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.[1]

Computer models

The Univac 1103 (1953) includes a repeat instruction (op code mnemonic: RPjnw) which executes the following instruction a fixed number of times, possibly incrementing one or both of the address fields of that instruction.[2] This compensates for the architecture's lack of index registers.[3]

The GE-600/Honeywell 6000 series (1964) supports a single-instruction repeat (RPT), a double-instruction repeat (RPTD), and a linked-list repeat (RPL).[4][5]

The x86 ISA, starting with the 8086, includes a series of special-purpose repeat instructions (REP(∅/E/Z/NE/NZ)) which are called "repeat string operation prefixes" and may only be applied to a small number of string instructions (INS, OUTS, MOVS, LODS, STOS, CMPS, SCAS). These instructions repeat an operation and decrement a counter until it is zero, or may also stop when a certain condition is met.[6]

The Texas Instruments TMS320 digital signal processor (1983) includes an instruction for repeating a single-cycle instruction or two single-cycle instruction in parallel (RPT) and an instruction for repeating a block of instructions (RPTC). These use special block-repeat counter registers (BRC0, BRC1).[7]

Semantics

The instruction or instruction pair to be executed follows the repeat instruction. Fields in the instruction determine the loop termination condition.[5] In the case of the TMS320, a block of up to 64Kbytes can be repeated.[7]

Notes

  1. ↑ Rossman, George E. (December 1975). "A Course of Study in Computer Hardware Architecture". IEEE Computer. 8 (12): 44–63. doi:10.1109/C-M.1975.218835. S2CID 977792., p. 50
  2. ↑ Reference Manual: Univac Scientific 1103A Computer (PDF). Remington Rand Univac. 1956. p. 30.
  3. ↑ Conrad Weisert, "Simulating an Obsolete Computer in 1960", Part 1: The main processor
  4. ↑ GE-635 System Manual (PDF). General Electric Computer Department. July 1964. p. A-6.
  5. 1 2 GE-625/635 Programming Reference Manual (PDF). General Electric Information Systems. July 1964.
  6. ↑ Intel Corporation, "Instruction Set Reference", Intel 64 and IA-32 Architectures Software Developer's Manual, Order Number 253667-060US, September 2016, 2B:4-550 full text
  7. 1 2 Kuo, Sen-Maw (2003). Real-time Digital Signal Processing: Implementations, Applications, and Experiments with the TMS320C55X. 清华大学出版社有限公司. pp. 67–68. ISBN 978-7-302-07700-8.
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