what is an opcode?

An opcode (abbreviated from operation code), is the portion of a machine language instruction that states the operation to be performed. Most instructions also specify the data that they will process, in the form of operands.

Opcodes for a given instruction set can be described through the use of an opcode table detailing all possible opcodes, see table 1. Apart from the opcode itself, an instruction normally has one or more specifiers for operands on which the operation should act, although some operations may have implicit operands or none at all.