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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 bytes. 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]]
[[Assembly language]], or just ''assembly'', is a [[low-level programming language]], which uses [[Assembly language#Opcode mnemonics and extended mnemonics|mnemonic]] instructions and operands to represent [[machine code]].<ref name="Hennessy_2017"/> 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="langpop"/> which are typically easier to read and write.<ref name="Hennessy_2017"/> These languages need to be compiled (translated into assembly language) by a [[Operating system|system]]-specific [[compiler]], or run through other compiled programs.<ref name="Swanson_2001"/>
=={{Anchor|SIS}}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 [[
== See also ==
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