Why is the x86 undefined instruction called ud2? Why 2?

The x86 instruction `ud2` is an architecturally undefined instruction that reliably triggers an "invalid opcode" exception, used by compilers to mark unreachable code so crashes occur instead of executing random instructions. The name comes from Intel's history: early programmers discovered two undocumented byte sequences (0F FF and 0F B9) that raised invalid opcode exceptions, but when Intel modified their processors, 0F FF no longer worked reliably, prompting the need for a standardized undefined instruction, which became `ud2`.