U3DSUNDERTALE on a real 3DS

What a GameMaker game actually is

Undertale ships as data.win — a 61 MB container of chunked, offset-addressed data plus BC16 bytecode. No source. No symbols. The port is a from-scratch interpreter for that bytecode and a loader for the chunk format: GEN8, OPTN, STRG, TXTR, TPAG, SPRT, BGND, ROOM, OBJT, CODE, FONT, SOND, AUDO, AGRP.

When the original 2016 game calls draw_self(), or adds a draw event at depth 12, or spawns a bullet with instance_create, it has to mean exactly what it meant then. The game is not reimplemented. It is executed.

Two screens, 268 MHz

400×240 up top, 320×240 below — and a GPU that wants everything power-of-two. Textures are packed into a fragmented per-page atlas with a room manifest, so a scene streams only the pages it needs. Draws batch; a residency budget caps memory.

On an Old 3DS the whole atlas cannot be resident. Eviction is demand-driven — eager flushing is exactly what makes a room seam visibly "reload" on every step.

Audio: the DSP is not your PC

Everything is ADPCM through the hardware DSP. Music streams from file in native ADPCM; sound effects live in a packed bank. There are 24 voices, total — and GameMaker retriggers a text blip faster than the 1.7-second sample can finish, so later sounds are dropped silently.

The fix is to steal the oldest non-stream voice when the pool is full, and always ask for a free channel first. Getting that order backwards silences every sound in the game.

THE HEAP. The 8 megabyte wall.

After parsing a 61 MB data.win, an Old 3DS has roughly 8.6 MB of app heap left. The 17.7 MB sound bank cannot be a single app-heap allocation — it goes to the separate linear heap.

And the cruel part: an allocation that fails and calls abort() shows no crash screen. It tears services down mid-frame, so the dump points somewhere absurd, like hidScanInput reading null shared memory. Every hot path had to learn to fail soft and log.

Three bugs that looked like something else

• The stack. A 3DSX gets a 32 KB main-thread stack. Deep script chains walked off it; the dump showed the stack pointer below the image end, mid-VM. Now 256 KB.
• The shadowing. The music resolver matched exact filenames, so a raw .ogg beside its converted .bcwav twin fed Vorbis bytes to the CWAV parser forever.
• The ghosts. 46 music cues — piano notes, the drum kit, the dial-up — exist only embedded inside data.win's AUDO chunk. No file to convert, so they were silent until the preprocessor learned to reach inside the chunk.

The pipeline, end to end

Your data.win goes in, and out comes a card-shaped tree:

data.win ─┬─▶ texture atlas  ▸ 522 pages .t3x
          ├─▶ streamed music ▸ 248 × .bcwav
          ├─▶ packed SFX     ▸ sound_bank.bin
          └─▶ 3ds/U3DS/      ◀ copy to card

The game's own data.win is never modified — it's parsed at runtime on the console. That's why this page can hand you a converter instead of a download.