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Document:	DECsystem10/20 ALGOL Programmer's Guide
Order Number:	AA-0196C-TK
Revision:	0
Pages:	144
Original Filename:

OCR Text

QecsysCeno/20
ALGOL PROGRAMMER’S GUIDE

decsyUsteno/20

ALGOL PROGRAMMER'S GUIDE
AA-0196C-TK

digital equipment corporation - maynard. massachusetts

First Printing:
Revised:

September
September
December

1971
1971
1971

May

1972

December

1972

July

1973

July

1974

May

1975

September

1975

March
April

1976
1977

Second Printing:
Revised:

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 document.

The software described in this document is furnished under a license
and may be used or copied only in accordance with the terms of such
license.

Digital Equipment Corporation assumes no responsibility for the use
or reliability of its software on equipment that is not supplied by
DIGITAL.

(©)

1971, 1972, 1973, 1974, 1975, 1976, 1977 by
Digital Equipment Corporation

The postage prepaid READER'S COMMENTS
document requests the user's critical
paring future documentation.

form on the last page of this
evaluation to assist us in pre-

The following are trademarks of Digital Equipment Corporation:

DIGITAL

DECsystem-10

MASSBUS

DEC

DECtape

OMNIBUS

DECUS

EDUSYSTEM

PHA

UNIBUS

FLIP

COMPUTER LABS

FOCAL

RSX

COMTEX
DDT
DECCOMM

INDAC
LAB-8
DECSYSTEM-20

TYPESET-8
TYPESET-10
TYPESET-11

PDP

DIRBROL

CHIP

0S/8

RSTS

CONTENTS

iy Sy WA

SYMBOLS

DELIMITER

WORDS

SPACING

AND

COMMENTARY

DECLARATICNS

(NI

IDENTIFIERS

SCALAR

w W

ENVIRONMENT

STRUCTURE

COMPOUND
USE

BASIC

DECLARATIONS

[]
[]

[

wWwwhNh
=~

[)

(S
A
-~
|]
o
e

NUMERIC

CONSTANTS

Integer

Constants

Real

Constants

Long

Real

Constants

Automatic

Conversion

Long

OCTAL

AND

ASCII

CONSTANTS

STRING

Real

Constants

BOOLEAN

CONSTANTS

Boolean

Evaluation

Arithmetic

Conditions

INTEGER

Operators

AND

OO
OV

STATEMENTS

W N

ASSIGNMENTS

STATEMENTS

EVALUATION

MULTIPLE

Boolean

BOOLEAN
AND

Variables

CONVEKSIONS

ASSIGNMENTS

EXPRESSICNS

iii

N
Cvix

EXPRESSIONS

)

BOOLEAN

EXPRESSIONS

Identifiers and Constants
Special Functions

w N

[

OO
)

ARITEMETIC

N I|
B WWNN

EXPRESSIONS
LU,

[~
-

OPERATING

IDENTIFIERS

(o))

ALGOL

TERMINOLOGY

THE

WWN
N

=
N

o
et et
et

Extensions
Restrictions

b WwwN

Compiler
Compiler

CHAPTER

ALGOL

COMPILER

CONSTANTS

CHAPTER

ALGOL

PROGRAM

CHAPTER

THE

W N

CHAPTER

GENERAL

DECSYSTEM-10/20

CHAPTER

INTRODUCTION

WNNDDODND

CHAPTER

—

Page

CONTENTS

COMPOUND
CONTROL

CHAPTER

STATEMENTS
TRANSFERS,

wN
-

UNCONDITIONAL
CONDITIONAL
AND

WHILE

FOR

STATEMENTS

Element

N
W

STATEMENT

GENERAL

GENERAL
(OGS I O

TRANSFERS

Element

WHILE

FOR

ARRAYS

CONTROL

STATEMENTS

WHILE

CHAFPTER

AND

LABELS

STEP-UNTIL

CHAPTER

LABELS,

STATEMENTS

00)

CONDITIONAL

CHAPTER

(Cont.)

NOTES

ARRAY

DECLARATIONS

ARRAY

ELEMENTS

BLOCK

STRUCTURE

GENERAL
ARRAYS

CHAPTER

BGUNDS

PROCEDURES

11.1

PAKAMETERS

CALLED

"VALUE"

11-1

PARAMETERS

CALLED

“"NAME"

11-1

11.3

PROCEDUKE

HEADINGS

11.4

PRGCEDURE

BODIES

11.5

PROCEDURE

11.6

ADVANCED

11.6.1

CHAPTER

DYNAMIC

11.2

11.6.2

CHAPTER

WITH

11-2
11-3
11-5

CALLS
USE

11-6

PROCEDURES

Jensen's Device
Recursion

11.7

LAYOUT

11.8

FORWARD

11.9

EXTERNAL

11.10

ADDITIONAL

11-6

DECLARATIONS

WITHIN

BLOCKS

11-9

PROCEDURES

11.10.1

Comment

11.10.2

Comments

METHODS

After

COMMENTARY

END

Within

11-7
11-8

REFERENCES

Procedure

Headings

11-10
11-10
11-10

SWITCHES

12.1

GENERAL

12.2

SWITCH

DECLARATIONS

12-1

12.3

USE

SWITCHES

12-1

STRINGS

12-1

13.1

GENERAL

13.2

STRING

13.3

BYTE

STRINGS

13.4

BYTE

SUBSCRIFING

13-2

13.5

NULL

STRINGS

13-2

13.6

STRING

COMPARISONS

13.7

LIBRARY

PROCLEDURES

13.7.1

13-1
EXPRESSIONS

AND

ASSIGNMENTS

13-1
13-1

13-3

Concatenation

13-3

13.7.2

Length

13-3

13.7.3

Copying

13-4

13.7.4

Newstring

13-4

and

Size

CONTENTS

13.7.5
CHAPTER

CHAPTER

(Cont.)

13-4

Delete

CONDITIONAL

EXPRESSIONS

AND

STATEMENTS

14-1

14.1

GENERAL

14.2

CONDITIONAL

OPERANDS

14-1

14.3

CONDITIONAL

STATEMENTS

14-2

14.4

DESIGNATIONAL

OWN

14-4

EXPRESSIONS

VARIABLES

GENEKAL
OWN

CHAPTER

TRANSMISSION

16.1

GENERAL
ALLOCATION

l6-1

Device

PERIPHERAL

Modes

Error

16.4

FILE

16-2

16-3

INPUT/OUTPUT

Error

16.6

BASIC

l6-5
16-5

DEVICES

INPUT/OUTPUT

16.6.1

Byte

16.6.2

Strina Output
Miscellaneous

Processing

PROCEDURES

Symbol

16.6.4. 1
16.6.4.

Numeric

Output

16.8

Input/Output
INPUT/OUTPUT
INPUT/OUTPUT

16.9

SPECIAL

OPERATIONS

16.10

I1/0

l6.11

TRANSFERRING

CURRENTLY

THE

17.1

MATHEMATICAL

17.2
17.3
17.3.1

STRING

17.3.2
17.3.3
17.4

Procedures

lo-8
16-9
l6-10

16-10
16-11
lo-11
16-12

STATUS
FILES

SELECTED

CHANNEL

DECSYSTEM-10/20

NUMBERS

OPERATING

17-1

PROCEDURES

17-2
17-2

PROCEDURES
Dimension Procedures

Array

17-2

Minima and Maxima Procedures
Field Manipulations
DATA

TRANSMISSION

PROCEDURES

17-4

FORTRAN

INTERFACE

GENERAL

INFORMATION

17.7

DATE

17-3

17-4

17.5

TIME

17-3

PROCEDURES

17.6

AND

16-13
16-13

ENVIRONMENT

PROCEDURE

UTILITY

lo-8

16-8

Data

Octal
DEFAULT
LOGICAL

16.12

16-6
16-6

and String
Input Data

CHANNEL

16-6

Procedures

Numeric
Numeric

16.6.4.
1o.7

l6-4
l6-4

Returns

RELEASING

16.6.4

CHANNELS

DEVICES

16.5

lo-1

l16-3

Returns

SELECTING

16.6.3

DEVICES

Buffering

16.3

16.4.1

CHAPTER

DATA

l6.2
16.2.1
16.2.2
16.2.3

CHAPTER

ARRAYS

17-5

ROUTINE

ASCII

17-5
17-6

FORMAT

17.8

RANDOM

NUMBER

17.9

ONTRACE

AND

17.10

PAUSE

17-6

17.11

DUMP

17-6

RUNNING

18.1
18.1.1
18.2

COMPILATION
LOADING

ALGOL

PROGRAMS

18.3

RUNNING

ALGOL

PROGRAMS

18.4

CONCISE

COMMAND

AND

ROUTINES

17-6

OFFTRACE

DEBUGGING
OF

Compilation

ALGOL

PROGRAMS

18-1

PROGRAMS

Free-Standing

LANGUAGE

Procedures

18-4
18-4
18-5

18-5

CONTENTS

(Cont.)

18.5

RUN-TIME

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

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

TECHNICAL

NOTES

CHAPTER

THE

DYNAMIC

CHAPTER

DIAGNOSTICS

ALGOL

20.1

SUMMARY

20.2

GENERAL

REMARKS

20.3

TYPEOUT

COMMANDS

20.4
20.5

CHANGING ALGOL
PAUSES

AND

DEBUGGING

18-5

18-7
18-7
18-7
18-8
18-8
18-8
18-9
18-9
18-11
18-11
18-11
18-12

SYSTEM

FEATURES

20-1
20-1
20-2

VARIABLES

20.6

EXECUTE

20.7
20.8
20.9

DUMP
MISCELLANEOUS COMMANDS
SUMMARY OF COMMANDS

THE

20-6
20-7

COMMANDS

20-12

DECSYSTEM-10

MACRO

21.1

GENERAL

21.2

PROCEDURE

HEADINGS

21.3

ACCESSING

FORMAL

21.4

RETURN

RESULTS

21.5

PROCEDURE

21.6

FORMATS

21.7

PROCEDURES

21.8

OF PARAMETERS
INCLUDING PROCEDURE

21.9

UTILITY

20-13
20-14
20-17
SUBROUTINES

21-1

PARAMETERS
FROM

21-3

TYPED

PROCEDURES

EXITS

21-5
21-6

VARIABLES
WITH

21-6

VARIABLE

NUMBER

21.9.2
21.9.2.1

Core
Input/Output
Device Open

21-7
21-8
21-8
21-8
21-8
21-8

21.9.2.2
21.9.2.3

File
File

21-9
21-9

21.9.2.4

Release

21.9.2.5

Select

21.9.1

THE

LIBRARY

ROUTINES

Getting

Open
Close
Channel

21-9

Channel

21-9

21.9.2.6

Read

21.9.2.7

Write

Byte

21-9

21.9.2.8

Break

Output

21-9

21.9.2.9
21.9.2.10

Read Number
Print Number

21.9.2.11
21.10

Byte

String
GENERAL

21-9

21-10
21-10

Output

Terminal

NOTES

21-10
21-10

TABLES

TABLE

2-1

DECsystem-10/20

2-2

Compound

2-3

Delimiter

5-1

Operator

ALGOL

Symbols

Words

2-1
2=-2

Used

Precedence
vi

DECsystem-10/20 ALGOL2-3
5-1

CONTENTS

(Cont.)

5-2
5-3
11-1

Function of Boolean Operators
Boolean Expressions
Parameters in a Procedure Call

l16-1

Standard Device Names

16-2

17-1

FORTRAN

17-4

Interface

Procedures

vii

5~4
5-5
11-1

CHAPTER

INTRODUCTION

1.1

GENERAL

DECsystem-10/20 ALGOL is an implementation of ALGOL-60;
ALGOL
1is
an
abbreviation
of
ALGOrithmic
Language,
and
1960 is the year it was
defined.
The authoritative definition of ALGOL-60 is contained in the

"Revised
Report
on
the Algorithmic Language ALGOL-60",
(1) 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
1in
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.
(2)

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
1is
wused
as
a
basis.

1.2

These points

are

discussed

in Chapter

19.

DECSYSTEM-10/20 ALGOL

The purpose of this manual is to
teach
the
wuse
of
DECsystem-10/20
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
1IV.
This
manual
1is
not
a
primer
1in
high-level

languages.
(3)

Readers not thoroughly familiar with
manual.
Readers
already
familiar
chapters except Chapters 5, 6, 7, 8,
be referred to only briefly.

1.3

ALGOL
should
read
the
entire
with
ALGOL-60
should
read all
9, 10, 11, 12, and 14, which need

THE ALGOL COMPILER

The DECsystem-10/20 ALGOL Compiler is that part of the DECsystem-10/20
ALGOL
System that reads programs written in DECsystem-10/20 ALGOL and
converts them into a form (relocatable binary) that is
acceptable
to
_the
DECsystem-10 or DECsytem-20 Linking Loader.
The compiler is also
responsible for finding
errors
1in
the
wuser's
source
program
and
reporting

them

the

user.

Slight constraints are imposed on the way the user writes
These
constraints,
made
to
gain
the
most
desirable
single-pass compiler, concern the order in which the user

1-1

his program.
feature of a
declares the

INTRODUCTION

identifiers
in
the program and
certain special circumstances.

the use of

forward declarations under

Such a compiler can
process
ALGOL
programs
rapidly
and
does
not
reguire
the use of any backing store.
The minor restrictions imposed
will not normally affect the user.

1.3.1

The

Compiler

following
1.

ALGOL-60

extensions

are

allowed

the

compiler:

LONG

REAL type, equivalent to FORTRAN's
double
precision,
added that gives the user power to handle double-precision

real

Extensions

numbers.

EXTERNAL procedure
procedures separately

A WHILE
statement,
statement, allow the

new

type

STRING

facility

from
and
user

allows

allows
the
user
main program.

the

an
abbreviated
form
greater flexibility of
the

user

of
the
FOR
iteration.

manipulate

strings

various
size
bytes.
1In addition, the user can
manipulate the bytes within a
string
by
means
subscripting facility.
5.

integer

Assignments

Delimiter

format

remainder

are

words

function

permitted
may

REM,

within

1is

individually
of
a
byte

provided.

expressions.

represented

non-reserved

compile

words

either

enclosed

reserved

single

word

quotes

(primes).
8.

The

Constants of type REAL may
and a decimal part only as

compiler

accepts

reserved

word

also
accept
programs
using
primes.
Refer to Chapter 18.

1.3.2

The

Compiler

compiler

be
in

expressed
FORTRAN.

delimiters

non-reserved

normal

delimiter

integer

mode,

words

part

but

can

enclosed

Restrictions

imposes

Numeric

All

Identifiers

Arrays

the

labels

formal

following
are

not

parameters

and

are

restrictions

on ALGOL-60:

permitted.

must

specified.

restricted

scalars

must

characters

declared

length.

before

switches

and

procedures.
5.

Forward

under
For

definitions

Revised

Report.

references

certain
of

the

for

procedures

and

labels

must

given

circumstances.
terms

used,

refer

section

1.5

and

the

INTRODUCTION

1.4

THE

ALGOL

Programs

OPERATING

compiled

environment
that
facilities for the
The

ALGOL
1.

ALGOL

which

linking
2.

The

environment

Object

Time

System,

the

smooth

services

allocation,

Chapters

TERMINOLOGY

Some

the

reader.

exists,

this

have

Delimiter

inherent
cannot

fault

for

enclosed

used

set

the

known

special

operating

input/output

run

descripion

this

routines,
program

ALGOTS
of

some
by

program

and
device

peripheral

case

the

program

time.

ALGLIB

manual,
and

may

where

in parentheses.

and

such

ALGOTS.

new
an

the

equivalent

a single,
English
1language
word
that
is
an
the structure of the ALGOL language.
Such words

used

for

other

purposes.

Examples:

BEGIN

ARRAY.

Identifier

represents

some

Label

1in

transferred

Procedure

the

Parameter

(Formal

Argument)

See

within
the

user

identifier

used

of
following

statement

Function)

the

result

may

Parameter

procedure

part

1In

general,

returned.

Procedure.

Dummy
-

that

called.

declaration,

that

program.

Control

“calling".

and

used

the

mark

1label.

the

not

which

may

supplied

Parameter

are

can

numeric

number,

Actual

Parameter

certain

program,

parameters

represents

execution

statement

Variable,

Formal

program

1is
similar
to
a FORTRAN
DECsystem-10/20 ALGOL.

arguments

when

within

progranm.

(Subroutine,

1invoked

established

Number)

label
which
available in

name,
quantity

(Statement

statement

the

responsible

the

management,

equivalent,

user‘'s

running

monitoring

including

of:

core

condition

FORTRAN

Word part of

and

words,

normally

such

error

and

following

Many

consists

organizing

1.5

run

loader.

encounters

are

services,

known as ALGLIB
incorporated
into

are

providing

Refer

compiler

Library,

ALGOL

for

ALGOL

provides
special
object program.

operating
The

the

ENVIRONMENT

argument

identifier

supplied

CHAPTER
PROGRAM

2.1

BASIC

STRUCTURE

SYMBOLS

DECsystem-10/20 ALGOL programs consist of a
the
DECsystem-10/20
ASCII
<character set.

characters given in Table

2-1,

sequence of
symbols
from
The meaning of individual

is much the same as in other high-level

languages.

Table

2-1

DECsystem-10/20

Symbol

ALGOL

Meaning

A-2

Used

a-2z

Lower
case
letters;
letters
except
when
and

0-9

ASCII

Decimal
and

construct

Symbols

Use

identifiers

and

delimiter

are
treated
as
upper
case
they appear in string constants

constants.

digits;

wused

to construct numeric

constants

identifiers.

Arithmetic

addition

Arithmetic

subtraction operator.

Arithmetic multiplication operator.

Arithmetic

division

Arithmetic

exponentiation

()

words.

operator.

operator.
operator.

Parentheses;
used in arithmetic expressions
and
to
enclose
parameters
in
procedure specifications and
calls.

[

]

Square brackets;
used to enclose subscript bounds
array declarations, and array subscript lists.

Comma;
general
separator,
placed
subscripts,
procedure
parameters,
lists, etc.

Decimal point;

subscripting.
identifiers.

used

Also,

in numeric

between
array
items
1in switch

constants

and

byte

used as a readability symbol

PROGRAM STRUCTURE

Table

2-1

(Cont.)
Symbols

DECsystem=10/20 ALGOL

Symbol

Meaning or Use

-e

Semicolon;

terminate

statements.

Colon;
wused to indicate labels, and
separate
and upper bounds in array declarations.

Equality;

Nonequality.

2.2

used

Less

than,

1lower

wused in arithmetic and string comparisons.

greater

than.

& @

Introduces exponent in floating-point numbers.

Prime, or single quote;
wused
to
enclose
delimiter
non-reserved word implementation 1s
the
when
zg;gs

Opening and closing string quotes.

Comment.

Introduces

an octal

constant.

Introduces

an ASCII

constant.

“

Alternative

COMPOUND

(refer

to Table

2-2).

SYMBOLS

Any
symbols.
basic
adjacent
two
of
consist
symbols
Compound
intervening
spaces
or
tabs
do
not affect their use.
The compound
symbols

are

shown

in Table

2-2.

Table
Compound

2-2

Symbols

Symbol

2.3

Usage

Assignment

Less

Greater

than

equal
or

equal

DELIMITER WORDS

Certain letter combinations are reserved as part of the
structure
of
the
language
and
may not be used as identifiers unless the compiler
option to accept delimiter words in single quotes is in use.
Such
an
option
1is selected by using a special switch option (refer to Chapter

2-2

PROGRAM STRUCTURE

18).

The

standard

reserved

words,

delimiter

word

method
1is

assumed

delimiter

word

throughout

representation,

this

manual.

For

that

1is,

example,

the

above

and

BEGIN

will

always

cannot

method

appear
used

in
as

the

text

identifier

representation

used,

this
it

manual
a

would

program.

appear

shown
If

the

list

alternative

'BEGIN'

and
BEGIN

could be used as an identifier.
Table
delimiter words used in the language.

Table

Delimiter

Words

Reserved

Word

Used

2-3

contains

2-3
DECsystem-10/20
Chapter

Reference

5.2.1

ARRAY

BEGIN

CHECKON

18
2.4

DIV

5.1

LISTOFF
LISTON

LONG

NOT

OR
OWN
PROCEDURE
REAL
REM

STEP
STRING

SWITCH
THEN

LINE

INTEGER
LABEL

)
e
NN

IMP

GOTO

FORWARD
GO

FALSE

FOR

)

EQV
EXTERNAL

END

.
N
w

ELSE

— .
N e

COMMENT

NHHEHEFOOMWHRFOULO, WHHFHWWONNNHO®
H U~

5.2

CHECKOFF

AND

BOOLEAN

ALGOL

all

the

PROGRAM STRUCTURE

Table 2-3 (Cont.)
Delimiter Words Used in DECsystem-10/20

Reserved Word

Chapter

2.4

USE

Reference

TRUE

4.2

UNTIL

VALUE

11
8

WHILE

SPACING

AND

ALGOL

COMMENTARY

The readability of ALGOL programs can be enhanced by the judicious use
of
spacing,
tab
formatting, and commentary.
Spaces, tabs, and form
feeds (page throws) may be used freely in a source program subject
to
the following constraints:
l.

