IBM 7090 User Manual

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Logically

speaking,

the

EXCLUSIVE OR

is

the

result

of OR

'ing

bits

from

either

one

word

or

the

other,

but

not

both.

The

computer,

which

can

only

AND

and

OR,

makes

use

of

the

following

logical

equation

to

develop

the

EXCLUSIVE OR:

EXCLUSIVE OR

= 2

(A

or

B)

- ( A +

B)

The

derivation

of

the

above

equation

can

be

shown

with

the

following

adder

table:

Factor

A

0

0 0

1 1

Factor

B 0 0 1 0

1

A

+ B

(carry

not

blocked)

0

1

0

0

0

Carry

to

be

blocked

0 0

0

1

0

A + B

(carry

blocked)

0 0 1

1

0

EXCLUSIVE OR

(A

+ B

carry

blocked)

0 0

1

1

0

A

or

B 0 0 1

1 1

2

(A

+

B)

1

0 0

0 0

2

(A

or

B)

0

1 1

1

0

From

the

table

it

can

be

seen

that

the

EXCLUSIVE OR

is

equal

to

the

sum

output

of

the

adders

with

all

carries

blocked.

The

carries

cannot

be

blocked,

but

the

EXCLUSIVE

OR

can

be

obtained

by

subtracting

an

amount

equal

to

the

blocked

carries

from

the

sum

of

two

words:

1.

EXCLUSIVE OR = ( A +

B)

-

blocked

carries

The

blocked

carries

can

be

simulated

by

subtracting

twice

the

OR

from

twice

the

sum

of

two

words:

2.

Blocked

carries

= 2

(A

+

B)

- 2

(A

or

B)

= 10000 - 01110 = 00010

Substituting

the

equation

2

in

equation

1:

3.

EXCLUSIVE

OR =

(A

+

B)

-[2

(A

+

B)

- 2

(A

or

B[]

= 01000 - 00010 = 00110

And

Simplifying

equation

3:

4.

EXCLUSIVE OR = 2 ( A

or

B)

- ( A +

B)

= 01110 - 01000 = 00110

5.3.12

Convert

Instructions

The

three

convert

instructions

can

materially

reduce

the

time

required

for

many

"housekeeping"

and

table-look-up

routines.

They

can

be

used

for

number

conversions,

for

preparing

print

fields,

and

even

for

adding

numbers

in

systems

other

than

binary.

The

convert

instructions,

like

variable-length

instructions,

include

a

count

field

as

well

as

the

operation

code,

address

and

tag

bit.

The

convert

instructions

cause

a

series

of

references

to

be

made

(usually

six)

and

the

address

specifies

the

starting

location

of

the

first

storage

table.

The

register

(accumulator

or

MQ)

from

which

the

refer-

ence

is

controlled

is

considered

to

be

made

up

of

six

6-bit

groups.

The

first

of

these

groups

is

added

to

the

instruction

address

to

give

the

location

of

the

first

storage

refer-

ence.

The

word

stored

at

this

location

must

contain,

in

addition

to

its

conversion

in-

formation,

the

starting

location

of

the

next

storage

table.

The

convert-by-replacement

instructions

shift

the

controlling

register

six

places,

clearing

the

six

places

on

the

132

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IBM 7090 Specifications

General

Category	Mainframe Computer
Introduced	1959
Transistor-based	Yes
Word Length	36 bits
Add Time	4.8 microseconds
Memory	Core memory
Memory (words)	32, 768 words