Digital Equipment Corporation ALGOL Programmer's Reference Manual

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ALGOL
PROGRAMME
REFERENCE MANUAL
gecsyUscennic
ALGOL
PROGRAMMER'S REFERENCE MANUAL
This manual reflects Version 6 of the ALGOL System.
Additional copies of this manual may be ordered from: Software Distribution Center,
Digital Equipment Corporation, Maynard, MA 01754 Order Code: DEC-10-LALMA-8-D
digital equipment corporation - maynard. massachusetts
First Printing, September 1971
Revised: September 1971
December 1971
May 1972
December 1972
July 1973
July 1974
May 1975
September 1975
Second Printing, March 1976
The information in this document is subject to change without notice and should not be construed
as a commitment by Digital Equipment Corporation. Digital Equipment Corporation assumes no
responsibility for any errors that may appear in this manual,
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CONTENTS
CHAPTER 1 INTRODUCTION
1.1 General
1.2 DECsystem-10 ALGOL
1.3 The ALGOL Compiler
1.3.1 Compiler Extensions
1.3.2 Compiler Restrictions
1.4 The ALG OL Operating Environment
1.5 Terminology
CHAPTER 2 PROGRAM STRUCTURE
2.1 Basic Symbols
2.2 Compound Symbols
2.3 Delimiter Words
2.4 Use of Spacing and Commentary
CHAPTER 3 IDENTIFIERS AND DECLARATIONS
3.1 Identifiers
3.2 Scalar Declarations
CHAPTER 4 CONSTANTS
4.1 Numeric Constants
4.1.1 Integer Constants
4.1.2 Real Constants
4.1.3 Long Real Constants
4.2 Octal and Boolean Constants
4.3 ASCII Constants
4.4 String Constants
CHAPTER 5 EXPRESSIONS
5.1 Arithmetic Expressions
5.1.1 Identifiers and Constants
5.1.2 Special Functions
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CONTENTS (Cont)
5.2 Boolean Expressions
5.2.1 Boolean Operators
5.2.2 Evaluation of Boolean Variables
5.2.3 Arithmetic Conditions
5.3 Integer and Boolean Conversions
CHAPTER 6 STATEMENTS AND ASSIGNMENTS
6.1 Statements
6.2 Assignments
6.3 Multiple Assignments
6.4 Evaluation of Expressions
6.5 Compound Statements
CHAPTER 7 CONTROL TRANSFERS, LABELS, AND
CONDITIONAL STATEMENTS
7.1 Labels
7.2 Unconditional Control Transfers
7.3 Conditional Statements
CHAPTER 8 FOR AND WHILE STATEMENTS
8.1 FOR Statements
8.1.1 STEP-UNTIL Element
8.1.2 WHILE Element
B.2 WHILE Statement
8.3 General Notes
~ CHAPTER 9 ARRAYS
9.1 General
9.2 Array Declarations
9.3 Array Elements
CHAPTER 10 BLOCK STRUCTURE
10.1 General
10.2 Arrays with Dynamic Bounds
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May 1975
CHAPTER 11
11.
11.
11.
11.
11.
11.
11.6.
11.6.
11.
11.
11.
11.10
11.10.1
11.10.2
OO Oo N "Go "O "O OO E “N —
CONTENTS (Cont)
PROCEDURES
Parameters Called By "Value"
Parameters Called By "Name"
Procedure Headings
Procedure Bodies
Procedure Calls
Advanced Use of Procedures
Jensen's Device
Recursion
Layout of Declarations Within Blocks
Forward References
External Procedures
Additional Methods of Commentary
Comment After END
Comments Within Procedure Headings
CHAPTER 12 SWITCHES
12.1
12.2
12.3
General
Switch Declarations
Use of Switches
CHAPTER 13 STRINGS
13.1 General
13.2 String Expressions and Assignments
13.3 Byte Strings
13.4 Byte Subscripting
13.5 Null Strings
13.6 String Comparisons
13.7 Library Procedures
13.7.1 Concatenation
13.7.2 Length and Size
13.7.3 Copying
13.7.4 Newstring
13.7.5 Delete
Version 5A ALGOL у
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Мау 1975
CONTENTS (Cont)
CHAPTER 14 CONDITIONAL EXPRESSIONS AND STATEMENTS
14.1 General
14.2 Conditional Operands
14.3 Conditional Statements
14.4 Designational Expressions
CHAPTER 15 OWN VARIABLES
15.1 General
15.2 Own Arrays
CHAPTER 16 DATA TRANSMISSION
16.1 General
16.2 Allocation of Peripheral Devices
16.2.1 Device Modes
16.2.2 Buffering
16.2.3 Error Returns
16.3 Selecting Input/Output Channels
16.4 File Devices
16.4.1 Error Returns
16.5 Releasing Devices
16.6 Basic Input/Output Procedures
16.6.1 Byte Processing Procedures
16.6.2 String Output
16.6.3 Miscellaneous Symbol Procedures
16.6.4 Numeric and String Procedures
16.6.4.1 Numeric Input Data
16.6.4.2 Numeric Output Data
16.6.4.3 Octal Input/Output
16.7 Default Input/Output
16.8 Logical Input/Output
16.9 Special Operations
16.10 I/O Channel Status
16.11 | Transferring Files
16.12 Currently Selected Channel Numbers
Version 5A ALGOL vi
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May 1975
CONTENTS (Cont)
CHAPTER 17 THE DECsystem-10 OPERATING ENVIRONMENT
17.1
17.2
17.3
17.3.1
17.3.2
17.3.3
17.4
17.5
17.6
17.7
17.8
Mathematical Procedures
String Procedure
Utility Procedures
Array Dimension Procedures
Minima and Maxima Procedures
Field Manipulations
Data Transmission Procedures
FORTRAN Interface Procedures
General Information Routine
Date and Time in ASCII Format
Random Number Routines
CHAPTER 18 RUNNING AND DEBUGGING PROGRAMS
18.1
18.1.1
18.2
18.3
18.4
18.5
18.5.1
18.5.1.1
18.5.1.2
18.5.1.3
18.6
18.7
18.8
18.8.1
18.8.2
18.9
18.9.1
18.9.2
Compilation of ALGOL Programs
Compilation of Free-Standing Procedures
Loading ALGOL Programs
Running ALGOL Programs
Concise Command Language
Run-Time Diagnostics and Debugging
Facilities to Aid in Program Debugging
Checking
Controlling Listing of the Source Program
Setting Line Numbers in Listings
Cross Reference Listing
Stack Analysis
Trace
Dynamic Trace
Post-Mortem Trace
Performance Analysis
Heap Space
Code Utilization
CHAPTER 19 TECHNICAL NOTES
Version 6 ALGOL
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September 1975
Version 6 ALGOL
CONTENTS (Cont)
TABLES
DECsystem-10 ALGOL Symbols
Compound Symbols
Delimiter Words Used in DECsystem-10 ALGOL
Operator Precedence
Function of Boolean Operators
Boolean Expressions
Parameters in a Procedure Call
Standard Device Names
FORTRAN Interface Procedures
Error Trap Numbers
viii
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September 1975
CHAPTER 1
INTRODUCTION
1.1 GENERAL
DECsystem-10 ALGOL is an implementation of ALGOL-60; ALGOL is an abbreviation of ALGOrithmic
Language, and 1960 is the year it was defined. The authoritative definition of ALGOL-60 is contain-
ed in the "Revised Report on the Algorithmic Language ALGOL-60" hereafter referred to as the
"Revised Report". This report leaves a number of ALGOL-60 features undefined, notably input/output,
and permits the implementer of the language some latitude in interpreting other features. Many of
these features have been discussed extensively since the publication of the Revised Report; some have
been given rigorous interpretations in various versions of ALGOL, particularly the ALGOL-68
Language.
Where there is need for interpretation in the Revised Report, such interpretations as seem reasonable
have been made in light of current ALGOL opinion. Where no guidelines exist, ALGOL-68 is used as
a basis. These points are discussed in Chapter 19.
1,2 DECsystem-10 ALGOL
The purpose of this manual is to teach the use of DECsystem-10 ALGOL. The manual is written both
for the user who is familiar with ALGOL implementations and for the user who has no knowledge of
ALGOL but is reasonably fluent in a high-level scientific programming language such as FORTRAN IV.
This manual is not a primer in high-level languages.
Revised Report on the Algorithmic Language ALGOL-60", Backus et al., Communications of the
ACM, 1963, vol. 6, no. 1, pp. 1-17.
? "Report on the Algorithmic Language ALGOL-68", A. Van Wijngaarden (Editor), B. J. Mailloux,
J. E. L. Peck, and C. H. A. Koster, Mathematisch Centrum, Amsterdam, MR101, October 1969.
“A Primer of ALGOL-60 Programming, E. W. Dijkstra, Academic Press, London, 1962.
1-1 December 1972
Readers not thoroughly familiar with ALGOL should read the entire manual. Readers already familiar
with ALGOL-60 should read all chapters except Chapters 5,6, 7, 8, 9, 10, 11, 12, and 14, which
need be referred to only briefly.
1.3 THE ALGOL COMPILER
The DECsystem-10 ALGOL Compiler is that part of the DECsystem-10 ALGOL System that reads pro-
grams written in DECsystem-10 ALGOL and converts them into a form (relocatable binary) that is
acceptable to the DECsystem-10 Linking Loader. The compiler is also responsible for finding errors in
the user's source program and reporting them to the user,
Slight constraints are imposed on the way the user writes his program. These constraints, made to gain
the most desirable feature of a single-pass compiler, concern the order in which the user declares the
identifiers in the program and the use of forward declarations under certain special circumstances.
Such a compiler can process ALGOL programs rapidly and does not require the use of any backing
store. The minor restrictions imposed will not normally affect the user.
1.3.1 Compiler Extensions
The following ALGOL-60 extensions are allowed by the compiler:
a. A LONG REAL type, equivalent to FORTRAN's double precision, is added that
gives the user power to handle double -precision real numbers.
b. An EXTERNAL procedure facility allows the user to compile procedures separately
from the main program.
с. A WHILE statement, and an abbreviated form of the FOR statement, allow the
user greater flexibility of iteration.
d. A new type STRING allows the user to manipulate strings of various size bytes,
In addition, the user can individually manipulate the bytes within a string by
means of a byte subscripting facility.
e. An integer remainder function REM, is provided,
f. Assignments are permitted within expressions.
g. Delimiter words may be represented in either reserved word format (upper case) or
as non-reserved words enclosed in single quotes (primes).
h. Constants of type REAL may be expressed as an integer
part and a decimal part only as in FORTRAN,
The compiler accepts reserved word delimiters in normal mode, but it can also accept programs
using non-reserved delimiter words enclosed in primes. Refer to Chapter 18,
1-2 July 1974
1.3.2 Compiler Restrictions
If the user is unfamiliar with any of the following terminology, he should refer to the Revised Report
and to Paragraph 1.5.
The compiler imposes the following restrictions on ALGOL-60:
a. Numeric labels are not permitted.
b. All formal parameters must be specified.
c. Identifiers are restricted to 64 characters in length.
d. Arrays and scalars must be declared before switches and procedures,
e. Forward references for procedures and labels must be given under certain circum-
stances.
1.4 THE ALGOL OPERATING ENVIRONMENT
Programs compiled by the ALGOL compiler are run in a special operating environment that provides
special services, including input/output facilities for the object program.
The ALGOL operating environment consists of:
a. The ALGOL Library, known as ALGLIB - a set of routines, some of which are
incorporated into the user's program by the linking loader.
b. The ALGOL Object Time System, known as ALGOTS - responsible for organizing
the smooth running of the program and providing services such as core manage-
ment, peripheral device allocation, and fault monitoring in case the program
encounters an error condition at run time.
Refer to Chapters 17 and 18 for a description of ALGLIB and ALGOTS.
1.5 TERMINOLOGY
Some of the following words, used in this manual , may be new to the reader. Many have a FORTRAN
equivalent; where such an equivalent exists, if is enclosed in parentheses.
Delimiter Word - a single, English language word that is an inherent part of the
structure of the ALGOL language. Such words cannot normally be used for other
purposes. Example: BEGIN IF ARRAY.
Identifier - a name, established by user declaration, that represents some quantity
within a program.
Label (Statement Number) - an identifier used to mark a certain statement in a program.
Control of program execution can be transferred to the statement following the label.
A numeric label (not available in DECsystem-10 ALGOL) is similar to a FORTRAN
statement number.
(continued on next page)
1-3 July 1974
Procedure (Subroutine, Function) = part of a program, which may be invoked by "calling".
In general , parameters are supplied as arguments and a result may be returned.
Parameter (Formal Parameter - Dummy Variable, Actual Parameter - Argument) See
Procedure. - A Formal Parameter is an identifier used within the procedure that repre-
sents the argument supplied when the procedure is called.
1-4 December 1971
CHAPTER 2
PROGRAM STRUCTURE
2.1 BASIC SYMBOLS
DECsystem-10 ALGOL programs consist of a sequence of symbols from the DECsystem-10 ASCII
character set. The meaning of individual characters, given in Table 2-1, is much the same as in other
high-level languages.
Table 2-1
DECsystem-10 ALGOL Symbols
Sy mbol Meaning or Use
A-Z Used to construct identifiers and delimiter words.
a-z Lower case letters; are treated as upper case letters except when they
appear in string constants and ASCII constants.
0-9 Decimal digits; used to construct numeric constants and identifiers.
+ Arithmetic addition operator.
- Arithmetic subtraction operator.
* Arithmetic multiplication operator.
/ Arithmetic division operator.
t Arithmetic exponentiation operator.
(>) Parentheses; used in arithmetic expressions and to enclose parameters
in procedure specifications and calls.
[ ] Square brackets; used to enclose subscript bounds in array declarations,
and array subscript lists.
, Comma; general separator, placed between array subscripts, procedure
parameters, items in switch lists, etc.
Decimal point; used in numeric constants and byte subscripting. Also,
used as a readability symbol in identifiers.
; Semicolon; used to terminate statements.
(continued on next page)
2-1 December 1971
Table 2-1 (Cont)
DECsystem-10 ALG OL Symbols
Symbol Meaning or Use
Colon; used to indicate labels, and separate lower and upper bounds in
array declarations.
Equality; used in arithmetic and string comparisons.
Is
Nonequality.
> Less than, greater than..
—
©
Introduces exponent in floating-point numbers.
Prime, or single quote; used to enclose delimiter words when the non-
reserved word implementation is used.
Opening and closing string quotes.
! Comment.
% Introduces an octal constant.
$ Introduces an ASCII constant.
+ Alternative to := (refer to Table 2-2).
2.2 COMPOUND SYMBOLS
Compound symbols consist of two adjacent basic symbols. Any intervening spaces or tabs do not affect
their use. The compound symbols are shown in Table 2-2,
Table 2-2
Compound Symbols
Symbol Usage
= Assignment
<= Less than or equal to
>= Greater than or equal to
2.3 DELIMITER WORDS
Certain upper-case letter combinations are reserved as part of the structure of the language and may
not be used as identifiers unless the compiler used is a version accepting delimiter words in single
| quotes. Such an option is selected by using a special switch option (refer to Chapter 18). It is assumed
Version 2 ALGOL 2-2 December 1971
throughout this manual that the standard method of delimiter word representation is used, that is,
reserved words,
For example, the delimiter word
BEGIN
will always appear in the text of this manual as shown above and cannot be used as an identifier in a
program. If the alternative method of representation is used, it would appear as
"HEC IN
and
RFCIN
could be used as an identifier. Table 2-3 contains a list of all the delimiter words used in the language.
