gameboy-rust/AGENTS.md

AGENTS.md

Orientation for coding agents. This is a map, not a rulebook: use your judgment, and read the linked doc for the area you are touching rather than everything.

What this is

gameboy-rust is a from-scratch Game Boy family emulator in Rust. The Game Boy Advance comes first; other systems (GB, GBC) may follow as separate cores. It cannot run commercial games yet. See docs/status.md for what works, what is deliberately missing, and what is next.

Crates

• crates/gba-core: the emulator (CPU, memory bus, I/O, timers, DMA, video, BIOS replacement). Keep it free of external dependencies and of window, audio, clock, and file-system code, so it stays portable to WebAssembly.
• crates/gba-demos: original ARM test programs and frame runners that drive the core.
• crates/desktop: minifb frontend and the gameboy-rust binary. Host-specific code belongs here.

docs/architecture.md has a file-by-file map.

Commands

Run through direnv so the Nix toolchain and macOS build flags apply:

direnv exec . cargo test --locked                                  # everything
direnv exec . cargo test --locked -p gba-core -p gba-demos         # skip the window dependency
direnv exec . cargo clippy --locked --all-targets -- -D warnings
direnv exec . cargo fmt --all
direnv exec . cargo run --locked --release -- --tile-demo          # --help lists all modes

Window smoke tests (--*-smoke-test) need a logged-in desktop session. Details are in docs/testing.md.

Where to read

Working on	Read
ARM/Thumb instructions, modes, exceptions, timing	docs/hardware/cpu.md
ROM files and bounded terminal execution	docs/hardware/cartridge.md
BIOS services (SWI)	docs/hardware/bios.md
Timers, IE/IF/IME, HALT/STOP	docs/hardware/timers-irq.md
DMA	docs/hardware/dma.md
VCOUNT/DISPSTAT, HBlank/VBlank, row capture	docs/hardware/display-timing.md
Backgrounds, sprites, mosaic, windows, blending, input	docs/hardware/video.md
Demos and key mapping	docs/demos.md
Build environment, minifb workarounds	docs/development.md

Where to write

• Behavior notes for a subsystem go in its docs/hardware/*.md file. Add a new file there when a new subsystem arrives (audio, cartridge, saves).
• Sources you consulted, including web searches, go in docs/references.md, or in a "References" section of the subsystem doc when the link explains one specific behavior.
• Update docs/status.md when a feature lands or a limit goes away.
• Keep README.md short: overview, quick start, and the docs index.
• Avoid hard-coded test counts in prose; they go stale with every change.

Conventions

• Version control is jj (colocated with git). History is a linear stack with one bookmark per change, named feature/<topic> or refactor/<topic>.
• Commit messages follow Conventional Commits, for example feat(bios): implement SoftReset restart service.
• New behavior comes with tests. Core integration tests live in crates/gba-core/tests/.
• The core currently reports unsupported hardware behavior as explicit errors rather than guessing. Prefer that to silent approximations, and note the gap in the docs.
• Everything in the repo is original. Do not add Nintendo BIOS code, game ROMs, or game assets.