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Gener5al Purpose Registers
The General Purpose Registers (GPRs) can be viewed as High Speed Devices.
The original implementation, on System/360, gave them a size of 4-bytes, but only three bytes were used for address calculation
due to the 16MB memory limit.
Extended Architecture (XA), which introduced 31-bit addressing was still accommodated in 4-byte registers, albeit with the
high-order bit reserved.
The advent of 64-bit addressing meant the size of the registers was increased to 8 bytes, but for compatibility only the right-most
4 bytes are used in 24 and 31 bit addressing modes.
The role of GPRs in addressing is discussed next.