Table 2-3
Delimiter Words Used in DECsystem-10 ALGOL
Reserved Word Chapter Reference
AND 5.2.1
ARRAY 9
BEGIN 10
BOOLEAN 5.2
CHECKOFF 18
CHECKON 18
COMMENT 2.4
DIV 5.1
DO 8
ELSE 7.3
END 10
EQV 5.2.1
EXTERNAL 11.9
FALSE 4.2
FOR 8
FORWARD 11.8
| GO 7.2
GOTO 7.2
IF 7.3
IMP 5.2.1
INTEGER 3.2
LABEL 11
LINE 18
LISTOFF 18
LISTON 18
LONG 3.2
(continued on next page)
Version 2A ALGOL 2-3 May 1972
DECsystem-10 ALG OL also permits the use of a decimal point as a "readability symbol" in the alpha-
betic portion of identifiers. These readability symbols can appear between two alphabetic characters
of an identifier and are ignored by the compiler. Thus:
ONCE «AGAIN
and
Pl.BY.TWC
have exactly the same effect as
ONCEAGA IN
and
PIBYTWC
respectively.
Note that
ALLFHA3 «5
and
BETA +22
are not identifiers, since the decimal point does not appear between two alphabetic characters.
3.2 SCALAR DECLARATIONS
A declaration reserves an identifier to represent a particular quantity used in a program. Such
declarations are mandatory in ALGOL. At any particular point during program execution, the form of
the variable or quantity associated with the identifier depends on the type of variable. The type of
variable is controlled by the type of identifier which represents it.
There are five scalar variables, that is, variables which contain a single value:
a. Integer
b. Real
c. Long Real
d. Boolean
e. String
Integer, real , and long real variables are capable of holding numerical values of the appropriate type
(and only of that type). The range of values is as follows: integer: -34,359,738,368 through
34,359 ‚738 ‚367; real and long real: approximately -1.7838 through 1.7838; values less than approx-
imately 1.4&-39 in magnitude are represented by zero.
3-2
Boolean variables (similar to FORTRAN's Logical variables) can hold a Boolean quantity, which is
| usually one of the states TRUE or FALSE but, in general , can be any pattern of 36 bits.
String variables are somewhat more complicated. A full discussion of their properties is presented in
| Chapter 13. At this point, it is sufficient to say that string variables are really pointers to byte strings.
All of the above variables can be declared for use by preceding a list of the identifiers to be used by
the appropriate delimiter word for their type. Throughout this manual, a "list of items" consists of
those items arranged sequentially and separated by commas.
Examples:
INTEGER I,JsK3
LONG REAL DCUBLE »PsQsELEPHANTs
BOCLEAN ISITREALLYTRUEs
STRING SoaT3
3-3 December 1971
CHAPTER 4
CONSTANTS
4.1 NUMERIC CONSTANTS
There are three forms of numeric constants:
a. Integer constants
b. Real constants
c. Long Real constants
4.1.1 Integer Constants
Integer constants consist of a number of adjacent decimal digits, subject to the constraint that the
number represented must be in the range O through 34,359,738 ,367.
NOTE
Any preceding sign that appears in the program is not
considered part of the constant.
Examples:
3
24
9276541
See also Section 4.3
4.1.2 Real Constants
Real constants consist of a decimal number (containing either an integral part or a fractional part, or
both) followed by an optional exponent. If the decimal number is unity, it may be omitted. The ex-
ponent consists of either the & or @ symbol followed by an optionally signed integer. This has the
effect of multiplying the decimal number by the power of ten specified in the exponent. If no decimal
number appears, a value of unity is assumed.
Version SA ALGOL 4-1 May 1975
The range of real constants is approximately 1.48-39 to 1.7838; numbers less than 1.4&-39 are repre-
sented by zero. Real numbers are stored to a significance of approximately eight and one-half decimal
digits.
Examples:
Representation Value
3.141592653589793 3.14159265
.0001 0.0001
4.3785 437000.0
5&-3 0.005
8-6 0.000001
4.1.3 Long Real Constants
Long real constants are used to represent numeric quantities to approximately twice the precision
available with real numbers: about seventeen decimal digits. Long real constants are formed by
writing a real constant in floating-point form, but replacing the & or @ by && or @@.
The range of long real constants is the same as that of real constants, except numbers below approxi-
mately 3.0&&-30 can only be represented to single precision due to hardware considerations.
Examples:
Representation Value
3.14159265358979323846&&0 3.1415926535897932
128:8-3 0.012
4.2 OCTAL AND BOOLEAN CONSTANTS
Octal constants consist of the symbol % followed by a number of octal digits. Up to twelve significant
digits may appear (leading zeros are ignored); these digits are right justified.
Examples:
2777777777774
20470
Octal constants may only be used in Boolean expressions.
Boolean constants consist of the words TRUE and FALSE. They are equivalent to the octal constants
7777777777777 and 000000000000, respectively.
4.3 ASCII CONSTANTS
Up to five ASCII symbols can be packed right justified to give an integer-type constant. The format is
a dollar sign ($), followed by up to five ASCII symbols enclosed within a delimiting symbol pair. The
leading delimiter symbol immediately follows the $, and may be a readable character or an invisible
one such as a space. Thus, the user can generate a single ASCII character constant by placing one
space on each side of it, and preceding the triplet by a dollar sign.
Examples:
Text Octal Value
$ А 000000 000101
$/01234/ 160713 516674
4,4 STRING CONSTANTS
String constants allow the user to store any reasonable length string of ASCII characters within a pro-
gram, The length of such a constant is restricted only by the amount of core storage available to the
user for the execution of the program. String constants may be used, typically, to output a message
during the execution of the program or as values assigned to string variables,
The string of symbols is enclosed within quotes ("). There are restrictions on the symbols that may
appear within the string.
a. [ ] ; and " may not appear alone.
b. [ and] may appear if they are properly paired.
c. Single occurrences of [ | ; and " are represented by [[ ]] ;; and "" respectively.
d. Where a string has to be broken across two or more lines of source, the carriage return
and line feed characters can be ignored by preceeding them with a control-back arrow
character,
Note that [[ and ]] are stored as such in the byte string generated by the compiler. ;; and "" are
stored as a single ; or ", respectively, The restriction on the representation of ; is made to
protect the user against quoting his whole program by missing outa ".
Square brackets are used to enclose symbols that have a specific effect when the string is output.
These are discussed in Paragraph 16.6.2,
Examples:
"ARCDEFGHTJKLYNOCPERSTIVWXYZ"
"REMEMBER THAT SPACES ETC. ARE SIGNIFICANT"
"CPSCIINPLT DATA:(5C]"
SEIN SS GREEN PS FELL
Version 5A ALGOL 4-3
September 1975
C HAPTER 5
EXPRESSIONS
5. 1 ARITHMETIC EXPRESSIONS
DE=Csystem-10 ALG OL arithmetic expressions are written in a form similar to that used in FORTRAN and
many other high-level scientific computer languages. The usual algebraic rules concerning precedence
of operators and brackets are followed (see Table 5-1).
Table 5-1
Operator Precedence
Priority
Operator (decreasing)
parentheses
exponentiation
multiplication and division
addition and subtraction
>> D N —
Th ere are two additional operators, DIV and REM, that indicate integer division and remainder,
respectively. They have the same precedence as ordinary division. Within the precedence scheme,
the= order of evaluation is always from left to right. For example:
XtYtZ means (XtY)tZ
ane
I DIV J KEV Kmeans (I DIV J) КЕМ К
Umm like FORTRAN, when ordinary division of one integer by another is performed, the real result is
no ¥ rounded to an integer value.
5-1 July 1974
The difference between the various types of division is clarified by the following examples:
7/4 yields a result of 1.75, whereas
7 DIV 4 yields a result of 1, and
7 REM 4 yields a result of 3
The interpretation of integer division for negative integers follows:
Let M, N > 0, then
-N DIV N = M DIV C-N) = -C(M DIV N)
-M DIV (=-N) = M DIV N
The integer remainder operator, REM, is defined so that for all integral M,N:
М КЕМ № = Мм = N*x(M DIV №)
5.1.1 Identifiers and Constants
Arithmetic expressions consist of operands, that is, identifiers and constants, of the three types,
integer, real and long real, together with the arithmetic operands + - * / DIV REM and t and
parentheses where necessary.
Identifiers are used to represent variables whose values are used when they appear in some calculation.
Since automatic conversion takes place as necessary when an expression is evaluated, the user may
freely mix the three different types of identifiers and constants.
Integer quantities may have more precision than can be represented in a real variable. The user must
beware of possible loss of significance in integral quantities used in mixed type expressions.
5.1.2 Special Functions
Three special functions are provided for use in arithmetic expressions. The first is the transfer function,
ENTIER, which converts a real or long real quantity into an integer quantity defined as the largest
integer value not exceeding the argument.
Thus
ЕМТТЕК (3.5) = 3
and
ENTIERC-3.5) = -4
5-2 December 1971
The special function ABS yields the absolute value (also known as the modulus) of its argument. The
argument may be any integer, real, or long real quantity; the result is always of the same type as the
argument.
Thus
ABS (=3e5) = 3.5
and
AES (-3) = 3
The special function SIGN is the signum function whose argument can be integer, real, or long real.
The result is always integral , being minus one or zero or plus one, depending on whether the argument
is negative, zero, or greater than zero, respectively.
Thus
SIGN(-3.5) = —-l
SIGN) = 0
SIGN(3.5) = 1
NOTE
ENTIER, ABS, and SIGN are not delimiter words. They
may be used for other purposes in a program.
Examples of simple arithmetic expressions follow:
X
I + 3
X*Y/Z
P+G/R
x2 + Y
XJ-4
J + ENTIERCK-2)
SIGNCENTIERCJ/K) + 1)
(X + Y) * C-I)
Version SA ALGOL 5-3 May 1975
5.2 BOOLEAN EXPRESSIONS
Boolean expressions involve Boolean identifiers, Boolean and octal constants, arithmetic conditions,
and Boolean operators interspersed in an order similar to that of arithmetic expressions.
5.2.1 Boolean Operators
| There are five Boolean operators listed here in decreasing order of precedence.
NOT (unary operator)
AND
OR
IMP (implication)
EQV (equivalence)
Ф © о — а
e e. e e oe
NOT is a unary operator that complements a Boolean quantity in the same way that a unary minus sign
negates an arithmetic quantity in an arithmetic expression. In this case, it changes FALSE to TRUE,
and vice versa.
Table 5-2 gives the result of A OP B where OP stands for one of the Boolean operators AND, OR,
IMP, or EQV, for all values of A and B.
Table 5-2
Function of Boolean Operators
A FALSE TRUE
B FALSE TRUE FALSE TRUE
A AND B FALSE FALSE FALSE TRUE
A ORB FALSE TRUE TRUE TRUE
A IMP B TRUE TRUE FALSE TRUE
A EQV B TRUE FALSE FALSE TRUE
In addition, the following theorems hold true:
A IMPB is equivalent to NOT A OR B,
A EQV B is equivalent to A AND B OR NOT A AND NOT B.
| 5.2.2 Evaluation of Boolean Variables
Actually, Boolean variables may have a value consisting of any pattern of bits, rather than be
confined to the values TRUE and FALSE. The logical operations operate on a bit-by-bit basis
according to the preceding rules,
Version 5A ALGOL 5-4 May 1975
The actual test employed to determine the truth of a Boolean expression such as
BAND C
is to evaluate it and regard it as true if its value is nonzero, i.e., at least one bit is set, otherwise it
is false.
This is particularly important when octal constants are used in Boolean expressions. For example, if
the user wishes to test a particular bit in a Boolean variable, an appropriate octal constant can be
used, for example:
B AND %]
is a Boolean expression that is true if and only if the bottom (least significant) bit of B is a one.
5.2.3 Arithmetic Conditions
Arithmetic conditions are used as operands in Boolean expressions. They consist of two arithmetic ex-
pressions coupled with a comparator. The comparator, which decides the particular type of test to be
performed on the two expressions, is one of the following:
< less than
<= less than or equal to
= equals
> greater than
>= greater than or equal to
#
not equal to
Such an arithmetic condition can be regarded as true or false according to whether the condition speci-
fied by the comparator is met when the arithmetic expressions on each side of it are evaluated. The
resulting condition may form part of a Boolean expression.
The following examples of Boolean expressions, shown in Table 5-3, also involve arithmetic conditions.
5.3 INTEGER AND BOOLEAN CONVERSIONS
An integer quantity can be converted to a Boolean quantity by means of the dummy function BOOL.
Similarly, the dummy function INT converts a Boolean quantity to an integer quantity.
Version 5A ALGOL 5-5 May 1975
Table 5-3
Boolean Expressions
Expression Meaning
NOT B NOT B
B AND NOT C B AND (NOT C)
A OR B AND С A OR (B AND C)
B EQV X<Y B EQV (X<Y)
X+Y<Z AND B OR P=Q (((X+Y)<Z) AND B) OR (P=Q)
The value passed by these functions is unchanged: the functions are included for semantic correctness.
Thus:
BOOL(1)
may be regarded as a Boolean operand, and
INT(8)
INT(9%400000000000)
as integer operands.
BOOL and INT are not reserved words. They can be used for other purposes by declaring them as re-
quired. However, this practice should be avoided since it could lead to confusion.
Version 5A ALGOL 5-6 May 1975
CHAPTER 6
STATEMENTS AND ASSIGNMENTS
6.1 STATEMENTS
The statement is the basic operational unit in ALGOL-60. It describes an operation to be performed at
run time, such as an assignment.
6.2 ASSIGNMENTS
Assignments convey the value produced by the execution of an expression to a destination variable of
the appropriate type. This is done by writing the destination identifier, followed first by the symbols
: and = and then by the expression to be evaluated. Thus
X := \ + 7
causes the result of the addition of the values contained in the variables Y and Z to be placed in the
variable X.
When an assignment is made to a variable type differing from that of the result of the expression, a
type conversion is performed. Integer, real and long real expressions may be assigned to variables of
any of these three types, but not to any other types. Boolean and string expressions can only be
assigned to a variable of their own type.
If a real or long real value is assigned to an integer type variable, a rounding process occurs.
I := X
results in an integral value equal to
ENTIERCX + 8.5)
being assigned to I.
When an integer expression is assigned to a real or long real variable, a conversion to that type is
performed. Real to long real conversion simply consists of zeroing the low-order precision word of
the long real result after assignment of the real result to the high-order part of the long real variable.
Long real to real assignments truncate the low-order part of the long real expression, after appropriat
rounding.
6.3 MULTIPLE ASSIGNMENTS
A value may be assigned simultaneously to several variables of the same type by a multiple assignment.
This takes a form such as
P := R := S = X + Y = 7
where the result of adding Y to X and subtracting Z is assigned to P, R, and S simultaneously.
All identifiers on the left-hand side of a multiple assignment must be of the same type. If the user
wishes fo assign a value to two or more different types of variables, the "assignment within expression”
(embedded assignment) feature must be used, as below.
A parenthesized assignment may be substituted for any operand in an expression. For example,
X 2= (Y t= P+0Q)/Z
This causes the embedded assignment to be made after the inner expression P+Q is evaluated. Where a
type conversion is performed as part of an embedded assignment, the operand type is the same as that
assigned to the variable in the embedded assignment. Thus
xX ¢= (1 t= 3.4)
sets I equal to 3 and X equal to 3.0.
6.4 EVALUATION OF EXPRESSIONS
All expressions in DECsystem=10 ALGOL are evaluated observing the normal algebraic rules of prece-
dence, including bracketing.
