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DEC-9A-AF40-D

1967

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Document:	FORTRAN IV Advanced System Software
Order Number:	DEC-9A-AF40-D
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Pages:	57
Original Filename:	http://bitsavers.org/pdf/dec/pdp9/DEC-9A-AF40-D.pdf

OCR Text

OEC ·9 A·AF40·D

Cl IGI T AL EQU IPM ENT CORPO F=l A TI ON • M A YN A R D , MASSAC H US ETTS

DEC -9A-AF40-D

FORTRAN IV LANGUAGE MANUAL

PDP-9
ADVANCED SYSTEM SOFTWARE

DIGITAL EQUIPMENT CORPORATION. MAYNARD, MASSACHUSETTS

PDP-9 FORTRAN IV

PREFACE

This manual describes the FORTRAN IV language for the PDP-9. It
provides users with the necessary information for writing FORTRAN programs for
compilation and execution in the PDP-9 ADVANCED Software System.
Operating instructions ,wi II be given in later publications. In paper
tape systems, the compiler along with an Input/Output Monitor is loaded from
paper tape. In larger systems with a bulk storage device such as DECtape, the
Keyboard Monitor accepts direct console commands to load the compiler in an
I/O device independent environment.
Several excellent texts are available for a more elementary approach
to FORTRAN programming. "A Guide to FORTRAN Programming, II by Daniel D.
McCracken (published by John Wiley and Sons, Inc.) is recommended.
This is essentially the language of USA Standard FORTRAN (X3.9-1966)
with the exceptions noted in appendix 2.

iii

PDP-9 FORTRAN IV

CONTENTS
Chapter
INTRODUCTION ••••••••••••••••••••.••.••••••.••••.••...••...••...••••

1 •1

FORTRAN •.•••••.•••••••••.••••..•.••••.••..•••..•.••••....•••....•

1 •1 • 1

Card Format ••••••••.•.••.•••.•••••••••.•••....•..••..•••••••.•

1 .1 .2

Paper Tape Format •.•••.••.•.•••.•••••••.•••••.••..•...•..••...•

2.1

ELEMENTS OF THE FORTRAN LANGUAGE
Constants •...........................

••••••••••••••••••••••••••••

2.1 .1

Integer Constants ••••••.•••••••••••••••••••••••.••••.••••.•••••••

2.1 .2

Real Constants (6-decimal-digit accuracy) ••••.•.•••••••.•••••••••.

2.1 .3

Double-Precision Constants (9-decimal-digit accuracy) ••.•••..••••••

2.1 .4

log jc a I Constan ts .................•............................

2.1 .5

Hollerith Constants ••••••••••.•••••••••••••••.••.••••.••.•••••••

Variables ........................................................ e

2.2.1

Variable Types •••••••••••••••••••••••••••.••.••••..••••.•••.•••

2.2.2

Integer Variables •....................................~ ........

e.a

2.2.3

Real Variables •......•.........................................

2.2.4

Double-Prec ision and Log ical Variables •••••••..••••••••••••••••••

Arrays and Subscripts •••••••••••••••••••••••••••••••••••••••••••••••

2.3.1

Arrangement of Arrays in Storage ••••••.••••••••••••••.•••.•••••••

2.3.2

Subscript Expressions ••••••••.•••••••••••••••••••.••••.••••••••••

2.3.3

Subscripted Variables •••.••••••••••••••••••••.•••••••••.••••..••

Expressions .......................................................•

2.2

2.3

2.4

2.4.1

Arithmetic Expressions

2.4.2

Re lational Expressions

••••••••••••••••••••••••••••••.••••••••.••

2.4.3

Log ical Expressions ••••••••••••••••••••••••••.••••.•••••••••••••

Statements .................................. , .................... .

ARITHMETIC STATEMENTS ••••••••••••••••••••••••••••••••••••••••••••••

CONTROL STATEMENTS •••••••••••••••••••••••••••••.•••...••.•.••.••••

4.1

Unconditional GO TO Statements •.•••••••••••••••••..•••.••...••.•.•

4.2

ASSIG N Statement ••••••••••••••••••••••••••••••••.••••.••••.••••••

4.3

Assigned GO TO Statement ••.•••••••.••••••••••••••..•••...•..••...•

2.5

PDP-9 FORTRAN IV

CON TEN T S (continued)
Chapter

4.4

Computed GO TO Statement •••••.•••••••••.•••••••••..••..••.•

4.5
4.6

Arithmetic IF Statement ••

000

•••••••••••

Log iccl IF Statement •••••••••••••.•••••••••••••••••••••••••••.••••••

4.7

DO Statemen t •••••••••••••••••••••••••••••••••••••••••••••••••••••

4.8

CONTINUE Statement •.•

••••••••

4.9

PAU SE Statemen t •

•••••••••••••••••••••••••••••••••••••••••••••••••

4.10

STOP Statement

•••••••••••••••••••••••••••••••••••••••••••••••••••

4.11

END Statement ••••••••••••••••••••••••••••••••••••••••••••••••.•••

INPUT/OUTPUT STATEMENTS •••••••.•••••••••••••••••••••••••••.•••••••

5.1

•

••••••••••

•••

••

•••••••••••

•••••••

•

••••

•

•••

General I/O Statements ••••••••••••••••••••••••••••••••

•

••••

•••

••••

••••••••••••

22
22

5.1 .1

Input/Output Argument Lists ••

5.1 .2

READ Statement •.••••••••••.••••••••••••••••••••••••••..•••••••

5.1 .3

WRITE Statement •••

5.2

FORMAT Statements o .

•••••••

••••••••••••••

••

••••••••••••

•••••••••••••

••••

•

5.2.1

Specifying FORMAT

5.2.2

Conversion of Numeric Data •..••.•••••

5.2.3

P-Scale Factor •.•••••

5.2.4

Conversion of Alphanumeric Data

5.2.5

Logical Fields, L <:onversion

5.2.6

Blank Fie Ids I X Conversion

5.2.7

FORTRAN Statements Read in at Object Time

5.2.8

Printing of a Formcltted Record •.•••••••

5.3

Auxiliary I/O Statements

•••••••••••••••• 0

••

•

••••••••

•••••••

•••••••••••••••••••••••••

•

•••

••••••••••••••••••••••••

••••

••

•••••

•••••••••

•••

•

•••

••••

••

•

••••••••

•••••

•••

5.3.3

ENDFILE Statement ••.

••

00.

SPECIFICATION STATEMENTS •••••••••••••••••

••

•

••••••

•••••••••••

.........................

••••••••••••••

•

••••••••••••••••

•••••

REWIND Statement ••••••••••••••••••••••••••

'• • •

.0.000

5.3.2

•.•••••••

••••••••••••••••••••

BACKSPACE Statement

••••••••••••••••••••

5.3.1

0.0

••

•••••••••••••

••••••••••••••••••••••••••

••••••••

•••••

••••••••••••••••••

0000."

••••••••••••••••••

. . . . . . . . . . . . . . 0.00.00.

••••••••••••••••••••••••••••••••

••

24
24

26
28
28
29
29
29
30

31
31
31
31
33
33

6.1

Type Statements

6.2

DIMENSION Statement ••••••••••••••••••••••••••

6.3

COMMON Statement ............................................... .

6.4

EQUIVALE NCE Statement

•

••••

o ••

•

••••••••••••••••••

411

••••••••••••••••••

• • •'

•

••

•

••••

•

PDP-9 FORTRAN IV

CON TEN T S (continued)
Chapter

6.5

EXTERNAL Statement ••••••••••••••••••••••••••••••••••••••••••••••••

6.6

DATA Statement •.•••••••••••..•..•.••.•••••••..•.•••.••..••••.•.•••

SUBPROGRAMS ..••••..................................................

7. 1

Statement Functions •••.••••..•••.••••••••••••••••••••••..•.••••••.••

7.2

Intrinsic or Library Functions •••••••••••••••••••••••.••••.•.•••••.••••

7.3

External Functions •••••••••••••••.•••••••••••••••••••..•...•••••..••

7.4
7.5

Subroutines ................ ~ ..............•........................

Block Data Subprogram ••••.••••••••••.••••••••••••.•••.•...•......••

Example of BLOCK DATA Subprogram ••••••••.••.••••..••••••••.••

7 .5.1
Appendix

SUMMARY OF PDP-9 FORTRAN IV ST ATEME NTS
2

••••••••••••••.•.•••••••.•

A NOTE ON USA STANDARD FORTRAN IV .............................. .

