View source for Opcode

{{short description|Part of a machine instruction}}
In [[Computing|computing]], an '''opcode'''<ref name="Barron_1978_Opcode"/><ref name="Chiba_2007"/> (abbreviated from '''operation code''',<ref name="Barron_1978_Opcode"/> also known as '''instruction machine code''',<ref name="Intel_1973_MCS-4"/> '''instruction code''',<ref name="Intel_1974_MCS-40"/> '''instruction syllable''',<ref name="Jones_1988_CISC"/><ref name="Domagała_2012"/><ref name="Smotherman_2013"/><ref name="Jones_2016_CISC"/> '''instruction parcel''' or '''opstring'''<ref name="Schulman_2005"/><ref name="Chiba_2007"/>) is the portion of a [[Machine code|machine language]] instruction that specifies the operation to be performed. Beside the opcode itself, most instructions also specify the data they will process, in the form of [[Operand|operand]]s. In addition to opcodes used in the [[Instruction set architecture|instruction set architecture]]s of various CPUs, which are hardware devices, they can also be used in [[Virtual machine#Process virtual machines|abstract computing machines]] as part of their byte code specifications.

==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|opcode table]] detailing all possible opcodes. Apart from the opcode itself, an instruction normally also has one or more specifiers for [[Operand|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|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 prefixes which add a subset of new instructions made up of existing opcodes following reserved byte sequences.{{Citation needed|date=February 2023}}

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

[[Assembly language]], or just ''assembly'', is a [[Low-level programming language|low-level programming language]], which uses [[Assembly language#Opcode mnemonics and extended mnemonics|mnemonic]] instructions and operands to represent [[Machine code|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|high-level programming language]]s,<ref name="langpop"/> which are typically easier for humans to understand and write.<ref name="Hennessy_2017"/> These languages need to be compiled (translated into assembly language) by a [[Operating system|system]]-specific [[Compiler|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 codes 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|Java Virtual Machine]] (JVM), the byte code used in [[Software:GNU Emacs|GNU Emacs]] for compiled [[Lisp (programming language)|Lisp]] code, .NET [[Common Intermediate Language]] (CIL), and many others.<ref name="bytecode"/>

==See also==
* Gadget (machine instruction sequence)
* [[Illegal opcode]]
* [[Syllable (computing)]]
* Fused operation

