Why is the x86 undefined instruction called ud2? Why 2?
Summary
The article explains why the x86 undefined instruction is called ud2 and traces the history behind it. It covers the competing ideas around 0F FF and 0F B9, and how Intel formalized a two-byte, no-operand instruction to reliably raise an invalid-opcode exception. The piece also highlights why ud2 is preferred over earlier approaches for debugging and safety.