AN 188: Custom Instructions for the Nios Embedded Processor

Custom Instructions
for the Nios Embedded
Processor
September 2002, ver. 1.2
Introduction
Application Note 188
With the Altera® Nios® embedded processor, system designers can
accelerate time-critical software algorithms by adding custom
instructions to the Nios instruction set. System designers can use custom
instructions to implement complex processing tasks in single-cycle
(combinatorial) and multi-cycle (sequential) operations. Additionally,
user-added custom instruction logic can access memory and/or logic
outside of the Nios system.
Using custom instructions, system designers can reduce a complex
sequence of standard instructions to a single instruction implemented in
hardware. System designers can use this feature for a variety of
applications, e.g., to optimize software inner loops for digital signal
processing (DSP), packet header processing, and computation-intensive
applications. The Nios CPU configuration wizard, which is accessed from
the SOPC Builder, provides a graphical user interface that system
designers can use to add up to five custom instructions to the Nios
processor.
This application note describes the Nios custom instruction feature,
provides a design example, and describes how to implement custom
instructions.
Understanding
Custom
Instructions
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AN-188-1.2
With custom instructions, system designers can add custom-defined
functionality to the Nios processor’s arithmetic logic unit (ALU) and
instruction set (see Figure 1 on page 2). Custom instructions consist of two
essential elements:
■
Custom logic block—Hardware that performs the operation. The Nios
processor can include up to five user-defined custom logic blocks.
The blocks become part of the Nios microprocessor’s ALU.
■
Software macro—Allows the system designer to access the custom
logic through software code.
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AN 188: Custom Instructions for the Nios Embedded Processor
Figure 1. Adding Custom Logic to the Nios ALU
To FIFO, Memory, or Other Logic
A
B
A
Custom
Logic
Nios
ALU
+
-
<<
>>
Out
&
B
Nios Embedded Processor
The custom logic block performs a user-defined operation (represented as
op in the custom instruction table of the Nios configuration wizard) on the
contents of two registers (Ra and Rb) and stores the result in Ra, for
example, Ra <- Ra op Rb. You can design a custom instruction logic
block to perform any function, as long as the logic block has the
appropriate interface as described in this document. See “Hardware
Interface” on page 3 for more details. See “Software Interface” on page 7
for information on accessing your custom instruction logic block from
software.
The Nios configuration wizard integrates the custom logic blocks with the
Nios processor’s ALU when building the Nios embedded processor (see
Figure 2). The Nios configuration wizard also creates software macros in
C/C++ and Assembly, providing software access to these custom logic
blocks. You must provide the name of the macro. If the custom instruction
is combinatorial, the number of clock cycles needed to perform the
instruction is fixed at 1. If the custom instruction is sequential, you must
provide the number of clock cycles.
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AN 188: Custom Instructions for the Nios Embedded Processor
Figure 2. Custom Instructions Tab in the Nios Configuration Wizard
Hardware Interface
You can create custom logic blocks using the following formats:
■
■
■
■
■
Verilog HDL
VHDL
EDIF netlist file
Quartus II Block Design File (.bdf)
Verilog Quartus Mapping File (.vqm)
Because the block connects directly to the ALU, it must provide an
interface with predefined ports and names (see Figure 3). The Nios
configuration wizard scans the custom logic blocks, searches for the
required ports, and connects these ports to the ALU.
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AN 188: Custom Instructions for the Nios Embedded Processor
Figure 3. Custom Logic Block Interface (32-Bit Nios Processor)
Optional FIFO, Memory, or Other Logic
32
dataa
32
Combinatorial
32
result
datab
clk
clk_en
reset
Multi-Cycle
start
11
prefix
Parameterized
You can implement custom logic blocks using one or a combination of
these four options:
■
■
■
■
Combinatorial logic
Multi-cycle logic
Parameterization
User-defined ports
Combinatorial Logic Option
Using the combinatorial logic option, the custom logic block must
complete the operation in a single clock cycle. Because the transaction
takes one clock cycle to complete, the block only requires data ports, and
does not require control signals (see Table 1).
1
4
If the block only requires one input port, use the dataa port.
