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An illegal opcode, also called an unimplemented operation, unintended opcode or undocumented instruction, is an instruction to a CPU that is not mentioned in any official documentation released by the CPU's designer or manufacturer, which nevertheless has an effect. Illegal opcodes were common on older CPUs designed during the 1970s, such as the MOS Technology 6502, Intel 8086, and the Zilog Z80. On these older processors, many exist as a side effect of the wiring of transistors in the CPU, and usually combine functions of the CPU that were not intended to be combined. On old and modern processors, there are also instructions intentionally included in the processor by the manufacturer, but that are not documented in any official specification.

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  • An illegal opcode, also called an unimplemented operation, unintended opcode or undocumented instruction, is an instruction to a CPU that is not mentioned in any official documentation released by the CPU's designer or manufacturer, which nevertheless has an effect. Illegal opcodes were common on older CPUs designed during the 1970s, such as the MOS Technology 6502, Intel 8086, and the Zilog Z80. On these older processors, many exist as a side effect of the wiring of transistors in the CPU, and usually combine functions of the CPU that were not intended to be combined. On old and modern processors, there are also instructions intentionally included in the processor by the manufacturer, but that are not documented in any official specification. The effect of many illegal opcodes, on many processors, is just a trap to an error handler. However, some processors that trap for most illegal opcodes do not do so for some illegal opcodes, and some other processors do not check for illegal opcodes, and, instead, perform an undocumented operation. (en)
  • Une instruction non documentée (en anglais, undocumented instruction ou illegal opcode) est une instruction-machine destinée à une unité centrale qui n'est pas mentionnée dans une documentation officielle publiée par le concepteur ou le fabricant de l'unité centrale, mais qui a néanmoins un effet. Les instructions non documentées étaient courantes sur les anciens processeurs conçus dans les années 1970, tels que le MOS Technology 6502, l'Intel 8086 et le Zilog Z80. Sur ces anciens processeurs, plusieurs instructions non documentées sont des effets secondaires du câblage des transistors dans l'unité centrale, et combinent généralement des fonctions de l'unité centrale qui n'étaient pas destinées à être combinées. Elles incluent la simplification du décodage des instructions par le processeur où certains bits du code d'instruction peuvent être ignorés.Les instructions concernées ont alors plusieurs codes possibles (des alias non officiels), mais un seul d'entre eux est officiellement documenté. Sur les processeurs anciens et modernes, il existe également des instructions intentionnellement incluses dans le processeur par le fabricant, mais qui ne sont documentées dans aucune documentation officielle. Certains programmes se servent d'instructions non documentées pour détecter la révision exacte du processeur. (fr)
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  • November 2021 (en)
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  • An illegal opcode, also called an unimplemented operation, unintended opcode or undocumented instruction, is an instruction to a CPU that is not mentioned in any official documentation released by the CPU's designer or manufacturer, which nevertheless has an effect. Illegal opcodes were common on older CPUs designed during the 1970s, such as the MOS Technology 6502, Intel 8086, and the Zilog Z80. On these older processors, many exist as a side effect of the wiring of transistors in the CPU, and usually combine functions of the CPU that were not intended to be combined. On old and modern processors, there are also instructions intentionally included in the processor by the manufacturer, but that are not documented in any official specification. (en)
  • Une instruction non documentée (en anglais, undocumented instruction ou illegal opcode) est une instruction-machine destinée à une unité centrale qui n'est pas mentionnée dans une documentation officielle publiée par le concepteur ou le fabricant de l'unité centrale, mais qui a néanmoins un effet. Elles incluent la simplification du décodage des instructions par le processeur où certains bits du code d'instruction peuvent être ignorés.Les instructions concernées ont alors plusieurs codes possibles (des alias non officiels), mais un seul d'entre eux est officiellement documenté. (fr)
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  • Illegal opcode (en)
  • Instruction non documentée (fr)
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