From f86e00f925cdd36f213b58743e82bb23cf301f8b Mon Sep 17 00:00:00 2001
From: alyosha-tas <alexei.f.k@gmail.com>
Date: Tue, 20 Mar 2018 21:17:57 -0400
Subject: [PATCH] PCE: only latch Y-scroll once per scanline after an offset.
 The latching behaviour is correct, but I'm unsure of what the offset value
 should be. fixes #771

---
 BizHawk.Emulation.Cores/Consoles/PC Engine/VDC.Render.cs | 9 +++++++--
 BizHawk.Emulation.Cores/Consoles/PC Engine/VDC.cs        | 1 +
 BizHawk.Emulation.Cores/Consoles/PC Engine/VPC.cs        | 8 +++++++-
 3 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/BizHawk.Emulation.Cores/Consoles/PC Engine/VDC.Render.cs b/BizHawk.Emulation.Cores/Consoles/PC Engine/VDC.Render.cs
index 0cdd634880..4122f23bc8 100644
--- a/BizHawk.Emulation.Cores/Consoles/PC Engine/VDC.Render.cs	
+++ b/BizHawk.Emulation.Cores/Consoles/PC Engine/VDC.Render.cs	
@@ -79,7 +79,7 @@ namespace BizHawk.Emulation.Cores.PCEngine
 					}
 				}
 
-				cpu.Execute(HBlankCycles);
+				cpu.Execute(24);
 
 				if (InActiveDisplay)
 				{
@@ -92,11 +92,16 @@ namespace BizHawk.Emulation.Cores.PCEngine
 							BackgroundY = Registers[BYR];
 							latch_bgy = false;
 						}
-
 						BackgroundY++;
 						BackgroundY &= 0x01FF;
+						
 					}
+				}
 
+				cpu.Execute(HBlankCycles - 24);
+
+				if (InActiveDisplay)
+				{
 					if (render) RenderScanLine();
 				}
 
diff --git a/BizHawk.Emulation.Cores/Consoles/PC Engine/VDC.cs b/BizHawk.Emulation.Cores/Consoles/PC Engine/VDC.cs
index eb4050f814..e8fda4374b 100644
--- a/BizHawk.Emulation.Cores/Consoles/PC Engine/VDC.cs	
+++ b/BizHawk.Emulation.Cores/Consoles/PC Engine/VDC.cs	
@@ -323,6 +323,7 @@ namespace BizHawk.Emulation.Cores.PCEngine
 
 			ser.Sync("ScanLine", ref ScanLine);
 			ser.Sync("BackgroundY", ref BackgroundY);
+			ser.Sync("latch_bgy", ref latch_bgy);
 			ser.Sync("RCRCounter", ref RCRCounter);
 			ser.Sync("ActiveLine", ref ActiveLine);
 			ser.EndSection();
diff --git a/BizHawk.Emulation.Cores/Consoles/PC Engine/VPC.cs b/BizHawk.Emulation.Cores/Consoles/PC Engine/VPC.cs
index 8ba32258b5..8c68463029 100644
--- a/BizHawk.Emulation.Cores/Consoles/PC Engine/VPC.cs	
+++ b/BizHawk.Emulation.Cores/Consoles/PC Engine/VPC.cs	
@@ -175,7 +175,7 @@ namespace BizHawk.Emulation.Cores.PCEngine
 					}
 				}
 
-				CPU.Execute(VDC1.HBlankCycles);
+				CPU.Execute(24);
 
 				if (InActiveDisplay)
 				{
@@ -203,10 +203,16 @@ namespace BizHawk.Emulation.Cores.PCEngine
 						VDC2.BackgroundY++;
 						VDC2.BackgroundY &= 0x01FF;
 					}
+				}
 
+				CPU.Execute(VDC1.HBlankCycles - 24);
+
+				if (InActiveDisplay)
+				{
 					if (render) RenderScanLine();
 				}
 
+
 				if (ScanLine == VBlankLine && VDC1.VBlankInterruptEnabled)
 					VDC1.StatusByte |= VDC.StatusVerticalBlanking;