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AN 188: Custom Instructions for the Nios Embedded Processor
Table 1. Ports for Combinatorial Custom Logic
Port Name
Width (Bits)
Direction
Description
dataa
CPU width
Input
Operand
datab
CPU width
Input
Operand (optional)
result
CPU width
Output
Result
Multi-Cycle Logic Option
For multi-cycle or sequential logic, the custom logic block must provide
the data ports shown in Table 1 as well as control signal ports (see
Table 2). These control signals provide status to the custom logic block
and synchronize it with the Nios processor. When you add the custom
instruction in the Nios configuration wizard, you must enter the number
of clock cycles the block requires to perform the operation.
Table 2. Additional Ports for Sequential Custom Logic
Port Name Width (Bits) Direction
Description
clk
1
Input
CPU master input clock, which is fed by the Nios system clock.
reset
1
Input
CPU master asynchronous reset, which is fed by the Nios master reset.
reset is only asserted when the Nios system is reset.
clk_en
1
Input
Clock qualifier. The custom logic block should use the clk_en signal as
a conventional clock qualifier signal and should ignore all rising clock
edges when clk_en is not asserted.
start
1
Input
Instructs the block to latch data and begin operation.
The CPU asserts start on the first clock cycle the operation executes
when the ALU issues the custom instruction. At this time, dataa and
datab hold valid values. The CPU waits for the required number of clock
cycles specified in the Nios configuration wizard and then reads result.
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AN 188: Custom Instructions for the Nios Embedded Processor
Parameterization Option
You can use the optional 11-bit prefix port (see Table 3) to pass a
parameter to the custom instruction. The prefix port uses the Nios PFX
instruction to load the 11-bit K register before the custom instruction
executes. The custom logic block receives the K register data upon
execution of the custom instruction.
Table 3. Prefix Port
Port Name
Width (Bits)
Direction
Description
prefix
11
Input
Payload of the K register.
You can use the prefix port with either combinatorial or multi-cycle
custom logic blocks. The interpretation of the prefix data is unspecified,
so you can use it for a variety of purposes. In extreme cases, you can
encode prefix to represent up to 2,048 different functions contained in a
single custom instruction.
User-Defined Ports Option
Optional user-defined ports allow the custom instruction to interact with
components outside of the Nios system. If the Nios configuration wizard
does not recognize a port, it routes the port to the top level of the system
module where external logic can access the signals.
Combining Options
Using a combination of multi-cycle ports, the prefix port, and userdefined ports, you can implement extremely versatile operations,
including cases in which a custom instruction’s operation is dependent
upon previous instances of the instruction. Example applications include:
6
■
A custom multiply accumulate (MAC) instruction. The result of each
multiplication is added to a running total. You can use multiple clock
cycles to pipeline the function, increasing system performance as
needed. The prefix port resets the MAC to zero (a value of zero in
software corresponds to a logic low in hardware).
■
A custom packet processing instruction. The custom instruction
receives up to 64 bits of data using dataa and datab in parallel.
Based on a command specified using prefix, the packet processing
logic could operate on the 64-bit packet (e.g., mask bits in the header)
and quickly inject the packet back into the data stream by outputting
the result via a user-defined port. A system can perform bit-compare,
bit-modify, and output-to-datastream (i.e., write-to-FIFO) operations
in one or two clock cycles.
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AN 188: Custom Instructions for the Nios Embedded Processor
Because the custom instruction logic is integrated with the Nios CPU, the
design of the custom instruction directly affects the fMAX performance of
the entire Nios CPU. You can pipeline the custom instruction logic so that
it does not become a performance bottleneck in a Nios system.
Software Interface
After you integrate a custom logic block into the Nios processor’s ALU,
you can access the custom logic through software. The Nios architecture
includes five user-definable opcodes (see Table 4) with which you can
access the custom logic block. You call these opcodes in software—written
in C/C++ or Assembly—using macros.
Table 4. User Opcode, Type & Format
Opcode
Type
Format
USR0
RR
Ra <- Ra op Rb
USR1
Rw
Ra <- Ra op %r0
USR2
Rw
Ra <- Ra op %r0
USR3
Rw
Ra <- Ra op %r0
USR4
Rw
Ra <- Ra op %r0
As described earlier, the custom instruction takes the contents of two
general-purpose registers (Ra and Rb) and performs an operation as
defined by the custom logic block. The result of this operation is then
stored in a general-purpose register (Ra).
