Merge pull request #152 from dshadoff/Fix_TIMER_IRQ_timing

Fix Timer IRQ Timing ( #141 )

Confirmed by Sour (author of MESEN):
Acknowledging the TIMER IRQ with a "STA $1403" immediately prevents a pending TIMER IRQ from taking control immediately after the "STA $1403".

This is different from the video interrupts (which would be acknowledged by accessing $0000 - in that case, the interrupt acknowledgement is delayed by an additional cycle.

This appears to be because the TIMER interrupt is internal to the CPU, so it is instant, however the VDC acknowledgement would be processed by the VDC (say, 1 VDC cycle) before de-asserting the interrupt line to the CPU.

mednafen/src/pce/huc6280.cpp

@@ -42,7 +42,10 @@ namespace MDFN_IEN_PCE
 INLINE void HuC6280::LastCycle(void)
 {
  /*assert(((P & I_FLAG) ? 0 : (uint32)~0) == PIMaskCache);*/
- IRQSample = (IRQlow & IRQMask) & PIMaskCache;
+
+ // TIMER interrupt to be tested after opcodes, not one cycle early
+ //
+ IRQSample = (((IRQlow & IRQMask) & PIMaskCache) & ~IQTIMER);
  IFlagSample = P & I_FLAG;
  ADDCYC(1);
 }
@@ -604,6 +607,11 @@ NO_INLINE void HuC6280::RunSub(void)
 	  }
          }
 
+         // TIMER interrupt to be tested after opcodes, not one cycle early
+         //
+         if (!IFlagSample)
+           IRQSample |= (IRQlow & IRQMask & IQTIMER);
+
 	 if(IRQSample | IRQlow)
 	 {
           if(MDFN_UNLIKELY(IRQSample & IQRESET))