GBA Lua SDK

Make games for the Game Boy Advance by writing a PICO-8-flavored Lua instead of C or ARM assembly.

The SDK ahead-of-time compiles your Lua to C, builds it with a bundled ARM toolchain against a libtonc + maxmod runtime, and produces a .gba ROM that runs in mGBA and on real hardware. No interpreter, no VM: your Lua becomes native ARM machine code.

If you know PICO-8 you'll feel at home (spr/btn/_init/_update/_draw, Lua syntax) - but this SDK leans into what the GBA can do that a fantasy console can't: 128 hardware sprites, rotate/scale (affine) sprites, four scrolling tile layers, Mode 7, hardware windows, alpha blending and fades, mosaic, per-scanline raster effects, battery saves, and streamed module music. Familiarity, not compatibility.

This is a fork of the GameTank Lua SDK; it reuses that compiler front-end and PICO-8 number model, and retargets the back end to native ARM.

Your first game

The whole hello, one main.lua with no asset files: a greeting plus a hardware sprite you move with the d-pad. _update60 runs the movement 60 times a second; _draw redraws the sprite every frame (examples/hello/main.lua):

-- The screen is 240x160. spr uses the built-in default sheet, so no art file
-- is needed. Colors are PICO-8-style indices 0-15 (1 dark-blue, 14 pink).
local x, y = 112, 72

function _update60()               -- 60fps input + movement
  if (btn(1)) then x += 2 end      -- right
  if (btn(0)) then x -= 2 end      -- left
  if (btn(3)) then y += 2 end      -- down
  if (btn(2)) then y -= 2 end      -- up
end

function _draw()
  cls(1)                           -- dark-blue background layer
  print("hello gba", 88, 24, 14)   -- greeting near the top, pink
  spr(0, x, y, 2, 2)               -- the hardware sprite, redrawn every frame
end

Build it and play it in a window - no external emulator needed:

node bin/gbalua.js run examples/hello/main.lua

gbalua run opens a window over the bundled mGBA core (arrows = d-pad, Z/X = B/A, Enter = START), via the shared romdev-core-runner SDL host. Or build a .gba ROM to ship:

node bin/gbalua.js build examples/hello/main.lua -o hello.gba

Run hello.gba in mGBA (or any GBA emulator) or flash it to a cartridge. That's the whole loop: write main.lua, run it, ship the .gba.

Colors are PICO-8-style indices 0-15 (0 black, 1 dark-blue, 10 yellow, 14 pink); pal() / spr_col() reach the full 15-bit BGR555 palette (32768 colors) at runtime when you want more.

Featured example: a full shmup

examples/starfall is a complete little shmup - hardware sprites, a scrolling starfield, music + sfx, a HUD, win/lose states - in one main.lua. That's what this SDK is for:

node bin/gbalua.js build examples/starfall/main.lua \
  --sheet examples/starfall/shmup_sheet.png \
  --map examples/starfall/space_bg.png -o starfall.gba

Requirements

• Node.js 24+
• nothing else — npm install brings the whole toolchain as dependencies: arm-gcc + libtonc + maxmod, all as WebAssembly (via romdev-platform-gba + romdev-maxmod). No devkitPro, no native tools to build or install.

The build runs the WASM toolchain in-process (cc1-arm → as → ld → objcopy) — no server, no daemons. It can also talk to a running romdev server instead (GBALUA_BACKEND=mcp, faster for rapid rebuilds since the server keeps warm toolchain workers). See compiler/build-gba.mjs.

gbalua run opens the window through romdev-core-runner (the one SDL host shared across the whole SDK family and the romdev playtest tool). The window needs @kmamal/sdl, an optional dependency of the runner (pulled by npm install, prebuilt for the common platforms); on a headless box or unsupported platform, run prints a clear message and you build a .gba for an external emulator instead. build never needs it.

The screen and the two rendering modes

The GBA screen is 240×160. Two ways to draw, and they are one either-or (a hardware mode bit); sprites, sound, and all the color effects compose over both.

• Tile mode (Mode 0) - the real game path. Four hardware tile layers that scroll for free, plus 128 hardware sprites on top. This is how scrolling GBA games work; it's what the flagship example (starfall) uses.
• Bitmap mode (Mode 4) - an immediate-mode 8bpp framebuffer for pset/rect/circ/line + text. Simplest to start with (the hello above), single-buffered, slower per-pixel. Sprites still compose on top.

You don't pick a mode explicitly - using tile-layer verbs (map_show, tileset, layer_*, mode7) puts you in tile mode; the immediate draw verbs use bitmap mode.

The PICO-8 contract

Define _update60() (60 fps) or _update() (30 fps), plus _draw(), and optionally _init(). The runtime latches input before each update and ends the frame after _draw() (OAM flush, vblank).

Numbers are PICO-8 numbers: 16.16 fixed point. sin/cos/atan2 use turns (0..1) with PICO-8's screen-space-inverted sin. The compiler infers which values stay integral and keeps them in fast 32-bit ints - an optimization, never a semantic change.

The dialect keeps PICO-8's syntax: +=-style compound assignment, one-line if (cond) stmt / while (cond) stmt, !=, \ floor division, // comments, hex/binary literals with fractions, and multiple assignment (x, y = 64, 32).

Arrays are 1-indexed (Lua/PICO-8 style): a = array(8), then a[1] is the first element (a[0] is out of bounds).

