Fix for "Mad Effect 1&2" demos (PR #725)

. 6502 interrupt delays 1 opcode when interrupt occurs on last cycle of opcode (#724).
. Only 1-cycle delay for VF_TEXT & VF_MIXED mode changes (#656).
NB. Mad Effect 1 still has a bit of flicker on Space Invader (left edge)

source/CPU.cpp

@@ -403,10 +403,23 @@ static __forceinline void NMI(ULONG& uExecutedCycles, BOOL& flagc, BOOL& flagn,
 #endif
 }
 
+static bool g_irqOnLastOpcodeCycle = false;
+static bool g_irqDefer1Opcode = false;
+
 static __forceinline void IRQ(ULONG& uExecutedCycles, BOOL& flagc, BOOL& flagn, BOOL& flagv, BOOL& flagz)
 {
 	if(g_bmIRQ && !(regs.ps & AF_INTERRUPT))
 	{
+		// if 6522 interrupt occurs on opcode's last cycle, then defer IRQ by 1 opcode
+		if (g_irqOnLastOpcodeCycle && !g_irqDefer1Opcode)
+		{
+			g_irqOnLastOpcodeCycle = false;
+			g_irqDefer1Opcode = true;	// if INT occurs again on next opcode, then do NOT defer
+			return;
+		}
+
+		g_irqDefer1Opcode = false;
+
 		// IRQ signals are deasserted when a specific r/w operation is done on device
 #ifdef _DEBUG
 		g_nCycleIrqStart = g_nCumulativeCycles + uExecutedCycles;
@@ -420,6 +433,8 @@ static __forceinline void IRQ(ULONG& uExecutedCycles, BOOL& flagc, BOOL& flagn,
 		UINT uExtraCycles = 0;	// Needed for CYC(a) macro
 		CYC(7)
 	}
+
+	g_irqOnLastOpcodeCycle = false;
 }
 
 const int IRQ_CHECK_OPCODE_FULL_SPEED = 40;	// ~128 cycles (assume 3 cycles per opcode)
@@ -435,7 +450,9 @@ static __forceinline void CheckInterruptSources(ULONG uExecutedCycles, const boo
 		g_fullSpeedOpcodeCount = IRQ_CHECK_OPCODE_FULL_SPEED;
 	}
 
-	MB_UpdateCycles(uExecutedCycles);
+	if (MB_UpdateCycles(uExecutedCycles))
+		g_irqOnLastOpcodeCycle = true;
+
 	if (sg_Mouse.IsActive())
 		sg_Mouse.SetVBlank( !VideoGetVblBar(uExecutedCycles) );
 }
@@ -536,11 +553,10 @@ DWORD CpuExecute(const DWORD uCycles, const bool bVideoUpdate)
 	//  >0  : Do multi-opcode emulation
 	const DWORD uExecutedCycles = InternalCpuExecute(uCycles, bVideoUpdate);
 
+	// NB. Required for normal-speed (even though 6522 is updated after every opcode), as may've finished on IRQ()
 	MB_UpdateCycles(uExecutedCycles);	// Update 6522s (NB. Do this before updating g_nCumulativeCycles below)
 										// NB. Ensures that 6522 regs are up-to-date for any potential save-state
 
-	//
-
 	const UINT nRemainingCycles = uExecutedCycles - g_nCyclesExecuted;
 	g_nCumulativeCycles	+= nRemainingCycles;
 

source/Mockingboard.cpp

@@ -129,6 +129,8 @@ struct SY6522_AY8910
 	SSI263A SpeechChip;
 	MockingboardUnitState_e state;	// Where a unit is a 6522+AY8910 pair
 	MockingboardUnitState_e stateB;	// Phasor: 6522 & 2nd AY8910
+	bool bLoadT1C;
+	bool bLoadT2C;
 };
 
 
@@ -376,6 +378,8 @@ 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;
@@ -433,7 +437,7 @@ static void SY6522_Write(BYTE nDevice, BYTE nReg, BYTE nValue)
 			UpdateIFR(pMB, IxR_TIMER1);
 
 			pMB->sy6522.TIMER1_LATCH.h = nValue;
-			pMB->sy6522.TIMER1_COUNTER.w = pMB->sy6522.TIMER1_LATCH.w;
+			pMB->bLoadT1C = true;
 
