From c932d4e37acf984ba04b3b4dc5828f93418d8368 Mon Sep 17 00:00:00 2001
From: Brandon Wright <bearoso@gmail.com>
Date: Sun, 27 May 2018 11:03:21 -0500
Subject: [PATCH] Add IRQ transition.

---
 cpu.cpp     |  1 +
 cpuexec.cpp | 34 ++++++++++++++++++++--------------
 ppu.cpp     |  6 +++++-
 3 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/cpu.cpp b/cpu.cpp
index c1e9aae7..8d33f64b 100644
--- a/cpu.cpp
+++ b/cpu.cpp
@@ -229,6 +229,7 @@ static void S9xSoftResetCPU (void)
 	CPU.PCBase = NULL;
 	CPU.NMIPending = FALSE;
 	CPU.IRQLine = FALSE;
+	CPU.IRQTransition = FALSE;
 	CPU.IRQExternal = FALSE;
 	CPU.IRQPending = Timings.IRQPendCount;
 	CPU.MemSpeed = SLOW_ONE_CYCLE;
diff --git a/cpuexec.cpp b/cpuexec.cpp
index 5904e87a..d045c3eb 100644
--- a/cpuexec.cpp
+++ b/cpuexec.cpp
@@ -223,7 +223,7 @@ void S9xMainLoop (void)
 				{
 					CPU.WaitingForInterrupt = FALSE;
 					Registers.PCw++;
-					CPU.Cycles += 14;
+					CPU.Cycles += ONE_CYCLE;
 					while (CPU.Cycles >= CPU.NextEvent)
 						S9xDoHEventProcessing();
 				}
@@ -232,24 +232,30 @@ void S9xMainLoop (void)
 			}
 		}
 
-		if ((CPU.Cycles >= Timings.NextIRQTimer || CPU.IRQExternal) && !CPU.IRQLine)
+		if (CPU.IRQTransition)
+		{
+			if (CPU.WaitingForInterrupt)
+			{
+				CPU.WaitingForInterrupt = FALSE;
+				Registers.PCw++;
+				CPU.Cycles += ONE_CYCLE;
+				while (CPU.Cycles >= CPU.NextEvent)
+					S9xDoHEventProcessing();
+			}
+
+			S9xUpdateIRQPositions();
+			CPU.IRQPending = Timings.IRQPendCount;
+			CPU.IRQTransition = FALSE;
+			CPU.IRQLine = TRUE;
+		}
+
+		if ((CPU.Cycles >= Timings.NextIRQTimer || CPU.IRQExternal) && !CPU.IRQLine && !CPU.IRQTransition)
 		{
 			if (CPU.IRQPending)
 				CPU.IRQPending--;
 			else
 			{
-				if (CPU.WaitingForInterrupt)
-				{
-					CPU.WaitingForInterrupt = FALSE;
-					Registers.PCw++;
-					CPU.Cycles += 14;
-					while (CPU.Cycles >= CPU.NextEvent)
-						S9xDoHEventProcessing();
-				}
-
-				S9xUpdateIRQPositions();
-				CPU.IRQPending = Timings.IRQPendCount;
-				CPU.IRQLine = TRUE;
+				CPU.IRQTransition = TRUE;
 			}
 		}
 
diff --git a/ppu.cpp b/ppu.cpp
index 9640f561..dba745af 100644
--- a/ppu.cpp
+++ b/ppu.cpp
@@ -334,7 +334,7 @@ void S9xUpdateIRQPositions (void)
 	else if (!PPU.HTimerEnabled && PPU.VTimerEnabled)
 	{
 		if (CPU.V_Counter == PPU.VTimerPosition)
-			Timings.NextIRQTimer = 0;
+			Timings.NextIRQTimer = Timings.IRQTriggerCycles;
 		else
 			Timings.NextIRQTimer = CyclesUntilNext (Timings.IRQTriggerCycles, PPU.VTimerPosition);
 	}
@@ -1556,7 +1556,10 @@ void S9xSetCPU (uint8 Byte, uint16 Address)
 					PPU.HTimerEnabled = FALSE;
 
 				if (!(Byte & 0x10) && !(Byte & 0x20))
+				{
 					CPU.IRQLine = FALSE;
+					CPU.IRQTransition = FALSE;
+				}
 
 				S9xUpdateIRQPositions();
 
@@ -1842,6 +1845,7 @@ uint8 S9xGetCPU (uint16 Address)
 			case 0x4211: // TIMEUP
 				byte = CPU.IRQLine ? 0x80 : 0;
 				CPU.IRQLine = FALSE;
+				CPU.IRQTransition = FALSE;
 				S9xUpdateIRQPositions();
 
 				return (byte | (OpenBus & 0x7f));