Implement proper RDY emulation --- fixes #42.

src/emucore/M6502.cxx

@@ -58,7 +58,9 @@ M6502::M6502(const Settings& settings)
     myLastSrcAddressA(-1),
     myLastSrcAddressX(-1),
     myLastSrcAddressY(-1),
-    myDataAddressForPoke(0)
+    myDataAddressForPoke(0),
+    myOnHaltCallback(0),
+    myHaltRequested(false)
 {
 #ifdef DEBUGGER_SUPPORT
   myDebugger = nullptr;
@@ -102,11 +104,18 @@ void M6502::reset()
   myLastSrcAddressS = myLastSrcAddressA =
     myLastSrcAddressX = myLastSrcAddressY = -1;
   myDataAddressForPoke = 0;
+
+  myHaltRequested = false;
 }
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 inline uInt8 M6502::peek(uInt16 address, uInt8 flags)
 {
+  if (myHaltRequested) {
+    myOnHaltCallback();
+    myHaltRequested = false;
+  }
+
   ////////////////////////////////////////////////
   // TODO - move this logic directly into CartAR
   if(address != myLastAddress)
@@ -159,6 +168,13 @@ inline void M6502::poke(uInt16 address, uInt8 value)
   myLastPokeAddress = address;
 }
 
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+void M6502::requestHalt()
+{
+  if (!myOnHaltCallback) throw runtime_error("onHaltCallback not configured");
+  myHaltRequested = true;
+}
+
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 bool M6502::execute(uInt32 number)
 {
@@ -310,6 +326,8 @@ bool M6502::save(Serializer& out) const
     out.putInt(myLastSrcAddressA);
     out.putInt(myLastSrcAddressX);
     out.putInt(myLastSrcAddressY);
+
+    out.putBool(myHaltRequested);
   }
   catch(...)
   {
@@ -358,6 +376,8 @@ bool M6502::load(Serializer& in)
     myLastSrcAddressA = in.getInt();
     myLastSrcAddressX = in.getInt();
     myLastSrcAddressY = in.getInt();
+
+    myHaltRequested = in.getBool();
   }
   catch(...)
   {

src/emucore/M6502.hxx

@@ -28,6 +28,8 @@
 
 class Settings;
 
+#include <functional>
+
 #include "bspf.hxx"
 #include "System.hxx"
 #include "Serializable.hxx"
@@ -50,6 +52,10 @@ class M6502 : public Serializable
   friend class CartDebug;
   friend class CpuDebug;
 
+  public:
+
+    using onHaltCallback = std::function<void()>;
+
   public:
     /**
       Create a new 6502 microprocessor.
@@ -83,6 +89,21 @@ class M6502 : public Serializable
     */
     void nmi() { myExecutionStatus |= NonmaskableInterruptBit; }
 
+    /**
+      Set the callback for handling a halt condition
+    */
+    void setOnHaltCallback(onHaltCallback callback) { myOnHaltCallback = callback; }
+
+    /**
+      RDY pulled low --- halt on next read.
+    */
+    void requestHalt();
+
+    /**
+      Pull RDY high again before the callback was triggered.
+    */
+    void clearHaltRequest() { myHaltRequested = false; };
+
     /**
       Execute instructions until the specified number of instructions
       is executed, someone stops execution, or an error occurs.  Answers
@@ -328,6 +349,12 @@ class M6502 : public Serializable
     /// Indicates the number of system cycles per processor cycle
     static constexpr uInt32 SYSTEM_CYCLES_PER_CPU = 1;
 
+    /// Called when the processor enters halt state
+    onHaltCallback myOnHaltCallback;
+
+    /// Indicates whether RDY was pulled low
+    bool myHaltRequested;
+
 #ifdef DEBUGGER_SUPPORT
     Int32 evalCondBreaks() {
       for(uInt32 i = 0; i < myBreakConds.size(); i++)

src/emucore/tia/TIA.cxx

@@ -186,6 +186,12 @@ void TIA::installDelegate(System& system, Device& device)
   for(uInt32 i = 0; i < 8192; i += (1 << System::PAGE_SHIFT))
     if((i & 0x1080) == 0x0000)
       mySystem->setPageAccess(i >> System::PAGE_SHIFT, access);
+
+  mySystem->m6502().setOnHaltCallback(
+    [this] () {
+      onHalt();
+    }
+  );
 }
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
@@ -428,17 +434,7 @@ bool TIA::poke(uInt16 address, uInt8 value)
   switch (address)
   {
     case WSYNC:
-      // It appears that the 6507 only halts during a read cycle so
-      // we test here for follow-on writes which should be ignored as
-      // far as halting the processor is concerned.
-      // See issue #42 for more information.
-      if (mySystem->m6502().lastAccessWasRead())
-      {
-        mySubClock += (228 - myHctr) % 228;
-        mySystem->incrementCycles(mySubClock / 3);
-        mySubClock %= 3;
-      }
-      myShadowRegisters[address] = value;
+      mySystem->m6502().requestHalt();
       break;
 
     case RSYNC:
@@ -1050,6 +1046,14 @@ void TIA::onFrameComplete()
     myConsole.setFramerate(myFrameManager.frameRate());
 }
 
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+void TIA::onHalt()
+{
+  mySubClock += (228 - myHctr) % 228;
+  mySystem->incrementCycles(mySubClock / 3);
+  mySubClock %= 3;
+}
+
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 void TIA::cycle(uInt32 colorClocks)
 {
@@ -1161,6 +1165,8 @@ void TIA::nextLine()
   myFrameManager.nextLine();
 
   if (myFrameManager.isRendering() && myFrameManager.getY() == 0) flushLineCache();
+
+  mySystem->m6502().clearHaltRequest();
 }
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

src/emucore/tia/TIA.hxx

@@ -401,6 +401,8 @@ class TIA : public Device
 
     void onFrameComplete();
 
+    void onHalt();
+
     void updateEmulation();
 
     void cycle(uInt32 colorClocks);