0xrsydn/gameboy-rust
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fix(memory): ignore CPU and DMA writes to unused I/O addresses

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Stores to I/O addresses with no register returned Unmapped and stopped the
machine. Hardware ignores them. Emulated writes there now pay their bus cost and
change nothing; host writes stay strict. The unmodeled internal memory control
register at 0x04xx0800 and all unused I/O reads remain diagnostics.

Rollback tests that used unused I/O as their failing word now run the block on
to a register that rejects the stored value: SIOMULTI2 outside multiplayer
mode, an RCNT GPIO serial IRQ, or 64-sample wave playback. Where no reachable
register can fail (JOYSTAT, POSTFLG/HALTCNT, IME, DMA3CNT), the test now checks
that the block commits and the unused word is ignored.

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 11:38:51 +07:00
commit 20176716d9
16 changed files with 166 additions and 92 deletions
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24
crates/gba-core/tests/dma.rs
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@ -749,13 +749,14 @@ fn failing_later_unit_preserves_earlier_data_but_not_partial_current_unit() {
}
#[test]
fn failed_io_write_does_not_partially_change_display_control() {
fn unused_io_destination_is_ignored_without_changing_display_control() {
let mut machine = machine();
machine.memory_mut().write32(SOURCE, 0xffff_ffff).unwrap();
configure(machine.memory_mut(), 3, SOURCE, 0x0400_0058, 1, 0x400);
assert!(machine.step().is_err());
units(&mut machine, 3, 1);
assert!(!enabled(&machine, 3));
assert_eq!(machine.memory().read16(DISPCNT).unwrap(), 0);
assert_eq!(machine.cycles(), 0);
assert!(machine.cycles() > 0);
}
#[test]
@ -787,11 +788,11 @@ fn cpu_can_enable_dma_with_a_combined_count_control_store() {
}
#[test]
fn failed_cpu_block_store_cannot_enable_dma_or_change_its_hidden_latches() {
fn cpu_block_store_past_dma3_control_enables_the_channel_and_ignores_unused_io() {
let rom = words(&[
0xe59f_000c, // LDR r0,[pc,#12] (DMA3CNT)
0xe59f_100c, // LDR r1,[pc,#12] (count/control)
0xe8a0_0006, // STMIA r0!,{r1,r2}; second word unmapped.
0xe8a0_0006, // STMIA r0!,{r1,r2}; the second word is unused I/O.
0xeaff_fffe,
0xe1a0_0000,
base(3) + 8,
@ -803,15 +804,14 @@ fn failed_cpu_block_store_cannot_enable_dma_or_change_its_hidden_latches() {
cpu.step(&mut bus).unwrap();
configure(&mut bus, 3, SOURCE, DEST, 2, 0);
bus.write16(base(3) + 10, 0).unwrap();
let before = cpu.clone();
bus.write32(SOURCE, 0x1234_5678).unwrap();
let mut machine = Machine::new(cpu, bus);
assert!(matches!(machine.step(), Err(MachineError::Cpu(_))));
assert_eq!(machine.cpu(), &before);
assert!(!enabled(&machine, 3));
assert_eq!(machine.cycles(), 0);
machine.memory_mut().write16(base(3) + 10, 0x8000).unwrap();
units(&mut machine, 3, 2); // The count latch is still 2, not 1.
assert_eq!(machine.step().unwrap(), StepKind::Instruction);
assert_eq!(machine.cpu().registers()[0], base(3) + 16);
assert!(enabled(&machine, 3));
units(&mut machine, 3, 1); // The store latched a count of 1.
assert!(!enabled(&machine, 3));
assert_eq!(machine.memory().read32(DEST).unwrap(), 0x1234_5678);
}
#[test]