Ibm 709 Configuration guide

~~

General

Information

Manual

iii

709·7090

Data

Processing System

MINOR

REVISION

(August, 1960)

This

edition,

Form

D22-6508-2, is a

minor

revISIon of

the

pre-

ceding

edition

but

does

not

obsolete

Form

D22-6508-1.

The

principal

change

in

this

edition

is

the

addition

of

a section

on

1401

operation

on

page 31.

INTRODUCTION

Binary

Notation

Octal

Notation

Magnetic

Cores

CENTRAL

PROCESSING

UNIT

Stored

Program

.

Assembly Programs .

Computer

Operations

Information

Paths

Indexing

and

Indirect

Addressing

Sample

Problems

.

Operator's

Consoles.

.

INPUT-OUTPUT

COMPONENTS

•

Magnetic

Tape

Storage.

Auxiliary

Equipment

.

Data

Synchronizer

IBM

755

Tape

Control.

Multiplexor

Data

Channel

. .

External

Signal

Direct

Data

Feature

.

Magnetic

Drum

Storage

Punched

Cards

Card

Reader

Card

Punch

..

Printer

Cathode

Ray

Tube

Equipment

SHARE

Organization

Programming

System

FORTRAN

AUTOMATIC

CODING

SYSTEM.

APPENDIX

Contents

5

6

7

10

13

15

16

18

20

22

24

30

31

35

36

37

39

40

41

42

44

46

47

IBM

7090

Data

Processing System

IBM

709·7090

Data Processing System

Data

processing

sy~tems

are

finding new

application

in

virtually

every

phase

of

science, business,

and

in-

dustry.

Rapidly

expanding

scientific investigations in-

volve

many

complex

calculations.

The

vast

amount

of

data

constantly

being

used

in

aircraft

industries,

government

agencies,

and

business establishments

of

all kinds

demand

machines to

compute,

select,

and

correlate

data

at

electronic speeds.

Figure I shows

the

kind

of

problem

that

computers

are solving for scientists today.

Many

similar

prob-

lems

are

encountered

and

solved

in

the

scientific field;

however,

the

computer

is

adaptable

to business

appli-

cations as well.

The

computer

can

be,

and

is,

used

for payroll,

material

inventory,

or

any

of

a

great

num-

ber

of

other

business

problems

requiring

prompt,

accurate results.

In

the

sample

problem,

the

computer

calculates

the

density

and

velocity

of

air

at

each

of

the

mesh-points

shown.

From

these

data,

engineers

can

evaluate

the

lifting

power

and

drag

of

the

wing

section. By solv-

ing

a

number

of

these problems,

an

optimum

new

wing design is developed.

The

mathematical

formulas used are:

4.

p=

f (p)

Figure 1. Aircraft

Application

Equations

1,

2

and

3

are

replaced

by difference

equations

over

a

network

as

shown

in

the

figure.

This

difference system

and

equation

4

form

a set

of

simul-

taneous

non-linear

algebraic equations.

These

are

solved

on

the

computer

by

repeating

calculations,

called

an

iteration

method.

In

this

one

typical prob-

lem,

there

are

800 equations,

100

iterations, 80,000

operations

per

iteration,

and

8,000,000

operations

per

solution.

Figure 2 shows a comparison,

in

time, between

man-

ual

methods

of

solution

and

solution

by computers.

The

times

are

approximate.

This

comparison

shows

that

computers

of

today now have

the

ability

to solve

problems

that

cannot

be solved

in

a lifetime

of

man-

ual

labor.

One

of

the

first things to be considered

in

a com-

puter

system is

an

understanding

of

the

functions

of

its various components. A simplified

computer

sys-

tem consists

of

three

main

units

as

shown

in

Figure

3.

In

a typical

computer

system, several types

of

input-

output

devices

are

needed.

These

may be

units

such

~

r\

\ Pencil and paper

{

15

years

Desk

calculator

80 weeks

IBM

701

2 minutes

IBM

704

30 seconds

IBM

709

25 seconds

IBM

7090

5 seconds

Figure

2.

Time

Comparison

of

Computation

Methods (Approxi-

mate)

Introduction

5

Figure

3.

Main

Computer

Components

as

punched

card

readers, a device to

read

or

write

on

magnetic

or

paper

tape, a

telephone

or

teletype line,

punched

card

recorders,

and

printing

equipment.

An

input

component

is

any

device

capable

of

feed-

ing

information

into

a

computing

and

data

handling

unit,

usually called

the

central

processing

unit.

A storage

unit

must

also be

provided

to store

or

"remember"

all

input

data

and

instructions (also

called orders) to

the

computer.

These

instructions are

then

used

to tell

the

computer

how

the

processing

of

data

is

to be executed.

The

central

processing

unit

has

an

arithmetic

sec-

tion

to accomplish all

arithmetic

and

logical opera-

tions

and

a section to modify

or

change instructions

to

adapt

the

processing

to

variations

within

the

es-

tablished procedure. A

control

section is also a

part

of

the

central

processing

unit

and

directs

the

com-

puter

in

its

operation

by

making

logical decisions.

An

example

of

one

of

these decisions

would

be a test to

determine

if

a

number

is positive

or

negative. As a

result

of this test,

the

computer

can

take

a different

processing course for each alternative. Simple tests

may be

combined

to

make

complex

logical decisions.

The

output

components

are

any devices

capable

of

recording

the

resultant

data

after

the

central

process-

ing

unit

has

operated

upon

it.

Binary

Notation

The

common

decimal

notation

of

the

commercial

and

scientific

world

is a

familiar

one.

This

notation

is so

familiar

that

its use is

hardly

questioned. However,

it

is possible

that,

for

some purposes,

other

number-

ing

systems

are

more

convenient.

What

numbering

system

to

use

is

entirely a

matter

of convenience. Decimal

notation

is

used

because

it

is

most

familiar

and

is

understood

by most people.

However,

had

our

primeval

ancestors developed

eight

fingers

instead

of

ten, we

would

probably

be

more

familiar

with

a

numbering

system based

on

eight,

rather

than

ten,

and

we

might

consequently

question

the

decimal system.

6

IBM

709-7090

The

decimal system,

with

its

ten

digits, is

learned

by most

people

early

in

their

training.

This

system

serves well for

counting

purposes.

Why

then

should

computers designed

to

assist

mathematicians,

engi-

neers

and

businessmen, be designed to use

the

binary

system

of

numbers?

The

reason

is

that

current

digital

computers

use

binary

circuits; therefore,

the

mathematics

of

com-

puters

is

binary

in

nature.

The

octal system

is

a

shorthand

method

of

writing

long

binary

numbers.

Octal

notation

is used

when

discussing

the

computer,

but

has

no

relation

to

the

internal

computer

circuits.

