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Fix period register writes disturbing the tone, noise and envelope counters #15

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Merged
lanceewing merged 1 commit into from
Jun 20, 2026
Merged

Fix period register writes disturbing the tone, noise and envelope counters #15

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11 changes: 8 additions & 3 deletions html/src/main/java/emu/joric/gwt/GwtAYPSG.java
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Original file line number Diff line number Diff line change
Expand Up @@ -340,7 +340,12 @@ public void writeRegister(int address, int value) {
int val = (((registers[(address << 1) + 1] & 0x0f) << 8) | registers[address << 1]) * updateStep;
int last = period[address];
period[address] = val = ((val < 0x8000) ? 0x8000 : val);
int newCount = count[address] - (val - last);
// Adjust the time remaining to the next flip-flop toggle so that the
// time already elapsed since the last toggle is preserved, i.e. the
// period write moves only the target, as on the real chip. If the
// elapsed time already exceeds the new period, the clamp below makes
// the overdue toggle happen straight away.
int newCount = count[address] + (val - last);
count[address] = newCount < 1 ? 1 : newCount;
break;
}
Expand All @@ -351,7 +356,7 @@ public void writeRegister(int address, int value) {
val *= 2;
int last = period[NOISE];
period[NOISE] = val = val == 0 ? updateStep : val;
int newCount = count[NOISE] - (val - last);
int newCount = count[NOISE] + (val - last);
count[NOISE] = newCount < 1 ? 1 : newCount;
break;
}
Expand Down Expand Up @@ -386,7 +391,7 @@ public void writeRegister(int address, int value) {
int val = (((registers[0x0C] << 8) | registers[0x0B]) * updateStep) << 1;
int last = period[ENVELOPE];
period[ENVELOPE] = val;
int newCount = count[ENVELOPE] - (val - last);
int newCount = count[ENVELOPE] + (val - last);
count[ENVELOPE] = newCount < 1 ? 1 : newCount;
break;
}
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