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

In a recent post on The Old New Thing, Raymond Chen explores the historical origins of the x86 'ud2' instruction, which is used by compilers to trigger an invalid opcode exception. The article clarifies that 'ud' stands for 'undefined instruction.' The inclusion of the number '2' is not a versioning indicator but rather a remnant of the instruction's evolution within the Intel architecture. Chen explains that the original 'ud' instruction (ud1) was reserved for future use, and when Intel needed a reliable way to force a processor exception for debugging or safety purposes, they introduced the second variant. This technical deep dive highlights how legacy naming conventions in processor architecture often persist long after their original context has faded, providing a fascinating look at the design decisions that shape modern computing hardware at the instruction set level.