The

binary,

or

"base two" system, uses

the

two sym-

bols 0

and

1 to

represent

all

quantities.

Counting

starts

in

the

same

manner

as

in

the

decimal system

with

a 0

for

zero

and

then

a I for one.

At

two, how-

ever, there

are

no

more

symbols to be used.

It

is

therefore necessary to take

the

same step

at

two

in

the

binary

system

that

is

taken

at

ten

in

the

decimal

sys-

tem.

This

step is to place a I

in

the

to

the

left

and

start

again

with

a 0

in

the

original

position. A

binary

10

is

equivalent

in

this respect to

a 2

in

the

decimal system.

Counting

is

continued

in

an

analogous

manner

with

a carry to

the

order

every

time

a two is reached

instead

of

every

time a

ten

is

reached.

Counting

in

the

binary

system

is

as

follows:

Binar) Decimal

Binary

Decimal

8 4 2 1

84

2 1

0 = 0 1 = 1

1 0 -2 1 1 -3

100

-4 1 0 1 -5

1 1 0 -6

11

1 = 7

1 0 o0 -8 1 0 o 1 -9

Octal

Notation

It

has

already

been

pointed

out

that

binary

numbers

require

about

three

times as

many

positions as deci-

mal

numbers

to express

the

equivalent

number.

This

is

not

a

problem

to

the

computer

itself,

but

in

talking

and

writing, these

binary

numbers

are bulky. A

long

string

of

ones

and

zeros

cannot

be effectively trans-

mitted

from

one

individual

to

another.

Some short-

hand

method

is necessary.

The

octal

number

system

fills this need. Because

of

its simple

relationship

to

binary,

numbers

can

be

converted from

one

system

to

another

by inspection.

The

base

of

the

octal system

is

8.

This

means

there

are

eight

symbols:

0,

1,

2,

3,

4, 5,

6,

and

7.

There

are

no

8's

or

9's.

The

important

relationship

is

that

three

binary

positions

are

equiva-

lent

to

one

octal position.

The

following

table

com-

bines

what

has

been

shown

concerning

decimal

and

binary

with

the

octal

numbers.

BINARY OCTAL

DECIMAL

0 0 0

1 1 1

10 2 2

11

3 3

100

4 4

101

5 5

110

6 6

III

7 7

At

this

point

a carry to

the

position

of

the

number

is necessary, since all

eight

symbols

in

the

octal system have

been

used.

1000

1001

1010

1011

10

II

12

13

8

9

10

11

As

far

as

the

internal

circuitry

of

the

computer

is

con-

cerned,

it

only

understands

binary

ones

and

zeros.

The

octal system

is

used

to

provide

a

shorthand

method

of

reading

and

writing

binary

numbers

so

that

the

contents

of

a register,

when

shown

on

the

operator's

panel,

may

be

read

directly.

This

is

shown

in

Figure 4,

using

a 36-digit

binary

number.

Reg;

ster contents (b;nary)

Octal

value

Figure

4.

Binary

Representation

of Register

Contents

Magnetic

Cores

The

main

storage

medium

for

many

computers

is

the

magnetic

core.

Each

magnetic core is a

ring

or

doughnut

shaped

piece

of

ferromagnetic

material.

The

cores "remem-

ber"

information

indefinitely,

and

can

recall

it

in

a

few

millionths

of

a second.

When

a wire

is

inserted

through

the

hollow

center

of

a core (Figure

5),

an

electrical

current

passed

along

the

wire sets

up

a mag-

netic

field

around

the

wire.

This

field magnetizes

the

core.

When

the

current

is removed,

the

core

remains

magnetized.

1£

a

current

is

passed

along

the

wire

in

Core

Is

Set

Figure

5. Magnetic Core Action

the

opposite

direction,

the

magnetic

field set

up

around

the wire is reversed.

When

this occurs,

the

core

is

said to have

"flipped"

or

changed

its mag-

netic state. A "sense" wire

is

inserted

through

the

core

and,

when

the

core flips, a small electrical voltage

is sent along

the

sense wire.

This

voltage

may

then

be amplified

and

used

in

the

computer

(Figure

6).

Computer

Figure 6.

Flipping

of a Magnetic Core

As

with

the

magnetic core, all

computer

elements

are

able

to

represent

two states.

These

two

distinct

states

provide

the basis by

which

these elements

hold

information.

For

this reason,

the

elements

are

called

bi-stable elements.

For

example,

one

state

may

be

interpreted

as

the

digit

0;

the

other,

as

1.

Similarly,

the

elements may be used to represent:

plus

or

minus,

on

or

off,

yes

or

no,

and

so on. Several of these ele-

ments are shown

in

Figure

7.

The

IBM

709

and

7090

Data

Processing Systems use

high-speed core storage units.

Each

unit

is

divided

into

distinct sections called locations.

Each

location

is

uniquely

identified by a

number

assigned to it.

This

identifying

number

is

called

an

address because

just

as

a street address denotes

the

precise

location

of

a

particular

building

on

that

street, this

number

de-

liD

..

State

Figure

7.

Bi·stable

Computer

Elements

Introduction

7

Number 1500

Figure

8.

Core Storage

Unit

notes the location of a

particular

item

of

data

inside

the core storage

unit

called a

"word"

(Figure

8).

Each word is

further

subdivided

into

elements

called bits. Each

bit

has a value

of

either

a 0

or

a

1.

Thus.

the bits

(36

in

each word)

are

the basic units

of

information

in

the computer. Figure 8 also shows

S6

"planes"

or

"stories"

in

the core storage "building."

The

operation

of locating a word

in

core storage may

be compared to the

operation

of

an

elevator

in

an

office

building.

The

elevator picks

up

passengers from

each floor;

in

core storage,

one

bit

of

information is

read

or

stored from each plane.

In

actual computer

operation, all

36

bits are read, stored,

or

operated

upon

simultaneously, one from each plane.

As

previously stated, one word contains

36

bits

of

information. Core storage contains two types of

words:

1.

A word

upon

which arithmetic

or

logical oper-

ations are to be performed

is

called a data-word.

2.

A word

interpreted

by the

computer

as a code

to

"order

or

instruct"

it

to perform a

particular

operation

is

called an instruction-word.

8

IBM

709-7090

Figure 9. Instructions

and

Data

in

Core Storage

Both types of words are combinations of

36

zeros

and

ones.

An

instruction is able to "direct"

the

computer

to perform some type

of

operation, e.g., read. write,

add, subtract,

or

test for zero.

If

a

data

word were

incorrectly used

as

an

instruction, the computer

might

perform

an

illegal

operation

depending

upon

the

data

bit

configuration.

