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,
logic instructions, and I/O
instructions. They may or may not be of fixed length. The set of operations
available in a particular computer is known as its operation code or order
code. See also instruction format.