IBM 1620 1 Manual page 17

Data Flow and Field Length Definition
Data is read serially from right to left (low-order to
high-order) until terminated by a flag bit defining the
high-order position of the field. For example, where
the data in a field is 285, the dash (flag bit) over the
high-order digit indicates a field mark. The Program
Testing section of this manual provides more specific
data flow information.
The minimum length of both the P and
Q fields is
two digits: a units digit which contains the sign, and
at least one higher-order digit which is needed for
field definition.
Arithmetic Indicators
Three arithmetic indicators and their associated con-
sole lights are controlled by Arithmetic instructions
and turned off by the Reset key on the 1620 console.
High/Positive. The High/Positive (H/P) indicator
is turned on at the beginning of each Arithmetic in-
struction and remains on if the result is positive and
not zero. It is turned off if the result is negative
or zero.
Equal/Zero. The Equal/Zero (E/Z ) indicatO'r is
turned on at the beginning of each Arithmetic instruc-
tion and remains on if the result is zero. It is turned
off if the result is not zero.
Arithmetic Overflow (O'flow). The Arithmetic
Check indicator is turned on during the execution of
Add, Subtract, and Compare instructions, if either
of the following conditions exists:
1. The number of digits in the
Q
data exceeds
the number of digits in the P data. Only the
number of digits in the Q data that equal the
number of digits in the P data are used in de-
veloping the result.
2. The result causes a carry beyond the high-
order position of the initial field at P. (The
carry is lost.)
This indicator is also turned on during a Divide
operation if more than nine successful subtractions
occur (ten or more subtractions indicate the divisor is
mispositioned) .
The Arithmetic Check indicator is turned off by
the execution of a Branch Indicator or Branch No
Indicator instruction, or by m'anual pressing of the
Reset key, and does not automatically turn on at the
beginning of each Arithmetic instruction.
Table Look-up
A unique method of doing arithmetic calculations is
used in the 1620. Two tables (Multiply and Add),
stored in the "table area" of core storage, are auto-
matically referred to by the computer during arith-
metic operations. The positions of core storage con-
taining the table data are addressable but must not
be altered; altering can cause incorrect arithmetic
operations to result.
Three hundred positions of core storage have been
assigned to the Table area. Two hundred positions,
00100 through 00299, are assigned to the storage of
the Multiply table. One hundred positions, 00300
through 00399, are assigned to the storage of the Add
table used in all arithmetic operations. (See Appen-
dix B.) A digit with a flag bit in the table indicates
that a carry is associated with that digit.
In addition, 20 positions, 00080 through 00099, are
used to receive the product or partial product in
Multiply operations.
Sign Analysis
Addition and Subtraction. The data in the Q field
is either true-added or complement-added to the data
in the P field. A true-add operation causes the
Q data
to be added just as it is. A complement-add operation
causes the Q data to be altered before addition, as
follows: the units digit is tens-complemented and the
remaining higher-order digits are nines-complemented
(95 becomes 05, 139 becomes 861, 2476 becomes
7524, etc.). The sign analysis chart in Figure 11
shows that (1) the Q data is complement-added dur-
ing addition and subtraction when the signs of the
P and Q fields are different and alike respectively,
( 2) if the
Q
field is complemented and if the value of
the original
Q
data is higher than the value of the
P data, the sum or difference is recomplemented, and
(3) if a recomplement occurs, the original sign of the
P field is changed.
For example:
Add
+15
(Q data) to
-35
(P data).
According to the sign control analysis chart
1. The Q data is complement-added
(15
becomes
85 ),
and
85
+
35
==
'2[
The hundreds carry is
lost. A carry is always lost when it causes the
sum or difference to exceed the size of the P
field. Arithmetic Check is not turned on since
the carry in this case indicates that recomple-
menting the P field is not required.
2. The sum is not recomplemented (15 is less
than 35).
3. The sign of the P field is not changed since
no recomplement occurred.
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