Within the precedence structure, expressions are always evaluated from left to right. For example, if
X is a scalar, and F a function procedure (see Chapter 11) that alters X,
X $= X+F
may have a different effect than
X 2= F+X
This is known as a "side effect".
Consider also:
The subscript I is always evaluated before I is incremented, as it is to the left of the embedded assign-
ment, within the statement. Thus the above expression is equivalent to
J = I3 I := I+13 ACJI := I
The user can always predict the order of evaluation of an expression and can count on such things as
X t= (P := P+C)/(P+R)
being evaluated correctly, thus giving the same result as
P := P+C
X
:= P/CP+RK)3
6.5 COMPOUND STATEMENTS
À compound statement consists of a number of statements, preceded by BEGIN, separated by semi-
colons, and terminated by END. ALGOL statements, unlike those in FORTRAN, are terminated by
a semicolon not by the end of a line of text.
For example:
BEGIN
I := 335 J := 43
K := I + Js;
x := K
END
is a compound statement. Semicolons do not have to appear after the BEGIN or before the END;
BEGIN and END act as a type of bracket.
The usefulness of compound statements will become apparent in later chapters.
CHAPTER 7
CONTROL TRANSFERS, LABELS, AND CONDITIONAL
STATEMENTS
7.1 LABELS
A label is a method of marking a place in a program so that control can be transferred to that point
from elsewhere in the program.
DECsystem-10 ALGOL uses identifiers as labels. These identifiers are placed before statements and are
followed by a colon. Numeric labels are permitted in the Revised Report, but are not implemented in
DECsystem-10 ALGOL. Most implementations of ALGOL-60 do not allow integer labels.
For example:
COMP: Х := X + Y
is a statement labeled by COMP.
More than one label can be attached to a statement if required; thus,
LARI: LABE: Y := 0
7.2 UNCONDITIONAL CONTROL TRANSFERS
A transfer of control, or "jump", to a statement in a program is effected by a GOTO statement. This
statement consists of the word GOTO followed by the name of the label attached to the relevant state-
ment. The two words GO TO can be used instead of the word GOTO in any statement where GOTO
can be used. Thus:
BEGIN INTEGER IsJ>K3
LAR: I = J := 33
K := I] + Js
GCTO LAR
END
is an example of a somewhat tedious program. Clearly, to write any reasonable program, it is neces-
sary to be able to jump conditionally.
Version 2A ALGOL 7-1 May 1972
8.1.1 STEP-UNTIL Element
This particular form deserves closer inspection, because it is not quite as simple as it appears. For
example, consider
FOR ] := 1 STEP I UNTIL N DC S
The statement S is obeyed with ! taking an initial value of 1, and being incremented by I until the final
value N is achieved. The question is, "Is the I after the STEP recalculated during each turn around
the loop, or does it have a constant value equal to the initial value of 1?"
The answer is slightly more complicated. Consider the general case
FCK М := El STEP E2 UNTIL E3 DC S
This is defined to have exactly the same effect as
V := El;
Li: IF (V - E3)xSIGN(EZ) > Y THEN GOTÚ Le;
5;
V := V + EZ3
GOTO Lis
|2:
Clearly, the value of I following the STEP in the previous example is evaluated, if necessary, twice
during each turn around the loop, once in the sign test at L1, and again to update V. ALGOL allows
the user to modify V, El, E2, and E3 freely throughout the loop, and takes account of all these
changes in the evaluation of the loop.
NOTE
DECsystem-10 ALGOL allows the user the abbreviated form
FOR V := El UNTIL E3 DO S
instead of
FOR V := El STEP 1 UNTIL E3 DO S
8.1.2 WHILE Element
A FOR statement with a single WHILE element takes the form
FOR V := Е WHILE B DOS
8-2 December 1972
This is interpreted as follows:
Ll: V := E,
IF NOT B THEN GOTO 12;
5;
GOTO Lls
Le :
Once again, the complexity of the loop may be affected by changing V and E within the loop.
8.2 WHILE STATEMENT
The WHILE statement is an enhancement of ALGOL-60 provided in DECsystem-10 ALGOL. It takes
the general form
WHILE B DGC S
and is interpreted as follows:
Ll: IF NOT E THEN СОТО 12;
5;
GCTO 115
|2:
8.3 GENERAL NOTES
1. Within a FOR statement of any kind, the user can change the controlling variable
or any other variable appearing within the action of the loop. Such changes pre-
dictably affect the execution of the loop by the rules given above.
2. On exit from a FOR statement either by jumping out of the loop or by exhausting
the FOR elements, the controlling variable has a well-defined value equal to
the last assigned value of the controlling variable. This may not be true of other
ALGOL-60 implementations. Section 4.6.4 of the Revised Report should be
studied carefully in this connection,
8-3 July 1974
CHAPTER 9
ARRAYS
9.1 GENERAL
Arrays are essentially collections of variables of the same type, allowing the user to address them
individually by means of a common name and a unique subscript or subscripts. In the simplest case, an
array is a vector and is known as a one-dimensional array. A matrix is a two-dimensional array, etc.
There is no limit to the number of subscripts allowed, other than those imposed by the ability of the
computer to store the array.
9.2 ARRAY DECLARATIONS
Arrays may be of type integer, real, long real, Boolean, or string. They are declared in a similar
fashion to scalar variables, except the size of the array must be stated. For each subscript that the
array possesses, a lower and an upper bound, called the "bound pair" for that subscript, must be given.
For example, to declare two one-dimensional integer arrays A and B with lower bound 1 and upper
bound 5:
INTEGER AKRAY A,B[1:5]
Note that the lower and upper bounds are enclosed in square brackets and separated by a colon.
When there are two or more subscripts, the declaration is similar, and the bound pairs are separated by
commas. Thus
LONG REAL ARRAY PsQsREL-5:2,0:101
declares three real arrays, P, Q and R, with the first subscript bounded by -5 and 2 and the second
subscript bounded by O and 10.
It is possible to declare arrays of different sizes in the same statement provided they are of the same
type:
REAL ARRAY ACI:103. B.+CC1:10.1:12])
9-1
Note also that in the case of real arrays, the REAL may be omitted in the declaration, and is assumed
by default, thus:
ARRAY ACI:10J.s B.»,CEI:ID,1:12)
The bounds in an array need not be static, as in the examples above. In general, they may be any
arithmetic expressions, which are evaluated to give an integral value for the individual bound pairs.
The use of such dynamic array declarations will become apparent later.
9.3 ARRAY ELEMENTS
An individual element of an array can be referred to by following the name of the array by a list of
subscripts in square brackets. The number of subscripts must be identical to the number in the array
declaration. Thus, a typical element of A used in the last declaration might be
ALS) orAL9lor generally, ALI]
where I is some integer expression or, in general, any expression whatsoever, with the limitation that
its value when used as a subscript and evaluated as an integer is in the range 1 through 10, the bounds
of the array A.
As an example of the use of arrays, consider the declaration
REAL ARRAY DsFEsF C1:1%>1:10]
and suppose that it was required to set F equal to the matrix product of D and E:
FOR 1 := 1 UNTIL 14 DO
FCk J := 1 UNTIL 16 DC
BEGIN X 2= 5
FCR K := 1 UNTIL 18 DO X := X + DCI.KIXECKoJJ3
FÜI>,J] := X
END
Note that X is used to accumulate the inner product of the multiplication for all values of I and J.
It would be very inefficient not to use such a variable, because F would otherwise be needlessly in-
volved in the inner loop of the computation.
Also , note that an element of an array of a particular type may be used anywhere that a scalar variable
of the same type may be used, even in such places as the controlling variable in a FOR statement.
9-2 December 1971
CHAPTER 10
BLOCK STRUCTURE
10.1 GENERAL
ALGOL program structure is somewhat more complicated than other high-level languages, such as
FORTRAN. An ALGOL program consists of a number of "blocks" arranged hierarchically; a block con-
sists of the words BEGIN and END enclosing declarations and (optionally) statements.
Thus:
BEGIN
REGIN
END
BEGIN
EEGIN
FND
END
END
is an ALGOL program, assuming appropriate declarations and statements in the blocks.
The block structure offers the user many interesting features not available in non-block structured
languages. For instance, the user may declare an identifier that appears to conflict with another
identifier in an enclosing block. Thus:
REGIN INTEGER Is
BEGIN INTEGER |;
END
END
10-1
In fact, there is no conflict as there are two different Is. The only I that statements in the outer block
can "see" is the one in the outer block. Similarly, any statements in the inner block will always use
the I in that block. Such a declaration in an inner block is known as a "local" variable; it takes
precedence over declarations occurring at an outer or more "global" level. In general, all variables
can be "seen" from any point in a program that is either in the same block as the declaration or in a
block that is enclosed by the block in which the declaration of the variable occurred. Note that a
more local variable is always taken in preference to a relatively global variable. Consider the follow-
ing example:
BEGIN INTEGER I,J3
(1)
BEGIN INTEGEK J>K
[2]
END 3
BEGIN INTEGER IsK
3)
END
END
Any statements occurring at point [1] can see the declarations of I and J, which are local, but cannot
see the declarations of J and K in the first inner block, or the declarations of I and K in the second
inner block. At [2], the local variables J and K can be seen, as can the global variable I in the outer
block. The global variable J is not seen because the local variable J takes precedence over it; the
variables I and K in the second inner block are not seen at all. A similar situation occurs at [3];
here both local variables I and K, as well as the global variable J, are seen.
Note that the "scope" of a variable is the set of all places in a program where it can be seen and
therefore used. This term will be used frequently throughout this text.
In general, it is more efficient to use local variables in preference to global ones. This statement is
also true of most ALGOL-60 implementations. Where a non-local variable is used frequently, it is
advisable to assign its value to a local variable and use that in preference. For example:
10-2
REGIN INTEGER 13
I >= eee...
BEGIN INTEGER 113
II := 1;
ce... II 0. 00.0
END
END
Here, in the inner block, a local variable II is used, and assigned the value of the global variable I
for use throughout the local block.
10.2 ARRAYS WITH DYNAMIC BOUNDS
The concept of the scope of a variable can be applied most usefully to arrays. In DECsystem-10
ALGOL, all arrays are constructed at execution time, that is, no fixed space is reserved for them by
the compiler, irrespective of whether their bounds are static or dynamic. When a declaration of an
array is encountered within a block, the space required to construct it is obtained and the array is laid
out. When the end of the block enclosing the array is reached, that is, the array variable is no longer
within scope, the space utilized by the array is recovered and can be used later for other arrays.
Consider the case of a problem in which the size of an array to be used in a calculation is dependent
on the data fo be processed. The programmer has the choice of making the array large enough to cope
with the worst case (in many languages he does not have any choice at all) or constructing the array
with dynamic bounds to suit the size required by the particular data. The first method has the disad-
vantage of wasting space on many occasions; the latter method only has the minor disadvantage of the
overhead needed fo construct the array. Such overhead is very small compared to the running time of
most programs; therefore, the second method is more desirable.
10-3
Consider the following example:
BEGIN INTEGEK N3
Le N
BEGIN AKKAY ACI1:N>1:N135
END3
GOTQ L
END
A value for N is calculated in this example, possibly dependent on some data read into the program,
and used to declare the array A, which is used to process the data in the inner block. When the end
of the inner block is reached, the space used by A is recovered and control passes to L, where another
value for N is calculated, and the process repeated.
10-4 December 1971
CHAPTER 11
PROCEDURES
Procedures are similar in concept to the FORTRAN subroutine, although more sophisticated and general
in their possible applications.
A "procedure" is a portion of an ALGOL program that is given a name to identify it and can be
"called" from any part of a program which is in the scope of the body of the procedure. A procedure
can execute a number of statements, or it can return a value if it is a function procedure. In addition,
it may or may not have parameters.
In DECsystem-10 ALGOL, a procedure can be one of the following types: integer, real, long real,
Boolean or string, or it may be typeless. The formal parameters of a procedure, known as "dummy
variables" in FORTRAN, can be one of the following types: integer, real, long real, Boolean or string,
as scalars, arrays or procedures, or label. There are seventeen different types of parameters. In
addition, all of these parameters may appear in two different modes, neither of which is the same as
FORTRAN's method of handling parameters.
11.1 PARAMETERS CALLED BY "VALUE"
Calling parameters by "value" is the most common and, with the exception of arrays, the most efficient
way to pass a parameter to a procedure. The value of the expression presented in a procedure call,
known as the actual parameter, is evaluated on entry to the procedure and assigned to a formal param-
eter within the procedure. This formal parameter acts exactly as if it were a local variable of the pro-
cedure which is initialized with the value of the actual parameter supplied in the call to the procedure.
Since, in the case of arrays or strings, a new copy of the array or string is made, this type of param-
eter-passing for arrays and strings (if they are very long) should be avoided unless it is specifically
required.
11.2 PARAMETERS CALLED BY "NAME"
Calling parameters by "name" is a very sophisticated method of passing a parameter to an ALGOL pro-
cedure. Whenever the formal parameter associated with the actual parameter in a procedure body
Version 2A ALGOL 11-1 May 1972
appears in the body of the procedure, the actual parameter is re-evaluated as if it appeared in the
procedure body at that point. For example, if the actual parameter were an array element such as
ACTI)
it would be re-evaluated using the value of I available each time the formal parameter is used, not the
value of I at the time the procedure body is entered.
Table 11-1 shows the different types of formal parameters, together with valid actual parameters that
can be substituted in a procedure call.
Table 11-1
Parameters in a Procedure Call
Formal Parameter Type
Permissible Actual Parameter
Integer
Real
Long Real
Boolean
String
Label
Switch
Integer Array
Real Array (or Array)
Long Real Array
Boolean Array
String Array
Procedure
Integer Procedure
Real Procedure
Long Real Procedure
Boolean Procedure
String Procedure
Any arithmetic expression
Any Boolean expression
Any string expression (refer to Chapter 13)
A label or switch element (refer to Chapter 12 and
Paragraph 14.4)
A switch
An array of type integer*
An array of type real *
An array of type long real *
An array of type Boolean
An array of type string
À non-type procedure
A procedure of type integer, real, or long real
A procedure of type Boolean
A procedure of type string
*In the case where the array parameter is called by value, any arithmetic type
(integer, real, or long real) array is allowed as an actual parameter. A type
conversion takes place during the copying process.
11.3 PROCEDURE HEADINGS
Procedure headings identify the type of procedure and the number and type of its parameters. They
precede the body of the procedure.
A procedure heading consists of:
a. The type of procedure (omitted in the case of typeless procedures).
b. The word PROCEDURE followed by the name of the procedure.
c. A semicolon if the procedure has no parameters; otherwise
d. A list of the formal parameters, enclosed in parentheses, and followed by a semi-
colon.
e. Specifications of the formal parameters. Omitting formal parameter specifications,
this looks like
LONG REAL PROCEDURE LkKs
RCOLEAN PROCEDLIKE BCCLCÓON CIsdosK)3
PROCEDURE CALCCTHETA3,X)3
The formal parameter specification that follows consists of a list of descriptions of the formal param-
eters, appearing in any order, and a value specification if any of the parameters are to be called by
value. (If this is omitted, the parameters, by default, will be called by name.) For example, the
specification of the formal parameters for the second example above might be:
VALUE 1,J3 INTEGER IsJsK3
meaning that all three formal parameters are of type integer (scalars), and I and J are to be called by
value, while K is to be called by name. A typical formal parameter specification for the third ex-
ample might be:
REAL PROCEDURE THETA3 ARRAY X3
11.4 PROCEDURE BODIES
The body of a procedure is that part that follows the procedure heading. It consists of a single state-
ment, a compound statement, or a block. In the last-mentioned case, there may be declarations of
local variables within the block, and also other blocks or procedures. Consider the following examples
of realistic procedures:
July 1974
A real procedure, squareroot, to calculate the square root of a real quantity. The
first parameter is the argument; the second is a label that is used as an escape if
the argument is found to be negative. The result of the procedure is the square
root of the argument. Note how the result of the calculation is assigned to the
procedure by placing the name of the procedure on the left-hand side of an
assignment.
