index register

index register

A high-speed circuit used to hold the current, relative position of an item in a table (array). At execution time, its stored value is added to the instructions that reference it. In a programming language, subscripts are an example of high-level statements that are ultimately implemented at machine level using index registers (see subscript).

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index register [′in‚deks ‚rej·ə·stər]

(computer science)

A hardware element which holds a number that can be added to (or, in some cases, subtracted from) the address portion of a computer instruction to form an effective address. Also known as base register; B box; B line; B register; B store; modifier register.

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(processor)

index register - A register found in some CPUs, whose contents can be added to the address operand to give the effective address. Incrementing the index register then allows the program to access the next location in memory and so on, making it very useful for working with arrays or blocks of memory. Index registers first appeared around April 1949 in the Manchester Mark I. The Mark I's index register's contents were simply added to the entire instruction, thus potentially changing the opcode (see The story of Mel)!