TABLES
Table

Intrinsic Functions

External Functions

vii

PDP-9 FORTRAN IV

CHAPTER 1
INTRODUCTION

T.1

FORTRAN
FORTRAN (for FORmula TRANslation) is the most widely used programming language for

engineering and scientific applications. It consists of precise procedures for expressing numerical
compilations.
The FORTRAN character set consists of the 26 letters:

A, B, C, D, E, F, G, H, I, J, K, L, M,
N, 0, P, Q, R, 5, T, U, V, W, X, Y, Z
the 10 digits:
0,1,2,3,4,5,6,7,8,9
and 11 spec ia I characters:
Blank
Equals
Plus
Minus
Asterisk
Slash
Left Parenthesis
Right Parenthesis
Comma
Decimal Point
Dollar Sign

1. 1. 1

*
/
(
)

Card Format (IBM Model 029 Keypunch codes)
The FORTRAN source program is written on a standard FORTRAN coding sheet, which consists

of the following fields: statement number field, I ine continuation field, statement field, and identification field.
The FORTRAN statement is written in columns 7-72. If the statement is too long for one line,
it can be continued in the statement field of as many lines as necessary if column 6 of each continuation
line contains any character other than blank or zero. There are two exceptions to this rule: (1) the DO
statement must be on one line; and (2) the equal sign (=) of an assignment statement must appear on the
first line.

PDP-9 FORTRAN IV

For one statement to be referenced by another, a statement number is placed in columns 1-5
of the first line of that statement. This number is made up of digits only, and may contain one to five
digits. Leading zeros and all blanks in this field are ignored. The stabement numbers are used for identification only, and they may be assigned in any order.
The FORTRAN compiler ignc)res the last eight columns (columns 73-80) which may be used
for program identification, sequencing, or any other purpose desired by the user. Comments may be
included in the program by putting a "e" in column 1 of each line cont1aining a comment (or continuation
of a comment). The compi ler ignores these comments except for printing them.
Blanks may be used to aid readability of a FORTRAN stateme!nt, except where indicated in this
manual.

1 .1.2

Paper Tape Format
When FORTRAN source program statements are prepared on paper tape, the sequence of char-

acters is exactly the same as for card input, and each I ine is terminated with a carriage return, I ine feed
sequence.
A statement number (all digits) may be written as the first five characters, or a "C" may be the
first character to indicate a comment line or a continuation of a comment line. For statement continuation
lines, any character other than blank or zero is written as the sixth character. The seventh character begins the statement and must be alphabetic. E'Jch line is terminated with a carriage return, line feed.
The TAB key can increase the speed of writing FORTRAN statements on paper tape. A TAB
followed by an alphabetic character begins the statement in column 7. A TAB followed by a digit places
the digit in column 6, indicating a statement continuation line. A stat'ement number less than five digits,
followed by a TAB, places the next character in column 6 if it is a digit or in column 7 if it is a letter.

PDP-9 FORTRAN IV

CHAPTER 2
ELEMENTS OF THE FORTRAN LANGUAGE

2.1

CONSTANTS
There are five types of constants a II owed in the FORTRAN source program: integer, rea I,

double-precision, logical, and Hollerith.

2. 1 . 1

Integer Constants
An integer constant is a number written without a decimal point, consisting of one to six

decimal digits. A + or - sign preceding the number is optional. The magnitude of the constant must
17
be less than or equal to 131071 (2
-1). Example:
+97

-2176
576

If the magn itude >217 -1, an error message wi II be output. Negative numbers are represented
1

in 2 s complement notation.

2.1.2

Real Constants (6-decimal-digit accuracy)
A real constant is an integer, fraction, or mixed format number and may be written in the

following forms:
a. A number consisting of one to six significant decimal digits with a decimal point
inc luded someplace with in the constant. A + or - sign preceding the number is optiona I.
b. A number followed by the letter E, indicating a decimal exponent, and a 1- or 2-digit
constant with magnitude less than 76*indicating the appropriate power of 10. A + or - sign
may precede the scale factor. The decimal point is not necessary in real constants having
a decimal exponent. Example:
352.
+12.03
-.0054
5.E-3
+5E7

* If the adjusted magnitude exceeds 75, an error results . . 999999E75 is legal, but 999. 999E73
is illegal.

PDP-9 FORTRAN IV

Real constants are stored in two words in the following formclt:

LOW ORDER
MANTISSA

SIGN OF
MANTISSA

EXPONENT
(2'S COMP.)

...._"'--_ _ _ _ _ _ _ ._ _ _H_IG_H_O_R_DE_R_M_A_NT_IS_S_A_ _ _ _ _ _ _ _ _ _ _--'

Negative mantissae are indicated with a change of sign.

2. 1.3

Double-Prec ision Constants (9-dec ima I-digit accuracy)
A double-prec ision constant is written as a rea I number followed by a decima I exponent, in-

dicated by the letter D and a 1- or 2-digit constant with magnitude not greater than 76. A + or - sign
may precede the constant and maya ISCI precede the sca Ie factor. A decima I point with in the constant is
optiona I. A double-precision constant is interpreted identica Ily to a n~a I constant, the only difference
be ing that the degree of accuracy is greater. Example:

-3.0DO
987.6542D15
32. 123D+7
Double-precision constants are stored in three PDP-9 words:

EXPONENT (2'S COMP.)
17

SIGN OF
MANTISSA

HIGH ORDER MANTISSA

NEGATIVE
MANTISSAE
ARE.
INDICATED
WITH A
CHANGE
OF
SIGN

LOW ORDER MANTISSA
17

2. 1 .4

Logical Constants
The two logical constants are the words TRUE and FALSE, eClch both preceded and followed

by a decimal point •
• TRUE.
.FALSE.

=777777
=0

PDP-9 FORTRAN IV

2.1.5

Hollerith Constants
A Hollerith constant is written as an unsigned integer constant, whose value, n, must be equa I

to or greater than one and less than or equal to five, followed by the letter H, followed by exactly n
characters, wh ich are the Hollerith data. Any FORTRAN character, including blank, is acceptable.
The Hollerith constants are used only in CALL and DATA statements and must be associated with real
variable names. The Hollerith constants are packed in 7-bit ASCII, five per two words of storage with
the righmost bit a Iways zero. Examples:
1HA
4H A$C

2. 2

VARIABLES
A variable is a symbolic representation of a numeric quantity whose values may change during

the execution of a program either by assignment or by computation. The symboPs representation, or name,
of the FORTRAN variable consists of one to six alphanumeric (alphabetic and numeric) characters, the
first of which must be alphabetic. Example:
X=Y+10.

Both X and Yare variables; X by computation, and Y by assignment
in some previous statement.

TEST
GAMMA

X 12345

2.2. 1

Variable Types
Variables in FORTRAN may represent one of the following types of quantities: integer, real,

double-precision, or logical. This corresponds to the type of constant the variable is supposed to represent.

2.2.2

Integer Variables
Variable names beginning with the letters I, J, K, L, M, or N are considered to be integer

variables. If the first letter is not one of the above letters, it is an integer variable only if it was named
in a previous integer type specification statement.

2.2.3

Real Variables
Variable names beginning with letters other than I, J, K, L, M, or N are considered to be real

variables. If the first character is one of the above letters, it is a real variable only if it was named in a
previous real type specification statement.

PDP-9 FORTRAN IV

2.2.4

Double-Precision and Logical Variables
A type specification statement' is the only way to assign a variable value to one of these two

types. This is done with either a double precision statement or a 10giceJI statement.

2.3

ARRAYS AND SUBSCRIPTS
An array is an ordered set of data identified by a symbolic nl:lme. Each individua I quantity

in th is set of data is referred to in terms of its position with in the array" This identifier is ca lied a subscript. For example,

A (3)
represents the third element in a one-dimensional array named A. To gleneralize further, in an array A
with n elements, A (I) represents the Ith element of the array A where I = 1, 2, .•• , n.
FORTRAN allows for one-, two-, and three-dimensional arr1oys, so there can be up to three
subscripts for the array, each subscript separated from the next by a comma. For example,
B (1, 3)
represents the va lue located in the first row and the th ird col umn of a two-dimensiona I array named B.
A dimension statement defining the size of the array (i .e., the maximum values each of its subscripts
can attain) must precede the array in the source program.

2.3.1

Arrangement of Arrays in Storage
Arrays are stored in column order in ascending absolute storage Ie cations. The array is stored

with the first of its subscripts varying most rapidly and the last varying least rapidly. For example, a
three-dimensional array A, defined in a DIMENSION statement as A(2,r 2, 2) will be stored sequentially
in th is order:

A( 1, 1, 1)
A(2, 1, 1)
A(l ,2, 1)
A(2, 2, 1)
A(l, 1,2)
A(2, 1,2)
A(l, 2, 2)
A(2, 2, 2)

ascending Clbsolute
storage locations

PDP-9 FORTRAN IV

2.3.2

Subscript Expressions
Subscripts may be written in any of the following forms:

V
C
V+k
V-k
C*V
C*V+k
C * V - k
where C and k represent unsigned integer constants and V represents an unsigned integer variable. Example:

I
13
IMOST + 3
ILAST - 1
5 * IFIRST
2* J+9
4 * M1 - 7

2.3.3

Subscripted Variables
A subscripted variable is a variable name followed by a pair of parentheses which contain

one to three subscripts separated by commas. Example:

A (1)
B (I, J - 3)
BETA (5 * J + 9, K + 7, 6 * JOB)

2.4

EXPRESSIONS
An expression is a combination of elements (constants, subscripted or nonsubscripted variables,

and functions) each of which is related to another by operators and parentheses. An expression represents
one single value which is the result of the calculations specified by the values and operators that make
up the expression. The FORTRAN language provides two kinds of expressions: arithmetic and logical.

