Skip to content
Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

opcode

The part of an instruction's bytes that says which operation to do, such as add or move.

The opcode (operation code) is the field of an instruction encoding that selects the operation; on x86-64 it is one to three bytes, on ARM64 and RISC-V a group of bit fields inside the 32-bit instruction word.

Examples

  • In the x86-64 add rax, rcx, bytes 48 01 c8, the opcode is 01.

See also

instruction encoding variable-length encoding instruction prefix REX prefix
Read the full section →