits, and those comprising the acceptable operators, for
example:
#if ^S YYY
int m;
#endif
/* what is this control characters doing here? */
(126) strings can’t be used in #if
(Preprocessor)
The preprocessor does not allow the use of strings in #if expressions, for example:
/* no string operations allowed by the preprocessor */
#if MESSAGE > "hello"
#define DEBUG
#endif
(127) bad syntax for defined() in #[el]if
(Preprocessor)
The defined() pseudo-function in a preprocessor expression requires its argument
to be a single name. The name must start with a letter and should be enclosed in
parentheses, for example:
/* oops -- defined expects a name, not an expression */
#if defined(a&b)
input = read();
#endif
(128) illegal operator in #if
(Preprocessor)
A #if expression has an illegal operator. Check for correct syntax, for example:
#if FOO = 6
/* oops -- should that be: #if FOO == 5 ? */
(129) unexpected "\" in #if
(Preprocessor)
The backslash is incorrect in the #if statement, for example:
#if FOO == \34
#define BIG
#endif
(130) unknown type "*" in #[el]if sizeof()
(Preprocessor)
An unknown type was used in a preprocessor sizeof(). The preprocessor can only
evaluate sizeof() with basic types, or pointers to basic types, for example:
#if sizeof(unt) == 2
i = 0xFFFF;
#endif
 2012-2016 Microchip Technology Inc.
/* should be: #if sizeof(int) == 2 */
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MPLAB® XC8 C Compiler User’s Guide
(131) illegal type combination in #[el]if sizeof()
(Preprocessor)
The preprocessor found an illegal type combination in the argument to sizeof() in a
#if expression, for example:
/* To sign, or not to sign, that is the error. */
#if sizeof(signed unsigned int) == 2
i = 0xFFFF;
#endif
(132) no type specified in #[el]if sizeof()
(Preprocessor)
Sizeof() was used in a preprocessor #if expression, but no type was specified. The
argument to sizeof() in a preprocessor expression must be a valid simple type, or
pointer to a simple type, for example:
#if sizeof()
i = 0;
#endif
/* oops -- size of what? */
(133) unknown type code (0x*) in #[el]if sizeof()
(Preprocessor)
The preprocessor has made an internal error in evaluating a sizeof() expression.
Check for a malformed type specifier. This is an internal error. Contact Microchip
Technical Support with details.
(134) syntax error in #[el]if sizeof()
(Preprocessor)
The preprocessor found a syntax error in the argument to sizeof in a #if expression.
Probable causes are mismatched parentheses and similar things, for example:
#if sizeof(int == 2)
i = 0xFFFF;
#endif
// oops - should be: #if sizeof(int) == 2
(135) unknown operator (*) in #if
(Preprocessor)
The preprocessor has tried to evaluate an expression with an operator it does not
understand. This is an internal error. Contact Microchip Technical Support with details.
(137) strange character "*" after ##
(Preprocessor)
A character has been seen after the token catenation operator ## that is neither a letter
nor a digit. Because the result of this operator must be a legal token, the operands must
be tokens containing only letters and digits, for example:
/* the ’ character will not lead to a valid token */
#define cc(a, b) a ## ’b
(138) strange character (*) after ##
(Preprocessor)
An unprintable character has been seen after the token catenation operator ## that is
neither a letter nor a digit. Because the result of this operator must be a legal token, the
operands must be tokens containing only letters and digits, for example:
/* the ’ character will not lead to a valid token */
#define cc(a, b) a ## ’b
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Error and Warning Messages
(139) end of file in comment
(Preprocessor)
End of file was encountered inside a comment. Check for a missing closing comment
flag, for example:
/* Here the comment begins. I’m not sure where I end, though
}
(140) can’t open * file "*": *
(Driver, Preprocessor, Code Generator, Assembler)
The command file specified could not be opened for reading. Confirm the spelling and
path of the file specified on the command line, for example:
xc8 @communds
should that be:
xc8 @commands
(141) can’t open * file "*": *
(Any)
An output file could not be created. Confirm the spelling and path of the file specified
on the command line.
(144) too many nested #if blocks
(Preprocessor)
#if, #ifdef, etc., blocks can only be nested to a maximum of 32.
(146) #include filename too long
(Preprocessor)
A filename constructed while looking for an include file has exceeded the length of an
internal buffer. Because this buffer is 4096 bytes long, this is unlikely to happen.
(147) too many #include directories specified
(Preprocessor)
A maximum of 7 directories can be specified for the preprocessor to search for include
files. The number of directories specified with the driver is too many.
(148) too many arguments for preprocessor macro
(Preprocessor)
A macro can only have up to 31 parameters, per the C Standard.
(149) preprocessor macro work area overflow
(Preprocessor)
The total length of a macro expansion has exceeded the size of an internal table. This
table is normally 32768 bytes long. Thus any macro expansion must not expand to a
total of more than 32K bytes.
(150) illegal "__" preprocessor macro "*"
(Preprocessor)
This is an internal compiler error. Contact Microchip Technical Support with details.
(151) too many arguments in preprocessor macro expansion
(Preprocessor)
There were too many arguments supplied in a macro invocation. The maximum
number allowed is 31.
(152) bad dp/nargs in openpar(): c = *
(Preprocessor)
This is an internal compiler error. Contact Microchip Technical Support with details.
 2012-2016 Microchip Technology Inc.
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MPLAB® XC8 C Compiler User’s Guide
(153) out of space in preprocessor macro * argument expansion
(Preprocessor)
A macro argument has exceeded the length of an internal buffer. This buffer is normally
4096 bytes long.
(155) work buffer overflow concatenating "*"
(Preprocessor)
This is an internal compiler error. Contact Microchip Technical Support with details.
(156) work buffer "*" overflow
(Preprocessor)
This is an internal compiler error. Contact Microchip Technical Support with details.
(157) can’t allocate * bytes of memory
(Code Generator, Assembler)
This is an internal compiler error. Contact Microchip Technical Support with details.
(158) invalid disable in preprocessor macro "*"
(Preprocessor)
This is an internal compiler error. Contact Microchip Technical Support with details.
(159) too many calls to unget()
(Preprocessor)
This is an internal compiler error. Contact Microchip Technical Support with details.
(161) control line "*" within preprocessor macro expansion
(Preprocessor)
A preprocessor control line (one starting with a #) has been encountered while
expanding a macro. This should not happen.
(162) #warning: *
(Preprocessor, Driver)
This warning is either the result of user-defined #warning preprocessor directive, or
the driver encountered a problem reading the map file. If the latter, contact Microchip
Technical Support with details
(163) unexpected text in control line ignored
(Preprocessor)
This warning occurs when extra characters appear on the end of a control line. The
extra text will be ignored, but a warning is issued. It is preferable (and in accordance
with Standard C) to enclose the text as a comment, for example:
#if defined(END)
#define NEXT
#endif END
/* END would be better in a comment here */
(164) #include filename "*" was converted to lower case
(Preprocessor)
The #include file name had to be converted to lowercase before it could be opened,
for example:
#include <STDIO.H>
/* oops -- should be: #include <stdio.h> */
(165) #include filename "*" does not match actual name (check upper/lower case)
(Preprocessor)
In Windows versions this means the file to be included actually exists and is spelled the
same way as the #include filename; however, the case of each does not exactly
match. For example, specifying #include "code.c" will include Code.c, if it is
found. In Linux versions this warning could occur if the file wasn’t found.
DS50002053G-page 444
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Error and Warning Messages
(166) too few values specified with option "*"
(Preprocessor)
The list of values to the preprocessor (CPP) -S option is incomplete. This should not
happen if the preprocessor is being invoked by the compiler driver. The values passed
to this option represent the sizes of char, short, int, long, float and double
types.
(167) too many values specified with -S option; "*" unused
Preprocessor)
There were too many values supplied to the -S preprocessor option. See message 166.
(168) unknown option "*"
(Any)
The option given to the component which caused the error is not recognized.
(169) strange character (*) after ##
(Preprocessor)
There is an unexpected character after #.
(170) symbol "*" in undef was never defined
(Preprocessor)
The symbol supplied as argument to #undef was not already defined. This warning
can be disabled with some compilers. This warning can be avoided with code like:
#ifdef SYM
#undef SYM
#endif
/* only undefine if defined */
(171) wrong number of preprocessor macro arguments for "*" (* instead of *)
(Preprocessor)
A macro has been invoked with the wrong number of arguments, for example:
#define ADD(a, b) (a+b)
ADD(1, 2, 3)
/* oops -- only two arguments required */
(172) formal parameter expected after #
(Preprocessor)
The stringization operator # (not to be confused with the leading # used for
preprocessor control lines) must be followed by a formal macro parameter, for
example:
#define str(x) #y
/* oops -- did you mean x instead of y? */
If you need to stringize a token, you will need to define a special macro to do it, for
example:
#define __mkstr__(x) #x
then use __mkstr__(token) wherever you need to convert a token into a string.
(173) undefined symbol "*" in #if; 0 used
(Preprocessor)
A symbol on a #if expression was not a defined preprocessor macro. For the
purposes of this expression, its value has been taken as zero. This warning can be
disabled with some compilers. Example:
#if FOO+BAR
/* e.g. FOO was never #defined */
#define GOOD
#endif
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MPLAB® XC8 C Compiler User’s Guide
(174) multi-byte constant "*" isn’t portable
(Preprocessor)
Multi-byte constants are not portable; and, in fact, will be rejected by later passes of the
compiler, for example:
#if CHAR == ’ab’
#define MULTI
#endif
(175) division by zero in #if; zero result assumed
(Preprocessor)
Inside a #if expression, there is a division by zero which has been treated as yielding
zero, for example:
#if foo/0
int a;
#endif
/* divide by 0: was this what you were intending? */
(176) missing newline
(Preprocessor)
A new line is missing at the end of the line. Each line, including the last line, must have
a new line at the end. This problem is normally introduced by editors.
(177) symbol "*" in -U option was never defined
(Preprocessor)
A macro name specified in a -U option to the preprocessor was not initially defined, and
thus cannot be undefined.
(179) nested comments
(Preprocessor)
This warning is issued when nested comments are found. A nested comment can indicate that a previous closing comment marker is missing or malformed, for example:
output = 0; /* a comment that was left unterminated
flag = TRUE; /* next comment:
hey, where did this line go? */
(180) unterminated comment in included file
(Preprocessor)
Comments begun inside an included file must end inside the included file.
(181) non-scalar types can’t be converted to other types
(Parser)
You cannot convert a structure, union, or array to another type, for example:
struct TEST test;
struct TEST * sp;
sp = test;
/* oops -- did you mean: sp = &test; ? */
(182) illegal conversion between types
(Parser)
This expression implies a conversion between incompatible types, i.e., a conversion of
a structure type into an integer, for example:
struct LAYOUT layout;
int i;
layout = i;
/* int cannot be converted to struct */
Note that even if a structure only contains an int , for example, it cannot be assigned
to an int variable, and vice versa.
