Information Paths - IBM 709 General Information Manual

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 next instruction to be
executed, and (2) conditional transfers performing a
test of some kind. The location of the next instruc-
tion then depends on the outcome of the test.
Unconditional
Conditional
Transfer (TRA)
2000.
The next instruction will
come from location 2000.
Transfer on If the contents of the ac-
plus (TPL) 2000. cumulator are plus. the
next instruction will be
taken from 2000. If not
plus. the next sequential
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 next instruction. Instead, the alternative location
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 next instruction (102) is skipped
and the instruction located after that
(103) is executed. If position P con-
tains a O. the next instruction (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