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feat(audio): implement nominal Direct Sound playback and FIFO DMA

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# Sound initialization subset
# Direct Sound subset
Sound synthesis and host audio output are not implemented.
The core now models disabled sound-register state so firmware can perform a functional sound reset.
This is not a silent-audio compatibility mode. Enabling sound returns an explicit diagnostic.
The core models timer-driven Direct Sound A/B playback and a bounded sound-DMA configuration.
It does not produce desktop audio. Programmable sound generator (PSG) synthesis remains unsupported.
`Memory::audio_level()` exposes the current digital stereo level for deterministic inspection, not a continuous sample stream.
## Supported state
## Registers and inactive PSG state
| Address | Behavior |
| --- | --- |
| `0x04000060..0x04000081` | Programmable sound generator (PSG) registers remain zero; writes have no effect while master sound is disabled |
| `0x04000082` (`SOUNDCNT_H`) | Retains mixing/routing/timer configuration with readback mask `0x770f` |
| `0x04000084` (`SOUNDCNT_X`) | Reads zero; status flags are read-only; a write setting bit 7 fails |
| `0x04000088` (`SOUNDBIAS`) | Retains bias/resolution configuration with mask `0xc3fe`; no analog output is generated |
| `0x04000090..0x0400009f` | Read/write wave RAM in the CPU-accessible inactive bank |
| `0x04000060..0x04000081` | Idle PSG configuration and documented read masks; writes are ignored while master sound is disabled |
| `0x04000082` (`SOUNDCNT_H`) | Mixing, routing, timer selection, and FIFO reset strobes; readback mask `0x770f` |
| `0x04000084` (`SOUNDCNT_X`) | Master enable in bit 7; read-only PSG activity flags remain zero |
| `0x04000088` (`SOUNDBIAS`) | Bias/resolution configuration with mask `0xc3fe` |
| `0x04000090..0x0400009f` | CPU wave RAM window, opposite the bank selected by `SOUND3CNT_L` bit 6 |
| `0x040000a0..0x040000a7` | FIFO A/B writes with byte, halfword, and word access boundaries preserved |
Unused mapped bytes through `0x0400008b` read zero and ignore writes.
The FIFO reset strobes in `SOUNDCNT_H` do not latch. No FIFO data can be queued in this subset.
FIFO addresses and the remaining sound-region holes stay unmapped.
All sound configuration starts at zero in raw memory. RegisterRamReset bit 6 sets the functional bias default to `0x0200`.
Unused mapped register bytes read zero and ignore writes.
FIFO reads and the `0x0400008c..0x0400008f` gap remain explicit unmapped-access errors; no FIFO open-bus value is invented.
Raw memory starts with zero sound state. RegisterRamReset bit 6 sets the functional bias default to `0x0200`.
With master sound disabled, `SOUND3CNT_L` stays zero. The CPU accesses wave bank 1.
Bank 0 cannot be selected or played and remains zero. Disabling master sound does not erase wave RAM.
The BIOS reset explicitly clears the accessible wave RAM through ordinary stores.
Future bank switching and channel playback need a complete sound engine, not a larger register array.
Master disable clears PSG configuration and the FIFO queues, but retains wave RAM and Direct Sound routing/bias.
The CPU then accesses wave bank 1. Idle bank selection can expose bank 0 after master enable.
The original BIOS reset clears bank 1 and both FIFO queues. It retains bank 0 and any separate in-flight playback word.
This is a functional reset subset, not a claim of complete firmware ordering or sound-reset equivalence.
## Diagnostics and atomicity
An enabled PSG channel trigger returns `MemoryError::UnsupportedIo` with the exact address and value.
This applies even when the configured channel would be inaudible. Disabled PSG trigger writes are ignored.
Length, envelope, sweep, frequency, and wave-bank playback clocks are not implemented.
Do not interpret successful idle configuration writes as PSG synthesis.
A write setting `SOUNDCNT_X` bit 7 returns `MemoryError::UnsupportedIo`, including the address, byte value, and operation.
Status bits and unused bits alone do not enable sound.
Validation occurs before any bytes in the access are written.
Block stores validate every register value before committing earlier writes. Failed CPU and DMA steps remain retryable.
## FIFO and timer behavior
Original tests cover disabled-register writes, masks, byte lanes, wave retention, and BIOS reset selection.
They also check rejected sound activation through byte/word accesses, ARM block stores, and DMA without partial state changes.
Each channel has a seven-word queue, a separate four-byte playback word, and a held signed sample.
Timer 0 or timer 1 overflow clocks the selected channel while master sound is enabled.
Timer 1 cascade overflows work. Timer IRQ enable and stereo routing do not gate sample consumption.
Timers 2 and 3 cannot clock Direct Sound.
## Remaining work
On each selected overflow:
Implement channel clocks, length/envelope/sweep state, wave banking, FIFO consumption, timer-triggered sound DMA, and mixing.
Add a host-independent sample interface before connecting a desktop audio backend.
Do not acknowledge active sound requests without implementing their device behavior.
1. Request sound DMA if the queue contains at most three words, before loading a playback word.
2. Load a queued word if the playback word is empty.
3. Latch the next signed eight-bit sample in little-endian order.
The held sample remains unchanged between overflows. An empty playback word supplies zero on the next overflow.
This underflow policy follows both consulted emulator implementations; the cited hardware report leaves it uncertain.
A bulk advance consumes all available samples and then reaches zero without looping over every empty overflow.
A partial FIFO write updates the addressed lanes of the next queue slot and enqueues that whole word.
Other lanes retain that slot's previous bytes. A write to a full queue clears the queue.
Reset strobes clear queues without changing the held sample or remaining playback bytes.
Queue clearing also zeros stored queue slots in this nominal model.