DS50002053G-page 446
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Error and Warning Messages
(183) function or function pointer required
(Parser)
Only a function or function pointer can be the subject of a function call, for example:
int a, b, c, d;
a = b(c+d);
/* b is not a function -did you mean a = b*(c+d) ? */
(184) calling an interrupt function is illegal
(Parser)
A function-qualified interrupt cannot be called from other functions. It can only be
called by a hardware (or software) interrupt. This is because an interrupt function
has special function entry and exit code that is appropriate only for calling from an interrupt. An interrupt function can call other non-interrupt functions.
(185) function does not take arguments
(Parser, Code Generator)
This function has no parameters, but it is called here with one or more arguments, for
example:
int get_value(void);
void main(void)
{
int input;
input = get_value(6);
/* oops -parameter should not be here */
}
(186) too many function arguments
(Parser)
This function does not accept as many arguments as there are here.
void add(int a, int b);
add(5, 7, input);
/* call has too many arguments */
(187) too few function arguments
(Parser)
This function requires more arguments than are provided in this call, for example:
void add(int a, int b);
add(5);
/* this call needs more arguments */
(188) constant expression required
(Parser)
In this context an expression is required that can be evaluated to a constant at compile
time, for example:
int a;
switch(input) {
case a: /* oops!
cannot use variable as part of a case label */
input++;
}
(189) illegal type for array dimension
(Parser)
An array dimension must be either an integral type or an enumerated value.
int array[12.5];
 2012-2016 Microchip Technology Inc.
/* oops -- twelve and a half elements, eh? */
DS50002053G-page 447
MPLAB® XC8 C Compiler User’s Guide
(190) illegal type for index expression
(Parser)
An index expression must be either integral or an enumerated value, for example:
int i, array[10];
i = array[3.5];
/* oops -exactly which element do you mean? */
(191) cast type must be scalar or void
(Parser)
A typecast (an abstract type declarator enclosed in parentheses) must denote a type
which is either scalar (i.e., not an array or a structure) or the type void, for example:
lip = (long [])input;
/* oops -- possibly: lip = (long *)input */
(192) undefined identifier "*"
(Parser)
This symbol has been used in the program, but has not been defined or declared.
Check for spelling errors if you think it has been defined.
(193) not a variable identifier "*"
(Parser)
This identifier is not a variable; it can be some other kind of object, i.e., a label.
(194) ")" expected
(Parser)
A closing parenthesis, ), was expected here. This can indicate you have left out this
character in an expression, or you have some other syntax error. The error is flagged
on the line at which the code first starts to make no sense. This can be a statement
following the incomplete expression, for example:
if(a == b
b = 0;
/* the closing parenthesis is missing here */
/* the error is flagged here */
(195) expression syntax
(Parser)
This expression is badly formed and cannot be parsed by the compiler, for example:
a /=% b;
/* oops -- possibly that should be: a /= b; */
(196) struct/union required
(Parser)
A structure or union identifier is required before a dot "." , for example:
int a;
a.b = 9;
/* oops -- a is not a structure */
(197) struct/union member expected
(Parser)
A structure or union member name must follow a dot “.” or an arrow (“->”).
(198) undefined struct/union "*"
(Parser)
The specified structure or union tag is undefined, for example:
struct WHAT what;
/* a definition for WHAT was never seen */
(199) logical type required
(Parser)
The expression used as an operand to if, while statements or to boolean operators
like ! and && must be a scalar integral type, for example:
struct FORMAT format;
if(format)
/* this operand must be a scaler type */
format.a = 0;
DS50002053G-page 448
 2012-2016 Microchip Technology Inc.
Error and Warning Messages
(200) taking the address of a register variable is illegal
(Parser)
A variable declared register cannot have storage allocated for it in memory, and thus it
is illegal to attempt to take the address of it by applying the & operator, for example:
int * proc(register int in)
{
int * ip = ∈
/* oops -- in cannot have an address to take */
return ip;
}
(201) taking the address of this object is illegal
(Parser)
The expression which was the operand of the & operator is not one that denotes memory storage (“an lvalue”) and therefore its address cannot be defined, for example:
ip = &8;
/* oops -- you cannot take the address of a literal */
(202) only lvalues can be assigned to or modified
(Parser)
Only an lvalue (i.e., an identifier or expression directly denoting addressable storage)
can be assigned to or otherwise modified, for example:
int array[10];
int * ip;
char c;
array = ip;
/* array is not a variable,
it cannot be written to */
A typecast does not yield an lvalue, for example:
/* the contents of c cast to int
is only a intermediate value */
(int)c = 1;
However, you can write this using pointers:
*(int *)&c = 1
(203) illegal operation on bit variable
(Parser)
Not all operations on bit variables are supported. This operation is one of those, for
example:
bit
b;
int * ip;
ip = &b; /* oops -cannot take the address of a bit object */
(204) void function can’t return a value
(Parser)
A void function cannot return a value. Any return statement should not be followed
by an expression, for example:
void run(void)
{
step();
return 1;
/* either run should not be void, or remove the 1 */
}
(205) integral type required
(Parser)
This operator requires operands that are of integral type only.
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MPLAB® XC8 C Compiler User’s Guide
(206) illegal use of void expression
(Parser)
A void expression has no value and therefore you cannot use it anywhere an
expression with a value is required, i.e., as an operand to an arithmetic operator.
(207) simple type required for "*"
(Parser)
A simple type (i.e., not an array or structure) is required as an operand to this operator.
(208) operands of "*" not same type
(Parser)
The operands of this operator are of different pointers, for example:
int * ip;
char * cp, * cp2;
cp = flag ? ip : cp2;
/* result of ? : will be int * or char * */
Possibly, you meant something like:
cp = flag ? (char *)ip : cp2;
(209) type conflict
(Parser)
The operands of this operator are of incompatible types.
(210) bad size list
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(211) taking sizeof bit is illegal
(Parser)
It is illegal to use the sizeof operator with the C bit type. When used against a type,
the sizeof operator gives the number of bytes required to store an object that type.
Therefore its usage with the bit type make no sense and it is an illegal operation.
(212) missing number after pragma "pack"
(Parser)
The pragma pack requires a decimal number as argument. This specifies the alignment of each member within the structure. Use this with caution as some processors
enforce alignment and will not operate correctly if word fetches are made on odd
boundaries, for example:
#pragma pack
/* what is the alignment value */
Possibly, you meant something like:
#pragma pack 2
(214) missing number after pragma "interrupt_level"
(Parser)
The pragma interrupt_level requires an argument to indicate the interrupt level.
It will be the value 1 for mid-range devices, or 1 or 2 or PIC18 devices.
(215) missing argument to pragma "switch"
(Parser)
The pragma switch requires an argument of auto, direct or simple, for example:
#pragma switch
/* oops -- this requires a switch mode */
Possibly, you meant something like:
#pragma switch simple
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Error and Warning Messages
(216) missing argument to pragma "psect"
(Parser)
The pragma psect requires an argument of the form oldname = newname where oldname is an existing psect name known to the compiler, and newname is the desired
new name, for example:
#pragma psect
/* oops -- this requires an psect to redirect */
Possibly, you meant something like:
#pragma psect text=specialtext
(218) missing name after pragma "inline"
(Parser)
The inline pragma expects the name of a function to follow. The function name must
be recognized by the code generator for it to be expanded; other functions are not
altered, for example:
#pragma inline
/* what is the function name? */
Possibly, you meant something like:
#pragma inline memcpy
(219) missing name after pragma "printf_check"
(Parser)
The printf_check pragma expects the name of a function to follow. This specifies
printf-style format string checking for the function, for example:
#pragma printf_check
/* what function is to be checked? */
Possibly, you meant something like:
#pragma printf_check sprintf
Pragmas for all the standard printf-like function are already contained in <stdio.h>.
(220) exponent expected
(Parser)
A floating-point constant must have at least one digit after the e or E, for example:
float f;
f = 1.234e;
/* oops -- what is the exponent? */
(221) hexadecimal digit expected
(Parser)
After 0x should follow at least one of the HEX digits 0-9 and A-F or a-f, for example:
a = 0xg6;
/* oops -- was that meant to be a = 0xf6 ? */
(222) binary digit expected
(Parser)
A binary digit was expected following the 0b format specifier, for example:
i = 0bf000;
(223) digit out of range
/* oops -- f000 is not a base two value */
(Parser, Assembler)
A digit in this number is out of range of the radix for the number, i.e., using the digit 8
in an octal number, or HEX digits A-F in a decimal number. An octal number is denoted
by the digit string commencing with a zero, while a HEX number starts with “0X” or “0x”.
For example:
int a = 058;
/* leading 0 implies octal which has digits 0 - 7 */
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MPLAB® XC8 C Compiler User’s Guide
(224) illegal "#" directive
(Parser)
An illegal # preprocessor has been detected. Likely, a directive has been misspelled in
your code somewhere.
(225) missing character in character constant
(Parser)
The character inside the single quotes is missing, for example:
char c = ";
/* the character value of what? */
(226) char const too long
(Parser)
A character constant enclosed in single quotes cannot contain more than one
character, for example:
c = ’12’;
/* oops -- only one character can be specified */
(227) "." expected after ".."
(Parser)
The only context in which two successive dots can appear is as part of the ellipsis symbol, which must have 3 dots. (An ellipsis is used in function prototypes to indicate a
variable number of parameters.)
Either .. was meant to be an ellipsis symbol which would require you to add an extra
dot, or it was meant to be a structure member operator which would require you to
remove one dot.
(228) illegal character (*)
(Parser)
This character is illegal in the C code. Valid characters are the letters, digits and those
comprising the acceptable operators, for example:
c = a;
/* oops -- did you mean c = ’a’; ? */
(229) unknown qualifier "*" given to -A
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(230) missing argument to -A
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(231) unknown qualifier "*" given to -I
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(232) missing argument to -I
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(233) bad -Q option "*"
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(234) close error
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
DS50002053G-page 452
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Error and Warning Messages
(236) simple integer expression required
(Parser)
A simple integral expression is required after the operator @, used to associate an
absolute address with a variable, for example:
int address;
char LOCK @ address;
(237) function "*" redefined
(Parser)
More than one definition for a function has been encountered in this module. Function
overloading is illegal, for example:
int twice(int a)
{
return a*2;
}
/* only one prototype & definition of rv can exist */
long twice(long a)
{
return a*2;
}
(238) illegal initialization
(Parser)
You cannot initialize a typedef declaration, because it does not reserve any storage
that can be initialized, for example:
/* oops -- uint is a type, not a variable */
typedef unsigned int uint = 99;
(239) identifier "*" redefined (from line *)
(Parser)
This identifier has already been defined in the same scope. It cannot be defined again,
for example:
int a;
int a;
/* a filescope variable called "a" */
/* attempting to define another of the same name */
Note that variables with the same name, but defined with different scopes, are legal;
but; not recommended.