The USR0 opcode is of type RR, which can use any of the general-purpose
registers for Ra and Rb. USR1 through USR4 are type Rw opcodes, where
Ra can be any of the general-purpose registers. However, Rb must be the
%r0 register.
f
For more information on opcodes and their usage, refer to the Nios 16-Bit
Programmer’s Reference Manual and the Nios 32-Bit Programmer’s Reference
Manual.
When writing C/C++ code, the register usage is transparent, because the
compiler automatically chooses the registers. However, in Assembly, you
must indicate which registers are used for a particular operation.
The Nios configuration wizard automatically builds the macro after you
add the custom instruction. The wizard supports macro naming for ease
of use and readability of your software code.
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AN 188: Custom Instructions for the Nios Embedded Processor
Using Custom Instructions in C/C++
When using custom instructions in C/C++, you access the custom
instruction with a function call. Your Nios system header file
(excalibur.h), which the SOPC Builder generates, includes the C/C++
macro definition. There are two different C/C++ macros, one for use with
prefix:
nm_<name>_pfx(prefix, dataa, datab)
and one for use without prefix:
nm_<name>(dataa, datab)
If your custom instruction uses the prefix port, the only valid input to
prefix is an immediate value of 11 bits or less. If the C/C++ macro does
not pass a value for prefix, the custom logic’s prefix port is loaded
with zero. Figure 4 shows an example using a custom instruction
(my_cust_inst) with and without a prefix.
Figure 4. C Code Using a Custom Instruction
int main(void)
{
unsigned short a = 4660;
unsigned short b = 17185;
unsigned int return_value = 0;
unsigned int return_value_1 = 0;
printf ("\n\nWill call the My_Cust_Inst function ...\n");
return_value = nm_my_cust_inst(a, b);
//If a prefix is not given, the
//value is 0x000
return_value_1 = nm_my_cust_inst_pfx(1, a, b);
printf("After the call, return_value = \n", return_value);
printf("After the call, return_value_1 = %d \n", return_value_1);
return (0);
}
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AN 188: Custom Instructions for the Nios Embedded Processor
Using Custom Instructions in Assembly
When using custom instruction in Assembly, you can use either the
Assembly macro or the opcode to call the custom instruction. To use the
macro, include the Nios header file (excalibur.h), which the SOPC Builder
generates. If you use prefix, the PFX instruction must precede the
custom instruction macro, otherwise zero is loaded into the prefix port.
Figure 5 shows an example using a custom instruction (my_cust_inst)
with the PFX instruction. When using an Rw opcode, the opcode
automatically loads the %r0 register for the datab port.
Figure 5. Assembly Code Using A Custom Instruction
LD %r1,[%L6]
LD %r0,[%L2]
PFX 1
nm_my_cust_inst %r1
ST [%L4],%r1
Design
Example
;
;
;
;
;
Load word at [%L6] into %r1
Load word at [%L2] into %r0
Only needed if using prefix
Macro calling a Rw opcode, r1 <- r1 “OP” r0
%L4 is the pointer, %r1 is stored
The following example, which is an excerpt of a reference design provided
with the Nios embedded processor, shows the benefits of using custom
instructions versus a software-only implementation. The design uses a
simple four-function floating-point unit (FPU) created in hardware as a
custom instruction. The FPU implements the signed multiply, multiply
with negate, absolute value, and negate floating-point operations. The
prefix port specifies the different operations of the FPU.
All four functions are compared with their respective software
implementation. A timer measures the clock cycles required for each
operation. Sample code for the multiplication is provided in Figure 6. For
the complete hardware and software design used in this example, refer to
the reference design in the <Nios installation
directory>\tutorials\CI_Tutorial\Cust_Inst_Example1 directory.
