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index register[′in‚deks ‚rej·ə·stər]
(also called register of modifications, address register, or address increment register), an assembly of the control unit of a digital computer designed for the reception, storage, and output of the codes used for automatic changing of instructions. Before the instruction is performed, any part of it, usually the address part, may be modified by multiple additions of codes contained in the index register. For example, the effective address in a modern digital computer with multiprogramming is formed as the sum of the base, index, and relative addresses, which are stored in the index register.
The number of index registers in a computer may reach several dozen; their speed of operation is usually an order higher than the speed of the primary immediate-access memory. The index register is made either as an independent block of flip-flop registers or as a high-speed memory block with ferrite cores, tunnel diodes, and so on; part of the computer’s immediate-access memory may also be assigned to the index register. The index register increases the output of the computer, reduces the volume of problem-solving programs, simplifies programming, and carries on automatic readdressing at the same time as other computer operations.
REFERENCEProektirovanie sverkhbyslrodeistvuiushchikh sistem. Moscow, 1965. (Translated from English.)
A. V. GUSEV
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)!