An opcode (operation code) 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 (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.