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AN 188: Custom Instructions for the Nios Embedded Processor
Figure 6. Multiplication with Custom Instructions
/******************/
/* Multiplication */
/******************/
a=-32.57;
b=300.84;
dwStartTick=GetTickCount(); /* record start time*/
res_a=a*b;
lTicksUsed=GetTickCount(); /* record end time */
printf("\nFor Nios
printf("\nValue of
printf("\nValue of
printf("\nValue of
software implementation:");
a is: %f", a);
b is: %f", b);
a*b is: %f", res_a);
CheckTimeStamp (dwStartTick, lTicksUsed);
our_dwStartTick=GetTickCount(); /* record start time*/
res_a = nm_fpu_pfx(2, a, b)
our_lTicksUsed=GetTickCount(); /* record end time */
printf("\nFor the floating point module: ");
printf("\nValue of a is: %f", a);
printf("\nValue of b is: %f", b);
printf("\nValue of a*b is: %f", res_a);
CheckTimeStamp (our_dwStartTick, our_lTicksUsed);
Table 5 compares the performance of the custom instruction versus a
software-only implementation. The custom instruction increases the
performance by as much as two orders of magnitude over the softwareonly implementations.
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AN 188: Custom Instructions for the Nios Embedded Processor
Table 5. Custom Instruction vs. Software-Only Performance Comparison (1)
Floating-Point Operation
CPU Clock Cycles
Speed
Increase
Software
Library
Custom
Instruction (FPU)
Multiplication a × b
2,874
19
151.26
Multiply and Negate -(a × b)
3,147
19
165.63
Absolute |a|
1,769
18
98.28
Negate -(a)
284
19
14.95
Note:
(1)
These performance calculations are compiler-dependant. They were taken using
the Cygnus compiler included in Version 2.1 of the Nios embedded processor.
Additionally, custom instructions reduce the software code needed to
perform the operations:
■
■
Custom instruction—200 bytes
Software-only—4 KBytes
In this example, a standard math library performs the software
implementation for floating-point math. However, there are many custom
applications for which a predefined library is not available. In these cases,
you must create the functionality in C/C++ or Assembly, further
increasing the complexity of the software code.
By using custom instructions, you only need to call the software macro to
interface with hardware that completes the task, reducing both the
complexity and size of the software code.
Implementing
Custom
Instructions
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This section describes the process of implementing a custom instruction
and includes the following steps:
■
■
■
“Create a Custom Logic Block” on page 12
“Instantiate the Custom Instruction” on page 14
“Create Software Code Using Macros” on page 18
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AN 188: Custom Instructions for the Nios Embedded Processor
Create a Custom Logic Block
To begin, create a logic block that performs the functionality you want.
You control the design and operation of the custom logic block, however,
you must follow these guidelines:
■
■
■
■
File Format
Port Naming
Port Operation
Multi-Cycle Signal Timing
File Format
When creating your logic block, you must use one of the following file
formats:
■
■
■
■
■
Verilog HDL
VHDL
EDIF netlist file
Quartus II .bdf
.vqm
1
If you black box design file(s), make sure to include the design
file(s) in your Quartus II project.
Port Naming
When designing the logic block, you must include all of the required ports
for the type of operation the block will perform (i.e., combinatorial or
multi-cycle). Table 6 shows the required ports for each type of operation.
1
You must use these predefined port names so that the ports
connect to the proper interface.
Table 6. Custom Instruction Ports
Port
Width (Bits)
Direction
Combinatorial
Multi-Cycle
dataa
CPU width
Input
Required
Required
datab
CPU width
Input
Optional
Optional
result
CPU width
Output
Required
Required
clk
1
Input
–
Required
reset
1
Input
–
Required
clk_en
1
Input
–
Required
start
1
Input
–
Required
prefix
11
Input
Optional
Optional
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AN 188: Custom Instructions for the Nios Embedded Processor
The ports your block requires depend on whether you implement
combinatorial or multi-cycle logic. Use combinatorial logic if the required
operation can execute in a single CPU clock cycle. If the logic block cannot
meet system frequency requirements for a single clock cycle or if is
intrinsically a multi-cycle operation, you can pipeline the block. In these
cases, you must use multi-cycle logic.
1
Adding custom logic blocks to the Nios ALU can affect the
system frequency. Even if the custom logic block meets the
system frequency requirements as standalone block, when you
place it in the Nios system, changes in place and route may affect
the performance.
Port Operation
You should design your logic block so that the ports operate as described
below:
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■
For combinatorial logic, the CPU presents the data from the dataa
and datab ports for execution on the rising edge of the CPU clock.
The CPU reads the result port on the following rising edge of the
CPU clock.
■
For multi-cycle logic, the CPU asserts the start port on the first
clock cycle of execution when the custom instruction issues through
the ALU. At this time, dataa and datab have valid values. For all
subsequent clock cycles, dataa and datab have undefined values.