2.4. 1

Arithmeti c Expressions
An arithmetic expression consists of arithmetic elements joined by the arithmetic operators +,

-, *, I, and **, which denote addition, subtraction, multiplication, division, and exponentiation,
respectively. An expression may consist of a single element (meaning a constant, a variable, or a
function name). An expression enclosed in parentheses is considered a single element. Compound
expressions use arithmetic operators to combine single elements.

PDP-9 FORTRAN IV

2.4. 1 . 1

Mode of an Expression - The type of quantities making up an expression determine its mode;

i • e., a simple expression consisting of an integer constant or an integer variable is said to be in the
integer mode. Similarly, real constants or variables produce a real mode of expression, and doubleprecision constants or variables produce a double-precision mode. The mode of an arithmetic expression
is important because it determines the accuracy of the expression.
In general, variables or constants of one mode cannot be combined with variables or constants
of another mode in the same expression. There are, however, exceptions to this rule.
a. The following examples show the modes of the valid arithmetic expressions involving
the use of the arithmetic operators +, -, *, and I. I, R, and D indicate integer, real,
and double-precision variables or constants. A + is used to Indicate anyone of the four
operators:

I+I

Integer resu It

R+R

Real result

R+D~
D+R
Double-prec ision resu It
D+D
b. When raising a value to a power, the mode of the power may be different than that
of the value being raised. The following examples show the modes of the valid arithmetic expressions using the arithmetic operator **. As above, I, R, and D indicate
integer, real, and double-precision.
1**1

Integer result

R**I
R**R

Real result

~::~}

D**R
D**D

Double-precision result

The subscript of a subscripted variable, which is a Iways an integer quantity, does not
affect the mode of the expression.

2.4.1.2

Hierarchy of Operations - The order in which the operations of an arithmetic expression are

to be computed is based on a priority rating. The operator with the higlhest priority takes precedence
over other operators in the expression. Parentheses may be used to detE~rmine the order of computation.

If no parentheses are used, the order is understood to be as follows:

PDP-9 FORTRAN IV

a.
b.
c.
d.
e.

Function reference
** (Exponentiation)
Unary minus evaluation
* and /(multiplication and division)
+ and -(addition and subtraction)

Within the same priority, operations are computed from left to right. Example:
FUNC + A*B/C-D(I,J) + E**F*G-H
interpreted as,
FUNC+ ((A*B)/C)-D(I,J) + (EF*G)-H

2.4. 1 .3

Rules for Constructing Arithmetic Expressions a. Any expression may be enc losed in parentheses.
b. Expressions may be preceded by a + or -- sign.
c. Simple expressions may be connected to other simple expressions to form a compound
expression, provided that:
(1) No two operators appear together.
(2) No operator is assumed to be present.
d. Only valid mode combinations may be used in an expression (described under Mode
of an Expression, section 2.4.1.1).
e. The expression must be constructed so that th~ priority scheme determines the order
of operation desired (described in section 2.4. 1 .2, Hierarchy of Operations).
Examples of arithmetic expressions follow:

A(I)
B + 7.3
C*D
A + (B*C) - D**2 + E/F

2.4.2

Re lationa I Expressions
A relational expression is formed with the arithmetic expressions separated by a relational

operator. The result value is either true or false (depending upon whether the condition expressed by
the relational operator is met or not met. The arithmetic expressions may both be of the integer mode
or they may be a combination of real and/or double-precision. No other mode combinations are legal.
The relational operators must be preceded and followed by a decimal point. They are:
9

PDP-9 FORTRAN IV

• LT •
.LE.
.EQ.
• NE.
.GT.
.GE.

Less than «)
Less than or equa I to (S)
Equal to (=)
Not equa I to (1=)
Greater than ~»
Greater than or equa I to (»

Examples:

N • LT.5
DELTA + 7.3 .LE. B/3E7
(KAPPA + 7/5 .NE. IOTA
1 • 736D-4. GT. BETA
X. GE. y* Z**2

2.4.3

Logica I Expressions
A logical expression consists of logical elements joined by logical operators. The value is

either true or false. The logical operator symbols must be preceded and followed by a decimal point.
They are:

2.4.3.1

.NOT.

Logical negation. Reverses the state of the logical quantity
that follows.

.AND.

Logical AND generates a logical result (TRUIE of FALSE) determined by two logical elements as follows:
T .AND. T generates T
T .AND. F generates F
F .AND. T generates F
F .AND. F ~~enerates F

.OR.

Logical OR generates a logical result determiined by two logical
elements as follows:
T • OR. T generates T
T • OR. F generates T
F • OR. T generates T
F • OR. F generates F

Rules for Construction Logical Expression
a. A logical expression may consist of a logical constant, a logical variable, a reference
to a logical function, a relational expr~ssion, or a complex logical expression enclosed
in parentheses.
b. The logical operator. NOT. need only be followed by a logical expression, while
the logica I operators .AND. and. OR. must be both preceded and followed by a
logical expression for form more complex logical expressions ..

PDP-9 FORTRAN IV

c. Any logica I expression may be enc losed in parentheses. The logica I expression
following the logical operator. NOT. must be enclosed in parentheses if it contains
more than one quantity.
d. No two logica r operators may appear in sequence, not separated by a comma or
parenthesis unless the second operator is • NOT. In addition, no two decimal points
may appear together, not separated by a comma or parenthesis, unless one belongs
to a constant and the other to a relational operator.

2.4.3.2

Hierarchy of Operations - Parentheses may be used as in normal mathematical notation to

specify the order of operations. Within the parentheses, or where there are no parentheses, the order
in which the operations are performed is as follows:
a. Eva luation of functions
b. ** (Exponentiation)
c. Evaluation of unary minus quantities
d. * and /(multiplication and division)
e. + and -(addition and subtraction)

• LT., • LE. , • EO., • NE., • GT., .GE.

g . .NOT.
h.

•AND. and .OR.

. = Replacement operator
I.

Unlike an arithmetic expression where sequence of elements of the same priority (i.e., operations being performed from left to right) is important for the end result of the expression, the order of
operation within the same priority in logical and relational expressions is unimportant.

2.5

STATEMENTS
Statements specify the computations required to carry out the processes of the FORTRAN

program. There are four categories of statements provided for by the FORTRAN language:
a. Arithmetic statements define a numerical calculation.
b. Control statements determine the sequence of operation in the program.
c. Input/output statements are used to transmit information between the computer
and related input/output devices.
d. Specification statements define the properties of variables, functions, and arrays
appearing in the source program. They a Iso enable the user to control the allocation
of storage.

PDP-9 FORTRAN IV

CHAPTER 3
ARITHMETIC STATEMENTS

An arithmetic statement is a mathematical equation written in the FORTRAN language which
defines a numerical or logical calculation. It directs the assignment of a calculated quantity to a given
variable. An arithmetic statement has the form

V= E
where V is a variable (integer, real, double-precision, or logical, subscripted or nonsubscripted) or any
array element name; = means replacement rather than equivalence, as opposed to the conventional mathematical notation; and E is an expression.
In some cases, the mode of the variable may be different from that of the expression. In such
cases an automatic conversion takes place. The rules for the assignment of an expression E to a variable
V are as follows:
E Mode

V Mode

Assignment Rule

Integer

Assign

Integer

Real

Fix and assign

Integer

Double-precision

Fix and assign

Real

Integer

Float and assign

Real

Assign

Real

Double-precision

Double-precision evaluate and real assign

Doubleprecision

Integer

Double-prec ision float
and assign

Doubleprecision

Real

Doul:-Ie-prec ision eva 1uate and assign

Doubleprecision

Double-prec ision

Assign

Logical

Assign

Mode conversions involving logical quantities are illegal unless the mode of both V and E
is logical. Examples of an assignment statement:
ITEM = ITEM + 1
A(I) = B(I) = ASSIN (C (I) )
V = .FALSE.
X =A.GT.B .AND. C .LE. G

A= B

PDP-9 FORTRAN IV

CHAPTER 4
CONTROL STATEMENTS

The statements of a FORTRAN program normally are executed as written. However, it is
frequently desirable to alter the norma I order of execution. Control statements give the FORTRAN user
this capability. This section discusses the reasons for control statements and the ways in which they may
be used.