Spaces,

appear
2.

Where
two
identifier

by one
3.
Comments

tabs,

Spaces,
are

line

feed,

within delimiter

delimiter
follows a

tabs

spaces

etc.,

introduced

are

form

feed

characters

may

not

words.

words
are
adjacent,
or
delimiter word, these must

and/or

the

where
an
separated

tabs.

significant within

either

word

COMMENT

string
or

constants.

the

symbol

(available
in
DECsystem-10/20
ALGOL,
but
not necessarily in other
implementations of ALGOL).
Such a comment may appear
anywhere
1n
a
program;
the
comment
text
1is terminated by a semicolon.
Refer
Section 11.10 for additional means to add comments to a program.

CHAPTER
IDENTIFIERS

3.1

AND

3
DECLARATIONS

IDENTIFIERS

An identifier must begin with an upper-case letter and
optionally
followed
by one or more upper-case letters and/or decimal digits.
identifier may not contain more than 64 characters.

be
An

NOTE

Unlike

FORTRAN,

attached
2.

All

there

have

use to
must

actual

implied

type

identifier.

identifiers

labels)

the
put

1in

program

"declared",

(except
that

is,

to
to

be
the

which identifiers are
be
specified,
prior

usage.

Examples:

The

following

are

identifiers:

I
ALPHA

P43

J4K5
HOUSEHOLDERTRIDIAGONALIZATION

The

following

are

not

identifiers:

does

not

BOOLEAN

unless

begin
the

representation
ONCE

AGAIN

DECsystem-10/20

space

ALGOL

also

with

not

letter

non-reserved

word

delimiter

used

allowed

permits

the

use

decimal

point

"readability
symbol" in the alphabetic portion of identifiers.
readability symbols can appear between two alphabetic characters
identifier and are ignored by the compiler.
Thus:
ONCE.AGAIN

These
of an

IDENTIFIERS

AND

DECLARATIONS

and
PI.BY.TWO

have

exactly

the

same

effect

ONCEAGAIN

and
PIBYTWO

respectively.
Note

that
ALPHA3.5

and
BETA.22

are
two

not identifiers, since
alphabetic characters.

3.2

SCALAR DECLARATIONS

declaration

reserves

the

decimal

point

1identifier

does

not

appear

represent

guantity used in a program.
Such declarations are
At any particular point during program
execution,

between

particular

mandatory in ALGOL.
the
form
of
the

variable
or
quantity
associated
with the identifier depends on
type of variable.
The type of variable is determined by the
type
identifier which represents 1it.
There are
contain a

five types of
single value:

scalar

variables,

that

1s,

variables

the
of

which

Integer

‘Real

Long

Real

Boolean

String
Integer,

real,

and

numerical

values

range of values
34,359,738,367;
1.7&38;

wvalues

represented

Boolean

but,

String
to

the

as
real

less

FORTRAN's

usually

are

for

items

any

Throughout
arranged

pattern

somewhat
full

capable

only

1.4&-39

can be

be
this

used

bits.

the

declared
by

the

manual,

sequentially

Logical

one

that

holding

type).

The

through
through

magnitude

variables)

the

complicated.

description

above variables

identifiers

those

approximately

variables

type.

than

are

(and

integer:
-34,359,738,368
real:
approximately -1.7&38

can

their

follows:
and
1long

type

which

general,

the

variables

(similar

quantity,

real

appropriate

are

zero.

variables

Chapter

All

1long

the

and

states

The

can

TRUE

user

hold

FALSE

referred

subject.

for

use by

appropriate
"list

separated

preceding

items"
by

list

word

for

consists

delimiter
commas.

IDENTIFIERS

AND

Examples:

INTEGER

LONG

I,J,K;

REAL

BOOLEAN
STRING

DOUBLE,P,Q,ELEPHANT;

ISITREALLYTRUE;
S,T;

DECLARATIONS

CHAPTER

CONSTANTS

4.1

NUMERIC

There

are

CONSTANTS

three

forms

Integer

Real

constants

Long

Real

4.1.1

Integer

range

constants:

constants

Constants

constants

subject

numeric

the

through

consist

constraint

number

that

the

number

34,359,738,367.

adjacent

decimal

represented must

digits,

the

either

NOTE

Any

preceding

program

sign
not

that

appears

considered

1in

part

the

constant.

Examples:

3
24

9276541
See

also

4.1.2

Real

constants

integral

Section

Constants
consist

part

exponent.

omitted,

and the
example

the
the
the

last
&

effect

4.3

the

decimal

fractional

value

part,

decimal

number

both)

number

(containing

followed
unity,

then

optional

this

may

real constant solely represented by the exponent (see
in this section).
The exponent
consists
of
either
symbol followed by an optionally signed integer.
This has

of multiplying the
specified
in
the exponent.
unity is assumed.

decimal

number

decimal

number

the

power

appears,

ten

value

CONSTANTS

The range of real constants 1is approximately 1.4&-39 to 1.7&38;
numbers 1less than 1.4&-39 are represented by zero. Real numbers are
stored to a significance of approximately eight and one-half decimal
digits.

Examples:

Representation

Value

3.141592653589793
.0001
4,37&5
5&-3
&—6

3.14159265
0.0001
437000.0
0.005
0.000001

Long

4.1.3

Real

Constants

Long real constants are wused to represent numeric quantities to
about
approximately twice the precision available with real numbers:
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
except numbers below approximately 3.0&&-30 can only be
constants,
represented to single precision due to hardware considerations.
Examples:

Value

Representation

3.14159265358979323846&&0 3.1415926535897932
0.012

12&&-3

4.1.3.1

Automatic Conversion of Constants to Long

Real

Constants -

The compiler keeps all real constants internally to the precision
available with 1long real variables and determines the precision
required by the context 1in which the constant is used. This is to
avoid loss of precision in arithmetic expressions involving long real
thus making easier the conversion of
variables and real constants,
As a result of this, a long real
variables.
real
programs to use long
constant only needs to be specified explicitly if it is desired to
force a calculation to be done to long real precision irrespectively.
For example, if LR is a long real variable and X is a real variable,
(slightly) different
the following two assignments would produce
results:

LR:=0.1+X

would perform the addition, then convert the result

" real and assign
2.

1long

this to LR.

LR:=0.1@@0+X

would take the value of
addition
perform the
If X was
result to LR.
would be no difference,
long

real precision

then
real,
1long
to
convert
X and
to long real precision and assign the
there
a long real variable, however,
as the addition would be performed to

in both

4-2

cases.

CONSTANTS

4.2

OCTAL AND BOOLEAN

CONSTANTS

Octal constants consist of the symbol % followed by
digits.
Up to twelve significant digits may appear
ignored);
these digits are right justified.

a number
(leading

of
octal
zeros are

Examples:

$777777777774

$0470
Octal

constants

Boolean

constants

equivalent
to
respectively.

4.3

may

ASCII

only

consist

the

used

octal

the

Boolean

words

constants

expressions.

TRUE

and

FALSE.

$777777777777

and

They

are

%000000000000,

CONSTANTS

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

user

can

space

dgenerate

each

side

invisible
single

it,

and

one

ASCII

such

character

preceding

the

space.

constant

triplet

Thus,

the

placing

one

dollar

sign.

Examples:

Text

Octal

S A
$/01234/

4.4

STRING

String
of

is
for

restricted

only

execution

typically,
as

allow

characters

the
values

000000

000101

160713

516674

CONSTANTS

constants

ASCII

the

user

within

the
of

output

assigned

amount
the

store

core

progranm.

a message

program.

string

during

string of symbols
1is
enclosed
restrictions on the symbols that may
1.

[

;

Single

]

;7
[

and
and

may

not

appear

occurrences

and

Where

]

any

reasonable

The

length

storage
String

the

length
such

available
constants

execution

[

within

appear

string
constant

the

user

may

used,

the

program

they

]

quotes
(").
There
within the string.

are

;

are

and

properly

are

paired.

represented

[[

respectively.

string

has

alone.

broken

across

source,

the

carriage

return

and

ignored

preceding

them

with

character.

variables.

The

Value

line

two
feed

1lines

characters

control-back

can

of
be

arrow

CONSTANTS

NOTE

[[{ 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
the
whole
£}

program

Square

effect
16.6.2.

brackets

when

are

the

by missing

used

string

out

enclose

symbols

output.

These

are

that

have

specific

discussed

1in

Paragraph

Examples:

"ABCDEFGHIJKLMNOPCRSTUVWXYZ"
"REMEMBER

THAT

"[P5C] INPUT
llllllA[[I]]

SPACES

ETC.

DATA:[5C]*
e =

0.1;;lllill

ARE

SIGNIFICANT"

CHAPTER

EXPRESSIONS

5.1

ARITHMETIC

EXPRESSIONS

DECsystem-10/20

ALGOL

similar

computer

languages.

that

operators

and

arithmetic

used

The

expressions

FORTRAN
usual

brackets

are

and

many

algebraic

followed

Table
Operator

are

written

other

rules

(see

high-level
concerning

Table

form

scientific

precedence

5-1).

5-1

Precedence

Priority
Operator

(decreasing)

parentheses

There

are
division
of

1
2

multiplication and division
addition and subtraction

3
4

two

additional

operators, DIV and REM, that
remainder,
respectively,
and
these

and

precedence

order

exponentiation

ordinary

evaluation

”~

Z2Z

DIV

REM

division.

means

always

Within

from

left

the

indicate

have

precedence

right.

For

integer
the
same

scheme,

the

example:

and

Unlike

means

DIV

FORTRAN, when ordinary
the real result is

performed,
The

difference

between

the

following

examples:

7/4

yields

the

result

REM

division of
not rounded

various

types

1.75,

whereas

DIV

yields

result

REM

yields

result

and

one integer
by
another
to an integer value.
of

division

clarified

EXPRESSIONS

The interpretation of integer division for negative integers follows:
Let M,

N>0,

then

-M DIVN =M DIV
-M DIV

(-N)

= -(M DIV N)

= M DIV N

The integer remainder operator,
integral

M REM N

5.1.1

REM,

defined

that

for

all

= M - N*¥(M DIV N)

Identifiers And Constants

1identifiers and
Arithmetic expressions consist of operands, that is,
real, together
long
and
real
integer,
constants, of the three types,
+ and parentheses
with the arithmetic operands + - * / DIV REM and
where

necessary.

Since automatic conversion takes place as necessary when an expression
the user may freely mix the three different types of
is evaluated,
identifiers and constants, (refer to section 4.1.3.1 for the effect of

mixing real or long real constants and variables).

1in
Integer quantities may have more precision than can be represented
The user must beware of possible 1loss of
variable.
real
a
significance in integral quantities used in mixed type expressions.

5.1.2

Special

Functions

arithmetic
in
use
for
Three special functions are provided
ENTIER, which
function,
transfer
1is the
first
The
expressions.
converts a real or long real quantity into an integer quantity defined
as the largest integer value not exceeding the argument.

Thus

ENTIER(3.5)

and

ENTIER(-3.5)

-4

the
The special function ABS yields the absolute value (also known as
The argument may be any integer, real, or
its argument.
of
modulus)
the
as
type
same
the
the result is always of
long real quantity;
argument.

Thus

ABS (-3.5)

ABS(-3)

3.5

and

The special function SIGN is the signum function whose argument can be
The result is always integral, being
real.
1long
or
real,
integer,
is
argument
the
on whether
depending
one,
plus
or
zero
or
one
minus
negative, zero, or greater than zero, respectively.
5-2

EXPRESSIONS

Thus
SIGN(-3.5)
SIGN(0)

SIGN(3.5)

-1

0
=1
NOTE
ENTIER,

Examples

ABS,

and

words.

They

purposes

simple

SIGN

may

are

not

used

delimiter

for

other

program.

arithmetic

expressions

follow:

I+

X*Y /7
P+Q/R
X2

XJ-4
J

ENTIER(K-2)

SIGN (ENTIER(J/K)

5.2

(X + Y)

BOOLEAN

EXPRESSIONS

Boolean

expressions

constants,
in an order

5.2.1

are

involve

arithmetic
similar to

Boolean

There

(-I)

five

Boolean

identifiers,

Boolean

<conditions, and Boolean operators
that of arithmetic expressions.

and

octal

interspersed

Operators

Boolean

operators

listed

here

decreasing

order

1in

the

precedence.

NOT
same

NOT

AND

IMP

(implication)

EQV

(equivalence)

(unary

unary

way

that

operator)

operator
a

unary

arithmetic expression.
vice versa.

that
minus

complements
sign

this

negates

case,

Boolean
an

FALSE

quantity

arithmetic

changed

quantity

TRUE,

EXPRESSIONS

Table 5-2 gives the result of A OP B where OP stands for
one
of
Boolean operators AND, OR, IMP, or EQV, for all values of A and B.

Table

Function

5-2

Boolean Operators

FALSE

B
A AND B
A OR

A IMP B
A EQV B

addition,

TRUE

FALSE
FALSE

TRUE
FALSE

FALSE
FALSE

TRUE
TRUE

FALSE

TRUE

TRUE
TRUE

TRUE
FALSE

FALSE
FALSE

TRUE
TRUE

the

following

theorems

hold

true:

IMP

1is

equivalent

NOT A OR B,

EQV B

1is

equivalent

A AND B

5.2.2

the

OR NOT A AND

NOT

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.

The
actual
test
expression such as
B

AND

employed

determine

the

truth

This is particularly
important
when
Boolean
expressions.
For
example,
particular bit in o Boolean variable,
be
B

used,
AND

Boolean

for

nonzero,

octal
constants
are
if
the
wuser
wishes
an
appropriate
octal

that

wused
1n
to test a
constant

example:

a Boolean

significant)

5.2.3

is to evaluate it and regard it as true if the value is
is, at least one bit is set, otherwise regard as false.

can

expression
bit

Arithmetic

that

true

and

only

the bottom

(least

one.

Conditions

Arithmetic conditions are used as
operands
in
Boolean
expressions.
They
consist of two arithmetic expressions 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

equal

EXPRESSIONS

Such

eguals

greater

than

greater

than

not

egual

equal

arithmetic

<condition

<can

regarded

true

false

according
to whether the condition specified by the comparator is
when the arithmetic expressions
on
each
side
are
evaluated.
resulting condition may form part of a Boolean expression.
The

following

examples

also

involve

arithmetic

Boolean

expressions,

Table

5.3

Meaning

NOT
NOT

EQV

X<KY

X+¥<Z

AND

INTEGER AND

C
B

P=Q

BOOLEAN

The

guantity

value

included

passed

for

these

semantic

AND

EQV

(NOT
(B

AND

(X<Y)

(((X+Y)<Z)

AND

(P=Q)

CONVERSIONS

an integer quantity can be converted
the dummy function BOOL.
Similarly,
Boolean

5-3,

5-3

AND

Table

Expressions

Expression

conditions.

Boolean

NOT

shown

met
The

integer

the

Boolean quantity by means of
function INT converts a

dummy

gquantity.

functions

correctness.

unchanged:

the

functions

are

Thus:

BOOL (I)

may

regarded

Boqlean

operand,

and

INT (B)

INT (%400000000000)

integer

BOOL

and

operands.

INT

are

"declaring"

avoided

not

them

prevent

reserved

words

required.

confusion.

and

used

for

However,

can

this

practice

other

purposes

should

CHAPTER
STATEMENTS

6.1

AND

ASSIGNMENTS

STATEMENTS

The

statement

operation,

6.2

the

such

basic

operational

assignment,

unit

ALGOL-60

performed

and

run

describes

time.

ASSIGNMENTS

Assignments

convey
the
value
produced
by
the
execution
expression to a destination variable of the appropriate type.
done by writing the destination
identifier,
followed
first
symbols

and

causes the result
variables Y and Z
When

the

result

assignment

Integer,

any

string

real

then

the

expression

addition

placed

made

in
a

the

variable

type

expression,
a
type
long real expressions may

and

three

expressions

types,

can

only

real

value

but

not

assigned

<contained

differing

other

real

rounding
I

results

long

process

assigned

the

that

variable

integer

type

performed.
variables
Boolean

and

the

same

variable,

ENTIER(X

being

occurs.

from
1is
to

types.

type.

Thus

conversion
be assigned

any

evaluated.

wvalues

variable

the

these

of
an
This is
by
the

assigned

integral
+

value

equal

0.5)

When

an integer
expression
1is
assigned
to
a
real
or
1long
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
1long
real
result
after
assignment
of
the real result to the
high-order part
of
the
1long
real
variable.
Long
real
to
real
assignments
truncate
the low-order part of the long real expression,
after

appropriate

rounding.

STATEMENTS AND ASSIGNMENTS

6.3

MULTIPLE

ASSIGNMENTS

A value may be assigned
same type by a multiple
P

where
R,

:(=

the

and

result of

simultaneously to
assignment.
This

+.Y

the

;

adding Y to X and subtracting

assigned

simultaneously.

All identifiers on the left-hand side
of the same type.
If the user wishes
different types
of
variables,
the
(embedded assignment) feature must be
A parenthesized assignment
expression.
For example,
X

several
variables
of
takes a form such as

may

of a multiple assignment must be
to assign a value to two or more
‘“assignment
within
expression"
used, as below.

substituted

for

any

operand

1in

P+Q)/Z;

This

causes the
embedded
assignment
to
be
made
after
the
1inner
expression
P+Q is evaluated.
Where a type conversion 1is performed as
part of an embedded assignment, the operand type is the same
as
that
assigned to the variable in the embedded assignment.
Thus
X

3.4)

sets

equal

6.4

EVALUATION

EXPRESSIONS

All

expressions

DECsystem-10/20

normal

algebraic

;

and

rules

equal

3.0.

ALGOL

precedence,

are

evaluated

including

observing

the

bracketing.

Within the precedence structure, expressions are always evaluated from
left
to
right.
For
example,
1if
X
1is a scalar, and F a function
procedure (see Chapter 11) that alters X,
X

may

have
X

This

X+F

different
F+X

than

;

known

Consider

effect

"side

always

effect".

also:

The

subscript

to
the

the
1left
of the embedded assignment,
above expression is eqguivalent to
J

order

of
X

evaluation
(P

I+1;

evaluated

A[J]

P+Q)/(P+R)

before

;

expression
;

The

and

incremented,

within

user

can

count

the

always

statement.

such

predict

things

1is

Thus

the

STATEMENTS

being

6.5
A

evaluated

thus

ASSIGNMENTS

giving

the

same

result

COMPOUND STATEMENTS

compound

BEGIN,

statement

separated

statements,

correctly,

AND

the

For

end

unlike

consists

by
those

line

number

semicolons,
in

and

FORTRAN,

are

statements,

terminated
terminated

preceded

by
by

END.

semicolon

text.

ALGOL
not

example:

(&
+

]

—

BEGIN

END

compound

BEGIN

The

usefulness

chapters.

statement.

before

the

END;

compound

Semicolons
BEGIN

and

not

have

END

act

type

statements

will

become

appear
of

after

the

bracket.

apparent

1later

CHAPTER

CONTROL

7.1

TRANSFERS,

LABELS,

AND

CONDITIONAL

STATEMENTS

LABELS

A label is a method of marking a
can be transferred to that point
DECsystem-10/20

ALGOL

uses

place in a program
so
that
control
from elsewhere in the progranm.

identifiers

labels.

These

1identifiers

are
placed
before
statements
and are followed by a colon.
Numeric
labels are permitted in the Revised Report, but are not implemented in
DECsystem-10/20
ALGOL.
Most implementations of ALGOL-60 do not allow

integer
For

labels.

example:
COMP:

statement
than
LABl:

7.2

one

label

LAB2:

;

labeled
by COMP.
can

UNCONDITIONAL

transfer

attached

CONTROL

control,

statement

required;

thus,

TRANSFERS

"jump",

statement

program

1is

effected
by
a
GOTO followed by
statement.
The

GOTO
statement.
This statement consists of the word
the name
of
the
1label
attached
to
the
relevant
two words GO TO can be used instead of the word GOTO

where

any

statement

GOTO

can

BEGIN

INTEGER

I,J,K;

LAB:

:=J

GOTO

used.

