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'In [[computer science]], an '''opcode''' ('''operation code''') is the portion of a [[Machine code|machine language]] [[instruction (computer science)|instruction]] that specifies the operation to be performed. ==ISA== Their specification and format are laid out in the [[instruction set architecture]] (ISA) of the processor in question (which may be a general [[central processing unit|CPU]] or a more specialized processing unit). Apart from the opcode itself, an instruction normally also has one or more specifiers for [[operand]]s (i.e. data) on which the operation should act, although some operations may have ''implicit'' operands, or none at all. There are instruction sets with nearly uniform fields for opcode and operand specifiers, as well as others (the [[x86]] architecture for instance) with a more complicated, varied length structure. <ref name="machine">[http://web.archive.org/web/20080213090055/http://www.atariarchives.org/mlb/introduction.php Machine Language For Beginners - Introduction<!-- Bot generated title -->]</ref> Depending on architecture, the '''[[operand]]s''' may be [[processor register|register]] values, values in the [[call stack|stack]], other [[memory]] values, [[I/O]] ports, etc., specified and accessed using more or less complex [[addressing mode]]s. The types of '''operations''' include [[arithmetic]]s, data copying, [[logical operation]]s, and program control, as well as special instructions (such as [[CPUID]] and others). ==Assembly== [[Assembly language]], or just ''assembly'', is a [[low-level programming language]], which uses [[Assembly language#Opcode mnemonics and extended mnemonics|mnemonic]]s, instructions and operands to represent [[machine code]]. This enhances the readability while still giving precise control over the machine instructions. Most programming is currently done using [[high-level programming language]]s,<ref name="tiobe">[http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html TIOBE Software Popularity Index]</ref><ref name=langpop>http://langpop.com/</ref> which are typically easier to read and write. These languages need to be compiled (translated into machine language), or run through other compiled programs.<ref name="asm">[http://www.swansontec.com/sprogram.htm Introduction to Assembly Language]</ref> ==Software instruction sets== Opcodes can also be found in so called [[byte code]]s and other representations intended for a software interpreter rather than a hardware device. These software based instruction sets often employ slightly higher-level data types and operations than most hardware counterparts, but are nevertheless constructed along similar lines. Examples include the byte code found in [[Java class file]]s which are then interpreted by the [[Java Virtual Machine]] (JVM), the byte code used in [[GNU Emacs]] for compiled [[LISP]] code, .NET [[Common Intermediate Language]] (CIL), and many others.<ref name="bytecode">[http://www.pcmag.com/encyclopedia_term/0,2542,t=bytecode&i=39108,00.asp bytecode Definition<!-- Bot generated title -->]</ref> vnvffvnvnnfv b vb bv == See also == * [[Illegal opcode]] * [[Opcode database]] ==References== {{reflist|30em}} [[Category:Machine code]]'