index register

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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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Index Register


(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.


Proektirovanie sverkhbyslrodeistvuiushchikh sistem. Moscow, 1965. (Translated from English.)


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

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)!
This article is provided by FOLDOC - Free Online Dictionary of Computing (

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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