The CPU waits for the required number of clock cycles specified in
the Nios configuration wizard for the custom instruction, and then
reads the result port. See Figure 7.
■
The Nios system clock feeds the block’s clk port, and the Nios
master reset feeds the reset port. reset is asserted only when the
whole Nios system is reset, e.g., during a watchdog timer time out.
The logic block should use the clk_en signal as a conventional clockqualifier signal and should ignore all clock rising edges during which
clk_en is deasserted.
■
You can use the optional prefix port with either combinatorial or
multi-cycle logic; the interpretation of the 11-bit prefix data is not
specified. The prefix port has a valid value on the first clock cycle
of the execution when the custom instruction issues through the
ALU. For all subsequent clock cycles, prefix has an undefined
value.
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AN 188: Custom Instructions for the Nios Embedded Processor
■
You can add other ports to your logic block, which allows your
custom logic block to access logic that is external to the Nios CPU.
Any ports that are not recognized by the Nios configuration wizard
are routed to the top level of the Nios system module. These ports are
labeled export.
Multi-Cycle Signal Timing
Before instantiating your logic block, you must determine the number of
CPU clock cycles your block requires. Combinatorial logic requires one
clock cycle. For multi-cycle logic, you can simulate the logic block to verify
the required number of clock cycles. For example, Figure 7 shows a logic
block that uses five clock cycles. The CPU executes the custom instruction
on the T0 clock edge, asserting the start bit and providing valid data for
the next clock edge (T1). Five clock cycles later, the custom instruction
provides a valid result. If you were instantiating the logic block in this
example, you would specify five clock cycles in the Nios wizard.
Figure 7. Multi-Cycle Timing Example (5 CPU Clock Cycles)
T0
T1
T2
T3
T4
T5
clk
clk_en
start
reset
dataa
datab
prefix
result
VALID
VALID
VALID
VALID
Instantiate the Custom Instruction
You use the SOPC Builder in the Quartus II software to create a Nios
embedded processor (CPU), configure system peripherals, and connect
these elements to make a Nios system module. When you add a Nios CPU
to the SOPC Builder, the Nios configuration wizard displays.
f
14
For detailed instructions on creating a Nios design, including using the
SOPC Builder and adding a Nios CPU, refer to the Nios Tutorial.
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AN 188: Custom Instructions for the Nios Embedded Processor
Perform the following steps to add a custom instruction to the CPU.
1.
Turn on Enable advanced configuration controls in the
Architecture tab. See Figure 8.
Figure 8. Nios CPU Architecture Tab
Turn on the Enable
advanced configuration
controls option
2.
Click the Custom Instructions tab.
3.
Select the opcode (USR0 through USR4) that you want to use for the
custom instruction.
4.
Click Import. See Figure 9. The Interface to User Logic displays.
Figure 9. Nios CPU Custom Instructions Tab
Select the
opcode
Import
Button
5.
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Click Add under Design Files. See Figure 10.
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AN 188: Custom Instructions for the Nios Embedded Processor
6.
Browse to the directory in which you saved the design file(s) for
your logic block.
7.
Select all of the logic block design files and click Open.
8.
Enter the name of logic block’s top-level module in the Top module
name box. See Figure 10.
9.
Click Populate Port Table. The wizard scans the files and imports
the port information. The ports are displayed under Port
Information. See Figure 10.
1
If the wizard does not scan the files correctly, you can
manually enter the port names, widths, directions, and
types.
Figure 10. Interface to User Logic
Add button
Logic block
design files
Base macro name
Imported port
information
Populate Port Table
button
10. If your logic block has a different file format than your Nios system
module, click the Instantiation tab. See Figure 11 on page 17.
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AN 188: Custom Instructions for the Nios Embedded Processor
11. Select Instantiate as Black Box. See Figure 11.
1
The Instantiate as Black Box option is turned on
automatically if your design files are .bdf, EDIF Input Files
(.edf), or .vqm.
Figure 11. Instantiation as Black Box
12. Click Add to System. You are returned to the Nios configuration
wizard Custom Instruction tab.
13. Enter the base name that you would like to use to call your custom
instruction in C/C++ or Assembly in the macro Name box. The
wizard inputs the first four letters of the top-level module by default.
