Registers
The IBM 709 had a 38 bit accumulator, a 36 bit multiplier quotient register, and three 15 bit index registers whose contents were subtracted from the base address instead of being added to it. Addition could be performed using two's complement arithmetic and instructions were provided for this purpose. All three index registers could participate in an instruction: the 3 bit tag field in the instruction was a bit map specifying which of the registers would participate in the operation, however if more than one index register was specified, their contents were combined by a logical or operation, not addition.p. 12
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