4.1

UNCONDITIONAL GO TO STATEMENTS
The form of the unconditional GO TO statement is
GO TO n

where n is a statement number. Upon the execution of th is statement, control is transferred to the statement identified by the statement number, n, which is the next statement to be executed. Example:
GO TO 17
4.2

ASSIGN STATEMENT
The general form of an ASSIGN statement is
ASSIGN n TO i

where n is a statement number and i is a nonsubscripted integer variable name which appears in a subsequently executed assigned GO TO statement. The statement number, n, is the statement to wh ich
control will be transferred after the execution of the assigned GO TO statement. Example:
ASSIGN 27 TO ITEST
4.3

ASSIGNED GO TO STATEMENT
Assigned GO TO statements have the form

where i is an nonsubscripted integer variable reference appearing in a previously executed ASSIGN statement, and n , n , ..•• , nm are the statement numbers which the ASSIGN statement may legally assign
1 2
to i. Examples:
ASSIGN 13 TO KAPPA
GO TO KAPPA (1, 13, 72, 100, 35)
There is no object time checking to ensure that the assignment is one of the legal statement
numbers.
15

POP-9 FORTRAN IV

4.4

COMPUTED GO TO STATEMENT
The format of a computed GO TO statement is
GO TO (n l' n , •••• , n ), i
2
m

where n l' n , •••• , nm are statement numbers and i is an integer vari<:Jble reference whose va lue is
2
greater than or equal to 1 and less than or equal to the number of statement numbers enclosed in parentheses. If the value of i is out of this range, the statement is effectively a CONTINUE statement.
Example:
GO TO (3, 17, 25, 50, 66), ITEM

If the value of ITEM is 2 at the time this GO TO statement is executed, the statement to which control
is transferred is the statement whose number is second in the series, i.e., statement number 17.

4.5

ARITHMETIC IF STATEMENT
The form of the arithmetic IF statement is

where e is an arithmetic expression and n , n , n3 are statement numbers. The IF statement evaluates
1 2
the expression in parentheses and transfers control to one of the referenc:ed statements. If the value of
the expression (e) is less than, equal to, or greater than zero, control is transferred to n l' n , or n3

respectively. Example:
IF (AUB (I) - B*O) 10, 7, 23

4.6

LOGICAL IF STATEMENT
The general format of a logical IF statement is
IF (e) s

where e is a logical expression and s is any executable statement other than a DO statement or another
logical IF statement. The logical expression is evaluated, and different statements are executed depending on whether the expression is true or false. If the logical expres:sion e is true, statement s is
executed and control is then transferred to the following statement (unless the statement s is a GO TO
statement or an arithmetic IF statement, in which cases control is transferred as indicated; or the statement s is a CALL statement, in which case control is transferred to the n(;!xt statement after return from
the subprogram). If the logica I expression e is fa Ise, statement s is ignored and control is transferred
to the statement following the IF statement. Example:
16

PDP-9 FORTRAN IV

IF (L l) I = I + 1
IF (L.LE.k) GO TO 17
IF (LOG.AND. (.NOT.LOGl)) IF (X) 3,5,5

4.7

DO STATEMENT
The DO statement is a command to execute repeatedly a specified series of statements. The

general format of the DO statement is

where n is a statement number representing the termina I statement or the end of the "range"; i is a nonsubscripted integer variable known as the II index"; and m l' m , and m3 are unsigned nonzero integer

constants or nonsubscripted integer variables, which represent the "initial, II "final, II and "increment"
values of the index. If m3 is omitted, as in the second form of the DO statement, its value is assumed
to be 1.
The DO statement is a command to execute repeatedly a group of statements following it up to
and including statement n. The initial value of i is m (m must be less than or equal to m ). Each suc2
1 1
ceeding time the statements are operated, i is increased by the value of m • When i is greater than m ,
3
2
control passes to the statement following statement number n.
The range of a DO statement is a series of statements to be executed repeatedly. It consists
of all statements immediately following the DO, up to and including statement n. Any number of statements may appear between the DO and statement n. The terminal statement (statement n) may not be a
GO TO (of any form), an arithmetic IF, a RETURN, a STOP, a PAUSE, or a DO statement, or a logical
IF statement containing any of these forms.
The index of a DO is the integer variable i wh ich is controlled by the DO statement in such a
way that its initial value is set to m ' and is increased each time the range of statements is executed by
1
m , until a further incrementation would cause the va lue of m to be exceeded. Throughout the range
2
3
of the DO, the index is available for computation either as an ordinary integer variable or as the variable
of a subscript. However, the index may not be changed by any statement within the DO range.
The initial value is the value of the index at the time the range is executed for the first time.
The final value is the value which the index must not exceed. When the condition is satisfied the DO is completed and control passes to the first executable statement following statement n.

PDP-9 FORTRAN IV

The increment is the amount by which the index is to be incrElased after each execution of
the range. If the increment is omitted, a value of 1 is implied. Examplle:
DO 72 I = 1, 10, 2
DO 15K = 1, 5
DO 23 I = 1, 11, 4
Any FORTRAN statement may appear with in the range of a DO statement, inc luding another
DO statement. When such is the case, the range of the second DO musl" be contained entirely within
the range of the first; i.e., it is not permissible for the ranges of DOs tel overlap. A set of DOs satisfying this rule is called a nest of DOs. It is possible for a terminal statement to be the terminal statement
for more than one DO statement. The following configuration, where brackets are used to represent the
range of the DOs, indicates the permissible and i lIega I nesting procedures.

ILLEGAL , . . - - - - - - - - DO

PERMISSIBLE , . . - - - - - - - DO

ir-=°O
oO

, . . - - - - - - - DO

LL
Transfer of control from within the range of a DO statement tC) outside its range is permitted
at any time. However, the reverse is not true; i. e., control cannot be "transferred from outside the
range of a DO statement to inside its range. The following examples show both valid and invalid transfers.

VALID

INVALID

[~
18

4.8

CONTINUE STATEMENT
The CONTINUE statement causes no action and generates no machine coding. It is a dummy

statement which is used for terminating DO loops when the last statement would otherwise be an illegal
terminal statement (i .e., GO TO, arithmetic IF, RETURN, STOP, PAUSE, or DO I or a logical IF containing any of these forms). The form consists of the single word
CONTINUE

4.9

PAUSE STATEMENT
A PAUSE statement is a temporary halt of the program at run time. The PAUSE statement has

one of the two forms
PAUSE
or
, PAUSE n
where n is an octal integer whose value is less than 777777 • The integer n is typed out on the console
8
Teletype for the purpose of determining which of several PAUSE statements was encountered. Program
execution is resumed by operator intervention, starting with the first statement following the PAUSE
statement.

4.10

STOP STATEMENT
The STOP statement is of one of the forms
STOP

or
STOP n
where n is an octa I integer whose va lue is less than 777777 • The STOP statement is placed at the
8
logical end of a program and causes the computer to type out on the console Teletype the integer nand
then to exit back to the Monitor. There must be at least one STOP statement per main program I but
none are allowed in subprograms.

4. 11

END STATEMENT
The END statement is placed at the physical end of a program or subprogram. The form con-

sists of the single word
END

PDP-9 FORTRAN IV

The END statement is used by the compiler and generates no code. It signals the compiler
that the processing of the source program is complete.
A control transfer type statement must precede END. This wii II be checked by the compi ler.

PDP-9 FORTRAN IV

CHAPTER 5
INPUT/OUTPUT ST_ATEME NTS

The input/output (I/O) statements direct the exchange of data between the computer and I/O
devices. The information thus transmitted by an I/O statement is defined as a logical record, wh ich may
be formatted or unformatted. A logical record, or records, may be written on a device as one or more
physical records. This is a function of the size of the logical record(s) and the physical device used.
The definition of the data which comprises a user's optimum physical record varies for each
I/O device, as fol lows:

Unit
or
Device

Formatted Physica I
Record Definition

Unformatted
(Binary) Physica I
Record Definition

Typewriter
(input and output)

One line of type is terminated by
a carriage return. Maximum of
72 printing characters per line

Undefined

Line printer

One line of printing. Maximum
of 120 characters per line

Undefined

Cards
(input and output)

One card. Maximum of 80 characters

50 words

Paper tape
(input and output)

One line image of 72 printing
characters

50 words

Magnetic tape

One line image of 630 characters

252 words

Disc/drum/
DECtape

One line imagr of 630 characters

252 words

Each I/O device is identified by an integer constant which is associated with a device assignment table in the PDP-9 Monitor. This table may be modified at system generation time, or just
before run time. For example, the statement
READ (u,f) list
requests one logical record from the device associated with slot u in the device assignment table.
The statement descriptions in this section use u to identify a specific I/O unit, f as the statement number of the FORMAT statement describing the type of data conversion, and I ist as a list of arguments to be input or output.

PDP-9 FORTRAN IV

5.1

GENERAL I/O STATEMENTS
These statements cause the transfer of data between the computer and I/O devices.

5. 1 . 1

Input/Output Argument Lists
An I/O statement which calls for the transmission of information includes a list of quantities

to be transmitted. In an input statement this list consists of the variables to which the incoming data is
to be assigned; in an output statement the list consists of the variables whose values are to be transmitted
to the given I/O device. The list is ordered, and the order must be that in which the data words exist
(input) or are to exist (output) in the I/O device. Any number of items may appear in a single list. The
same statement may transmit integer and rea I quantities. If the data to be transmitted exceeds the items
in the list, only the number of quantities equa I to the number of items in the list are transmitted. The
remaining data is ignored. Conversely, if the items in the list exceed the data to be transmitted, succeeding superfluous records are transmitted until all items specified in the list have been transmitted.

