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IBM 7090/94 Register Set

The 7090/94 contained registers that could be directly addressed and modified or queried by the programmer and other registers that were only used by the machine during the instruction cycle but were not directly addressable by the user.

The contents of these registers were all viewable, to the programmer or machine operator, in real-time by indicator lights on the console control panel.

The convention on the 7090/94 was to number bits from left to right with the left most bit being assigned Bit-1 (1, 2, 3, ..., 36).

7090/94 Non-Addressable Registers
'Instruction Counter'15-BitsHeld address of the next instruction to be executed.
'Address Register'15-BitsHeld address of the data or instruction to be read from memory.
'Storage Register'36-BitsServed as a buffer between core storage and the central processor -- Data read from memory was first placed in the Storage Register before being used.
'Instruction Register'18-BitsHeld the operation portion of an instruction word while the instruction was being executed.

7090/94 User-Addressable Registers
'AC' (Accumulator) Register38-BitsUsed primarily for addition, subtraction, shifting, masking, etc. One data word was loaded into the register and the other data words were added or subtracted from it or the AC was shifted, logically masked, etc. The results were retained in the AC register from which they were then moved by an instruction. The AC register had 38-bits with the two extra bits used to detect overflow from arithmetic operations.
AC Bit Layout -- [Left to Right] Sign-Bit('S'), Second Overflow-Bit('Q'), First Overflow-Bit('P'), 35-Bit Data Value => 'S, Q, P, 35-Bits DATA (Bit-1,... Bit-35)' (36-Bit Data Word Including the Sign Bit -- 38-Bits Total with Overflow Indicator Bits)
'MQ' (Multiplier-Quotient) Register36-BitsUsed primarily with the AC for multiplication and division. In multiplication, the multiplier was placed in the MQ register and the results were developed in the MQ and the rest in the AC. In division, the MQ would contain the quotient and the AC would retain the calculated remainder.
'XR' (3 or 7 Index Registers for the 7090 or the 7094 respectively)15-BitsThere were three index registers in the 7090 and seven in the 7094. The index registers were primarily used for address modification.
'SI' (Sense Indicator)36-BitsUsed primarily for reading and writing to and from the sense lights/switches on the control console that could be used to control and query running programs on-the-fly. -- Last Revision: 23 August 2001
Copyright © 1996 - 2017 Jack Harper (unless otherwise noted)
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