Data

words form records. fields,

amounts, results,

and

so

on. Figure 9 is a schematic

of core storage

with

both

types

of

words contained

in

separate locations.

Core storage

units

are available to provide the com-

puters

with

a capacity

of

4096, 8192,

or

32,768 word

locations of

36

information

bits each. As a decimal

digit is expressed by three

of

these bits, a total of

II

significant decimal digits may be expressed by each

word.

In

BCD coding (alphamerical characters),

one

word contains six numerical

or

alphabetic characters.

Thus,

the 32,768 words of core storage may contain

the binary equivalent to 360,448 decimal digits of stor-

age

or

the BCD representation of 196,608 characters.

The

actual capacity of storage is thus directly related

to the type of coding used.

Figure

10

shows the three core storage units

that

are available for the 709

and

7090

Data

Processing

Systems.

The

IBM

737

Core Storage, used

with

the 709

system, has 4,096 words

of

storage.

The

IBM

738

and

the

IBM

7302 Core Storage have 32,768 words

of

stor-

age

and

are used

with

the

709

and

the 7090 systems,

respectively.

Figure

10.

IBM Core Storage

Units

Introduction

9

Central Processing

Unit

The

central processing

unit

can

be divided

into

two

basic parts,

information

processing elements

and

exe-

cution

controls.

The

information

processing elements

are normally referred to

as

the

arithmetic

section.

The

execution controls are called simply

the

control

section.

The

arithmetic

section

is

the

computer's problem-

solving

unit.

The

operations

of

addition, subtraction,

multiplication,

and

division, as well as shifting, trans-

ferring,

and

storing

of

results are executed

in

this

section. Figure

11

shows a few of

the

operations per-

formed

in

the

arithmetic

unit.

The

control section guides the

computer

through

the

operations necessary to complete

the

various in-

structions.

The

execution

of

a

group

of

instructions may be

compared to

compounding

a chemical formula.

The

arithmetic

section would be

the

laboratory where the

compounding

takes place;

it

would

call

on

core stor-

age for materials needed to

produce

the

result.

The

control section would follow directions given

in

the

formula.

It

would instruct

the

chemist as to

what

ingredients to mix, how

long

to

mix

them,

and

in

what

order

they should be combined.

Of

necessity, then, the control section contains a

device called

the

"instruction

sequencer."

This

de-

vice locates

the

proper

instruction to be executed;

then, while

the

instruction

is

being executed, this de-

vice sets

up

the

conditions for

the

At

different times

in

the program,

the

to

be

executed may

depend

on

the result of a test in-

struction, for example,

whether

an

error

indicator

is

on

or

off (Figure

12)

.

The

arithmetic

operations which computers are

capable

of

performing

treat

each

part

of

a word

in

014623

ADD

541034

555657

12345

Sub.

11111

01234

':"_

,,'_"'~"~

Unit

Control

Un

it

Figure

II.

Arithmetic

Operations

10

IBM

709·7090

12345

SHIFT~

~

STORE

3416

Arithmetic Section

"test the indicator!!

error

indicator«.t-.

"if

the indicator

is

on obtain the next instruction-from location

1000.

If

the indicator

is

off obtain the next instruction

from

location

2000."

"the

indicator

is

on,

put 1000 into the instruction sequencer"

"get

the instruction located

at

address 1000 and

execute

it."

- - - -

COntral

se~iia;;

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

--

11000!;nstructian sequencer

Figure

12.

Conditional

Testing

and

Transferring

core storage as

either

a 1

or

a

O.

Thus,

a

computer

word

consists

of

36 zero

and

one

digits

and

is said to

be

expressed

in

the

"binary

system."

An

example

of

a

binary

word

as

it

appears

in

the

arithmetic

unit

is

001101110010011011001001111001001001 which is sim-

ply

a string

of

zeros

and

ones. By

doing

its arith-

metic

in

the

binary system,

the

computer

is able to

achieve

greater

speeds

and

efficiencies

than

would

otherwise be possible.

Normally,

in

solving a problem,

the

numerical

data

are

originally

entered

into

the

computer

in

decimal

or

IBM

card

coding.

The

computer

is

then

instructed

to translate

the

data

into

binary form, go

through

the

desired computations,

and

then

translate

the

results

back

into

decimal form (Figure 13). However,

data

may be

entered

into

the

computer

in

any form desired

by the programmer.

Instruction

0011010101111

ADD

11010001111,00

10000011101011

PRINT

Central

P(ocessing Unit

Figure

13. Decimal

and

Binary

Data

Flow

Stored

Program

The

work accomplished by the

computer

in

solving

a

problem

or

processing

data

consists

of

executing

many

instructions

at

high

speed.

To

solve a problem,

it

must

first

be

reduced to equations

or

instructions

that

the

computer

is capable

of

performing.

The

en-

tire set

of

instructions used

in

solving a

problem

form

a program for

the

computer. Because these instruc-

tions are

held

in

the computer's storage

unit,

it

is

called a stored program system.

Normally, instructions

are

taken

from

sequentially

ascending locations

of

core storage. However,

the

exe-

cution

of

instructions does

not

necessarily have

to

occur

sequentially.

It

is possible,

when

control

or

transfer

instructions

are

given, for

the

computer

to

alter

the

process

of

sequential

execution

and

to

indi-

cate

some

particular

location

in

core storage contain-

ing

the

word

to

be

executed.

In

this

way,

the

execution

of

any

instruction

or

block

of

in-

structions

may

be

repeated

as

often

as desired.

For

some

control

instructions,

whether

the

struction

is

taken

in

sequence

or

from

some

other

specified location

may

depend

on

the

result

of

a test

(Figure

12).

In

this case,

the

control

operations

pro-

vide

the

program

with

decision-making abilities.

The

logical

path

followed by

the

program

(that

is,

the

precise sequence

of

instructions to

be

executed) may

be

controlled

by a series

of

tests

applied

at

various

points

in

the

program.

Doing

this gives a

stored

pro-

gram

the

ability

to

change

its course of execution.

These

conditional

operations

increase

immeasurably

the

scope

of

the

system's

application.

Most

computer

instructions

have

an

address

part

indicating

the

location

in

core storage

subjected

to

some

arithmetic

or

logical

operation.

This

address

part,

or

field, always occupies

bit

positions

21

through

35 (Figure 14)

in

an

instruction

word.

Address

part

I

s,

I

Figure

14. Address

Part

of

an

Instruction

The

15-bit address field is large

enough

to

hold

the

number

32,767 -

the

largest address

in

core storage.

This

number

expressed

in

the

binary

system is simply

15

consecutive ones (Figure 15) .

II

III

I I

11111

S,1

2021

35

Figure

15. Address

Part

Showing

Maximum

Address

In

a

data

processing system

with

8,192 words of

core storage,

the

largest address (8191)

is

contained

in

only

13

bit

positions

of

the

instruction

word.

