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In computer science, an instruction is a single operation of a processor 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.

On traditional architectures, an instruction includes an opcode specifying the operation to be performed, such as "add contents of memory to register", and zero or more operand specifiers, which may specify registers, memory locations, or literal data. The operand specifiers may have addressing modes determining their meaning or may be in fixed fields.

In very long instruction word (VLIW) architectures, which include many microcode 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 microcontrollers to many hundreds of bits in some VLIW systems. Most modern processors used in personal computers, mainframes, and supercomputers have instruction sizes between 16 and 64 bits. In some architectures, notably most Reduced Instruction Set Computers, 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 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 or, more commonly, may be generated by compilers.