(240) too many initializers
(Parser)
There are too many initializers for this object. Check the number of initializers against
the object definition (array or structure), for example:
/* three elements, but four initializers */
int ivals[3] = { 2, 4, 6, 8};
(241) initialization syntax
(Parser)
The initialization of this object is syntactically incorrect. Check for the correct placement
and number of braces and commas, for example:
int iarray[10] = {{’a’, ’b’, ’c’};
/* oops -- one two many {s */
 2012-2016 Microchip Technology Inc.
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MPLAB® XC8 C Compiler User’s Guide
(242) illegal type for switch expression
(Parser)
A switch operation must have an expression that is either an integral type or an
enumerated value, e.g:
double d;
switch(d) { /* oops -- this must be integral */
case ’1.0’:
d = 0;
}
(243) inappropriate break/continue
(Parser)
A break or continue statement has been found that is not enclosed in an appropriate
control structure. A continue can only be used inside a while, for, or do while
loop, while break can only be used inside those loops or a switch statement, for
example:
switch(input) {
case 0:
if(output == 0)
input = 0xff;
} /* oops! this should not be here; it closed the switch */
break;
/* this should be inside the switch */
(244) "default" case redefined
(Parser)
Only one default label is allowed to be in a switch statement. You have more than
one, for example:
switch(a) {
default:
b = 9;
break;
default:
b = 10;
break;
/* if this is the default case... */
/* then what is this? */
(245) "default" case not in switch
(Parser)
A label has been encountered called default, but it is not enclosed by a switch
statement. A default label is only legal inside the body of a switch statement.
If there is a switch statement before this default label, there could be one too many
closing braces in the switch code. That would prematurely terminate the switch
statement. See message 246.
DS50002053G-page 454
 2012-2016 Microchip Technology Inc.
Error and Warning Messages
(246) case label not in switch
(Parser)
A case label has been encountered, but there is no enclosing switch statement. A
case label can only appear inside the body of a switch statement.
If there is a switch statement before this case label, there might be one too many
closing braces in the switch code. That would prematurely terminate the switch
statement, for example:
switch(input) {
case ’0’:
count++;
break;
case ’1’:
if(count>MAX)
count= 0;
}
/* oops -- this shouldn’t be here */
break;
case ’2’:
/* error flagged here */
(247) duplicate label "*"
(Parser)
The same name is used for a label more than once in this function. Note that the scope
of labels is the entire function, not just the block that encloses a label, for example:
start:
if(a > 256)
goto end;
start:
if(a == 0)
goto start;
/* error flagged here */
/* which start label do I jump to? */
(248) inappropriate "else"
(Parser)
An else keyword has been encountered that cannot be associated with an if
statement. This can mean there is a missing brace or other syntactic error, for example:
/* here is a comment which I have forgotten to close...
if(a > b) {
c = 0;
/* ... that will be closed here, thus removing the "if" */
else
/* my "if" has been lost */
c = 0xff;
(249) probable missing "}" in previous block
(Parser)
The compiler has encountered what looks like a function or other declaration, but the
preceding function was ended with a closing brace. This probably means that a closing
brace has been omitted from somewhere in the previous function, although it might not
be the last one, for example:
void set(char a)
{
PORTA = a;
void clear(void)
{
PORTA = 0;
}
 2012-2016 Microchip Technology Inc.
/* the closing brace was left out here */
/* error flagged here */
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MPLAB® XC8 C Compiler User’s Guide
MESSAGES 250-499
(251) array dimension redeclared
(Parser)
An array dimension has been declared as a different non-zero value from its previous
declaration. It is acceptable to redeclare the size of an array that was previously
declared with a zero dimension; but, not otherwise, for example:
extern int array[5];
int array[10];
/* oops -- has it 5 or 10 elements? */
(252) argument * conflicts with prototype
(Parser)
The argument specified (argument 0 is the left most argument) of this function definition
does not agree with a previous prototype for this function, for example:
/* this is supposedly calc’s prototype */
extern int calc(int, int);
int calc(int a, long int b) /* hmmm -- which is right? */
{
/* error flagged here */
return sin(b/a);
}
(253) argument list conflicts with prototype
(Parser)
The argument list in a function definition is not the same as a previous prototype for
that function. Check that the number and types of the arguments are all the same.
extern int calc(int);
int calc(int a, int b)
{
return a + b;
}
/* this is supposedly calc’s prototype */
/* hmmm -- which is right? */
/* error flagged here */
(254) undefined *: "*"
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(255) not a member of the struct/union "*"
(Parser)
This identifier is not a member of the structure or union type with which it used here, for
example:
struct {
int a, b, c;
} data;
if(data.d)
/* oops -there is no member d in this structure */
return;
(256) too much indirection
(Parser)
A pointer declaration can only have 16 levels of indirection.
(257) only "register" storage class allowed
(Parser)
The only storage class allowed for a function parameter is register, for example:
void process(static int input)
DS50002053G-page 456
 2012-2016 Microchip Technology Inc.
Error and Warning Messages
(258) duplicate qualifier
(Parser)
There are two occurrences of the same qualifier in this type specification. This can
occur either directly or through the use of a typedef. Remove the redundant qualifier.
For example:
typedef volatile int vint;
/* oops -- this results in two volatile qualifiers */
volatile vint very_vol;
(259) object can’t be qualified both far and near
(Parser)
It is illegal to qualify a type as both far and near, for example:
far near int spooky;
/* oops -- choose far or near, not both */
(260) undefined enum tag "*"
(Parser)
This enum tag has not been defined, for example:
enum WHAT what;
/* a definition for WHAT was never seen */
(261) struct/union member "*" redefined
(Parser)
This name of this member of the struct or union has already been used in this struct
or union, for example:
struct {
int a;
int b;
int a;
} input;
/* oops -- a different name is required here */
(262) struct/union "*" redefined
(Parser)
A structure or union has been defined more than once, for example:
struct {
int a;
} ms;
struct {
int a;
} ms;
/* was this meant to be the same name as above? */
(263) members can’t be functions
(Parser)
A member of a structure or a union cannot be a function. It could be a pointer to a
function, for example:
struct {
int a;
int get(int);
} object;
/* should be a pointer: int (*get)(int); */
(264) bad bitfield type
(Parser)
A bit-field can only have a type of int (or unsigned), for example:
struct FREG {
char b0:1;
char
:6;
char b7:1;
} freg;
 2012-2016 Microchip Technology Inc.
/* these must be part of an int, not char */
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MPLAB® XC8 C Compiler User’s Guide
(265) integer constant expected
(Parser)
A colon appearing after a member name in a structure declaration indicates that the
member is a bit-field. An integral constant must appear after the colon to define the
number of bits in the bit-field, for example:
struct {
unsigned first: /* oops -- should be: unsigned first; */
unsigned second;
} my_struct;
If this was meant to be a structure with bit-fields, then the following illustrates an
example:
struct {
unsigned first : 4;
unsigned second: 4;
} my_struct;
/* 4 bits wide */
/* another 4 bits */
(266) storage class illegal
(Parser)
A structure or union member cannot be given a storage class. Its storage class is determined by the storage class of the structure, for example:
struct {
/* no additional qualifiers can be present with members */
static int first;
} ;
(267) bad storage class
(Code Generator)
The code generator has encountered a variable definition whose storage class is
invalid, for example:
auto int foo; /* auto not permitted with global variables */
int power(static int a) /* parameters cannot be static */
{
return foo * a;
}
(268) inconsistent storage class
(Parser)
A declaration has conflicting storage classes. Only one storage class should appear in
a declaration, for example:
extern static int where;
/* so is it static or extern? */
(269) inconsistent type
(Parser)
Only one basic type can appear in a declaration, for example:
int float input;
/* is it int or float? */
(270) variable can’t have storage class "register"
(Parser)
Only function parameters or auto variables can be declared using the register
qualifier, for example:
register int gi;
/* this cannot be qualified register */
int process(register int input) /* this is okay */
{
return input + gi;
}
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Error and Warning Messages
(271) type can’t be long
(Parser)
Only int and float can be qualified with long.
long char lc;
/* what? */
(272) type can’t be short
(Parser)
Only int can be modified with short, for example:
short float sf;
/* what? */
(273) type can’t be both signed and unsigned
(Parser)
The type modifiers signed and unsigned cannot be used together in the same
declaration, as they have opposite meaning, for example:
signed unsigned int confused;
/* which is it? */
(274) type can’t be unsigned
(Parser)
A floating-point type cannot be made unsigned, for example:
unsigned float uf;
/* what? */
(275) "..." illegal in non-prototype argument list
(Parser)
The ellipsis symbol can only appear as the last item in a prototyped argument list. It
cannot appear on its own, nor can it appear after argument names that do not have
types; i.e., K&R-style non-prototype function definitions. For example:
/* K&R-style non-prototyped function definition */
int kandr(a, b, ...)
int a, b;
{
(276) type specifier required for prototyped argument
(Parser)
A type specifier is required for a prototyped argument. It is not acceptable to just have
an identifier.
(277) can’t mix prototyped and non-prototyped arguments
(Parser)
A function declaration can only have all prototyped arguments (i.e., with types inside
the parentheses) or all K&R style arguments (i.e., only names inside the parentheses
and the argument types in a declaration list before the start of the function body), for
example:
int plus(int a, b)
int b;
{
return a + b;
}
/* oops -- a is prototyped, b is not */
(278) argument "*" redeclared
(Parser)
The specified argument is declared more than once in the same argument list, for
example:
/* cannot have two parameters called "a" */
int calc(int a, int a)
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(279) initialization of function arguments is illegal
(Parser)
A function argument cannot have an initializer in a declaration. The initialization of the
argument happens when the function is called and a value is provided for the argument
by the calling function, for example:
/* oops -- a is initialized when proc is called */
extern int proc(int a = 9);
(280) arrays of functions are illegal
(Parser)
You cannot define an array of functions. You can, however, define an array of pointers
to functions, for example:
int * farray[]();
/* oops -- should be: int (* farray[])(); */
(281) functions can’t return functions
(Parser)
A function cannot return a function. It can return a function pointer. A function returning
a pointer to a function could be declared like this: int (* (name()))(). Note the many
parentheses that are necessary to make the parts of the declaration bind correctly.
(282) functions can’t return arrays
(Parser)
A function can return only a scalar (simple) type or a structure. It cannot return an array.
