diff --git a/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj b/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj
index 48ff2d409b..184d1d2653 100644
--- a/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj
+++ b/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj
@@ -181,6 +181,7 @@
     <Compile Include="Consoles\Atari\2600\Mappers\m3E.cs" />
     <Compile Include="Consoles\Atari\2600\Mappers\m3F.cs" />
     <Compile Include="Consoles\Atari\2600\Mappers\m4A50.cs" />
+    <Compile Include="Consoles\Atari\2600\Mappers\MapperBase.cs" />
     <Compile Include="Consoles\Atari\2600\Mappers\mAR.cs" />
     <Compile Include="Consoles\Atari\2600\Mappers\mDPC.cs" />
     <Compile Include="Consoles\Atari\2600\Mappers\mDPCPlus.cs" />
diff --git a/BizHawk.Emulation.Cores/Consoles/Atari/2600/Atari2600.Core.cs b/BizHawk.Emulation.Cores/Consoles/Atari/2600/Atari2600.Core.cs
index 2817d31ab8..1bc92522ca 100644
--- a/BizHawk.Emulation.Cores/Consoles/Atari/2600/Atari2600.Core.cs
+++ b/BizHawk.Emulation.Cores/Consoles/Atari/2600/Atari2600.Core.cs
@@ -193,7 +193,8 @@ namespace BizHawk.Emulation.Cores.Atari.Atari2600
 					_mapper = new mSB();
 					break;
 
-				default: throw new InvalidOperationException("mapper not supported: " + this._game.GetOptionsDict()["m"]);
+				default:
+					throw new InvalidOperationException("mapper not supported: " + _game.GetOptionsDict()["m"]);
 			}
 
 			_mapper.Core = this;
@@ -314,30 +315,4 @@ namespace BizHawk.Emulation.Cores.Atari.Atari2600
 			return value;
 		}
 	}
-
-	public class MapperBase
-	{
-		public Atari2600 Core { get; set; }
-		
-		public virtual byte ReadMemory(ushort addr)
-		{
-			return Core.BaseReadMemory(addr);
-		}
-
-		public virtual byte PeekMemory(ushort addr)
-		{
-			return Core.BasePeekMemory(addr);
-		}
-
-		public virtual void WriteMemory(ushort addr, byte value)
-		{
-			Core.BaseWriteMemory(addr, value);
-		}
-
-		public virtual void SyncState(Serializer ser) { }
-
-		public virtual void Dispose() { }
-
-		public virtual void ClockCpu() { }
-	}
 }
\ No newline at end of file
diff --git a/BizHawk.Emulation.Cores/Consoles/Atari/2600/Mappers/MapperBase.cs b/BizHawk.Emulation.Cores/Consoles/Atari/2600/Mappers/MapperBase.cs
new file mode 100644
index 0000000000..186b8b4320
--- /dev/null
+++ b/BizHawk.Emulation.Cores/Consoles/Atari/2600/Mappers/MapperBase.cs
@@ -0,0 +1,30 @@
+using BizHawk.Common;
+
+namespace BizHawk.Emulation.Cores.Atari.Atari2600
+{
+	public class MapperBase
+	{
+		public Atari2600 Core { get; set; }
+
+		public virtual byte ReadMemory(ushort addr)
+		{
+			return Core.BaseReadMemory(addr);
+		}
+
+		public virtual byte PeekMemory(ushort addr)
+		{
+			return Core.BasePeekMemory(addr);
+		}
+
+		public virtual void WriteMemory(ushort addr, byte value)
+		{
+			Core.BaseWriteMemory(addr, value);
+		}
+
+		public virtual void SyncState(Serializer ser) { }
+
+		public virtual void Dispose() { }
+
+		public virtual void ClockCpu() { }
+	}
+}