computing, an opcode  (abbreviated from  operation code, also known as  instruction machine code,  instruction code,  instruction syllable,     instruction parcel or opstring  ) is the portion of a  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 operands. In addition to opcodes used in the instruction set architectures of various CPUs, which are hardware devices, they can also be used in abstract computing machines as part of their byte code specifications.
Specifications and format of the opcodes are laid out in the instruction set architecture (
ISA) of the processor in question, which may be a general CPU or a more specialized processing unit. 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 operands (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, variable-length structure.  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.
Depending on architecture, the
operands may be register values, values in the stack, other memory values, I/O ports (which may also be memory mapped), etc., specified and accessed using more or less complex addressing modes. The types of operations include arithmetics, data copying, logical operations, and program control, as well as special instructions (such as CPUID and others). 
Assembly language, or just assembly, is a low-level programming language, which uses mnemonic 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 languages, which are typically easier to read and write.  These languages need to be compiled (translated into assembly language) by a  system-specific compiler, or run through other compiled programs. 
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 files 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. 
^ a b
Barron, David William (1978) [1971, 1969]. "2.1. Symbolic instructions". Written at University of Southampton, Southampton, UK. In Floretin, J. John (ed.). Assemblers and Loaders. Computer Monographs (3 ed.). New York, USA: Elsevier North-Holland Inc. p. 7. ISBN . 0-444-19462-2 LCCN 78-19961. (xii+100 pages)
^ a b
Chiba, Shigeru (2007) . "Javassist, a Java-bytecode translator toolkit". Archived from the original on 2020-03-02 . Retrieved .
"Appendix B - Instruction Machine Codes" (PDF). MCS-4 Assembly Language Programming Manual - The INTELLEC 4 Microcomputer System Programming Manual (Preliminary ed.). Santa Clara, California, USA: Intel Corporation. December 1973. pp. B-1-B-8. MCS-030-1273-1. Archived (PDF) from the original on 2020-03-01 . Retrieved .
Raphael, Howard A., ed. (November 1974). "The Functions Of A Computer: Instruction Register And Decoder" (PDF). MCS-40 User's Manual For Logic Designers. Santa Clara, California, USA: Intel Corporation. p. viii. Archived (PDF) from the original on 2020-03-03 . Retrieved . [...] Each operation that the processor can perform is identified by a unique binary number known as an instruction code. [...]
Jones, Douglas W. (June 1988). "A Minimal CISC". ACM SIGARCH Computer Architecture News. New York, USA: Association for Computing Machinery (ACM). 16 (3): 56-63. doi: 10.1145/48675.48684. S2CID 17280173.
Domaga?a, ?ukasz (2012). "7.1.4. Benchmark suite". . Gliwice, Poland: Jacek Skalmierski Computer Studio. pp. 80-83 . Application of CLP to instruction modulo scheduling for VLIW processors ISBN . 978-83-62652-42-6 Archived from the original on 2020-03-02 . Retrieved .
Smotherman, Mark (2016) . "Multiple Instruction Issue". School of Computing, Clemson University. Archived from the original on 2016-05-28 . Retrieved .
Jones, Douglas W. (2016) . "A Minimal CISC". Computer Architecture On-Line Collection. Iowa City, USA: The University of Iowa, Department of Computer Science. Archived from the original on 2020-03-02 . Retrieved .
Schulman, Andrew (2005-07-01). "Finding Binary Clones with Opstrings & Function Digests". . Part I. Vol. 30 no. 7. Dr. Dobb's Journal CMP Media LLC. pp. 69-73. ISSN 1044-789X. #374. Archived from the original on 2020-03-02 . Retrieved ; Schulman, Andrew (2005-08-01). "Finding Binary Clones with Opstrings & Function Digests". . Part II. Vol. 30 no. 8. Dr. Dobb's Journal CMP Media LLC. pp. 56-61. ISSN 1044-789X. #375. Archived from the original on 2020-03-02 . Retrieved ; Schulman, Andrew (2005-09-01). "Finding Binary Clones with Opstrings & Function Digests". . Part III. Vol. 30 no. 9. CMP Media LLC United Business Media. pp. 64-70. ISSN 1044-789X. #376. Archived from the original on 2020-03-02 . Retrieved .
^ a b c d e f
Hennessy, John L.; Patterson, David A.; Asanovi?, Krste; Bakos, Jason D.; Colwell, Robert P.; Bhattacharjee, Abhishek; Conte, Thomas M.; Duato, José; Franklin, Diana; Goldberg, David; Jouppi, Norman P.; Li, Sheng; Muralimanohar, Naveen; Peterson, Gregory D.; Pinkston, Timothy M.; Ranganathan, Parthasarathy; Wood, David A.; Young, Cliff; Zaky, Amr (2017-11-23). Computer architecture: A quantitative approach (6 ed.). Cambridge, Massachusetts, USA: Morgan Kaufmann Publishers. ISBN . 978-0-12811905-1 OCLC 983459758.
Mansfield, Richard (1983). "Introduction: Why Machine Language?". . Machine Language For Beginners Compute! Books (1 ed.). Greensboro, North Carolina, USA: COMPUTE! Publications, Inc., American Broadcasting Companies, Inc.; Small System Services, Inc. ISBN . 0-942386-11-6 Archived from the original on 2008-02-13 . Retrieved .
"Programming Language Popularity". langpop.com. 2013-10-25. Archived from the original on 2015-04-11 . Retrieved .
Swanson, William (2001). "Introduction to Assembly Language". Swanson Technologies. Archived from the original on 2020-03-02 . Retrieved .
"bytecode Definition". . PC Magazine Encyclopedia. Archived from PC Magazine the original on 2012-10-06 . Retrieved .