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Encyclopedia > Illegal opcode

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 or earlier, such as the MOS Technology 6502 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 inteded 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 didn't last very long. CPU can stand for: in computing: Central processing unit in journalism: Commonwealth Press Union in law enforcement: Crime prevention unit in software: Critical patch update, a type of software patch distributed by Oracle Corporation in Macleans College is often known as Ash Lim. ... The 1970s decade refers to the years from 1970 to 1979, inclusive. ... The MOS Technology 6502 is an 8-bit microprocessor designed by MOS Technology in 1975. ... Zilog Z80 microprocessor. ... A computer game is a game composed of a computer-controlled virtual universe that players interact with in order to achieve a defined goal or set of goals. ... 1980 (MCMLXXX in Roman) was a leap year starting on Tuesday. ... Copy prevention, also known as copy protection, is any technical measure designed to prevent duplication of information. ... Software cracking is the modification of software to remove encoded copy prevention. ... In cryptography and computer security, security through obscurity (sometimes security by obscurity) is to some a controversial principle in security engineering, which attempts to use secrecy (of design, implementation, etc. ...

A danger associated with the use of illegal instructions was that, given the fact that the manufacturer didn't 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 don't work correctly on the newer Apple IIc, because the latter uses a newer CPU revision that does away with illegal opcodes. The Apple II was one of the most popular personal computers of the 1980s. ... The Apple II was one of the most popular personal computers of the 1980s. ...

Modern CPUs, such as the i386 and its decendants or the Power PC processor, usually don't have illegal opcodes any more. 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 (equivalent to the CPU's official NOP instruction); this method is used by the 65816 processor, among others. The Intel 80386 is a microprocessor which was used as the central processing unit (CPU) of many personal computers from 1986 until 1994 and later. ... PowerPC is a RISC microprocessor architecture created by the 1991 Apple-IBM-Motorola alliance, known as AIM. Originally intended for personal computers, PowerPC CPUs have since become popular embedded and high-performance processors as well. ... An operating system is a program required for the user to manage the system and to run third-party application software for that system. ... NOP or NOOP (short for No OPeration) is an assembly language instruction, sequence of programming language statements, or computer protocol command that does nothing at all. ... The W65816 (also: 65C816), a 16-bit microprocessor developed by the Western Design Center (WDC), is an expanded and compatible successor to the venerable MOS Technology 6502. ...

Today, the details of these instructions are mainly of interest for exact emulation of older systems. This article is about emulation in computer science. ...


  • Illegal opcodes on the 6502: http://www.ffd2.com/fridge/docs/6502-NMOS.extra.opcodes
  • Illegal opcodes on the Z80: http://www.myquest.nl/z80undocumented/



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