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Instruction (computer science)
 
 
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In computer science
Computer science

Computer science is the study of the theoretical foundations of information and computation, and of practical techniques for their implementation and application in computer systems....
, an instruction is a single operation of a processor
Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
 defined by an instruction set architecture. In a broader sense, an "instruction" may be any representation of an element of an executable program, such as a bytecode
Bytecode

Bytecode is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software Interpreter as well as being suitable for further compilation into machine language....
.

On traditional architectures, an instruction includes an opcode
Opcode

In computer technology, an opcode is the portion of a machine language instruction that specifies the operation to be performed. Their specification and format are laid out in the instruction set architecture of the processor in question ....
 specifying the operation to be performed, such as "add contents of memory to register", and zero or more operand
Operand

An operand is one of the inputs of an operator in mathematics. The following arithmetic expression shows an example of operators and operands:...
 specifiers, which may specify registers
Processor register

In computer architecture, a processor register is a small amount of Computer storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere....
, memory locations, or literal data.






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In computer science
Computer science

Computer science is the study of the theoretical foundations of information and computation, and of practical techniques for their implementation and application in computer systems....
, an instruction is a single operation of a processor
Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
 defined by an instruction set architecture. In a broader sense, an "instruction" may be any representation of an element of an executable program, such as a bytecode
Bytecode

Bytecode is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software Interpreter as well as being suitable for further compilation into machine language....
.

On traditional architectures, an instruction includes an opcode
Opcode

In computer technology, an opcode is the portion of a machine language instruction that specifies the operation to be performed. Their specification and format are laid out in the instruction set architecture of the processor in question ....
 specifying the operation to be performed, such as "add contents of memory to register", and zero or more operand
Operand

An operand is one of the inputs of an operator in mathematics. The following arithmetic expression shows an example of operators and operands:...
 specifiers, which may specify registers
Processor register

In computer architecture, a processor register is a small amount of Computer storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere....
, memory locations, or literal data. The operand specifiers may have addressing mode
Addressing mode

Addressing modes are an aspect of the instruction set architecture in most central processing unit designs. The various addressing modes that are defined in a given instruction set architecture define how Machine code Instruction in that architecture identify the operand of each instruction....
s determining their meaning or may be in fixed fields.

In very long instruction word
Very long instruction word

Very Long Instruction Word or VLIW refers to a Central processing unit architecture designed to take advantage of instruction level parallelism ....
 (VLIW) architectures, which include many microcode
Microcode

Microcode is a layer of lowest-level instructions involved in the implementation of machine code instructions in many computers and other processors; it resides in a special high-speed memory and translates machine instructions into sequences of detailed circuit-level operations....
 architectures, multiple simultaneous operations and operands are specified in a single instruction.

The size or length of an instruction varies widely, from as little as four bits in some microcontroller
Microcontroller

A microcontroller is a small computer on a single integrated circuit consisting of a relatively simple CPU combined with support functions such as a crystal oscillator, timers, watchdog, serial and analog I/O etc....
s to many hundreds of bits in some VLIW systems. Most modern processors used in personal computer
Personal computer

A personal computer is any general-purpose computer whose original sales price, size, and capabilities make it useful for individuals, and which is intended to be operated directly by an end user, with no intervening computer operator....
s, mainframe
Mainframe

Mainframe may refer to one of the following:* Mainframe computer, large data processing systems* Mainframe Entertainment, a Canadian computer animation and design company....
s, and supercomputer
Supercomputer

A supercomputer is a computer that is at the frontline of current processing capacity, particularly speed of calculation. Supercomputers introduced in the 1960s were designed primarily by Seymour Cray at Control Data Corporation , and led the market into the 1970s until Cray left to form his own company, Cray Research....
s have instruction sizes between 16 and 64 bits. In some architectures, notably most Reduced Instruction Set Computer
Reduced instruction set computer

The acronym RISC , for reduced instruction set computing, represents a CPU design strategy emphasizing the insight that simplified instructions that "do less" may still provide for higher performance if this simplicity can be utilized to make instructions execute very quickly....
s, instructions are a fixed length, typically corresponding with that architecture's word size. In other architectures, instructions have variable length, typically integral multiples of a byte
Byte

A byte is a basic unit of measurement of Computer storage in computer science. In many computer architectures it is a Byte addressing memory address space....
 or a halfword.

The instructions constituting a program are rarely specified using their internal, numeric form; they may be specified by programmers using an assembly language
Assembly language

An assembly language is a low-level language for programming computers. It implements a symbolic representation of the numeric machine codes and other constants needed to program a particular CPU architecture....
 or, more commonly, may be generated by compiler
Compiler

A compiler is a computer program that transforms source code written in a programming language into another computer language . The most common reason for wanting to transform source code is to create an executable program....
s.

Some exotic instruction sets do not have an opcode field (such as Transport Triggered Architectures
Transport Triggered Architectures

The transport triggered architecture is an application-specific instruction-set processor architecture template that allows easy customization of microprocessor designs....
 or the Forth virtual machine), only operand(s). Other unusual "0-operand
Instruction set

An instruction set is a list of all the instruction , and all their variations, that a processor can execute.Instructions include:* Arithmetic such as add and subtract...
" instruction sets lack any operand specifier fields, such as some stack machine
Stack machine

In computer science, a stack machine is a model of computation in which the computer's memory takes the form of one or more stack s. The term also refers to an actual computer implementing or simulating the idealized stack machine....
s including NOSCs.

See also

  • Command (computing)
    Command (computing)

    In computing, a command is a directive to a computer program acting as an interpreter of some kind, in order to perform a specific task. Most commonly a command is a directive to some kind of command line interface, such as a shell ....
  • Data (computing)
    Data (computing)

    In computer science, data is anything in a form suitable for use with a computer. Data is often distinguished from computer programs. A program is a set of instruction that detail a task for the computer to perform....
  • Machine language