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- Illegal_opcode abstract "An illegal opcode, also called an 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. They 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.While most illegal instructions have useless or even highly undesirable effects (such as crashing the computer), a few might by accident do something that can be useful in certain situations. Such instructions were sometimes exploited in computer games of the 1970s and 1980s to speed up certain time-critical sections. Another common use of them was in the ongoing battle between copy protection implementations and cracking. Here, they were a form of security through obscurity, and their secrecy usually did not last very long.A danger associated with the use of illegal instructions was that, given the fact that the manufacturer did not guarantee their existence and function, they might disappear or behave differently with any change of the CPU internals or any new revision of the CPU, rendering programs that use them incompatible with the newer revisions. For example, a number of older Apple II games do not work correctly on the newer Apple IIc, because the latter uses a newer CPU revision — 65C02 — that does away with illegal opcodes.More recent CPUs, such as the 80186, 80286, 68000 and its descendants or the PowerPC processor, usually do not have illegal opcodes. Their manufacturers guarantee that the CPU will behave in a well-defined way when it finds an unknown opcode in the instruction stream: usually, this means triggering a certain exception or fault condition. The operating system's exception or fault handler will then usually terminate the application that caused the fault. Another, less common way of handling illegal instructions is by defining them to do nothing except taking up time and space (equivalent to the CPU's official NOP instruction); this method is used by the TMS9900 and 65C02 processor, among others.Today, the details of these instructions are mainly of interest for exact emulation of older systems.".
- Illegal_opcode wikiPageExternalLink 6502-NMOS.extra.opcodes.
- Illegal_opcode wikiPageExternalLink z80undocumented.
- Illegal_opcode wikiPageExternalLink ?p=39.
- Illegal_opcode wikiPageID "3956469".
- Illegal_opcode wikiPageLength "2966".
- Illegal_opcode wikiPageOutDegree "24".
- Illegal_opcode wikiPageRevisionID "583654699".
- Illegal_opcode wikiPageWikiLink 65C02.
- Illegal_opcode wikiPageWikiLink 68000.
- Illegal_opcode wikiPageWikiLink 80186.
- Illegal_opcode wikiPageWikiLink 80286.
- Illegal_opcode wikiPageWikiLink Apple_II.
- Illegal_opcode wikiPageWikiLink Apple_IIc.
- Illegal_opcode wikiPageWikiLink Backdoor_(computing).
- Illegal_opcode wikiPageWikiLink CPU.
- Illegal_opcode wikiPageWikiLink Category:Machine_code.
- Illegal_opcode wikiPageWikiLink Central_processing_unit.
- Illegal_opcode wikiPageWikiLink Computer_game.
- Illegal_opcode wikiPageWikiLink Copy_protection.
- Illegal_opcode wikiPageWikiLink Easter_egg_(media).
- Illegal_opcode wikiPageWikiLink Emulator.
- Illegal_opcode wikiPageWikiLink Intel_80186.
- Illegal_opcode wikiPageWikiLink Intel_80286.
- Illegal_opcode wikiPageWikiLink Intel_8086.
- Illegal_opcode wikiPageWikiLink MOS_Technology_6502.
- Illegal_opcode wikiPageWikiLink Motorola_68000.
- Illegal_opcode wikiPageWikiLink NOP.
- Illegal_opcode wikiPageWikiLink Operating_system.
- Illegal_opcode wikiPageWikiLink PC_game.
- Illegal_opcode wikiPageWikiLink PowerPC.
- Illegal_opcode wikiPageWikiLink Security_through_obscurity.
- Illegal_opcode wikiPageWikiLink Software_cracking.
- Illegal_opcode wikiPageWikiLink TMS9900.
- Illegal_opcode wikiPageWikiLink Texas_Instruments_TMS9900.
- Illegal_opcode wikiPageWikiLink Undocumented_feature.
- Illegal_opcode wikiPageWikiLink WDC_65C02.
- Illegal_opcode wikiPageWikiLink Zilog_Z80.
- Illegal_opcode wikiPageWikiLinkText "Illegal opcode".
- Illegal_opcode wikiPageWikiLinkText "Undefined opcodes".
- Illegal_opcode wikiPageWikiLinkText "illegal opcode".
- Illegal_opcode wikiPageWikiLinkText "undocumented instructions".
- Illegal_opcode wikiPageWikiLinkText "undocumented opcodes".
- Illegal_opcode hasPhotoCollection Illegal_opcode.
- Illegal_opcode wikiPageUsesTemplate Template:Multiple_issues.
- Illegal_opcode wikiPageUsesTemplate Template:Reflist.
- Illegal_opcode subject Category:Machine_code.
- Illegal_opcode hypernym Instruction.
- Illegal_opcode type Article.
- Illegal_opcode type Language.
- Illegal_opcode type ProgrammingLanguage.
- Illegal_opcode type Article.
- Illegal_opcode type Language.
- Illegal_opcode comment "An illegal opcode, also called an 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.".
- Illegal_opcode label "Illegal opcode".
- Illegal_opcode sameAs m.0b8kh7.
- Illegal_opcode sameAs Q16250980.
- Illegal_opcode sameAs Q16250980.
- Illegal_opcode wasDerivedFrom Illegal_opcode?oldid=583654699.
- Illegal_opcode isPrimaryTopicOf Illegal_opcode.