IdeaWell

Project ideas from Hacker News discussions.

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

Original Article

Hacker News Discussion

📝 Discussion Summary (Click to expand)

Theme 1 – Preference for a dedicated undefined opcode (UD2) over software interrupts
Many commenters argue that using a specific UD opcode is simpler, smaller, and more portable than invoking an interrupt handler.
- “It's basically a convention. The alternative is to raise interrupts of course, but that might be application specific…” – js8
- “There's a bit of convention and practicality… Needing to set register to identify a fatal error is not great for code size…” – sweetjuly

Theme 2 – Technical reasons for having UD0/UD1/UD2 instead of using INT to trigger #UD
The discussion explains why Intel defined explicit undefined opcodes: they avoid the need to manually set up the stack frame that INT requires and give a guaranteed, OS‑independent exception.
- “Already since Intel 8086, x86 has the instruction 'INT vector_number'… you need to setup the stack in such a way… If Intel had not defined an official opcode that is guaranteed to remain unused forever… reserving an opcode at Intel and AMD was simpler.” – adrian_b
- “Nowadays UD0 UD1 UD2 are in the SDM and APM. We also got UDB (D6), the one‑byte variant that arrived with x86‑64 for 64‑bit mode.” – 349ru3h4f03

Theme 3 – Historical legacy of floppy drives shaping DOS/Windows drive letters
The assignment of A: and B: to floppy disks (and consequently C: to the first hard disk) is explained as a compatibility relic from early PCs.
- “It's like why the first (hard) drive letter is C.” – qbane
- “Yeah it's a relic from when computers booted off floppy and hard disks were rare and expensive.” – alightsoul
- “The floppies got A and B because hard drives were expensive and for a time a lot of people got by without them…” – raldi

🚀 Project Ideas

UD Trap

Summary

  • Provides a single‑header library that emits the guaranteed undefined opcode UD2 (or architecturally appropriate trap) to trigger a CPU exception for assertions/fatal errors without register setup.
  • Core value: tiny, portable, zero‑overhead way to raise #UD (or equivalent) across x86, ARM, RISC‑V for debugging, guard pages, and sanitizer‑like checks.

Details

Key Value
Target Audience Systems programmers, compiler writers, low‑level library developers
Core Feature Macro TRAP() that expands to UD2 on x86, BRK #0 on ARMv8, etc., with optional immediate payload
Tech Stack C/C++ header‑only (optional Rust crate), inline assembly snippets
Difficulty Low
Monetization Hobby

Notes

  • Quote: “UD2 stops instruction fetching … nice for userspace to have something that doesn't need active OS support.” – fweimer
  • Potential: Could be adopted in sanitizers, fuzzers, or as a lightweight assert in kernels, sparking discussion on safe trap mechanisms.

OpcodeMap

Summary

  • Interactive reference and CLI that lists undefined/reserved opcodes for major ISAs, showing their documented behavior, CPUID feature bits, and safe usage.
  • Core value: eliminates guesswork when choosing a trap instruction, provides vendor‑specific safety matrix and community‑vetted examples.

Details

Key Value
Target Audience OS/kernel developers, emulator writers, security researchers
Core Feature Searchable database + CLI tool to query e.g. “x86 UD2” or “ARM BRK” and get encoding, exception type, notes
Tech Stack React frontend, Node.js backend, SQLite; CLI in Go
Difficulty Medium
Monetization Revenue-ready: “Freemium: free tier + paid API for CI integration”

Notes

  • Quote: “It's basically a convention … need to raise interrupts … but they might work differently on other processor types.” – js8
  • Potential: Helps avoid UD0/UD1 confusion, sparks discussion on ISA design and safe undefined‑opcode usage.

FloppyLetter

Summary

  • Utility that maps virtual floppy disk images to the legacy A: and B: drive letters on Windows/Linux, automatically handling the DOS‑style duplicate‑drive semantics for single‑image use.
  • Core value: lets retro‑computing enthusiasts and legacy software run without manually fiddling with subst or DOSBox config, preserving the classic A/B floppy experience.

Details

Key Value
Target Audience Retro‑gaming community, DOSBox users, embedded‑system maintainers using floppy‑based bootloaders
Core Feature GUI/CLI that assigns an .img file to A: (and optionally B:) and creates the necessary driver/shim (e.g., Windows Substituted Drive or Linux loop + udev rule) to mimic two‑drive behavior
Tech Stack Electron/Tauri GUI, Rust backend, uses libvfdloop or similar
Difficulty Medium
Monetization Hobby

Notes

  • Quote: “A: and B: were not specific to the type of drive … first hard‑drive in a system was made C:” – dspillett
  • Potential: Useful for preserving legacy workflows, could be bundled with DOSBox or discussed on HN retro threads.

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