Building a Game Boy Emulator in OCaml

CAMLBOY is a Game Boy emulator written in OCaml that runs in the browser at 60 FPS even on mobile devices.

The emulator uses OCaml’s module system, functors, GADTs, and first-class modules for clean structure, testability, and runtime cartridge selection.

The architecture comprises CPU, timer, and GPU modules synchronized via a bus that routes 8-bit and 16-bit reads/writes through shared interfaces.

Synchronization is achieved with a main loop that advances timer and GPU in a “catch up” method based on CPU cycle counts.

Game Boy instructions are represented using OCaml GADTs for type-safe argument handling and simplified execution logic.

Integration tests use test ROMs and ppx_expect to enable regression testing and exploratory development.

The emulator compiles to JavaScript with js_of_ocaml and Brr for a browser UI implementation.

Profiling identified GPU and other modules as bottlenecks, and sequential optimizations improved FPS from 20 to 60+ in browsers and 100+ on PC.

Benchmarks in headless mode compare performance across OCaml compiler backends.

The author praises OCaml’s strengths (static typing, variants, modules, ecosystem) and notes challenges in dependency management and abstraction costs.