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AppleWin
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Fixed 6522 timer interrupt which was occurring 1 cycle late (#711)

This commit is contained in:
tomcw 2019-11-17 12:26:14 +00:00
parent d1b595f7bd
commit b82de931a1
1 changed files with 10 additions and 12 deletions
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22
source/Mockingboard.cpp
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@ -378,8 +378,6 @@ static void UpdateIFR(SY6522_AY8910* pMB, BYTE clr_ifr, BYTE set_ifr=0)
CpuIrqDeassert(IS_6522);
}
#define DEFER_T1C_LOAD
static void SY6522_Write(BYTE nDevice, BYTE nReg, BYTE nValue)
{
g_bMB_Active = true;
@ -1801,6 +1799,9 @@ void MB_PeriodicUpdate(UINT executedCycles)
static bool CheckTimerUnderflowAndIrq(USHORT& timerCounter, int& timerIrqDelay, const USHORT nClocks, bool* pTimerUnderflow=NULL)
{
if (nClocks == 0)
return false;
int oldTimer = timerCounter;
int timer = timerCounter;
timer -= nClocks;
@ -1810,23 +1811,20 @@ static bool CheckTimerUnderflowAndIrq(USHORT& timerCounter, int& timerIrqDelay,
if (timerIrqDelay) // Deal with any previous counter underflow which didn't yet result in an IRQ
{
timerIrqDelay -= nClocks;
if (timerIrqDelay <= 0)
{
timerIrqDelay = 0;
timerIrq = true;
}
// don't re-underflow if TIMER = 0xFFFF or 0xFFFE (so just return)
_ASSERT(timerIrqDelay == 1);
timerIrqDelay = 0;
timerIrq = true;
// don't re-underflow if TIMER = 0x0000 (so just return)
}
else if (oldTimer >= 0 && timer < 0) // Underflow occurs for 0x0000 -> 0xFFFF
{
if (pTimerUnderflow)
*pTimerUnderflow = true; // Just for Willy Byte!
if (timer < -2)
if (timer <= -2) // TIMER = 0xFFFE (or less)
timerIrq = true;
else // TIMER = 0xFFFF or 0xFFFE
timerIrqDelay = 3 + timer; // ...so 2 or 1 cycles until IRQ
else // TIMER = 0xFFFF
timerIrqDelay = 1; // ...so 1 cycle until IRQ
}
return timerIrq;