From 1ca15b4a4f2cbbf3df64892eea41cdefc0a1ceb0 Mon Sep 17 00:00:00 2001
From: Brandon Wright <bearoso@gmail.com>
Date: Mon, 20 Aug 2018 12:53:20 -0500
Subject: [PATCH] Proper write-twice behavior for $2122.

---
 ppu.h        | 18 +++++-------------
 snapshot.cpp |  1 +
 snapshot.h   |  2 +-
 3 files changed, 7 insertions(+), 14 deletions(-)

diff --git a/ppu.h b/ppu.h
index 2ad0aa15..8cce413d 100644
--- a/ppu.h
+++ b/ppu.h
@@ -294,6 +294,7 @@ struct SPPU
 	bool8	CGFLIP;
 	uint8	CGFLIPRead;
 	uint8	CGADD;
+	uint8	CGSavedByte;
 	uint16	CGDATA[256];
 
 	struct SOBJ OBJ[128];
@@ -719,12 +720,12 @@ static inline void REGISTER_2122 (uint8 Byte)
 {
 	if (PPU.CGFLIP)
 	{
-		if ((Byte & 0x7f) != (PPU.CGDATA[PPU.CGADD] >> 8))
+		if ((Byte & 0x7f) != (PPU.CGDATA[PPU.CGADD] >> 8) || PPU.CGSavedByte != (uint8) (PPU.CGDATA[PPU.CGADD] & 0xff))
 		{
 			FLUSH_REDRAW();
-			PPU.CGDATA[PPU.CGADD] &= 0x00ff;
-			PPU.CGDATA[PPU.CGADD] |= (Byte & 0x7f) << 8;
+			PPU.CGDATA[PPU.CGADD] = (Byte & 0x7f) << 8 | PPU.CGSavedByte;
 			IPPU.ColorsChanged = TRUE;
+			IPPU.Red[PPU.CGADD] = IPPU.XB[PPU.CGSavedByte & 0x1f];
 			IPPU.Blue[PPU.CGADD] = IPPU.XB[(Byte >> 2) & 0x1f];
 			IPPU.Green[PPU.CGADD] = IPPU.XB[(PPU.CGDATA[PPU.CGADD] >> 5) & 0x1f];
 			IPPU.ScreenColors[PPU.CGADD] = (uint16) BUILD_PIXEL(IPPU.Red[PPU.CGADD], IPPU.Green[PPU.CGADD], IPPU.Blue[PPU.CGADD]);
@@ -734,16 +735,7 @@ static inline void REGISTER_2122 (uint8 Byte)
 	}
 	else
 	{
-		if (Byte != (uint8) (PPU.CGDATA[PPU.CGADD] & 0xff))
-		{
-			FLUSH_REDRAW();
-			PPU.CGDATA[PPU.CGADD] &= 0x7f00;
-			PPU.CGDATA[PPU.CGADD] |= Byte;
-			IPPU.ColorsChanged = TRUE;
-			IPPU.Red[PPU.CGADD] = IPPU.XB[Byte & 0x1f];
-			IPPU.Green[PPU.CGADD] = IPPU.XB[(PPU.CGDATA[PPU.CGADD] >> 5) & 0x1f];
-			IPPU.ScreenColors[PPU.CGADD] = (uint16) BUILD_PIXEL(IPPU.Red[PPU.CGADD], IPPU.Green[PPU.CGADD], IPPU.Blue[PPU.CGADD]);
-		}
+		PPU.CGSavedByte = Byte;
 	}
 
 	PPU.CGFLIP ^= 1;
diff --git a/snapshot.cpp b/snapshot.cpp
index c62c5079..45974249 100644
--- a/snapshot.cpp
+++ b/snapshot.cpp
@@ -433,6 +433,7 @@ static FreezeData	SnapPPU[] =
 	INT_ENTRY(6, CGFLIP),
 	INT_ENTRY(6, CGFLIPRead),
 	INT_ENTRY(6, CGADD),
+	INT_ENTRY(11, CGSavedByte),
 	ARRAY_ENTRY(6, CGDATA, 256, uint16_ARRAY_V),
 #define O(N) \
 	INT_ENTRY(6, OBJ[N].HPos), \
diff --git a/snapshot.h b/snapshot.h
index a88d6123..c219fa70 100644
--- a/snapshot.h
+++ b/snapshot.h
@@ -198,7 +198,7 @@
 #define SNAPSHOT_MAGIC			"#!s9xsnp"
 #define SNAPSHOT_VERSION_IRQ    7
 #define SNAPSHOT_VERSION_BAPU   8
-#define SNAPSHOT_VERSION		10
+#define SNAPSHOT_VERSION		11
 
 #define SUCCESS					1
 #define WRONG_FORMAT			(-1)