==References==
{{Reflist|40em|refs=
<ref name="Barron_1978_Opcode">{{cite book |author-first=David William |author-last=Barron |editor-first=J. John |editor-last=Floretin |title=Assemblers and Loaders |date=1978 |orig-year=1971, 1969 |edition=3 |publisher=Elsevier North-Holland Inc. |series=Computer Monographs |publication-place=New York, USA |location=[[Organization:University of Southampton|University of Southampton]], Southampton, UK |isbn=0-444-19462-2 |lccn=78-19961 |chapter=2.1. Symbolic instructions |page=7}} (xii+100 pages)</ref>
<ref name="Jones_2016_CISC">{{cite web |title=A Minimal CISC |work=Computer Architecture On-Line Collection |author-first=Douglas W. |author-last=Jones |publisher=The University of Iowa, Department of Computer Science |date=2016 |orig-year=2012 |location=Iowa City, USA |url=http://homepage.cs.uiowa.edu/~jones/arch/cisc/ |access-date=2016-05-28 |url-status=live |archive-url=https://web.archive.org/web/20200302190911/http://homepage.cs.uiowa.edu/~jones/arch/cisc/ |archive-date=2020-03-02}}</ref>
<ref name="Jones_1988_CISC">{{cite journal |title=A Minimal CISC |author-first=Douglas W. |author-last=Jones<!-- |other=The University of Iowa, Department of Computer Science, Iowa City, USA --> |journal=ACM SIGARCH Computer Architecture News |publisher=[[Organization:Association for Computing Machinery|Association for Computing Machinery]] (ACM) |location=New York, USA |date=June 1988 |volume=16 |issue=3 |pages=56–63 |doi=10.1145/48675.48684 |s2cid=17280173 |doi-access=free }}</ref>
<ref name="Domagała_2012">{{cite book |title=Application of CLP to instruction modulo scheduling for VLIW processors |chapter=7.1.4. Benchmark suite |author-first=Łukasz |author-last=Domagała |publisher=Jacek Skalmierski Computer Studio |date=2012 |isbn=978-83-62652-42-6 |pages=80–83 [83<!-- relevant page -->] |location=Gliwice, Poland |url=https://books.google.com/books?id=e6apNOED26kC |chapter-url=https://books.google.com/books?id=e6apNOED26kC |access-date=2016-05-28 |url-status=live |archive-url=https://web.archive.org/web/20200302192452/https://books.google.de/books?hl=de&id=e6apNOED26kC&jtp=83 |archive-date=2020-03-02}}</ref>
<ref name="Smotherman_2013">{{cite web |title=Multiple Instruction Issue |author-first=Mark |author-last=Smotherman |publisher=School of Computing, Clemson University |date=2016 |orig-year=2013 |url=https://people.cs.clemson.edu/~mark/330/ilp.txt |access-date=2016-05-28 |url-status=live |archive-url=https://web.archive.org/web/20160528142545/https://people.cs.clemson.edu/~mark/330/ilp.txt |archive-date=2016-05-28}}</ref>
<ref name="Schulman_2005">{{cite magazine |title=Finding Binary Clones with Opstrings & Function Digests |author-first=Andrew |author-last=Schulman |magazine=Dr. Dobb's Journal |volume=30 |issue=7 |id=#374 |issn=1044-789X |publisher=CMP Media LLC |date=2005-07-01 |series=Part I |pages=69–73 |url=http://www.drdobbs.com/finding-binary-clones-with-opstrings-fu/184406152 |access-date=2020-03-02 |url-status=live |archive-url=https://web.archive.org/web/20200302175401/https://www.drdobbs.com/finding-binary-clones-with-opstrings-fu/184406152 |archive-date=2020-03-02 |postscript=none}}; {{cite magazine |title=Finding Binary Clones with Opstrings & Function Digests |author-first=Andrew |author-last=Schulman |magazine=Dr. Dobb's Journal |volume=30 |issue=8 |id=#375 |issn=1044-789X |publisher=CMP Media LLC |date=2005-08-01 |series=Part II |pages=56–61 |url=http://www.drdobbs.com/finding-binary-clones-with-opstrings-fu/184406203 |access-date=2016-05-28 |url-status=live |archive-url=https://web.archive.org/web/20200302185255/https://www.drdobbs.com/finding-binary-clones-with-opstrings-fu/184406203 |archive-date=2020-03-02 |postscript=none}}; {{cite magazine |title=Finding Binary Clones with Opstrings & Function Digests |author-first=Andrew |author-last=Schulman |magazine=CMP Media LLC |volume=30 |issue=9 |id=#376 |issn=1044-789X |publisher=United Business Media |date=2005-09-01 |series=Part III |pages=64–70 |url=http://www.drdobbs.com/tools/finding-binary-clones-with-opstrings-fu/184406247 |access-date=2016-05-28 |url-status=live |archive-url=https://web.archive.org/web/20200302185646/https://www.drdobbs.com/tools/finding-binary-clones-with-opstrings-fu/184406247?pgno=3 |archive-date=2020-03-02}}</ref>
<ref name="Chiba_2007">{{cite web |title=Javassist, a Java-bytecode translator toolkit |author-first=Shigeru |author-last=Chiba |date=2007 |orig-year=1999 |url=http://www.docjar.org/html/api/javassist/bytecode/InstructionPrinter.java.html |access-date=2016-05-27 |url-status=live |archive-url=https://web.archive.org/web/20200302185725/http://www.docjar.org/html/api/javassist/bytecode/InstructionPrinter.java.html |archive-date=2020-03-02}}</ref>
