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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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5
docs/hardware/cartridge.md
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@ -33,9 +33,10 @@ It does not decode archives or ELF executables. Supply a raw binary with an ARM
Headerless original programs are accepted.
Reads past supplied cartridge bytes remain explicit memory errors, not open-bus values.
All three Game Pak ROM windows use the same supplied bytes and existing wait-state rules.
Ordinary cartridge bytes remain read-only.
Ordinary cartridge bytes remain read-only. CPU and DMA writes to the `0x08`–`0x0c` ROM windows pay their bus cost and are ignored, as on hardware.
Writes to the `0x0d` window remain `ReadOnly` diagnostics until EEPROM is modeled there. Host `Memory::write*` calls stay strict everywhere.
Optional `--save-type flash64` or `--save-type flash128` enables [nominal Flash programming, erase, and reads](saves.md).
Without `--save-file PATH`, changes remain volatile. With it, the frontend loads an exact-size image and persists changes on clean exit.
Without `--save-file PATH`, changes remain volatile. With it, the frontend loads an exact-size image and persists changes on clean exit or when a ROM window ends on a diagnostic.
Missing images start erased. Existing images receive unique backups before replacement; incomplete Flash operations refuse persistence.
Read the [storage safeguards and limits](saves.md#desktop-persistence) before using a valuable save.
Optional `--rtc` selection enables the bounded [Game Pak GPIO and RTC calendar interface](cartridge-gpio.md).