The

contents

of

the

two left-most positions

of

the

address

field

are

ignored

during

the

execution

of

an

instruc-

tion

to

decode

an

address. Similarly, a

computer

with

4,096 words

of

core storage uses

only

bit

positions 24

through

35

as

its address field.

The

operation

part

of

an

instruction

normally

is

not

fixed

in

length

but

may

vary

depending

upon

the

instruction

itself.

Figure

16

shows

the

bit

pattern

for

an

instruction

specifying

the

addition

of

the

contents

S,1

35

Figure

16.

Add

Instruction

of

core storage

location

0001.

When

this

instruction

is

executed

by

the

computer,

the

contents

of

core loca-

tion

0001

are

added

to

the

contents

of

the

accumulator

register.

For

example, assume

that

the

accumulator

contains

the

number

+1.

If

the

number

in

location

0001

is

+2,

the

result

of

executing

the

ADD

instruction

(Fig-

ure

17) is a 1

bit

in

positions 34

and

35,

and

0 bits

in

all

other

positions

of

the

accumulator;

(II

is

the

binary

expression for

the

sum,

3).

The

first

position

of

the

word

is

used

to express

the

sign

of

the

amount

or

result.

Instruction

(Add)

location 1

(+2)

000000000000000000000000000000000010

, ,

AC

before

(+1)

aaa

000

aaaa0 a

000

aa0 a0 0 a0 0 0 a0 0 a

00

a0 a

01

,

AC

after

(+3)

1010000

a0 0 0 0 0 aa0 0 0 aa0 aaa0 a0 0 a

00000

aa1

Ii

s 1

J5

Figure

17.

Execution

of

an

Add

Instruction

A register is defined

as

a device

with

the

ability

to

accept

data,

hold

them,

and

transfer

the

data

to

an-

other

register

or

device.

The

registers

are

given dif-

ferent

names according to

their

functions.

Thus,

the

accumulator

register

"accumulates"

arithmetic

results.

The

multiplier-quotient

register

holds

either

the

mul-

tiplier

or

the

quotient

in

an

arithmetic

operation.

On

all

arithmetic

operations,

the

sign positions

of

the

registers

in

use

are

automatically

adjusted.

The

accumulator

register has 38 positions.

They

allow

the

register to

handle

a 35-bit

word

with

its

sign.

Two

extra

positions, P

and

Q,

are

provided

to

keep

track

of

overflow conditions.

If

two 35-bit

num-

bers

are

added

together,

it

is possible

that

the

result

would

be

larger

than

35 bits, as

shown

in

Figure

18.

The

accumulator

is

used to

hold

one

factor

during

arithmetic

operations.

The

other

factor usually is

Accumulator

register

Storage register

Combined in

the

adder

and

returned

to

the

accumulator

register.

S Q P I

35

! ! !10000000000000000000000000000000001

1100000000000000000000000000000000111

Q P 1

35

I!

Ii

000000000000000000000000000000001001

!

iIi

000000000000000000000000000000001001

S Q P 1

35

}1igure 18.

Accumulator

Overflow

Condition

Central

Processing

Unit

11

Accumulator register

before a left

shift.

5 Q P 1 35

I ! ! :100000000000000000000000000000000001

5 Q P

1-----------35

Accumulator register I I I i I

after

a left shift

of

I

11

100000000000000000000000000000000000

one

place.

Figure

19.

Accumulator

Shifting

held

in

the

storage register.

The

two factors

are

com-

bined

in

another

register-type device called

the

adder.

The

adder

consists

of

35-bit positions

and

the

two

extra

ones, P

and

Q.

Thus,

any overflow

out

of

posi-

tion

1 will

be

placed

in

position

P. Likewise, any

overflow

out

of

position P will

be

placed

in

position

Q.

An

overflow

out

of

position Q will, however, be

lost.

Information

in

the

accumulator

may be shifted to

the

right

or

to the left.

1£

a left shift is involved,

an

overflow

condition

may

be

recorded

in

the

same man-

ner

as

in

an

arithmetic

operation

(Figure

19).

Not

only

is

the

entire

number

shifted

to

the

left,

but

zeros

are

inserted (position 35)

in

the

positions vacated.

Thus,

no

matter

what

the

content

of

the

accumulator,

if

a

left

shift

of

37

or

more

places is executed,

the

contents

of

the

accumulator

are

replaced by zeros.

Information

may also

be

shifted

into

the

accumu-

lator

from

the

MQ

register,

one

bit

at

a time,

or

from

the

accumulator

to the

MQ,

one

bit

at

a time.

The

effect is to create a register

that

is

74 positions

in

length,

38

positions for the

accumulator

and

36 for

the

MQ

register.

The

multiplier-quotient

(MQ)

register has

36

posi-

tions.

During

a

multiply

operation,

this register con-

tains

the

multiplier;

during

a divide,

it

receives

the

quotient.

The

register

can

store

and

shift a full

36-

bit

word.

In

addition

to shifting

right

or

left,

in

the

same

manner

as

the

accumulator,

it

can

also

"ring

shift."

That

is, bits shifted

out

of

the

sign position

enter

position

35

(Figure

20)

.

In

addition

to its

arithmetic

and

shifting functions,

the

MQ

register serves

as

a sending

and

receiving regis-

ter

for some

input-output

operations.

For

the

709

system, this means

that

information

coming from

the

magnetic

drum

or

going to

the

drum

and

CRT

equip-

ment

passes

through

the

MQ

register.

When

a

word

is

entered

into

or

taken

from a loca-

tion

in

core storage, a storage reference is said to

be

51

35

MQ

register before 111111000000000000111000111000111

000

a

ring-shift

of

three

places.

51

35

MQ

register

after

shift 11110000000000001110001110001110001111

Figure

20.

MQ

Register

"Ring

Shift"

12

IBM

709-7090

made.

This

operation

is non-destructive.

That

is,

the

contents

of

that

location

are

left

unaltered

after

the

operation.

However,

if

the

storage reference causes

new

information

to

be

entered

into

a location,

the

prior

contents

of

that

location

are

automatically re-

placed

by

the

new

information.

Only

one

such storage reference

can

be

made

during

any single

computer

cycle.

The

basic

computer

cycle

(or

internal

speed) for

the

709 is 12 microseconds

long

(12

millionths

of

a

second).

The

basic cycle for

the

7090 is 2.18 microseconds.

Normal

computer

operations

start

with

an

instruc-

tion

(I) cycle.

This

cycle does

the

following:

1.

It

obtains

the

instruction

to

be

executed from

the

core location designated

by

the

instruction

counter.

2.

