Opcode: Difference between revisions

Revision as of 23:08, 6 July 2021

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==Overview==

Specifications and format of the opcodes are laid out in the instruction set architecture ([[instruction set architecture|ISA]]) of the processor in question, which may be a general [[central processing unit|CPU]] or a more specialized processing unit.<ref name="Hennessy_2017"/> Opcodes for a given instruction set can be described through the use of an [[opcode table]] detailing all possible opcode bytesopcodes. 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.<ref name="Hennessy_2017"/> 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, variable-length structure.<ref name="Hennessy_2017"/><ref name="Mansfield_1983"/> Instruction sets can be extended through the use of [[opcode prefix]]es which add a subset of new instructions made up of existing opcodes following reserved byte sequences.

===Operands===

Depending on architecture, the ''operands'' may be [[processor register|register]] values, values in the [[call stack|stack]], other [[memory]] values, [[I/O]] ports (which may also be [[Memory-mapped I/O|memory mapped]]), etc., specified and accessed using more or less complex [[addressing mode]]s.{{citation needed|date=October 2015}} The types of ''operations'' include [[arithmetic]], data copying, [[logical operation]]s, and program control, as well as special instructions (such as [[CPUID]] and others).<ref name="Hennessy_2017"/>