 			StartTimer1(pMB);
 			CpuAdjustIrqCheck(pMB->sy6522.TIMER1_LATCH.w);	// Sync IRQ check timeout with 6522 counter underflow - GH#608
@@ -529,7 +533,8 @@ static BYTE SY6522_Read(BYTE nDevice, BYTE nReg)
 			nValue = pMB->sy6522.DDRA;
 			break;
 		case 0x04:	// TIMER1L_COUNTER
-			nValue = pMB->sy6522.TIMER1_COUNTER.l;
+			// NB. GH#701 (T1C:=0xFFFF, LDA T1C_L, A==0xFC)
+			nValue = (pMB->sy6522.TIMER1_COUNTER.w - 3) & 0xff;		// -3 to compensate for the (assumed) 4-cycle STA 6522.T1C_H
 			UpdateIFR(pMB, IxR_TIMER1);
 			break;
 		case 0x05:	// TIMER1H_COUNTER
@@ -1541,7 +1546,8 @@ void MB_Reset()	// CTRL+RESET or power-cycle
 
 static BYTE __stdcall MB_Read(WORD PC, WORD nAddr, BYTE bWrite, BYTE nValue, ULONG nExecutedCycles)
 {
-	MB_UpdateCycles(nExecutedCycles);
+	if (g_bFullSpeed)
+		MB_UpdateCycles(nExecutedCycles);
 
 #ifdef _DEBUG
 	if(!IS_APPLE2 && MemCheckINTCXROM())
@@ -1604,7 +1610,8 @@ static BYTE __stdcall MB_Read(WORD PC, WORD nAddr, BYTE bWrite, BYTE nValue, ULO
 
 static BYTE __stdcall MB_Write(WORD PC, WORD nAddr, BYTE bWrite, BYTE nValue, ULONG nExecutedCycles)
 {
-	MB_UpdateCycles(nExecutedCycles);
+	if (g_bFullSpeed)
+		MB_UpdateCycles(nExecutedCycles);
 
 #ifdef _DEBUG
 	if(!IS_APPLE2 && MemCheckINTCXROM())
@@ -1828,24 +1835,45 @@ static bool CheckTimerUnderflowAndIrq(USHORT& timerCounter, int& timerIrqDelay,
 // Called by:
 // . CpuExecute() every ~1000 @ 1MHz
 // . CheckInterruptSources() every opcode (or every 40 opcodes at full-speed)
-// . MB_Read() / MB_Write()
+// . MB_Read() / MB_Write() (only for full-speed)
-void MB_UpdateCycles(ULONG uExecutedCycles)
+bool MB_UpdateCycles(ULONG uExecutedCycles)
 {
 	if (g_SoundcardType == CT_Empty)
-		return;
+		return false;
 
 	CpuCalcCycles(uExecutedCycles);
 	UINT64 uCycles = g_nCumulativeCycles - g_uLastCumulativeCycles;
+	if (uCycles == 0)
+		return false;		// Likely when called from CpuExecute()
+	_ASSERT(uCycles > 1);
+
+	const bool isOpcode = (uCycles > 1 && uCycles <= 7);		// todo: better to pass in a flag?
+
 	g_uLastCumulativeCycles = g_nCumulativeCycles;
 	_ASSERT(uCycles < 0x10000);
 	USHORT nClocks = (USHORT) uCycles;
 
+	bool bIrqOnLastOpcodeCycle = false;
+
 	for (int i=0; i<NUM_SY6522; i++)
 	{
 		SY6522_AY8910* pMB = &g_MB[i];
 
