Computer Operations - IBM 709 General Information Manual

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
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