More fixes for DPC+ jitter issues, thanks to Spiceware.

Changes.txt

@@ -181,6 +181,10 @@
   * Fixed bug in DPC+ scheme; 'fast fetch mode' was enabled at startup,
     when it should be disabled by default.
 
+  * Some more work on DPC+ playfield 'jitter' effect for certain older DPC+
+    driver versions; more ROMs are now detected properly.  Special thanks
+    to SpiceWare for his research in this area.
+
   * Added proper Retron77 port.
 
   * Added proper libretro port, and fixed display for OpenGLES renderers.

src/debugger/gui/CartDPCPlusWidget.cxx

@@ -35,7 +35,8 @@ CartridgeDPCPlusWidget::CartridgeDPCPlusWidget(
        << "DPC registers accessible @ $F000 - $F07F\n"
        << "  $F000 - $F03F (R), $F040 - $F07F (W)\n"
        << "Banks accessible at hotspots $FFF6 to $FFFB\n"
-       << "Startup bank = " << cart.startBank() << "\n";
+       << "Startup bank = " << cart.startBank() << "\n"
+       << "Ver = " << cart.myDriverMD5;
 
 #if 0
   // Eventually, we should query this from the debugger/disassembler

src/emucore/CartDPCPlus.cxx

@@ -57,9 +57,20 @@ CartridgeDPCPlus::CartridgeDPCPlus(const ByteBuffer& image, size_t size,
        Thumbulator::ConfigureFor::DPCplus,
        this);
 
-  // Currently only one known DPC+ ARM driver exhibits a problem
-  // with the default mask to use for DFxFRACLOW
-  if(MD5::hash(image, 3_KB) == "8dd73b44fd11c488326ce507cbeb19d1")
+  // Currently 4 DPC+ driver versions have been identified:
+  //   17884ec14f9b1d06fe8d617a1fbdcf47  Jitter  Encore Compatible
+  //   5f80b5a5adbe483addc3f6e6f1b472f8  Stable  Encore Compatible
+  //   8dd73b44fd11c488326ce507cbeb19d1  Stable  NOT Encore Compatible
+  //   b328dbdf787400c0f0e2b88b425872a5  Jitter  Encore Compatible
+  //
+  // Jitter/Stable refers to the appearance of the playfield in bB games if
+  // the DFxFRACINC registers are not updated before every drawscreen.
+  //
+  // The default mask for DFxFRACLOW implements the Jitter behavior. This
+  // changes the mask to implement the Stable behavior.
+  myDriverMD5 = MD5::hash(image, 3_KB);
+  if(myDriverMD5 == "5f80b5a5adbe483addc3f6e6f1b472f8" ||
+     myDriverMD5 == "8dd73b44fd11c488326ce507cbeb19d1" )
     myFractionalLowMask = 0x0F0000;
 
   setInitialState();

src/emucore/CartDPCPlus.hxx

@@ -272,6 +272,10 @@ class CartridgeDPCPlus : public Cartridge
     // Indicates the offset into the ROM image (aligns to current bank)
     uInt16 myBankOffset{0};
 
+    // MD5 value of the 3K DPC+ driver. Used to determine which mask to use,
+    // and shown in the Cartridge tab of the debugger
+    string myDriverMD5;
+
     // 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