Convert Instructions - IBM 7090 Instruction-Reference

Logically speaking, the EXCLUSIVE OR is the result of OR 'ing bits from either one
word or the other, but not both. The computer, which can only AND and OR, makes
use of the following logical equation to develop the EXCLUSIVE OR:
EXCLUSIVE OR
=
2 (A or B) - ( A
+
B)
The derivation of the above equation can be shown with the following adder table:
Factor A 0 0 0 1 1
Factor B 0 0 1 0
1
A + B (carry not blocked) 0 1 0 0 0
Carry to be blocked 0 0
0
1 0
A + B (carry blocked)
0 0 1
1
0
EXCLUSIVE OR (A + B carry blocked) 0 0 1
1
0
A or B 0 0 1 1 1
2 (A + B)
1
0 0 0 0
2 (A or B) 0 1
1
1 0
From the table it can be seen that the EXCLUSIVE OR is equal to the sum output of
the adders with all carries blocked. The carries cannot be blocked, but the EXCLUSIVE
OR can be obtained by subtracting an amount equal to the blocked carries from the sum
of two words:
1. EXCLUSIVE OR
= (
A + B) - blocked carries
The blocked carries can be simulated by subtracting twice the OR from twice
the sum of two words:
2. Blocked carries
=
2 (A + B) - 2 (A or B)
=
10000 - 01110
=
00010
Substituting the equation 2 in equation 1:
3. EXCLUSIVE OR
=
(A + B) -[2 (A + B) - 2 (A or B[]
=
01000 - 00010
=
00110
And Simplifying equation 3:
4. EXCLUSIVE OR
=
2 ( A or B) - ( A
+
B)
=
01110 - 01000
=
00110
5.3.12 Convert Instructions
The three convert instructions can materially reduce the time required for many
"housekeeping" and table-look-up routines. They can be used for number conversions,
for preparing print fields, and even for adding numbers in systems other than binary.
The convert instructions, like variable-length instructions, include a count field as
well as the operation code, address and tag bit. The convert instructions cause a series
of references to be made (usually six) and the address specifies the starting location
of the first storage table. The register (accumulator or MQ) from which the refer-
ence is controlled is considered to be made up of six 6-bit groups. The first of these
groups is added to the instruction address to give the location of the first storage refer-
ence. The word stored at this location must contain, in addition to its conversion in-
formation, the starting location of the next storage table. The convert-by-replacement
instructions shift the controlling register six places, clearing the six places on the
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