Why is the x86 undefined instruction called ud2? Why 2?
If you look at x86 compiler output (or if, like me, you’re looking at a crash caused by some software that tried to detour an API), you may see an instruction ud2. The ud2 instruction is an architecturally undefined instruction, guaranteed to raise an “invalid opcode” exception. Some compilers generate it to mark “unreachable” code, so that if execution somehow manages to reach it, you get a crash rather than executing random instructions.
- ▪If you look at x86 compiler output (or if, like me, you’re looking at a crash caused by some software that tried to detour an API), you may see an instruction ud2.
- ▪The ud2 instruction is an architecturally undefined instruction, guaranteed to raise an “invalid opcode” exception.
- ▪Some compilers generate it to mark “unreachable” code, so that if execution somehow manages to reach it, you get a crash rather than executing random instructions.
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| Original publisher | The Old New Thing |
| Canonical URL | https://devblogs.microsoft.com/oldnewthing/20260910-00/?p=112689 |
| Publication time | Sun, 13 Sep 2026 12:30:15 +0000 |
| Retrieval time | 2026-09-13T15:16:50.737Z |
| Last seen | 2026-09-13T15:16:50.737Z |
| Headline source | Publisher (no WeSearch rewrite) |
| Excerpt source | publisher body |
| Excerpt method | First ~120 words (~800 chars) of extracted publisher body, fair-use limited. |
| Summary | WeSearch · cerebras-chat (WeSearch summarizer) |
| Summary source text | contentText |
| Citation coverage | Summary is a WeSearch-generated derivative; primary citation is the original publisher URL. |
| Cluster | 5YLbUBV-h-xP · 1 stories |
| Cluster logic | Grouped by semantic title/content similarity across sources within a rolling window. Same-publisher template collisions are excluded from coverage comparison. |
| Ranking reason | Story pages are not engagement-ranked. Hub feeds use recency, with optional source-diversified chronological ordering (cap consecutive stories per source). No personalized ranking. |
| Publisher visit | Yes — open original |
| Substitutes article? | No — link-out required for full text |
Rights status (four layers)
WeSearch handling by dimension
| Indexing | May the item be indexed (stored, ranked, made findable)? | Allowed |
| Snippet | May a short excerpt of the publisher's text be shown? | Allowed |
| AI summary | May WeSearch generate its own short summary of the article? | Limited |
| Retrieval / RAG | May the content be exposed for third-party retrieval-augmented generation? | Not asserted |
| Model training | May the content be used to train AI models? | Not asserted |
| Commercial reuse | May the content be reused commercially? | Not permitted |
Basis: Derived from the published RSS/Atom feed. Contact: [email protected]. Reviewed: 2026-07-24.
Opening excerpt (first ~120 words) tap to expand
If you look at x86 compiler output (or if, like me, you’re looking at a crash caused by some software that tried to detour an API), you may see an instruction ud2. What’s up with that? The ud2 instruction is an architecturally undefined instruction, guaranteed to raise an “invalid opcode” exception. Some compilers generate it to mark “unreachable” code, so that if execution somehow manages to reach it, you get a crash rather than executing random instructions. For example, if a function marked [[noreturn]] somehow returns, the compiler will put a ud2 after the call so that the program crashes instead of falling through to the next function.
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Excerpt limited to ~120 words for fair-use compliance. The full article is at The Old New Thing.
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