<ref name="Hennessy_2017">{{cite book |title=Computer architecture: A quantitative approach |author-last1=Hennessy |author-first1=John L. |author-last2=Patterson |author-first2=David A. |author-last3=Asanović |author-first3=Krste |author-last4=Bakos |author-first4=Jason D. |author-last5=Colwell |author-first5=Robert P. |author-last6=Bhattacharjee |author-first6=Abhishek |author-last7=Conte |author-first7=Thomas M. |author-last8=Duato |author-first8=José |author-last9=Franklin |author-first9=Diana |author-last10=Goldberg |author-first10=David |author-last11=Jouppi |author-first11=Norman P. |author-last12=Li |author-first12=Sheng |author-last13=Muralimanohar |author-first13=Naveen |author-last14=Peterson |author-first14=Gregory D. |author-last15=Pinkston |author-first15=Timothy M. |author-last16=Ranganathan |author-first16=Parthasarathy |author-last17=Wood |author-first17=David A. |author-last18=Young |author-first18=Cliff |author-last19=Zaky |author-first19=Amr |date=2017-11-23 |edition=6 |publisher=Morgan Kaufmann Publishers |location=Cambridge, Massachusetts, USA |isbn=978-0-12811905-1 |oclc=983459758 }}</ref>
<ref name="Mansfield_1983">{{cite book |title=Machine Language For Beginners |chapter=Introduction: Why Machine Language? |author-first=Richard |author-last=Mansfield |date=1983 |series=Compute! Books |publisher=COMPUTE! Publications, Inc., American Broadcasting Companies, Inc.; Small System Services, Inc. |location=Greensboro, North Carolina, USA |isbn=0-942386-11-6 |edition=1 |url=https://www.atariarchives.org/mlb/index.php |chapter-url=http://www.atariarchives.org/mlb/introduction.php |access-date=2016-05-28 |url-status=live |archive-url=https://web.archive.org/web/20080213090055/http://www.atariarchives.org/mlb/introduction.php |archive-date=2008-02-13}}</ref>
<ref name="langpop">{{cite web |title=Programming Language Popularity |website=langpop.com |date=2013-10-25 |url=http://langpop.com/ |access-date=2015-10-10 |archive-url=https://web.archive.org/web/20150411135441/http://langpop.com/ |archive-date=2015-04-11}}</ref>
<ref name="Swanson_2001">{{cite web |title=Introduction to Assembly Language |work=Swanson Technologies
|author-first=William |author-last=Swanson |date=2001 |url=http://www.swansontec.com/sprogram.htm |access-date=2015-10-10 |url-status=live |archive-url=https://web.archive.org/web/20200302185840/https://www.swansontec.com/sprogram.html |archive-date=2020-03-02}}</ref>
<ref name="bytecode">{{cite magazine |title=bytecode Definition |series=PC Magazine Encyclopedia |magazine=PC Magazine |url=https://www.pcmag.com/encyclopedia_term/0,2542,t=bytecode&i=39108,00.asp |access-date=2015-10-10 |url-status=dead |archive-url=https://web.archive.org/web/20121006015213/http://www.pcmag.com/encyclopedia_term/0%2C2542%2Ct%3Dbytecode%26i%3D39108%2C00.asp |archive-date=6 October 2012 }}</ref>
<ref name="Intel_1973_MCS-4">{{cite book |title=MCS-4 Assembly Language Programming Manual - The INTELLEC 4 Microcomputer System Programming Manual |chapter=Appendix B - Instruction Machine Codes |edition=Preliminary |publisher=Intel Corporation |date=December 1973 |location=Santa Clara, California, USA |id=MCS-030-1273-1 |pages=B-1 – B-8 |chapter-url=http://bitsavers.trailing-edge.com/components/intel/MCS4/MCS-4_Assembly_Language_Programming_Manual_Dec73.pdf |access-date=2020-03-02 |url-status=live |archive-url=https://web.archive.org/web/20200301235541/http://bitsavers.trailing-edge.com/components/intel/MCS4/MCS-4_Assembly_Language_Programming_Manual_Dec73.pdf |archive-date=2020-03-01}}</ref>
<ref name="Intel_1974_MCS-40">{{cite book |title=MCS-40 User's Manual For Logic Designers |chapter=The Functions Of A Computer: Instruction Register And Decoder |publisher=Intel Corporation |editor-first=Howard A. |editor-last=Raphael |date=November 1974 |location=Santa Clara, California, USA |page=viii |chapter-url=http://bitsavers.trailing-edge.com/components/intel/MCS40/MCS-40_Users_Manual_Nov74.pdf |access-date=2020-03-03 |url-status=live |archive-url=https://web.archive.org/web/20200303024244/http://bitsavers.trailing-edge.com/components/intel/MCS40/MCS-40_Users_Manual_Nov74.pdf |archive-date=2020-03-03 |quote=[…] Each operation that the processor can perform is identified by a unique binary number known as an instruction code. […]}}</ref>
}}

== Further reading ==
* {{cite book |author-last=Hyde |author-first=Randall |date=2004 |title=Write Great Code: Thinking Low-level, Writing High-level |volume=1 |work=Understanding the Machine |page=passim |location=San Francisco, California, USA |publisher=No Starch Press |isbn=1-59327003-8 |url=https://books.google.com/books?isbn=1593270658 |access-date=2015-10-10}}

{{X86 assembly topics}}

[[Category:Machine code]]

{{Sourceattribution|Opcode}}