diff --git a/BizHawk.Emulation/Consoles/Atari/2600/Mappers/mEF.cs b/BizHawk.Emulation/Consoles/Atari/2600/Mappers/mEF.cs
index 19d9daac2e..6c27803a02 100644
--- a/BizHawk.Emulation/Consoles/Atari/2600/Mappers/mEF.cs
+++ b/BizHawk.Emulation/Consoles/Atari/2600/Mappers/mEF.cs
@@ -17,6 +17,44 @@ namespace BizHawk.Emulation.Consoles.Atari._2600
 
 	class mEF : MapperBase 
 	{
+		int toggle = 0;
 
+		public override byte ReadMemory(ushort addr)
+		{
+			Address(addr);
+			if (addr < 0x1000) return base.ReadMemory(addr);
+			return core.rom[toggle * 4 * 1024 + (addr & 0xFFF)];
+		}
+		public override void WriteMemory(ushort addr, byte value)
+		{
+			Address(addr);
+			if (addr < 0x1000) base.WriteMemory(addr, value);
+		}
+
+		public override void SyncState(Serializer ser)
+		{
+			base.SyncState(ser);
+			ser.Sync("toggle", ref toggle);
+		}
+
+		void Address(ushort addr)
+		{
+			if (addr == 0x1FE0) toggle = 0;
+			if (addr == 0x1FE1) toggle = 1;
+			if (addr == 0x1FE2) toggle = 2;
+			if (addr == 0x1FE3) toggle = 3;
+			if (addr == 0x1FE4) toggle = 4;
+			if (addr == 0x1FE5) toggle = 5;
+			if (addr == 0x1FE6) toggle = 6;
+			if (addr == 0x1FE7) toggle = 7;
+			if (addr == 0x1FE8) toggle = 8;
+			if (addr == 0x1FE9) toggle = 9;
+			if (addr == 0x1FEA) toggle = 10;
+			if (addr == 0x1FEB) toggle = 11;
+			if (addr == 0x1FEC) toggle = 12;
+			if (addr == 0x1FED) toggle = 13;
+			if (addr == 0x1FEE) toggle = 14;
+			if (addr == 0x1FEF) toggle = 15;
+		}
 	}
 }