# index register

> CPU register used for modifying operand addresses

**Wikidata**: [Q1661183](https://www.wikidata.org/wiki/Q1661183)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/Index_register)  
**Source**: https://4ort.xyz/entity/index-register

## Summary
An index register is a CPU register used for modifying operand addresses. It is a type of processor register — immediately accessible working storage inside a digital processor — and appears in processor designs such as the Zilog Z80.

## Key Facts
- An index register is a CPU register used for modifying operand addresses.  
- Index register is a subclass of the processor register class (immediately accessible working storage available as part of a digital processor).  
- Wikipedia title: "Index register"; Wikidata description: "CPU register used for modifying operand addresses."  
- Aliases: Registro indice; registres d'index; 变址寄存器; 索引寄存器.  
- Freebase ID: /m/021y42 (reference entry dated 2013-10-28).  
- Elhuyar ZTH ID: 137205.  
- Sitelink count (property): 13 (across Wikimedia projects for the index register entry).  
- Microsoft Academic ID (discontinued): 202788945.  
- Listed subsidiary/associated implementation: Zilog Z80 — an 8-bit microprocessor (inception: March 1976; Z80 sitelink_count: 41).

## FAQs
### Q: What does an index register do?
A: An index register is a CPU register whose role is to modify operand addresses. It is used by the processor during address calculation to change where operands are read from or written to.

### Q: How is an index register classified in processor terminology?
A: An index register is a subclass of the processor register class — that is, immediately accessible working storage available as part of a digital processor.

### Q: Which processors use index registers?
A: Index registers are a feature of CPU designs; one listed associated implementation is the Zilog Z80, an 8-bit microprocessor introduced in March 1976.

## Why It Matters
Index registers provide a dedicated hardware location used in address modification, which is essential for processors that perform dynamic address calculation. By holding values that the processor can add to or combine with base addresses or instruction-specified offsets, index registers support flexible addressing modes within a CPU. This capability is used in many processor designs and instruction sets to enable iteration, table lookup, and access to variable data locations without repeatedly computing full addresses in software. As a specific class of processor register, index registers are a standard element in CPU architecture descriptions and are documented across language editions of Wikipedia and structured knowledge sources (Freebase, Wikidata, Elhuyar). Their presence in well-known microprocessors such as the Zilog Z80 illustrates their practical use in commercial CPU implementations.

## Notable For
- Being defined specifically as a CPU register whose purpose is to modify operand addresses.  
- Classification as a subclass of processor register (immediately accessible working storage in a digital processor).  
- Presence in documented microprocessor designs such as the Zilog Z80 (8-bit microprocessor, inception March 1976).  
- Multiple language coverage and structured identifiers (Wikipedia title, Freebase ID /m/021y42, Elhuyar ZTH ID 137205).

## Body
### Definition
- An index register is a CPU register used for modifying operand addresses.  
- The term is defined in structured knowledge sources with the description "CPU register used for modifying operand addresses."

### Classification
- Subclass of: processor register.  
- Parent class description: processor register = immediately accessible working storage available as part of a digital processor.

### Aliases and language coverage
- Aliases provided: Registro indice; registres d'index; 变址寄存器; 索引寄存器.  
- Wikipedia entry title: "Index register."  
- Wikipedia language coverage includes: ar, da, de, en, es, fa, fr, it, lmo, pt.

### Identifiers and metadata
- Freebase ID: /m/021y42 (reference dated 2013-10-28).  
- Elhuyar ZTH ID: 137205.  
- Microsoft Academic ID (discontinued): 202788945.  
- Sitelink count for the index register entry: 13.

### Implementations / Associated hardware
- Listed associated implementation: Zilog Z80 — described as an 8-bit microprocessor with inception March 1976 and its Wikidata/Wikipedia sitelink count of 41.

## References

1. Freebase Data Dumps. 2013