Add an "UnusedMemory" class to better allow "gaps" in the memory map.

Signed-off-by: Adrian Conlon <Adrian.conlon@gmail.com>

src/Bus.cpp

@@ -93,6 +93,5 @@ uint8_t& EightBit::Bus::reference(const uint16_t address) {
 	const uint16_t offset = (address - mapped.begin) & mapped.mask;
 	if (mapped.access == MemoryMapping::ReadOnly)
 		return DATA() = mapped.memory.peek(offset);
-	Ram& ram = (Ram&)(mapped.memory);
-	return ram.reference(offset);
+	return mapped.memory.reference(offset);
 }

src/EightBit.vcxproj

@@ -156,6 +156,7 @@
     <ClInclude Include="..\inc\Register.h" />
     <ClInclude Include="..\inc\Signal.h" />
     <ClInclude Include="..\inc\TestHarness.h" />
+    <ClInclude Include="..\inc\UnusedMemory.h" />
     <ClInclude Include="stdafx.h" />
   </ItemGroup>
   <ItemGroup>

src/EightBit.vcxproj.filters

@@ -62,6 +62,9 @@
     <ClInclude Include="..\inc\MemoryInterface.h">
       <Filter>Header Files</Filter>
     </ClInclude>
+    <ClInclude Include="..\inc\UnusedMemory.h">
+      <Filter>Header Files</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="stdafx.cpp">