5. 1 • 1 • 1

Simple Lists - The list uses the form
C 1 'C 2 ' •... ,C

where each C. is a variable, a subscripted variable, or an array identifier. Constants are not allowed
I

as list items. The list reads from left to right. When an array identifier appears in the list, the entire
array is to be transmitted before the next item in the list. Examples of Simple Lists:

Y,Y,Z

A, B (3), C, D (I + 1 , 4)

5. 1 .1.2

DO-Implied Lists - Indexing similar to that of the DO statement may be used to control the

number of times a group of simple lists is to be repeated. The list elements thus controlled, and the
index control itself, are enclosed in parentheses, and the contents of the parentheses are regarded as
a single item of the I/O list. Example:

W, X (3), (Y (I), Z (I, K), I = 1, 10)

5.1.2

READ Statement
The READ statement is used to transfer data from any input device to the computer. The gen-

eral READ statement can be used to read either BCD or binary information. The form of the statement
determines what kind of input will be performed.

PDP-9 FORTRAN IV

5. 1 .2. 1

Formatted READ - The formatted READ statements have the genera I form
READ (u,f) list

or
READ (u, f)
Execution of this statement causes input from device u to be converted as specified by format statement f,
the resulting values to be assigned to the items specified by list, if any.

5. 1 .2.2

Unformatted READ - An unformatted READ statement has the general form
READ (u) list

or
READ (u)
Execution of this statement causes input from device u, in binary format, to be assigned to the items specified by
list. If no I ist is given, one record wi II be read, but ignored. If the record contains more information
words than the list requires, that part of the record is lost. If more elements are in the list than are in
one record, additional records are read until the list is satisfied. Example of READ:
READ (3,13) A,B,C
READ (2,10) A, (B (I), 1=1,5)
READ (1,3)
READ (5) I,J,K
READ (8)

5.1.3

WRITE Statement
The WRITE statement is used to transmit information from the computer to any I/O device.

The WRITE statement closely parallels the READ statement in both format and operation.

5. 1 .3. 1

Formatted WRITE - The formatted WRITE statement has the genera I form
WRITE (u,f) list

or
WRITE (u, f)
Execution of this statement causes the list elements, if any, to be converted according to format statement f, and output into device u.

5. 1 .3.2

Unformatted WRITE - The unformatted WRITE statement has the genera I form
WRITE (u) list

Execution of this statement causes output onto device u, in binary format, of all words specified by the
list. If the list elements do not fill the record, the remaining part of the record is filled with blanks.

PDP-9 FORTRAN IV

If the list elements more than fill one rE~cord, successive records are written until all elements of the
list are satisfied, the last record padded with blanks if necessary. Examples of WRITE:
WRITE (1,10) A, (B (I), (C (I,J), J=2,10,2), 1=1,5)
WRITE (2,7) A,B,C
WRITE (5) W ,X(3), Y(I + l,4),Z

5. 2

FORMAT STA TEME NTS
These statements are used in conjunction with the general I/O statements. They specify the

type of conversion which is to be performed between the internal machine language and the external
notation. FORMAT statements are not executed. Their function is to supply information to the object
program.

5.2.1

Specifying FORMAT
The general form of the FORMAT statement is
FORMAT (Sl' S2' •••• , Sn)

where S 1 •••• Snare data fie Id descripf'ions. Break ing th is format down further, the basic data field
descriptor is written in the form
nkw.d
where n is a positive unsigned integer indicating the number of successive fields for which the data conversion will be performed according to the same specification. This is ellso known as the repeat count.

If n is equal to 1, it may be omitted. The control character k indicates which type of conversion will
be performed. This character may be I,E,F,D,P,L,A,H, or X. The nonzero integer constant w specifies
the width of the field. The integer constant d indicates the number of digits to the right of the decimal
point.
Six of the nine control characters listed above provide for data conversion between internal
mach ine language and externa I notation.

Internal

External

Integer variable

Decimal integer

Real variable

Floating-point, scaled

Rea I variable

Floating-point

Double-precision
variable

Floating-point, scaled

Logical variable

Letter T or F

Alphanumeric

Alphanumeric (BCD) characters

PDP-9 FORTRAN IV

The other three control types are spec ia I purpose control characters:
Type

Purpose

Used to set a sea Ie factor for use with E, F, and D
conversions.

Provides for skipping characters in input or specifying blank characters in output.

Designates Hollerith fields

FORMAT statements are not executed and therefore may be placed anywhere in the source
program. Because they are referenced by READ or WRITE statements, each FORMAT statement must be
given a statement number.
Commas (,) and slashes (/) are used as field separators. The comma is used to separate field
descriptors, with the exception that a comma need not follow a field specified by an H or X control
character. The slash is used to specify the termination of formatted records. A series of slashes is also
a field separator. Multiple slashes are the equivalent of blank records between output records, or
records skipped for input records. If the series of n slashes occurs at the beginning or the end of the
FORMAT specifications, the number of input records skipped or blank lines inserted in output is n. If
the series of n slashes occurs in the middle of the FORMAT specifications, this number is n-l. A comma
may precede and/or follow a slash, but is not necessary. An integer value cannot precede a slash.
For all field descriptors (with the exception of H and X), the field width must be specified.
For those descriptors of the w.d type (see next page), the d must be specified even if it is zero. The
field width should be large enough to provide for all characters (including decimal point and sign) necessary to constitute the data va lue as we II as blank characters needed to separate it from other data va lues.
Since the data value within a field is right ju.stified, if the field specified is too small, the most significant characters of the va lue wi II be lost.
Successive items in the I/O list are transmitted according to successive descriptors in the
FORMAT statement, until the entire I/O list is satisfied. If the Iist contains more items than descriptors
in the FORMAT statement, a new record must be begun. Control is transferred to the preceding left
parenthesis where the same spec ifications are used again unti I the Iist is complete.
Field descriptors (except H and X) are repeated by preceding the descriptor with an unsigned
integer constant (the repeat count). A group repeat count is used to enable the repetition of a group of
field descriptors or field separators enclosed in parentheses. The group count is placed to the left of the
parenthesis. Two levels of parentheses (not including those enclosing the FORMAT specification) are
permitted.

PDP-9 FORTRAN IV

The field descriptors in the FORMAT must be the same type as the corresponding item in the
I/O list; i.e., integer quantities require integer (I) conversion; real qucmtities require real (E or F) conversion, etc. Example:
FORMAT (17, FlO. 3)
FORMAT (13, I7/E10.4, E10.4)
FORMAT (214, 3(15, D 10.3))

5 .2.2

Conversion of Numeric Data

5.2.2.1

I-Type ConversionField descriptor: Iw or nIw
The number of characters specified by w is'converted as a decimal integer.
On input, the number in the input field by w is converted to a binary integer. A minus sign

indicates a negative number. A plus sign, indicating a positive number, is optional. The decimal point
is illegal. If there are blanks, ,they must prece'de the sign or first digit. All imbedded blanks are interpreted as zero digits.
On output, the converted number is right justified. If the nUlmber is sma lIer than the field w
allows, the leftmost spaces are filled with blanks. If an integer is too I,arge, the most significant digits
are truncated and lost.

Negative numbers have a minus sign just preceding their most significant digit

if suffic ient spaces have been reserved. No sign indicates a positive number. Examples (b indicates
blank):
Format
Descriptor

5.2.2.2

Internal

Output

bbbbb

+00000

bbbbO

-b5

-05

b-5

bbb12345

+12345

bbb12345

E-Type Conversion Field descriptor: Ew. d or nEw. d
The number of characters specified by w is converted to a floating~point number with d spaces

reserved for the digits to the right of the decimal point. The w includes field d, spaces for a sign, the
decimal point, plus four spaces for the exponent (written E ± XX) in addition to space for optional sign
and one digit preceding the decimal point.

PDP-9 FORTRAN IV

The input format of an E-type number consists of an optiona I sign, followed by a string of
digits containing an optiona I decima I point, followed by an exponent. Input data can be any number of
digits in length, a Ithough it must fa II within the range of 0 to ± 10±39.
E output consists of a minus sign if negative (blank if positive), the digit 0, a decima I point,
a string of digits rounded to d significant digits, followed by an exponent of the form E ± XX. Examples:

5.2.2.3

Format
Descriptor

Input

E10.4

00.2134E03

213.4

0.2134E+03

E9.4

0.2134E02

21.34

.2134E+02

E10.3

bb-23.0321

-23.0321

-0.230E+02

Interna I

Output

F-TypeConversionField descriptor: Fw.d or nFw.d
The number of c~aracters specified by w is converted as a floating-point mixed number with d

spac~s reserved for the digits to the right of the decimal point.
Input for F:"'type conversion is basica IIy the same as that for E-type conversion, described
above.
The output consists of a minus sign if the number is negative (blank if positive), the integer
portion of the number, a decimal point, and the fractional part of the number rounded to d significant
digits. Examples:

Format
Descriptor

5.2.2.4

Internal

Output

F6.3

b13457

13.457

F6.3

313457

313.457

13.457

F9.2

-21367.