(283) dimension required
(Parser)
Only the most significant (i.e., the first) dimension in a multi-dimension array cannot be
assigned a value. All succeeding dimensions must be present as a constant
expression, for example:
/* This should be, for example: int arr[][7] */
int get_element(int arr[2][])
{
return array[1][6];
}
(284) invalid dimension
(Parser)
The array dimension specified is not valid. It must be larger than 0.
int array[0];
// oops -- you cannot have an array of size 0
(285) no identifier in declaration
(Parser)
The identifier is missing in this declaration. This error can also occur when the compiler
has been confused by such things as missing closing braces, for example:
void interrupt(void)
{
}
(286) declarator too complex
/* what is the name of this function? */
(Parser)
This declarator is too complex for the compiler to handle. Examine the declaration and
find a way to simplify it. If the compiler finds it too complex, so will anybody maintaining
the code.
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 2012-2016 Microchip Technology Inc.
Error and Warning Messages
(287) arrays of bits or pointers to bit are illegal
(Parser)
It is not legal to have an array of bits, or a pointer to bit variable, for example:
bit barray[10];
bit * bp;
/* wrong -- no bit arrays */
/* wrong -- no pointers to bit variables */
(288) the type 'void' is applicable only to functions
(Parser)
A variable cannot be void. Only a function can be void, for example:
int a;
void b;
/* this makes no sense */
(289) the specifier 'interrupt' is applicable only to functions
(Parser)
The qualifier interrupt cannot be applied to anything except a function, for example:
/* variables cannot be qualified interrupt */
interrupt int input;
(290) illegal function qualifier(s)
(Parser)
A qualifier has been applied to a function which makes no sense in this context. Some
qualifier only make sense when used with an lvalue, i.e., const or volatile. This
can indicate that you have forgotten a star * that is indicating that the function should
return a pointer to a qualified object, for example:
const char ccrv(void) /* const * char ccrv(void) perhaps? */
{
/* error flagged here */
return ccip;
}
(291) K&R identifier "*" not an argument
(Parser)
This identifier, that has appeared in a K&R style argument declarator, is not listed inside
the parentheses after the function name, for example:
int process(input)
int unput;
/* oops -- that should be int input; */
{
}
(292) a function is not a valid parameter type
(Parser)
A function parameter cannot be a function. It can be a pointer to a function, so perhaps
a "*" has been omitted from the declaration.
(293) bad size in index_type()
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(294) can’t allocate * bytes of memory
(Code Generator, Hexmate)
This is an internal compiler error. Contact Microchip Technical Support with details.
(295) expression too complex
(Parser)
This expression has caused overflow of the compiler’s internal stack and should be
rearranged or split into two expressions.
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(296) out of memory
(Objtohex)
This could be an internal compiler error. Contact Microchip Technical Support with
details.
(297) bad argument (*) to tysize()
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(298) end of file in #asm
(Preprocessor)
An end of file has been encountered inside a #asm block. This probably means the
#endasm is missing or misspelled, for example:
#asm
MOV
MOV
}
r0, #55
[r1], r0
/* oops -- where is the #endasm */
(300) unexpected end of file
(Parser)
An end-of-file in a C module was encountered unexpectedly, for example:
void main(void)
{
init();
run();
/* is that it? What about the close brace */
(301) end of file on string file
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(302) can’t reopen "*": *
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(303) can’t allocate * bytes of memory (line *)
(Parser)
The parser was unable to allocate memory for the longest string encountered, as it
attempts to sort and merge strings. Try reducing the number or length of strings in this
module.
(306) can’t allocate * bytes of memory for *
(Code Generator)
This is an internal compiler error. Contact Microchip Technical Support with details.
(307) too many qualifier names
(Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(308) too many case labels in switch
(Code Generator)
There are too many case labels in this switch statement. The maximum allowable
number of case labels in any one switch statement is 511.
(309) too many symbols
(Assembler, Parser)
There are too many symbols for the assembler’s symbol table. Reduce the number of
symbols in your program.
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Error and Warning Messages
(310) "]" expected
(Parser)
A closing square bracket was expected in an array declaration or an expression using
an array index, for example:
process(carray[idx);
/* oops -should be: process(carray[idx]); */
(311) closing quote expected
(Parser)
A closing quote was expected for the indicated string.
(312) "*" expected
(Parser)
The indicated token was expected by the parser.
(313) function body expected
(Parser)
Where a function declaration is encountered with K&R style arguments (i.e., argument
names; but, no types inside the parentheses) a function body is expected to follow, for
example:
/* the function block must follow, not a semicolon */
int get_value(a, b);
(314) ";" expected
(Parser)
A semicolon is missing from a statement. A close brace or keyword was found following
a statement with no terminating semicolon , for example:
while(a) {
b = a-- /* oops -- where is the semicolon? */
}
/* error is flagged here */
Note: Omitting a semicolon from statements not preceding a close brace or keyword
typically results in some other error being issued for the following code which the parser
assumes to be part of the original statement.
(315) "{" expected
(Parser)
An opening brace was expected here. This error can be the result of a function
definition missing the opening brace, for example:
/* oops! no opening brace after the prototype */
void process(char c)
return max(c, 10) * 2; /* error flagged here */
}
(316) "}" expected
(Parser)
A closing brace was expected here. This error can be the result of a initialized array
missing the closing brace, for example:
char carray[4] = { 1, 2, 3, 4;
/* oops -- no closing brace */
(317) "(" expected
(Parser)
An opening parenthesis , (, was expected here. This must be the first token after a
while , for , if , do or asm keyword, for example:
if a == b
b = 0;
 2012-2016 Microchip Technology Inc.
/* should be: if(a == b) */
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(318) string expected
(Parser)
The operand to an asm statement must be a string enclosed in parentheses, for
example:
asm(nop);
/* that should be asm("nop");
(319) while expected
(Parser)
The keyword while is expected at the end of a do statement, for example:
do {
func(i++);
}
if(i > 5)
end();
/* do the block while what condition is true? */
/* error flagged here */
(320) ":" expected
(Parser)
A colon is missing after a case label, or after the keyword default. This often occurs
when a semicolon is accidentally typed instead of a colon, for example:
switch(input) {
case 0;
state = NEW;
/* oops -- that should have been: case 0: */
(321) label identifier expected
(Parser)
An identifier denoting a label must appear after goto, for example:
if(a)
goto 20;
/* this is not BASIC -- a valid C label must follow a goto */
(322) enum tag or "{" expected
(Parser)
After the keyword enum, must come either an identifier that is, or will be, defined as an
enum tag, or an opening brace, for example:
enum 1, 2;
/* should be, for example: enum {one=1, two }; */
(323) struct/union tag or "{" expected
(Parser)
An identifier denoting a structure or union or an opening brace must follow a struct
or union keyword, for example:
struct int a;
/* this is not how you define a structure */
You might mean something like:
struct {
int a;
} my_struct;
(324) too many arguments for printf-style format string
(Parser)
There are too many arguments for this format string. This is harmless, but can
represent an incorrect format string, for example:
/* oops -- missed a placeholder? */
printf("%d - %d", low, high, median);
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Error and Warning Messages
(325) error in printf-style format string
(Parser)
There is an error in the format string here. The string has been interpreted as a
printf() style format string, and it is not syntactically correct. If not corrected, this will
cause unexpected behavior at runtime, for example:
printf("%l", lll);
/* oops -- possibly: printf("%ld", lll); */
(326) long int argument required in printf-style format string
(Parser)
A long argument is required for this format specifier. Check the number and order of
format specifiers and corresponding arguments, for example:
printf("%lx", 2);
// possibly you meant: printf("%lx", 2L);
(327) long long int argument required in printf-style format string
(Parser)
A long long argument is required for this format specifier. Check the number and
order of format specifiers and corresponding arguments, for example:
printf("%llx", 2);
// possibly you meant: printf("%llx", 2LL);
Note that MPLAB XC8 does not provide direct support for a long long integer type.
(328) int argument required in printf-style format string
(Parser)
An integral argument is required for this printf-style format specifier. Check the number
and order of format specifiers and corresponding arguments, for example:
printf("%d", 1.23); /* wrong number or wrong placeholder */
(329) double argument required in printf-style format string
(Parser)
The printf format specifier corresponding to this argument is %f or similar, and requires
a floating-point expression. Check for missing or extra format specifiers or arguments
to printf.
printf("%f", 44);
/* should be: printf("%f", 44.0); */
(330) pointer to * argument required in printf-style format string
(Parser)
A pointer argument is required for this format specifier. Check the number and order of
format specifiers and corresponding arguments.
(331) too few arguments for printf-style format string
(Parser)
There are too few arguments for this format string. This would result in a garbage value
being printed or converted at runtime, for example:
printf("%d - %d", low);
/* oops! where is the other value to print? */
(332) "interrupt_level" should be 0 to 7
(Parser)
The pragma interrupt_level must have an argument from 0 to 7; however,
mid-range devices only use level 1. PIC18 devices can use levels 1 or 2. For example:
#pragma interrupt_level 9 /* oops -- the level is too high */
void interrupt isr(void)
{
/* isr code goes here */
}
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(333) unrecognized qualifier name after "strings"
(Parser)
The pragma strings was passed a qualifier that was not identified, for example:
/* oops -- should that be #pragma strings const ? */
#pragma strings cinst
(334) unrecognized qualifier name after "printf_check"
(Parser)
The #pragma printf_check was passed a qualifier that could not be identified, for
example:
/* oops -- should that be const not cinst? */
#pragma printf_check(printf) cinst
(335) unknown pragma "*"
(Parser)
An unknown pragma directive was encountered, for example:
#pragma rugsused myFunc w
/* I think you meant regsused */
(336) string concatenation across lines
(Parser)
Strings on two lines will be concatenated. Check that this is the desired result, for
example:
char * cp = "hi"
"there";
/* this is okay,
but is it what you had intended? */
(337) line does not have a newline on the end
(Parser)
The last line in the file is missing the newline (operating system dependent character)
from the end. Some editors will create such files, which can cause problems for include
files. The ANSI C standard requires all source files to consist of complete lines only.
(338) can’t create * file "*"
(Any)
The application tried to create or open the named file, but it could not be created. Check
that all file path names are correct.
