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Most commonly, "register" refers to a piece of the processor that acts like a small block of memory. Though processor architectures vary in the number (usually some multiple or power of 2) and use of their registers, many of them follow certain conventions. For instance, some registers have data types (integers, booleans, floating points, etc.) and others are used for storing control commands/instructions. Sometimes, the registers which store commands act as pointers to other registers, and they often have the same size as registers that are used to store integers.
At a higher level, registers can be used to fetch information from RAM, perform simple calculations, and generally move data around. Languages that are closest to the assembly language of the architecture (C, for instance) often reveal a bit about how registers on a particular system work.
it is the processor used to fetch information in the memory