Definition of Opcode

In computer science, an opcode (Operation Code) is the portion of a machine language instruction that specifies the operation to be performed. Their specification and format will be laid out in the instruction set architecture (ISA) of the computer hardware component in question—normally a CPU, but possibly a more specialized unit. A complete machine language instruction contains an opcode and, optionally, the specification of one or more operands—what data the operation should act upon. Some operations have implicit operands, or indeed none. Some ISAs have instructions with defined fields for opcodes and operands, while others (e.g. the Intel x86 architecture) have a more complicated and ad-hoc structure.