It

locates

the

operand

(number

to

be

worked

with

or

upon),

if

any, as specified

by

the in-

struction's address

part.

An

example

would

be

the

instruction

ADD 0002 (add

the

contents

of

storage location 0002 to

the

contents

of

the

accumulator

register), shown

in

Figure

21. All

data

handling

concerned

with

storage

and

the

central

processing

unit

is accomplished

in

parallel.

This

means

that

all operations

are

concerned

with

a full

word

of

36 bits.

As

an

example, refer to Figure

17

where

an

ADD

operation

is performed.

Even

though

the

numbers

involved use

only

a few bits

of

the

entire

word,

the

remainder

of

the

word

is filled

with

zeros

and

all 36

positions

of

both

words

are

then

added

in

one

opera-

tion.

Thus

the

time

required

to

add

two

I-bit

num-

bers is

the

same

as

the

time

required

to

add

two

36-

bit

numbers.

This

principle

of

parallel

operation

is

one

of

the

basic differences between

the

709

and

7090 computers

when

compared

with

the

705 system series, which is

said

to

operate

serially (normally om;

position

or

character

at

a time) .

(ADD 0002) =

~

Instruction Register

5torage Register

~

Address Register

Adders

1-+004121503051

I (number in accumulator)

Accumulator Register

Figure

21. Schematic, Decoding

an

Add

Instruction

The

instruction

ADD 0002

is

brought

from

core

storage

into

the

storage register.

At

this

point,

the

operation

part

is sent to

the

instruction register

to

determine

the

type

of

operation

to be executed.

At

the

same time,

the

address

portion

of

the

storage reg-

ister is

taken

to

the

address register so

that

the

proper

core storage

location

(operand) is

added

during

the

All

numbers

in

Figure

21

are

shown

in

octal

notation.

The

referred

to as

an

execution

cycle,

is used

when

a reference to core storage is

required

to

obtain

another

word, e.g., a

number

to be added.

Assume

that

the

address register has

the

address

of

the

core storage location to be added.

During

this second

(reference) cycle, the contents

of

that

storage location

are

brought

from core storage to

the

storage register,

and

then

to

the

adders.

At

the

same time,

the

pre-

vious

number

in

the

accumulator register

is

taken

to

the

adders. A

new

number

is

formed

in

the

adders

and

is

then

placed

back

in

the

accumulator

register

(Figure

22).

The

operation

is

now

finished

and

the

computer

is

ready

to execute

the

instruction.

The

type

of

operation

just

described is

called

fixed

point

arithmetic

or

integer

arithmetic.

The

computer

can

also

perform

operations

in

floating

point

arithme-

tic. A

complete

set

of

floating

point

instructions

is

provided

to

increase

the

range

of

numbers

used

and

to

reduce

programming

time.

In

integer

arithmetic,

the

size

of

the

numbers

used

is fixed by

the

design

of

the

computer

(36-bit

word

size) . By

using

floating

point

arithmetic,

a

larger

num-

ber

may

be

expressed

and

operated

upon,

because a

part

of

the

word

is used to express

the

exponent

(char-

acteristic)

and

another

part

of

the

word

is

used

for

the

fraction

(mantissa).

A

comparison

of

number

size shows

that

the

largest

number

the

computer

can

use

with

fixed

point

opera-

tion

is

I X

1011.

This

is

equal

to 100 billion.

With

floating

point

operation,

however, 3 X

10

35

is

the

larg-

Instruction Register

+132540020425 (number

from

location 0002)

+1

32540020425

Accumulator

Register

Storage Register

Address

Reg

ister

(number

from

location 0002)

(previous

number

in accumulator)

(resulting number

in

accumulator)

Figure

22. Schematic,

Performing

an

Addition

I

Characteristic

Fraction

51

8 9 35

Figure

23.

Floating-Point

Word

Format

est

number.

This

number

is too

high

to be expressed

in

common

language

terms.

In

the

computer,

a floating-point

number

is stored

in

a

word

as shown

in

Figure 23.

The

fraction

is

con-

tained

in

bit

positions 9

through

35

with

the

sign of

the

fraction

contained

in

position

S

of

the

word.

The

characteristic

is

contained

in

bit

positions 1

through

8

and

is

formed

by

adding

+128

to

the

ex-

ponent.

For

example,

an

exponent

of

-32

would

be

represented

by a characteristic

of

128-32

or

96.

An

exponent

of

+ 1

00

would

be

represented

by a charac-

teristic

of

100+128

or

228.

Most

integer

arithmetic

instructions

have

a floating-

point

counterpart

instruction.

Thus,

it

is

possible to

ADD

or

to

perform

a floating-point ADD

(FAD).

To

summarize, a floating-point

binary

number

(X)

may be

represented

as

a signed

proper

fraction (B)

times some integral power (b) of

2.

Examples:

X B 2b

-.001

=-.100

X 2-"

.100 .100 x

2"

l.l00

.110 X

2'

110.000 .110 X 23

Assembly

Programs

The

writing

of a complete

program

for

the

computer

in

its

machine

language

would

be

rather

awkward.

For

example, to

write

only

the

one

instruction

re-

quired

to

subtract

the

contents

of

core

location

0003

from

the

contents

of

the

accumulator

register

requires

the

recording

of

36 zeros

and

ones,

as

shown

in

Figure

24.

This

instruction

can

be

written

as

SUB

0003, a

more

convenient

form

of

expressing

the

instruction.

The

writ-

ing

of

instructions

in

this form is

an

improvement

over

the

system

of

writing

36

zeros

and

ones.

In

addi-

tion

to

reducing

the

amount

of

information

which

must

be

written,

it

has.

more

meaning

to

the

reader.

Namely,

"subtract

the

contents

of

location

0003."

However, to

determine

the

entire

machine

operation,

the

programmer

or

person

reading

the

program

must

Operation

Address

Figure

24.

Subtract

Instruction

Central

Processing

Unit

13

know

what

is stored

in

location 0003.

To

assist

the

programmer,

the

use

of

symbols

can

be

extended

to

in-

clude addresses as well as

the

operation

part

of

an

in-

struction.

Assuming

that

the

net

pay

for a payroll calculation

is

stored

in

location 0003,

the

instruction

might

now

be

written

as

"subtract

net."

This

clarifies to the pro-

grammer

the

operation

to be performed,

and

also

what

quantity

is involved.

Thus,

symbolic

programming

employs instruction-by-instruction coding

in

a lan-

guage

that

is a representation

of

the

basic language

of

the

computer

itself.

The

computer

can

execute instructions only

if

,they

are stored

in

machine language.

Thus,

the

instruction

"subtract

net"

must be converted to

the

basic

machine

language before its execution.

