Fix: Floating bus not returned for empty slot $Cnxx addresses (Fix for A2VGA.DSK: Apple][VGA card detection).

AppleWin/docs/History.txt

@@ -19,6 +19,12 @@ Restrictions/bugs:
 - For an original Apple //e, 80-column (PR#3) and INVERSE, it still appears to be mousetext character, but it should be inverted upper-case from $40 to $5F.
 
 
+1.xx.0 - ?? ??? 2012
+--------------------
+Fixes:
+. Floating bus not returned for empty slot $Cnxx addresses (Fix for A2VGA.DSK: Apple][VGA card detection).
+
+
 1.20.1 - 17 Jul 2011
 --------------------
 Fixes:

AppleWin/source/Memory.cpp

@@ -471,6 +471,8 @@ inline bool IsPotentialNoSlotClockAccess(const WORD address)
 			  (!SW_SLOTCXROM && (AddrHi == 0xC8)) );					// Internal ROM at [$C100-CFFF]               && AddrHi == $C8
 }
 
+static bool IsCardInSlot(const UINT uSlot);
+
 // Enabling expansion ROM ($C800..$CFFF]:
 // . Enable if: Enable1 && Enable2
 // . Enable1 = I/O SELECT' (6502 accesses $Csxx)
@@ -579,6 +581,14 @@ BYTE __stdcall IORead_Cxxx(WORD programcounter, WORD address, BYTE write, BYTE v
 		}
 	}
 
+	if (address >= APPLE_SLOT_BEGIN && address <= APPLE_SLOT_END)
+	{
+		// NB. Currently Mockingboard/Phasor is never unplugged, just disabled. See MB_Read().
+		const UINT uSlot = (address>>8)&0x7;
+		if (uSlot != 3 && !IsCardInSlot(uSlot))
+			return IO_Null(programcounter, address, write, value, nCyclesLeft);
+	}
+
 	if ((g_eExpansionRomType == eExpRomNull) && (address >= FIRMWARE_EXPANSION_BEGIN))
 		return IO_Null(programcounter, address, write, value, nCyclesLeft);
 	else
@@ -661,6 +671,17 @@ void RegisterIoHandler(UINT uSlot, iofunction IOReadC0, iofunction IOWriteC0, io
 	ExpansionRom[uSlot] = pExpansionRom;
 }
 
+static bool IsCardInSlot(const UINT uSlot)
+{
+	if (IORead[uSlot+8] == IO_Null &&
+		IOWrite[uSlot+8] == IO_Null &&
+		IORead[uSlot*16] == IORead_Cxxx &&
+		IOWrite[uSlot*16] == IOWrite_Cxxx)
+		return false;
+
+	return true;
+}
+
 //===========================================================================
 
 //// Only called by MemSetFastPaging()

AppleWin/source/Mockingboard.cpp

@@ -1382,8 +1382,8 @@ static BYTE __stdcall MB_Read(WORD PC, WORD nAddr, BYTE bWrite, BYTE nValue, ULO
 	if(!IS_APPLE2 && !MemCheckSLOTCXROM())
 		return mem[nAddr];
 
-	if(g_SoundcardType == SC_NONE)
+	if(g_SoundcardType == SC_NONE)	// TODO: Should really unplug the card from the slot and let IORead_Cxxx() return the floating bus
-		return 0;
+		return MemReadFloatingBus(nCyclesLeft);
 
 	BYTE nMB = (nAddr>>8)&0xf - SLOT4;
 	BYTE nOffset = nAddr&0xff;
@@ -1391,26 +1391,31 @@ static BYTE __stdcall MB_Read(WORD PC, WORD nAddr, BYTE bWrite, BYTE nValue, ULO
 	if(g_bPhasorEnable)
 	{
 		if(nMB != 0)	// Slot4 only
-			return 0;
+			return MemReadFloatingBus(nCyclesLeft);
 
-		BYTE nRes = 0;
 		int CS;
-
 		if(g_nPhasorMode & 1)
 			CS = ( ( nAddr & 0x80 ) >> 6 ) | ( ( nAddr & 0x10 ) >> 4 );	// 0, 1, 2 or 3
 		else															// Mockingboard Mode
 			CS = ( ( nAddr & 0x80 ) >> 7 ) + 1;							// 1 or 2
 
+		BYTE nRes = 0;
+
 		if(CS & 1)
 			nRes |= SY6522_Read(nMB*NUM_DEVS_PER_MB + SY6522_DEVICE_A, nAddr&0xf);
 
 		if(CS & 2)
 			nRes |= SY6522_Read(nMB*NUM_DEVS_PER_MB + SY6522_DEVICE_B, nAddr&0xf);
 
-		if((nOffset >= SSI263_Offset) && (nOffset <= (SSI263_Offset+0x05)))
+		bool bAccessedDevice = (CS & 3) ? true : false;
-			nRes |= SSI263_Read(nMB, nAddr&0xf);
 
-		return nRes;
+		if((nOffset >= SSI263_Offset) && (nOffset <= (SSI263_Offset+0x05)))
+		{
+			nRes |= SSI263_Read(nMB, nAddr&0xf);
+			bAccessedDevice = true;
+		}
+
+		return bAccessedDevice ? nRes : MemReadFloatingBus(nCyclesLeft);
 	}
 
 	if(nOffset <= (SY6522A_Offset+0x0F))
@@ -1420,7 +1425,7 @@ static BYTE __stdcall MB_Read(WORD PC, WORD nAddr, BYTE bWrite, BYTE nValue, ULO
 	else if((nOffset >= SSI263_Offset) && (nOffset <= (SSI263_Offset+0x05)))
 		return SSI263_Read(nMB, nAddr&0xf);
 	else
-		return 0;
+		return MemReadFloatingBus(nCyclesLeft);
 }
 
 //-----------------------------------------------------------------------------