The description of an operation that is to be performed by a computer. It consists of a statement of an operation to be performed and some method of specifying the operands (or their locations) and the disposition of the result of the operation. Instructions are often divided into classes such as arithmetic instructions, program control instructions, logic instructions, and I/O instructions. They may or may not be of fixed length. The operation code or order code of an instruction specifies one of the set of operations available in a particular computer. See also instruction format.