IBM 7090 Instruction-Reference page 29

3.1.01 Storage Register (SR) (Systems 2.01. 00. 1-2.01. 37. 1)
The storage register is a 37 -position shift cell register and the register positions are
labeled (S), (1-35), and (Q). All instructions and data coming into or leaving the CPU
go through the storage register, positions (S), (1-35). Storage register positions (S),
(1-35), and (Q) are also used for holding factors and partial results during the execu-
tion of many instructions.
3. 1. 02 Accumulator Register (AC) (Systems 2. 03. 00. 1-2. 03. 37. 1)
The accumulator is a 38-position shift cell register used in nearly every arithmetic
operation. The register positions are labeled (S), (Q), (P), (1-35). Positions (S), (1-35)
accommodate the word in standard operations. Positions (Q) and (P) are used as
overflow positions because the sum of two 35-position numbers can be greater than 35
positions. Postion P also holds the S bit of a word during logical operations. As the
name implies, the accumulator is the unit into which results accumulate; it does not
perform the addition.
3.1.03 Multiplier-Quotient Register (MQ) (Systems 2.04.00. 1-2.04.35.1)
The multiplier-quotient register is a 36-position shift cell register with the positions
labeled (S), (1-35). This register has several uses. During a multiply operation it holds
the multiplier; after the multiply operation it holds the least significant half of the prod-
uct. During a divide operation, it holds the least significant half of the dividend; after
the divide operation it holds the quotient. In several floating point operations, the MQ
holds the least significant 35 bits of the result.
3.1. 04 Sense Indicator Register (SI) (Systems 2.06.00.1-2.06.35.1)
The sense indicator register is a 36-position trigger register, labeled (0-35). Each
position of the register contains two triggers. One of the triggers is used to retain in-
formation in the register, and the other trigger is used as a remembering device during
an invert operation. The sense indicator register is controlled completely by the pro-
gram and is not used by the computer as a part of its arithmetic operations. It can be
used as a set of switches which are set and tested by the program to check the progress
or direction of the program. The
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register may also be used to store words or parts
of words temporarily and, in this way, it is useful for altering and testing words. The
SI register inputs and outputs go directly to the storage register.
3.1. 05 Index Registers (XR) (Systems 2.07.01.1-2.07.15.1)
There are three 15-position shift cell registers called index registers A, B, and C.
The register positions are labeled (3-17). The three registers are identical in opera-
tion and are used for instruction address modification .• They are activated by the tag
positions of an instruction and can be used singly or in parallel or combinations. They
modify an address by adding the complement of their contents to the address; in effect,
the address is reduced by the contents of the index register.
There are many instructions which operate on the index registers, thereby making
these registers useful programming tools for counting, word alteration, program loop
control and so forth.
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