From 21791170f4213676febb1b7392a7640485939cbf Mon Sep 17 00:00:00 2001
From: Sour <mesenemu@gmail.com>
Date: Sun, 24 Feb 2019 09:38:22 -0500
Subject: [PATCH] PPU: Fixed VRAM read behavior

---
 Core/Ppu.cpp | 15 +++++++++++----
 Core/Ppu.h   |  4 +++-
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/Core/Ppu.cpp b/Core/Ppu.cpp
index 1983bcc..54f1379 100644
--- a/Core/Ppu.cpp
+++ b/Core/Ppu.cpp
@@ -1036,6 +1036,11 @@ void Ppu::LatchLocationValues()
 	_locationLatched = true;
 }
 
+void Ppu::UpdateVramReadBuffer()
+{
+	_vramReadBuffer = _vram[_vramAddress << 1] | (_vram[(_vramAddress << 1) + 1] << 8);
+}
+
 uint8_t Ppu::Read(uint16_t addr)
 {
 	switch(addr) {
@@ -1063,9 +1068,9 @@ uint8_t Ppu::Read(uint16_t addr)
 
 		case 0x2139: {
 			//VMDATALREAD - VRAM Data Read low byte
-			uint8_t returnValue = _vramReadBuffer;
-			_vramReadBuffer = _vram[_vramAddress << 1];
+			uint8_t returnValue = (uint8_t)_vramReadBuffer;
 			if(!_vramAddrIncrementOnSecondReg) {
+				UpdateVramReadBuffer();
 				_vramAddress = (_vramAddress + _vramIncrementValue) & 0x7FFF;
 			}
 			return returnValue;
@@ -1073,9 +1078,9 @@ uint8_t Ppu::Read(uint16_t addr)
 
 		case 0x213A: {
 			//VMDATAHREAD - VRAM Data Read high byte
-			uint8_t returnValue = _vramReadBuffer;
-			_vramReadBuffer = _vram[(_vramAddress << 1) + 1];
+			uint8_t returnValue = (uint8_t)(_vramReadBuffer >> 8);
 			if(_vramAddrIncrementOnSecondReg) {
+				UpdateVramReadBuffer();
 				_vramAddress = (_vramAddress + _vramIncrementValue) & 0x7FFF;
 			}
 			return returnValue;
@@ -1281,11 +1286,13 @@ void Ppu::Write(uint32_t addr, uint8_t value)
 		case 0x2116:
 			//VMADDL - VRAM Address low byte
 			_vramAddress = (_vramAddress & 0x7F00) | value;
+			UpdateVramReadBuffer();
 			break;
 
 		case 0x2117:
 			//VMADDH - VRAM Address high byte
 			_vramAddress = (_vramAddress & 0x00FF) | ((value & 0x7F) << 8);
+			UpdateVramReadBuffer();
 			break;
 
 		case 0x2118:
diff --git a/Core/Ppu.h b/Core/Ppu.h
index 6561542..31f0318 100644
--- a/Core/Ppu.h
+++ b/Core/Ppu.h
@@ -70,7 +70,7 @@ private:
 	uint8_t _vramIncrementValue;
 	uint8_t _vramAddressRemapping;
 	bool _vramAddrIncrementOnSecondReg;
-	uint8_t _vramReadBuffer = 0;
+	uint16_t _vramReadBuffer = 0;
 	
 	uint16_t _cgramAddress;
 	uint8_t _cgram[Ppu::CgRamSize];
@@ -198,6 +198,8 @@ private:
 	bool ProcessMaskWindow(uint8_t activeWindowCount, int x);
 	void ProcessWindowMaskSettings(uint8_t value, uint8_t offset);
 
+	void UpdateVramReadBuffer();
+
 	void SendFrame();
 
 public: