From 4ad09233998869169ce7e0886702a804aabf86cf Mon Sep 17 00:00:00 2001
From: tomcw <tomcw@users.noreply.github.com>
Date: Sun, 3 Nov 2019 15:05:28 +0000
Subject: [PATCH] Fix video tearing by only updating during the Apple II VBL
 (#711)

---
 source/Applewin.cpp | 2 +-
 source/NTSC.cpp     | 8 ++++++--
 source/Video.cpp    | 6 ++++++
 source/Video.h      | 1 +
 4 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/source/Applewin.cpp b/source/Applewin.cpp
index b6cd6409..01579acd 100644
--- a/source/Applewin.cpp
+++ b/source/Applewin.cpp
@@ -356,7 +356,7 @@ static void ContinueExecution(void)
 	//
 
 	const UINT dwClksPerFrame = NTSC_GetCyclesPerFrame();
-	if (g_dwCyclesThisFrame >= dwClksPerFrame)
+	if (g_dwCyclesThisFrame >= dwClksPerFrame && !VideoGetVblBar())
 	{
 		g_dwCyclesThisFrame -= dwClksPerFrame;
 
diff --git a/source/NTSC.cpp b/source/NTSC.cpp
index 87e8853f..38691c63 100644
--- a/source/NTSC.cpp
+++ b/source/NTSC.cpp
@@ -2003,8 +2003,12 @@ void NTSC_VideoInit( uint8_t* pFramebuffer ) // wsVideoInit
 //===========================================================================
 void NTSC_VideoReinitialize( DWORD cyclesThisFrame, bool bInitVideoScannerAddress )
 {
-	_ASSERT(cyclesThisFrame < g_videoScanner6502Cycles);
-	if (cyclesThisFrame >= g_videoScanner6502Cycles) cyclesThisFrame = 0;	// error
+	if (cyclesThisFrame >= g_videoScanner6502Cycles)
+	{
+		// Possible, since ContinueExecution() loop waits until: cycles > g_videoScanner6502Cycles && VBL
+		cyclesThisFrame %= g_videoScanner6502Cycles;
+	}
+
 	g_nVideoClockVert = (uint16_t) (cyclesThisFrame / VIDEO_SCANNER_MAX_HORZ);
 	g_nVideoClockHorz = cyclesThisFrame % VIDEO_SCANNER_MAX_HORZ;
 
diff --git a/source/Video.cpp b/source/Video.cpp
index a29c6e49..4440ec30 100644
--- a/source/Video.cpp
+++ b/source/Video.cpp
@@ -874,6 +874,7 @@ WORD VideoGetScannerAddress(DWORD nCycles, VideoScanner_e videoScannerAddr /*= V
 //===========================================================================
 
 // TODO: Consider replacing simply with: return g_nVideoClockVert < kVDisplayableScanLines
+// - will this work in full-speed mode?
 bool VideoGetVblBar(const DWORD uExecutedCycles)
 {
 	// get video scanner position
@@ -888,6 +889,11 @@ bool VideoGetVblBar(const DWORD uExecutedCycles)
 	return nCycles < kVDisplayableScanLines * kHClocks;
 }
 
+bool VideoGetVblBar(void)
+{
+	return g_nVideoClockVert < kVDisplayableScanLines;
+}
+
 //===========================================================================
 
 #define MAX_DRAW_DEVICES 10
diff --git a/source/Video.h b/source/Video.h
index fe566322..0bb88578 100644
--- a/source/Video.h
+++ b/source/Video.h
@@ -189,6 +189,7 @@ void    VideoResetState ();
 enum VideoScanner_e {VS_FullAddr, VS_PartialAddrV, VS_PartialAddrH};
 WORD    VideoGetScannerAddress(DWORD nCycles, VideoScanner_e videoScannerAddr = VS_FullAddr);
 bool    VideoGetVblBar(DWORD uExecutedCycles);
+bool    VideoGetVblBar(void);
 
 bool    VideoGetSW80COL(void);
 bool    VideoGetSWDHIRES(void);