RFA} PROCFNIIKFE SOUAKEKQOOTCX3>L 33
VALL'F X3 REAL As LABEL Ls
НЕС ТК KFAL №, 7 3
Е ОХ < © О ТНЕМ СОТС |;
(1 + X)/25
Y 3
IT: Z 2= (A/Y + Y)I/23
IF ARSZ - Y) < 1&-6 THEN GOTO Ок;
Y := ¢5 GOTO ITs;
~
Ок: SQOIUAKEKOOT :2= Z
END
The previous example uses the Newton-Rapheson method of finding the square
root of a number: taking an initial approximation (1 + X)/2 and iterating until
the difference between successive approximations is less than 18-6. Although
this is a very simple procedure, it is more enlightening with the aid of some
commentary. The DECsystem-10 ALGOL alternative method of commentary
(refer to Chapter 2) is used for brevity:
REAL PROCEDURE SGLAREROCTCX,L);
VALUE X3 REAL Xs LABEL Ls
BEGIN ! CALCULATES THE VALL'E OF SGERTCA)
USING THE NEWTON-RAPHESON METHOD.
L IS USED FOR AN ESCAPE IF x < 03
REAL YoZs:
IF X < O THEN GCTO Li ! EXIT IF X < 03
Y 2= (1+X)/23 ! FIRST APPROXIMATION:
IT:
Z 2= (X/Y + Y)/2: ! ITERATE;
IF ABS(Z-Y) < 18-6
THEN GOTO Ok: ' TEST FOKk CONVEKGENCE 3
Y := Z3 GOTO IT: ! OTHERWISE CONTINUE;
(continued on next page)
OK :
SQUAREROCT := Z3 ! FINAL RESULT:
END
b. This function evaluates the sum of the values of any real procedure G over the
integers 1 ..... N, where N is also a parameter of the procedure.
REAL PROCEDURE SUMCG,N)3
VALUE Ns REAL PROCEDURE Gs INTEGER Ns
REGIN INTEGER Is REAL X3
FOR I := 1 UNTIL N DO X := X + GN);
wn
C
<
И
x
END
Notice in this example how the formal parameter G is invoked so that the actual
procedure that is substituted for G is called.
11.5 PROCEDURE CALLS
In the preceding example, the procedure G was "called". Since G is a function procedure, it is only
necessary for its name to appear in an expression for the procedure to be entered with the actual
parameters specified substituted for the formal parameters.
The procedure squareroot can be called in a similar way, for example:
P := SOUARFROOT(Z + 4.5)
causes the square root of Z + 0.5 to be calculated.
An example of the use of the procedure sum can be used to calculate the sums of the square roots of
the first J integers, with the result squared, as follows:
X := SUMCSQUAREKOOT >J)t23
11-5
Here is a further example of a procedure and its calls:
PKOCEDUKE MATRIXMULT CA>B>C>ND)3
VALUE N3
BEGIN
ARRAY A,B,C
INTEGER I3>J>K3
5 INTEGER №}
REAL X3
COMMENT THIS PROCEDURE PERFOKWS THE MATRIX
MULTIPLICATION OF B AND C AND PUTS THE RESULT
IN A.
THE ARRAYS ARE ASSUMED TO BE SQUARE
AND OF BOUNDS 1:N>1:N35
BEGIN X := 03
UNTIL N DO X
UNTIL N DO
UNTIL N DO
и
x
+
ВЕСТ, К) ЖС (К, 73;
АСТ» 3
END
END
A typical call for this procedure might be
MATR IXMLLTCE »F GIN);
or
MATRIXMULTCF>F>F3,N)3
Since the arrays are called by name, a call such as MATRIXMULT(E,E,F, N); would give rather interest-
ing results.
This call could be made to work by calling B and C by value. However, this would increase the over-
head of the procedure considerably.
11.6 ADVANCED USE OF PROCEDURES
11.6.1
Jensen's Device
This method of using a procedure exploits the power and flexibility of the call-by-name concept.
Consider the following example:
t= X
December 1971
REAL PROCEDURE SUM(IsNsX)3 VALUE Ns INTEGER I>N3 KEAL X3
BEGIN REAL Y;
Y := Ds
FOR TI 2= 1 UNTIL N DO Y t= Y + Хх;
SUM := Y
END
On the surface, the procedure appears to calculate the value of N*X. However, consider the call
Z 2= SUMCJII210,ACJI):;
and remember that J and A[J] are parameters called by name. Since I and consequently J take new
values, each X in the loop is evaluated as a particular value of A[J], using the value of J just assign-
ed. Hence the above call calculates
All] + A(2] + eceeee + A[10].
Similarly, the call
Z 2= SUMCK2MsACT,KIXBLCK,J]);
calculates the (I,J)th inner product of A and B.
11.6.2 Recursion
ALGOL procedures have the inherent ability of recursion, that is, they may call themselves, directly
or indirectly, to any reasonable depth. (The only restriction is the amount of core storage available
to the object program.)
An often-quoted and very inefficient method of calculating the factorial function of a small positive
integer N is:
INTEGER PROCEDURE FACTORIAL(N); VALUE N3 INTEGEK N3
IF № = 1 THEN FACTORIAL := 1
ELSE FACTORIAL := N*FACTORIALCN-1)3
Note that this procedure has only a single statement, but no local variables. Therefore, it can be
written in a very compact form. A call such as
J 2= FACTORIALC6)3
causes the procedure to be entered with N equal to 6. The call to FACTORIAL inside FACTORIAL
enters the procedure a second time with N equal to 5, but this N is different from the one to the pre-
vious N, which retains its value of 6, as it is stored in a different space. In this particular case,
FACTORIAL is entered six times, the last time with N equal to 1.
11.7 LAYOUT OF DECLARATIONS WITHIN BLOCKS
Declarations must always be made at the head of a block, before any assignments, procedure calls,
etc., in the following order: 1) scalars and arrays and 2) procedures and switches (see Chapter 12).
Any procedure bodies that occur in a block should follow the declarations at the head of the block,
although this is only enforced when necessary. Consider the following example:
BEGIN
PROCEDURE PCX)3 VALUE X3 KEAL X3
BEGIN INTEGEK J3
FND 3
INTEGER 1;
The assignment of I to J within the body of P utilizes the I that is declared following the body of P,
rather than some global I. However, the compiler has not yet seen this I and, therefore, cannot take
any rational action. In a case such as this, the user must declare I before the body of P:
BEGIN INTEGER 135
PROCEDURE P(X)5 VALUE X3 KEAL Xj
BEGIN INTEGER 3
END 3
If the user neglects to declare I before P, the compiler can easily detect the condition, because either
I is unknown at the time of the assignment to J, or else there is a more global I available, whereupon
an error message will occur when the declaration of I is found following the body of P.
Version 2A ALGOL 11-8 May 1972
11.8 FORWARD REFERENCES
Although most ALGOL-60 compilers operate in two or more passes, the DECsystem-10 ALGOL compiler
operates in one pass. Consequently, it has to make some minor restrictions to ALGOL-60 in order not
to restrict the user in other ways.
A forward reference for a procedure has to be given when a procedure is called (either directly, or in-
directly, by passing its name as an actual parameter in a procedure call) before its body is encountered
by the compiler. In most cases the user can avoid this situation by a minor re-ordering of the program.
However, in rare cases like the following, where procedure P calls procedure Q, and vice versa, a
forward reference, as shown, must be given.
BEGIN
| FORWARD REAL PROCEDURE @3
PROCEDURE PCX)3 VALUE X3 REAL X;
BEGIN KEAL Y3
END 3
REAL PROCEDURE GCZ)3 VALUE Z3 REAL 23
BEGIN RKEAL F3
END 3
In general, a forward reference consists of the word FORWARD, followed by the type of the procedure
| (omitted if the procedure is typeless), the word PROCEDURE, and the name of the procedure.
For example:
| FORWARD LONG REAL PROCEDURE INTEGRATE
or
FORWARD PRCCEDURE PROBLEM
Version 2A ALGOL 11-9 May 1972
Note that the forward reference must occur in the same block as the procedure body to which it refers.
| A forward reference has to be given for a label in one of the following rare cases:
a. The label is used as an actual parameter in a procedure call, and has not yet
appeared in the program.
b. A variable of identical name has appeared in the program and is in the scope of
the procedure call.
For example:
BEGIN REAL Ls
BEGIN FORWARD L35
PCL)
|: ооооо
END 3
In this case, a forward reference for L must be given.
11.9 EXTERNAL PROCEDURES
If it is required to compile a procedure independently of a program (see Paragraph 18.1.1), an
EXTERNAL declaration must be made in the program instead of the procedure. The form of this
is the same as that of a FORWARD declaration, but with the word FORWARD replaced by EXTERNAL.
For example:
EXTERNAL INTEGER PROCEDURE CALC
Such an EXTERNAL declaration can be made in any block within the program, and has the same scope
as if the procedure appeared at that point.
At present all EXTERNAL procedure names referenced in a program or scanned in a library must differ
in their first six characters, as only the first six characters are available to LINK.
Version 5A ALGOL 11-10 May 1975
11.10 ADDITIONAL METHODS OF COMMENTARY
Two further ways of writing commentary are available to the user in addition to COMMENT and |
described in Section 2.4.
11.10.1 Comment After END
Following the delimiter word END, the user may add commentary, terminated by a semicolon, with the
following restrictions:
1. The commentary may only contain letters and digits.
2. If the reserved delimiter word mode of compilation is employed, any words
appearing in the comment may not be delimiter words.
For example:
END OF PKOC INVERT 3
11.10.2 Comments Within Procedure Headings
This method of commentary allows the user to comment formal parameters in a procedure heading. This
is done by enclosing the commentary, which may consist of letters only, between the symbols ) and :(
and omitting the comma on the left of the formal parameter. This cannot apply to the first formal
parameter.
The example in Section 11.6.1 can thus be rewritten:
REAL PRCCEDURE SUMCI) CCUNT:(N) INCKENMENT:(C(X);
In a similar fashion, a call to such a procedure can be commented. The following example uses the
call to SUM in Section 11.6.1:
Z:=SUM(K) CCUNTER:(M) CROSS PRODUCT: CACI,»KI*BCK.J]);
11-11 December 1972
CHAPTER 12
SWITCHES
12.1 GENERAL
Switches enable the user to jump to one of a number of labels, depending on the value of an arithmetic
expression. In addition, they provide an automatic detection when such an expression is out of range
for the switch.
12.2 SWITCH DECLARATIONS
A switch declaration takes the form of the word SWITCH followed by (a) the name of the switch, (b)
an assignment (:=), and (c) a list of labels, called switch elements, all of which must be in the scope
of the switch declaration. For example:
SWITCH SW := LAB>L1>L2>0K’>STOP
À switch name must follow the usual rules of scope with regard to its use and, therefore, must not
conflict with any local variable of the same name.
In addition to the example above, a switch element itself may be one of the labels in the switch
declaration.
12.3 USE OF SWITCHES
À jump to a particular label in a switch declaration is made by following the word GOTO with the
name of the switch and an arithmetic expression in square brackets. Thus:
GOTO SW(1I]
This causes control to pass to the I'th label in the switch declaration, unless I is negative or zero, or is
larger than the number of switches in the switch declaration. In either case, there is no transfer of
control. If the expression in square brackets is not integral, it is evaluated and rounded as usual .
12-1
Consider the following more complicated example:
SWITCH SW := LAB>L1>L2>0K,STOP3
SWITCH TW := L3,SWCJJ,Las
GOTO TWCI13
If I has the value 3, a jump to L4 occurs. If I has the value 2 and J has the value 1, a jump to LAB
occurs, via SW.
More sophisticated switch elements are described in Chapter 14.
12-2
CHAPTER 13
STRINGS
13.1 GENERAL
DECsystem-10 ALGOL-60 includes a major extension to the string features defined in the
Revised Report. Users wishing to run their programs on machines other than the DECsystem-10
should check whether the compiler they will use offers similar facilities. Scalar, array or
procedure variables may be of type STRING, and are declared by the delimiter word STRING.
The byte size, length and contents of string variables are defined via the various assignment
statements described below.
Typical string declarations might be:-
STRING S,T; STRING ARRAY SACI:101;
STRING PROCEDURE BCX)s VALUE X: REAL X:
13.2 STRING EXPRESSIONS AND ASSIGNMENTS
String expressions are limited to a single string variable, a string procedure call or a string
constant. (For a full description of string constants see Section 4.4.) The only string operators
are the comparison operators and the assignment operator. All other operations are achieved via
the string library procedures described in Section 13.7.
String expressions can be assigned only to string variables. For example:-
S:=T:
SALI]: =SAL[83]):
SAL? ): =B (7):
T:="ANY ""*oL,p"" IRON":
Version 5A 13-1 May 1975
13.3 BYTE STRINGS
The value associated with a string variable is a byte string. A byte string is a sequence of bytes
of a uniform size between one and thirty-six, which can be efficiently handled by the
DECsystem-10 hardware. In some ways they can be thought of as arrays, but the most important
difference between an array and a byte string is that the size of a byte string can vary con-
tinuously. Thus when a byte string is created by means of a READ statement, the programmer
need not know how long the string will be. The routine starts accepting characters after it
encounters the opening quotation marks and continues until it finds the closing ones.
When one string is assigned to another, e.g.,
then a copy of T is made to which S will refer, Any previous value S had is destroyed (and the
heap space it occupied is released). Subsequent changes to the value of T will not affect S, or
vice versa, unless a further assignment is made from one to the other.
\
Please note that this is an important change from the implementation of strings prior to Version
>.
13.4 BYTE SUBSCRIPTING
Byte strings can be modified by means of the byte subscripting mechanism, Individual bytes in
a string are referenced by following the string variable name by a decimal point and then the
subscript number enclosed in square brackets, For example
Se[I]
refers to the I'th byte of string S. The subscript may be any arithmetic expression and is eval-
vated in exactly the same way as an array subscript.
Byte-subscripted string variables are regarded as being of type integer, having an integer value
equivalent to the byte to which they refer. It follows that to change the value of a particular
Version 5A ALGOL 13-2 May 1975
byte in a string, a byte-subscript must appear on the left-hand side of an arithmetic statement
with the appropriate new value on the right-hand side. Should the new value be too large to
be held in the byte, it is simply truncated. No warning is given.
13.5 NULL STRINGS
Until a value is assigned to a string by the program, it is assumed to have the value null. That
is, it is assumed to contain no bytes. Any attempt to reference it by a byte-subscript will
result in a fatal run-time error, though it can be used on the right-hand side of a string assign-
ment, in which case the variable to which it is assigned similarly becomes null,
13.6 STRING COMPARISONS
Two byte strings can be compared with each other using the usual comparison operators. For
example
IN S< T THEN RO TO |;
where S and T are string variables, string constants or string procedures. The effect of the
comparison is to compare the strings byte-by -byte, the "lesser" string being that with the
first lower value byte, working from left to right. Thus "ABCD" is less than "ABCE" or
"ABCDE", Where the strings to be compared are of different byte sizes, then the smaller bytes
are regarded as being extended on the left by null bits,
In the special case of ASCII strings (strings of byte size 7, like string constants), trailing nulls
and trailing blanks, or any mixture thereof, are treated as equal. Similarly ASCII strings of
different lengths will compare equally if the extra length comprises only spaces and nulls. In
all other cases strings of unequal length can only be regarded as equal if the extra length is
comprised entirely of null bytes.
