opcode

An opcode, short for operation code, is the portion of a machine language instruction that identifies the operation the processor should perform. In conjunction with operand fields, the opcode determines the action, such as arithmetic, data movement, comparison, or control transfer. The set of all opcodes available to a processor is defined by its instruction set architecture (ISA).

Instructions are encoded as binary or hexadecimal bit patterns. In many architectures, the opcode occupies a fixed-width field, while other designs combine the opcode with bits that specify addressing mode or immediate data. Some modern CPUs use extended or multi-byte opcodes, prefixes, or variable-length formats that allow more operations but complicate decoding.

During fetch-decode-execute, the processor reads the opcode, decodes it via the control unit, and emits the necessary control signals to perform the operation and access operands. In some designs, opcodes map directly to hardware circuits; in others, the instruction is translated into a sequence of micro-operations or microcode that implement the higher-level operation.

Common contrasts arise between RISC and CISC philosophies. RISC architectures typically employ a small, regular set of opcodes and fixed-length instructions, while CISC historically used a larger and more variable opcode space. Nevertheless, both rely on opcodes to specify the operation, with operands supplied separately as registers or memory addresses.

Tools such as disassemblers, emulators, and debuggers interpret opcodes to present human-readable assembly language. OpCode handling interacts with addressing modes, exceptions, and privileged instructions, influencing compiler design, virtualization, and system security.