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opcode
In computing, an opcode (abbreviated from operation code) is the portion of a machine language instruction that specifies the operation to be performed. Beside the opcode itself, instructions usually specify the data they will process, in form of operands. In addition to opcodes used in 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.
== Overview ==
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. 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.
Depending on architecture, the ''operands'' may be register values, values in the stack, other memory values, I/O ports, 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 mnemonics, 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), or run through other compiled programs.〔(【引用サイトリンク】title=Introduction to Assembly Language )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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