13.7 LIBRARY PROCEDURES
Section 16,6 deals with the input and output procedures that are applicable to strings.
The procedures LINK, LINKR and TAIL that were included in the library until Version 3B have
been dispensed with,
Version 5A ALGOL 13-3 May 1975
13.7.1 Concatenation
A string can be assigned the concatenated value of two strings with the string procedure
CONCAT, For example
5; = СОМСАТСТ, Ш) ;
Se: =CONCATCS, T);
If, in the first example, T had a different byte size from U, then the size of the first string
encountered (T in this case), would be adopted by S. The bytes copied from U would be trun-
cated or filled with null bits as appropriate.
13.7.2 Length and Size
The primary attributes of a string, its length in bytes and byte size in bits, are returned by the
integer LENGTH and SIZE, respectively.
Thus
I:=LENGTH(S)3 J:=SIZE(S3
would return the number of bytes in string S in |, and the number of bits in each byte in J.
In the case of a null string both procedures return zero.
13.7.3 Copying
The new byte string can be generated from an existing one by means of the string procedure
COPY. This procedure can have one, two or three parameters.
a. The effect of copy with one parameter is precisely
the same as a simple string assignment, but this
feature has been retained for the sake of continuity.
b. Where there are two parameters, e.g.,
5: = СОРУ СТ, М);
where M is an arithmetic expression, then S is
assigned the value of the first through M'th bytes of T.
Version 5A ALGOL 13-4 May 1975
Ce If there are three parameters, e.g.,
S:=COPY (ToI1,N);
where both M and N are arithmetic expressions,
then S is assigned the value of the M'th through
N'th bytes of T.
13.7.4 Newstring
Although it is not possible to refer to bytes in a null string (see Section 13.5 above), it is
possible to create a string of any appropriate byte size but containing nulls, by using this
string procedure. NEWSTRING takes two parameters, the first being the number of null
bytes to be assigned to the string, and the second their size.
For example
S:=NENSTRINGC192,7):
causes a null string of 100 ASCII nulls to be assigned to S. Notice that although S is 100 bytes
long at this point, and thus byte subscripts up to and including S.[100] are valid, if a subsequent
assignment of another string were made to S, both its length and its byte-size might change
(depending, of course, on the length and byte-size of the other string!).
13.7.5 Delete
In Section 13.5 it was explained that if a null string is assigned to another string, then that
string also becomes null, and its previous value is lost, Any space that the previous value
occupied is returned to the heap, as with any ordinary string assignment, The typeless
procedure DELETE has the same effect on the string passed to it as a parameter as the assignment
of a null string would have. Deleting a null string has no effect, beyond using computer time.
Version 5A ALGOL 13-5 May 1975
CHAPTER 14
CONDITIONAL EXPRESSIONS AND STATEMENTS
14.1 GENERAL
ALGOL-60 allows great flexibility in the construction of expressions and conditions.
Consider, for example, if a user wanted to set a variable I equal to 0 or 1 according to the value of a
Boolean variable B, he could write:
I := (35
IF B THEN I := 13
Also, consider the case where a user wants to perform some action, depending on the value of B:
IF B THEN X1 := Y3 IF NCT B THEN X@ := Y3
14.2 CONDITIONAL OPERANDS
ALGOL-60 allows the user to substitute a conditional operand for any operand in an expression by the
use of a construction involving IF ..... THEN ..... ELSE.
For instance, the first example above can be rewritten
I := IF B THEN @ ELSE 13
Clearly, this is more compact and of great use in cases such as:
J 2= J + CIF K <1 THEN 1-K ELSE K-1)3
Note that the conditional operand must be bracketed. It may be unbracketed only when it forms the
complete expression itself.
In general, a conditional operand may replace an operand in any arithmetic or Boolean expression. It
may also be used in place of a label as the element in a switch list, for example:
SWITCH SW := L!> IF B THEN L2 ELSE L3, La;
14-1 December 1971
It is also permitted, of course, in an array subscript (and also in a byte subscript), for example:
X 2= ACI, IF L = Q THEN J ELSE J+1)5
Since a conditional operand may replace any operand in an expression, it may also replace operands
in conditional expressions. Consider the following example:
IF IF B THEN Bl ELSE B2 THEN I := 1 + 15
This looks complicated but is really quite simple if brackets are inserted for clarity. Thus:
IF CIF B THEN BI ELSE 82) THEN 1 := I + 13
14.3 CONDITIONAL STATEMENTS
The reader was introduced to conditional statements of the form
IF B THEN S1 ELSE S2
| in Chapter 7. The full power of this type of statement can now be demonstrated.
First, S1 and S2 can be compound statements or blocks. For example:
IF 1 < @ THEN
BEGIN I := -Is B := FALSE
END ELSE
BEGIN I := I + 13 GOTO 12
END
Second, the whole structure of the IF ..... THEN ..... ELSE statement can be made more powerful
by using conditional statements within themselves. For example:
IF X <O THEN X := @ ELSE IF B THEN GOTO |
This is equivalent to the simple sequence of statements:
IF NOT X < Y THEN GOTO Lls
X := 0% GOTC Les
Li: IF NOT B THEN GOTO 1-2;
GOTO Ls
12:
Clearly the former method of expression is both briefer and more elegant.
Version 2A ALGOL 14-2 May 1972
Conditional statements take the general form
IF B THEN S1 ELSE S2
where 51 and 52 may themselves be conditional statements with the provision that if there is ambiguity,
bracketing using BEGIN and END must be used to remove it. Consider the following illegal example:
IF B THEN IF X = @ THEN Y := Z ELSE P := G3
This could be interpreted as
i
Ss
=)
xX
™
zZ
<
Н
A
IF B THEN BEGIN IF X END ELSE P := Qs
or
IF B THEN BEGIN IF X = Y THEN Y := 7 ELSE P := Q END;
The first case is interpreted as:
че
IF NOT B THEN GOTO Ll
IF NOT X = Y THEN GOTO Les
Y := Zs GOTO 12;
The second case is interpreted as:
IF NOT B THEN GOTO LZ;
IF NOT X = @ THEN GOTO |1;
Y := Zs GOTO Les
ALGOL-60 forbids such ambiguities by forbidding the sequence THEN IF ..... THEN ..... ELSE.
14.4 DESIGNATIONAL EXPRESSIONS
A designational expression is something that acts as an argument in a GOTO statement, either directly
or indirectly, via a formal procedure parameter of type label. It may simply be a label or a switch
element. Thus the following are designational expressions:
L
IF B THEN L1 ELSE L2
IF X < O THEN SWCI) ELSE IF X+Y >= Z THEN TWCJ)] ELSE L
Version 24 ALGOL 14-3 May 1972
These designational expressions would be used in the following manner:
GOTO Ls
GOTO IF B THEN LI ELSE Les
GOTO IF X < Y THEN SWLIJ ELSE IF X+Y >= Z THEN TWCJ] ELSE Ls
Version 2A ALGOL 14-4 May 1972
CHAPTER 15
OWN VARIABLES
15.1 GENERAL
| Own variables are a special kind of ALGOL variable, and may be of type integer, real, long real,
Boolean or string, either scalar or array. They have the following properties:
a. Although they follow the normal scope rules, they are not recursive, the same
copy of each variable being used in all occurrences of a procedure or block.
. b. The values they contain when control passes out of a block are retained and
are still available when the block is re-entered.
c. They are initialized to zero before execution of the program. (FALSE in the
case of Boolean own variables.) OWN STRINGS are initialized to possess no
byte string.
Own variables are declared by writing the usual declaration with the word OWN preceding it. For
example:
OWN INTEGER I,JsK3
OWN REAL ARRAY THETAL1l:M]
15.2 OWN ARRAYS
Own arrays are implemented in a completely dynamic fashion in DECsystem-10 ALGOL. The
declaration proceeds according to the following rules.
a. If this is the first time the array is declared, space is obtained and then the array
laid out. If the array has been laid out before, proceed to Step b.
b. The bounds are examined to see if they are identical to those of the previous con-
struction of this array. If they are the same, the array is left unaltered; otherwise,
proceed to Step c.
c. A new array is constructed and those elements that it has in common, if any, with
the old array are copied into it; the remaining elements are zeroed. The old array
is then deleted and the space used by it is recovered for future use.
15-1 December 1971
For example, if an own array A is declared as follows:
OWN REAL ARRAY AL[1:MsM:NJ3
where M = 2 and N = 5 the first time, and M = 1 and N = 4 the second time, the elements [1,2],
[1,3] and [1,4] are copied over, and the remaining elements of the new array are zeroed.
15-2
CHAPTER 16
DATA TRANSMISSION
16.1 GENERAL
Data transmission encompasses the input and output of data between the user's program and peripheral
devices, such as disk, DECtape, magnetic tape, card reader, card punch, and line printer. The
DECsystem-10 ALGOL object-time system, in conjunction with the ALGOL library, provides the user
with a set of basic procedures for handling data from most DECsystem-10 devices in a uniform fashion.
The user may also perform input/output operations with virtual peripherals that manifest themselves as
byte strings in the user's program.
All peripheral devices that the user requires are under his control completely and can be allocated or
released at any time throughout the execution of the program. The user can handle up to sixteen de-
vices simultaneously (seventeen, if one of them is the terminal attached to his job), any number of
which may be file devices (disk, DECtape) and have an independent file open.
16.2 ALLOCATION OF PERIPHERAL DEVICES
Peripheral devices are allocated to the user's program by calls to the library procedures INPUT or
OUTPUT. A call to one of these procedures usually has two parameters. The first is the channel num-
ber, an integer in the range 0 to 15, on which the device is to operate. Only one device at a time
may be operated on a channel; a channel provides either input or output facilities, except in the case
of a terminal, where the input and output functions are performed simultaneously on the same channel.
The second parameter is either a string or a string constant. The text contained in the string is the
logical name of the device to be allocated to this channel.
The DECsystem-10 Users Handbook should be consulted for an explanation of what constitutes a logical
device name. In the simplest case, it may be the actual name of the peripheral device. The device
names shown in Table 16-1 are recognized as standard.
16-1 December 1971
Table 16-1
Standard Device Names
Device Name Peripheral
DSK Disk
DTA DECtape
MTA Magnetic tape
CDR Card reader
CDP Card punch
LPT Line printer
PTR Paper-tape reader
PTP Paper-tape punch
PLT Plotter
TTY Terminal
For example, to allocate the card reader for use as an input device on channel 5, the user would use
the statement
INPUT CS >" CDkK'") 3
or, if S were a string possessing a byte string that had the characters CDR in it,
INPUT (55S) 3
Similarly, if the disk were to be used as an output device on channel 9:
OUTPUT C9>"DSK'") 3;
Note that with the exception of terminals, all devices are allocated to operate in one direction only;
thus, if the user wants input and output from the disk, he must use two separate channels.
Terminals are always allocated bidirectionally , irrespective of whether the user uses INPUT or OUTPUT.
For example,
INPUT CO," ТТУ");
allocates the user's terminal for input and output on channel 0.
16.2.1 Device Modes
Normally, a device is allocated in ASCII mode, that is, when the user reads a character from the de-
vice it is a 7-bit byte representing readable text, such as a stored source program or data. To allocate
the device in a different mode, a third parameter is specified in the call to the INPUT or OUTPUT pro-
cedure. Thus, to allocate a disk to channel 9 in image binary mode (the mode used for the storage of
binary data on a disk), the user can use
OUTPUT (9>"DSK'' > 113
16-2
The DECsystem-10 Assembly Language Handbook should be consulted for a full explanation of
the different modes used with peripheral devices. The INPUT and OUTPUT procedures allow
the user to allocate any standard peripheral device in any buffered mode.
16.2.2 Buffering
The INPUT and OUTPUT procedures normally allocate two buffers for each allocated device;
terminals are allocated two buffers for input and two for output. The user may desire to use
either one or more than one buffer for a device. For example, in a non-compute bound job
that uses a lot of disk transfers at odd intervals, four or even eight buffers may be desirable to
increase the speed of execution of the program,
The number of buffers to be used can be controlled by adding a fourth parameter to the procedure
call, Thus, to allocate a disk on channel 14 in mode O with eight buffers, the call is
OUTPUT C14>"DSK'" >f >8) 3
Note that the mode must always be specified in this case, otherwise there would be an ambiguity
in the third parameter.
16.2.3 Error Returns
Normally, if the device allocation fails (for example if the device is in use by another job), a
suitable message is typed and the program terminated. The user can prevent this by providing,
as the fifth parameter to INPUT or OUTPUT, a label to which control is to be passed in the
event of an error. Note that the third and fourth parameters must be present in this case to
avoid ambiguity: they may be zero when the default values will be used. For example:
QUTPUTCIA, MITA" ,0,6,ERROR.LABEL):
If the actual label parameter is a switch whose subscript is out of range, the procedures behave
as though the label parameter were absent.
16.3 SELECTING INPUT/OUTPUT CHANNELS
Before a user uses a device to transfer data, assuming that the device has already been allocated
to some channel, the appropriate input or output channel must be "selected" for use as the input
or output channel, All data input and output always occurs on the currently selected input
channel and output channel, respectively. The user may change the selection of channels at
any time, switching from one channel to another without loss of data, irrespective of whether
Version 5A ALGOL 16-3 May 1975
complete lines (or records) of data have been read or not. In fact, the DECsystem-10 input/
output system does not assume any structure in the data: all input and output channels are re-
garded as pipelines through which the user pulls or pushes data,
To select an input channel, a call to the procedure SELECTINPUT must be made. This has one
parameter, which is the channel number. Thus
SELECTINPUT CS) 3
causes input channel 5 to be selected.
Similarly, the procedure SELECTOUTPUT is used to select an output channel.
16.4 FILE DEVICES
Some peripheral devices, such as disk and DECtape, require the opening of a specifically named
file before any input or output operations can be performed. This optionally may be performed
on spooled devices (refer to Chapter 3 of DECsystem Operating System Commands for a description
of spooling). The opening of this file is performed by means of the procedure OPENFILE, which
is called after the device has been allocated to a channel. The procedure call has two
parameters: the channel number on which the device has been allocated and a string variable
possessing a byte string or a string constant, the text of which is the name of the file.
The user can also specify a protection and/or project-programmer number of a file by means of
optional third and fourth Boolean or integer parameters, For example, to open a file with
protection 177 on disk area [11,50] the user could write
OPENF ILE (9," TEST «DAT» %177,%0000110000G50) 3
When a user has finished with a file it should be closed. A file is closed by using the procedure
CLOSEFILE, with a parameter that is the channel number on which the file is open. Thus,
CLOSEFILEC9)3
closes the file that is open on channel 9.
Version SA ALGOL 16-4 May 1975
The user may also rename or delete existing files: if a file is already open, use of OPENFILE
causes the file to be renamed with the new name supplied. Thus the sequence
OPENF ILE (S,"TESTI1.DAT');
OPENF ILE (S>"TEST2.DAT');
causes the file with name TESTI .DAT to be renamed TEST2 .DAT.