One

way

of

doing this

would be to

manually

convert all symbolic instruc-

tions to machine language before

entering

them

into

the

computer.

This

obviously would be a laborious

task. A more practical solution

is

to have

the

com-

puter

perform

the

conversion.

This

is accomplished

through

use

of

a symbolic assembly program.

An

assembly

program

performs

the

necessary trans-

lation

or

conversion from

the

symbolic language to

the machine language.

At

the

same time,

the

program

assigns absolute core storage locations to

the

imtruc-

Convert

the

Instruction

"SUBTRACT NET"

Becomes

000100000010000000000000100000000

No Yes

Print

and

punch

the

resulting

program

Figure 25. Flow

Chart

of

an

Assembly

Program

14

IBM

709-7090

tions

and

data

contained

in

the

symbolic program.

Figure

25

shows a flow

chart

of

the

process involved

when using

an

assembly program. Assume

that

the

instructions to

be

assembled are

on

punched

cards

and

that

the

assembly

program

itself is

on

a magnetic tape

(assembly tape) .

The

assembly

program

normally provides certain re-

strictions

upon

the

length

and

type

of

symbols

that

may

be used by

the

programmer.

It

assigns

an

abso-

lute

core storage location for

the

first symbolic instruc-

tion

and

increases this location by

one

for each sub-

sequent

instruction decoded.

In

this manner, the de-

coded

program

instructions are assigned sequential lo-

cations. Space

is

provided for

the

data

that

are

a

part

of

the

assembled program. Normally,

the

constants

and

other

data

are placed

at

the

end

of

the

symbolic

program

and

are assigned sequential locations

that

follow

the

instruction area.

During

assembly,

the

first reference to a symbolic

address assigns this address to

an

absolute storage lo-

cation. Any subsequent references to this symbolic ad-

dress will also be assigned

the

same address.

The

man-

ner

in

which

the

assembly proceeds is described below.

Each

symbolic

instruction

is

read

from

the

card

reader

into

the central processing

unit.

The

"sub-

tract"

portion

of

the

first instruction is

matched

against

the

contents

of

the

assembly tape

and

is de-

coded

into

the

proper

bit

configuration for a

subtract

operation

000100000010.

The

"net"

portion

of

the

instruction

is assigned a location

in

core storage

that

is

not

being

used.

This

bit

configuration (assume

000100000000)

is

then

inserted

into

the

address

portion

of

the

decoded

subtract

instruction.

The

full instruc-

tion

word

(000100000010000000000000000I00000000),

both

operation

and

address parts,

is

then

written

as

the

first instruction

on

the

program

tape.

The

original

"subtract

net"

is stored

so

that

it

may

appear

in

the

final

printing

of

the

program.

The

assembly

program

then

tests to

determine

if

more instructions

remain

to

be

decoded.

If

there

are more,

the

above

process

is

repeated

until

all symbolic instructions have

been

decoded

and

written

on

the

program

tape.

When

the

assembly process is complete,

the

resultant

as-

sembled

program

is

printed,

together

with

the

original

symbolic instructions.

The

print-out

takes

the

follow-

ing

form:

SYMBOLIC

INSTRUCTION DECODED

MACHINE

EQUIVALENT

SUBTRACT

NET

000100000010000000000000100000000

When

the

printing

process is complete,

the

assembly

program

furnishes the

programmer

with

a

group

of

punched

cards

containing

the

machine

language

trans-

lation

so

that

if

the

program

is used again,

the

as-

sembly process

need

not

be

repeated.

Computer

Operations

The

format

of

the

instruction

word

is, for

the

most

part,

a precise one.

Although

slight

variations

exist,

in

general

the

format

is

as

shown

in

Figure

26.

Operation

part

Flag

Tag

Address part

I I I I

5,

1

11

12-13 18-20

21

35

Figure 26.

Instruction

I'onnat

The

operation

part

usually

is

contained

in

positions

S,

1-11

of

the

instruction

word.

In

the

following text

and

in

program

writing,

an

alphabetic

code

is

used

to

identify

the

instruction

operation

rather

than

the

full

name

of

the

instruction.

Thus,

the

code

CLA

signifies

CLear

and

Add,

or

SUB

means SUBtract.

If

the

numeri-

cal

machine

language

code is to be used,

it

is given

in

the

octal system.

For

example,

+0500

is

the

code

for

the

clear

and add

operation;

+0402

denotes subtrac-

tion.

All

abbreviations are simply

shorthand

methods

used to

reduce

the

manual

task

of

writing

a

program.

The

flag

part,

or

flag bits, is

contained

in

positions

12

and

13

of

the

word.

It

specifies

that

indirect

ad-

dressing is

to

take place.

This

operation

may

be

per-

formed

only

on

instructions

that

use

index

registers.

Both

indirect

addressing

and

the

use

of

index

registers

are

explained

later

in

the

text.

The

tag

bits

are

contained

in

positions 18, 19,

and

20.

They

specify

which

index

registers

are

to

be used.

Only

a few

instructions

may

not

use

indexing.

The

address

part

of

the

instruction

is

contained

in

positions

21

through

35

of

the

word.

It

tells

the

com-

puter

the

location

or

"operand"

that

is to be

used

with

the

instruction.

In

the

case

of

shifting

operations,

the

address

part

contains

the

number

of

places to

be

shifted.

For

other

instructions,

the

address

part

may

be a

part

of

the

operation

itself. Such a case

would

have

the

address

part

expressed as a four-digit num-

ber; read-1200 means

the

reading

operation

will

take

place

and

the

1200 will tell

which

input-output

unit

is

being

used.

In

this case,

the

tape

unit

numbered

0

on

data

channel

A

would

be used.

In

executing

the

instruction

ADD

1000,

the

computer

assumes

that

the

augend

is

in

the

accumulator

and

the

addend

(the

number

being

added) is specified

by

the

address

part

of

the

ADD

instruction.

The

sign

of

the

numbers

(S

position) is,

of

course, considered

dur-

ing

an

add

operation.

When

two

numbers

of

the

same

magnitude

are

being

added,

the

sign

of

the

result

is

taken

from

the

number

in

the

accumulator.

ACCUMULATOR

+6

-6

+

STORAGE

-6

+6 =

=

RESULT

IN

ACCUMULATOR

+0

-0

The

clear-and-add

(CLA)

instruction

is

similar

to

the

add

instruction

except

that

the

accumulator

is

cleared to zeros

and

the

contents

of

the

location

speci-

fied by

the

address

part

of

the

CLA

instruction

are

placed

in

the

accumulator.

Add

magnitude

(ADM)

is

another

similar

instruc-

tion.

Its

operation

is

the

same

as

ADD

except

that

the

sign

of

the

number

is

ignored

and

the

number

is

treated

as positive.