 		bool bTimer1Underflow = false;	// Just for Willy Byte!
-		const bool bTimer1Irq = CheckTimerUnderflowAndIrq(pMB->sy6522.TIMER1_COUNTER.w, pMB->sy6522.timer1IrqDelay, nClocks, &bTimer1Underflow);
+		bool bTimer1Irq = false;
+		bool bTimer1IrqOnLastCycle = false;
+
+		if (isOpcode)
+		{
+			bTimer1Irq = CheckTimerUnderflowAndIrq(pMB->sy6522.TIMER1_COUNTER.w, pMB->sy6522.timer1IrqDelay, nClocks-1, &bTimer1Underflow);
+			bTimer1IrqOnLastCycle  = CheckTimerUnderflowAndIrq(pMB->sy6522.TIMER1_COUNTER.w, pMB->sy6522.timer1IrqDelay, 1, &bTimer1Underflow);
+			bTimer1Irq = bTimer1Irq || bTimer1IrqOnLastCycle;
+		}
+		else
+		{
+			bTimer1Irq = CheckTimerUnderflowAndIrq(pMB->sy6522.TIMER1_COUNTER.w, pMB->sy6522.timer1IrqDelay, nClocks, &bTimer1Underflow);
+		}
+
 		const bool bTimer2Irq = CheckTimerUnderflowAndIrq(pMB->sy6522.TIMER2_COUNTER.w, pMB->sy6522.timer2IrqDelay, nClocks);
 
 		if (!pMB->bTimer1Active && bTimer1Underflow)
@@ -1864,6 +1892,7 @@ void MB_UpdateCycles(ULONG uExecutedCycles)
 		if (pMB->bTimer1Active && bTimer1Irq)
 		{
 			UpdateIFR(pMB, 0, IxR_TIMER1);
+			bIrqOnLastOpcodeCycle = true;
 
 			MB_Update();
 
@@ -1880,6 +1909,9 @@ void MB_UpdateCycles(ULONG uExecutedCycles)
 				// - Ultima4/5 change ACCESS_TIMER1 after a couple of IRQs into tune
 				pMB->sy6522.TIMER1_COUNTER.w += pMB->sy6522.TIMER1_LATCH.w;	// GH#651: account for underflowed cycles too
 				pMB->sy6522.TIMER1_COUNTER.w += 2;							// GH#652: account for extra 2 cycles (Rockwell, Fig.16: period=N+2cycles)
+																			// EG. T1C=0xFFFE, T1L=0x0001
+																			// . T1C += T1L = 0xFFFF
+																			// . T1C +=   2 = 0x0001
 				if (pMB->sy6522.TIMER1_COUNTER.w > pMB->sy6522.TIMER1_LATCH.w)
 				{
 					if (pMB->sy6522.TIMER1_LATCH.w)
@@ -1891,6 +1923,12 @@ void MB_UpdateCycles(ULONG uExecutedCycles)
 			}
 		}
 
+		if (pMB->bLoadT1C)
+		{
+			pMB->bLoadT1C = false;
+			pMB->sy6522.TIMER1_COUNTER.w = pMB->sy6522.TIMER1_LATCH.w;
+		}
+
 		if (pMB->bTimer2Active && bTimer2Irq)
 		{
 			UpdateIFR(pMB, 0, IxR_TIMER2);
@@ -1913,6 +1951,8 @@ void MB_UpdateCycles(ULONG uExecutedCycles)
 			}
 		}
 	}
+
+	return bIrqOnLastOpcodeCycle;
 }
 
 //-----------------------------------------------------------------------------

source/Mockingboard.h

@@ -11,7 +11,7 @@ void    MB_Demute();
 void    MB_StartOfCpuExecute();
 void    MB_PeriodicUpdate(UINT executedCycles);
 void    MB_CheckIRQ();
-void    MB_UpdateCycles(ULONG uExecutedCycles);
+bool    MB_UpdateCycles(ULONG uExecutedCycles);
 SS_CARDTYPE MB_GetSoundcardType();
 bool    MB_IsActive();
 DWORD   MB_GetVolume();

source/Video.cpp

@@ -660,9 +660,10 @@ BYTE VideoSetMode(WORD, WORD address, BYTE write, BYTE, ULONG uExecutedCycles)
 	if (!IS_APPLE2)
 		RGB_SetVideoMode(address);
 
-	bool delay = true;
+	// Only 1-cycle delay for VF_TEXT & VF_MIXED mode changes (GH#656)
-	if ((oldVideoMode ^ g_uVideoMode) & VF_PAGE2)
+	bool delay = false;
-		delay = false;	// PAGE2 flag changed state, so no 1 cycle delay (GH#656)
+	if ((oldVideoMode ^ g_uVideoMode) & (VF_TEXT|VF_MIXED))
+		delay = true;
 
 	NTSC_SetVideoMode( g_uVideoMode, delay );