If the string containing the new name is null, the original file is deleted. Thus,
OPENFILE (5,"TEST3.DAT'")3;
OPENF ILE (S9''')3
causes the file TEST3.DAT to be deleted,
16.4.1 Error Returns
Normally, if the operation requested fails (for example an input file does not exist), a suitable
message is typed and the program terminated, The user can prevent this by providing a label
and an optional integer variable as the fifth and sixth parameters to OPENFILE. In the event
of an error, control will be passed to the label, with an error-code set into the integer
variable if present, The error-codes are those returned by the ENTER and LOOKUP UUO'S:
refer to Appendix E of the DECsystem-10 Monitor Calls Manual.
Note that the third and fourth parameters must be present if the error return parameter is
specified: they may be zero in which case the default values will be used.
If the actual label parameter is a switch whose subscript is out of range, the procedure behaves
as though the label parameter were absent, The integer error-code parameter is called by name.
16.5 RELEASING DEVICES
The procedure RELEASE is used to release a device from a channel, Thus,
RELEASE (5);
Version 5A ALGOL 16-5 May 1975
releases the device allocated to channel 5. If the device is a file device, and a file is still open on
the device, it is automatically closed. Releasing a device on a channel causes a channel to become
free; if this channel is currently selected for input or output operations, it is deselected.
If an attempt is made to allocate a device to a channel that already has a device allocated, the allo-
cated device is first released and, if a file is open on it, it is closed before releasing the device.
If a user terminates his program without releasing devices on channels, they are automatically released.
16.6 BASIC INPUT/OUTPUT PROCEDURES
16.6.1 Byte Processing Procedures
The following procedures may be used with any device to handle bytes of any standard size (1 to 36
bits). However, because they are normally used with devices supplying or receiving ASCII bytes, they
are "symbol" oriented.
a. INSYMBOL(S); - (where S is usually some integer variable) causes the next byte
to be read from the currently selected input channel and stored in 5.
b. OUTSYMBOL(J); - (where J is usually some integer expression) causes the value
of J to be output as a byte to the currently selected output channel. If J is too
large for the byte size of the device in use, it is truncated to size.
c. NEXTSYMBOL(S); - acts in exactly the same way as INSYMBOL except that
the byte pointer for the input channel is not advanced to the next available
byte. This gives the user a look-ahead facility of one byte.
d. SKIPSYMBOL; - causes the next byte from the selected input channel to be
read and ignored.
e. BREAKOUTPUT; - causes all bytes in the buffer of a device to be sent immedi-
ately to it. This procedure is normally used to conduct a question-and-answer
dialogue on a terminal, with the question and answer on the same line. Nor-
mally, a block of data is sent to a device only when the buffer is full (the
exception being the terminal, where a break is sent at the end of each line).
16.6.2 String Output
A byte string may have its contents transferred to the currently selected output channel by means of
the procedure WRITE, whose single parameter is either a string constant or a string variable that
possesses the string to be output. For example:
WRITECS)3
or
WRITE("THE MOON IS MADE OF GREEN CHEESE"):
16-6
With exceptions explained in the following paragraphs, all of the bytes in the string are output literal-
ly, with the exception, of course, of the quotes in a string constant, which are not in fact stored in
the byte string at all. The important thing to remember is that, unlike some other ALGOL implemen-
tations, spaces and other non-printing symbols in byte strings are meaningful.
Special editing characters are permitted within square brackets within the text of a byte string. These
have a special function:
P Page throw
Cor N New line (C stands for carriage return, line feed)
T Tab
S Space
B Break output
Any combination of these characters, with optional repetition counts preceding them, can appear with-
in square brackets in a byte string and are output as their special interpretation demands. For example:
URITE C'ABCDEP2CSSJEFGH");
causes the following to be output:
a. the symbols ABCD followed by a page throw
b. two new lines and five spaces
c. the symbols EFGH.
In order to output the symbols
[ ] ‘or;
they must appear in the form
[[ ]] ""or;;
respectively. Thus
WRITE C'"ACEIDD := 33311");
causes the text
ACIJ := 33"
to be output.
16.6.3 Miscellaneous Symbol Procedures
The procedures SPACE, TAB, PAGE, and NEWLINE cause the appropriate number of spaces, tabs,
page throws, or new lines to be output, depending on their single parameter, which is an integer ex-
pression. If the parameter is omitted a value of one is assumed. Thus
Version 2A ALGOL 16-7 May 1972
SPACE (5)3
causes five spaces to be output, whereas
SPACE 3
or
SPACECI)s
causes one space to be output.
16.6.4 Numeric and String Procedures
Numeric procedures are used fo read and print numeric quantities. They will normally be used with a
device that is operating in ASCII mode. They are capable of processing integer, real, or long real
quantities in fixed-point and floating-point representations.
16.6.4.1 Numeric Input Data - Numeric data for input can be represented in any format that would
be acceptable as a numeric constant in a program, irrespective of the type of variable involved. When
a number is read, an automatic type conversion is performed, giving a result of the same type as if an
assignment of the data represented as a constant in the program had been executed.
There is a minor restriction in that no spaces, tabs, or other non-printing symbols may appear in such
numeric data except between the exponent sign (& or @ for real, && or @@ for long real) and the
exponent. Otherwise, any symbol that is not itself a part of a numeric quantity may act as a terminator
for such a quantity. It is strongly recommended that spaces, tabs, or new lines be used as separators.
For example:
3.4 -9.6 1.36 -52
Y 14.9
Note that in reading a numeric quantity, the terminating symbol, that is, the first symbol that is not
part of the number, is lost.
DECsystem-10 ALG OL also allows the user to input floating-point data written in FORTRAN format,
that is, using E for & or @, and D for && or @@. Note, however, that no other special effects
inherent in FORTRAN formatting are introduced.
The procedure READ is used to input numeric data and also strings. This procedure may have any num-
ber of parameters, of type integer, real, long real, Boolean, or string.
Version 2 ALGOL 16-8 December 1971
The effect is as follows:
a. For integer, real and long real variables, a number is read and converted to the
type appropriate to the parameter and then assigned to the variable.
b. For Boolean, a number is read as if for an integer variable, and assigned to the
variable.
c. For astring variable, the data text is scanned until a quote (") is found, and the
text following this up to but not including the next free quote is read in and a
byte string generated, which is then possessed by the string variable.
If the sequence
is found, a single
" is stored, and reading of the string continues.
16.6.4.2 Numeric Output Data = Numeric data is output by means of the procedure PRINT. This
procedure may have one, two, or three parameters, the first of which is the variable to be printed.
| This variable may be an integer, real, or long real. The second and third parameters determine the
format to be used and are integer expressions. If they are omitted, they are assumed to be zero. The
effect of the various combinations of the format integers, M and N, is as follows:
М >0, N> O0:
M>0, N=0:
M=0, N>0:
Fixed-point printing, M places before the decimal
point, N places after. A sign, space if positive,
- if negative appears before the number. Zeros
before the decimal point are replaced by spaces and
the sign moved up to the number.
This format always outputs M+N+2 symbols.
The same as the preceding except that (a) no frac-
tional part appears, and (b) the decimal point is
suppressed.
This format always outputs M+1 symbols.
Floating-point format, consisting of a sign, a decimal
digit, a decimal point, N more decimal digits, and an
exponent consisting of & for real, && for long real
followed by the exponent sign and a two-digit exponent,
zero suppressed from the left.
This format outputs N+7 symbols for real and N+8 sym-
bols for long real quantities.
If only two parameters appear, format M,0 is assumed for integer variables, and format 0,N for real
and long real quantities, where M and N are, respectively, the values of the second parameter.
If only one parameter appears, the format is interpreted as 0,0 which assumes standard printing modes
of 11,0 for integer quantities, 0,9 for real quantities, and 0,17 for long real quantities.
Version 2B ALGOL
16-9 December 1972
If the user requests more digits to be printed than are significant in real or long real numbers, the
appropriate number of zeros follow a properly rounded printing of the number to the maximum precision
available.
16.6.4.3 Octal Input/Output = The procedures READOCTAL and PRINTOCTAL, respectively, allow
the user to input and output quantities in octal format.
On input, for single precision variables, up to 12 octal digits are read, preceded by the symbol %, the
terminator being any non-numeric symbol. For long real variables, two such octal numbers must be
| presented for input, each preceded by the symbol %.
On output, 12 octal digits, preceded by the symbol %, are printed for single precision variables. For
long real variables, two quantities each with 12 octal digits are printed, with a space separating them:
The foregoing procedures have one scalar parameter which may be of type integer, real, long real or
Boolean.
16.7 DEFAULT INPUT/OUTPUT
If the user does not select any input or output channels, input and output occur via an "invisible"
channel from and to the user's terminal. Thus, for simple programs where the user wishes to input a
few numbers and print a few results, he simply uses READ, types in the data on line through his
terminal, and gets back the results from PRINT.
16.8 LOGICAL INPUT/OUTPUT
In addition to the 16 channels used to communicate with peripheral devices, an additional 16 channels,
numbered from 16 to 31, are provided. These are input or output channels that use byte strings as a
means of storage.
By means of the procedures INPUT or OUTPUT, the user can attach a channel to a byte string possessed
by a string variable, and can read and write bytes from and to this byte string, either to or from a
peripheral device, or to and from another byte string.
INPUT (2055);
or
OUTPUT (20,5) 3
cause the byte string possessed by the string variable S to be used as logical channel 20; this channel
may subsequently be selected for input or output, as appropriate.
Version 2 ALGOL 16-10 December 1971
The user is still free, of course, to manipulate the individual bytes within the byte string by means of
the byte-subscripting facilities available. Such facilities enable the user to read a file from a
peripheral device into a string, process it in any way whatsoever, and output it again.
16.9 SPECIAL OPERATIONS
These procedures are used on channels assigned to magnetic tapes. They consist of the procedures
BACKSPACE, ENDFILE and REWIND, each having one parameter, i.e., the channel number on which
the operation is to be performed.
Since there is no implicit structure on a magnetic tape, these procedures enable the user to build up
formats in any way he chooses.
16.10 1/0 CHANNEL STATUS
The status of any input or output channel can be determined at any time by means of the Boolean pro-
cedure IOCHAN, which takes as its single parameter an integer quantity which is the channel number.
The status returned is bit coded as follows:
Bit
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Value
%400000
%200000
%100000
%040000
7020000
010000
%004000
%002000
%001000
%000400
%000200
%000100
%000040
%000020
%000010
%000004
%000002
%000001
Meaning if Set
Device is physical (i.e., not logical)
Directory device
Terminal device
ASCII mode
Magnetic tape
Plotter
Set for default TTY on channel -1
Device is spooled
Device can do input
Device is initialized for input
File is open for input
End of file encountered
Input ok status
Device can do output
Device is initialized for output
File is open for output
Device quota (exceeded)
Output status ok
Some of these bits are of little use to the user, but, for example, if a device is allocated, and the user
does not know whether or not it is a file device, he can use IOCHAN to determine this. The bits of
particular use to the user are the input and output end-of-file (note that end-of-file on output is a
logical status indicating that, for example, a disk quota is exceeded or a DECtape is full, or in the
case of a logical device, the byte string is full).
Version 2B ALGOL
16-11 December 1972
When IOCHAN is used, the end-of-file flags are always cleared, if set, so that the user may proceed
to read a magnetic tape after an end-of-file marker is found.
The following example shows how the user would handle an unknown device whose name is given to the
program via the user's terminal:
BEGIN
STRING DEVICE, FILES INTEGEK CHANNEL 3
WRITE ("CHANNEL NO: '")3; EBERKEAK.QUTPUTS
READ (CHANNEL)
WKITE ("L[CIDEVICE NAME: ")3 BREAK.CUTPUT:
READ (DEVICE);
QUTPUT (CHANNEL > DEVICE);
IF TOCHAN (CHANNEL) AND 2209000 THEN
BEGIN
WRITE ("[CIFILE NAME: ")5 BREAK.CUTPUT;
READ (FILE);
OPENF ILE (CHANNEL, FILE)
E ND 3
END
NOTE
When using Boolean expressions involving IOCHAN,
the rules for evaluation followed in this implementa-
tion should be borne in mind. See Section 5.2.1.
16.11 TRANSFERRING FILES
Once a device has been allocated to an input or an output channel, a complete file of information
may be transferred between them automatically by calling the parameter-less procedure TRANSFILE.
This procedure copies bytes from one device to another from the currently selected input channel to
the currently selected output channel, until an end-of-file status is raised on either the input or out-
put channel.
16.12 CURRENTLY SELECTED CHANNEL NUMBERS
The number of the channel currently selected for input or output may be obtained by use of the integer
procedures INCHAN or OUTCHAN.
Version 5A ALGOL 16-12 May 1975
CHAPTER 17
THE DECsystem-10 OPERATING ENVIRONMENT
The operating environment of DECsystem-10 ALGOL programs consists of those procedures in the
DECsystem-10 ALGOL Library required by the user's program, and the DECsystem-10 ALGOL Object
Time System,
The former are those procedures detailed in Chapters 13 and 16, together with those described below.
These procedures can be thought of as existing in a block surrounding the user's program, and, there-
fore, are available when called. Their names, however, are in no sense reserved as are words such as
BEGIN.
Note also that these procedures are only present in the user's program when required. They are loaded
by the DECsystem-10 Linking Loader when so directed by the DECsystem-10 ALGOL Compiler. The
user is not required to take any action to include these procedures, other than make a call to them.
À complete list of library procedures is given below.
17.1 MATHEMATICAL PROCEDURES
| The following procedures expect one argument, of real type, and yield a real type result,
Procedure Name Function
SIN Sine
COS Cosine
ARCTAN Arctangent
SQRT Square root
EXP Exponential
LN Logarithm (fo base e)
TAN Tangent
ARCSIN Arcsine
ARCCOS Arccosine
SINH Sinh
COSH Cosh
TANH Tanh
Note that if arguments of type integer or long real are given in an ALGOL call to these
procedures, the compiler plants the appropriate conversion code.
July 1974
The following procedures expect one argument, of long real type, and yield a long real type result.
Note that theyare formed by adding an L before the equivalent single precision procedure.
Procedure Name Function
LSIN Sine
LCOS Cosine
LARCTAN Arctangent
LSQRT Square root
LEXP Exponential
LLN Logarithm (to base e)
The functions ENTIER, ABS and SIGN are also available, as described in Section 5.1.2
17.2 STRING PROCEDURES
For details of the procedures CONCAT, LENGTH, SIZE, COPY, NEWSTRING and DELETE, see
Paragraph 13.7.
17.3 UTILITY PROCEDURES
17.3.1 Array Dimension Procedures
The integer procedure DIM, which takes as its parameter the name of an array of any type, yields a
result that is the number of dimensions of the array. This is most useful when the user passes an array
as a parameter and wishes to check if it is, for example, a matrix.
The integer procedures LB and UB also take as first parameters the name of an array; their second
parameter is the subscript number. The result is the lower or upper bound, respectively , of the sub-
script specified by the second parameter. The following procedure uses these to clear real matrices.
PKCCEDURE ZEKQOCA)3 AKRAY A3
BEGIN
INTEGER 1,J3
IF DIMCA) = 2 THEN
REGIN
TETEGER L1,L2,U1,U2;
Li
LBCA3>1)33 UI := UBCA>1 335
Le
LECA>2)3 UP, := UK(A»2);
FOR I := L1 UNTIL Ul DO
FOR J t= 12 UNTIL U2 DC ALTI,J) := 0
F ND
END
Version 5A ALGOL 17-2 May 1975
17.3.2 Minima and Maxima Procedures
The integer procedures IMIN and IMAX, the real procedures RMIN and RMAX, and the long real pro-
cedures LMIN and LMAX are used, respectively, to determine the minimum or maximum of a number
of arguments of the appropriate type. These procedures normally accept up to ten parameters (this
figure may be changed by re-assembling the ALGOL library with a different parameter.
