Pass channel_1_freq_change_timing
@@ -681,6 +681,21 @@ void GB_apu_write(GB_gameboy_t *gb, uint8_t reg, uint8_t value)
case GB_IO_NR14:
case GB_IO_NR24: {
unsigned index = reg == GB_IO_NR24? GB_SQUARE_2: GB_SQUARE_1;
+
+ /* TODO: When the sample length changes right before being updated, the countdown should change to the
+ old length, but the current sample should not change. Because our write timing isn't accurate to
+ the T-cycle, we hack around it by stepping the sample index backwards. */
+ if ((value & 0x80) == 0 && gb->apu.is_active[index]) {
+ /* On an AGB, as well as on CGB C and earlier (TODO: Tested: 0, B and C), it behaves slightly different on
+ double speed. */
+ if (gb->model == GB_MODEL_CGB_E /* || gb->model == GB_MODEL_CGB_D */ || gb->apu.square_channels[index].sample_countdown & 1) {
+ if (gb->apu.square_channels[index].sample_countdown >> 1 == (gb->apu.square_channels[index].sample_length ^ 0x7FF)) {
+ gb->apu.square_channels[index].current_sample_index--;
+ gb->apu.square_channels[index].current_sample_index &= 7;
+ }
+ }
+ }
+
gb->apu.square_channels[index].sample_length &= 0xFF;
gb->apu.square_channels[index].sample_length |= (value & 7) << 8;
if (index == GB_SQUARE_1) {
