From a813eb07e367845754722e3fdd2ce25bdfcad377 Mon Sep 17 00:00:00 2001
From: David Shadoff <david.shadoff@gmail.com>
Date: Tue, 9 Apr 2024 18:13:23 -0400
Subject: [PATCH] Fix Timer IRQ Timing ( #141 )

---
 mednafen/src/pce/huc6280.cpp | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mednafen/src/pce/huc6280.cpp b/mednafen/src/pce/huc6280.cpp
index 9a63333..cf64a13 100644
--- a/mednafen/src/pce/huc6280.cpp
+++ b/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))