Thus:

LAB

END

i1s

example of a somewhat tedious
program, it is necessary

reasonable

program.
Clearly,
to be able to jump

write

any

conditionally.

CONTROL TRANSFERS,

CONDITIONAL

7.3

LABELS,

AND CONDITIONAL STATEMENTS

STATEMENT

Conditional statements provide a
method
to
make
the
execution
of
either a statement or a compound statement dependent on some condition
in the program, such as the value of a variable.
The simplest form of
a conditional
IF

where B

statement

THEN

1is

some

Boolean

expression,

and

statement.

For

example:

Here,

which
if

THEN

the Boolean expression and

the

is obeyed if and only if the Boolean condition is true,

A more

statement

that

1is,

negative.

general

THEN

form of

a conditional

ELSE

statement

In this case, the statement S1 is obeyed if and only
1if
the
Boolean
expression B is true, and S2 is obeyed if and only if it is false.
1In
order to eliminate the "dangling ELSE ambiguity"”
(a
construction
1in
which
an
ELSE could be paired with either of two THENs), S1 must not
be conditional, FOR, or WHILE statement which ends in an ELSE
clause.
(Refer to Chapter 14 for more complete information.)

A control transfer,
statement.
BEGIN

LAB:

a type of

statement,

can appear

conditional

Thus:
INTEGER

100

THEN

GOTO

LAB

END

is a simple way
of
counting
to
methods are shown in Chapter 14.

one

hundred.

sophisticated

CHAPTER

FOR AND WHILE

8.1

FOR

The

for

statement

fashion

general
FOR V

where

FORLIST

can

commas).

STATEMENTS

enables

the

similar,

format

iterate

portion

sophisticated

than,

the

program

FORTRAN's

FORLIST

variable

and

consist
FOR

user

but more

loop.

The

any

element

statement

(compound

number

FOR

one

the

following

taking

the

form:

takes

elements

otherwise).
(separated

forms:

expression:
E

STEP-UNTIL
El

STEP

WHILE
E

Any

number

serially.

8.1.1

This

FOR

UNTIL

element

WHILE

where

element

taking

FOR

:=1,2,5

3,5,10

some

Boolean

the

may

STEP

FOR

statement

and

executed

.....

Element

deserves

FOR

STEP

examples:

.....

UNTIL

form

appear

.....

particular
I

form:

expression.

following

2.5,5.0,10.0

STEP-UNTIL

the

elements

Consider

UNTIL

closer
N

inspection.

Consider

The

statement S is obeyed with I taking an initial
value
of
1,
being
incremented
by
I
until
the
final value N is achieved.
question is , "Is the I after the STEP recalculated during
each

8-1

and
The

turn

FOR AND WHILE STATEMENTS

around the loop, or does it have a constant value equal to the initial

value of

I?"

The answer is slightly more complicated.
FOR V

Consider the general case

:= E1 STEP E2 UNTIL E3 DO S

This is defined to have exactly the same effect as

(V - E3)*SIGN(E2)

Ll:

El;

> 0 THEN GOTO L2;

S;
E2;

:=V +

L1;

GOTO
L2:

Clearly, the value of I following the STEP in the previous example

if necessary, twice during each turn around the loop, once
evaluated,
ALGOL allows the user
in the sign test at L1, and again to update V.

modify

E2,

E1,

and E3 freely throughout the loop, and takes

account of all these changes in the evaluation of the loop.
NOTE

the

abbreviated
FOR V

El1

the

allows

DECsystem-10/20 ALGOL

user

form

UNTIL

of
instead

FOR V

WHILE

8.1.2

:= E1 STEP

UNTIL E3

DO S

Element

A FOR statement with a single WHILE element takes the form
FOR V

This

E WHILE

interpreted as

Ll:

B DO S

follows:

IF NOT B THEN GOTO L2Z;

S;
GOTO

L1;

L2:

Once again, the complexity of the loop may be affected by
and E within

the

1loop.

changing

FOR AND WHILE

8.2

WHILE

STATEMENT

The
WHILE
statement
DECsystem-10/20 ALGOL.
WHILE

and

STATEMENTS

of
the

ALGOL-60
provided
statement is

interpreted

Ll:

1is
an
enhancement
The geaneral form of

as
NOT

follows:
B

THEN

GOTO

L2:

S;
GOTO

L1;

L2:

GENERAL

NOTES

Within a FOR statement of any kind, the user can
change
the
controlling
wvariable
or any other variable appearing within
the action of the loop.
Such changes predictably affect
the
execution of the loop by the rules given above.
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

Report

should

implementations.
be

studied

Section

carefully

4.6.4

this

the

connection.

Revised

CHAPTER

ARRAYS

9.1

GENERAL

Arrays

are

essentially

collections

allowing
the user to address each
common name and a unique subscript

variables

case, an array is a vector and is known
matrix is a two-dimensional array, etc.
There

those

imposed

9.2

ARRAY

Arrays

may

these

are

the

limit

the

number

ability

the

one-dimensional

subscripts

the

same

computer

allowed,

store

type,

by means of a
the
simplest
array.

other

the

than

array.

DECLARATIONS
be

type

declared

array

must

possesses,

that

variable individually
or
subscripts.
In

For

example,

lower

and

must

and

INTEGER ARRAY

long

scalar

For

real,

each

upper

Boolean,

variables,

bound,

except

string

the

subscript

that

called

"bound

the

size

the

and

array

pair"

for

given.

declare

bound

real,

stated.

lower

subscript,

with

integer,

similarly

two

one-dimensional

upper

bound

integer

arrays

and

A,B[1:5]

NOTE

The

1lower

enclosed
separated

When

the

there

two

pairs

are

LONG

REAL

ARRAY

declares
bounded
Arrays
same

are

bound

three
by

long

-5

and

the

same

and

upper

square

bounds

the

commas.

ARRAY

declaration

similar,

and

Thus

P,Q,R[-5:2,0:10]

real

arrays,

and

the

second

type

but

and

A[1l:10],

subscript

different

statement.

REAL

and

colon.

subscripts,

separated

must

brackets

B,C[1:10,1:12]

with

the

bounded

sizes

may

first
by

subscript

and

declared

10.
in

the

ARRAYS

NOTE

the

case

omitted

assumed

real

default,

ARRAY A(1:10],

arrays,

the

REAL may

the declaration,

and

thus:

B,C[1:10,1:12]

The bounds in an array need not be static, as in the
examples
above,
but
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.
No bound may
exceed

9.3

131,072

ARRAY

in magnitude.

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
1in
the
array
declaration.
Thus,
a
typical
element
of
A
wused
1in
the
1last
declaration might

A[5]

where

A[9]

some

generally,

A[I]

integer expression

or,

whatsoever,
with
the
subscript and evaluated

1limitation
that
as an integer is

the

bounds

an example
REAL

the

ARRAY

array

the

use

D,E,F

arrays,

and

UNTIL

FOR J

UNTIL

BEGIN

FOR K

any

expression

the

as a
10,

declaration

[1:10,1:10]

FOR

general,

its
wvalue
when
used
in the range
1
through

consider

and suppose the operation required,
product

was

to set F equal

the

UNTIL

X +

D[I,K]*E[K,J];

END

NOTE

In
the
above
example
X
1is
wused
to
accumulate
the
1inner
product
of
the
multiplication for all values of
I
and
J.
The variable X was used instead of F
to facilitate the computation.

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
1in
such places as the controlling
variable in a FOR statement.

9-2

matrix

CHAPTER

BLOCK

10.1

GENERAL

ALGOL

program

structure

1is

STRUCTURE

somewhat

complicated

than

high-level languages, such as FORTRAN.
An ALGOL program consists
number of "blocks" arranged hierarchically.
A block consists
of

words

BEGIN

and

END

enclosing

the

declarations

other

of a
the

and

(optionally)

and

statements

statements.

Thus:
BEGIN

BEGIN
END

BEGIN
BEGIN
END
END
END

is
in
The

an ALGOL program,
the blocks.
block

structure

available
may

1in

declare

identifier

assuming

offers

non-block

the

user

structured

many

that

enclosing

appears

block.

Thus:

INTEGER

BEGIN

declarations

interesting

languages.

1identifier

BEGIN

appropriate

For

features

instance,

<conflict

the

with

not

user

another

INTEGER

END
END

fact,

there

that

statements

outer
use

known

block.
the

occurring
variables

1in

outer

the

there

that

are

block

Similarly,

"local"
at
can

conflict

two
can

different
"see",

Is.
the

The
one

only
the

any statement in the inner block will
always
block.
Such a declaration in an inner block is

variable

an
outer
or
be "seen" from

and

takes
precedence
over
declarations
more
‘“"global"
level.
In general, all
any point in a program that is either
in

10-1

BLOCK STRUCTURE

by
the same block as the declaration or in a block that is enclosedNote
.
occurred
e
wvariabl
the
of
the block in which the declaration
that a more 1local variable is always taken 1in preference to a

relatively global variable.
BEGIN

Consider the following example:

INTEGER I,J;

[1]
INTEGER J,K

BEGIN

[2]
END;

INTEGER I,K

BEGIN

[3]
END
END

Any statements occurring at point [l] can see the declarations of I
and J, which are local, but cannot see the declarations of J and K 1in
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 wvariable J is
not seen because the local variable J takes precedence over it; the

A
variables I and K in the second inner block are not seen at all.
K,
and
I
variables
local
similar situation occurs at [3]; here both
are seen.

as well as the global variable J,

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 freguently throughout this text.

In general, local variables are more efficient to use than global
This statement is also true of most ALGOL-60 implementations.
ones.
Where a global variable is used frequently, a local variable should be
For example:
assigned as having the same value and used instead.
BEGIN

INTEGER

BEGIN

.....

I1;

INTEGER
IT

.....

END

and assigned
Here, in the inner block, a local variable II is used,
the value of the global variable I for use throughout the local block.
10-2

BLOCK

10.2

ARRAYS WITH

DYNAMIC

STRUCTURE

BOUNDS

The concept of the scope of a variable can be applied most usefully to
arrays.
In
DECsystem-10/20
ALGOL,
all
arrays
are constructed at
execution
time
(that
1s,
no
fixed
space
is
reserved
during
compile-time),
irrespective
of
whether
their
bounds are static or
dynamic.
When a declaration of
an
array
1s
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
1is
reached,
that
1is,
the
array
variable
1is
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
1in
a calculation is dependent on the data to be processed.
The
programmer has the choice of making the array
1large
-enough
to
cope
with
the worst case, or constructing the array with dynamic bounds to
suit the size required by the particular data.
The first
method
has
the
disadvantage
of
wasting
space
on
many occasions.
The latter
method only has the minor
disadvantage
of
the
overhead
needed
to
construct
the
array.
Such
overhead
1is very small compared to the

running time
desirable.

Consider

following

the

most

programs,

BEGIN

INTEGER

therefore,

the

second

method

1is

example:
N;

END;

GOTO

END

A value
some

for

data

read

calculated
1into

the

this

example,

program,

which is used to process the data in
the
1inner
block
1is
reached,
the

control

passes

process

repeated.

where

another

and

declare

the inner block.
space used by A

value

10-3

possibly

used

for

dependent
the

array

on
A,

When the end of
is recovered and

calculated,

and

the

CHAPTER

PROCEDURES

Procedures are similar
more sophisticated and

in concept to the FORTRAN
general applications.

subroutine,

but

with

"procedure" is a portion of an ALGOL program that is
given
a
name
for
identification
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, and in the case of function procedures
can return a value to the procedure body.
In addition, it may or
may
not

have

parameters.

In DECsystem-10/20
types:
integer,

ALGOL,

a procedure

real,

1long

formal

parameters

FORTRAN),

real,
There

<can

real,

procedure

one

the

can
be
Boolean,
(know

following

one
of
string or

"dummy

types:

the

following
typeless.
The
variablesTM

1integer,

real,

Boolean or string, as scalars, arrays or procedures,
or
are eighteen different types of parameters.
1In addition,

these parameters may
appear
neither
of
which
1is
the

1in
same

two

different

modes

and

FORTRAN's

method

1in
1long

label.
all of

strings,
handling

parameters.

11.0.1

PARAMETERS

CALLED

Calling parameters by
exception
of
arrays
parameter

"VALUE"

“value"
1is
the
most
and
strings,
the most

common
and,
with
the
efficient way to pass a

a procedure.
The value of the expression presented in
a
known as the actual parameter, is evaluated on entry
and
assigned
to
a
formal
parameter
within
the

procedure
<call,
to the procedure

procedure.
This
formal
parameter
acts
as a local variable in
procedure which is initialized, the initial value being
that
of
actual parameter supplied in the call to the procedure.
Since,
string

in the case of
1is made, this

should

11.1

avoided

PARAMETERS

arrays or strings, a new copy
type of parameter-passing for

unless

specifically

CALLED BY

the
the

of
the
array
or
arrays and strings

required.

"NAME"

Calling parameters by "name" is a useful method of passing a parameter
to
an ALGOL procedure.
Whenever the formal parameter associated with
the actual parameter in a procedure body appears in the
body
of
the
procedure,
the
actual parameter is re—-evaluated as if 1t appeared in
the procedure
body
at
that
point.
For
example,
1if
the
actual
parameter

were

array

element

such

A[TI]

11-1

PROCEDURES

the element 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

entered.

Table 11-1 shows the different types of formal parameters, with
actual parameters that can be substituted in a procedure call.

Parameter

Formal

Table 11-1
in a Procedure

Permissible

Parameter
Type

valid

Call

Actual

Parameter

Integer

Real

Any

Long

arithmetic

expression

Real

Boolean

Any Boolean

String

Any

Label

A label or switch element
12 and Paragraph 14.4)

Switch

A switch

Integer

Array

string

expression

(refer

array

type

integer*

Array) | An

array

type

real®*

array

type

long

array

type

Boolean

String Array

array

type

string

Procedure

non-type

Real

Array

(or

Long

Real

Array

Boolean

Integer

Array

Procedure

A procedure

Long

Real

real

String

(refer

13)

Chapter

real*

procedure

Procedure

Real

Boolean

to Chapter

Procedure

type

integer,

A procedure

type

Boolean

A procedure

type

string

real,

long

any
value,
by
*In the case where the array parameter 1is
called
(integer, real or long real) array is allowed as
arithmetic
type
A type conversion
takes
place
during
the
an actual parameter.
copying

11.2

PROCEDURE HEADINGS

Procedure
type

process.

headings

identify

the

type

parameters.

11-2

procedure,

the

number

and

the

PROCEDURES

procedure

heading

type

The

consists

of:

procedure

(omitted

the

case

procedures).

The

semicolon

list

word

PROCEDURE

followed
5.

the

REAL

BOOLEAN

the

has

name

parameters,

the

procedure.

parameters,

enclosed

typeless

otherwise

parentheses,

and

semicolon.

formal

LONG

procedure

formal

Specifications

Omitting

followed

the

formal

parameter
PROCEDURE

PROCEDURE

parameters.

specifications,

this

looks

LR;

BOOLEAN

(I,J,K);

CALC (THETA,X) ;

The

formal parameter specification that follows consists of a list
of
descriptions
of
the formal parameters, appearing in any order, and a
value specification if any of the
parameters
are
to
be
called
by

value.

(If

called

this

1is

name.)

parameters

for

VALUE

the

I,J;

example
REAL

parameters,

by
specification

the

second

example

above

INTEGER

that
all
(scalars),
and
I
called by name.
A

the

example,

meaning

third

omitted,

For

PROCEDURE

the

will

formal

be:

I,J,K;

three

formal

and

are

J
typical

might

default,

parameters

to be called by
formal
parameter

are
of
type
value, while K
specification

be:
THETA;

ARRAY

integer
is to be
for
the

NOTE

Procedure

11.3

PROCEDURE

The body
heading,

the

headings

must

precede

the

body

procedure.

BODIES

procedure

is
that
part
which
follows
the
procedure
of a single statement, 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:
1.

and

A
a

consists

real

procedure,

real

quantity.

SQUAREROOT,
The

first

second

label

used

The

is
found

square

root

that is
negative.

the

quantity.

calculation
of

the

calculate

parameter

square

the

root

guantity,

of
the

an escape if
the
quantity
result of the procedure is the

Note

how

1is assigned to the procedure
procedure on the left-hand side of

11-3

the

result

placing

the

the
assignment.

name

PROCEDURES

REAL PROCEDURE SQUAREROOT (X,L);

Y, Z;

REAL

BEGIN

IT:

GOTO L;

THEN

(1 + X)/2;

(X/Y + Y)/2;
< 1l&-6 THEN GOTO OKj;

IF ABS(Z - Y)
Z;:;

OK:

LABEL L;

REAL X;

VALUE X;

IT;

GOTO

SQUAREROOT

example

uses

END

;The previous

Newton-Rapheson

the

method

finding the sgquare root of a number by taking an initial
approximation (1 + X)/2 and iterating until the difference
between successive approximations 1is 1less then l&-6. The
with the aid of some
procedure is again described below,
alternative method of
ALGOL
0/20
DECsystem-1
The
commentary.
commentary (refer to Chapter 2)

is used for brevity:

REAL PROCEDURE SQUAREROOT (X,L) ;
VALUE

BEGIN

LABEL

REAL X;

CALCULATES THE VALUE OF SQRT (X)

USING THE NEWTON-RAPHESON METHOD.

L IS USED FOR AN ESCAPE
REAL

Y, Z;

IF X < 0 THEN GOTO L;

IF X <

EXIT IF X <

(1+4X)/2;

FIRST APPROXIMATION;

(X/Y + Y)/2;

ITERATE;

IT:

IF ABS(Z-Y)

1l&-6

TEST FOR CONVERGENCE;

THEN GOTO OK;
Y

2Z2;

GOTO

IT;

OTHERWISE CONTINUE;

FINAL

OK:

SQUAREROOT

RESULT;

END

any real
This function evaluates the sum of the values of
procedure G over the integers 1 ..... N (that is, iterating N
times)

where N is also a parameter of the procedure.

11-4

PROCEDURES

REAL

PROCEDURE

VALUE
BEGIN

SUM(G,N)
;

REAL

INTEGER

PROCEDURE

REAL

UNTIL

INTEGER

FOR

SUM

G(N);

END

NOTE

In
is

this example, the formal parameter
G
1invoked so that the actual procedure

substituted

11.4

PROCEDURE

for

called.

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
P

causes

SQUAREROOT(Z

the

example

the

sums

squared,

Here

square
of

of
as

SQUAREROOT
+

root

the

use

can

called

the

0.5

procedure

roots

the

similar

way,

for

SUM

first

can

used

integers,

with

SUM(SQUAREROOT,J)
" 2;
further

example

procedure

MATRIXMULT(A,B,C,N);

VALUE

ARRAY

A,B,C

;

INTEGER

I,J,K;

COMMENT

THIS

THE

REAL

ARRAYS

the

calls:

PERFORMS

THE

MATRIX

STORES

THE

AND

ARE

AND

FOR

UNTIL

FOR

UNTIL

BEGIN

BOUNDS

and

INTEGER

PROCEDURE

MULTIPLICATION
IN

example:

calculated.

follows:

PROCEDURE

BEGIN

0.5,ERR)

square

AND

ASSUMED

1:N,1:N;

11-5

RESULT

SQUARE

calculate

the

result

PROCEDURES

FOR

UNTIL

B[I,K]*C[K,J];
A[I,J]

END
END

typical

call

for

this

procedure

might

MATRIXMULT(E,F,G,N);
or

MATRIXMULT (E,F,F,N);

Since

the

arrays

are

MATRIXMULT(E,E,F,N);

would

called
by
rather

name,

give

This call could be made to work by calling B and C
B,
C,
N) by value.
However, this would increase
procedure considerably.

11.5

ADVANCED

11.5.1

USE

Jensen's

interesting

call

such

MATRIXMULT

the

overhead

(4,
the

results.

PROCEDURES

Device

This method of using a procedure exploits the power and flexibility
the call-by-name concept.
Consider the following example:
REAL

PROCEDURE

BEGIN

REAL

FOR

SUM(I,N,X);

VALUE

INTEGER

I,N;

REAL

UNTIL

END

the

surface,

However,
Z

the

consider

procedure

the

appears

calculate

the

value

N*X.

call

SUM(J,10,A[J]);

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 assigned.
Hence
the above call calculates
A[l]

Similarly,
Z

A[2]

the

.....

