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MB/6522: improve T1 underflow with latch

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This commit is contained in:
tomcw 2021-02-12 19:16:14 +00:00
parent ff29b055ce
commit 84a705d20d
1 changed files with 23 additions and 17 deletions
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40
source/Mockingboard.cpp
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@ -696,13 +696,15 @@ static void SY6522_Write(BYTE nDevice, BYTE nReg, BYTE nValue)
//-----------------------------------------------------------------------------
static USHORT GetTimer1Counter(BYTE reg, USHORT counter, USHORT latch)
static bool CheckTimerUnderflow(USHORT& counter, int& timerIrqDelay, const USHORT nClocks);
static int OnTimer1Underflow(USHORT& counter, USHORT latch);
static USHORT GetTimer1Counter(BYTE reg, USHORT counter, USHORT latch, int timerIrqDelay)
{
const UINT opcodeCycleAdjust = GetOpcodeCyclesForRead(reg) - 1; // to compensate for the 4/5/6 cycle read opcode
int timer = (short)counter - opcodeCycleAdjust;
while (timer < -1)
timer += (latch + kExtraTimerCycles);
return (USHORT)timer;
if (CheckTimerUnderflow(counter, timerIrqDelay, opcodeCycleAdjust))
OnTimer1Underflow(counter, latch);
return counter;
}
static USHORT GetTimer2Counter(BYTE reg, USHORT counter)
@ -734,11 +736,11 @@ static BYTE SY6522_Read(BYTE nDevice, BYTE nReg)
break;
case 0x04: // TIMER1L_COUNTER
// NB. GH#701 (T1C:=0xFFFF, LDA T1C_L[4cy], A==0xFC)
nValue = GetTimer1Counter(nReg, pMB->sy6522.TIMER1_COUNTER.w, pMB->sy6522.TIMER1_LATCH.w) & 0xff;
nValue = GetTimer1Counter(nReg, pMB->sy6522.TIMER1_COUNTER.w, pMB->sy6522.TIMER1_LATCH.w, pMB->sy6522.timer1IrqDelay) & 0xff;
UpdateIFR(pMB, IxR_TIMER1);
break;
case 0x05: // TIMER1H_COUNTER
nValue = GetTimer1Counter(nReg, pMB->sy6522.TIMER1_COUNTER.w, pMB->sy6522.TIMER1_LATCH.w) >> 8;
nValue = GetTimer1Counter(nReg, pMB->sy6522.TIMER1_COUNTER.w, pMB->sy6522.TIMER1_LATCH.w, pMB->sy6522.timer1IrqDelay) >> 8;
break;
case 0x06: // TIMER1L_LATCH
nValue = pMB->sy6522.TIMER1_LATCH.l;
@ -2166,15 +2168,15 @@ void MB_PeriodicUpdate(UINT executedCycles)
//-----------------------------------------------------------------------------
static bool CheckTimerUnderflow(USHORT& timerCounter, int& timerIrqDelay, const USHORT nClocks)
static bool CheckTimerUnderflow(USHORT& counter, int& timerIrqDelay, const USHORT nClocks)
{
if (nClocks == 0)
return false;
int oldTimer = timerCounter;
int timer = timerCounter;
int oldTimer = counter;
int timer = counter;
timer -= nClocks;
timerCounter = (USHORT)timer;
counter = (USHORT)timer;
bool timerIrq = false;
@ -2197,6 +2199,15 @@ static bool CheckTimerUnderflow(USHORT& timerCounter, int& timerIrqDelay, const
return timerIrq;
}
static int OnTimer1Underflow(USHORT& counter, USHORT latch)
{
int timer = (int)(short)(counter);
while (timer < -1)
timer += (latch + kExtraTimerCycles); // GH#651: account for underflowed cycles / GH#652: account for extra 2 cycles
counter = (USHORT)timer;
return (timer == -1) ? 1 : 0; // timer1IrqDelay
}
// Called by:
// . CpuExecute() every ~1000 cycles @ 1MHz
// . MB_SyncEventCallback() on a TIMER1/2 underflow
@ -2222,12 +2233,7 @@ void MB_UpdateCycles(ULONG uExecutedCycles)
const bool bTimer1Underflow = CheckTimerUnderflow(pMB->sy6522.TIMER1_COUNTER.w, pMB->sy6522.timer1IrqDelay, nClocks);
if (bTimer1Underflow)
{
int timer = (int) (short) (pMB->sy6522.TIMER1_COUNTER.w);
while (timer < -1)
timer += (pMB->sy6522.TIMER1_LATCH.w + kExtraTimerCycles); // GH#651: account for underflowed cycles / GH#652: account for extra 2 cycles
pMB->sy6522.TIMER1_COUNTER.w = (USHORT)timer;
}
pMB->sy6522.timer1IrqDelay = OnTimer1Underflow(pMB->sy6522.TIMER1_COUNTER.w, pMB->sy6522.TIMER1_LATCH.w);
// No TIMER2 latch so "after timing out, the counter will continue to decrement"
CheckTimerUnderflow(pMB->sy6522.TIMER2_COUNTER.w, pMB->sy6522.timer2IrqDelay, nClocks);