The actual macro name will be nm_<base name>. See Figure 12.
1
The wizard makes all macro names lower case.
14. Enter the number of clock cycles required to perform the operation
in the Cycle Count box. See Figure 12.
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AN 188: Custom Instructions for the Nios Embedded Processor
Figure 12. Macro Name & Clock Cycles
Base
macro
name
Enter the
clock cycles
15. Repeat steps 3 through 14 for any additional custom instructions.
16. Click Finish when you are finished adding custom instructions.
17. Generate your Nios system module.
Create Software Code Using Macros
When you generate your Nios system module, the SOPC Builder creates
software macros that you can use in your software code. The macros have
the prefix nm_, which stands for Nios macro. The following sections
describe how to use these macros in your C/C++ or Assembly code.
Using Macros in C/C++
For C/C++, you use the macros as a function call. Before using the macro,
you must include the file excalibur.h, which is created by the SOPC
Builder, in your code. Figure 13 shows an example macro the SOPC
Builder created for a custom instruction.
Figure 13. C/C++ Example Macro
#define nm_fpu(_x,_y) ({\
int __x = (_x), __y = (_y);\
asm volatile("usr0 %0,%2 ; does fpu" \
: "=r" (__x) \
: "0" (__x), "r" (__y));\
__x;\
})
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AN 188: Custom Instructions for the Nios Embedded Processor
When the SOPC Builder creates the macro, it assigns the inputs and
outputs to type int (integer). If the inputs and outputs are not integers,
you can manually edit the macro in the excalibur.h file to change int to
another type (e.g., float). Alternatively, you can use type casting, which
you would perform on the macro inputs and outputs. Figure 14 shows an
example in which a macro takes in a signal of type float and changes it
to type int (and vice versa) without changing any of the data bits.
Figure 14. Changing Signal Types
#define take_float_as_int32(x) (*((int *)(&(x))))
#define take_int32_as_float(i) (*((float *)(&(i))))
When including a macro in your code, use the following format:
result = nm_<name>(dataa, datab);
If you only want to use one input port, leave out the datab input:
result = nm_<name>(dataa);
If you are using the prefix port, the SOPC Builder creates a second
macro with the suffix _pfx. The only valid input for prefix is an
immediate value, which is a literal constant from 0 to 2,047 (11-bits). You
must enter this value directly into the macro; it cannot be defined
previously. Figure 15 provides an example macro with a prefix.
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AN 188: Custom Instructions for the Nios Embedded Processor
Figure 15. C/C++ Example Macro with Prefix
#define nm_fpu_pfx(_p,_x,_y) ({\
int __x = (_x), __y = (_y);\
asm volatile("pfx " #_p "\n\tusr0 %0,%2 ; does fpu" \
: "=r" (__x) \
: "0" (__x), "r" (__y));\
__x;\
})
When including a macro with a prefix in your code, use the following
format:
result = nm_<name>_pfx(prefix, dataa, datab);
or
result = nm_<name>_pfx(prefix, dataa);
If your custom instruction has a prefix port but you do not want to use
it, you should use the format for the non-prefix macro. In this case, an 11bit zero is provided to the prefix port of the custom instruction
operation. This scenario is useful for situations in which your default
prefix is zero because you do not have to load the prefix register
before your custom instruction executes.
Using Macros in Assembly
In Assembly, you can call your custom instruction using a macro or an
opcode. To use the macro, you must include the excalibur.s file, which is
created by the SOPC Builder, in your code.
You assign the opcode (USR0 through USR4) when you instantiate the
custom instruction (refer to “Instantiate the Custom Instruction” on
page 14). The macro calls the associated USR opcode and it is used in the
same manner as the opcode.
For USR0, which is of type RR, you can input any of the 32 generalpurpose registers for %Ra and %Rb. The resulting value is stored in the first
register, %Ra. The format to use is:
nm_<name> %Ra %Rb
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AN 188: Custom Instructions for the Nios Embedded Processor
For USR1 through USR4, which are of type Rw, you can input any of the 32
general-purpose registers for %Ra. %Rb must be %r0 explicitly, and must
be loaded before the opcode executes. The resulting value is stored in the
general-purpose register %Ra. See Figure 16.
