0xrsydn/gameboy-rust
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fix(memory): ignore CPU and DMA writes to the Game Pak ROM windows

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Stores to 0x08-0x0c returned ReadOnly and stopped the machine. Hardware ignores
them, and a second game's boot code stores to a ROM mirror. Emulated writes now
pay their bus cost and change nothing. The 0x0d window stays a diagnostic until
EEPROM is modeled there, and host writes stay strict.

Tests that used a ROM store as their failing write now cross an ignored ROM
word into the 0x0d window.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhfHfHEuzv2AUTd6erW1gU
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0xrsydn 2026-10-14 08:26:03 +07:00
commit 258fbff2fa
25 changed files with 86 additions and 74 deletions
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docs/research/emerald-visible-progress.md
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@ -91,6 +91,9 @@ folding plus write-ignore semantics need independent hardware evidence and
a deliberate policy decision against the explicit-error convention. See
[the usability roadmap](../usability-roadmap.md) for sequencing.
Update: emulated writes to the `0x08`–`0x0c` ROM windows are now ignored, which removes the
`0x09fe2ffe` store diagnostic. Reads past the end of the image remain explicit errors.
## References
- [GBATEK memory map](https://problemkaputt.de/gbatek-gba-memory-map.htm):