The apu module's upload path runs once, at boot. This example adds the missing capability: replacing the running APU program at any time. Press A for the play_noise drum kit, B for the pitch_mod vibrato cello — the kit cuts, the cello blooms, and back, forever. The backdrop tracks the active program (red = drums, amber = cello).

The method is krom (Peter Lemon)'s PlayTwoSong protocol, productized: apuReset() writes APU_RESET_MAGIC to I/O port 0; the SPC700 program polls for it inside its wait loops (the APU_CHECK_RESET idiom from templates/memmap_spc700.inc), silences the DSP, acks on its output ports, and jumps back to the IPL boot ROM at $FFC0 — which is re-entrant, so the full boot handshake works again and the next apuUpload()/apuExecute() proceeds exactly like the first.

krom's original demo swaps between two songs whose samples are ripped from commercial games (Axelay, Final Fantasy VI, the Halken logo), so this port swaps between two OpenSNES-native programs instead — same protocol, zero rights issues.

ROM mode: LoROM (project default).

SNES Concepts

Cooperative APU reset: CPU→APU messaging over $2140 after boot
The IPL ROM is re-entrant — re-enable it (CONTROL bit 7), jump to $FFC0, and the $AA/$BB + $CC upload protocol runs again
Hot-swap hygiene: the DSP arrives dirty in the second program (this example's cello inherited the hi-hat's ADSR on its LFO voice and lost its vibrato until the program zeroed V0ADSR explicitly). Every register a voice depends on must be written — never rely on reset defaults after a swap.
Two independent APU programs in one ROM, uploaded on demand

Validation

Functional probe (tools/luna-test/probes/apu_switch.py): boot state, non-silent drums, and the B-press swap asserted on current_song via WRAM. Audio parity of each program vs its source example verified by capture (drums RMS/onsets; cello 174.6 Hz, vibrato ±1.1 %, LFO 5 Hz — identical to pitch_mod).

How to Build

make

Modules Used

console, apu, input