For example:
IMINCJI,2K)s
bt
ll
KEMAX CY+Z > KNINCY-Z>0))3
>
И
17.3.3 Field Manipulations
The procedures GFIELD and SFIELD enable the user to manipulate a field within any integer, real,
long real, Boolean or string variable. The integer parameters I and J specify a byte of length J bits
whose leftmost bit is the I'th bit (counting from zero at the left-hand side). The byte specified may
be from 1 to 36 bits in length and may be at any position in the variable.
For single word variables (integer, real, Boolean), I may range from 0 to 35, with the constraint
| + ) <= 36. For double word variables (long real and string), I may range from O to 71, with the
constraint [ + J <=72,
The integer procedure GFIELD uses I and J as the second and third parameters; the first parameter is
the variable. The result is the value of the byte (right justified) specified by I, J.
Thus
K := GFIELDCA,3,5)s
gives the value of the byte consisting of bits 3 through 7 of A.
The procedure SFIELD sets a byte specified by the second and third parameters I, J to the value speci-
fied by the fourth parameter, of type integer. Thus
SFIELD(CA33>5>0h)5
zeros the byte specified in the first example.
17.4 DATA TRANSMISSION PROCEDURES
For details of these procedures refer to Chapter 16.
17-3 July 1974
17.5 FORTRAN INTERFACE PROCEDURES
F-10 or F-40 FORTRAN subroutines may be incorporated in ALGOL object programs by loading these
subroutines with the ALGOL main program (and any other separate ALGOL procedures).
Such FORTRAN subroutines should be specified by an EXTERNAL declaration in the ALGOL program
and, depending on which compiler was used to compile them, are called by the procedures shown in
the table.
Table 17-1
YPE NONTYPE INTEGER REAL LONG REAL BOOLEAN
FORTRAN (DOUBLE PR.) (LOGICAL)
F-10 F1GCALL FIBICALL FIGRCALL FIGDCALL FIBLCALL
F-40 CALL ICALL RCALL DCALL LCALL
The first parameter in these procedure calls must be the name of the FORTRAN subroutine. Subsequent
parameters are taken as the arguments to the procedures.
CALL and FIBCALL are used as single statements, for example:
is equivalent to
CALL FORT (XsY)
in a FORTRAN program.
CALL CFORT2X,Y)
ICALL etc. must appear in the appropriate context in an expression, thus
:= € + ICALLCZ)
NOTE
The parameters of CALL, ICALL, etc., are restricted to
integer, real, long real, or Boolean expressions with the
restriction that if the expression is a single variable, it
must be a local scalar or a format parameter called by
value.
Version 5A ALGOL
17-4
May 1975
17.6 GENERAL INFORMATION ROUTINE
The integer procedure INFO provides information about various aspects of the environment, depending
on the value of the parameter with which it is called.
parameter returns integer value of
core size in words
date (15-bit format)
time (ticks since midnight)
time (micro-seconds since midnight)
runtime (milliseconds)
processor type (1=KA, 2=KI, 3=KL)
number of stack shifts so far
compiler version word
NN ÑOMRWN-O
For example
PRINTCINFOCA)):
might produce
1134608
- the job's runtime up to now in milliseconds.
17.7 DATE AND TIME IN ASCII FORMAT
Three routines are provided for returning the current time and date in string format suitable for
printing without modification. The two date routines, FDATE and VDATE, give the option of a
standard three-character abbreviation for the month (FDATE), or a variable-length string with the
name of the month in full (VDATE). In both cases the year is given in full. String procedure TIME
gives an eight-character string with the current time as HH:MM:SS.
For example
URITECVDATE); NEWLINE; WRITECTINE)
might produce
27-JANUARY- 1975
2:16:55
17.8 RANDOM NUMBER ROUTINES
Three routines have been included to provide a random number capability. The number generator
is similar to that used in the FORTRAN library, which is documented in the Science Library
manual, The ALGOL version is, however, initialized randomly. If it is required to generate a
Version SA ALGOL 17-5 May 1975
sequence of pseudo-random numbers in a repeatable way, then procedure SETRAN should be called
before the first reference to RAND, with the required initial value. For example, to generate the
same sequence as a FORTRAN program using the default starting value currently used by FORTRAN,
SETRAN(-1); should be included in the ALGOL program. The third procedure is SAVRAN which
returns the value of the last random number without invoking the number generator.
RAND and SAVRAN are INTEGER procedures; SETRAN is non-type.
Version 5A ALGOL 17-6 May 1975
CHAPTER 18
RUNNING AND DEBUGGING PROGRAMS
18.1 COMPILATION OF ALGOL PROGRAMS
DECsystem-10 ALGOL programs are compiled by the ALGOL compiler under the standard DECsystem-10
timesharing monitor. The compiler is called by typing
R ALGCL
at monitor command level.
The DECsystem-10 ALGOL Compiler responds by typing an asterisk on the user's terminal. The user
then types a command string to the compiler, specifying the source file(s) from which the program is to
be compiled, and the output files for listing and output of relocatable binary. The command string
takes the form:
OUTPUT-FILE, LISTING -FILE=SOURCE-FILES
followed by a carriage-return (ALTMODE cannot be used to terminate a command string).
A file takes one of the forms
DEVICE: FILE- NAME. FILE= EXTENSION
or
DEVICE:FILE. NAME
for directory devices (disk and DECtape)
or
FILE-NAME. FILE-EXTENSION
or
FILE-NAME
where DSK is assumed to be a default device.
Version 2A ALGOL 18-1 May 1972
In the case of non-directory devices, the format is simply
DEVICE:
In cases where no FILE-EXTENSIONS are specified, the standard defaults REL for the relocatable
binary output file, LST for the listing file, and ALG for the source file are assumed.
SOURCE-FILES
consists of one file or a list of files separated by commas. If a DEVICE is specified for the first file,
and not for succeeding files, the second and following files are taken from the same device as the
first.
Example:
EULER>TTY:=EULER
[read source from DSK:EULER.ALG, write relocatable binary on DSK:EULER.REL and listing on the
user's terminal].
MTAN : ,DSK:SIMP6=S INP6>PAKAMeTST
[read source from DSK:SIM26.ALG, DSK:PARAM.TST, write relocatable binary on device MTAO,
and listing on file DSK:SIM26.LST].
Certain switches may be set by the user in the command string. These are:
BUFFERS:n Set number of buffers in compiler's 1/O buffer-ring to n.
CHECKON Compile run-time array-bound checking (see 18.5.1.1).
CHECKOFF Do not compile run-time array-bound checking, regardless
of any CHECKON statements in the source.
HEAP:n Set the initial size of the dynamic core area (which is used
for 1/O buffers, strings and OWN arrays) to n words. This
area is dynamically expanded at run-time if necessary; its
final size is typed out at the end of execution if the object
program is loaded with DDT.
HELP *Type helpful text,
КАТО Produce code to run on the KA10 processor.
KITO Produce code to run on the KI10 processor.
KL10 Produce code to run on the KL10 processor.
LIST *List the source program (default if a listing-device is
specified in the command-string).
NOERRORS Do not type error-messages on the terminal.
NOLIST Do not list the source program.
NONUMBERS The source program does not have line sequence numbers in
columns 73 to 80 (default).
NOQUOTES Delimiter words are not in quotes (default).
Version 5A ALGOL
18-2 May 1975
NUMBERS
PRODUCTION
QUOTED
TEMPCODE:n
TRACE
The source program has line sequence numbers in
columns 73 to 80.
Do not compile trace information or output expanded
symbol-table to the .REL file (not yet implemented).
*Delimiter words are in quotes.
Set length of TEMPCODE area in compiler to n words:
this is only necessary if the compiler produces a message
to that effect, which may happen for some very complicated
statement constructs,
Control tracing. (Not yet implemented.)
Switches may be shortened to a unique abbreviation: those marked with an asterisk (*) in the
above list may also be given as a single character. Values (n) are in decimal.
These switches are set by preceding them with a / after a file-specification, for example:
PROD, PROD: PRADI/L, PROD2/NOL/HEAP: 2005
causes file PROD] to be compiled with a listing, PROD2 to be compiled without listing, and the
initial size of the run-time dynamic core area to be set to 2000 words (the default size is 521
words).
Version 5A ALGOL
18-3 May 1975
The ALGOL compiler reports all source program errors both on the user's terminal and in the listing
device (if it is other than the terminal). After compiling a program, the compiler returns with another
asterisk, whereupon the user may compile another program, or type 1C to return to monitor level.
18.1.1 Compilation of Free-Standing Procedures
DECsystem-10 ALG OL allows the user to compile procedures independent of programs that call them.
Such procedures may either follow the main program in the source file (but may not appear before it),
or may be in an independent source file either singly or together. The user uses exactly the same
process to compile such files.
If the user requires to call those procedures from the main ALGOL program, the appropriate EXTERNAL
declarations must be made (refer to Paragraph 11.9).
18.2 LOADING ALGOL PROGRAMS
ALGOL programs are loaded by means of the DECsystem-10 Linking Loader in exactly the same way as
programs generated by MACRO-10 and FORTRAN (for details, refer to the DECsystem-10 Assembly
Language Handbook).
LINK-10 automatically causes all procedures required from the ALGOL Library (ALGLIB) to be
incorporated into the user's program.
For example, consider the source file MAIN. ALG which contains the ALGOL main program and
the files SUB1.ALG and SUB2.ALG which contain free-standing procedures.
The user may compile these files to give one relocatable binary file by typing the following command
string to the ALGOL compiler,
MAIN, MAINZMAIN, SUBI, SUB2
and loading the resulting program by giving the command string
eI N/GO
to LINK-10. Alternatively, the three source files can be compiled independently by typing three
command strings to the ALGOL compiler, for example:
MAIN, MAINZMALN
SUB1, SUBI =SUBI
S5UB2 , SUB2=S UBZ
Version 5A ALGOL 18-4 May 1975
and giving LINK-10 the command string.
MAIN, SUBI , SUB2/CG
After a program has been loaded, it may be executed.
18.3 RUNNING ALGOL PROGRAMS
ALGOL programs are executed by typing the console command
START
or any of its valid abbreviations. If the program executes successfully, it finishes by printing the exe-
cution time statistics (core store used and execution and elapsed times) on the user's terminal , and
returns to monitor command level,
18.4 CONCISE COMMAND LANGUAGE
The concise command language (CCL) features in the DECsystem-10 monitor may be used to facilitate
the compilation and execution of ALGOL programs. They are used in exactly the same way as for
programs written in DECsystem-10 FORTRAN. For details, refer to the DECsystem-10 Users Handbook.
Switches to the ALGOL compiler are enclosed in parentheses and separated by slashes; for example:
EXECUTE FOOCQUOTED/HEAP: 2004/KI)
18.5 RUN-TIME DIAGNOSTICS AND DEBUGGING
If a run-time error occurs during the execution of an ALGOL object program, an error message is pro-
duced, detailing the type of error, and its address within the user's program. Such errors fall into two
categories - fatal and non-fatal.
A mechanism has been provided by which the user can trap non-fatal errors, and, when they occur,
transfer control to a label within the user's program. Each such error has a unique number, and a
table of these appears below. The Library procedure TRAP, used to trap non-fatal error has the follow -
ing specification:
PROCEDURE TRAP (N, L); VALUE N, L;
INTEGER N; LABEL L;
Where N is the number of the error to be trapped, and L is a label to which control is required to be
passed when the error occurs.
Version 5A ALGOL 18-5 May 1975
Once such a trap is set up by a call to TRAP, it remains in force until another call to TRAP sets a trap
to a different label, or until the trap is turned off by
TRAP (N)
i.e., omitting the label parameter in a trap call.
Note that the trap label is a formal parameter by value.
Table 18-1
Error Trap Numbers
TRAP NO. ERROR
18 FLOATING POINT OVERFLOW
19 FIXED POINT OVERFLOW
32 INPUT OR OUTPUT DEVICE UNAVAILABLE
33 ILLEGAL MODE FOR INPUT OR OUTPUT DEVICE
34 INPUT OR OUTPUT ON UNDEFINED CHANNEL
35 ATTEMPT TO READ OR WRITE ON DIRECTORY DEVICE WITHOUT FILE OPEN
37 FILE NOT AVAILABLE OR RENAME FAILURE
38 ATTEMPT TO READ OR WRITE OVER END-OF-FILE
39 ERROR CONDITION ON INPUT OR OUTPUT
40 ILLEGAL CHARACTER IN NUMERIC DATA
4] OVERFLOW IN NUMERIC DATA
42 ERROR CONDITION ON CLOSING FILE
43 ILLEGAL INPUT-OUTPUT OPERATION
44 [-O CHANNEL NUMBER OUT OF RANGE
48 SQRT ARGUMENT NEGATIVE
49 LN ARGUMENT ZERO OR NEGATIVE
50 EXP ARGUMENT TOO LARGE
51 INVERSE MATHS FUNCTION ARGUMENT OUT OF RANGE
52 TAN ARGUMENT TOO LARGE
18.5.1 Facilities to Aid in Program Debugging
18.5.1.1 Checking - The directive
| CHECKON
when placed anywhere in a user's program causes all array subscripts from this point onward in the pro-
gram to be checked at run-time for being in range. The directive
| CHECKOFF
nul lifies this action. Note that use of this facility causes the generated program to be slightly larger,
and to run slower.
Version 5A ALGOL 18-6 May 1975
The compiler switches /CHECKON and /CHECKOFF may also be used: they override any CHECKON
or CHECKOFF statements in the source program,
NOTE
Most inexplicable errors arising during the execution of an
ALGOL program are caused by an array subscript being out
of range. Whenever such errors occur, the program should
be recompiled with the array bound check feature on, and
rerun.
18.5.1.2 Controlling Listing of the Source Program - Normally, a listing of the source program is
output with the object program during compilation. The user can suppress this listing entirely by means
of the /NOLIST compiler switch. However, if the user wishes to suppress only part of the listing and
then continue listing, he can control the listing from within his program by means of the statements
LISTOFF
LISTON
The LISTOFF statement causes listing to be suppressed from the point in the program where LISTOFF
was encountered to either the end of the program or until a LISTON statement is encountered. The
LISTON statement causes listing to continue after it had been suppressed by a LISTOFF statement.
The LISTON and LISTOFF statements have no effect if the /NOLIST switch is included in the com-
piler command string.
18.5.1.3 Setting Line Numbers in Listings - Ordinarily, the lines in the listing file are numbered
sequentially starting at 1 and incrementing by 1. The user can, however, change the line numbers by
placing sequence numbers in columns 73 through 80 of the source program and compiling with the
/NUMBERS switch. Another way in which the user can change the line numbers is by means of the
LINE statement. The statement
LINE n
causes the next line number to be set to n, which is a decimal integer. The line numbers that follow
are incremented by 1 until either another LINE statement is encountered or the program terminates.
Version 5A ALGOL 18-7 May 1975
18.6 CROSS REFERENCE LISTING
The /CREF switch has been implemented, causing ALGOL to generate output for CREF. Output
takes the following formats:
Variables and Labels: each occurrence of a variable or label name is recorded, with a #
whenever a defining reference is made. In the case of labels the line reference
is to the line which causes code for the label to be generated, which may
follow the line where the label itself appears. No distinction is made amongst
different incarnations of a variable at various block levels,
Blocks: the messages "START OF BLOCK n" and "END OF BLOCK n" are suppressed in the
CREF listing. However, there is a separate CREF of blocks on the first page
following the program, As with labels, the line-numbers refer to those
causing code to be generated, not necessarily those on which BEGIN or END
appear in the source.
