From b360d27f225ea2db1a87408d3eaee6b41b29244b Mon Sep 17 00:00:00 2001
From: Andrea Odetti <mariofutire@gmail.com>
Date: Thu, 12 Nov 2020 19:31:35 +0000
Subject: [PATCH] split

---
 source/frontends/sa2/emulator.cpp | 31 ++++++++++++++++++++++++-------
 source/frontends/sa2/emulator.h   |  2 ++
 source/frontends/sa2/main.cpp     |  1 +
 3 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/source/frontends/sa2/emulator.cpp b/source/frontends/sa2/emulator.cpp
index b1c834ce..30b327e3 100644
--- a/source/frontends/sa2/emulator.cpp
+++ b/source/frontends/sa2/emulator.cpp
@@ -182,21 +182,38 @@ Emulator::Emulator(
   , myFPS(getFPS())
   , myMultiplier(1)
   , myFullscreen(false)
+  , myExtraCycles(0)
 {
 }
 
 void Emulator::executeOneFrame()
 {
-  const DWORD uCyclesToExecute = int(g_fCurrentCLK6502 / myFPS);
-  const UINT dwClksPerFrame = NTSC_GetCyclesPerFrame();
   const bool bVideoUpdate = true;
-  const DWORD uActualCyclesExecuted = CpuExecute(uCyclesToExecute, bVideoUpdate);
-  g_dwCyclesThisFrame = (g_dwCyclesThisFrame + uActualCyclesExecuted) % dwClksPerFrame;
+  const int nExecutionPeriodUsec = 1000;		// 1.0ms
+  const double fUsecPerSec = 1.e6;
+  const double fExecutionPeriodClks = int(g_fCurrentCLK6502 * (nExecutionPeriodUsec / fUsecPerSec));
 
-  GetCardMgr().GetDisk2CardMgr().UpdateDriveState(uActualCyclesExecuted);
-  MB_PeriodicUpdate(uActualCyclesExecuted);
-  SpkrUpdate(uActualCyclesExecuted);
+  int totalCyclesExecuted = 0;
+  const DWORD uCyclesToExecute = int(g_fCurrentCLK6502 / myFPS) + myExtraCycles;
+  const UINT dwClksPerFrame = NTSC_GetCyclesPerFrame();
 
+  while (totalCyclesExecuted < uCyclesToExecute)
+  {
+    // go in small steps of 1ms
+    const DWORD uActualCyclesExecuted = CpuExecute(fExecutionPeriodClks, bVideoUpdate);
+    totalCyclesExecuted += uActualCyclesExecuted;
+    g_dwCyclesThisFrame = (g_dwCyclesThisFrame + uActualCyclesExecuted) % dwClksPerFrame;
+
+    GetCardMgr().GetDisk2CardMgr().UpdateDriveState(uActualCyclesExecuted);
+    MB_PeriodicUpdate(uActualCyclesExecuted);
+    SpkrUpdate(uActualCyclesExecuted);
+  }
+
+  myExtraCycles = uCyclesToExecute - totalCyclesExecuted;
+}
+
+void Emulator::refreshVideo()
+{
   // SDL2 seems to synch with screen refresh rate so we do not need to worry about timers
   const SDL_Rect srect = refreshTexture(myTexture);
   renderScreen(myRenderer, myTexture, srect);
diff --git a/source/frontends/sa2/emulator.h b/source/frontends/sa2/emulator.h
index ef1ecda6..54c794ce 100644
--- a/source/frontends/sa2/emulator.h
+++ b/source/frontends/sa2/emulator.h
@@ -13,6 +13,7 @@ public:
     );
 
   void executeOneFrame();
+  void refreshVideo();
   void processEvents(bool & quit);
 
 private:
@@ -27,4 +28,5 @@ private:
 
   int myMultiplier;
   bool myFullscreen;
+  int myExtraCycles;
 };
diff --git a/source/frontends/sa2/main.cpp b/source/frontends/sa2/main.cpp
index 76e3aec9..a9e4b721 100644
--- a/source/frontends/sa2/main.cpp
+++ b/source/frontends/sa2/main.cpp
@@ -216,6 +216,7 @@ void run_sdl(int argc, const char * argv [])
     SDirectSound::writeAudio();
     emulator.processEvents(quit);
     emulator.executeOneFrame();
+    emulator.refreshVideo();
   } while (!quit);
 
   SDirectSound::stop();