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In [[computer science]], an '''Opcode''' is the portion of a [[machine language]] [[instruction (computer science)|instruction]] that specifies the operation to be performed. The term is an abbreviation of '''Op'''eration '''Code'''. Their specification and format will be laid out in the [[instruction set architecture]] (ISA) of the [[computer hardware]] component in question—normally a [[central processing unit|CPU]], but possibly a more specialised unit. A complete machine language instruction contains an opcode and, optionally, the specification of one or more [[operand]]s—what data the operation should act upon. Some operations have ''implicit'' operands, or indeed none. Some ISAs have instructions with defined fields for opcodes and operands, while others (e.g. the [[Intel x86]] architecture) have a more complicated and ad-hoc structure.
The operands upon which opcodes operate may, depending on CPU architecture, consist of [[processor register|registers]], values in [[memory]], values stored on the [[Stack (computing)|stack]], [[I/O]] ports, the [[Computer bus|bus]], etc. The operations an opcode may specify can include arithmetic, data copying, logical operations, and program control.
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