Index register

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An index register in a computer's CPU is a processor register (or an assigned memory location)^[1] used for pointing to operand addresses during the run of a program. It is useful for stepping through strings and arrays. It can also be used for holding loop iterations and counters. In some architectures it is used for read/writing blocks of memory. Depending on the architecture it may be a dedicated index register or a general-purpose register.^[2] Some instruction sets allow more than one index register to be used; in that case additional instruction fields may specify which index registers to use.^[3]

Generally, the contents of an index register is added to (in some cases subtracted from) an immediate address (that can be part of the instruction itself or held in another register) to form the "effective" address of the actual data (operand). Special instructions are typically provided to test the index register and, if the test fails, increments the index register by an immediate constant and branches, typically to the start of the loop. While normally processors that allow an instruction to specify multiple index registers add the contents together, IBM had a line of computers in which the contents were or'd together.^[4]

Index registers has proved useful for doing vector/array operations and in commercial data processing for navigating from field to field within records. In both uses index registers substantially reduced the amount of memory used and increased execution speed.