Figure 16. Assembly Example Macro
MOV %r0, %Rb
nm_<name> %Ra
If your custom instruction uses the prefix port, you must use the PFX
instruction to enter data into this port. In the code, the PFX instruction
must be located before the Assembly macro or the opcode because the
PFX instruction only affects the next instruction. If you do not use the PFX
instruction, an 11-bit zero is entered into your custom logic’s prefix
port. See Figure 17.
Figure 17. Assembly Example Macro with Prefix
MOV %r0, %Rb
PFX IMM11
nm_<name> %Ra
When you are finished writing your software code, compile it using the
nios-build utility as you would with any other Nios program.
Custom
Instruction
Templates
Altera Corporation
Figures 18 and 19 provide VHDL and Verilog HDL template files that you
can reference when writing custom instructions in these languages. You
can download these template files from the Altera web site at
http://www.altera.com/nios.
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AN 188: Custom Instructions for the Nios Embedded Processor
Figure 18. VHDL Template
LIBRARY __library_name;
USE __library_name.__package_name.ALL;
ENTITY __entity_name IS
PORT(
signal
signal
signal
signal
signal
signal
signal
signal
clk : IN STD_LOGIC;
-- CPU's master-input clk <required for multi-cycle>
reset : IN STD_LOGIC; -- CPU's master asynchronous reset <required for multi-cycle>
clk_en: IN STD_LOGIC; -- Clock-qualifier <required for multi-cycle>
start: IN STD_LOGIC;
-- True when this instr. issues <required for multi-cycle>
dataa: IN STD_LOGIC_VECTOR (31 DOWNTO 0);
-- operand A <always required>
datab: IN STD_LOGIC_VECTOR (31 DOWNTO 0);
-- operand B <optional>
prefix: IN STD_LOGIC_VECTOR (10 DOWNTO 0);
-- prefix <optional>
result : OUT STD_LOGIC_VECTOR (31 DOWNTO 0)
-- result <always required>
);
END __entity_name;
ARCHITECTURE a OF __entity_name IS
signal clk: IN STD_LOGIC;
signal reset : IN STD_LOGIC;
signal clk_en: IN STD_LOGIC;
signal start: IN STD_LOGIC;
signal dataa: IN STD_LOGIC_VECTOR (31 DOWNTO 0);
signal datab: IN STD_LOGIC_VECTOR (31 DOWNTO 0);
signal prefix: IN STD_LOGIC_VECTOR (10 DOWNTO 0);
signal result : OUT STD_LOGIC_VECTOR (31 DOWNTO 0);
BEGIN
-- Process Statement
-- Concurrent Procedure Call
-- Concurrent Signal Assignment
-- Conditional Signal Assignment
-- Selected Signal Assignment
-- Component Instantiation Statement
-- Generate Statement
END a;
22
Altera Corporation
AN 188: Custom Instructions for the Nios Embedded Processor
Figure 19. Verilog HDL Template
module __module_name(
clk,
//
reset,
//
clk_en,
//
start,
//
dataa,
//
datab,
//
prefix,
//
result
//
);
input
input
input
input
input [31:0]
input [31:0]
input [10:0]
CPU's master-input clk <required for multi-cycle>
CPU's master asynchronous reset <required for multi-cycle>
Clock-qualifier <required for multi-cycle>
True when this instr. issues <required for multi-cycle>
operand A <always required>
operand B <optional>
prefix <optional>
result <always required>
clk;
reset;
clk_en;
start;
dataa;
datab;
prefix;
output [31:0]result;
// Port Declaration
// Wire Declaration
// Integer Declaration
// Concurrent Assignment
// Always Construct
endmodule
Conclusion
Custom instructions are a powerful tool that allow you to customize the
Nios embedded processor for a particular application. This customization
can increase the performance of Nios systems dramatically, while
reducing the size and complexity of the software.
Documentation
Feedback
Altera values your feedback. If you would like to provide feedback on this
document—e.g., clarification requests, inaccuracies, or inconsistencies—
send e-mail to nios_docs@altera.com.
Altera Corporation
23
AN 188: Custom Instructions for the Nios Embedded Processor
101 Innovation Drive
San Jose, CA 95134
(408) 544-7000
http://www.altera.com
Applications Hotline:
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Literature Services:
lit_req@altera.com
24
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