0xrsydn/gameboy-rust
1
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

fix(serial): gate RCNT interrupt and Joybus controls by mode

Browse source
This commit is contained in:
0xrsydn 2026-10-07 03:10:04 +07:00
commit 86f8663904
7 changed files with 203 additions and 20 deletions
Download patch file Download diff file Expand all files Collapse all files

22
docs/research/emerald-reset.md
View file

@ -33,13 +33,18 @@ The earlier reset-only implementation stopped at master sound enable (`0x0400008
The Direct Sound increment then reached a PSG channel 1 trigger (`0x04000065`, PC `0x082e03bc`).
Channel 1 support reached the channel 2 trigger (`0x0400006d`, PC `0x082e03c0`).
Both pulse channels then reached the noise trigger (`0x0400007d`, PC `0x082e03c4`).
With noise channel 4 implemented, both runs now stop at general-purpose input/output (GPIO) serial interrupt enable:
Noise channel 4 support then reached an incorrectly rejected RCNT write: `0x0100` at `0x04000134`, PC `0x082e2aac`.
RCNT bit 15 was clear, so bit 8 did not enable general-purpose input/output (GPIO) interrupts.
GBATEK documents bits 8 and 14 as writable but unused in normal mode.
The serial model now retains these inactive bits. Effective GPIO interrupt enable and Joybus selection still return explicit errors.
Both runs pass that write and now stop in the original BIOS replacement:
```text
Steps: 395420; instructions: 393690; IRQ entries: 2; DMA units: 1728; HALT idle: 0
Nominal cycles: 1328057
PC=0x082e2aac
unsupported GPIO serial interrupt enable: write 0x01 at 0x04000135
Steps: 398019; instructions: 396289; IRQ entries: 2; DMA units: 1728; HALT idle: 0
Nominal cycles: 1334271
PC=0x00000378
read-only memory at 0x00000000
```
The native run captures five startup frames. These do not establish a working title screen or gameplay.
@ -54,9 +59,10 @@ Workspace debug/release tests, strict Clippy, public ARM/Thumb/memory/BIOS check
Native ROM-window tests and graphics smoke modes pass on Darwin arm64.
The pinned Pong debug/release reports match.
The next observed requirement is [serial control-register behavior](../hardware/serial.md), specifically RCNT bit 8.
Check mode gating and interrupt-enable semantics before changing the current rejection.
This diagnostic alone does not establish that the game needs an active link transfer.
The next investigation is the BIOS replacement's attempted write to address zero at PC `0x00000378`.
Trace the service arguments and original firmware routine before deciding whether the caller or service behavior is wrong.
The diagnostic alone does not establish the root cause. Do not make BIOS memory writable to bypass it.
The corrected RCNT write does not establish a need for active link transfers or GPIO interrupts.
Noise now has deterministic counter clocks and shares tested length/envelope logic with the independent pulse channels.
Sweep applies only to channel 1. Direct Sound has FIFO clocks and nominal DMA requests.