Other

arithmetic

instructions are

treated

in

a like

manner

since all

arithmetic

processes

are

accomplished

by

addition,

and

complementing

a

result

where

neces-

sary.

Subtraction

occurs

in

the

following

manner:

7 = 000111

Number

in

Accumulator

5 = 000101

Number

to

be

Subtracted

(Operand)

111000

Complement

of

Number

in

Accumulator

+ 000101

111101

Recomplement

the

Result

(No

High-Order

Carry)

000010

This

is

the

answer, 5

subtracted

from 7 =2.

Multiplication

is accomplished by testing

the

low-

order

position

of

the

multiplier

and

adding

the

multi-

plicand

if this low-order position is a

1.

After

each

test,

the

answer is shifted

one

place

to

the

left.

This

is

repeated

until

there

are

no

more

numbers

in

the

mul-

tiplier. 5 = 000101

Multiplicand

X3=

000011

Multiplier

000101

000000

15

=00001111

Product

Division is accomplished

in

a like

manner

but

shift-

ing

occurs

in

the

opposite

direction. By

shifting

a

binary

number

one

place

to

the

left,

the

result

is

the

same as

multiplying

by 2. A

number

shifted

one

place

to

the

right

has

been

divided

by

2.

A

group

of

word

transmission

instructions

is also

provided.

These

instructions

are

concerned

with

the

movement,

at

high

speed,

of

words

or

parts

of

words

from

one

location

or

register

to

another.

In

particu-

lar,

information

may

be

either

stored

or

taken

from

locations

in

core storage

and

various registers

in

the

central

processing

unit.

Since

the

word

transmission

instructions

are

concerned

with

the

movement

of

data,

they

are

used

frequently.

The

store

(STO)

instruction

stores

the

contents

of

the

accumulator

in

the

location specified by

the

ad-

Central

Processing

Unit

15

dress

part

of

the

store instruction.

Other

store instruc-

tions accomplish

the

storing

of

the address portion,

tag,

and

other

portions

of

both

the

accumulator

and

the

multiplier-quotient registers.

The

computer

also has certain load instructions

to

accomplish the same

end

as

the

store instructions, ex-

cept

in

reverse. A store instruction places the contents

of

a register

in

storage. Conversely a

load

instruction

takes the contents

of

a storage location

and

places

it

in

a register.

There

are also

load

instructions for

the

index

registers.

With

transmit, store,

or

load

type instructions,

the

contents

of

the storage location

or

register from which

the

data

are being moved

remain

unchanged.

Shift instructions are used to move the contents

of

the accumulator

and/or

the

multiplier-quotient regis-

ters

either

to the

right

or

the left

of

their original posi-

tions.

With

the exception

of

the

rotate-MQ-Ieft instruc-

tion, zeros are automatically inserted

in

the vacated

positions of the register.

Thus,

a shift larger

than

the

bit

capacity

of

the register causes the contents

of

the

register to be lost

and

replaced by zeros.

When

a shift

instruction is interpreted, the

amount

of

the shift is

determined by

bit

positions

28

through

35

of

the

shift

instruction.

An

example is given

in

Figure

27.

Instruction

Accumulator left shift

(ALS)

Operation part Address part

+0767 00000100 = 4

SQPl

35

Accumulator contents

,,0..Q01010oo100111oo101001110111ooog.Y),01

before shift

LOS;W

4'$

Accumulator contents 01000100111001010011101110000111010000

after shift

Figure

27.

Accumulator Shifting

Control instructions are defined

as

instructions gov-

erning

the flow

of

a program.

They

are the instruc-

tions

that

cause the

computer

to alter the

normal

process of taking its instructions from sequential stor-

age locations. Control

or

"transfer" instructions may

be divided

into

two types:

(1)

unconditional transfers

specifying the location of

the

executed,

and

(2)

conditional transfers performing a

test

of

some kind.

The

location

of

the

tion

then

depends

on

the

outcome

of

the test.

Unconditional

Conditional

Transfer

(TRA)

2000.

The

come from location 2000.

Transfer

on

If

the

contents

of

the

ac-

plus

(TPL)

2000.

cumulator

are plus.

the

be

taken from 2000.

If

not

plus.

the

instruction will be taken.

Certain

test instructions exist

and

are similar to

conditional transfer instructions

in

that

they cause

16

IBM

709-7090

some test to be performed. Unlike conditional trans-

fers,

the

test instructions

do

not

specify a location for

the

is fixed relative to the position

of

the test instruction

in

the

program.

An

example is shown

in

the follow-

ing

program.

LOCATION

INSTRUCTION

100

Add

10l

P-BitTest

102 Shift

Right

103

Store

REMARKS

If

the

P

bit

of

the

accumulator

is

a

I.

the

(102)

is skipped

and

the

instruction located

after

that

(103)

is executed.

If

position P con-

tains a

O.

the

(102)

is executed.

Many

of

the indicators may be tested

in

this

manner

and

the

flow

of

the

program

adjusted to

fix

the cause

of

the

indication

or

to proceed

around

it.

Another

important

group

of

instructions

is

the

one

concerned

with

the indexing operations.

These

in-

structions are explained

under

the indexing section of

the manual.

Information

Paths

The

core storage

unit

is normally connected directly

to

the

central processing

unit.

It

is

also the site

of

the

stored program

that

controls the entire

computer

sys-

tem.

On

the

other

hand,

the auxiliary storage

and

in-

put-output

devices are normally disconnected (not

physically

but

logically) from the system.

They

be-

come connected only by execution of certain stored

program

instructions.

The

contents

of

these devices

may control the computer only after being transmitted

to core storage.

Thus,

data

flows

between

input-output

devices

and

core storage

through

the central processing

unit

in

the

704

system (Figure

28)

.

T

Figure 28. Information Flow.

IBM

704 System

Using Figure 28, assume

that

the

magnetic

tape

unit

marked

"1"

is

reading

data.

Because

the

tape

data

are

arranged

in

six-bit

groups

across

the

width

of

the

tape,

it

takes six

of

these

groups

to

make

up

a 36-bit

word. As these

groups

are

read

from

tape, they

are

as-

sembled

in

the

tape

control.

When

a full

word

is

as-

sembled, this

word

is

sent

to

the

central

processing

unit

and

from

there

to

the

core storage location speci-

fied by

the

instruction

that

caused

the

tape

to

be

read.

Actually, two

instructions

are

required

to

read

data

from

any

input-output

device

into

core storage.

The

first is a read-tape-I

instruction

with

"read"

as its op-

eration

part

and

"tape

1" as its address

part.

The

execution

of

this

instruction:

(1) selects

the

proper

tape

unit,

(2)

puts

it

into

read

status,

and

(3) starts

the

tape

moving

in

the

proper

direction.