A[1l0].

call

SUM(K,M,A[I,K]*B[K,J]);

11-6

PROCEDURES

calculates

11.5.2
ALGOL

the

(I,J)th

procedures

program.)

recursive,

often-quoted

function

INTEGER

PROCEDURE

ELSE

and

are

indirectly,
the amount of

factorial

This

product

and

Recursion

directly
or
restriction is
the

inner

THEN

has

therefore

and

is,

themselves,

(The
only
the
object

they

may

inefficient

positive

FACTORIAL(N);

method

integer

VALUE

calculating

is:

INTEGER

N*FACTORIAL (N-1);

only
be

call

any
reasonable
depth.
storage
available
to

very

small

FACTORIAL

procedure
can

that

to
core

single

written

statement,
a

compact

but

form.

1local
call

variables,

such

FACTORIAL(6)
;

causes the
FACTORIAL

procedure

to be entered with
FACTORIAL enters the

1inside

N equal to
procedure a

6.
The
call
to
second time with N

equal to 5, but this N is different from the one to
the
previous
N,
which
retains its value of 6, and is stored in a different space.
In
this particular case, FACTORIAL is entered six times,
the
1last
time

with

11.6

equal

LAYOUT OF

Declarations

Procedure
the

DECLARATIONS WITHIN BLOCKS

must

assignments,
1) scalars and

bodies
head

necessary.

always

procedure
arrays and
that

occur

the

Consider

made

block,

the

head

calls,
etc.,
2) procedures and
a

block

should

although

following

block,

in
the
switches

this

before

any

following
order:
(see Chapter 12).

the

1is

only

declarations
enforced

when

example:

BEGIN

PRGCEDURE
BEGIN

P (X);

VALUE

INTEGER

REAL

the

body

END;

INTEGER

The

assignment

within

utilizes

the

declared following the body of P, rather than some global I.
the compiler has not yet "seen" this I and, therefore, cannot

rational
before

the

action.
body

case

such

11-7

this,

the

user

must

that

However,
take any

declare

PROCEDURES

BEGIN

INTEGER

PROCEDURE
BEGIN

P(X);

VALUE

INTEGER

REAL

END

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 1is found
following

11.7

the

body

FORWARD

REFERENCES

Although

most ALGOL-60
DECsystem-10/20
ALGOL

some

minor

restrict
A

user

reference

operate

in two or
more
operates in one pass.

compiler

restrictions

the

forward

compilers

have

other
for

made

ALGOL-60

has

when

passes,
the
Consequently,

order

not

ways.
procedure

given

procedure

is
called
(either
directly, or indirectly, by passing the procedure
name as an actual parameter in a procedure call) before
1its
body
1is
encountered
by
the
compiler.
1In most cases the user can avoid this
situation by a minor re-ordering of the
program.
However,
1in
rare
cases
1like
the
following,
where procedure P calls procedure Q, and
vice versa, a forward reference, as shown, must be given.
BEGIN

FORWARD

REAL

PROCEDURE
BEGIN

REAL

PROCEDURE

P (X);

VALUE

REAL

VALUE

END;

REAL
BEGIN

PROCEDURE
REAL

Q(Z);

END;

11-8

REAL

%2;

PROCEDURES

general,

by
the
the word

forward

reference

consists

type of the procedure (omitted
PROCEDURE, and the name of the

FORWARD

LONG

FORWARD

PROCEDURE

REAL

PROCEDURE

the

word

FORWARD,

followed

if the procedure is typeless),
procedure.
For example:

INTEGRATE

PROBLEM

NOTE

forward
following

this

label

and

has

the

REAL

the

given

used

yet

must

occur

procedure

for

actual

appeared

1label

parameter

the

body.

either

a procedure

call,

program.

of identical name has
scope of the procedure

appeared
call.

the

program

and

forward

reference

for

must

given.

EXTERNAL PROCEDURES
procedure

instead
FORWARD

18.1.1),

the

compiled

EXTERNAL

procedure.

declaration,

but

The

independently

declaration

form of

with

the

this

word

program,

point.

INTEGER

EXTERNAL
and

PROCEDURE

declaration

has

the

same

program

must

made

the

same

FORWARD

example:
EXTERNAL

Such

not

variable

case,

Paragraph

For

has
cases:

reference
as

example:

11.8
If

block

The

BEGIN

forward

same

reference

rare

1s
For

The

replaced

(see

the

program

that

EXTERNAL.

CALC

can
scope

be
as

11-9

made
if

the

any

block

procedure

within

appeared

the
that

PROCEDURES

1in a program Or
At present all EXTERNAL procedure names referenced six
characters, as
first
their
in
differ
must
scanned in a library
only the first six characters are available to LINK.
ADDITIONAL METHODS OF COMMENTARY

11.9

le to
Two further ways of writing commentary are availab
n 2.4.
addition to COMMENT and ! described in Sectio

11.9.1

wuser

the

Comment After END

add commentary,
Following the delimiter word END, the user may tions:
restric
ng
followi
the
with
on,
terminated by a semicol
1. The commentary may only contain letters and digits.

ation is
If the reserved delimiter word mode of compil
not be
may
t
commen
the
1in
ing
appear
words
any
employed,

delimiter words.
For

example:

END OF PROC

11.9.2

INVERT;

Comments Within Procedure Headings

parameters
This method of commentary allows the user to comment formal comment
ary,
the
ng
enclosi
by
This is done
in a procedure heading.
and
:(
and
)
symbols
the
between
only,
letters
of
which may consist
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 which is:
REAL PROCEDURE SUM

(1, N, X);

can thus be rewritten as

(X) ;
REAL PROCEDURE SUM(I) COUNT: (N) INCREMENT:

In a similar fashion, a call to such a

procedure

can

commented.

The following example uses the call to SUM in Section 11.6.1:
7

:= SUM

(K, M, A[I,K]*B[K,J]);

to be commented as:

(M) CROSS PRODUCT:
7 :=SUM (K) COUNTER:

11-10

(A[I,K]*B[K,J]):

CHAPTER

SWITCHES

12.1

GENERAL

Switches enable the user to
jump
to
one
of
a
number
of
1labels,
depending
on
the value of an arithmetic expression, and in addition,
provide automatic detection when such an expression is
out
of
range
for

the

12.2

switch.

SWITCH

switch

name

DECLARATIONS

declaration

the

are called switch
declaration.
For
SWITCH

switch

must

not

takes

switch,

name

the

form

assignment

elements,
example:

and

the

(:=),

must

word

and

jump

with

USE

SWITCHES

particular

following

the

word

expression

square

GOTO
This

list

the

any

the

usual

local

rules

variable

In addition to the example above, a switch
the labels in the switch declaration.

12.3

SWITCH

followed

scope

labels.

the

These

switch

LAB,L1,L2,0K,STOP;

must

conflict

label

GOTO with

1in

the

name

brackets.

and,

same

element

switch

scope

the

may

also

declaration

the

switch

therefore,

name.

and

z2n

one

made

arithmetic

Thus:

SWI[I]

causes

control

pass

the

1I'th

1label

the

switch

declaration,
unless
I
1s
negative
or
zero, or is larger than the
number of switches in the switch declaration.
1In either
<case,
there
is
no
transfer
of control.
If the expression in square brackets is
not integral, it is evaluated and
rounded
as
usual.
Consider
the
following more complicated example:
SWITCH

LAB,L1,L2,0K,STOP;

SWITCH

L3,SW[J],L4;

GOTO

TWI[I];

12-1

SWITCHES

If I has the value
has the value 1, a

3, a
jump

switch

sophisticated

jump to L4 occurs.
to LAB occurs, via
elements

are

12-2

If
SW.

described

has

the

value

Chapter

14.

and

CHAPTER

STRINGS

13.1

GENERAL

DECsystem-10/20

ALGOL-60

includes

major

extension

the

string

features
defined
1in
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

STRING

S,T;

STRING

PROCEDURE

13.2

STRING

String

STRING

expressions

procedure
call
string constants

expressions

be:

SA[1l:10];

VALUE

REAL

AND ASSIGNMENTS

are

or
see

limited

single

a
string
constant.
Section 4.4.) The only

comparison
operators
operations are achieved
Section 13.7.

String

ARRAY

B (X);

EXPRESSIONS

might

variable,

(For a
string

and
the
assignment
operator.
via the string library procedures

can be

assigned

only

string

full description of
operators
are
the

string

All
other
described in

variables.

For

example:

SA[I]:=SA[3];
SA[2]:=B(2);
T:="ANY

13.3
The

BYTE
value

""OLD""

IRON";

STRINGS
associated

with

string

variable

byte

string.

byte

string
1is
a
sequence
of
bytes
of
a uniform size between one and
thirty-six, which can be efficiently handled
by
the
DECsystem-10/20
hardware.
1In some ways, byte strings can be thought of as arrays, but
the most important difference is that the size of a
byte
string
can
vary

continuously.

Thus

when

byte

13-1

string

created

by means

STRINGS

READ statement, the programmer need not know how long the string will
The routine starts accepting characters after encountering the
be.
open quotation marks and continues until the close quotation marks
have

been

read.

When one string is assigned to another,

e.g.,

S:=T;

then a copy of T is made to which S will refer. Any previous value of
(and the memory space occupied 1is released).
destroyed
is
S
Subsequent changes to the value of T will not affect S5, or vice versa,
unless a further assignment is made from one to the other.
NOTE

change
important
This is an
strings
of
implementation
Version

13.4

the
from
to
prior

BYTE SUBSCRIPTING

Byte strings can be modified by means of the byte subscripting
Individual bytes in a string are referenced by following
mechanism.
the string variable name by a decimal point and then the subscript
number enclosed in square brackets.

For example

S.[I]

array

may

any

being

type

subscript

string

The

variables

are

regarded

refers to the I'th byte

and is evaluated in exactly the same way as an

expression

arithmetic

subscript.

Byte-subscripted string

having an integer value equivalent to the byte to which they
integer,
1in a
Therefore, to change the value of a particular byte
refer.
on the left-hand side of an
appear
a byte-subscript must
string,
arithmetic statement with the appropriate new value on the right-hand
If the new value is too large to be held in the byte, this 1is
side.
simply truncated.

13.5

No warning is given.

STRINGS

NULL

Until a value is assigned to a string by the program, the string takes
Any attempt
That is, it is assumed to contain no bytes.
null value.
to reference the string by a byte-subscript will result 1in a fatal
though it can be used on the right-hand side of a
run—-time error,
string assignment, in which case the variable to which it 1is assigned
similarly becomes

13.6

null.

STRING COMPARISONS

Two byte strings can be compared
comparison operators.
IF

For

T THEN GO TO

example

with

Lj;

13-2

each

other

using

the

usual

STRINGS

where

and

are

string

The

byte-by-byte,

the "lesser" string
working
from
left

value

byte,

"ABCE"

byte
the

or "ABCDE".
sizes,
then

left

the

null

special

string

nulls.

regarded

13.7

case

ASCII

trailing

treated
compare

LIBRARY

procedures

LINK,

until

Version

13.8

CONCATENATION

string

the

can

with
the
Thus "ABCD"

be compared
regarded as

or
the

string
strings

first
1lower
is less than

are of
different
being extended on

cases
extra

(strings

and

trailing

Similarly

ASCII

the

length

extra

byte

size

blanks,

any

1like

mixture

strings

of
different
comprises only spaces

strings of unequal length can
only
be
length consists entirely of null bytes.

PROCEDURES

16.6 deals with
applicable to strings.

strings

nulls

as equal.
equally if

Section

The

being
that
to right.

constants
to compare

bits.

1In all other
equal if the

Where the strings to
the smaller bytes are

constants),

thereof, are
lengths will
and

effect

variables,
string
the comparison is

procedures.

procedure

the

LINKR

have

been

assigned

CONCAT.

1input

and

output

that

were

dispensed

the

For

TAIL

and

procedures

included

that

the

are

1library

with.

concatenated

value

two

strings

with

example

S:=CONCAT(T,U);

S:=CONCAT(S,T);

If,

size

the

adopted

with

null

13.8.1

first

the

example,

first

The

bits

had

bytes

different

encountered

copied

from

byte

would

size

1in
be

appropriate.

Length And

from

this case),
truncated

then

the

would

filled

Size

The

primary

attributes

size

1in

are

bits,

string

returned

string,

the

that

is,

integer

length

procedures

bytes

and

LENGTH

and

byte

SIZE,

respectively.

Thus
:=LENGTH(S);

would

bits

return

13.8.2
A

new

the

each

the

byte

J:=SIZE(S);

number

bytes

string

and

the

number

means

Copying
byte

string

can

procedure

generated

COPY.

This

from

procedure

parameters.

13-3

existing

can

have

one

one,

two

three

STRINGS

The

effect of

COPY with

one

parameter

1is precisely

as
a
simple
string
assignment,
but
retained for the sake of continuity.
2.

Where

there

are

two

parameters,

this

the

same

feature has been

e.qg.,

S:=COPY (T,M) ;

where M is an arithmetic expression, then S
value of the first through M'th bytes of T.
3.

there

are

three

parameters,

assigned

the

e.qg.,

S:=COPY (T,M,N) ;
where both M and N are
arithmetic
expressions,
then
assigned the value of the M'th through N'th bytes of T.

13.8.3

1is

Newstring

Although bytes in a null string may not be referred
to
(see
Section
13.5
above),
a string containing nulls of any appropriate byte sizes
can be created 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:=NEWSTRING(100,7);

causes a null string of 100 ASCII nulls to be assigned to S.
Although
S
1is 100 bytes long at this point (and thus byte subscripts up to and
including S.[100] are valid), any
subsequent
assignment
of
another
string to S may vary both the length and byte-size of S.

13.8.4

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 the value
previously held is lost.
Any space that the previous
wvalue
occupied
is
returned
to
memory, 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.
/

13-4

CHAPTER
CONDITIONAL

14.1

EXPRESSIONS

AND

STATEMENTS

GENERAL

ALGOL-60

allows
conditions.

and

Consider,
according

great

flexibility

for example,
to the value

a variable I
of a Boolean

the

construction

which could
variable B:

expressions

be set equal to 0 or 1
this could be written

as:

THEN

Also,

consider

depending
IF

14.2

the

THEN

allows

1in

THEN

.....

Clearly,
J

Note

:=J

that

general,

arithmetic
may

also

the

unbracketed
In

the

this

user

(IF

NOT

wants

THEN

substitute

ELSE

perform

some

action,

the

use

above

can

conditional

operand

for

construction

any

involving

compact
1

THEN

forms

Boolean

and
1

great

-K

ELSE

operand

must

the

complete

operand

expression.

label

rewritten

conditional

example

conditional

replace

user

ELSE.

first

THEN

when

expression

.....

where

OPERANDS

the

instance,
I

case

CONDITIONAL

operand

value

ALGOL-60

For

and

act

use

cases

such

as:

K-1);

bracketed,

expression

may

replace

addition,

element

and

may

only

any

itself.
an

operand

conditional
a

switch

operand

1list,

for

example:
SWITCH

L1,

It is also permitted, in
subscript), for example:
X

A[I,

THEN

ELSE

L3,

L4;

array

subscript

J+1];

ELSE

14-1

(and

also

byte

CONDITIONAL

Since a conditional
operands
may
also
the

following
IF

EXPRESSIONS AND

operand may replace any operand in an
expression,
be replaced in conditional expressions.
Consider

example:
THEN

ELSE

THEN

This looks complicated but is
inserted for clarity.
Thus:
IF

14.3

The

(IF

THEN

CONDITIONAL

reader

was

ELSE

really

B2)

THEN

quite

simple

1if

brackets

are

STATEMENTS

introduced

THEN

The

Chapter

STATEMENTS

ELSE

conditional

statements

the

form

full

power

this

type

statement

<can

now

demonstrated.

First,

and

THEN

can

BEGIN
END

compound

¢:=

-I;

statements

blocks.

For

example:

FALSE

ELSE

BEGIN

GOTO

END

Second,

can

the

whole

made

themselves.

This

For
0

THEN

equivalent
IF

NOT

GOTO

powerful

the

using

.....

THEN

.....

conditional

ELSE

statement

statements

within

example:

L1:

structure

0;
B

the

ELSE

following

THEN

GOTO

LZ2;

THEN

GOTO

THEN

GOTO

sequence

statements:

L1;

L2;

L2:

Clearly

the

elegant.
IF

former

method

Conditional
B

THEN

expression

statements

ELSE

take

1is

the

both

general

briefer

and

form

where S1 and S2 may both be conditional statements.
However, if there
is
any
ambiguity,
bracketing
wusing
BEGIN
and END must be used to
clarify this.
Consider the following example:
IF

THEN

ELSE

14-2

CONDITIONAL

This

could
IF

interpreted

EXPRESSIONS

AND

STATEMENTS

THEN

BEGIN

THEN

ELSE

THEN

END

BEGIN

THEN

case

ELSE

END

The

first

L1:

interpreted

NOT

THEN

NOT

GOTO

THEN

GOTO

L2;

as:
L1;

GOTO

L2;

L2:
The

second

Ll:

case

interpreted

NOT

THEN

NOT

GOTO

THEN

GOTO

L2;

as:

L2;

GOTO

L1;

L2:
ALGOL-60

forbids

THEN

.....

14.4

such

.....

DESIGNATIONAL

ambiguities

forbidding

the

sequence

THEN

ELSE.

EXPRESSIONS

A designational expression is something that acts as an argument in
a
GOTO
statement, either directly, or indirectly via a formal procedure
parameter

element.

Thus

type

the

label.

This

following

may

are

simply

1label

designational

expressions:

X+Y

TW([J]

THEN

ELSE

SW[I]

ELSE

14-3

THEN

ELSE

switch

CONDITIONAL

These

designational
GOTO

GOTO

IF
IF

GOTO

THEN

EXPRESSIONS AND

expressions

would

L1 ELSE L2;
THEN SW[I] ELSE

14-4

STATEMENTS

used

X+Y

the

THEN

following manner:

TW([J]

ELSE

CHAPTER

OWN

15.1
OWN

variables

OWN

15.2

are

real,

variables

Although

the

following

all

occurrences

control

and

are

The

initial

the

the
of a

and

either

may

scalar

type

array.

normal

scope
of
or

of a
when

variables
being

are

used

block, the values
are
retained
the block is re-entered.

the
OWN

are

declared

OWN

INTEGER

OWN

REAL

it.

For

the

preceding

set

rules,

each variable
block.

program.
(FALSE
1n the case of Boolean OWN variables.)
STRINGS are initialized to possess no byte string.

OWN

variable,

string,

same copy
procedure

out
available

value

properties:

passes

still

ALGOL

Boolean

following

recursive;

When

kind

real,

not

2zero

by writing
example:

the

before

usual

execution

declaration

with

dynamic

fashion

the

I,J,K;

ARRAY

THETA[1:M];

OWN ARRAYS

OWN

arrays
are
DECsystem-10/20

following

The

1in

completely

declaration

proceeds

1If this is the first time the array
1is
obtained
and then the array laid out.

The

out

before,

proceed

Step

new

array

copied

constructed

from

zeroed.

The

space

recovered

example,
REAL

1if

ARRAY

OWN

old

the
array

array

for
A

and

the

0ld

array;

then

future
is

15-1

declared,
If

the

common

the

elements

remaining

deleted

array

space
has

1in

the

is
been

these are identical to
of this array, and the
of the same dimension:

and

use.

declared

A[l1:M,M:N];:

according

bounds are examined to ensure that
the
ones of the previous construction
array 1is left unaltered if found to be
otherwise, proceed to Step 3.

are

OWN

1implemented

ALGOL.

rules.

laid

For

special

1long

have

variables

word

VARIABLES

GENERAL

integer,
The

follows:

elements

the

any,

are

allocated

OWN

where

M = 2 and N = 5
time,
the
elements
remaining elements of

VARIABLES

the

first time, and M = 1 and N = 4
the
[1,2],
[1,3] and [1,4] are copied over,
the new array are zeroed.

15-2

second
and the

CHAPTER
DATA

16.1

TRANSMISSION

GENERAL

Data

transmission

user's

tape,

program

card

ALGOL

encompasses

and

the

peripheral

reader,

card

object-time

punch,

system,

input

devices,

and

1in

and

output

such

line

printer.

conjunction

disk,

data

The

with

between

DECtape,

the

magnetic

DECsystem-10/20

the

ALGOL

library,

provides the user with a set of basic
procedures
for
handling
data
from
most
DECsystem-10 or DECsystem-20 devices in a uniform fashion.
The
user
may
also
perform
input/output
operations
with
virtual

peripherals

that

appear

All

peripheral

devices

allocated

byte

are

under

released

strings
the

any

program.

the

user's

time

user's

control

throughout

program.

completely

the

and

can

the

execution

The user can handle up
to
sixteen
devices
simultaneously
(seventeen,
1if
one of them is the terminal attached to the job), any

number

independent

16.2

which

may

files

open.