-21367.00

F7.2

-21367.

1367.00

D-Type Conversion Field descriptor: Dw.d or nDw.d
The number of characters specified by w is converted as a double-precision floating-point

number with the number of digits specified by d to the right of the decimal point.

PDP-9 FORTRAN IV

The input and output are the same as those for E-type conversion except that a D is used in
place of the E in the exponent. Examples:

5.2.3

Format
Descriptor

Input

D12.6

bb+21 345 D 03

21.345

0.213450D+02

D12.6

b+3456789012

3456.789012

0.345678D+04

D12.6

-12345 .6D-02

-123.456

O. 123456D+03

Interna I

Output

P-Sca Ie Factor
Field descriptor: nP or -nP
This scale factor n is an integer constant. The scale factor has effect only on E-, F-, and

D-type conversions. Initially, a scale factor of zero is implied. Once a P field descriptor has been
processed, the scale factor established by n remains in effect for all subsequent E, F, and D descriptors
within the same FORMAT statement until another scale factor is encountered.
For F, E, and D input conversions {when no exponent exists in the external field} the scale
factor is defined as externa I quantity = interna I quantity x 1On •
The scale factor has no effect if there is an exponent in the external field.
The definition of sca Ie factor for F output conversion is the same (lS it is for F input. For E
n
and D output, the fractional part is multiplied by 10 and the exponent is reduced by n. Examples:
Format
Descriptor

Input

Scale
Factor

Internal

F6.3

123456

-3

+123456.

23.456

E12.4

123456

-3

+12345.6

bbO.0001E+08

D10.4

12.3456

+1 .23456

1.2345D+00

5.2.4

Conversion of Alphanumeric Data.

5.2.4. 1

A-Type Conversion {7-Bi't ASCII, Handled As REAL Variables} -

Output

Field descriptor: Aw or nAw
The number of alphanumeric characters specified by w is transmitted according to list
spec ifications.
If the field width specified for A input is greater than or equal to five {the number of char-

acters representable in two mach ine words}, the rightmost five characters arE! stored interna lIy. If w is
less than five, 5-w trailing blanks are added.

PDP-9 FORTRAN IV

For A output, if w is greater than five, w-5 leading blahks are output followed by.five alphanumeric characters. Ifw is less than or equal to five, the leftmost w characters are output.

5.2.4.2

H-Field Descriptor (7-Bit ASCII) Field descriptor: nHa a a ••• a
1 2 3
n
The number of characters specified by n immediately following the H descriptor are transmitted

to or from the external device. Blanks may be included in the alphanumeric string. The value of n must
be greater than o.
On Hollerith input, n characters read from the external device replace the n characters following the letter H.
In output mode, the n characters following the letter H, inc luding blanks, are output.
Examples:
3HABC
17H THIS IS AN ERROR
16H JANUARY 1, 1966

5.2.5

Logical Fields, L Conversion
Field descriptor: Lw or nLw
The external format of a logical quantity is T or F. The internal format is 7777778 for T or

o for F.
On L input, the first nonblank character must be a T or F. Leading blanks are ignored. A
nonblank character is illegal.
For L output, if the internal value is 0, an F is output. Otherwise a T is output. The For T
is preceded by w-1 leading blanks.

5.2.6

Blank Fields, X Conversion
Field descriptor: nX
The value of n is an integer number greater than o. On X input, n characters are read but

ignored. On X output, n spaces are output.

5.2.7

FORTRAN Statements Read in at Object Time
FORTRAN provides the facility of including the formatting data along with the input"data.

This is done by using an array name in place of the reference to a FORMAT statement label in any of
the formatted I/O statements. For an array to be referenced in such a manner, the name of the variable
FORMAT specification must appear in a DIMENSION statement, even if the size of the array is 1 •

PDP-9 FORTRAN IV

The statements have the genera I form:
READ (u, name)
READ (u, name) list
WRITE (u, name)
WRITE (u, name) list
The form of the FORMAT specification which is to be inserted in the array is the same as the source program FORMAT statement, except that the word FORMAT is omitted and the nH field descriptor may not
be used. The FORMAT specification may be inserted in the array by usiing a data initialization statement:
or by using a READ statement together with an A format.
For example, this fac i lity can be used to specify at object time the format of a deck of cards to be
read. The first card of the deck wou Id contain the format statement,

'10

(I7,FIO.3)
the subsequent cards would contain delta in the general form,

(
13

xxxx

DIMENSION AA (10)
FORMAT (10A5)
READ (3, 13) (AA(I) , I = 'I, 10)

READ (3,AA) JJ,BOB
With the card reader assigned to device number 3, the first READ places the format statement
from the first card into the array AA, and the second READ statement cCluses data from the subsequent
cards to be read into JJ and BOB with format specifications 17 and FlO. 3 respectively.

5.2.8

Printing of a Formatted Record
When formatted records are prepared for printing, the first character of the record is not

printed. The first character is used instead to determine vertical spacing as follows:
Character

Vertical Spacing Before Printing

Blank

One line
Two lines
Skip to first I ine of next page
No advance

o
1
+

Output of formatted records to other devices considers the first charactelr as an ordinary character in
the record.
30

PDP-9 FORTRAN IV

5.3

AUXILIARY 1/0 STATEMENTS
These statements manipulate the 1/0 file oriented devices. The u is an unsigned integer

constant or integer variable specifying the device.

5.3.1

BACKSPACE Statement
The BACKSPACE statement has the general form
BACKSPACE u

Execution of this statement causes the 1/0 device identified by u to be positioned so that the record
which had been the preceding record becomes the next record. If the unit u is positioned at its initial
point, execution of th is statement has no effect.

5.3.2

REWIND Statement
The REWIND statement has the genera I form
REWIND u

Execution of this statement causes the I/O device identified by u to be positioned at its initial point.

5.3.3

ENDFILE Statement
The ENDFILE statement has the general form
ENDFILE u

Execution of this statement causes an endfile record to be written on the I/O device identified by u.

PDP-9 FORTRAN IV:

CHAPTER 6
SPECIFICATION STATEMENTS

Specification statements are nonexecutable because they do not generate instructions in the
object program. They provide the compiler with information about the nature of the constants and variables used in the program. They also supply the information required to allocate locations in storage for
certain variables and/or arrays. All SPECIFICATION statements, with the exception of the DATA
statement, must appear before any executable code generating statement. They must appear in this
order: type statements, DIMENSION statements, COMMON statements, and EQUIVALENCE statements.

EXTERNAL statements may appear anywhere after all type statements and before the executable

code generat i ng statements. The DATA statements may appear anywhere in the source program.

6.1

TYPE STATEMENTS
The type statements are of the forms
INTEGER a, b, c
REAL a, b, c
DOUBLE PRECISION a, b, c
LOGICAL a, b, c

where a, b, and c are variable names which may be dimensional or function names. A type statement
is used to inform the compiler that the identifiers listed are variables or functions of a specified type,
i.e., INTEGER, REAL, etc. It overrides any implicit typing; i.e., identifiers which begin with the
letters I, J, K, L, M, or N are implicitly of the INTEGER mode; those beginning with any other letter
are impl ic itly of the REAL mode. The type statement may be used to supply dimension information.
The variable or function names in each type statement are defined to be of that specific type throughout the program; the type may not change.

Examples:

INTEGER ABC, UK, XYZ
REAL A (2, 4), I, J, K
DOUBLE PRECISION ITEM, GROUP
LOGICAL TRUE, FALSE

6.2

DIMENSION STATEMENT
The DIMENSION statement is used to declare arrays and to provide the necessary informa-

tion to allocate storage for them in the object program.

PDP-9 FORTRAN IV

The general form of the DIMENSION statement is:

where each V is the name of an array and each i is composed of one, two, or three unsigned integer
constants separated by commas.

The number of constants represents the number of dimensions the array

contains; the value of each constant represents the maximum size of each dimension. If the dimension
information for the variable is given in a type statement or a COMMON statement, it must not be included in a DIMENSION statement. Example:
DIMENSION ITEM (150), ARRAY (50, 50)
When arrays are passed to subprograms, they must be redeclared in the subprogram. The mode, number
of dimensions, and size of each dimension must be the same as that dec lared by the call ing program.

