Bus snooping.

src/emucore/CartELF.cxx

@@ -330,7 +330,9 @@ inline uInt8 CartridgeELF::driveBus(uInt16 address, uInt8 value)
 
   if (myIsBusDriven) value |= myDriveBusValue;
 
-  runArm();
+  myVcsLib.updateBus(address, value);
+
+  if (!myVcsLib.isSuspended()) runArm();
 
   return value;
 }
@@ -493,7 +495,7 @@ void CartridgeELF::runArm()
 
   const CortexM0::err_t err = myCortexEmu.run(cyclesGoal, cycles);
 
-  if (err && (CortexM0::getErrCustom(err) != ERR_QUEUE_FULL))
+  if (err && (CortexM0::getErrCustom(err) != ERR_STOP_EXECUTION))
     FatalEmulationError::raise("error executing ARM code: " + CortexM0::describeError(err));
 }
 

src/emucore/elf/ElfEnvironment.hxx

@@ -112,7 +112,8 @@ namespace elfEnvironment {
 
   constexpr uInt32 RETURN_ADDR_MAIN = 0xffffdead;
 
-  constexpr uInt32 ERR_QUEUE_FULL = 1;
+  constexpr uInt32 ERR_STOP_EXECUTION = 1;
+
   constexpr uInt32 QUEUE_SIZE_LIMIT = 10;
 
   enum class Palette: uInt8 {pal, ntsc};

src/emucore/elf/VcsLib.cxx

@@ -60,6 +60,10 @@ VcsLib::VcsLib(BusTransactionQueue& transactionQueue) : myTransactionQueue(trans
 void VcsLib::reset()
 {
   myStuffMaskA = myStuffMaskX = myStuffMaskY = 0x00;
+  myIsWaitingForRead = false;
+  myWaitingForReadAddress = 0;
+  myCurrentAddress = 0;
+  myCurrentValue = 0;
 }
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
@@ -124,7 +128,7 @@ CortexM0::err_t VcsLib::fetch16(uInt32 address, uInt16& value, uInt8& op, Cortex
   CortexM0::err_t err;
 
   if (myTransactionQueue.size() >= elfEnvironment::QUEUE_SIZE_LIMIT)
-    return CortexM0::errCustom(ERR_QUEUE_FULL);
+    return CortexM0::errCustom(ERR_STOP_EXECUTION);
 
   myTransactionQueue.setTimestamp(cortex.getCycles());
 
@@ -170,7 +174,8 @@ CortexM0::err_t VcsLib::fetch16(uInt32 address, uInt16& value, uInt8& op, Cortex
       return returnFromStub(value, op);
 
     case ADDR_VCS_NOP2:
-      FatalEmulationError::raise("unimplemented: vcsNop2");
+      myTransactionQueue.injectROM(0xea);
+      return returnFromStub(value, op);
 
     case ADDR_VCS_NOP2N:
       vcsNop2n(cortex.getRegister(0));
@@ -234,7 +239,28 @@ CortexM0::err_t VcsLib::fetch16(uInt32 address, uInt16& value, uInt8& op, Cortex
       return returnFromStub(value, op);
 
     case ADDR_VCS_READ4:
-      FatalEmulationError::raise("unimplemented: vcsRead4");
+      if (myIsWaitingForRead) {
+        if (myTransactionQueue.size() > 0 || myCurrentAddress != myWaitingForReadAddress)
+          return CortexM0::errCustom(ERR_STOP_EXECUTION);
+
+        myIsWaitingForRead = false;
+        cortex.setRegister(0, myCurrentValue);
+
+        return returnFromStub(value, op);
+      } else {
+        arg = cortex.getRegister(0);
+
+        myIsWaitingForRead = true;
+        myWaitingForReadAddress = arg;
+
+        myTransactionQueue
+          .injectROM(0xad)
+	        .injectROM(arg & 0xff)
+	        .injectROM(arg >> 8)
+          .yield(arg);
+
+        return CortexM0::errCustom(ERR_STOP_EXECUTION);
+      }
 
     case ADDR_RANDINT:
       FatalEmulationError::raise("unimplemented: randint ");
@@ -277,6 +303,12 @@ CortexM0::err_t VcsLib::fetch16(uInt32 address, uInt16& value, uInt8& op, Cortex
   }
 }
 
+void VcsLib::updateBus(uInt16 address, uInt8 value)
+{
+  myCurrentAddress = address;
+  myCurrentValue = value;
+}
+
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 CortexM0::err_t VcsLib::returnFromStub(uInt16& value, uInt8& op)
 {

src/emucore/elf/VcsLib.hxx

@@ -20,8 +20,7 @@
 
 #include "bspf.hxx"
 #include "CortexM0.hxx"
-
-class BusTransactionQueue;
+#include "BusTransactionQueue.hxx"
 
 class VcsLib: public CortexM0::BusTransactionDelegate {
   public:
@@ -31,6 +30,13 @@ class VcsLib: public CortexM0::BusTransactionDelegate {
 
     CortexM0::err_t fetch16(uInt32 address, uInt16& value, uInt8& op, CortexM0& cortex) override;
 
+    void updateBus(uInt16 address, uInt8 value);
+
+    inline bool isSuspended() const {
+      return
+        myIsWaitingForRead && (myTransactionQueue.size() > 0 || (myWaitingForReadAddress != myCurrentAddress));
+    }
+
     void vcsWrite5(uInt8 zpAddress, uInt8 value);
     void vcsCopyOverblankToRiotRam();
     void vcsStartOverblank();
@@ -48,6 +54,12 @@ class VcsLib: public CortexM0::BusTransactionDelegate {
     uInt8 myStuffMaskX{0x00};
     uInt8 myStuffMaskY{0x00};
 
+    bool myIsWaitingForRead{false};
+    uInt16 myWaitingForReadAddress{0};
+
+    uInt16 myCurrentAddress{0};
+    uInt8 myCurrentValue{0};
+
   private:
     VcsLib(const VcsLib&) = delete;
     VcsLib(VcsLib&&) = delete;