Engineer built Fame Boy, a working Game Boy emulator in F# with sound, running on desktop and web, after hundreds of hours and prior CHIP-8 and Nand-to-Tetris groundwork.
Key Takeaways
Core interface is minimal: a 160x144 framebuffer, a 32768 Hz ring audio buffer, stepEmulator(), and a joypad callback, enabling both desktop and web frontends.
F# discriminated unions compressed 512 raw opcodes into 58 typed instructions using From/To types, with illegal states ruled out at compile time.
A 16-line Flags module using inline pure functions replaced a DU-based setFlags array approach, improving FPS by ~10% with zero heap allocation.
Single-threaded stepper function sequences CPU, timers, serial, APU, and PPU steps in the correct T-cycle ratios to simulate parallel hardware.
AI was used only to generate CPU unit test cases from the spec, not to write emulator logic, enabling true TDD while preserving learning.
Hacker News Comment Review
Commenters with F# experience noted that [<Struct>] discriminated unions in Instructions.fs could reduce allocations further, and flagged redundant byte-masking in register setters.
Consensus praised the use of genuine human effort and functional-language domain modelling for hardware emulation, a pairing most consider harder than imperative approaches.
A side thread asked about cycle-accurate emulators with visual CPU step panels (NES/SNES era), unresolved but distinct from this project’s goals.
Notable Comments
@debugnik: Suggests [<Struct>] on DUs in Instructions.fs and notes a &&& 0xFFuy is a no-op on byte registers.
@cermicelli: “Finally someone putting in actual human effort to learn something” – sharp contrast to LLM-built project posts.
