ud2

Hold

Techniques

A permanently undefined x86 instruction that reliably raises an invalid opcode exception.

Why it's here

Placed in Hold: 1 article(s) of evidence from 1 source(s), led by research-stage coverage, with 1 in the last 30 days. Confidence 24%. Low accumulated evidence, so it defaults conservatively pending more signal.

Evidence (1)

2Hacker News·9/13/2026research
Why x86 uses ud2 for undefined instructions
The article explains the history behind x86's ud2 instruction, which is a guaranteed invalid opcode used to trigger a crash when execution reaches unreachable code. It traces how older byte sequences like 0F FF and 0F B9 were used informally before Intel standardized ud2 for consistent behavior across processors.