index register

Definition

Noun:
- A specialized storage location within a central processing unit (CPU): An index register is a small, high-speed memory circuit inside a computer's processor. Its primary function is to hold a numerical value (an index or offset) that is used to modify, or calculate, a memory address during program execution.

Usage

In Programming and Computer Architecture: The term is used when describing low-level operations, assembly language programming, or CPU design. It specifies a register whose contents are added to (or sometimes subtracted from) another address to form the effective address of an operand (data).
- The programmer loaded a value into the index register to iterate through the array.
- The instruction uses the index register to calculate the address of the next data element.

Advanced Usage

Base-Index Addressing: A common addressing mode where the effective address is the sum of a base address (often in a base register) and an offset (in the index register).
- The base register held the start of the structure, and the index register was used to access individual fields.
Auto-increment/Decrement: Some processor architectures automatically increment or decrement the value in an index register after it is used, facilitating loops.
- The post-increment feature of the index register streamlined the loop code.

Variants and Related Words

Indexed Addressing (n): The method of address calculation that employs an index register.
- The compiler generated efficient code using indexed addressing.
Base Register (n): Another CPU register often used in conjunction with an index register to hold a starting memory address.

Synonyms

Modifier Register: A less common synonym emphasizing its role in modifying addresses.
Offset Register: Highlights its use for holding an offset value.

Related Phrases

To index into an array: A phrase describing the action of using an index value to access an array element, which is the practical application of an index register.
- The program uses a loop counter to index into the array efficiently.

Noun