Why is the x86 undefined instruction called ud2? Why 2?
The x86 UD2 instruction is a deliberately undefined opcode that triggers an invalid‑opcode exception when executed. Its name derives from “undefined,” and the “2” differentiates it from earlier undefined opcodes such as UD0. Introduced in later Intel and AMD processors, UD2 is commonly used by compilers and developers to embed runtime traps or to mark unreachable code.