From 415441a069d8b0b55d217c9d4af614a6c98cdb51 Mon Sep 17 00:00:00 2001
From: vailkyte <133282358+vailkyte@users.noreply.github.com>
Date: Thu, 18 May 2023 14:14:46 -0500
Subject: [PATCH] Backport Fixed PPU bus address on cycle 0 + improved MMC3 IRQ
 timing

---
 Core/MMC3.h  | 20 +++++++++++++++-----
 Core/PPU.cpp | 24 ++++++++++++++++--------
 2 files changed, 31 insertions(+), 13 deletions(-)

diff --git a/Core/MMC3.h b/Core/MMC3.h
index 36f3fa8c..a0ca4b80 100644
--- a/Core/MMC3.h
+++ b/Core/MMC3.h
@@ -4,7 +4,6 @@
 #include "BaseMapper.h"
 #include "CPU.h"
 #include "EmulationSettings.h"
-#include "A12Watcher.h"
 
 
 class MMC3 : public BaseMapper
@@ -28,7 +27,7 @@ class MMC3 : public BaseMapper
 		bool _wramEnabled;
 		bool _wramWriteProtected;
 
-		A12Watcher _a12Watcher;
+		uint64_t _a12LowClock = 0;
 
 		bool _forceMmc3RevAIrqs;
 
@@ -189,9 +188,8 @@ class MMC3 : public BaseMapper
 		{
 			BaseMapper::StreamState(saving);
 			ArrayInfo<uint8_t> registers = { _registers, 8 };
-			SnapshotInfo a12Watcher{ &_a12Watcher };
 			Stream(_state.Reg8000, _state.RegA000, _state.RegA001, _currentRegister, _chrMode, _prgMode,
-				_irqReloadValue, _irqCounter, _irqReload, _irqEnabled, a12Watcher,
+				_irqReloadValue, _irqCounter, _irqReload, _irqEnabled, _a12LowClock,
 				_wramEnabled, _wramWriteProtected, registers);
 		}
 
@@ -265,10 +263,22 @@ class MMC3 : public BaseMapper
 			_console->GetCpu()->SetIrqSource(IRQSource::External);
 		}
 
+		bool IsA12RisingEdge(uint16_t addr)
+		{
+			if(addr & 0x1000) {
+				bool isRisingEdge = _a12LowClock > 0 && (_console->GetCpu()->GetCycleCount() - _a12LowClock) >= 3;
+				_a12LowClock = 0;
+				return isRisingEdge;
+			} else if(_a12LowClock == 0) {
+				_a12LowClock = _console->GetCpu()->GetCycleCount();
+			}
+			return false;
+		}
+
 	public:
 		virtual void NotifyVRAMAddressChange(uint16_t addr) override
 		{
-			if(_a12Watcher.UpdateVramAddress(addr, _console->GetPpu()->GetFrameCycle()) == A12StateChange::Rise) {
+			if(IsA12RisingEdge(addr)) {
 				uint32_t count = _irqCounter;
 				if(_irqCounter == 0 || _irqReload) {
 					_irqCounter = _irqReloadValue;
diff --git a/Core/PPU.cpp b/Core/PPU.cpp
index 6ef367bd..32e15d6a 100644
--- a/Core/PPU.cpp
+++ b/Core/PPU.cpp
@@ -112,7 +112,7 @@ void PPU::Reset()
 
 	_firstVisibleSpriteAddr = 0;
 	_lastVisibleSpriteAddr = 0;
-	
+
 	memset(_oamDecayCycles, 0, sizeof(_oamDecayCycles));
 	_enableOamDecay = _settings->CheckFlag(EmulationFlags::EnableOamDecay);
 
@@ -999,7 +999,7 @@ void PPU::ProcessScanline()
 		}
 	} else if(_cycle == 337 || _cycle == 339) {
 		if(IsRenderingEnabled()) {
-			ReadVram(GetNameTableAddr());
+			_nextTile.TileAddr = ReadVram(GetNameTableAddr());
 
 			if(_scanline == -1 && _cycle == 339 && (_frameCount & 0x01) && _nesModel == NesModel::NTSC && _settings->GetPpuModel() == PpuModel::Ppu2C02) {
 				//This behavior is NTSC-specific - PAL frames are always the same number of cycles
@@ -1270,12 +1270,20 @@ void PPU::Exec()
 		}
 
 		//Cycle = 0
-		if(_scanline == -1) {
-			_statusFlags.SpriteOverflow = false;
-			_statusFlags.Sprite0Hit = false;
-			
-			//Switch to alternate output buffer (VideoDecoder may still be decoding the last frame buffer)
-			_currentOutputBuffer = (_currentOutputBuffer == _outputBuffers[0]) ? _outputBuffers[1] : _outputBuffers[0];
+		if(_scanline < 240) {
+			if(_scanline == -1) {
+				_statusFlags.SpriteOverflow = false;
+				_statusFlags.Sprite0Hit = false;
+
+				//Switch to alternate output buffer (VideoDecoder may still be decoding the last frame buffer)
+				_currentOutputBuffer = (_currentOutputBuffer == _outputBuffers[0]) ? _outputBuffers[1] : _outputBuffers[0];
+			} else if(_prevRenderingEnabled) {
+				if(_scanline > 0 || (!(_frameCount & 0x01) || _nesModel != NesModel::NTSC || _settings->GetPpuModel() != PpuModel::Ppu2C02)) {
+					//Set bus address to the tile address calculated from the unused NT fetches at the end of the previous scanline
+					//This doesn't happen on scanline 0 if the last dot of the previous frame was skipped
+					SetBusAddress((_nextTile.TileAddr << 4) | (_state.VideoRamAddr >> 12) | _flags.BackgroundPatternAddr);
+				}
+			}
 		} else if(_scanline == 240) {
 			//At the start of vblank, the bus address is set back to VideoRamAddr.
 			//According to Visual NES, this occurs on scanline 240, cycle 1, but is done here on cycle for performance reasons