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Core Storage
Add Instruction
Op
Reg
I
A
I
p
I
Q
I
~
~~------------~
MARS
Figure 60.
E Cycle of Add Operation
Subtraction
The following steps with reference to Figure 61 indi-
cate how 52 is subtracted from 185:
70
l. The machine automatically inserts 003 into the
high-order positions of
MAR.
The Add table
( 00300-00399) is used for subtraction also.
2.
The
P
units digit
(5)
goes into the tens posi-
tion of
MAR.
3.
The Q units digit (2) is tens-complemented
and the complement (8) goes into the units
position of
MAR.
4. The difference (3), which is located at 00358
of the Add table, replaces the
P
units digit.
A carry is also present at 00358.
5. The P tens digit (8) goes into the tens position
of
MAR.
6.
The
Q
tens digit (:5) is nines-complemented
and the complement (4), plus the stored carry
(step 4), causes a five (5) to go into the
units position of
MAR.
7. The difference (3), which is located at 00358
of the Add table, replaces the P tens digit. A
carry is also present at 00385.
8.
The P hundreds digit (I) goes into the tens
position of
MAR.
9.
There is no
Q
hundreds digit, but a zero is
inserted internally. The zero is nines-comple-
mented and the nine, plus the carry stored in
step 7, causes a zero (0) to go into the units
position of
MAR.
10. The difference (1), which is located at 00310
of the Add table, replaces the P hundreds
digit.
Data flow for subtract is the same as that for add
(Figure 60) except that the
Q
digits are comple-
mented as they are routed from the Digit and Branch
register to
MAR.
The console Complement light is turned on when
a digit is complemented.
Multiplication
Multiplication is accomplished in the 1620 by combin-
ing the digits to be multiplied into a Multiply table
address. The answer is then "looked up" in the Multi-
ply table. Reference to the table in Appendix
B
will
aid in following the procedure shown in Figure 62,
which shows how each multiplier digit is used with
the multiplicand. Twelve is multiplied by 12 as
follows:
1.
The machine automatically inserts 00 into the
two high-order positions of
MAR.
(The Multi-
ply table is located in core storage positions
00100-00299. )
2.
The P units digit (2) goes into the tens posi-
tion of
MAR.
3. The
Q
units digit (2) is routed through the
Multiply register and the doubler. The doubler
is an internal device that doubles the units
digit of the multiplier. The doubler increases
the 2 to 04 and rautes its units digit (4)
to'
the units pasitian af
MAR.
The tens digit af
the daubler (0) is incremented by. ane and
rauted to the hundreds pasitian of
MAR.
4.
The praduct is "laO' ked up" in the Multiply
table. The develaped
MAR
address is 00124
and the digit 4 is located at that address of
the Multiply table. Internal machine aperation
causes the twa digits within the heavy black
vertical lines of the Multiply table
to'
be re-
versed and routed aut. In this example, a
4
is lacated at 00124. A zero
is
'n'ext to the 4 and
bath are faund within the ~arne heavy ver-
tical lines. Thus 04 is rauteq O'4t of the Mul-
tiply table.
5.
The 04 is added
to'
00 in the product area
(the prO' duct area was reset to zeros as a result
of the multiply instructian). Twa add cycles
are necessary to' accamplish this addition.
6. The P tens digit (I) gaes intO' the tens pasi-
tian af
MAR.
7. The
Q
units digit (2) is doubled, etc., as de-
scribed in Step 3.
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