ALLOCATION OF

Peripheral devices
library procedures
usually

has

integer

one

device

either

two

the
at

parameters.

time

may

output

DECtape)

constant.

The

text

the user's
A call to

first

on which

operated

facilities,

The

DECsystem-10

appropriate,

device

name.

In
peripheral

recognized

contained

allocated
or

this

1is

the

channel

the

device

channel.

except

device.
standard.

the

string

Users

explanation

simplest

have

program by calls to the
one of these procedures

to
A

the

case,

Device

the

Handbook
of

what

name may
names
shown

16-1

number,

operate.

channel

case

Only

provides

terminal,

simultaneously

string
logical

channel.

DECsystem-20

for

the

and

DEVICES

The

15,

(disk,

the input and output functions are performed
same channel.
The second parameter is either a

device

the

devices

are allocated to
INPUT or OUTPUT.

where

the

file

PERIPHERAL

range
a

input

should

name

constitutes

1in

the

on
string

the

consulted,
a

logical

actual name of
Table
16-1
are

DATA TRANSMISSION

Table 16-1
Standard Device Names

Device

Name

Peripheral

DSK
DTA
MTA
CDR
CDP
LPT
PTR
PTP
PLT
TTY

Disk
DECtape

Magnetic tape
reader
punch
Line printer
Paper-tape reader
Paper-tape punch
Plotter
Terminal
Card
Card

For example, to allocate the card
channel 5, the user would use the

reader

for

use

input

device

string

that

had

the

statement

INPUT (5,"CDR") ;

or, if S
were
a
characters CDR in

string
it,

possessing

byte

INPUT(5,S5)
;

Similarly,

the

disk were

used

output

device

channel

9:
OUTPUT
(9, "DSK") ;

NOTE

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, two
separate

channels

must

used.

Terminals
are
always
allocated
bi-directionally,
whether the user uses INPUT or OUTPUT.
For example,

irrespective

INPUT(O,"TTY")
;

allocates

16.2.1

the

user's

Device

Modes

terminal

for

input

and

output on channel

Normally, a device is allocated in ASCII mode, that is, when the
user
reads a character from the device, the readable text, such as a stored
source program or data is represented by a 7-bit
byte.
To
allocate
the
device in a different mode, a third parameter is specified in the
call to the INPUT or OUTPUT procedure.
Thus, to allocate
a
disk
to
channel
9
in
binary
image
mode
(the mode used for the storage of
binary data on a disk), the user can use
OUTPUT(9,"DSK",11)
;

16-2

DATA

The

DECsystem-10

TRANSMISSION

DECsystem-20

Assembly

Language

Handbook

should

consulted,
as
appropriate,
for
a full explanation of the different
modes used with peripheral devices.
The INPUT and
OUTPUT
procedures
allow
the
wuser
buffered mode.

16.2.2

allocate

any

standard

peripheral

device

any

Buffering

The

INPUT
allocated
two

for

and

OUTPUT procedures normally allocate two buffers for each
device
(terminals
are allocated two buffers for input and

output).

The

user

may

desire

use

either

one

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

parameter
14 in mode

buffers

to the
0 with

used

can

controlled

procedure call.
Thus, to allocate
eight buffers, the call is

adding

fourth

disk

channel

OouTPUT(14,"DSK",0,8);

NOTE

The

mode

must

allocating
would
be

always

buffer space,
an
ambiguity

specified

when

otherwise
in
the

there
third

parameter.

16.2.3

Error

Normally,

in
use
by
terminated.
parameter

the

event

Returns
the

device

allocation

fails

(for

example

the

another job), a suitable message is typed
The user can prevent this
by
providing,

and
as

INPUT

error.

OUTPUT,

For

label

which

control

device

the program
the
fifth
to

passed

example:

ouTPUT (14,"MTA",0,0,ERROR.LABEL)
:
If

the

range,

actual
the

label

parameter

procedures

behave

switch

whose

though

the

subscript
1label

absent.

NOTE

The third and fourth parameters must
be
specified
in
this
case
to
avoid
ambiguity.
However,
if
Zeros
are
specified,
then
default values will be
taken.
The default being ASCII mode and
2

buffers

parameter

for

the

third

respectively.

16-3

and

fourth

out

parameter

of
were

DATA

16.3

SELECTING

TRANSMISSION

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
1input
or
output
channel.
All
data
1input
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
1loss
of
data,
1irrespective
of
whether
complete
1lines
(or records) of data have been read or not.
1In fact,
the DECsystem-10/20 input/output system does not assume any
structure
in
the data:
all input and output channels are regarded as pipelines
through
To

which

select

be made.

the
input

This

has

user

pulls

channel,

one

or
a

pushes

call

parameter,

data.
the

which

procedure

the

SELECTINPUT

channel

number.

must

Thus

SELECTINPUT
(5) ;

causes

input

Similarly,

channel

the

procedure

selected.

SELECTOUTPUT

wused

select

output

channel.

16.4

FILE

DEVICES

Some
of a

peripheral devices, such as disk and DECtape, require the opening
specifically named file before any input or output operations can
be performed.
This optionally may be
performed
on
spooled
devices
(refer
to
the
appropriate
Operating
System
Commands manual for a
description of spooling).
The opening of this file
1is
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
variable possessing a byte string or a string constant,
the
which is the name of the file.

a string
text
of

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
OPENFILE

(9,"TEST.DAT",%177,%000011000050)
;

When a user has finished with a file it should be closed.
closed
by using the procedure CLOSEFILE, with a parameter
channel number on which the file 1is open.
Thus,

A
file
that 1is

1is
the

CLOSEFILE
(9) ;

closes

the

The user
already
new name

file

that

open

may also rename or delete
existing
files:
open,
use of OPENFILE causes the file to be
supplied.
Thus the sequence

OPENFILE

(5,"TEST1.DAT");

OPENFILE

(5,"TESTZ2.DAT");

causes
string

channel

if
a
renamed

the file with name TEST1.DAT to be renamed TESTZ2.DAT.
containing the new name is null, the original file is

l16-4

file
with

1is
the

If
the
deleted.

DATA

TRANSMISSION

Thus,
OPENFILE

(5,"TEST3.DAT");

OPENFILE

(5,"");

causes

the

16.4.1

file

Error

TEST3.DAT

deleted.

Returns

Normally,

if the operation requested fails (for example an input
file
not
exist),
a
suitable
message
is
typed
and
the
program
terminated.
The user can prevent this by providing
a
1label
and
an

does

optional

1integer

OPENFILE.

1In

the

variable

event

the

error,

fifth

and

sixth

control

will

parameters
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

Appendix

the

E
of
the
DECsystem-10 Monitor Calls Manual or
DECsystem-20 Monitor Calls Manual, as appropriate).

NOTE

The third and fourth parameters must
be
present if the error return parameter is
specified.
Defaults will
be
taken
if
both parameters are specified as zero.

the

actual

range,

the

absent.

16.5
The

The

label

parameter

procedure

integer

behaves

error-code

switch

whose

though

the

subscript
1label

parameter

called

release

device

1is

out

parameter

of
were

name.

RELEASING DEVICES
procedure

RELEASE

used

Thus,

from

channel.

RELEAS
(5)E
;

releases the device allocated to channel 5.
If
device,
and
a
file
1is
still
open
on
the
automatically closed.
Releasing
a
device
on
channel
to
become
free;
if this channel is
input or output operations, it is deselected.

device is
a
file
device,
this will be
a
channel
causes
a
currently selected for

attempt

has

device

channel

file

is made to allocate a device to a
allocated, the allocated device is

open on

the

device

closed

before

If
a
user
terminates
his
program
without
channels, these are automatically released.

16-5

the

first

the

that

released

already
and,

release.

releasing

devices

DATA TRANSMISSION
16.6

BASIC

16.6.1

INPUT/OUTPUT PROCEDURES

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 accepting
ASCII
bytes,
they
are

"symbol"

oriented.

INSYMBOL(S);
causes
the
input

channel

- (where S is usually
some
integer
variable)
next byte to be read from the currently selected
and

stored

in S.

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 1is truncated to size.

NEXTSYMBOL(S);

acts

exactly

one

byte.

the

except
that
the
byte
pointer for
advanced to the next available byte.
look-ahead

facility

same

way

INSYMBOL

the input channel is not
This gives the
wuser
a

SKIPSYMBOL;
- causes the next byte
channel to be read and ignored.

BREAKOUTPUT;
- causes all bytes in the buffer
of
an
output
device
to
be
sent
immediately
to
it.
This procedure 1is
normally used to conduct a question-and-answer dialogue on
a
terminal,
with
the
question
and
answer on the same line.
Normally, a block of data is sent to a device only
when
the
buffer
1is
full
(the
exception being the terminal, where a
break is sent ‘at the end of each line).

16.6.2

A byte

String

from the

selected

input

Output

string may have

1its

selected

output channel

possesses

the

parameter is either a
string

contents

by means of

string
be

transferred

constant

output.

the

the procedure WRITE,

For

example:

string

currently

whose

variable

single

that

WRITE
(S) ;
or

WRITE ("THE

MOON

MADE

GREEN

CHEESE")
;

With exceptions explained in the
following
paragraphs,
bytes
1in
the
string
are
output
1literally, with the
course, of the quotes in a string constant,
which
are
stored in the bytes string at all.

NOTE
Unlike some other ALGOL implementations,
spaces and other non-printing symbols in
byte
strings
are
meaningful
in
DECsystem-10/20 ALGOL.

16-6

all
of
the
exception, of
not
1in
fact

DATA TRANSMISSION

Special editing characters
the text of a byte string.
P

Any

Page

New

and

line

Tab

Space

Break

combination

are

stands

for

carriage

return,

line

feed)

output

counts,

output

within

throw

repetition

are permitted within square brackets
These have a special function:

these

can

characters,

appear

within

their.special

with

square

optional

brackets

interpretation

demands.

preceding

byte

For

string

example:

WRITE ("ABCD[P2C5S]EFGH")
;

causes

the

following

the

symbols

two

new

the

symbols

output

[

the

these must

([

followed

and

five

a page

throw

spaces

EFGH.

or;
appear

"" or

;;

the

form

Thus

:= 3;:""");

3;"

output.

16.6.3
The

output:

text

"A[I}
to

ABCD

lines

WRITE ("""A[[I]]
the

symbols

respectively.

causes

Miscellaneous

procedures

SPACE,

Symbol
TAB,

Procedures

PAGE,

and

NEWLINE

cause

number
of spaces, tabs, page throws, or new lines
number is specified by a single integer parameter.
is omitted a value of one is assumed.
Thus
SPACE (5) ;

causes

five

spaces

output,

output.

whereas

SPACE:;
or

SPACE (1) ;
cause

one

space

16-7

the

appropriate

to be output.
This
If
the
parameter

DATA TRANSMISSION

16.6.4

Numeric and String Procedures

Numeric procedures are used to read and print numeric quantities.

The

ASCII mode, and are capable of processing integer, real, or long
gquantities in fixed-point and floating-point representation.

real

normally

will

procedures

be used with a device that 1is operating 1in

be
can
input
for
data
Input Data - Numeric
16.6.4.1 Numeric
that would be acceptable as a numeric
in any format
represented
1involved.
constant in a program, irrespective of the type of variable
an automatic type conversion is performed,
read,
1is
number
a
When
data
the
of
assignment
an
if
giving a result of the same type as
represented as a constant in the program had been executed.

There is a minor
restriction in
that
no
spaces,
tabs,
or
other
appear in such numeric data except between
may
symbols
non-printing

the exponent sign (& or @ for real, && or @@ for long

the

and

real)

exponent.
Otherwise,
any
symbol
that
1is
not a part of a numeric
guantity may act as a terminator for such a quantity.
It is
strongly
spaces,

that

recommended

For

tabs,

or new lines be used as separators.

example:

3.4
0

-9.6

1.36

-52

14.9

NOTE

In
reading
a
numeric
quantity,
the
terminating
symbol,
that is, the first
symbol that is not part of
the
number,
is

lost.

DECsystem-10/20 ALGOL also allows the
user
to
input
floating-point
data written in FORTRAN format, that is, using E for & or @, and D for
FORTRAN
in
inherent
effects
special
other
However, no
&& or @@.
formatting

are

introduced.

The procedure READ is used to input numeric
This

procedure

may

have

any

installation-dependent maximum),

Boolean,
The

effect

string.

number

of type

data

and

also

parameters

integer,

real,

(up

strings.

1long

real,

follows:

For integer, real and long real variables, a number
1is
read
the type appropriate to the parameter and
to
converted
and
then assigned to the variable.

For Boolean,

For a string variable, the data text is scanned until a quote
following this up to but not
text
the
and
found,
1is
(")
including the next free quote is read in and
a
byte
string
generated, which is then possessed by the string variable.

and

assigned

If the sequence
string

a number
to

is read as if for an integer

the variable.

is found,

a single

continues.

16-8

is stored,

wvariable,

and reading of

the

DATA

16.6.4.2

Numeric

procedure
parameters,

variable

the

may

parameters

Data

This

first

Numeric

procedure
which

integer,

determine

expressions.
effect of the
as follows:
M>0,

Output

PRINT.

TRANSMISSION

the

real,

format

data

may

variable

long

Fixed-point

output

one,
to

second

and

This

and

are

the

three

printed.

The

used

means

two,

real.

If omitted, both parameters
various combinations of the

N>O0:

have

third

integer

are assumed to be zero.
The
format integers, M and N, is

printing,

places

before

the

decimal

point, N places after.
A sign, space if positive,
- 1f negative appears before
the
number.
Zeros
before
the
decimal
point are replaced by spaces
and the sign moved up to the number.
This
M>0,

N=O0:

The

format
same

always

outputs

the

preceding

fractional part
i1s suppressed.
This
M=0,

N>O:

format

always

Floating-point

decimal
digits,
&&

for

appears,

M+N+2

format,

except

that

(2)

decimal

and

outputs

symbols.

M+l

the

(1)

point

symbols.

consisting

sign,

digit,
a
decimal
point, N more decimal
and an exponent consisting of & for
real,
long

real

two-digit

followed

exponent,

the

2zero

exponent

suppressed

sign

and

from

the

and

N+8

left.

This

format

symbols

only

two

parameters

and

respectively,

only

one

format

parameter

assumes

standard

for

quantities,

real
the

real

user
or

properly

real

rounded

O0,N

the

and

real

value

appears,

the

for

long

the

format
of

symbols

for

assumed

real

second
is

11,0

integer

the

number

0,0

which

quantities,

0,9

quantities.

digits to be printed than are
numbers, the appropriate number of

integer

where

parameter.

interpreted

for

real

for

quantities,

printing

real

quantities.

M,0

and

modes

0,17

N+7

real

format

for

printing

requests

1long

long

appear,

variables,
take,

outputs

for

the

significant in
zeros follow a

maximum

precision

available.

16.6.4.3

Octal

PRINTOCTAL,

quantities
On

input,

read,
symbol.

octal

for

single

preceded
For

presented

output,

Input/Output

respectively,

precision
the

real

symbol

input,

each

octal

digits,

The

foregoing

type

integer,

procedures
the

variables,
%,

the

variables,

for

single
precision
each with 12 octal

The

input

and

octal

digits

preceded

variables.
digits are

such

the

being

octal

symbol

the

have

one

scalar

real,

real

Boolean.

long

any

16-9

output

are

non-numeric

numbers

must

symbol

are

printed

For long real variables, two
printed separated by a space.

procedures

and

terminator

two

preceded

READOCTAL

wuser

format.

long

allow

parameter

which

for

quantities

may

DATA

DEFAULT

16.7

TRANSMISSION

INPUT/OUTPUT

If the user does not select any input or output
channels,
1input
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

devices,

provided.
a means

additional

These are

<channels,

communicate

numbered

with

from

peripheral

16 to 31,

are

input or output channels that use byte strings as

storage.

By means of the procedures INPUT or OUTPUT,
the
wuser
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
and
from
a
peripheral device, or to and from another byte string.
INPUT (20,S) ;
or

OUTPUT (20,S) ;

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.

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

16.9

again.

SPECIAL OPERATIONS

These procedures are
perform
operations

takes one

operation

parameter,

used on channels assigned to magnetic tapes,
and
of BACKSPACE, ENDFILE and REWIND.
Each procedure

that

1is,

the

channel

Since there is
no
implicit
structure
on
a
procedures enable the user to build up formats

16.10

I/0 CHANNEL

The status of any
time
by means of
channel number as

number

which

the

performed.

magnetic
tape,
these
in any way he chooses.

STATUS

input or output channel can
be
determined
at
any
the Boolean procedure IOCHAN, which takes an integer
parameter.
The status
returned
1is
bit
coded
as

follows:

16-10

DATA

Some

TRANSMISSION

Bit

Value

$400000

Device

$200000

directory

devices

(disk

and

DECtape)

FILE-NAME.FILE-EXTENSION
or

FILE-NAME

where
In

DSK

the

case

assumed
of

non-directory

default
devices,

DEVICE:

18-1

device.
the

format

simply

terminate

RUNNING AND DEBUGGING PROGRAMS
In cases where no FILE-EXTENSIONS are specified,
REL
for the relocatable binary output file, LST
and ALG for the source file are assumed.

the
for

standard defaults
the listing file,

SOURCE-FILES

consist of one file or a list of files
separated
by
commas.
If
a
DEVICE
1is 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

DSK:EULER.REL

from
and

DSK:EULER.ALG,

listing

the

write

user's

relocatable

binary

terminal].

MTAQ: ,DSK:SIM26=SIM26 ,PARAM.TST

[read

source

from
DSK:SIM26.ALG,
MTAO, and listing

DSK.PARAM.TST,
write
on DSK:SIM26.LST].

binary on device

Certain

switches may be

set

relocatable

file

the

user

the

command

string.

These

compiler's

1/0

checking

(see

are:

BUFFERS:n

Set

number

buffer-ring

CHECKON

Compile

of
n.

buffers

run—-time

array-bound

18.5.1.1).

CHECKOFF

Do
not
compile
regardless
of

run-time
array-bound
checking,
any
CHECKON
statements
1in
the

source.

HEAP:n

Set

the
(which
arrays)

initial

size
wused
for
n
words.

1s
to

the
dynamic
core
area
I/0 buffers, strings and OWN
This
area
1is
dynamically

expanded at run-time if necessary;
1its
is typed out at the end of execution if
program

loaded with

helpful

final size
object

the

DDT.

HELP

*Type

KAlQ

Produce

code

run

the

KAl(O

processor.

KI10

Produce

code

run

the

KI1l0

processor.

KL10

Produce

code

run

the

KL1l0

processor.

LIST

*List
the
source
listing-device
is
command-string).

NOERRORS

not

type

error-messages

NOLIST

not

list

the

NONUMBERS

The

source

numbers
NOQUOTES

Delimiter

text.

program

(default
specified
1n

source

does

not

columns

are

18-2

a
the

terminal.

program.

program

words

the

not

have

1line

sequence

(default).
quotes

(default).

RUNNING AND

NOSYMBOL

Suppress

NUMBERS

output

.REL

file.

The

source

columns
PRODUCTION

not

TEMPCODE:n

Set

expanded

has

line

trace

words

symbol

length

words:

this

produces

happen

for

symbol

table

sequence

the

numbers

80.

<compile

Include

PROGRAMS

symbol-table

*Delimiter

SYMBOL

program

expanded
QUOTED

DEBUGGING

information

are

table

the

.REL

TEMPCODE

area

1is

necessary

only
to

some

output

quotes.

information

message

file.

that

very

.REL

file.

compiler

the

effect,

which

complicated

compiler
may

statement

constructs.
TRACE

Control

tracing.

NOTE

Since only one processor type, the
KA20
is
available
for
the DECsystem-20, no
switch options are
therefore
available
(unlike
the
DECsystem-10 which can run
on

one

KI10

the

and

three

KL10,

processors,

and

therefore

KAlo0,

has

options).

Switches
an

may

character.
Switches
For

asterisk

shortened

(*)

1in

the

Values

(n)

are

after

unique

above

three

abbreviation:

1list

may

also

those

marked

given

as a

with

single

decimal.

file-specification

are ‘set

preceding

example:

PROD, PROD=PROD1 /L, PROD2/NOL/HEAP:2000

causes

file

PROD1
1listing,

without
area

The

ALGOL

user's

set

type

18.1.1

be
and

2000

compiler

terminal

terminal.
another

words

return

ALGOL

independent

programs

the

source

file

either

process

compile

main

program

device

program,

the

singly
such

the

user

may

level.

allows

source

Free-Standing

DECsystem-10/20

size

1listing

whereupon

default

the

to monitor

Compilation

(the

all

compiling

asterisk,

with a listing,
1initial size of the

the

reports

and

After

compiled

PROD2

to
run-time
521

words).

errors

be
compiled
dynamic core

both

other

than

the
compiler
returns
with
compile another program, or

Procedures

the

user

that

compile

call

them.

