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The 1620
CPu
(Figure 1) contains the arithmetic and
logic section of the system.
Data Representation
Data can be classified as digits, fields, or records,
depending upon the operation in which the data is
addressed.
Digits
BCD Bit Array. Each core storage position is ad-
dressable and can store one digit of information in
BCD
fonn (C, F, 8, 4, 2, and 1). The bit positions of
each digit consist of four numerical bits, one flag (F)
bit, and one check (C) bit.
Check
Flag
Bit
Bit
Numerical Bits
~--~----------/
C
F
8
4
2
The value of a decimal digit is the sum represented
by the bits present in the 8, 4, 2, and 1 numeric bit
positions. Only bit combinations whose sum is nine
or less are used. A negative numeric expression has
a sign flag in the units position of its field. Consider-
ing only the numeric bit positions, the decimal 6 is
Figure
1.
IBM 1620 Modell Central Processing Unit
1620 Central Processing Unit, Model 1
represented as 0110, the decimal 7 as 0111, etc., as
shown in Figure 2.
Check Bit (C). Each digit position within the com-
puter must contain an odd number of coded bits, in-
cluding a flag bit, if there is one, for correct parity.
To create this odd-bit number, a C bit is automatically
added, when required, to each digit position as data
enters core storage. Thereafter, during processing, a
digit position with an even number of bits causes the
machine to signal a parity error. A C bit alone repre-
sents a plus zero.
Flag Bit (F). Depending on its location and the
operation performed, the flag bit is used in five ways:
o
1
2
3
4
5
6
7
8
9
1. Sign Control
A numeric data field is negative if the units
(low-order) digit contains a flag bit, and posi-
tive if the units digit does not contain a Hag
bit. Minus five is shown as
5;
plus five is shown
as 5. The
BCD
representations for minus and
plus five are F-4-1 and C-4-1, respectively.
2.
Field Mark
A flag bit as a field mark defines the leftmost
(high-order) digit of a numeric data field. A
field is shown as XXXX where the dash over
the high-order digit is the field mark.
3. Carries
Flag bits present in certain digits of the Add
table (Appendix B) are interpreted in arith-
metic operations as carries. For example, an
eight with a carry is shown as
8.
Flag bits
are contained in table storage and transferred
automatically, as required.
4.
Minus Zero
The F bit alone represents a minus zero (0).
C
F
8
4
2
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Figure 2.
BCD Digits 0-9
3
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