Glossary
Opcode
The numeric byte(s) in machine code that identify which operation the CPU should execute.
An opcode (operation code) is the raw numeric byte or bytes at the start of a machine instruction that tell the CPU which operation to perform. For example, on x86, the byte 0x90 is the opcode for NOP (no operation). A disassembler maps opcodes back to human-readable mnemonics. Understanding opcodes is essential when analyzing shellcode or hand-crafted exploits where instructions are written as raw bytes.
90 ; opcode 0x90 → NOP
B8 01 00 00 00 ; opcode 0xB8 → MOV EAX, 1