The

second

instruction

needed

is calleda copy instruction.

"Copy"

is

the

operation

part

with

the

location

that

the

data

should

enter

(core storage) as

the

address

part.

If

a

copy-1000

instruction

were executed,

the

data

read

would

be

entered

into

core storage location 1000. A

single copy

instruction

moves

one

full

word

into

stor-

age.

If

two

or

more

words

are

to

be moved, two

or

more

copy instructions

must

be

furnished, each

with

a different address

part.

INSTRUCTION

ADDRESS

Read

Tape

I

Copy 1000

Copy

1001

Copy 5000

REMARKS

Gets

tape

1 ready

and

moving

Puts

that

word

into

location 1000

Puts

second word

into

location

1001

Puts

third

word

into

location 5000

This

type

of

instruction

group

is called a "copy

loop"

or

a copy

routine.

A

true

"loop"

is fully ex-

plained

under

the

heading

"Index

Registers."

The

information

paths

and

the

components used

in

the

709 system are

shown

in

Figure

29.

Comparing

Dota

Synchronizer

Unit

Channel

II

A"

Channel"

BII

Figure

29.

Information

Flow,

IBM

709 System

Figures

28

and

29,

note

that

the

major

difference be-

tween

the

two systems

is

the

fact

that

data

coming

from

or

going

to core storage

do

not

pass

through

the

central

processing

unit.

Instead,

a

new

device called

a

data

synchronizer is used as

the

go-between for in-

put-output

devices

and

core storage.

With

the

709

computer,

the

stored

program

starts

an

input-output

operation

by

defining

which

device

is

to

transmit

(read tape)

and

what

area

in

storage

is

to

receive

the

data;

then

the

central

processing

unit

is

free

to

do

other

calculations.

Note

the

main

differ-

ence between

the

704

and

the

709.

In

the

704,

the

cen-

tral

processing

unit

must

wait

until

all

data

have

been

transmitted

while

the

709 merely starts

the

operation

and

is

then

free

to

do

other

work.

This

subject

is

ex-

panded

under

the

section

"IBM

766

Data

Synchro-

nizer."

The

7090

computer

system uses basically

the

same

information

paths

as

does

the

709.

Two

new

units

are

added

in

the

7090 system.

They

replace

the

data

syn-

chronizer's

operation.

They

are

called

"multiplexor"

and

"data

channel."

Their

operation

is

explained

in

the

section

"IBM

766

Data

Synchronizer." Figure 30

shows

information

flow

and

components

in

the

7090

system.

There

may be a

maximum

of

eight

data

channels at-

tached

to

the

multiplexor.

Each

data

channel

may

have

the

same

complement

of

input·output

devices as

shown

in

Figure 30.

This

feature

gives a

maximum

of

80

magnetic

tape

units,

eight

card

readers,

eight

card

punches,

and

eight

printers.

Figure

30 shows

that

the

central

processing

unit

of

the

7090 system has even less

control

over

core storage

and

the

input·output

devices

than

does

the

709 system.

The

main

difference

in

all

three

systems is

the

pro-

gression from a synchronous

computer

(704) to

an

asynchronous

computer

(709-7090),

as

far

as

the

in-

put·output

controls

are

concerned.

Here,

synchronous

is

defined

as

one

operation

happening

after

another

has finished, while asynchronous means

simultaneous

occurrence

of

several operations.

L

~~~~J~-"""1Central

Processing

Unit

Figure

30.

Information

Flow,

IBM

7090 System

Central

Processing

Unit

17

Indexing

and

Indirect Addressing

The

copy

routine,

described previously, is acceptable

if

a

limited.

number

of

words are to

be

copied.

For

example, 1,000 copy instructions are needed to copy

1,000 words.

This

wastes storage space. However,

the

copy address

could

be altered by

the

program

each

time

it

is

used.

A

program

to copy 1,000 words from tape

number

I

and

place

them

into

consecutive storage locations

could be as

in

Figure 31.

In

this program,

thirteen

instructions

and

three constants are

required

tomodify

the address

of

the

copy instruction

and

test for

the

end

of

the

copy loop.

The

program

is considerably

improved over the

routine

in

which 1,000 copy in-

structions are needed. However,

the

use

of

an

index

register makes

the

program

much

more

efficient.

The

computers

contain

three

index

registers

that

are

important

to

the

system's

operational

abilities.

These

registers are

termed

A, B,

and

C

or

1,2,

and

4.

The

latter

terminology is more convenient for

the

programmer

because

the

numbers used are

the

octal

representation

of

the

"addresses"

of

the

three

index

registers.

Inst

.

Loc. Instruction

Remarks

50

Read

tape 1

Get

tape I ready

to

read

51

Copy

200

Put

the lst

word

into location

200

52 CLA62 Clear the accumulator

to

zeros and add

10

it the

contents

of

location

62

(0000----000)

53

ADD

64

Add

to

the accumulator contents the contents of lo-

cation 64 (0000----001).

These

instructions

make a counter which

is

increased

by

one

for

each

copy instruction

executed.

54

STO

62

Store the result

in

location

62.

55 CLA63 Clear and add into the accumulator, the contents

of

location

63

(0000----1000).

56

SUB

62

Subtract

from

the accumulator (1000) the contents

of location 62.

(This

is

the location that will in-

crease

with

each copy instruction

execution.)

57

TRZ

65

Transfer when the accumulator contents are

zero.

This

will occur

when

1000

copies have been

ex-

ecuted.

58 CLA51 Clear the accumulator and add the contents of lo-

cation

51

(copy 200).

59

ADD

64

Add

the contents

of

location 64

to

the contents of

the accumulator.

This

will increase the copy

address

from

200 to

201

the first time.

The

ad-

dress will then be increased

by

one each time the

instruction

is

executed.

60

STO

51

Store the result back

at

location

51

•

61

TR

51

Transfer back

to

location

51

so

that the next

word

will

wi

II

be read in.

(Remember

that the copy address

has

been increased

by

one.)

62 00---0000 Constants:

63 00---1000

64 00---0001

65

STOP

The

program will reach this instruction after

1000

words

have been copied and will stop.

Figure 31. Sample

Program

18

IBM

709-7090

operation

tag-bits

address

1-1

___

.....

1-=0_0"--'1

.....

1

__

,---_1

register

specification

octal

alphabetic

A

10 1 0 I

11

0 0 I

S,2

11

18

2021

35

Figure 32. Index-Register

Tag

Bits

2

4

B

c

The

addresses

of

index

registers are

stipulated

in

a

part

of

the

instruction

word

known as

the

tag field.

Such addresses are normally referred to as tags.

The

IBM 709 Configuration guide

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