(339) initializer in extern declaration
(Parser)
A declaration containing the keyword extern has an initializer. This overrides the
extern storage class, because to initialize an object it is necessary to define (i.e.,
allocate storage for) it, for example:
extern int other = 99;
/* if it’s extern and not allocated
storage, how can it be initialized? */
(340) string not terminated by null character
(Parser)
A char array is being initialized with a string literal larger than the array. Hence there is
insufficient space in the array to safely append a null terminating character, for
example:
char foo[5] = "12345"; /* the string stored in foo won’t have
a null terminating, i.e.
foo = [’1’, ’2’, ’3’, ’4’, ’5’] */
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Error and Warning Messages
(343) implicit return at end of non-void function
(Parser)
A function that has been declared to return a value has an execution path that will allow
it to reach the end of the function body, thus returning without a value. Either insert a
return statement with a value, or if the function is not to return a value, declare it
void, for example:
int mydiv(double a, int b)
{
if(b != 0)
return a/b;
/* what about when b is 0? */
}
/* warning flagged here */
(344) non-void function returns no value
(Parser)
A function that is declared as returning a value has a return statement that does not
specify a return value, for example:
int get_value(void)
{
if(flag)
return val++;
return;
/* what is the return value in this instance? */
}
(345) unreachable code
(Parser)
This section of code will never be executed, because there is no execution path by
which it could be reached, for example:
while(1)
process();
flag = FINISHED;
/* how does this loop finish? */
/* how do we get here? */
(346) declaration of "*" hides outer declaration
(Parser)
An object has been declared that has the same name as an outer declaration (i.e., one
outside and preceding the current function or block). This is legal, but can lead to
accidental use of one variable when the outer one was intended, for example:
int input;
/* input has filescope */
void process(int a)
{
int input;
/* local blockscope input */
a = input;
/* this will use the local variable.
Is this right? */
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(347) external declaration inside function
(Parser)
A function contains an extern declaration. This is legal but is invariably not desirable
as it restricts the scope of the function declaration to the function body. This means that
if the compiler encounters another declaration, use, or definition of the extern object
later in the same file, it will no longer have the earlier declaration and thus will be unable
to check that the declarations are consistent. This can lead to strange behavior of your
program or signature errors at link time. It will also hide any previous declarations of
the same thing, again subverting the compiler’s type checking. As a general rule,
always declare extern variables and functions outside any other functions. For
example:
int process(int a)
{
/* this would be better outside the function */
extern int away;
return away + a;
}
(348) auto variable "*" should not be qualified
(Parser)
An auto variable should not have qualifiers such as near or far associated with it. Its
storage class is implicitly defined by the stack organization. An auto variable can be
qualified with static, but it is then no longer auto.
(349) non-prototyped function declaration for "*"
(Parser)
A function has been declared using old-style (K&R) arguments. It is preferable to use
prototype declarations for all functions, for example:
int process(input)
int input;
/* warning flagged here */
{
}
This would be better written:
int process(int input)
{
}
(350) unused * "*" (from line *)
(Parser)
The indicated object was never used in the function or module being compiled. Either
this object is redundant, or the code that was meant to use it was excluded from compilation or misspelled the name of the object. Note that the symbols rcsid and
sccsid are never reported as being unused.
(352) float parameter coerced to double
(Parser)
Where a non-prototyped function has a parameter declared as float, the compiler
converts this to a double float. This is because the default C type conversion conventions provide that when a floating-point number is passed to a non-prototyped function, it is converted to double. It is important that the function declaration be consistent
with this convention, for example:
double inc_flt(f)
float f;
{
return f * 2;
}
DS50002053G-page 468
/* f will be converted to double */
/* warning flagged here */
 2012-2016 Microchip Technology Inc.
Error and Warning Messages
(353) sizeof external array "*" is zero
(Parser)
The size of an external array evaluates to zero. This is probably due to the array not
having an explicit dimension in the extern declaration.
(354) possible pointer truncation
(Parser)
A pointer qualified far has been assigned to a default pointer, or a pointer qualified near,
or a default pointer has been assigned to a pointer qualified near. This can result in
truncation of the pointer and loss of information, depending on the memory model in
use.
(355) implicit signed to unsigned conversion
(Parser)
A signed number is being assigned or otherwise converted to a larger unsigned
type. Under the ANSI C “value preserving” rules, this will result in the signed value
being first sign-extended to a signed number the size of the target type, then converted to unsigned (which involves no change in bit pattern). Thus, an unexpected
sign extension can occur. To ensure this does not happen, first convert the signed value
to an unsigned equivalent, for example:
signed char sc;
unsigned int ui;
ui = sc;
/* if sc contains 0xff,
ui will contain 0xffff for example */
will perform a sign extension of the char variable to the longer type. If you do not want
this to take place, use a cast, for example:
ui = (unsigned char)sc;
(356) implicit conversion of float to integer
(Parser)
A floating-point value has been assigned or otherwise converted to an integral type.
This could result in truncation of the floating-point value. A typecast will make this
warning go away.
double dd;
int i;
i = dd;
/* is this really what you meant? */
If you do intend to use an expression like this, then indicate that this is so by a cast:
i = (int)dd;
(357) illegal conversion of integer to pointer
(Parser)
An integer has been assigned to, or otherwise converted to, a pointer type. This will
usually mean that you have used the wrong variable. But, if this is genuinely what you
want to do, use a typecast to inform the compiler that you want the conversion and the
warning will be suppressed. This can also mean that you have forgotten the & address
operator, for example:
int * ip;
int i;
ip = i;
/* oops -- did you mean ip = &i ? */
If you do intend to use an expression like this, then indicate that this is so by a cast:
ip = (int *)i;
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(358) illegal conversion of pointer to integer
(Parser)
A pointer has been assigned to, or otherwise converted to, a integral type. This will usually mean that you have used the wrong variable. But, if this is genuinely what you want
to do, use a typecast to inform the compiler that you want the conversion and the warning will be suppressed. This can also mean that you have forgotten the * dereference
operator, for example:
int * ip;
int i;
i = ip;
/* oops -- did you mean i = *ip ? */
If you do intend to use an expression like this, indicate your intention by a cast:
i = (int)ip;
(359) illegal conversion between pointer types
(Parser)
A pointer of one type (i.e., pointing to a particular kind of object) has been converted
into a pointer of a different type. This usually means that you have used the wrong variable, but if this is genuinely what you want to do, use a typecast to inform the compiler
that you want the conversion and the warning will be suppressed, for example:
long input;
char * cp;
cp = &input;
/* is this correct? */
This is a common way of accessing bytes within a multi-byte variable. To indicate that
this is the intended operation of the program, use a cast:
cp = (char *)&input;
/* that’s better */
This warning can also occur when converting between pointers to objects that have the
same type, but which have different qualifiers, for example:
char * cp;
/* yes, but what sort of characters? */
cp = "I am a string of characters";
If the default type for string literals is const char *, then this warning is quite valid.
This should be written:
const char * cp;
cp = "I am a string of characters";
/* that’s better */
Omitting a qualifier from a pointer type is often disastrous, and almost certainly not what
you intend.
(360) array index out of bounds
(Parser)
An array is being indexed with a constant value that is less than zero, or greater than
or equal to the number of elements in the array. This warning will not be issued when
accessing an array element via a pointer variable, for example:
int i, * ip, input[10];
i = input[-2];
ip = &input[5];
i = ip[-2];
DS50002053G-page 470
/* oops -- this element doesn’t exist */
/* this is okay */
 2012-2016 Microchip Technology Inc.
Error and Warning Messages
(361) function declared implicit int
(Parser)
When the compiler encounters a function call of a function whose name is presently
undefined, the compiler will automatically declare the function to be of type int, with
unspecified (K&R style) parameters. If a definition of the function is subsequently
encountered, it is possible that its type and arguments will be different from the earlier
implicit declaration, causing a compiler error. The solution is to ensure that all functions
are defined (or at least declared) before use, preferably with prototyped parameters. If
it is necessary to make a forward declaration of a function, it should be preceded with
the keywords extern or static, as appropriate. For example:
/* I can prevent an error arising from calls below */
extern void set(long a, int b);
void main(void)
{
/* at this point, a prototype for set() has already been seen */
set(10L, 6);
}
(362) redundant "&" applied to array
(Parser)
The address operator & has been applied to an array. Because using the name of an
array gives its address anyway, this is unnecessary and has been ignored, for example:
int array[5];
int * ip;
/* array is a constant, not a variable; the & is redundant. */
ip = &array;
(363) redundant "&" or "*" applied to function address
(Parser)
The address operator “&” has been applied to a function. Because using the name of
a function gives its address anyway, this is unnecessary and has been ignored, for
example:
extern void foo(void);
void main(void)
{
void(*bar)(void);
/* both assignments are equivalent */
bar = &foo;
bar = foo; /* the & is redundant */
}
(364) attempt to modify object qualified *
(Parser)
Objects declared const or code cannot be assigned to or modified in any other way
by your program. The effect of attempting to modify such an object is compiler specific.
const int out = 1234;
out = 0;
 2012-2016 Microchip Technology Inc.
/* "out" is read only */
/* oops -writing to a read-only object */
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MPLAB® XC8 C Compiler User’s Guide
(365) pointer to non-static object returned
(Parser)
This function returns a pointer to a non-static (e.g., auto) variable. This is likely to
be an error, because the storage associated with automatic variables becomes invalid
when the function returns, for example:
char * get_addr(void)
{
char c;
/* returning this is dangerous;
the pointer could be dereferenced */
return &c;
}
(366) operands of "*" not same pointer type
(Parser)
The operands of this operator are of different pointer types. This probably means you
have used the wrong pointer, but if the code is actually what you intended, use a
typecast to suppress the error message.
(367) identifier is already extern; can’t be static
(Parser)
This function was already declared extern, possibly through an implicit declaration. It
has now been redeclared static, but this redeclaration is invalid.
void main(void)
{
/* at this point the compiler assumes set is extern... */
set(10L, 6);
}
/* now it finds out otherwise */
static void set(long a, int b)
{
PORTA = a + b;
}
(368) array dimension on "*[]" ignored
(Preprocessor)
An array dimension on a function parameter has been ignored because the argument
is actually converted to a pointer when passed. Thus arrays of any size can be passed.
Either remove the dimension from the parameter, or define the parameter using pointer
syntax, for example:
/* param should be: "int array[]" or "int *" */
int get_first(int array[10])
{
/* warning flagged here */
return array[0];
}
(369) signed bitfields not supported
(Parser)
Only unsigned bit-fields are supported. If a bit-field is declared to be type int, the
compiler still treats it as unsigned, for example:
struct {
signed int sign: 1;
signed int value: 7;
} ;
DS50002053G-page 472
/* oops -- this must be unsigned */
 2012-2016 Microchip Technology Inc.
