• A processor register (CPU register) is one of the computer processor's modest collection of data storage areas.
• An instruction, a storage address, or any other type of data can be stored in a register (such as a bit sequence or individual characters). Some instructions include instructions that define registers.
• A register must be large enough to hold an instruction; in a 64-bit computer, for example, a register must be 64 bits long. Smaller registers, such as half-registers, are used in some computer systems for shorter instructions. Registers can be numbered or given arbitrary names, depending on the processor design and language restrictions.

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