Exact stale-lane behavior across overflow/reset remains unverified on hardware.
HALT continues timer/audio progress. STOP freezes it.
Master disable stops sample consumption; route disable only mutes the mixer path.
## Sound DMA
Special timing supports DMA1 to FIFO A and DMA2 to FIFO B.
Each request transfers four 32-bit words with a fixed destination, regardless of programmed count, width, or destination mode.
Source address control, channel priority, repeat enable, completion IRQ, and retained DMA data still apply.
Without repeat, the channel disables after the fourth word. With repeat, it waits for another timer request.
Other channel/FIFO pairings and non-FIFO destinations remain explicit `DmaError::UnsupportedControl` cases.
The references disagree about request routing outside the supported pairing; hardware reports permit non-FIFO destinations.
The core does not silently guess those configurations. Video capture and Game Pak DRQ remain unsupported.
Requests become visible at successful machine-step boundaries. Active DMA blocks ignore further requests.
Device-only bulk advances coalesce unserviced requests; they do not execute or replay DMA blocks.
Sub-instruction DMA arbitration and exact refill latency are not modeled.
## Mixing and timing ownership
Each routed sample contributes `sample * 2` at 50% volume or `sample * 4` at 100% volume.
The mixer adds the ten-bit bias, clips each side to `0..1023`, and subtracts 512.
`StereoLevel` therefore contains signed levels in `-512..511`. Master disable returns zero.
Bias resolution bits retain their values but do not yet drive pulse-width modulation (PWM) sampling or quantization.
There is no resampling, analog filtering, sample queue, or host output backend.
Polling once per video frame will not reconstruct audio.
Timer and sound state share the staged CPU/DMA clock.
Reads and writes observe their nominal bus-completion phase, after earlier timer overflows.
Successful steps commit once. Failed steps discard FIFO consumption, held samples, register writes, and refill requests.
Block-store validation simulates sound enable/disable in a temporary state before committing any RAM or I/O writes.
Other devices retain their existing scheduling limits.
## Validation and remaining work
Original tests cover signed samples, byte order, queue capacity, partial writes, resets, underflow, stereo routing, and clipping.
They also cover cascaded timers, DMA block/repeat behavior, source progress, completion IRQs, HALT/STOP, and clock batching.
CPU/DMA failure tests verify transactional rollback and sound writes at bus completion.
BIOS tests cover disabled FIFO reset and the supported wave-bank policy.
These tests validate the documented nominal model, not hardware audio fidelity.
Next work: PSG clocks and synthesis, a timestamped or fixed-rate output stream, PWM/mixer sampling, and a Darwin host backend.
The local Emerald startup now passes master enable and stops at a PSG channel 1 trigger.
See [the local runtime result](../research/emerald-reset.md).
## References
- [GBATEK sound control registers](https://problemkaputt.de/gbatek-gba-sound-control-registers.htm): disabled PSG behavior, master enable, masks, FIFO reset strobes, and bias.
- [GBATEK](https://mgba-emu.github.io/gbatek/): channel 3 wave-bank access and register layout.
- [mGBA BIOS service implementation at 3a5e34be](https://github.com/mgba-emu/mgba/blob/3a5e34be33dc7f8f707e5bc9db69e8a430046f21/src/gba/bios.c): functional reset defaults. No upstream implementation was copied.
- [GBATEK Direct Sound](https://problemkaputt.de/gbatek-gba-sound-channel-a-and-b-dma-sound.htm): registers, timer selection, signed samples, and DMA blocks.
- [GBATEK sound control](https://problemkaputt.de/gbatek-gba-sound-control-registers.htm): master enable, read masks, reset strobes, routing, and bias.
- [GBATEK mirror](https://mgba-emu.github.io/gbatek/): wave-bank access and register layout.
- [gbadev Direct Sound overview](https://gbadev.net/gbadoc/audio/directsound.html): introductory FIFO/timer/DMA behavior; not authority for queue edge cases.
- [Gericom's hardware findings, mGBA issue 1847](https://github.com/mgba-emu/mgba/issues/1847): seven-word queue, separate playback buffer, request order, partial writes, and overflow observations.
- [mGBA audio at 3a5e34be](https://github.com/mgba-emu/mgba/blob/3a5e34be33dc7f8f707e5bc9db69e8a430046f21/src/gba/audio.c): playback, reset, mixing, and destination-based request routing comparison.
- [NanoBoyAdvance APU at 55b5cf0a](https://github.com/nba-emu/NanoBoyAdvance/blob/55b5cf0ae3d929582ac5bfd486558173502b8354/src/nba/src/hw/apu/apu.cc): master gating, playback word, underflow, and mixing.
- [NanoBoyAdvance FIFO](https://github.com/nba-emu/NanoBoyAdvance/blob/55b5cf0ae3d929582ac5bfd486558173502b8354/src/nba/src/hw/apu/channel/fifo.hh), [registers](https://github.com/nba-emu/NanoBoyAdvance/blob/55b5cf0ae3d929582ac5bfd486558173502b8354/src/nba/src/hw/apu/registers.cc), and [DMA](https://github.com/nba-emu/NanoBoyAdvance/blob/55b5cf0ae3d929582ac5bfd486558173502b8354/src/nba/src/hw/dma/dma.cc): queue/reset policy and channel-based request routing comparison.
- [mGBA BIOS at 3a5e34be](https://github.com/mgba-emu/mgba/blob/3a5e34be33dc7f8f707e5bc9db69e8a430046f21/src/gba/bios.c): functional reset defaults.
Reset values are source-backed. Exact Nintendo firmware ordering, timing, and analog behavior are not established.
The Rust implementation and regression programs are original. No upstream implementation or external test ROM was copied into the repository.