Such

procedures

the

source

file

together.

files.

18-3

the

The

may

user

be
uses

procedures

may

either

independent

exactly

the

same

RUNNING AND

DEBUGGING

PROGRAMS

NOTE

Free-standing procedures must not appear
before
the
main
program.
If the user
requires to call those
procedures
from

the

main ALGOL program,

EXTERNAL
(refer to

18.2

LOADING ALGOL

ALGOL

programs

DECsystem-20

the

declarations
must
Paragraph 11.9).

appropriate
be

PROGRAMS

are

loaded

Linking

Loader

1in

means

exactly

the
the

generated by MACRO-10/20 and FORTRAN are loaded
the

made

DECsystem-10/20 Assembly Language

DECsystem-10/20
same

(for details,

programs

refer

Handbook).

LINK-10 or LINK-20 automatically causes all procedures
the ALGOL Library (ALGLIB) to be incorporated into the
For example,
ALGOL
main

way

required
from
user's program.

consider the source
file
MAIN.ALG
which
contains
the
program and the files SUBl.ALG and SUB2.ALG which contain

free-standing

procedures.

The user may compile these files to give one relocatable
binary
by typing the following command string to the ALGOL compiler,

file

MAIN,MAIN=MAIN,SUB1,SUB2

and

loading the

resulting program by giving

the command

string

MAIN/GO

to LINK-10 or LINK-20.
Alternatively, the three source files
can
be
compiled
1indevpendently
by
typing three command strings to the ALGOL
compiler,

for

example:

MAIN,MAIN=MAIN

SUB1,SUB1=SUB1
SUB2,SUB2=SUB2

and giving

LINK-10

LINK-20

the

command

string.

MAIN,SUB1,SUB2/GO

After

program has

18.3

RUNNING

ALGOL programs

ALGOL

are

been

loaded,

it may

executed.

PROGRAMS

executed

typing

the

console

command

START

or
any
of
1its
successfully,
a
the program will

wvalid
abbreviations.
If
the
message
will be printed on the
return to monitor command level.

18-4

program
executes
user's terminal, and

RUNNING

18.4

CONCISE

COMMAND

AND

execution
same

way

monitor

FORTRAN.

may

programs.

for

programs

For

details,

(CCL)
be

ALGOL

PROGRAMS

LANGUAGE

The Concise Command Language
DECsystem-20

DEBUGGING

features

used

These

features

written

refer

1in

the

facilitate
in

are

DECsystem-10

compilation

and

the

used

DECsystem-10

exactly

DECsystem-20

Users

Handbook.

Switches

separated

the

EXECUTE

18.5
If

run-time
an

error

occurs

error

has

AND

during

message

been

the user's
non-fatal.

provided

within

the

user's

program.

Each

table

these

appears

below.

errors

PROCEDURE

TRAP

INTEGER

Where

which

control

Once

such

the

trap

has

LABEL

that

is,

parentheses

ang

set

an
the

type

errors

fall

into

DECsystem-10

occur,

such

transfer

error

The

has

Library

following

VALUE

Such

users

unique

procedure

the

label

The

passed

call

sets

trapped,

when

trap

parameter

trap

available

value.
caused

number,
TRAP,

trap
label

and

used

a
to

specification:

the

TRAP,

and

error
it

label

The
the

only
a

DECsystem-10.

formal

number

jump

obtained

procedure

TRAPNO.

call.

the

calling

18-5

parameter

the
1label

the

trap
may

remains

different

trap

label

occurs.

NOTE

TRAP

two

N,L;

error

TRAP
by

object
error,

can

control

(N)

omitting

ALGOL

required

until
another call to
the trap is turned off
TRAP

detailing

program.

they

the

(N,L);

number

when

execution

which

and

non-fatal

the

produced,

errors,

enclosed

DEBUGGING

non-fatal

trap

are

example:

DIAGNOSTICS

and its address within
categories - fatal and
A mechanism

compiler
For

FOO (QUOTED/HEAP:2000/KI)

RUN-TIME

program,

ALGOL

slashes.

the

DECsystem-20

that
be

integer

label,

1in

force

until

RUNNING AND DEBUGGING PROGRAMS

Table 18-1
Error Trap Numbers

TRAP

NO.

ERROR

FLOATING POINT OVERFLOW

FIXED

INPUT OR OUTPUT DEVICE

ILLEGAL MODE FOR INPUT OR OUTPUT DEVICE

INPUT OR OUTPUT ON

ATTEMPT TO READ OR WRITE ON DIRECTORY DEVICE

POINT OVERFLOW

WITHOUT

FILE

UNAVAILABLE

UNDEFINED CHANNEL

OPEN

FILE

ATTEMPT TO

ERROR CONDITION ON

ILLEGAL

OVERFLOW

ERROR CONDITION

ILLEGAL INPUT/OUTPUT OPERATION

I/0

SOQRT

LN ARGUMENT

EXP

INVERSE

TAN

18.5.1

Facilities

18.5.1.1

NOT AVAILABLE
READ

OR WRITE

CHARACTER
IN

CLOSING

MATHS

ARGUMENT

FILE

RANGE

NEGATIVE

ZERO

ARGUMENT

DATA

NUMBER OUT OF

ARGUMENT

OVER END-OF-FILE

IN NUMERIC

FAILURE

INPUT OR OUTPUT

NUMERIC

CHANNEL

to Aid

OR RENAME

TOO

OR NEGATIVE
LARGE

FUNCTION ARGUMENT OUT OF
TOO

RANGE

LARGE

in Program Debugging

Array Bound Checking

- The

directive

CHECKON

when placed anywhere in a user's program causes all array subscripts
from
this point onward in the program to be checked at run-time for
being

range.

The

directive

CHECKOFF

nullifies

this

action.

18-6

RUNNING

The compiler
override any

switches
CHECKON

AND

DEBUGGING

PROGRAMS

/CHECKON

and /CHECKOFF may also be used:
they
CHECKOFF statements in the source program.

NOTE

The

CHECKON,

the

larger,

CHECKOFF

generated
and

run

Most

inexplicable

the

execution

caused

causes

slightly

slower.

errors

arising
during
an ALGOL program are

array

range.

the

program should
array
bound

the

facility

program

subscript

Whenever

such

being

errors

out

occur,

recompiled

with

check

feature

and

on,

re-run.

18.5.1.2

Controlling Listing
of
the
Source
Program - Normally,
a
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

listing

only

part

listing

the

listing

from within

his

and

then

continue

program by means

listing,

the

can

control

the

statements

LISTOFF

LISTON

The

LISTOFF

the

program

statement
LISTOFF

the

until
causes

/NOLIST

program

and

can

statement.

The

Line

statement
continue
and

included

The

the

/CREF
for

suppressed

1is

after

LISTOFF

the

from

either

the

encountered.
it

had

been

statements

compiler

point

end

The
no

with

change

the

/NUMBERS

1line

switch.

numbers

string.

line

the

by
source

which

way

means

statement

1in
and

numbers

Another

effect

in Listings - Ordinarily, the lines
numbered
segquentially
starting
at
1

the

LISTON

suppressed
have

command

The user can, however, change the
numbers
in
columns
73
through

line

The

1line

another

CROSS

output

Numbers

are

LINE

number

numbers

REFERENCE

switch
CREF.

has

set

that

statement

follow are

1is

terminates.

18.6

encountered

the

LINE

integer.
either

was

LISTON

compiling

wuser

LINE

The

file

by 1.
sequence

listing

LISTON

switch

incrementing

causes

LISTOFF

listing

Setting

listing

placing
the

statement.

18.5.1.3
the

statement

program where

which

incremented by

encountered

decimal

until

the

program

ALGOL

generate

LISTING
been

Output

implemented,

takes

the

causing

following

18-7

formats.

RUNNING AND DEBUGGING PROGRAMS

Variables

and

Labels:

Each occurrence of a variable or
1label
name
is
recorded,
with
a
# whenever a defining
reference is made.
In the case of labels the
line
reference
1is
to the line which causes
code for the label to be generated.
This may
follow
the line where the label 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 1is 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
1in
the
source.

18.7

STACK ANALYSIS

The stack
the stack
a typlical

analysis takes two forms - if a
is scanned to give the names of
error message now reads:

?RUN-TIME

procedure

ADDRESS

000162

OPENFILE

from

procedure

PQRSTUVWXYZ

Called

from

procedure

THISPROCEDUREISNEXTTOINNERMOST

Called

from

procedure

THISONEISNEARLYTHEOUTERMOST

Called

from

procedure

THISISTHEOUTERMOSTPROCEDURE

Called

from MAIN

PROGRAM

DSK:NOSUCH.FLE

above type of
second
type

The

Called

File

The

ERROR

program stops with an error,
the active procedures.
Thus

not

available

analysis 1is
1is invoked

rename

failure

channel

automatic whenever an error 1is detected.
the Debugging System command PROFILE.

This

causes
a
1list
of
all
the
procedures,
1labels
and
1library
procedures
referenced
1in
the program to be printed, together with a
count of the actual number of times each
was
referenced
(or
passed
through

1in

the

case

be distinguished by
trailing
asterisk
must

identified

same

the

for

from

special

scanned,

case

labels).

a trailing
(*),
but
the

with

the

trace

order

which

they

appear,

overlaid

programs

only

the

>>PROF

COUNT

labels

can

which

1is

the

order.

example:

PROFILE

print,

colon (:) and library procedures
by
a
different procedures with the same name

PRINT.

NAME

POQRSTUVWXY?Z

THISPROCEDUREISNEXTTOINNERMOST

18-8

root

segment

RUNNING

18.8

The

DEBUGGING

PROGRAMS

THISONEISNEARLYTHEOUTERMOST

THISISTHEOUTERMOSTPROCEDURE

READ*

INPUT*

OUTPUT*

OPENFILE*

TRACE

18.8.1
Two

AND

Dynamic

types

Trace

are

available:

dynamic

user who is at a terminal and wishes
his program should type the following
.LOAD

and

post-mortem.

to see a full
dynamic
command sequence:

trace

TRTST

ALGOL: TRTST
LINK:

EXIT

.REE

ALGOL

DIAGNOSTIC

FACILITIES

SYSTEM

FOR

HELP)?

>>ONTRACE

>>START

This

produces
follows:

(at

1least)

one

1line

for

* %% % x*SECTION
*kk*xGQ
s

k*kkk**x*xPHASE
*kkkkkkkkkkx*kWRITE*

PHASE

kkkkkkkx*kkx**PRINT*

kkkkkkkkkhkk*xWRITE*

BEGIN

kkx*kkk***PHASE

khkkkkkkkkkkkWRITE*

END

** % ***SECTION
kkk*xG7
.

*kkk*kk**PHASE

*kkkkkkkkkkkkXWRITE*

PHASE

kkkkkkkkkx**DRINT*

khkkkkkkkk kX *WRITE*

BEGIN

kkk*x**k**PARFOR
kkkkkxk*PHASFE

*kkkkkkkkkkk*xWRITE*

END

18-9

each

trace

reference,

RUNNING AND DEBUGGING PROGRAMS

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
1line.
Note
that
a
notional
block
surrounds
the
main
program
and each procedure.
A
maximum of sixty asterisks are printed
before
the
1line
1s
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
1line
of
asterisks.
As with the profile, library procedures are distinguished
by a trailing asterisk, and labels by a trailing colon.
Procedures are traced at centry but not
exit;
exit can be inferred from the block levels.

however,

procedure

NOTE

Ordinary TTY
output
1in
Dynamic
Trace
mode
may
be interspersed amongst Trace
output
(as
1in
the
example
above)
.
Dynamic

off

trace

procedures

18.8.2

may

also

turned

use

the

ONTRACE

and

dynamically

Post-Mortem

and

1library

OFFTRACE.

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 Debugging System offers

the
wuser various options.
The TRACE command produces a trace similar
to the one described
under
Dynamic
Trace
above,
but
with
spaces
instead of asterisks to represent block levels, and without, any other
TTY output.
The
post-mortem
trace
entries
are
maintained
1n
a
circular
buffer in the Heap.
The default length of the buffer is 100
(decimal) but this can be altered by using the /TRACE
switch
to
the

compiler,
with
an appropriate value.
Tracing
users can compile their programs without trace

is the default option:
information
by
using

the
/PRODUCTION
switch
to
the
compiler.
However, the Trace entry
mechanism is quite efficient, and users are urged not to
exclude
1it,
as
this also has the effect of making the stack analysis less useful.
Library

18.9

procedures

PERFORMANCE

are

always

traced.

ANALYSIS

Various features are provided which are designed
to evaluate the performance of their programs.

18.9.1

Heap

to help

users

wishing

Space

DECsystem-10/20 ALGOL is very flexible in its use
of
memory:
space
for
arrays,
strings,
I/0
buffers and so on is allocated in an area
called the Heap, which lies between the program code
and
the
stack.
The
default
1initial
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
applied by the system).

18-10

RUNNING

For

most

and

programs

this

computation.

AND

provides

However,

DEBUGGING

the

PROGRAMS

reasonable

stack

balance

shifting

between

mechanism

core

can

use

very

expensive if used often and for small expansions.
The user
<can
find
how
the Heap 1s being used by typing the STATISTICS command (see
Chapter 20, section 20.8.6) to the Debugging System after the
program
has
executed
(or simply typing CONTINUE immediately after the End of
out

execution

message).

EXECUTION

(The

This

TIME:

MAXIMUM

HEAP

SIZE:

MAXIMUM

USED

item
and

is
is

a measure of the
only produced if

the

last
heap

present

FTGETCHK

judicious

program's
INFO

can

the

following

format:

STACK

THE

SHIFTS:

HEAP

TABLE:

fragmentation of the free space
in
the optional Heap Integrity Checker

system,

i.e.,

when

assembly

switch

shifts

is larger than, say, ten, an improvement
expected from re-compiling the program with
to the
maximum
heap
size
given
by
the

be
set

Utilization

placement

profile,

particular

on.)

the number of stack
in
performance
could
the /HEAP switch value
Statistics type-out.

Code

WORDS

object-time

turned

18.9.2

output

TIME:

ELAPSED

the

produces

sections
used

the

1labels

code

obtain

and

frequency

can
more

with

subsequent
which

monitored.

detailed

18-11

timing

The

printing

program

1library

information.

the

executes

procedure

CHAPTER
TECHNICAL

NOTES

These notes
concern
the
authors'
particular
interpretation
"Revised
Report
on
the
Algorithmic
Language
ALGOL-60"

implementation.
1.

all

times,

employed.
construed

strict

some

expressions

many

ALGOL-60

Section

4.3.5

Statement

left-to-right

Section

3.4.6

experts

not

of
to

mean

required.

programs

Revised

with
subscript

the
a

evaluation

the

Revised

that

Report

designational

there
rely

requires

the
its

statements

Report

has

left-to-right

However,

existence

of
and

are

is
been

evaluation

undoubtedly

this

that

feature.

GOTO

expression which is a switch
regarded as a dummy statement.

with a
out of range be
Neither
DECsystem-10/20
ALGOL
nor
any
other
ALGOL-60
implementations, to the knowledge of the authors,
follow
this
rule; there
1is a side-effect involved in the evaluation of the
subscript.

19-1

CHAPTER
THE

20.1
In

SUMMARY OF

ALGDDT,

the

ALGOL

DYNAMIC

FEATURES

ALGOL

programmer

program

facilities

Set

Examine

and

Examine

various

Automatically

Continue

program

execution

from where

Continue

program

execution

from

pauses

GENERAL

and

clear

alter

for

(.SYM)

has

20.2.2

ALGOL

system

type

release

ALGDDT

this

complete

find

"Stopped
a

scope

parameters

ALGOL

variables

after

pause

halted

appropriate

at runtime to
a
file
LINK.
If
1t cannot

name,

and

label

resides

wunder

supply its name,
is none, it will

terminal

the

running

the

current

Program

ALGOL

execution

.SYM

file,

line

nnn

run-time

called
Programname.SYM
access the file (perhaps

another

etc.
ALGDDT
ask the user

PPN)

also needs a
to
nominate

use.

detected,

executing

ALGDDT
from

1is

ALGDDT.

unable

variables

File

another

Entering

enter

When

them

REMARKS

Symbol

because

execution

opportunity
1is
given
to
free 1/0 channel:
1if there

After

has

ALGDDT requires access
that
1s
produced
by

SYSTEM

Interrupt

20.2.1

DEBUGGING

20.2

one

error,

the

statement

before

passing

[,statement

where

the

type

will

program,

Debugger

halted

following

ALGDDT

ALGOL

Dynamic

and,

message

unless

will

°C.

attempt

to
control

ALGDDT

was

typed:

n]"
entered

(except

1in

batch

job).
However,

there

are

cases

current

20-1

statement

never

completes.

THE

For

example,

reads

data

DYNAMIC

typed

DEBUGGING

during

the

execution

terminal,

will
be waiting at
To overcome this, a

the other
second "C

end for the system
is required.

may

REEnter

also

command.

The

entered

before

The

program

facility.

determining
BEGIN of the

20.2.3

this

syntax

case,

the

will

Command

the

can

then

effective
1is

statement which

forthcoming

enter

program

started

"current

the

user

ALGDDT

mode.

use

with

the

statement-,

1immediately

before

the

START

for

the

first

Format

ALGDDT

all

execution

scope
of
1identifiers,
main program.

ALGDDT

commands;

data

SYSTEM

the

ALGDDT

from

ALGOL

commands
which

has

are

been

designed

terminated

resemble

ALGOL

semicolon

(or
carriage-return), and the lists
associated
with
AUTO
commands
are
bracketed
by
a
BEGIN-END
pair.
However
the
resemblance
1is
superficial, and for the most part only simple commands can
be
given
in
a
single
"statement".
When
the
debugger is ready to accept a
command, ">>" is prompted.
All commands and options may be
shortened
to
a
unique
abbreviation (except where noted).
Blanks and tabs are

ignored between elements of a command, as
are
‘“readability
symbols"
(periods) in ALGOL identifiers.
A command may be continued on another
line by typing a control-backarrow or control-underline.
Comments may
be
introduced
by
a
preceding
!
and the rest of the line will be
ignored.

20.2.4

Line

Several

ALGDDT

Numbers
commands

require

line

numbers.

The

compiler

only

puts

line
numbers into the symbol table accessed by ALGDDT for those lines
where code is generated (including
BEGIN
and
END
statements
of
a
block,
but
not
for
blank
1lines, declarations, etc.).
If the user
gives an "unknown" line number, ALGDDT will scan forward to
the
next
known line number and use that;
this usually has the desired effect.
In

can

program

qualify
>>

The

six
main

PAUSE

LINKed

from

line

number

module-name

characters),
program.
If

than

with

one

.REL

use the
message

can

have

("module"),

preceded

the

procedure

the

user

"IN":

FOO

the

or
the

name

(truncated

the
main program-name (.REL-file name) for
module-name is not specified, the default is

external

the
the

current module (that in which execution was
error, or the main program before execution
program

file

a module-name

than

one that is also
will be typed.

the

one

module

main

with

program,

Additionally, in the OBJECT command,
by a preceding number-sign (#).

20-2

stopped by PAUSE, "C or
is started).
Although,
the

octal

same

any;

name,

ALGDDT

otherwise

address

may

an
a

will

error

specified

THE ALGOL

20.3

TYPEOUT

DYNAMIC

DEBUGGING

SYSTEM

COMMANDS

The command TYPE is used to display the value
array
element
on the terminal.
The keyword

of
is

an ALGOL
followed

one or more variable names, separated by commas.
The
terminated
by
a semicolon.
If more than one name is
the names are repeated by the debugger.

variable
by a list

whole
given,

or
of

1list
then

1is
all

size

Examples:

TYPE

REAL.VARIABLE;

1.2345

TYPE

REAL.VARIABLE,

INTEGER.VARIABLE;

REAL.VARIABLE

1.2345

INTEGER.VARIABLE

12345

20.3.1
When

the

String

Typeout

string typeout is requested, the length
string are output first, in parentheses,
1.

the

<case

representation

ASCII

the

and

SIXBIT

string

decimal

byte

then
strings,

typed

the

character

in quotes,

example:

For

TYPE

STRING.VAR;

(10,7)

string<CR><LF>"

TYPE

SIXBIT.VAR;

(15,6)

SIXBIT

strings

octal
comma,

with

byte

representation
is typed.

STRING"

size

other

than

six

the

byte

values,

each

separated

seven,

example:
>>

TYPE

NONASCIISTRING;

(8,5)

10,37,17,17,22,1,10,0
Extra

bits

20.3.2
If

the

TYPE

command

TYPE

an
the

words

are

ignored.

