An index register is a circuit that receives, stores, and outputs instruction-changing codes in a computer. This circuit is also called an address register or a register of modifications.
A register is a data-holding place in a computer processor. A register can hold an instruction, a storage address, or any kind of data. The effective address of an entity in a computer running multiple programs comprises the base, index, and relative addresses, all stored in the index register, and any of which can be modified by the index register before the entity is acted upon (for example, an instruction is carried out).
A typical computer contains several index registers, sometimes more than a dozen. Index registers operate much faster than primary storage (main memory), although part of the primary storage may be assigned to the index register if needed. The index register improves computer performance by speeding up simple operations.