In
computer instruction set architectures (ISA), a repeat instruction is a
machine language instruction which repeatedly executes another instruction a fixed number of times, or until some condition is met.
Since it is an instruction that operates on other instructions like the
execute instruction, it has been classified as a meta-instruction.
Computer models
The
Univac 1103 (1953) includes a ''repeat'' instruction (
op code mnemonic: ) which executes the following instruction a fixed number of times, possibly incrementing one or both of the address fields of that instruction. This compensates for the architecture's lack of
index registers.
The
GE-600/
Honeywell 6000
The Honeywell 6000 series computers were rebadged versions of General Electric's 600-series mainframes manufactured by Honeywell International, Inc. from 1970 to 1989. Honeywell acquired the line when it purchased GE's computer division in 19 ...
series (1964) supports a single-instruction repeat (), a double-instruction repeat (), and a
linked-list repeat ().
The
x86 ISA, starting with the
8086, includes a series of special-purpose repeat instructions () which are called "repeat string operation prefixes" and may only be applied to a small number of string instructions (). These instructions repeat an operation and decrement a counter until it is zero, or may also stop when a certain condition is met.
[Intel Corporation, "Instruction Set Reference", ''Intel 64 and IA-32 Architectures Software Developer's Manual'', Order Number 253667-060US, September 2016, 2B:4-55]
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The Texas Instruments TMS320 digital signal processor (1983) includes an instruction for repeating a single-cycle instruction or two single-cycle instruction in parallel () and an instruction for repeating a block of instructions (). These use special block-repeat counter registers ().[{{Cite book , last=Kuo , first=Sen-Maw , url=https://books.google.com/books?id=70scPIwK7cUC&newbks=0&printsec=frontcover&q=block+repeat&hl=en , title=Real-time Digital Signal Processing: Implementations, Applications, and Experiments with the TMS320C55X , date=2003 , publisher=清华大学出版社有限公司 , isbn=978-7-302-07700-8 , pages=67-68 , language=en]
Semantics
The instruction or instruction pair to be executed follows the ''repeat'' instruction. Fields in the instruction determine the loop termination condition.
Notes
Central processing unit
Instruction processing
Instruction set architectures