18,7 STACK AÑALYSIS
The stack analysis takes two forms - if a program stops with an error, the stack is scanned to
give the names of the active procedures. Thus a typical error message now reads:
?RUN-=TIME ERROR AT ADDRESS 09091 62
IN PROCEDURE OPENFILE
CALLED FROM PROCEDURE PQRSTUVWXYZ
CALLED FROM PROCEDURE THISPROCEDUREISNEXTTOINNERMOST
CALLED FROM PROCEDURE THISONEISNEARLYTHEOUTERMOST
CALLED FROM PROCEDURE THISISTHEOUTERMOSTPROCEDURE
CALLED FROM MAIN PROGRAM
FILE DSK: NOSUCH.FLE NOT AVAILABLE OR RENAME FAILURE ON CHANNEL # 1
The above type of analysis is automatic whenever an error is detected; the second type is in-
voked by a REEnter command after program execution, followed by typing P (for Profile). This
causes a list of all the procedures, labels and library procedures referenced in the program to
be printed, together with a count of the actual number of times each was referenced (or passed
through in the case of labels). As with the trace print, labels can be distinguished by a trailing
colon (:) and library procedures by a trailing asterisk (*), but different procedures with the same
name must be identified from the order in which they appear, which is the same as the order in
which they were loaded.
Note that in the special case of overlaid programs only the root segment is scanned.
Version 6 ALGOL 18-8 September 1975
ACTION CH FOR HELP)? P
PROFILE PRINT.
COUNT NAME
1 PQRS TUVWXYZ
1 THISPRROCEDUREISNEXTTOINNERMOST
1 THISONEISNEARLYTHEOUTERMOST
l THISISTHEOUTERMOSTPROCEDURE
0 READx
| INPUTx
0 OUTPUT*
| OPENFILE*
18.8 TRACE
18.8.1 Dynamic Trace
Two types of Trace are available: dynamic and post-mortem.
The user who is at a terminal and wishes to see a full dynamic trace of his program should type
the following command sequence:
„LOAD TRTST
ALGOL: TRTST
LINK: LOADING
EXIT
REE
ALGOL DIAGNOSTIC SYSTEM
FACILITIES (H FOR HELP)? T
FACILITIES (H FOR HELP)? G
Version 6 ALGOL 18-9 September 1975
This produces (at least) one line for each trace reference, as follows:
ok ok SECTION
x xkkS 62
sk sk desk kkkk PHAS E
sk dk kok ok WRI TE* PHASE
sk ek ee E PRI NTE 6
sk ok okkdokkkkkkWRI TEx 2 BEGIN
kkk kkk PHAS E
кжжЖжЖжЖжЖжЖжЖжЖжЖЖ МВ] ТЕЖ END
ax SECTION
sk kkkS 78
40 Koko PHAS E
ooo WAI TEX PHASE
sk depor PRI NT* 7
sk ok Sfeok desk dOk AK WRITEX 2 BEGIN
sk rojo PAR FOR
skxkkkskkkkPHASE
Seok skok kk skokskkokk WRI TEx END
Dynamic trace output always clears the terminal output buffer and begins a new line. The
current dynamic block level is represented by two asterisks for each level, printed along the
line. Note that a notional block surrounds the main program and each procedure. A maximum
of sixty asterisks are printed before the line is folded: folding is shown by a number in the first
two columns representing multiples of thirty dynamic block levels.
The name of the procedure or label appears at the end of the line of asterisks. As with the
profile, library procedures are distinguished by a trailing asterisk, and labels by a trailing
colon,
Procedures are traced at entry but not exit; however, a procedure exit can be inferred from the
block levels,
Note that in Dynamic Trace mode, ordinary TTY output may be interspersed amongst Trace
output (as in the example above).
18.8.2 Post-Mortem Trace
A post-mortem trace will be printed automatically if a batch job gives a run-time error. Under
timesharing, after typing out the location and type of error, and the stack analysis, the
diagnostic system offers the user various options. Of these T (for Trace print), produces a trace
Version 6 ALGOL 18-10 September 1975
similar to the one described under Dynamic Trace above, but with spaces instead of asterisks to
represent block levels, and without, of course, any other TTY output, The post-mortem trace
entries are maintained in a circular buffer in the Heap. The default length of the buffer is 100
(decimal) but it can be altered by using the /TRACE switch to the compiler, with an appropriate
value. Tracing is the default option: users can compile their programs without trace information
by using the /PRODUCTION switch to the compiler. However, the Trace entry mechanism is
quite efficient, and users are urged not to exclude it, as this also has the effect of making the
stack analysis less useful, Library procedures are always traced.
18.9 PERFORMANCE ANALYSIS
Various features are provided which are designed to help users wishing to evaluate the perfor-
mance of their programs,
18.9.1 Heap Space
DECsystem-10 ALGOL is very flexible in its use of memory: space for arrays, strings, 1/O
buffers and so on is allocated in an area called the Heap, which lies between the program code
and the stack. The default initial Heap size is 521 (decimal) words: if a program needs more
Heap than is currently available the object-time system moves the stack up in memory to make
more room, obtaining more core from the Monitor as necessary (subject to over-riding constraints
such as the user's COREMAX)\
For most programs this provides a reasonable balance between core use and computation.
However, the stack shifting mechanism can be very expensive if it is used often and for small
expansions. The user can find out how the Heap is being used by typing REENTER, and then $
(for statistics) after the program has executed (or simply typing CONTINUE immediately after
the End of execution message). The produces output of the following format:
EXECUTION TIME:
ELAPSED TIME:
MAXIMUM HEAP SIZE: — , * OF STACK SHIFTS:
Maximum * of used words in the heap table:
(The last figure is a measure of the fragmentation of the free space in the heap: it is only pro-
duced if the optional Heap Integrity Checker is present in the object-time system; i.e., when
assembly switch FTGETCHK is turned on.)
Version 6 ALGOL 18-11 - September 1975
If the number of stack shifts is larger than, say, ten, an improvement in performance could be
expected from re-compiling the program with the /HEAP switch value set to the maximum heap
size given by the Statistics type-out.
18.9.2 Code Utilization
By judicious placement of labels and subsequent printing of the program's profile, the frequency
with which a program executes particular sections of code can be monitored. The new library
procedure INFO can be used to obtain more detailed timing information,
Version 6 ALGOL 18-12 September 1975
CHAPTER 19
TECHNICAL NOTES
These notes concern the authors' particular interpretation of the "Revised Report on the Algorithmic
Language ALGOL-60" and its implementation.
a. At all times, strict left-to-right evaluation of statements is employed. Section
3.4.6 of the Revised Report has been construed by some experts to mean that
left-to-right evaluation of expressions is not required. However, there are un-
doubtedly many ALGOL-60 programs in existence that rely on this feature.
b. Section 4.3.5 of the Revised Report requires that a GOTO Statement with a
designational expression which is a switch with a subscript out of range be
regarded as a dummy statement. Neither DECsystem-10 ALGOL nor any
other ALGOL-60 implementations, to the knowledge of the authors, follow
this rule if there is a side-effect involved in the evaluation of the subscript.
19-1 December 1971
ABS, 5-3
ALGOL-60, 1-1, 6-1, 7-1
ALGOL-68, 1-1
AND, 5-4
ARCCOS, 17-1
ARCSIN, 17-1
ARCTAN, 17-1
Arithmetic Conditions, 5-5
Array Elements, 9-2
Arrays, 9-1
Arrays, OWN, 15-1
ASCII Constants, 4-3
Assignments, 1-2, 6-1
BACKSPACE, 16-11
BEGIN, 6-3, 10-1
Block Structure, 10-1
BOOLEAN, 3-2
Boolean Constants, 4-2
Boolean Conversions, 5-5
Boolean Expressions, 5-4, 5-5
Boolean Operators, 5-4
Boolean Variables, 3-3
Brackets, 4-3, 9-1
BREAK OUTPUT, 16-6
Buffering, 16-3
Byte Manipulations, 17-3
Byte Processing, 16-6
Byte String Copying, 13-4
Byte Strings, 13-2
Byte Subscripting, 13-2
Channels, 16-3, 16-10
Channel Status, 16-11
Checking Array Subscripts, 18-2
Version 6 ALGOL
ALGOL INDEX
CHECKOFF, 18-2
CHECKON, 18-2
CLOSEFILE, 16-4
COMMENT, 2-4
Commentary, 2-4, 11-11
Compiler Commands, 18-1
Compiler Extensions, 1-2
Compiler Restrictions, 1-3
Compiler Switches, 18-2
Compound Statements, 6-3
Compound Symbols, 2-2
CONCAT, 13-4
Concatenate, 13-4
Conditional Expressions, 14-1
Conditional Statements, 7-2
Constants, 4-1, 4-2, 4-3, 5-2
Continuation from I/O errors, 16-3, 16-5
Controlling Listing of the Source Program, 18-7
Control Transfers, 7-1
COPY, 13-4
COS, 17-1
COSH, 17-1
CREF, 18-8
Cross-referencing, 18-8
Data, 16-8, 16-9
Data Transmission, 16-1
Debugging, 18-5
Declarations, 3-2
Default 1/0, 16-10
DELETE, 13-5
Deleting Files, 16-5
Delimiter Word, 1-3
Delimiter Words, 1-2, 2-2
Designational Expressions, 14-3
Index-1 September 1975
Device Allocation, 16-1
Device modes, 16-2
Devices, 16-1
DIV, 5-1
DO, 8-1
Dummy Statement, 19-1
Dynamic Bounds, 10-3
Dynamic Trace, 18-9
Editing Characters, 16-7
ELSE, 7-2
END, 6-3, 10-1
End-Of-File, 16-11
ENDFILE, 16-11
ENTIER, 5-2
EQV, 5-4
EXP, 17-1
Exponents, 4-1, 16-8
Expressions, 6-2
EXTERNAL, 11-10
FALSE, 4-2
Fault Monitoring, 1-3, 18-5
Field Manipulations, 17-3
File Deletion, 16-5
File Devices, 16-4
File Names, 16-4
File Protection, 16-4
FOR & WHILE Statements, 8-1, 8-2
Format Parameters, 1-3, 1-4, 11-1
Forward References, 1-3, 11-9
GFIELD, 17-3
GLOBAL, 10-2
Version 6 ALGOL
ALGOL INDEX (Cont)
GO, 2-3, 7-1
GO TO, 2-3, 7-1
Heap, 18-11, 18-12
Identifier, 1-3, 1-4, 3-1, 5-2
IF, 7-2
IMAX, 17-3
IMIN, 17-3
IMP, 5-4
INFO, 18-12
INPUT, 16-2
INT, 5-5, 5-6
Input Data, 16-8
Input/Output Channels, 16-3
INSYMBOL, 16-6
INTEGER, 3-2, 4-1
Integer Constants, 4-1
Integer Conversions, 5-5
Integer Remainder, 1-2
IOCHAN, 16-11
I/O Channel Status, 16-11
Jensen's Device, 11-6
LABEL, 11-1
Label, 1-3, 7-1
LARCTAN, 17-1
LCOS, 17-1
LENGTH, 13-5
LEXP, 17-1
Library, 1-3, 17-1
Library Procedures, 13-3
LINE, 18-7
Index-2 September 1975
Line Numbers in Listings, 18-7
Linking Loader, 1-2, 18-4
Listing the Source Program, 18-7
LISTOFF, 18-7
LISTON, 18-7
LLN, 17-1
LMIN, 17-3
LN, 17-1
Loading Procedures, 18-8
Local, 10-2
Logical 1/O, 16-10
LONG REAL, 1-2, 3-2, 4-2
Long Real Constants, 4-2
LSIN, 17-1
LSQRT, 17-2
Mathematical Procedures, 17-1
Matrix, 9-1
Mode, 16-2
Name, 11-1
NEWLINE, 16-7
NEWSTRING, 13-5
Newton-Rapheson, 11-4
NEXTSYMBOL, 16-6
NOT, 5-4
Numeric Constants, 4-1
Numeric Labels, 1-3
Numeric Procedures, 16-8
Object Time System, 17-1, 18-5
Octal Constants, 4-2
Octal I/O, 16-10
OPENFILE, 16-5
Operating Environment, 1-3
Version 6 ALGOL
ALGOL INDEX (Cont)
OR, 5-4
OUTPUT, 16-1
Output Data, 16-9
OUTSYMBOL, 16-6
OWN Arrays, 15-1
OWN Variables, 15-1
PAGE, 16-7
Parameter, 1-4, 11-1
Performance Analysis, 18-11, 18-12
Peripherals, 16-1
Post-mortem trace,
PRINT, 16-9, 16-10
PRINTOCTAL, 16-10
PROCEDURE, 1-4
- Procedure Bodies, 11-3
Procedure Call Parameters, 11-2
Procedure Calls, 11-5
Procedure Headings, 11-3
Procedures, 11-1
Procedures, Advanced, 11-6
/PRODUCTION,
Profile, 18-8, 18-9
Profile Print, 18-8, 18-9
Protection, 16-4
READ, 16-8
READOCTAL, 16-10
REAL, 3-2, 4-1
Real Constants, 4-1
Recursion, 11-7
REEnter command, 18-8, 18-11
RELEASE, 16-5
Relocatable Binary, 1-2
REM, 5-1
Index-3 September 1975
ALGOL INDEX (Cont)
Rename, 16-5
Reserved Words, 2-3
Revised Report, 1-1, 19-1
REWIND, 16-11
RMAX, 17-3
RMIN, 17-3
Run-time diagnostics, 18-5
Run-time error, 18-10
Scalar, 3-2
Scope, 10-2
Select, 16-3
SELECTINPUT, 16-4
SELECTOUTPUT, 16-4
Semicolon, 6-3
Separators, 2-1
Setting Line Numbers in Listings, 18-7
SFIELD, 17-3
Side-effect, 19-1
SIGN, 5-3
SIN, 17-1
Single-pass Compiler, 1-2
SINH, 17-1
SIZE, 13-4
SKIPSYMBOL, 16-6
SPACE, 16-7
Spacing, 2-4
SQRT, 17-1
Stack Analysis, 18-8, 18-9
Stack Shifts, 18-11
Statements, 6-1
Statistics, 18-11
STEP - UNTIL, 8-2
STRING, 3-3
String Comparisons, 13-3
Version 6 ALGOL
String Constants, 4-4
String Output, 16-6
String Procedures, 16-7
String Variables, 3-3
Strings, 1-2, 13-1, 13-2, 13-3, 13-4, 13-5,
16-6
Strings, Byte, 13-2, 13-5
Subscripting, Byte, 13-2
Switches, 12-1, 14-1
Symbol Procedures, 16-7
Symbols, 2-1
Symbols, Compound, 2-2
TAB, 16-7
TAN, 17-1
TANH, 17-1
Terminology, 1-3
TRANSFILE, 16-12
Trapping Errors, 18-5
TRAP procedure, 18-5
TRUE, 4-2
Type Conversion, 5-2
UNTIL, 8-2
VALUE, 11-1
WHILE, 1-2, 8-2, 8-3
While Element, 8-3
WRITE, 16-6
Index-4 September 1975
DECsystem-10 ALGOL
Programmer's Reference Manual
DEC-10-LALMA-B-D
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