Error and Warning Messages
(370) illegal basic type; int assumed
(Parser)
The basic type of a cast to a qualified basic type could not be recognized and the basic
type was assumed to be int, for example:
/* here ling is assumed to be int */
unsigned char bar = (unsigned ling) ’a’;
(371) missing basic type; int assumed
(Parser)
This declaration does not include a basic type, so int has been assumed. This declaration is not illegal, but it is preferable to include a basic type to make it clear what is
intended, for example:
char c;
i;
/* don’t let the compiler make assumptions, use : int i */
func(); /* ditto, use: extern int func(int); */
(372) "," expected
(Parser)
A comma was expected here. This could mean you have left out the comma between
two identifiers in a declaration list. It can also mean that the immediately preceding type
name is misspelled, and has been interpreted as an identifier, for example:
unsigned char a;
/* thinks: chat & b are unsigned, but where is the comma? */
unsigned chat b;
(373) implicit signed to unsigned conversion
(Parser)
An unsigned type was expected where a signed type was given and was implicitly
cast to unsigned, for example:
unsigned int foo = -1;
/* the above initialization is implicitly treated as:
unsigned int foo = (unsigned) -1; */
(374) missing basic type; int assumed
(Parser)
The basic type of a cast to a qualified basic type was missing and assumed to be int.,
for example:
int i = (signed) 2; /* (signed) assumed to be (signed int) */
(375) unknown FNREC type "*"
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(376) bad non-zero node in call graph
(Linker)
The linker has encountered a top level node in the call graph that is referenced from
lower down in the call graph. This probably means the program has indirect recursion,
which is not allowed when using a compiled stack.
(378) can’t create * file "*"
(Hexmate)
This type of file could not be created. Is the file, or a file by this name, already in use?
(379) bad record type "*"
(Linker)
This is an internal compiler error. Ensure that the object file is a valid object file. Contact
Microchip Technical Support with details.
 2012-2016 Microchip Technology Inc.
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MPLAB® XC8 C Compiler User’s Guide
(380) unknown record type (*)
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(381) record "*" too long (*)
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(382) incomplete record: type = *, length = *
(Dump, Xstrip)
This message is produced by the DUMP or XSTRIP utilities and indicates that the
object file is not a valid object file, or that it has been truncated. Contact Microchip
Technical Support with details.
(383) text record has length (*) too small
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(384) assertion failed: file *, line *, expression *
(Linker, Parser)
This is an internal compiler error. Contact Microchip Technical Support with details.
(387) illegal or too many -G options
(Linker)
There has been more than one linker -g option, or the -g option did not have any
arguments following. The arguments specify how the segment addresses are
calculated.
(388) duplicate -M option
(Linker)
The map file name has been specified to the linker for a second time. This should not
occur if you are using a compiler driver. If invoking the linker manually, ensure that only
one instance of this option is present on the command line. See Section 4.8.7 “-M:
Generate Map File” for information on the correct syntax for this option.
(389) illegal or too many -O options
(Linker)
This linker -o flag is illegal, or another -o option has been encountered. A -o option
to the linker must be immediately followed by a filename with no intervening space.
(390) missing argument to -P
(Linker)
There have been too many -p options passed to the linker, or a -p option was not followed by any arguments. The arguments of separate -p options can be combined and
separated by commas.
(391) missing argument to -Q
(Linker)
The -Q linker option requires the machine type for an argument.
(392) missing argument to -U
(Linker)
The -U (undefine) option needs an argument.
(393) missing argument to -W
(Linker)
The -W option (listing width) needs a numeric argument.
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Error and Warning Messages
(394) duplicate -D or -H option
(Linker)
The symbol file name has been specified to the linker for a second time. This should
not occur if you are using a compiler driver. If invoking the linker manually, ensure that
only one instance of either of these options is present on the command line.
(395) missing argument to -J
(Linker)
The maximum number of errors before aborting must be specified following the -j
linker option.
(397) usage: hlink [-options] files.obj files.lib
(Linker)
Improper usage of the command-line linker. If you are invoking the linker directly, refer
to Section Section 7.2 “Operation” for more details. Otherwise, this could be an internal
compiler error and you should contact Microchip Technical Support with details.
(398) output file can’t be also an input file
(Linker)
The linker has detected an attempt to write its output file over one of its input files. This
cannot be done, because it needs to simultaneously read and write input and output
files.
(400) bad object code format
(Linker)
This is an internal compiler error. The object code format of an object file is invalid.
Ensure it is a valid object file. Contact Microchip Technical Support with details.
(402) bad argument to -F
(Objtohex)
The -F option for objtohex has been supplied an invalid argument. If you are not
invoking this tool directly, this is an internal compiler error, and you should contact
Microchip Technical Support with details.
(403) bad -E option: "*"
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(404) bad maximum length value to -<digits>
(Objtohex)
The first value to the OBJTOHEX -n,m HEX length/rounding option is invalid.
(405) bad record size rounding value to -<digits>
(Objtohex)
The second value to the OBJTOHEX -n,m HEX length/rounding option is invalid.
(406) bad argument to -A
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(407) bad argument to -U
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(408) bad argument to -B
(Objtohex)
This option requires an integer argument in either base 8, 10, or 16. If you are not invoking this tool directly, this is an internal compiler error, and you should contact Microchip
Technical Support with details.
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(409) bad argument to -P
(Objtohex)
This option requires an integer argument in either base 8, 10, or 16. If you are not invoking this tool directly, this is an internal compiler error, and you should contact Microchip
Technical Support with details.
(410) bad combination of options
(Objtohex)
The combination of options supplied to OBJTOHEX is invalid.
(412) text does not start at 0
(Objtohex)
Code in some things must start at zero. Here it doesn’t.
(413) write error on "*"
(Assembler, Linker, Cromwell)
A write error occurred on the named file. This probably means you have run out of disk
space.
(414) read error on "*"
(Linker)
The linker encountered an error trying to read this file.
(415) text offset too low in COFF file
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(416) bad character (*) in extended TEKHEX line
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(417) seek error in "*"
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(418) image too big
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(419) object file is not absolute
(Objtohex)
The object file passed to OBJTOHEX has relocation items in it. This can indicate it is the
wrong object file, or that the linker or OBJTOHEX have been given invalid options. The
object output files from the assembler are relocatable, not absolute. The object file
output of the linker is absolute.
(420) too many relocation items
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(421) too many segments
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(422) no end record
(Linker)
This object file has no end record. This probably means it is not an object file. Contact
Microchip Technical Support if the object file was generated by the compiler.
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Error and Warning Messages
(423) illegal record type
(Linker)
There is an error in an object file. This is either an invalid object file, or an internal error
in the linker. Contact Microchip Technical Support with details if the object file was
created by the compiler.
(424) record too long
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(425) incomplete record
(Objtohex, Libr)
The object file passed to OBJTOHEX or the librarian is corrupted. Contact Microchip
Technical Support with details.
(427) syntax error in list
(Objtohex)
There is a syntax error in a list read by OBJTOHEX. The list is read from standard input
in response to an option.
(428) too many segment fixups
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(429) bad segment fixups
(Objtohex)
This is an internal compiler error. Contact Microchip Technical Support with details.
(430) bad specification
(Objtohex)
A list supplied to OBJTOHEX is syntactically incorrect.
(431) bad argument to -E
(Objtoexe)
This option requires an integer argument in either base 8, 10, or 16. If you are invoking
objtoexe directly then check this argument. Otherwise, this can be an internal
compiler error and you should contact Microchip Technical Support with details.
(432) usage: objtohex [-ssymfile] [object-file [exe-file]]
(Objtohex)
Improper usage of the command-line tool objtohex. If you are not invoking this tool
directly, this is an internal compiler error, and you should contact Microchip Technical
Support with details.
(434) too many symbols (*)
(Linker)
There are too many symbols in the symbol table, which has a limit of * symbols.
Change some global symbols to local symbols to reduce the number of symbols.
(435) bad segment selector "*"
(Linker)
The segment specification option (-G) to the linker is invalid, for example:
-GA/f0+10
Did you forget the radix?
-GA/f0h+10
(436) psect "*" re-orged
(Linker)
This psect has had its start address specified more than once.
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(437) missing "=" in class spec
(Linker)
A class spec needs an = sign, e.g., -Ctext=ROM. See Section 7.2.2 “-Cpsect=class” for
more information.
(438) bad size in -S option
(Linker)
The address given in a -S specification is invalid, it should be a valid number, in decimal, octal, or hexadecimal radix. The radix is specified by a trailing O, for octal, or H for
HEX. A leading 0x can also be used for hexadecimal. Case in not important for any
number or radix. Decimal is the default, for example:
-SCODE=f000
Did you forget the radix?
-SCODE=f000h
(439) bad -D spec: "*"
(Linker)
The format of a -D specification, giving a delta value to a class, is invalid, for example:
-DCODE
What is the delta value for this class? Possibly, you meant something like:
-DCODE=2
(440) bad delta value in -D spec
(Linker)
The delta value supplied to a -D specification is invalid. This value should an integer of
base 8, 10, or 16.
(441) bad -A spec: "*"
(Linker)
The format of a -A specification, giving address ranges to the linker, is invalid, for
example:
-ACODE
What is the range for this class? Possibly, you meant:
-ACODE=0h-1fffh
(442) missing address in -A spec
(Linker)
The format of a -A specification, giving address ranges to the linker, is invalid, for
example:
-ACODE=
What is the range for this class? Possibly, you meant:
-ACODE=0h-1fffh
(443) bad low address "*" in -A spec
(Linker)
The low address given in a -A specification is invalid: it should be a valid number, in
decimal, octal, or hexadecimal radix. The radix is specified by a trailing O (for octal) or
H for HEX. A leading 0x can also be used for hexadecimal. Case in not important for
any number or radix. Decimal is default, for example:
-ACODE=1fff-3fffh
Did you forget the radix?
-ACODE=1fffh-3fffh
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Error and Warning Messages
(444) expected "-" in -A spec
(Linker)
There should be a minus sign, -, between the high and low addresses in a -A linker
option, for example:
-AROM=1000h
Possibly, you meant:
-AROM=1000h-1fffh
(445) bad high address "*" in -A spec
(Linker)
The high address given in a -A specification is invalid: it should be a valid number, in
decimal, octal, or hexadecimal radix. The radix is specified by a trailing O, for octal, or
H for HEX. A leading 0x can also be used for hexadecimal. Case in not important for
any number or radix. Decimal is the default, for example:
-ACODE=0h-ffff
Did you forget the radix?
-ACODE=0h-ffffh
See Section 7.2.1 “-Aclass =low-high,...” for more information.
(446) bad overrun address "*" in -A spec
(Linker)
The overrun address given in a -A specification is invalid: it should be a valid number,
in decimal, octal, or hexadecimal radix. The radix is specified by a trailing O (for octal)
or H for HEX. A leading 0x can also be used for hexadecimal. Case in not important for
any number or radix. Decimal is default, for example:
-AENTRY=0-0FFh-1FF
Did you forget the radix?
-AENTRY=0-0FFh-1FFh
(447) bad load address "*" in -A spec
(Linker)
The load address given in a -A specification is invalid: it should be a valid number, in
decimal, octal, or hexadecimal radix. The radix is specified by a trailing O (for octal) or
H for HEX. A leading 0x can also be used for hexadecimal. Case in not important for
any number or radix. Decimal is default, for example:
-ACODE=0h-3fffh/a000
Did you forget the radix?
