IBM 709 General Information Manual page 13

Accumulator register
before a left shift.
5 Q P 1 35
I
! !
:100000000000000000000000000000000001
5 Q P 1 - - - - - - - - - - - 3 5
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