From ea89ef01b4c8fca80121202467cc9dce26206753 Mon Sep 17 00:00:00 2001
From: Stephen Anthony <sa666666@gmail.com>
Date: Wed, 24 Apr 2019 17:11:37 -0230
Subject: [PATCH] Refactor DPC+ code wrt 'jitter'.  The code and commenting now
 properly indicates that it is a difference in mask values, and not actually
 jitter (TV 'jitter' effect is actually something else, and is not causing the
 screen shaking here).

---
 Changes.txt                 | 10 +++++-----
 src/common/StateManager.cxx |  2 +-
 src/emucore/CartDPCPlus.cxx | 20 +++++++-------------
 src/emucore/CartDPCPlus.hxx | 11 ++++++-----
 4 files changed, 19 insertions(+), 24 deletions(-)

diff --git a/Changes.txt b/Changes.txt
index 27791b955..cc3b433a2 100644
--- a/Changes.txt
+++ b/Changes.txt
@@ -63,12 +63,12 @@
 
   * Fixed 'Dancing Plate (Unknown) (PAL)' to use joystick.
 
-  * Allow the DPC+ scheme to not enable 'jitter' effect for certain older
-    DPC+ driver versions; this allows 'Epic Adventure' ROM to finally
-    work in Stella.
+  * Allow the DPC+ scheme to not enable playfield 'jitter' effect for
+    certain older DPC+ driver versions; this allows 'Epic Adventure' ROM
+    to finally work in Stella.
 
-  * PNG image support is now conditionally compiled into Stella.  All
-    major ports (Linux/macOS/Windows) have it enabled by default.
+  * PNG/ZIP image support is now conditionally compiled into Stella.
+    All major ports (Linux/macOS/Windows) have it enabled by default.
 
 -Have fun!
 
diff --git a/src/common/StateManager.cxx b/src/common/StateManager.cxx
index b175639ff..d5b82621d 100644
--- a/src/common/StateManager.cxx
+++ b/src/common/StateManager.cxx
@@ -27,7 +27,7 @@
 
 #include "StateManager.hxx"
 
-#define STATE_HEADER "06000003state"
+#define STATE_HEADER "06000002state"
 // #define MOVIE_HEADER "03030000movie"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
diff --git a/src/emucore/CartDPCPlus.cxx b/src/emucore/CartDPCPlus.cxx
index 37022e1e2..eecdc8af9 100644
--- a/src/emucore/CartDPCPlus.cxx
+++ b/src/emucore/CartDPCPlus.cxx
@@ -37,7 +37,7 @@ CartridgeDPCPlus::CartridgeDPCPlus(const BytePtr& image, uInt32 size,
     myARMCycles(0),
     myFractionalClocks(0.0),
     myBankOffset(0),
-    myJitterEnabled(true)
+    myFractionalLowMask(0x0F00FF)
 {
   // Image is always 32K, but in the case of ROM > 29K, the image is
   // copied to the end of the buffer
@@ -65,10 +65,10 @@ CartridgeDPCPlus::CartridgeDPCPlus(const BytePtr& image, uInt32 size,
        Thumbulator::ConfigureFor::DPCplus,
        this);
 
-  // Disable jitter on certain DPC+ driver versions
-  string driverMD5;
-  driverMD5 = MD5::hash(image, 3*1024);
-  myJitterEnabled = driverMD5 != "8dd73b44fd11c488326ce507cbeb19d1";
+  // Currently only one known DPC+ ARM driver exhibits a problem
+  // with the default mask to use for DFxFRACLOW
+  if(MD5::hash(image, 3*1024) == "8dd73b44fd11c488326ce507cbeb19d1")
+    myFractionalLowMask = 0x0F0000;
 
   setInitialState();
 }
@@ -403,10 +403,10 @@ bool CartridgeDPCPlus::poke(uInt16 address, uInt8 value)
 
     switch(function)
     {
-      //DFxFRACLOW - fractional data pointer low byte
+      // DFxFRACLOW - fractional data pointer low byte
       case 0x00:
         myFractionalCounters[index] =
-          (myFractionalCounters[index] & (myJitterEnabled ? 0x0F00FF : 0x0F0000)) | (uInt16(value) << 8);
+          (myFractionalCounters[index] & myFractionalLowMask) | (uInt16(value) << 8);
         break;
 
       // DFxFRACHI - fractional data pointer high byte
@@ -685,9 +685,6 @@ bool CartridgeDPCPlus::save(Serializer& out) const
 
     // Clock info for Thumbulator
     out.putLong(myARMCycles);
-
-    // Jitter effect
-    out.putBool(myJitterEnabled);
   }
   catch(...)
   {
@@ -749,9 +746,6 @@ bool CartridgeDPCPlus::load(Serializer& in)
 
     // Clock info for Thumbulator
     myARMCycles = in.getLong();
-
-    // Jitter effect
-    myJitterEnabled = in.getBool();
   }
   catch(...)
   {
diff --git a/src/emucore/CartDPCPlus.hxx b/src/emucore/CartDPCPlus.hxx
index 740585767..8594382da 100644
--- a/src/emucore/CartDPCPlus.hxx
+++ b/src/emucore/CartDPCPlus.hxx
@@ -270,11 +270,12 @@ class CartridgeDPCPlus : public Cartridge
     // Indicates the offset into the ROM image (aligns to current bank)
     uInt16 myBankOffset;
 
-    // Indicates whether 'jitter' should be emulated
-    // This is the default option, and should always be enabled
-    // However, certain ROMs were released with older DPC+ ARM drivers,
-    // before the scheme was finalized; this allows those ROMs to run
-    bool myJitterEnabled;
+    // Older DPC+ driver code had different behaviour wrt the mask used
+    // to retrieve 'DFxFRACLOW' (fractional data pointer low byte)
+    // ROMs built with an old DPC+ driver and using the newer mask can
+    // result in 'jittering' in the playfield display
+    // For current versions, this is 0x0F00FF; older versions need 0x0F0000
+    uInt32 myFractionalLowMask;
 
   private:
     // Following constructors and assignment operators not supported