-ACODE=0h-3fffh/a000h
(448) bad repeat count "*" in -A spec
(Linker)
The repeat count given in a -A specification is invalid, for example:
-AENTRY=0-0FFhxf
Did you forget the radix?
-AENTRY=0-0FFhxfh
(449) syntax error in -A spec: *
(Linker)
The -A spec is invalid. A valid -A spec should be something like:
-AROM=1000h-1FFFh
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(450) psect "*" was never defined, or is local
(Linker)
This psect has been listed in a -P option, but is not defined in any module within the
program. Alternatively, the psect is defined using the local psect flag, but with no
class flag; and, so, cannot be linked to an address. Check the assembly list file to
ensure that the psect exists and that it is does not specify the local psect flag.
(451) bad psect origin format in -P option
(Linker)
The origin format in a -p option is not a validly formed decimal, octal, or HEX number,
nor is it the name of an existing psect. A HEX number must have a trailing H, for
example:
-pbss=f000
Did you forget the radix?
-pbss=f000h
(452) bad "+" (minimum address) format in -P option
(Linker)
The minimum address specification in the linker’s -p option is badly formatted, for
example:
-pbss=data+f000
Did you forget the radix?
-pbss=data+f000h
(453) missing number after "%" in -P option
(Linker)
The % operator in a -p option (for rounding boundaries) must have a number after it.
(454) link and load address can’t both be set to "." in -P option
(Linker)
The link and load address of a psect have both been specified with a dot character.
Only one of these addresses can be specified in this manner, for example:
-Pmypsect=1000h/.
-Pmypsect=./1000h
Both of these options are valid and equivalent. However, the following usage is
ambiguous:
-Pmypsect=./.
What is the link or load address of this psect?
(455) psect "*" not relocated on 0x* byte boundary
(Linker)
This psect is not relocated on the required boundary. Check the relocatability of the
psect and correct the -p option. if necessary.
(456) psect "*" not loaded on 0x* boundary
(Linker)
This psect has a relocatability requirement that is not met by the load address given in
a -p option. For example, if a psect must be on a 4K byte boundary, you could not start
it at 100H.
(459) remove failed; error: *, *
(Xstrip)
The creation of the output file failed when removing an intermediate file.
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Error and Warning Messages
(460) rename failed; error: *, *
(Xstrip)
The creation of the output file failed when renaming an intermediate file.
(461) can’t create * file "*"
(Assembler, Code Generator)
This is an internal compiler error. Contact Microchip Technical Support with details.
(464) missing key in avmap file
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(465) undefined symbol "*" in FNBREAK record
(Linker)
The linker has found an undefined symbol in the FNBREAK record for a non-reentrant
function. Contact Microchip Technical Support if this is not handwritten assembler
code.
(466) undefined symbol "*" in FNINDIR record
(Linker)
The linker has found an undefined symbol in the FNINDIR record for a non-reentrant
function. Contact Microchip Technical Support if this is not handwritten assembler
code.
(467) undefined symbol "*" in FNADDR record
(Linker)
The linker has found an undefined symbol in the FNADDR record for a non-reentrant
function. Contact Microchip Technical Support if this is not handwritten assembler
code.
(468) undefined symbol "*" in FNCALL record
(Linker)
The linker has found an undefined symbol in the FNCALL record for a non-reentrant
function. Contact Microchip Technical Support if this is not handwritten assembler
code.
(469) undefined symbol "*" in FNROOT record
(Linker)
The linker has found an undefined symbol in the FNROOT record for a non-reentrant
function. Contact Microchip Technical Support if this is not handwritten assembler
code.
(470) undefined symbol "*" in FNSIZE record
(Linker)
The linker has found an undefined symbol in the FNSIZE record for a non-reentrant
function. Contact Microchip Technical Support if this is not handwritten assembler
code.
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(471) recursive function calls:
(Linker)
These functions (or function) call each other recursively. One or more of these functions
has statically allocated local variables (compiled stack). Either use the reentrant
keyword (if supported with this compiler) or recode to avoid recursion, for example:
int test(int a)
{
if(a == 5) {
/* recursion cannot be supported by some compilers */
return test(a++);
}
return 0;
}
(472) non-reentrant function "*" appears in multiple call graphs: rooted at "*" and "*"
(Linker)
This function can be called from both main-line code and interrupt code. Use the
reentrant keyword, if this compiler supports it, or recode to avoid using local variables or parameters, or duplicate the function, for example:
void interrupt my_isr(void)
{
scan(6);
/* scan is called from an interrupt function */
}
void process(int a)
{
scan(a);
/* scan is also called from main-line code */
}
(473) function "*" is not called from specified interrupt_level
(Linker)
The indicated function is never called from an interrupt function of the same interrupt
level, for example:
#pragma interrupt_level 1
void foo(void)
{
...
}
#pragma interrupt_level 1
void interrupt bar(void)
{
// this function never calls foo()
}
(474) no psect specified for function variable/argument allocation
(Linker)
The FNCONF assembler directive which specifies to the linker information regarding the
auto/parameter block was never seen. This is supplied in the standard runtime files if
necessary. This error can imply that the correct run-time startup module was not linked.
Ensure you have used the FNCONF directive if the runtime startup module is hand-written.
(475) conflicting FNCONF records
(Linker)
The linker has seen two conflicting FNCONF directives. This directive should be specified only once, and is included in the standard runtime startup code which is normally
linked into every program.
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Error and Warning Messages
(476) fixup overflow referencing * * (location 0x* (0x*+*), size *, value 0x*)
(Linker)
The linker was asked to relocate (fixup) an item that would not fit back into the space
after relocation. See the following error message (1356) for more information.
(477) fixup overflow in expression (location 0x* (0x*+*), size *, value 0x*)
(Linker)
The linker was asked to relocate (fixup) an item that would not fit back into the space
after relocation. See the following error message (1356) for more information.
(478) * range check failed (location 0x* (0x*+*), value 0x* > limit 0x*)
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(479) circular indirect definition of symbol "*"
(Linker)
The specified symbol has been equated to an external symbol which, in turn, has been
equated to the first symbol.
(480) function signatures do not match: * (*): 0x*/0x*
(Linker)
The specified function has different signatures in different modules. This means it has
been declared differently; i.e., it can have been prototyped in one module and not
another. Check what declarations for the function are visible in the two modules
specified and make sure they are compatible, for example:
extern int get_value(int in);
/* and in another module: */
/* this is different to the declaration */
int get_value(int in, char type)
{
(481) common symbol "*" psect conflict
(Linker)
A common symbol has been defined to be in more than one psect.
(482) symbol "*" is defined more than once in "*"
(Assembler)
This symbol has been defined in more than one place. The assembler will issue this
error if a symbol is defined more than once in the same module, for example:
_next:
MOVE r0, #55
MOVE [r1], r0
_next:
; oops -- choose a different name
The linker will issue this warning if the symbol (C or assembler) was defined multiple
times in different modules. The names of the modules are given in the error message.
Note that C identifiers often have an underscore prepended to their name after
compilation.
(483) symbol "*" can’t be global
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
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(484) psect "*" can’t be in classes "*" and "*"
(Linker)
A psect cannot be in more than one class. This is either due to assembler modules with
conflicting class= options to the PSECT directive, or use of the -C option to the linker,
for example:
psect final,class=CODE
finish:
/* elsewhere: */
psect final,class=ENTRY
(485) unknown "with" psect referenced by psect "*"
(Linker)
The specified psect has been placed with a psect using the psect with flag. The psect
it has been placed with does not exist, for example:
psect starttext,class=CODE,with=rext
; was that meant to be with text?
(486) psect "*" selector value redefined
(Linker)
The selector value for this psect has been defined more than once.
(487) psect "*" type redefined: */*
(Linker)
This psect has had its type defined differently by different modules. This probably
means you are trying to link incompatible object modules, i.e., linking 386 flat model
code with 8086 real mode code.
(488) psect "*" memory space redefined: */*
(Linker)
A global psect has been defined in two different memory spaces. Either rename one of
the psects or, if they are the same psect, place them in the same memory space using
the space psect flag, for example:
psect spdata,class=RAM,space=0
ds 6
; elsewhere:
psect spdata,class=RAM,space=1
(489) psect "*" memory delta redefined: */*
(Linker)
A global psect has been defined with two different delta values, for example:
psect final,class=CODE,delta=2
finish:
; elsewhere:
psect final,class=CODE,delta=1
(490) class "*" memory space redefined: */*
(Linker)
A class has been defined in two different memory spaces. Either rename one of the
classes or, if they are the same class, place them in the same memory space.
(491) can’t find 0x* words for psect "*" in segment "*"
(Linker)
One of the main tasks the linker performs is positioning the blocks (or psects) of code
and data that is generated from the program into the memory available for the target
device. This error indicates that the linker was unable to find an area of free memory
large enough to accommodate one of the psects. The error message indicates the
name of the psect that the linker was attempting to position and the segment name
which is typically the name of a class which is defined with a linker -A option.
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Error and Warning Messages
Section 5.15.2 “Compiler-Generated Psects” lists each compiler-generated psect and
what it contains. Typically psect names which are, or include, text relate to program
code. Names such as bss or data refer to variable blocks. This error can be due to
two reasons.
First, the size of the program or the program’s data has exceeded the total amount of
space on the selected device. In other words, some part of your device’s memory has
completely filled. If this is the case, then the size of the specified psect must be
reduced.
The second cause of this message is when the total amount of memory needed by the
psect being positioned is sufficient, but that this memory is fragmented in such a way
that the largest contiguous block is too small to accommodate the psect. The linker is
unable to split psects in this situation. That is, the linker cannot place part of a psect at
one location and part somewhere else. Thus, the linker must be able to find a contiguous block of memory large enough for every psect. If this is the cause of the error, then
the psect must be split into smaller psects if possible.
To find out what memory is still available, generate and look in the map file, see
Section 4.8.7 “-M: Generate Map File” for information on how to generate a map file.
Search for the string UNUSED ADDRESS RANGES. Under this heading, look for the
name of the segment specified in the error message. If the name is not present, then
all the memory available for this psect has been allocated. If it is present, there will be
one address range specified under this segment for each free block of memory. Determine the size of each block and compare this with the number of words specified in the
error message.
Psects containing code can be reduced by using all the compiler’s optimizations, or
restructuring the program. If a code psect must be split into two or more small psects,
this requires splitting a function into two or more smaller functions (which can call each
other). These functions can need to be placed in new modules.
Psects containing data can be reduced when invoking the compiler optimizations, but
the effect is less dramatic. The program can need to be rewritten so that it needs less
variables. If the default linker options must be changed, this can be done indirectly
through the driver using the driver -L- option, see Section 4.8.6 “-L-: Adjust Linker
Options Directly”. Section 4.8.7 “-M: Generate Map File” has information on interpreting the map file’s call graph if the compiler you are using uses a compiled stack. (If the
string Call graph: is not present in the map file, then the compiled code uses a hardware stack.) If a data psect needs to be split into smaller psects, the definitions for variables will need to be moved to new modules or more evenly spread in the existing
modules. Memory allocation for auto variables is entirely handled by the compiler.