API at a glance

lifecycle	_init _update _update60 _draw
bitmap draw	cls camera color pset pget sset clip rect rectfill circ circfill line
16-bit bitmap	mode15() true color · rgb15(r,g,b) · cls15 pset15 flip15 - a 160×128 BGR555 framebuffer
sprites	spr(n,x,y,[w,h],[fx,fy]) hardware OBJ · spr8(t,x,y,[flip]) · spr_pal spr_prio
affine sprites	sprr(n,x,y,angle,scale) rotate+scale · sprr2(n,x,y,angle,sx,sy) non-uniform
tile layers	map_show tileset tilemap layer_show layer_pri layer_scroll parallax camera mget/tget/tset
mode 7	mode7() · mode7_cam(x,y,angle,[zoom]) · mode7_off() - an affine plane on BG2
affine BG	abg_setup(tiles,ntiles,map,msize,[pal]) · abg_cam(x,y,angle,[zoom]) · abg_off() - your own rotate/scale layer
windows	window(x0,y0,x1,y1) spotlight · window_inside/window_outside/window_obj · window_off
color effects	fade(amount,[white]) · blend(layer,alpha) · blend_off · mosaic/mosaic2 · backdrop · screen_off/screen_on
palette	pal(i,r,g,b) BG · spr_col(i,r,g,b) OBJ · hgradient(table) per-scanline backdrop
animation	anim(slot,first,last,fps) loop · anim_once · anim_pingpong · anim_reset · anim_done
input	btn(i,[pl]) btnp(i,[pl]) - 0-3 d-pad, 4=A, 5=B, 6=L, 7=R, 8=START, 9=SELECT
math	flr ceil abs sgn sqrt min max mid sin cos atan2 rnd srand t/time + bit ops
data	array(n,[v]) 16.16 · array8(n,[v]) bytes 0-255 · pool(n) · dma(dst,src,n)/dma_fill fast moves
save	save(slot,array8,n) · load(slot,array8,n) - battery SRAM, 16 slots × 1 KB
timer	timer_start() · timer_read() - free-running Timer 3, sub-frame profiling
sound	music(n,[loop]) module music · sfx(n,[ch]) · sfx_ex(n,vol,pan,pitch) · sfx_volume

See docs/CHEATSHEET.md for every verb with signatures.

Assets

--sheet sprites.png imports a sprite sheet, --map level.png a tilemap, and --mode7 plane.png an affine plane, via a self-contained PNG → tile converter (compiler/png-tiles.mjs). All three also accept the formats artists actually work in — Aseprite (.ase/.aseprite, frame 0 flattened) and Tiled (.tmx, visible layers composited; embed the tileset in the map) — imported, never re-invented. They're CLI flags on build:

node bin/gbalua.js build --target gba mygame/main.lua \
  --sheet mygame/sprites.ase --map mygame/level.tmx -o mygame/game.gba

The examples/ directory shows each subsystem in use:

example	shows
showcase	a scene-cycling tour of the whole feature set (L/R to switch scenes)
starfall	a complete shmup - tile mode, sprite HUD, module music + SFX
effects	blend / fade in bitmap mode
mode7	an affine plane you rotate, zoom, and drive over
windows	a hardware spotlight over the Mode 7 plane
anim	frame-range animation helpers
hwtest	hgradient raster gradient, SRAM save/load, the timer

Sound

Music is a streamed module (maxmod), and SFX are sampled one-shots - trigger both by index, PICO-8 style:

function _init()
  music(0)                       -- start module 0, looping
end
function _update60()
  if btnp(4) then sfx(0) end      -- A -> a sound effect
  if btnp(5) then sfx_ex(1, 512, 200, 1.5) end  -- B -> louder, panned, +pitch
end

music(-1) stops; music(n, false) plays once. sfx_ex(n, vol, pan, pitch) gives per-shot volume (0-1024), pan (0-255, 128 center), and pitch (a 16.16 multiplier); sfx_volume(v) sets the master SFX level.

Bring your own music with --music song.xm on build (repeatable — music(0) plays the first module, music(1) the second, …; .xm/.mod/ .it/.s3m all work). The soundbank is compiled at build time by romdev-maxmod, a faithful pure-JS port of devkitPro's mmutil. Or link a prebuilt bank with --soundbank bank.bin. With neither flag, the default soundbank (assets/soundbank.bin) ships a chiptune as module 0.

Not-Lua walls (loud, never silent)

Conditions must be boolean (if x ~= 0 then, not if x then - Lua calls 0 truthy, C doesn't, and the compiler refuses to guess). No nil, closures, metatables, coroutines, string concatenation, or goto. Every unsupported feature is a compile-time error that says what to write instead.

Repo layout

compiler/ Lua→C compiler + the GBA build driver (build-gba.mjs) and PNG importer (png-tiles.mjs) · gba-sdk/ the C runtime (thin libtonc/maxmod wrappers: gba_api.c frame+sprites+input, gba_bg.c tile layers, gba_mode7.c, gba_win.c, gba_fx.c effects, gba_sound.c, gba_hw.c save+timer, gba_text.c, gba_anim.c, gba_math.c) · assets/ the default soundbank + its generators · bin/gbalua.js CLI · examples/.

Docs

doc	what
docs/CHEATSHEET.md	the full GBA Lua API reference
docs/CHEATSHEET_FOR_PICO8_USERS.md	per-function PICO-8 → GBA map

License

MIT. gba-sdk/ wraps libtonc and maxmod (both zlib/BSD-style); the compiler front-end derives from the GameTank Lua SDK (MIT).