6.3

COMMON STATEMENT
The COMMON statement provides a means of sharing memory storage between a program

and its subprograms. The general form of the COMMON statement is:

where each x is a variable which is a COMMON block name, or it can be blank. If xl is blank, the
first two slashes are optional. Each a represents a I ist of variables and arrays separated by commas.
The list of elements pertaining to a block name ends with a new block name, with a blank COMMON
block designation (two slashes), or the end of the statement.
The elements of a COMMON block, which are listed following the COMMON block name
(or the blank name), are located sequentially in order of their appearance in the COMMON statement.
An entire array is assigned in sequence. Block names may be used more than once in a COMMON
statement, or may be used in more than one COMMON statement within the program. The entries so
assigned are strung together in the given COMMON block in order of their appearance. Labeled
COMMON blocks with the same name appearing in several program,s or subprograms executed together
must contain the same number of total words. The elements within the blocks, however, need not
agree in name, mode, or order. A blank COMMON may be any length. Examples:
COMMON A, B, C/XX/X, Y, Z
COMMON/A/X{3, 3), Y{2, 5)//Z{5, 10, 15)

PDP-9 FORTRAN IV

The COMMON statement is a means of transferring data between programs. If one program
contains the statements,
COMMON /N/AA,BB,CC
AA=3
BB=4
CC=5
and another program which is called later contains the statement,
COMMON /N/XX, VY,ZZ
then the latter program will find the values 3,4, and 5 in its variables XX, VY, and ZZ, respectively,
since variables in the same relative positions in COMMON statements shc~ the same registers in memory.

6.4

EQUIVALENCE STATEMENT
The EQUIVALENCE statement is used to permit two or more entities of the same size and

type to share the same storage location. The general format of the EQUIVALENCE statement is:

where each k represents a Iist of two or more variables or subscripted vari,cbles separated by commas.
Each element in the list is assigned the same memory storage location.
An EQUIVALENCE statement may lengthen the size of a COMMON block. The size can·
only be increased by extending the COMMON block beyond the last assignment for that block made
directly by a COMMON statement. A variable cannot be made equivalemtto an element of an array
if it causes the array to extend past the beginn ing of the COMMON block.
When two variables or array elements share the same storage Ic)cation because of the use of
an EQUIVALENCE statement, they may not both appear in COMMON statements within the same
program. Example:
EQUIVALENCE (A, B), (C(10), D(10), E(15))

6.5

EXTERNAL STATEMENT
An EXTERNAL statement is used to pass a subprogram name on to another subprogram. The

general form of an EXTERNAL statement is:
EXTERNAL y, z, •••
Example:

EXTERNAL ISUM, ISUB

PDP-9 FORTRAN IV

6.6

DATA STATEMENT
The DATA statement is used to set variables or array elements to initial values at the time

the object program is loaded. The general form of the DATA initialization statement is:

where each k is a list of variables or array elements (with constant subscripts) separated by commas, and
each d is a corresponding list-of constants with optional signs. The k list may not contain dummy
arguments. There must be a one-to-one correspondence between the name I ist and the data list, except
where the data I ist consists of a sequence of identical constants. In such a case, the constant need be
written only once, preceded by an integer constant indicating the number of repeats and an asterisk.
A Hollerith constant may appear in the data list.
Variable or array elements appearing in a DATA statement may not be in blank COMMON.
They may be in a labeled COMMON block and initially defined only in a BLOCK DATA subprogram.
Example:
DATA A, B, C/3*2.0/
2

DATA X(l), X(2), X(3), X(4)/0.0, 0.1, 0.2, 0.3/Y(]), Y(2)
Y(3), Y(4)/1.0E2, 1.0E-2, 1.0E4, 1.0E-4/

PDP-9 FORTRAN IV

CHAPTER 7
SU BPROGRAMS

A subprogram is a series of instructions wh ich another program uses to perform complex or
frequently used operations. Subprograms are stored only once in the computer, regardless how many
times they are referred to by another program.
There are five categories of subprograms:
a.
b.
c.
d.
e.

Statement Functions
Intrinsic or Library Functions
External Functions
External Subroutines
Block Data Subprograms

The first three categories of subprograms are referred to as functions. The fourth category is
referred to as subroutines. Functions and subroutines differ in the following two respects. Functions can
return only a single value to the call ing program; subroutines can return more than one value.

Functions

are called by writing the name of the function and an argument I ist in a standard arithmetic expression;
subroutines are called by using a CALL statement. The last category is a spec ial purpose subprogram
used for data in itial ization purposes.

7.1

STATEMENT FUNCTIONS
A statement function is defined by a sing Ie statement sim i lar in form to that of an arithmetic

assignment statement. It is defined internally to the program un it by wh ich it is referenced. Statement
functions must follow all spec ification statements and precede any exectuable statements of the program
un it of wh ich they are a part. The general format of a statement function is:
f (a , a , ••• , an) = e
1
2
where f is a function name; the a's are nonsubscripted variables, known as dummy arguments, wh ich are
to be used in evaluating the function; and e is an expression.
The value of a function is a real quantity unless the name of the function begins with I, J,
K, L, M, or N; in wh ich case it is an integer quantity, or the function type may be defined by using the
appropriate spec ification statement.
Since the arguments are dummy variables, their names are unimportant, except to indicate
mode, and may be used elsewhere in the program, including within the expression on the right side of
the statement function.

PDP~9 FORTRAN IV

, The expression- of a statement function, in addition to containing nonsubscripted dummy
arguments, may only contain:a.
b.
c.
d.
e.

Non- Hollerith constants
Variable references
Intrinsic function references
References to previously defined statement functions
External function references

A statement function is called any time the name of the- function appears in any FORTRAN arithmetic
expression. The actual arguments must agree in order, number, and type with the corresponding dummy
arguments.
Execution of the statement function reference resu Its in the computations ind icated by the
function defin ition. The resu Iting quantity is used in the expression wh ich contains the function reference. Examples:
A(X) = 3.2+SQRT (5.7* X**2)
SUM (A, B, C) = A+B+C
FUNC (A, B) ~ 3. * A/B**2 .+Z

7.2

INTRINSIC OR LIBRARY FUNCTIONS
Intrinsic orl ibrary functions are predefined subprograms that are a part of the FORTRAN

system library. The type of each intrinsic function and its arguments are predefined and cannot be
changed.
An intrinsic function is referenced by using its function name with the appropriate arguments
in an arithmetic statement. The arguments may be arithmetic expressions, subscripted or simple variables,
constants, or other intrinsic functions (see table 1).

PDP-9 FORTRAN IV

TABLE 1

Intrinsic Functions

Absolute value

INTRINSIC FUNCTIONS

Definition

No. of
Arguments

Symbolic
Name

Type of
Argument

Type of
Function

ABS
lABS
DABS

Real
Integer
Double

AINT
INT
IDINT

Real
Real
Double

Real
Integer
Integer

AMOD
MOD

Real
Integer

Ia I

Truncation

Sign of a times largest
integer'::; a

Remaindering*

0 (mod O )
1
2

Choosing largest
value

Max (0 , O ,

... )

AMAXO
AMAXI
MAX 0
MAXI
DMAXI

Integer
Real
Integer
Real
Double

Real
Real
Integer
Integer
Double

Choosing smallest
value

Min (0 , O , ••. )

AMINO
AMINI
MINO
MINI
DMINI

Integer
Real
Integer
Real
Double

Real
Real
Integer
Integer
Double

Float

Conversion from
integer to real

FLOAT

Integer

Real

Fix

Conversion from real
to integer

IFIX

Real

Integer

Transfer of sign

Sign of O

SIGN
ISIGN
DSIGN

Real
Integer
Double

DIM
IDIM

Real
Integer

Obtain most significant part of double
precision argument

SNGL

Double

Real

Express sing Ie prec ision argument in
double precision
form

DBLE

Real

Double

Positive difference

I I

I0 1 I

times

0 - Min (0 , O )
1 2
1

*The function MOD or AMOD (01, 02) is defined as 01 - [01/02] 02, where [x] is the integer whose
magn itude does not exceed the magn itude of x and whose sign is the same (:IS X.

PDP-9 FORTRAN IV

7.3

EXTERNAL FUNCTIONS
An external function is an independently written program wh ich is executed whenever its name

appears in another program

The general form in wh ich an external function is written is:

t FUNCTION NAME (a , a , •• 0, an)
1
2
(FORTRAN statements)
NAME = final calculation
RETURN
END
where t is either INTEGER, REAL, DOUBLE PRECISION, LOGICAL, or is blank; NAME is the symbol ic
name of the function to be defined; and the a's are dummy arguments wh ich are nonsubscripted variable
names, array names, or other external function names.
The first letter of the function name impl ic itly determ ines the type of function.

If that letter

is I, J, K, L, M, or N, the value of the function is INTEGER. If it is any other letter, the value is
REAL. Th is can be overridden by preceding the word FUNCTION with the spec ific type name.
The symbol ic name of a function is one to six alphanumeric characters, the first of wh ich must
be the alphabetic name and must not appear in any nonexecutable statement of the function subprogram
except in the FUNCTION statement where it is named. The function name must also appear at least once
as a variable name with in the subprogram. During every execution of the subprogram, the variable must
be defined before leaving the function subprogram. Once defined, it may be referenced or redefined.
The value of this variable at the time any RETURN statement in the subprogram is encountered is called
the value of the function.
There must be at least one argument in the FUNCTION statement. There must be nonsubscripted variable names. If a dummy argument is an array name, an appropriate DIMENSION statement
is necessary. The dummy argument names may not appear in an EQUIVALENCE, COMMON, or DATA
statement in the function subprogram.
The function subprogram may contain any FORTRAN statements with the exception of a BLOCK
DATA, SUBROUTINE, or another FUNCTION statement • It, of course, cannot contain any statement
which references itself, either directly or indirectly.
A function subroutine must contain at least one RETURN statement. The general form is:
RETURN
Th is signifies the log ical end of the subprogram and returns control and the computed value to the call ing
program.
An END statement, described in section 4.11, signals the compiler that the physical end of
the subprogram has been reached.

