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=
1.
This meanS that any output posi-
tion will have a bit piesent only when
both A and B input positions have bits.
Table 1, in Figure 2- 28, shows that LM
and Connect controls are CN resulting in
these control lines, plus N being up.
Any of the three conditions, A without
B, B without A, or neither A nor B will
cause the output of AOI
1
to be down and
will allow the output of ACI 2 to be up.
The other condition, both A and B will
cause the opposite outputs with ACI
1
up
and ACI 2 down.
For the logical AND
operation, the Not Carry In line is
always up causing Sum
1
output if ACI
1
is up and a Not Sum
1
output if ACI 2 is
up.
The Connect control is effective in
preventing a carry output as explained
under Exclusive Cr operation.
(iR
Cperation
The logical -CR- function will cause a
bit in any output position in which
there is a bit in either input A or
input B or both.
The control lines up
for the operation will be Connect with
LM and N.
Thus the output of ACI
1
will
be down only if no bits are present in A
or B.
The output of ACI 2 will be down
under the three other possible condi-
tions, a bit in either A or B or both.
AS all logic operations use Not Carry
In, the up output of ACI
1
will switch
at ACI 4 to create a Sum, and the up
output of ACI 2 will switch at AOI 3 to
create the Not Sum.
Again, the correct
control eliminates the carry as pre-
viously described.
CARRY OPERATION
•
A Carry from one position to another
occurs only in ALU circuits during
Hex or Decimal Add operations.
•
Carries do not occur in the Decimal
Corrector or Excess-6 circuits.
Line Levels are -switched- for cor-
rect answers.
•
The Carry Latch is the third posi-
tion of the S-Register (S-3).
Carries into and out of ALU involve
several circuits.
As shown in the exam-
ples previously given, a carry can occur
from any pOSition of the byte, zero
through seven.
Within ALU, carries only
occur into positions six through zero.
(Carries into position seven will be
2-18
confirmed later).
The vehicle for
transferring the carry frQm one position
to the next is simply a line labelled
Carry
Bit.
The line Carry 1 Bit,
for example, is made active in the ALU 7
Bit circuitry and ties back to the input
of ALU 6 Bit to switch with Control
Lines, A-register inputs and True-
Complement inputs.
Developed by this
circuit are the lines +L Sum 6 Bit, -L
Sum 6 Bit, +L Carry 6 Bit and -L Carry 6
Bit.
See Figure 2-29.
Control~
1
and
Inputs
5-Bit
Position
Controls
!
and
Inputs
+l Carry 7 Bit
and
Position '""+'"'l""'S,..u-m.:.:7:-:B,.,.it---.....
Controls
I
7-Bit
-l Carry 7 Bit
Inputs
- - - - - - - i
-l Sum 7 Bit
Carry- {
In
Figure 2-29.
ALU Carry
ALU is the only circuit where a carry
can occur from one position to another.
Although carries were shown in the
excess-6 and the decimal corrector
operations of Figures 2-24 and 2-25, a
carry line does not actually exist.
The
switching of various lines and levels
result in. the desired output.
For exam-
ple in Figure 2-24, Bit position 4, the
final answer shows a 4 Bit ON.
The
result was obtained by investigating the
4, 5, and 6 positions as shown in Figure
2-30.
Other outputs from these correc-
tion circuits are determined by the
input lines of Carry 4.
The same type
of switching is done in the excess-6
circuitry.
In this manner most of the
arithmetic operations require just one
pass through ALU.
A recomplement cycle
will be the next micro-program step
however. if there was no carry out of
the high order position.
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