DSB2 IRQ 2 now fires at 1KHz rather than once per frame, improving

music timing in Daytona USA 2 and Sega Rally 2. DSB1 CPU timing 
increased from 1MHz to 4MHz, improving music fade timing in Scud Race. 
Thanks to gm_matthew for these discoveries.

Src/Model3/DSB.cpp

@@ -1,7 +1,8 @@
 /**
  ** Supermodel
  ** A Sega Model 3 Arcade Emulator.
- ** Copyright 2011 Bart Trzynadlowski, Nik Henson 
+ ** Copyright 2011-2021 Bart Trzynadlowski, Nik Henson, Ian Curtis,
+ **                     Harry Tuttle, and Spindizzi
  **
  ** This file is part of Supermodel.
  **
@@ -438,7 +439,7 @@ void CDSB1::RunFrame(INT16 *audioL, INT16 *audioR)
 	}
 
 	// While FIFO not empty, fire interrupts, run for up to one frame
-	for (cycles = (4000000/60)/4; (cycles > 0) && (fifoIdxR != fifoIdxW);  )
+	for (cycles = (4000000/60); (cycles > 0) && (fifoIdxR != fifoIdxW);  )
 	{
 		Z80.SetINT(true);	// fire an IRQ to indicate pending command
 		//printf("Z80 INT fired\n");
@@ -1015,12 +1016,27 @@ void CDSB2::RunFrame(INT16 *audioL, INT16 *audioR)
 
 		M68KSetIRQ(1);	// indicate pending command
 		//printf("68K INT fired\n");
-		M68KRun(500);
+		m_totalCyclesElapsed += M68KRun(500);
 	}
 
-	// Per-frame interrupt
+  // gm_matthew made the interesting discovery that IRQ2 may in fact be a timer interrupt
+  // rather than a per-frame interrupt.For Daytona 2 and Sega Rally 2, assuming a value
+  // of 1KHz fixes music fade outs and some timing issues. It is very likely this is a
+  // configurable timer and we should be on the look-out for games which appear to use
+  // different values. It is equally likely that all games share a similar code base and
+  // use 1KHz as the timer rate.
+  while (m_totalCyclesElapsed < m_nextFrameEndCycles)
+  {
+    if (m_totalCyclesElapsed >= m_nextTimerInterruptCycles)
+    {
+      // Fire timer interrupt and schedule next one
       M68KSetIRQ(2);
-	M68KRun(4000000/60);
+      m_nextTimerInterruptCycles = (m_totalCyclesElapsed + k_timerPeriod) - (m_totalCyclesElapsed + k_timerPeriod) % k_timerPeriod;
+    }
+    int cyclesToRun = (std::min)(m_nextTimerInterruptCycles, m_nextFrameEndCycles) - m_totalCyclesElapsed;
+    m_totalCyclesElapsed += M68KRun(cyclesToRun);
+  }
+  m_nextFrameEndCycles = (m_totalCyclesElapsed + k_framePeriod) - (m_totalCyclesElapsed + k_framePeriod) % k_framePeriod;
 
 	M68KGetContext(&M68K);
 
@@ -1080,6 +1096,10 @@ void CDSB2::Reset(void)
 	//printf("DSB2 PC=%06X\n", M68KGetPC());
 	M68KGetContext(&M68K);
 
+	m_totalCyclesElapsed = 0;
+	m_nextFrameEndCycles = k_framePeriod;
+	m_nextTimerInterruptCycles = k_timerPeriod;
+
 	DebugLog("DSB2 Reset\n");
 }
 
@@ -1162,6 +1182,12 @@ void CDSB2::LoadState(CBlockFile *StateFile)
 	M68KLoadState(StateFile, "DSB2 68K");
 	M68KGetContext(&M68K);
 
+	// Technically these should be saved/restored rather than being reset but that would mean
+	// the save state format has to be modified and the difference would be imperceptible anyway
+	m_totalCyclesElapsed = 0;
+	m_nextFrameEndCycles = k_framePeriod;
+	m_nextTimerInterruptCycles = k_timerPeriod;
+
 	// Restart MPEG audio at the appropriate position
 	if (isPlaying)
 	{

Src/Model3/DSB.h

@@ -1,7 +1,8 @@
 /**
  ** Supermodel
  ** A Sega Model 3 Arcade Emulator.
- ** Copyright 2011 Bart Trzynadlowski, Nik Henson 
+ ** Copyright 2011-2021 Bart Trzynadlowski, Nik Henson, Ian Curtis,
+ **                     Harry Tuttle, and Spindizzi
  **
  ** This file is part of Supermodel.
  **
@@ -317,6 +318,11 @@ private:
 
 	// M68K CPU
 	M68KCtx	M68K;
+	static const int k_framePeriod = 4000000/60;
+	static const int k_timerPeriod = 4000000/1000; // 1KHz timer
+	int m_totalCyclesElapsed;
+	int m_nextFrameEndCycles;
+	int m_nextTimerInterruptCycles;
 };