PDP-9 FORTRAN IV

An external function is called by using its function name, followed by an actual argument list
enc losed in parentheses, in an arithmetic or log ical expression. The actual arguments must correspond
in number, order, and type to the dummy arguments. An actual argument may be one of the following:
a.
b.
c.
d.
e.

A variable name
An array element name
An array name
Any other expression
The name of an external function or subroutine

Table 2 contains the basic external functions suppl ied by the FORTRAN System.
TABLE 2
Basic
External Function

Definition

EXTERNAL FUNCTIONS
No. of
Arguments

Symbolic
Name

Type of
Argument

Type of .
Function

EXP
DEXP

Real
Double

ALOG
DLOG

Real
Double

ALOG10
DLOG10

Real
Double

Exponential

Natural logarithm

log

Common logarithm

10910 (a)

Trigonometric sine

sin (a)

SIN
DSIN

Real
Double

Trigonometric cosine

cos (a)

COS
DCOS

Real
Double

Hyperbol ic tangent

tanh (a)

TANH

Real

Square root

(0)1/2

SQRT
DSQRT

Real
Double

Arctangent

arctan (a)

1
1
2
2

ATAN
DATAN
ATAN2
DATAN2

Real
Double
Real
Double

DMOD

Double

(a)
1
2

arctan (0 /0 )
1 2
Remaindering*

(mod O )
2

*The function DMOD (01, 02) is defined as 01 - [01/02 J 02, where [x] is the integer whose magnitude'
does not exceed the magnitude of x and whose sign is the same as the sign of x.

7.4

SUBROUTINES
A subroutine is defined externally to the program unit which references it. It is similar to an

external function in that both contain the same sort of dummy arguments, c:md both requ ire at least one

PDP-9 FORTRAN IV

RETURN statement and an END statement. A subroutine, however, may have mu Itiple outputs. The
general form of a subroutine is:
SUBROUTINE NAME (a , a , ••• , an)
l
2
or
SUBROUTINE NAME
where NAME is the symbol ic name of the subroutine subprogram to be defined; and the a's are dummy
arguments (there need not be any) which are nonsubscripted variable names, array names, or the dummy
name of another subroutine or external function.
The name of a subroutine consists of one to six alphanumeric characters, the first of which is
alphabetic. The symbo I ic names of the subroutines cannot appear in any statement of the subroutine
except the SUBROUTINE statement itself.
The dummy variables represent input and output variables. Any arguments used as output
variables must appear on the left side of an arithmetic statement or an input I ist with in the subprogram.

If an argument is the name of an array, it must appear in a DIMENSION statement with in the subroutine.
The dummy argument names may not appear in an EQUIVALENCE, COMMON, or DATA statement in
the subprogram.
The subroutine subprogram may contain any FORTRAN subprograms with the exception of
FUNCTION, BLOCK DATA, or another SUBROUTINE statement.
The logical term ination of a subroutine is a RETURN statement. The physical end of the subroutine is an END statement.
A subroutine is referenced by a CALL statement, wh ich has the general form
CALL NAME (a , a , ••• , an)

or
CALL NAME
where NAME is the symbol ic name of the subroutine subprogram being referenced, and the a's are the
actual arguments that are be ing suppl ied to the subroutine. The actual arguments in the CALL statement
must agree in number, order, and type with the corresponding arguments in the SUBROUTINE subprogram.
The array sizes must be the same. An actual argument in the CALL statement may be one of the following:
a.
b.
c.
d.
e.
f.

A Hollerith constant
A variable name
An array element name
An array
Any other expression
The name of an external function or subroutine

PDP-9 FORTRAN IV

7.5

BLOCK DATA SUBPROGRAM
The BLOCK DATA subprogram is a spec ial subprogram used to enter data into a COMMON

block during compilation. A BLOCK DATA statement takes the form
BLOCK DATA
This special subprogram contains only DATA, COMMON, EQUIVALENCE, DIMENSION, and TYPE
statements. It cannot contain any executable statements. It can be used to initialize data only in a
labeled COMMON block area; not in a blank COMMON block area.
All elements of a given COMMON block must be listed in the COMMON statement, even
if they don It all appear in a DATA statement. Data may be entered in more than one COMMON block

in a single BLOCK DATA subprogram.
An END statement signifies the termination of a BLOCK DATA subprogram.

7.5.1

Example of BLOCK DATA Subprogram
BLOCK DATA
DIMENSION X(4), Y(4)
COMMON /NAME/A, B,C,I,J ,X, Y
DATA A, B, C/3*2.0/

DATA X(1L X(2), X(3) , X(4)/0.O, 0.1, 0.2, 0.3/Y(1), Y(2),
2

Y(3), Y(4)/1.OE2, 1.OE-2, 1.0E4, 1.OE-4/
END

PDP-9 FORTRAN IV

APPENDIX 1
SUMMARY OF PDP-9 FORTRAN IV STATEMENTS

CONTROL STATEMENTS
ASSIGN n to i
CALL nome (° 1, 02'

... )

CONTINUE

DO n i=m , m , m3
1
2
GO TO n

GO TO if, ~n1' n2, ••• ) •.•••••••••• ~. • •••.•• ~ •.•••• • • • •• • • • •••• • • • ••

GQ TO (~ll' n , ••• ), i .....•.....................................
2
IF (e ) nt, n , n3 •..••••.••.•••.•.•••••••••••••.•••••••.•.•.••.•••

IF (e).~

PAUSE

PAUSE n

STOP ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••

STOP n

END

RETURN

SUBROUTINE NAME (° , 02' ••• on) ••••••••.••••••••••••..•..••..••

1
FUNCTION NAME (° , 02' ••• 0 n) •••••••••••••••••••••••.•••••••••
1

INPUT/OUTPUT ST ATEME NTS
BACKSPACE u ••••••••••••.,.......................................

END FILE u

FORMAT (51' 52' ••• , sn)

READ (u, f)

READ (u, f) list •••••••••••••••••••••••••••••••••••••••••••••••••••

READ (u) ••••••••••••••••••••••••••••••••••••••••••••••••••.••••••

READ(u)list ••••••••.••••••••••••••••••••••••••••••••••..••••••••••

READ (u, nome)
READ (u, name) list ••••••••••••••••••••••••••••••••••••••••••••••• ~

29
30

REWI N D u •.••••••••••••••••••••••••••••••••••••••••••••••••••••••

WRITE (u I f) list ••.•..•••••.•••••.•••••••.•••••••••••••••••••••••••

PDP-9 FORTRAN IV

WRITE (u, f) .••.••••.••.••.•••••••••••.••••••••••.••.••••••.•••••

WRITE (u) list .••••..•••••.••••••••••.•.••••••••••••••.••••••••••.

WRITE (u, name) ..••...•••.......••.•.•••.•.••..••.•••..••••.•.•.

WRITE (u, name) list .••••••.••.•.••••••••••••••••••••••••.••••••••

SPECIFICAnON ST ATEME NTS
COMMON /x1/o1/x!0!. • ./xn/o n •••.•••••••.•••••••.•.•••...•••

DA TA k 1/d 1/, k 2/d2 /, • • •kn/dn/

.................................

DIMENSION V1(i ), V (i ), ••• Vn(i ) ••.•.••.••••••..••••.•.. .••.•
n
1
2 2
DOU BlE PRECISION a, b, c ••...•..••••.•••••.•••••••••....•.••.•

EQUIVALENCE (k ), (k ) ••• (k ) ••...•....•••••••.••.•••....•....•
1
2
n
EXTERNAL y, z, ••• , •.•••••••••..•...••...•.••..••••..••......••

INTEGER a, b, c •.••••••••••.••.••.•.•..•..••••.•••••••...••.•••

lOG ICAl a, b, c

•. • •••• • . . . • ••••• •• . ••••• • . ••. •• •• . • . . • . . ••• . . .

REAL 0, b, c ....................................................

BLOCK DATA.................... • • . • • . • • • • • • . . . • • • . . . . . • • . • . . • •

33
35

PDP-9 FORTRAN IV

APPENDIX 2
A NOTE ON USA STANDARD FORTRAN IV

The FORTRAN language used in th is manual is essentially the language of USA Standard
FORTRAN (X3.9-1966) with the exception of the following features which are modified to allow the
compi ler to operate in 8192 words of core storage:
a. All references to complex arithmetic are illegal.
b. The size of arrays in subprograms is not ad justable to the size spec ified by the call ing
program.
c. Blank COMMON is treated as named COMMON.
d. The impl ied DO feature is not legal in a DATA statement.

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