1.7
2.3

[2,0]

5.4

The

contents of
location of the
TYPE

used

array
1is
structure

with

array

typed.
The
of the array:

name,

then

format

typed

the

entire

typeout

will

specifying

the

ARRAYNAME;

[0,0]
[1,0]

ends

Array Typeout

contents
of
correspond to
>>

the

2.3
1.7
...

a single
element:

5.4
5.4

1.32
8.1

ETC.

location

ELEMENT[1,3,5,200];

may

0.0

20-3

THE

ALGOL

DYNAMIC

DEBUGGING

SYSTEM

NOTE

ALGOL

wvariables
may
not
be
used
as
subscripts.
All
subscripts
must
be
simple constants.
Array bound
checking
is
1imposed
on
ALGDDT
commands
where
appropriate.

Rectangular
the

portions

relevant

arrays

may

selectively

typed

specifying

ranges:

TYPE

ARRAYNAME
5.4

8.1

6.8

[1,2]

[0:1,2:3];

The whole of a particular
e.g., A[l:20,*,1:5].

dimension

can

represented

asterisk,

NOTE

There

must be a specification for
every
dimension of a multi-dimensional array.

20.3.3

The

Displaying

current

DIMENSION

dimensions

Dimensions

array

can

displayed

wusing

the

command:

>>DIM A;

20.3.4

Current Array

[0:17,1:5]

Typeout of Boolean Variables

Boolean variables (or arrays) can be typeada using the
above
commands.
Since FALSE is represented by zero, zero elements or variables will be
displayed as 'FALSE';
non-zero variables having
values
of
-1
(the
value
used
1in
directly
generated assignments), are either typed as
'"TRUE'
For

octal

for

other

values.

example:

TYPE

Bl1l,B2;

Bl=

False

B2=

012345671234 (True)

20.3.5

General

Points

On Typeout

All

ALGOL variables in scope will be available for typeout,
1including
formals,
but the possibility of side-effects should be borne in mind.
The only exception 1s a variable that is declared but never referenced
in the program.
variables.

For

internal

reasons,

the

debugger

cannot

access

such

Long, unwanted type-outs (of arrays, etc.) may be
aborted
by
typing
two
“Cs.
Occasionally
this
may
return control to the monitor, 1in
which case CONTINUE or REENTER may be typed to return to ALGDDT.

20-4

THE

20.3.6

Typeout

Object

The user may request
statement, using the
>>

OBJECT

PAUSE

After

this

executed

next.

current

statement

error

ALGOL

types

out

typed

marked

DEBUGGING

SYSTEM

Code

a typeout
command

After

occurred

DYNAMIC

the

code

run-time

out

with

Macro

for

the

error,

instead
an

code

and

the

statement

the

asterisk.

for

code

current ALGOL

that

will

generated

for

the

instruction

The

user

can

OBJECT

octal

1line

number

address:

case

the
>>

the

#275;

"IN

following
OBJECT

decimal

rather

(16)

module"

would

number

than

number

22;

OBJECT

this

thus

the
other

statements by using the OBJECT command with the relevant
(and statement number if necessary).
For example:
>>

where

examine

dump

specified,

the

address

absolute;

accumulators:

#0;

line

ALGOL

statements

number)

(words

output

may

1f
be

octal

specified

address

typing

in parentheses:

OBJECT

(17)

22;

OBJECT

(13);

the typeout
formats may
>>
where

normally in octal,
specified by using

OBJECT

the
7

is
be

a's

line-number
are

SIXBIT

Symbolic
or

Decimal

O
or

Line-number
external
>>

a,a,a...;

from:

Instructions
Integer

Real

Number

Long

Real

may

OBJECT

will
and

A
27,3

ALGOL

SIXBIT

the

word

complete

(18)

dump
27

(each

qualified

procedures.

line

ASCII

Other

Octal

symbolic
instructions.
MODES option:

ASCII

This

chosen

MODES

and
the

and

MODES

statements

characters,

and

next).

module-name

example

EXTPRG

external

the

symbolic

the

program

has

therefore:

A,6,S;

starting

procedure

20-5

EXTPRG.

the

fourth

The

instructions.

dump

statement
will

THE

20.3.7

System

ALGOL

DYNAMIC

DEBUGGING

SYSTEM

Parameters

ALGDDT maintains a table of certain symbolic
variables
wused
1in
the
system.
The
contents
of
these system variables distinguished by a
preceding % can be displayed in the usual way.
Most
of
the
system
varliables,
as defined in ALGPRM, will be availilable.
A few are listed
below as examples.
$DB

current

$SP

stack

$CONDL

context

"DL"

$DL

pointer

$VERSHN

version

ALGDDT

20.4

The

does

not

CHANGING

values

assignment
>>

data-base

pointer

permit

ALGOL

current
word

display
compiler

alteration

used

system

parameters.

VARIABLES

ALGOL

variable

may

but
the

altered

INTVARIABLE:=

BOOL

INT:=1;

only
a
single
assignment:

INT

REAL:=1.2345&7;

A[l,3]:=18.4;

S[1,7].[3]

that

A:=B;

C[I,Jd]:= 20
D[1,6].[I]
:=

following

are

not

legal:

66;

nature,
some
extra
length are specified
Type-in consists of

bytes

separated by commas.
If the numbers typed
byte-size in force, then they are truncated, and

string enclosed

the byte-size
typing
two
only

typing

1is

64;

the

>>
>>

The

constant

:=1.0;

however,

ALT-modes,

simple

003007;

Since strings are essentially dynamic
in
apply.
Unless
a
new
byte
size and/or
type-in, then the current values are used.

if
by

:=TRUE;
:=

wusing

1235;

l1ssued.

statement:

Type conversion will
take
permitted on the right-hand

Note,

1s six or
quotes.

semicolons,

exceptions

are

!s,

single

rules
in the
octal

are too large for
a warning message

double quotes

also

the

1is

allowed

seven respectively.
A quote may be entered
All characters, including carriage returns,
and

on,

are

control-backarrow,

entered

exactly

20-6

acts

usual;

typed.

and a
quote, which terminates the string.
If more bytes than the
specified
length
are
typed, then
the length is extended.
1If fewer, then the
extra bytes are zeroed.
The string may be continued on
another
1line

control-underline.

which

THE

ALGOL

DYNAMIC

DEBUGGING

SYSTEM

NOTE
Square

brackets

have

in
strings
that
Section 16.6.2).

20.5

special

are

meaning

output

(see

PAUSES

By setting
interrupted

20.5.1

PAUSES,
the
wuser
automatically when

Setting

Pauses

may

ALGDDT

command:

may
cause
program
execution
specific points are reached.

PAUSEs

applied

before

any

executable

ALGOL

statement

the

of
Note
If

this
that
the

PAUSE

line

PAUSE

label:

PAUSE

PROCEDURE

[,statement

no]

[IN

module-name];

[IN

module-name];

When

this

point

"Pause

name

reached

line

nnnn

[IN

the

module-name];

message

[,statement

module

name"

typed.

The pause remains in effect, so
piece
of
<code will also cause
the portions
of
the
command

that subsequent
activations
this message to be printed.
in
brackets
are
optional.

statement
number
1is
absent,
then
the
first
(or
only) statement
beginning on that line is assumed.
If
module
name
is
absent,
the
current
module
1is
assumed;
the main program is the current module
before program execution starts.
If the statement specified does
not
exist,
or
has
no
generated
<code,
the PAUSE is placed on the next
suitable statement, if any.
Labels
and
procedures
must,
1like
all
other identifiers, be in scope.
There

number

then

the

upper
>>

(n)

limit
PAUSE

may

number

parentheses

pause

also

will

pauses

after

bypassed

specified

for

n-1

PAUSE

user

can

PAUSE
times

the

establish.

instruction,
before

form

program

(n:m);

cause

the

m'th

location.

On
be

will

the

halted.

This will
including
PAUSE

typed

resultant

execution
An

restriction

break

to be taken
time
that
the
completion of the break

killed

PAUSE;

with

current

statement

line

from

the

program
action

automatically.
number

(after

label

has

specified,

been

20-7

typed).

n'th

time
up
to
and
reaches the breakpoint
after
the
m'th
pass,

sets

PAUSE

the

THE ALGOL DYNAMIC

20.5.2

When

Resetting

a pause

PAUSE

Proceed

established

PAUSE

(n);

PAUSE

(n:nl);

using

DEBUGGING

SYSTEM

Count

the ALGDDT command:

This pause will occur on the n'th time after the program
reaches
the
breakpoint location.
Once the break has occurred, it will repeat each
time the program reaches this point.
To stop
subsequent
unnecessary
pauses, the count can be reset by specifying the command
>>

CONTINUE

mj;

This will result in the pause to be taken only on the m'th
subsequent
pass and the m'th subseguent pass after that and so on until the PAUSE
is

KILLed.

NOTE

This command may be given
as
a
direct
ALGDDT command, or 1in an AUTO-list.

20.5.3

Clearing

A KILL command
varieties
>>

kills

are

PAUSEs

1is

provided

available.

for

After

<clearing

pauses.

Three

possible

Pause,

KILL;

the

current

pause.

KILL

line-number

KILL

label:;

KILL

[,statement-number]

[IN

module];

(at

kills

the

specified
>>

kills

procname;

must

specific pause
does

KILL

all

PROCEDURE

least

not

ALL;

currently

typed)

referred

to,

results

the

exist."”
!

ALL

set

must

typed

full;

pauses.

NOTE

there are PAUSEs set on
labels
or procedures with

two

the

or
same

name

in the same module, a block-number
(the
one
given
1in
listings in the START OF
BLOCK n messages, or 1in the block
CREF)
must

given,

thus:
20-8

message

"Pause

THE

20.5.4

The
of

currently
LIST

lists

KILL

DYNAMIC

LABEL:

DEBUGGING

SYSTEM

[IN

module];

thereby

resolving

the

ambiguity.

Listing

the

ALGOL

KILL

lkills

AUTO-list

block-number;

may

listed

PAUSEs

set

PAUSEs,

command,

and

which

their

has

the

AUTO-lists,
following

use

formats:

LIST;

all

PAUSEs

and

the

names

all

DEFINEd AUTO-lists.

LIST

line-number(,statement-number];

LIST

label:;

lists

the
>>

lists

LIST

etc.;

specified

together

with

its

AUTO-list,

any.

AUTO-1list A.
>>

lists
For

PAUSE

LIST

all

ALL;!

PAUSEs

ALL

and

all

must

DEFINEd

typed

full;

AUTO-lists.

example:
>>

LIST

Proceed-count

Autolist

Private

27,3

LABELl1:

wWhere

293

PROC1

Defined
>>

Autolists:

LIST

27,3;

Proceed

count

DIM
TYPE

!Type

19,

the

FOO
in

MUMBLE

FOO

A,C-£,J,L,S-0Q

private

dimensions

Autolist:
of

array

I,J,K;

CONTINUE;
>

See

note

1in

Section

20.5.2

about

ambiguous

20-9

labels

and

procedure-names.

THE

20.5.5
Any

Automatic

pause

that

can

are

ALGOL

DYNAMIC

Execution

have

executed

DEBUGGING

Commands After

associated
whenever

AUTO-list,

the

pause

would

have

the

same

PAUSE

that
1s

contain almost any ALGDDT commands.
These
immediately
after
declaring
a PAUSE, or
single letter and referred to indirectly.

SYSTEM

list

reached.

commands

AUTO-lists

can

may
either
be
typed
in
they may be identified by a
The following two
examples

effect:

(direct)
>>

PAUSE

BEGIN;

TYPE

END;

(indirect)
>>

DEFINE

TYPE

END;

PAUSE

AUTO

NOTE

The

prompt is followed
reading an AUTO-list in

The

advantage

referred
AUTO-list
does

not
>>

destroyed

check
PAUSE

only

and

list.

commands,

different

BEGIN

obeyed

;

but

The

action

detecting

semicolons

be
controlled
by
means
However, this switch only
from

the

IGNORE

that

the

direct

KILL;

the

when

mode.

pause

same

1list

method
is

instruction

would

checked

syntax

for

semantics

(scope

error

for
of

kill

both

(legality

identifiers,

of
the PAUSE from which
by an END and elements in

during

ALGDDT

END

KILL

carriage

be
the

KILLed.

however,

can
used,

thus

are,

depends
on
the
context
AUTO-lists are terminated

separated

Tab

controlling

since

not

a
either

consistency,

TYPE

once,

are

AUTO-1list

the

for

AUTO-lists

etc.),

method

pauses.

when

AUTO-lists

pause

as
this
invoked.

indirect

from

would
of

the

etc.)

they are
the list

returns.

execution

AUTO-list

of
a
switch on the AUTO or BEGIN
controls the action for invocation

can

keyword.
of
this

PAUSE.
No

error

message,

continue

element

AUTO-1list.
CONTINUE

Type

error

message,

continue

AUTO-1list.
KILL

Type

error

AUTO-1list
AUTO-1list
STOP

Type

message,

KILL

reference

from this PAUSE.
If this is
(defined by BEGIN)
it itself

error

message,

debugger

this

a "private"
is KILLed.

command

level

(default).

PAUSE

sometimes
1is

CONTINUE

useful

reached
command);

the
AUTO
or
still be typed

suppress

(especially
this

may

BEGIN keyword.
unless /IGNORE

if
be

the

the
done

"Pause

at..."

associatied
by

using

the

message

AUTO-list
/SILENT

ends

when
with

switch

Note, however, that error messages
is also used.
For example, suppose

20-10

a
a
on

will
that

THE

program

the

start

(and

was

PAUSE

CONTINUE;

variable

25);

SYSTEM

(I)

was

the

following

the

user

not

would

initialized

temporary

END;

START;

20.5.6

DEFINE

command
a

because

DEBUGGING

BEGIN/SILENT;

I1:=0;

DYNAMIC

(on line
cure:

>>
>>

use

failing

loop

i1nefficient!)
>>

This

ALGOL

Command

provided

subsequent
DEFINE

!Or

(ALGDDT

END;

enable

PAUSE

any

valid

AUTO

command.

AUTO-list

define

The

an AUTO-list

format

for

is:

name;

commands)

20.5.7
EXTEND

DEFINEd
>>

EXTEND Command
enables

additional

AUTO-list.
EXTEND

The

commands

format

appended

previously

is:

(ALGDDT

END;

commands)

This

command

therefore

can

also

used

instead

used
of

the

define

DEFINE

AUTO-1list,

and

can

command.

NOTE

AUTO-list

command
will not
printed.

20.5.8
A

ending

with

EXTENDed,
executed.

the

new

error

CONTINUE

commands

will

AUTO Command

command

format

1is
be

is,

is provided to
for example:

AUTO

invoke

DEFINEd

AUTO-list

directly.

NOTE

AUTO

may

AUTO-1list.

nesting

appear

this

kind

within

(decimal)

are

20-11

another

levels

permitted.

The

THE

20.6

EXECUTE

20.6.1

ALGOL

DYNAMIC

CONTINUE

simplest execute
command
is
execution
of
the
program until

types

20.6.2
GOTO

SYSTEM

COMMANDS

The

user

DEBUGGING

CONTINUE.
This
continues
it terminates, meets a pause,

normal

the

“C.

GOTO

(or

TO)

can

take

LABEL:;

line

two

forms,

either

scope,

case,

number

the

otherwise

command

the

"Identifier

[,statement

does

only

number];

valid

the

destination

within

message
not

exist,

out

scope"

is
is

printed.
No ALGOL code is executed, and the status of the
program
unchanged.
Unless a break-point has been set to the destination,
program execution will proceed as if CONTINUE had been specified.
Formals

may

20.6.3

START

the

specified

debugging

sequence,

20.6.4

system

program

was

executlion

labels.

entered

may

way

started

the

initial

REEnter

using

the

START

command.

entered

and

executed

RETRY Command

This

command

the

beginning

enables

the

ALGOL

program

statement

which

error

has

from

occurred.

NOTE

RETRY

differs

from

CONTINUE

the

latter
continues program execution from
the point within the statement where the
error has occurred.

20.6.5

The

statement.

command

allows

the

user

Typing

has

the

step

the "next" statement and then typing
kills itself when reached).
Even in

same

through

program

effect

setting

statement

PAUSE

CONTINUE (except that
the
PAUSE
cases where the current statement

transfers
control
to
any
point
other
than
the
next
sequential
statement
(either
explicitly
by
a
GOTO or implicitly as part of a
conditional IF or iterative
FOR/WHILE
statement),
NEXT
remains
1in
operation until the KILL command is issued.
20-12

THE

ALGOL

DYNAMIC

DLEBUGGING

SYSTEM

NOTE

If an ALGOL statement includes
embedded
assignments,
extra
1line numbers may be

generated.
This will cause NEXT to stop
after
completing
the assignment rather
than

the

second
and
all

20.7
The

end

the

command

statement.

will

rectify

enable pauses to
be
taken
subsequent statements.

this

between

DUMP
DUMP

command

may

active variables.
The
SCALARS keyword.
This
absent,
then only the

used

output

the

wvalues

all

currently

output of arrays may be suppressed by using the
command has an optional parameter.
If this
is
variables declared in the current block will be

dumped.

If a numeric parameter, n, is specified, then
all
variables
declared in the n enclosing blocks will also be dumped.
If "DUMP ALL"
is specified, then the variables declared in all the currently
active

blocks

will

dumped.

NOTE

DUMP

static

nature,

variables
enclosed
recursively-activated
only
have
the
wvalue
occurrence

20.7.1

This

command

directed
filename
file.
The

REDIRECT

the

any

all

ALGDDT

[proj,prog]

output

to
the
device designated.
1If
is specified, then the resultant

command

directed

1is,

dumped.

Device:filename.ext

causes

that

in
blocks
in
procedures
will
of
the "latest"”

with

arguments

from

DUMP

commands

causes

DUMP

output

TTY.

NOTE

The

FINISH

instead

REDIRECT

file

20.8

MISCELLANEOUS

will

command

1is

not

exit

force,

should

closed.

COMMANDS

20-13

used

from ALGDDT when
else

the device is a disk and the
output is appended
to
that

the

output

THE

20.8.1

Accessing

In many cases,

ALGOL

"hidden"

there

are

DYNAMIC

DEBUGGING

Variables

variables

SYSTEM

- UNWIND and BACK

that

cannot

accessed

Dbecause

they
are
"hidden"
1in the current context, either because there 1is a
variable of the same name in an inner block or in cases of
recursion.

The UNWIND command is provided to allow access (for display or change)
to these variables.
It refers to the dynamic block
1levels
that
are

typed

the history trace

It has

command).
>>

UNWIND

for

program execution was
UNWIND

the

context out

Also,

the

command

UNWIND;

BACK;

variable

accessing to level

nj;

"out”

from the true context

(that at

which

stopped);

O0;

moves

or by the WHERE

=-n;

moves the context n levels

"C,

moves ALGDDT's context
>>

(produced on error or

formats:

three

the outermost

block.

returns

the

context

20.8.2

EXPERT

the

true

context.

and NOVICE

All error messages have a full and an abbreviated form.
Normally
the
full form is typed, but the EXPERT command causes the short form to be
used.
The NOVICE command causes ALGDDT to revert to
the
full
form.
In
addition,
the
wuser
may type a ?
after a short message, and the
full message will

typed.

The EXPERT mode also prevents ALGDDT from typing the procedure history
(after “C or error) which can be obtained by using the WHERE command.
If

line

ALGDDT/EXPERT

appears in the file SWITCH.INI the user's area, ALGDDT will be entered
with EXPERT mode in force (until changed by a NOVICE command)
.

20.8.3

WHERE

The WHERE command causes the

produced

when the error

particular use

for

system to

(if any)

retype

occurred.

the stack

users of visual display terminals:

Where

On
In

line 5 in module 8
procedure PROC1l (level

20-14

trace

that was

This is intended to be of

THE

ALGOL

DYNAMIC

DEBUGGING

Called

from

line

procedure

Called

from

line

main

program

PROX

for

use

the

UNWIND

SYSTEM

(level

(The

"levels”TM

20.8.4
The

are

command,

see

section

20.8.1.)

