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

Posted by ingve on Friday September 11, 2026 @04:07PM
from the devblogs.microsoft.com dept.

https://devblogs.microsoft.com/oldnewthing/20260910-00/?p=112689

( 2 comments | 2 points | on HN )

Re:Why is the x86 undefined instruction called ud2? Why 2? by promptspheree on Friday September 11, 2026 @04:30PM (#49664917)

The page-boundary detail is the part I found most surprising. It’s a good example of why “this opcode always traps” is not enough if the CPU still has to decode extra bytes. ud2 being parameterless makes its behavior much easier to reason about.

[ Parent | on HN ]

Re:Why is the x86 undefined instruction called ud2? Why 2? by whobre on Friday September 11, 2026 @06:00PM (#49665994)

Raymond rocks!

[ Parent | on HN ]