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An instruction register holds a machine instruction that is currently being executed. In general, a register sits at the top of the memory hierarchy. A variety of registers serve different functions in a central processing unit (CPU) – the function of the instruction register is to hold that currently queued instruction for use.
In a typical CPU, in addition to an accumulator, there are registers such as an address register, a data register and an index register, along with the instruction register. The CPU performs fetch, decode and execute operations on memory units according to its use of the registers. All of this serves the purpose of the memory processing that is at the heart of the CPU’s raison d’etre, which is why some experts call registers “the most important part of the CPU.” In a sense, the instruction register is particularly important in that it holds the “active” memory value that is being worked on at a given time.