Other than reducing the number of these variables used, the programmer has little control over their operation. This applies whether the compiled code uses a hardware or
compiled stack.
For example, after receiving the message:
Can’t find 0x34 words (0x34 withtotal) for psect text
in segment CODE (error)
look in the map file for the ranges of unused memory.
UNUSED ADDRESS RANGES
CODE
RAM
00000244-0000025F
00001000-0000102f
00300014-00301FFB
In the CODE segment, there is 0x1c (0x25f-0x244+1) bytes of space available in one
block and 0x30 available in another block. Neither of these are large enough to accommodate the psect text which is 0x34 bytes long. Notice, however, that the total amount
of memory available is larger than 0x34 bytes.
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(492) attempt to position absolute psect "*" is illegal
(Linker)
This psect is absolute and should not have an address specified in a -P option. Either
remove the abs psect flag, or remove the -P linker option.
(493) origin of psect "*" is defined more than once
(Linker)
The origin of this psect is defined more than once. There is most likely more than one
-p linker option specifying this psect.
(494) bad -P format "*/*"
(Linker)
The -P option given to the linker is malformed. This option specifies placement of a
psect, for example:
-Ptext=10g0h
Possibly, you meant:
-Ptext=10f0h
(495) use of both "with=" and "INCLASS/INCLASS" allocation is illegal
(Linker)
It is not legal to specify both the link and location of a psect as within a class, when that
psect was also defined using a with psect flag.
(497) psect "*" exceeds max size: *h > *h
(Linker)
The psect has more bytes in it than the maximum allowed as specified using the size
psect flag.
(498) psect "*" exceeds address limit: *h > *h
(Linker)
The maximum address of the psect exceeds the limit placed on it using the limit
psect flag. Either the psect needs to be linked at a different location or there is too much
code/data in the psect.
(499) undefined symbol:
(Assembler, Linker)
The symbol following is undefined at link time. This could be due to spelling error, or
failure to link an appropriate module.
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Error and Warning Messages
MESSAGES 500-749
(500) undefined symbols:
(Linker)
A list of symbols follows that were undefined at link time. These errors could be due to
spelling error, or failure to link an appropriate module.
(501) program entry point is defined more than once
(Linker)
There is more than one entry point defined in the object files given the linker. End entry
point is specified after the END directive. The runtime startup code defines the entry
point, for example:
powerup:
goto start
END powerup ; end of file and define entry point
; other files that use END should not define another entry point
(502) incomplete * record body: length = *
(Linker)
An object file contained a record with an illegal size. This probably means the file is
truncated or not an object file. Contact Microchip Technical Support with details.
(503) ident records do not match
(Linker)
The object files passed to the linker do not have matching ident records. This means
they are for different device types.
(504) object code version is greater than *.*
(Linker)
The object code version of an object module is higher than the highest version the
linker is known to work with. Check that you are using the correct linker. Contact
Microchip Technical Support if you have not patched the linker.
(505) no end record found inobject file
(Linker)
An object file did not contain an end record. This probably means the file is corrupted
or not an object file. Contact Microchip Technical Support if the object file was
generated by the compiler.
(506) object file record too long: *+*
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(507) unexpected end of file in object file
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(508) relocation offset (*) out of range 0..*-*-1
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(509) illegal relocation size: *
(Linker)
There is an error in the object code format read by the linker. This either means you are
using a linker that is out of date, or that there is an internal error in the assembler or
linker. Contact Microchip Technical Support with details if the object file was created by
the compiler.
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(510) complex relocation not supported for -R or -L options
(Linker)
The linker was given a -R or -L option with file that contain complex relocation.
(511) bad complex range check
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(512) unknown complex operator 0x*
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(513) bad complex relocation
(Linker)
The linker has been asked to perform complex relocation that is not syntactically
correct. Probably means an object file is corrupted.
(514) illegal relocation type: *
(Linker)
An object file contained a relocation record with an illegal relocation type. This probably
means the file is corrupted or not an object file. Contact Microchip Technical Support
with details if the object file was created by the compiler.
(515) unknown symbol type *
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(516) text record has bad length: *-*-(*+1) < 0
(Linker)
This is an internal compiler error. Contact Microchip Technical Support with details.
(520) function "*" is never called
(Linker)
This function is never called. This cannot represent a problem, but space could be
saved by removing it. If you believe this function should be called, check your source
code. Some assembler library routines are never called, although they are actually execute. In this case, the routines are linked in a special sequence so that program
execution falls through from one routine to the next.
(521) call depth exceeded by function "*"
(Linker)
The call graph shows that functions are nested to a depth greater than specified.
(522) library "*" is badly ordered
(Linker)
This library is badly ordered. It will still link correctly, but it will link faster if better
ordered.
(523) argument to -W option (*) illegal and ignored
(Linker)
The argument to the linker option -w is out of range. This option controls two features.
For warning levels, the range is -9 to 9. For the map file width, the range is greater than
or equal to 10.
(524) unable to open list file "*": *
(Linker)
The named list file could not be opened. The linker would be trying to fixup the list file
so that it will contain absolute addresses. Ensure that an assembler list file was generated during the compilation stage. Alternatively, remove the assembler list file
generation option from the link step.
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Error and Warning Messages
(525) too many address (memory) spaces; space (*) ignored
(Linker)
The limit to the number of address spaces (specified with the PSECT assembler
directive) is currently 16.
(526) psect "*" not specified in -P option (first appears in "*")
(Linker)
This psect was not specified in a -P or -A option to the linker. It has been linked at the
end of the program, which is probably not where you wanted it.
(528) no start record; entry point defaults to zero
(Linker)
None of the object files passed to the linker contained a start record. The start address
of the program has been set to zero. This can be harmless, but it is recommended that
you define a start address in your startup module by using the END directive.
(529) usage: objtohex [-Ssymfile] [object-file [HEX-file]]
(Objtohex)
Improper usage of the command-line tool objtohex. If you are not invoking this tool
directly, this is an internal compiler error, and you should contact Microchip Technical
Support with details.
(593) can’t find 0x* words (0x* withtotal) for psect "*" in segment "*"
(Linker)
See message (491).
(594) undefined symbol:
(Linker)
The symbol following is undefined at link time. This could be due to spelling error, or
failure to link an appropriate module.
(595) undefined symbols:
(Linker)
A list of symbols follows that were undefined at link time. These errors could be due to
spelling error, or failure to link an appropriate module.
(596) segment "*" (*-*) overlaps segment "*" (*-*)
(Linker)
The named segments have overlapping code or data. Check the addresses being
assigned by the -P linker option.
(599) No psect classes given for COFF write
(Cromwell)
CROMWELL requires that the program memory psect classes be specified to produce a
COFF file. Ensure that you are using the -N option.
(600) No chip arch given for COFF write
(Cromwell)
CROMWELL requires that the chip architecture be specified to produce a COFF file.
Ensure that you are using the -P option.
(601) Unknown chip arch "*" for COFF write
(Cromwell)
The chip architecture specified for producing a COFF file isn’t recognized by
CROMWELL. Ensure that you are using the -P option, and that the architecture is
correctly specified.
(602) null file format name
(Cromwell)
The -I or -O option to CROMWELL must specify a file format.
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(603) ambiguous file format name "*"
(Cromwell)
The input or output format specified to CROMWELL is ambiguous. These formats are
specified with the -i key and -o key options respectively.
(604) unknown file format name "*"
(Cromwell)
The output format specified to CROMWELL is unknown, for example:
cromwell -m -P16F877 main.HEX main.sym -ocot
and output file type of cot, did you mean cof?
(605) did not recognize format of input file
(Cromwell)
The input file to CROMWELL is required to have a Cromwell map file (CMF), COD, Intel
HEX, Motorola HEX, COFF, OMF51, ELF, UBROF or HI-TECH format.
(606) inconsistent symbol tables
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(607) inconsistent line number tables
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(608) bad path specification
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(609) missing device spec after -P
(Cromwell)
The -p option to CROMWELL must specify a device name.
(610) missing psect classes after -N
(Cromwell)
CROMWELL requires that the -N option be given a list of the names of psect classes.
(611) too many input files
(Cromwell)
To many input files have been specified to be converted by CROMWELL.
(612) too many output files
(Cromwell)
To many output file formats have been specified to CROMWELL.
(613) no output file format specified
(Cromwell)
The output format must be specified to CROMWELL.
(614) no input files specified
(Cromwell)
CROMWELL must have an input file to convert.
(616) option -Cbaseaddr is illegal with options -R or -L
(Linker)
The linker option -Cbaseaddr cannot be used in conjunction with either the -R or -L
linker options.
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Error and Warning Messages
(618) error reading COD file data
(Cromwell)
An error occurred reading the input COD file. Confirm the spelling and path of the file
specified on the command line.
(619) I/O error reading symbol table
(Cromwell)
The COD file has an invalid format in the specified record.
(620) filename index out of range in line number record
(Cromwell)
The COD file has an invalid value in the specified record.
(621) error writing ELF/DWARF section "*" on "*"
(Cromwell)
An error occurred writing the indicated section to the given file. Confirm the spelling and
path of the file specified on the command line.
(622) too many type entries
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(623) bad class in type hashing
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(624) bad class in type compare
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(625) too many files in COFF file
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(626) string lookup failed in COFF: get_string()
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(627) missing "*" in SDB file "*" line * column *
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(629) bad storage class "*" in SDB file "*" line * column *
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(630) invalid syntax for prefix list in SDB file "*"
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(631) syntax error at token "*" in SDB file "*" line * column *
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
(632) can’t handle address size (*)
(Cromwell)
This is an internal compiler error. Contact Microchip Technical Support with details.
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(633) unknown symbol class (*)
(Cromwell)
CROMWELL has encountered a symbol class in the symbol table of a COFF, Microchip
COFF, or ICOFF file which it cannot identify.
(634) error dumping "*"
(Cromwell)
Either the input file to CROMWELL is of an unsupported type or that file cannot be
dumped to the screen.
(635) invalid HEX file "*" on line *
(Cromwell)
The specified HEX file contains an invalid line. Contact Microchip Technical Support if
the HEX file was generated by the compiler.
(636) error in Intel HEX file "*" on line *
(Cromwell, Hexmate)
An error was found at the specified line in the specified Intel HEX file. The HEX file may
be corrupt.
(637) unknown prefix "*" in SDB file "*"
(Cromwell)
This is an internal compiler warning. Contact Microchip Technical Support with details.
(638) version mismatch: 0x* expected
(Cromwell)
The input Microchip COFF file wasn’t produ">