TRACE

buffer.

command

This

procedures

causes

ALGDDT

consists

encountered,

of
with

the
the

type

the

names

most

recent

contents

the

most

first.

the

recent
(The

trace

labels

typeout

and
may

as usual be aborted by typing two "Cs, or a "0.) The number of entries
in the buffer is 100 (decimal) by default.
This value may be
changed
by giving the /TRACE switch to the compiler.
Labels are distinguished
by a : and library procedures by a *.
The indentation
of
the
names
gives the dynamic block level (two spaces per level and each procedure
is enclosed by an extra notational level):
>>

TRACE

!ALGOL

postmortem

trace

(latest

first)

PRINT*

LABEL1:
LABEL2:
LABELl:
FOO

OPENFILE*

LABEL1l:
MAIN.PROGRAM
>>

20.8.5

PROFILE

This command types the history of the program in terms of
the
number
of times each label and procedure has been encountered;
typeout is in
the same order as the occurrence of the objects in
the
program.
As
for TRACE, labels are marked by a : and library procedures by a *.
example:
PROFILE

Profile

Count

name

OFHMHFOHFHOMFEW

LABEL1:
LABEL2:
LABEL3:
FOO
BAR

PRINT*

OPENFILE*
SELECTOUTPUT*
\4

For

20-15

THE

20.8.6

command

the

program.

20.8.7

all

DYNAMIC

DEBUGGING

SYSTEM

STATISTICS

This

ALGOL

types

the

execution

time,

elapsed

system

exit

time,

and

core

size

FINISH

command

files

causes

and

the

releasing

all

the

Monitor,

first

closing

devices.

NOTE

This

must

REDIRECT
output

20.8.8

ONTRACE

and

two

(see

Chapter

while

executing ALGDDT
by

otherwise

20.8.9
A

HELP

This

"C,

losing

a
the

this

commands

control

the

connection

(this

dynamic

that

trace

tracing

significant

cause

execution

pieces

provided

which

types

the

as
of

facility

suppressed

accessing

code

that

would

1is

contents

file

HLP:ALGDDT.HLP.

SOURCE

types

SOURCE

The

defaults
those

than

avoid

commands.

provided

Note
may

command

when

20.8.11

DUMP

HELP

command
>>

rather

force

traced).

SYS:ALGDDT.HLP

20.8.10

are

18).

name

from

used,
in

OFFTRACE

These

formal

commands

be
1is

the

Indirect

response

file.

The

format

is:

device:filename.ext[proj,progl;

are

any ASCII

command file by
enable commands
specified
then
1s particularly

DSK:ALGDDT.ALG;
user

will

Command
prompt

PROJ,

used.

PROG,

S.F.D.s

are

both
not

may

omitted

accepted.

Files

from

ALGDDT,

the

wuser

may

use

indirect

specifying the filename with a preceding @.
This will
to
be
read
from
that
file.
If
no
filename
1is
DSK:ALGDDT.CMD will be taken as default.
This feature
useful for inputting frequently used AUTO-lists.

20-16

THE

20.9

ALGOL

DYNAMIC

DEBUGGING

SYSTEM

SUMMARY OF ALGDDT COMMANDS

AUTO

BACK

BREAK

CONTINUE

DEFINE

DIMENSION

DUMP

END

EXPERT

FINISH

GOTO

(OR GO TO)

HELP

KILL

LIST

NOVICE

OBJECT
OFFTRACE

ONTRACE
PAUSE

PROFILE
REDIRECT

SOURCE

START

STATISTICS
TRACE
TYPE

UNWIND

WHERE

The

commands

abbreviated

marked
to

with

single

asterisk

character,

(*)

thus

20-17

in
S

the

means

above

list

START,

not

may

also be
STATISTICS.

CHAPTER
MACRO

21.1

SUBROUTINES

GENERAL

The

subroutines

one

the

which

following

may

called

from

The

routine
must
specification,
and

obey

the
called

for
example,
information can be

Further

True

ALGOL-like

call

This

OTS

results

with
be

(in

case

details

THE

PROCEDURE

the

exit

TYPE

implementation.
for

examples

correctly

executable

instruction

PARAM.

describe

the
parameters.

This

type

the

MACRO

the

procedure

followed

procedure

and

the

PROCEDURE
STRING

header

P(A,B,C);

VALUE

type

INTEGER

similar

the

following:

cEXIT==

A==

.B==4
JSP

AX,PARAM

EXP

PMB

XWD

0,11

XWD

SPRO!SI!ISSIM
4

21-1

should

set

Example:

MACRO

ALGPRM,ALGSYS

routine

have

definitions

written

HEADING

OTS

would

ALGOL

first

PARAM

procedure).

ALGPRM

the

INTEGER

for

and

the

The

REAL

environment

and

be included at the beginning
material uses symbolic names.

formal

on.

17.5.

instructions:

an
a

section

the

which

with
set

interface
interface

and

17,

have

ALGSYS

subroutines.

21.2

must

files

for

Fl0ICALL

Chapter

provide

the

specific
made

ALGLIB

begin

parameters;

and

program

calling
the FORTRAN

F1l0CALL,

found

must

any

deals

way

PARAM;

any

should

FORTRAN

routines

return

chapter

ALGOL

routine

obtain

Reference
etc.,

the

attributes:

procedures,

must

the

rest

be a call
to
descriptor words
of each
of
the

MACRO

XWD
XWD
XWD
An

SUBROUTINES

SVAR!SI!SFOV,
.A
SVAR!$R!SFON,
.B
SVAR!$S!ISFON,.C

explanation

follows:

The
first
word
must
normally
be
JSP
AX,PARAM
(AX
is
accumulator
16;
PARAM is a macro defined in ALGSYS, by the
ALGDIR macro, and which expands to @%ALGDR+1l,
1i.e.
@400011
which contains the address of the OTS routine PARAM.)

The second word is the address of the post-mortem
block
(or
zero
1f
none).
The post mortem block 1s used by the TRACE
features, and is laid out as follows, in the low segment:
PMB :

0
XWD

WORDS ,CHARS

SIXBIT/NAME/

;

THE

;

PROFILE

;

PRINTED

THE

WORD

NAME

TRACE

ETC.

NOTE

The
OTS
expects
the
name
to
be
terminated
by
a zero byte ("SIXBITZ").
If
the
name
1is
a
multiple
of
Six
characters
1in
length, an extra word of
zeroes may have to be supplied.

The third word is the length of
the
fixed
stack
reqguired.
There
must
be
enough
space
for
the
exit formal and the
parameters, see below.
More space can be requested, and
may
be used for any purpose as local storage by the procedure.
Length

fixed

For

the

For

stack

exit

typed

instruction

procedure

(Integer,

For

Real,

typed procedure
(Long

For

needed:

each

Real,

(always

with

1l-word

needed)

word

result

1l word

result

Boolean)

with

2-word

words

String)

parameter

called

name

3 words

(Any type)
For

The
the

each parameter called by value:
Integer,Real,Boolean,Label,Procedure
Array,String,Long Real

fourth word describes the procedure in the
number of parameters +1 in the right half.

must

always

"SPRO!$SIMITYPE"

where
SN

"type"

one

of:

NON-TYPE

S$1

INTEGER

REAL

SLR

LONG

$S
SB

REAL
STRING
BOOLEAN

21-2

1 word
2 words

left
The

nhalf, and
left half

MACRO

The

remaining

PARAM

where

giving

the

Kind

words

describe

put

kind,

type

one

SUBROUTINES

the

them.

and

formal

The

status

left

parameters,
half

the

Real

Long

and

bit

tell

pattern,

parameter:

of:

SVAR Variable
SARR Array
SPRO Procedure
Type
or

1is

one

$I,

SR,

SLR,

described

above,

of:

SL
SWV
SAB

Any type
Arithmetic

Label

$IB

Integer

Boolean

Boolean
Real or Long Real
Arithmetic (Integer,

SWF
SWA

Status

must

one

SFON Formal
$SFOV formal

by
by

"name"”
"value"

Real)

of:

NOTE

All

three

fields

must

included.

The right half of each descriptor word is the offset
1in
the
fixed stack where PARAM is to store the elaborated parameter.
Conventionally, these are in
ascending
order
of
parameter
position,
but this is not necessary, and gaps may be left if
desired.
PARAM obeys these offsets implicitly (to check them
would
be inefficient).
If insufficient words are allocated,
or insufficient fixed stack is requested for each
parameter,
parameters
will
overwrite each other and be lost, or worse.
The space required for each parameter is as described above.
It
in

is conventional to
the example above.

use

symbolic

names

for

the

offsets,

NOTE

Word
and
and
for

21.3

ACCESSING

Parameters

case,

may

PARAM

specified

by
A

MOVE

will

always the
2
(l-word

(2-word result) must
be
reserved
result of a typed procedure.

the

PARAMETERS

called

the

right

the

exit
instruction,
result) or words 2

"name"

actual

half

example

value

of
into

the

"value".

the

fixed

descriptor

the

"value"

stack

the

location

words:

thus

1load

A3,

A3,.A(DL)

suffice.

stack.

the
in

FORMAL

places

parameter

word

Obviously,

(accumulator

two word

15)

parameters

always
21-3

the

base

occupy
of

the

two

words

fixed

stack.

MACRO

SUBROUTINES

Accessing
formal
parameters
by
"name"
Essentially,
PARAM
stores instructions on

procedure

XCT's.

The

three

locations

are

which,

when

is
more
complicated.
the fixed stack, which the

known

F[0],

F[l]

and

will

fetch

the

F[2].

F[0]

contains

instruction

parameter
1into
accumulator
load parameter B's value into
XCT

executed,

O
(and 1 for a two word value).
So,
accumulator 0, in the example, write:

.B (DL)

NOTE

The

instruction
case)
may
be

AQ,value"

for

parameter,

OTS

routine

parts
All

be
shifted.

various name
in
order
1left
into a formal by
the
evaluation

XCTA

(XCT

parameter
store

written,

F[0]

A2,

store

the

and

actual

complicated

turn

program

except

destroyed,

written.

must

and

perhaps

set

parameters,
store
the

contents

This

which

(from

cases of actual
instruction
to

may

user

call

other

("thunks"

DL,

the

stack

and

may

must be evaluated exactly once
each,
and
to right, to obey the Algol rules.
Therefore storing
name is a two-step process.
At the correct point
1in
of
parameters
in
the
left
to
right sequence, an

with

constant

PUSHJ

which
the

in
this
a "MOVEI

parameters

1into

value

accumulators

may

The

in F[0]
(.B(DL)
anything
from

now

saved.

the

When

address

Al)

into

the

parameter,

it was

after

the

F[O0].

A2 has
wvalue

into

elaborates

no meaning
directly,

parameter

PUSH

SP,A3

;

THE

XCTA

.B (DL)

;

SET

POP

SP,AQ

XCT

.B+1 (DL)

;

STORE

MAY

XCT

F[1l]

(In very

simple

and
F[l]
contains
e.g.
a MOVEM.) Thus,
the example, write:

CLOBBER ANY

the

required

an
to

AC!!

AQ0

INTO

NOTE

The

XCTA

the
F[l]

F[2]

never

PARAM

simple

case
I:=

Then

the

must

1left
need

referenced

store

would

to
not

right
be.

directly

the

1information

needed

be:

the

suppose

proper

sequence

the

place

but

procedure

example

1in

the

XCT

evaluate

procedure

the

P were

P(I,123456789,"ABC");
three

words

for

the

fixed

stack

would

be:

.B(DL)

F[O]

MOVE

AQ, .B+2(DL)

.B+1 (DL)
.B+2 (DL)

F[1l]
F[2]

SYSERI1
"D123456789

11,

21-4

1is

used

parameters.

called

MACRO

(The

SYSER1

used

left

UUO

store

produces

when

actual

complicated

half,

the

cases,

delocated

Special considerations
strings.
The

code

XCT

.L (DL)
A2, (A2)

zero

F[2]

and

apply

JUMPN

will

error-message,

DL),

formal

SUBROUTINES

used

formal

F[0]

used

formal

arrays,

for

label

actual

arrays,

the

header

word

pair

(as

arrays

are

always

switch

obey

actual

formal

return

procedure,

parameter

with

address:

but

not

cannot

context

(in

the

thunk.

procedures

and

F[O]

whose

(see

subscript

section

static):

the

descriptor

formal

labels,

XCT

were

F[0]

just

stored

should

not

containing the string
and XCT F[1l] are used to
byte
1into
an
existing

will
point
and
nowhere
is a dynamic

out

XCTA
store a

words,

and

1is

21.6)

XCT's

arrays.

string.
In
some
circumstances
A2
string header will be in A0 and
Al,
happens
when
the
actual
parameter
call of a string procedure.
To

the

half)

For strings, XCT F[0] will
header
and
A2 containing
store a new string header,

is:

the

F[l]

store

(right

and

for

address

bounds.
For

because

constant.)

1is

to A0Q, that is, the
else.
This
only
expression, i.e.
a

coded,

though

followed

normal

procedure

being called, with the PUSHJ replaced by the XCT F[0].
The first
desriptor
1is
XWD type of procedure wanted, number of actuals+l.
The
remaining words, one for each actual, are coded with bits 1 to
11
as
described
above
for
formals.
Bit
0 is set if dynamic, that is, a

formal actual, or
constant
(simple

constant

(regular

static

the

address

fixed

thunk.

static

stack

for

Bits 18-35
expression),

expression)

dynamic

the

address

actual

dynamic
expression),
the
right
Before using dynamic variables and
formal
code

procedures,

(with

ALGDDT)

the

reader

similar

RETURNING

The

procedure

the

case

stores

FROM TYPED

the value
two-word result)

MOVEM

the

RESULTS

and

bits

advised
1in

immediate

non-immediate
own)

variable,

the

12-17,

offset

the

is, the offset
variable.
For a thunk

level,

the

(e.g.

address
that

1is
the address
expressions as actual
case

complications.

21.4

The

half

1is

the

value for
for a

static

variable,

are
the appropriate procedure
current display of the context DL of

the

variables.

address,

are
the

inspect

question,

(a
of the thunk.
parameters to

some
as

generated
there

are

PROCEDURES

into the second word (and
of the fixed stack, thus:

A7,.EXIT+1 (DL)

example.

21-5

;where

third

.EXIT

word

MACRO

21.5

PROCEDURE

The first word
return
to the
the result may

SUBROUTINES

EXITS

in the fixed stack
contains
an
call-site (via PARAM:
the stack
need its type converted), so the

appropriate
Jjump
must be "“unwound",
procedure exits by

to
and
use

of:
JRST

21.6

.EXIT(DL)

FORMATS

;Where

.EXIT

VARIABLES

Integer and Real variables are obvious.
the
format appropriate to the CPU type
variables are zero
for
"false",
and
values)

String

for

are
1in
Boolean
negative

"true'".

variables

Long real
variables
in use (KA or KI/KL).
non-zero
(including

are

passed

two

word

header.

The

format

1is:

—————— -

address

—————— -t
-

flags

length

1in

bytes

______ g
S
S
S

The

first

get

the

The

the

word

first

second

the

byte-size.

word

last word

bit
bit

1
2

byte

byte).

pointer

This word

mostly

may

taken

rubbish).

the

zero
by

string

for

the

The

(such

null

length

flags

a new value 1is assigned to
deleted.
The safest way to do
(string
assign),
which
takes

1in

bytes

and

on,

thus:

SP,word-0-of-new-string's-header

SP,word-l-of-new-string's-header

PUSH
PUSH

SP,word-0-of-old-string's-header
SP,word-1l-of-old-string's-header

PUSHJ

SP,STRASS

Result

A(0,Al

NOTE

STRASS
1s set

are

ILDB

will

(odd

bytes

string, any old string must first be
this 1is to call the OTS routine STRASS
the
necessary
care
not
to
delete

PUSH

Arrays

are:

PUSH

;

that

string.

dynamic (not a constant)
result of a string-type procedure

When

constants

also

passed

will copy the string unless
1n the new string's header.

two

word

21-6

header,

thus:

bit

MACRO

SUBROUTINES

_______________________ +________-_____—__—____

type

origin

_______________________ +__..__—..______—__—_—___

subscripts

address

_______________________ +___..-____________—_-__

where:

"type"

integer

etc.

(coded

descriptor

above).
2.

see

"origin" is the address of the (possibly
imaginary)
=zero'th
element
1in
the array (if one-dimensional), or in the N-1'th
Iliffe vector (if N-dimensional);
see below.

"DV

address"

for

the

subscript

words

per

address

checking

dimension,

the

(and

containing

Dope
the

the

Vector,

Debugging

low

and

each.

This

words,

applies

to a vector.
However, for a
matrix
(two
right half of word 0 is the address of the zero'th
Iliffe vector, which is as follows:

-----------------

origin

row

a+l

origin

used
two

(dimension

bound

dimensional),
element

the

a:b)

etc

System):

high

the

------------------

which

row

where the origins are addresses of the
(possibly
imaginary)
=zero'th
elements.
For more than two dimensions, a hierarchy of Iliffe vectors
exists.
(The purpose of this is to allow addresses of elements to
be
calculated

constant

block
(the

21.7

without

strings)

are
first

multiplications).

are

allocated

deleted
at block
word of the header

PROCEDURES

the

Arrays

Heap,

and

strings

(except

occurences

local

exit.
Strings are initialised
is zero) at block entry.

WITH A VARIABLE

NUMBER OF

for
to

a
"null"

PARAMETERS

This

facility
1is
not
provided
by
the
Algol-60
language
but
1is
available
in the DECsystem-10/20 ALGOL run-time system.
It is needed
for some library procedures (IMAX etc., READ, PRINT, etc.)

The

procedure

PROC:

must

have

special

heading,

XWD

DL,offset

JSP

AX,PAROQ

viz:

etc.

where "offset" is a
of fshoot
of
PARAM)
of
If

actuals allowed is
wild
types
were

location

on
the
fixed
stack
where
PARO
(an
the number of actuals.
The maximum number
determined by the number of formal
descriptors.
used in the formal descriptors, the actual types

stores

21-7

MACRO SUBROUTINES

can only be obtained by
contalns
(PRGLNK (DL)

words
descriptor
actuals'
the
up
picking
the
over
advanced
address,
call-site
the

actuals.)

INCLUDING THE PROCEDURE

21.8

IN THE LIBRARY

the

The procedure may simply be added to
or

FUDGE2

1library

(ALGLIB.REL)

with

MAKLIB.

If the procedure has a different version for KA and KI/KL processors,
An entry
LIBENT macro should be used (refer to file ALGSYS.MAC).
the

must also be made in a table in ALGSTB (in the compiler), to associate
this is done by using the LIB macro, which
the name with the alias:
is described in comments

in ALGSTB.MAC.

UTILITY ROUTINES AVAILABLE

21.9

A number of the routines in the OTS are available

for

wuse

Macro

procedures.

21.9.1

Getting

Core

any amount may be
Core may be obtained in the Heap by calling GETOWN;
as
stack etc.
the
shift
program,
the
expand
will
GETOWN
and
had,
necessary.
To

get

Calls

are:

core:

MOVEI

AQ,amount wanted

or GETCLR if wanted zero‘'d
SP,GETOWN ;
PUSHJ
on return, Al = address of core.
s+

return

core:

MOVEI

AQ,0

PUSHJ

SP,GETOWN

MOVE

21.9.2

Al,address of piliece
;

not

GETCLR!

Input/Output

the
because
Buffered mode input/output may not be done directly,
later
may
which
stack,
the
above
buffers
the
allocate
monitor will
Direct
have to expand or be shifted to allow the heap to expand.
access to the OTS routines is however allowed,

21.9.3

as follows.

Device Open

MOVE

AQ, [SIXBIT/device/]

HRRI

Al,channel-number

HRLI

MOVEI
PUSHJ

Al,#-buffers-required
A2 ,mode
SP,INPT

21-8

;

0 will give default

+
s

0 1s ASCII
or OUTPT

MACRO

return,

instruction
etc.

obtain

starting

21.9.2.2

zero

which

free

File

successful.

XCT'd

channel

%IODR(DB))

SUBROUTINES

will

number,

for

give

scan

zero

Al,channel

A2, [SIXBIT/Filename/]

MOVE

A3, [SIXBIT/Extension/]
A4, [<protection>B9]

MOVE

A5, [project, ,programmer]

PUSHJ

SP,OPFILE

channel

still

Al ,channel

PUSHJ

SP,CLFILE

Channel

Al ,channel

PUSHJ

SP,RELESE

;jnon-skip if
;skip return

and

the

not,
or

Channel

standard

;

May

give

error

number

error

(channel

not

use)

Select

A?,channel-#

HRLM

A?,%CHAN (DB)

Output:

MOVE1l

A?,channel-#

HRRM

A?,3CHAN (DB)

PUSHJ

Read Byte
SP,INBYTE

snon-skip

shere

contains
plus

with

;

USES

Al10-Al3

end of file
byte in Al3

21-9

100

error

obeying:

Input:

21.9.2.6

ENTER)

number

MOVE1

For

Al,

1If

LOOKUP

Release

MOVEI

For

from

MOVEI

21.9.2.5