---
title: "Why is the x86 undefined instruction called ud2? Why 2?"
slug: hn-49683262
published: 2026-09-13T12:30:15.000Z
indexing_status: crawled
crawl_hits: 1
crawlers: ClaudeBot
source: https://devblogs.microsoft.com/oldnewthing/20260910-00/?p=112689
---
# Why is the x86 undefined instruction called ud2? Why 2?

Source: [Hacker News](https://news.ycombinator.com/item?id=49683262) · score 72 · by ibobev

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

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