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
This commit is contained in:
0xrsydn 2026-10-14 08:26:03 +07:00
commit 258fbff2fa
25 changed files with 86 additions and 74 deletions
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8
crates/gba-core/tests/dma/latch.rs
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@ -145,12 +145,12 @@ fn zero_is_known_data_not_an_unknown_latch() {
fn failed_units_preserve_the_old_latch_without_new_data_or_irq() {
for (source, destination, control) in [
(0x0e00_0000, DEST, 0x4400), // Unmapped reads do not use a known latch.
(SOURCE, ROM_START, 0x4400),
(SOURCE, 0x0d00_0000, 0x4400),
(SOURCE, 0x0400_0058, 0x4400),
(SOURCE, DMA_BASE, 0x4400),
(SOURCE, 0x0400_0300, 0x4400),
(SOURCE, DEST, 0x4580), // Unsupported source mode.
(0, ROM_START, 0x4400), // A blocked source cannot bypass destination checks.
(SOURCE, DEST, 0x4580), // Unsupported source mode.
(0, 0x0d00_0000, 0x4400), // A blocked source cannot bypass destination checks.
] {
let mut machine = machine();
seed(&mut machine, 3, DATA);
@ -175,7 +175,7 @@ fn failed_units_preserve_the_old_latch_without_new_data_or_irq() {
fn failed_destination_does_not_seed_a_cold_channel() {
let mut machine = machine();
machine.memory_mut().write32(SOURCE, DATA).unwrap();
configure(machine.memory_mut(), 3, SOURCE, ROM_START, 1, 0x400);
configure(machine.memory_mut(), 3, SOURCE, 0x0d00_0000, 1, 0x400);
assert!(machine.step().is_err());
machine.memory_mut().write16(base(3) + 10, 0).unwrap();
configure(machine.memory_mut(), 3, 0, DEST, 1, 0x400);