diff --git a/BizHawk.Emulation/Consoles/Atari/2600/Mappers/mF4.cs b/BizHawk.Emulation/Consoles/Atari/2600/Mappers/mF4.cs
index a04a69fa51..13b0c7b85c 100644
--- a/BizHawk.Emulation/Consoles/Atari/2600/Mappers/mF4.cs
+++ b/BizHawk.Emulation/Consoles/Atari/2600/Mappers/mF4.cs
@@ -7,5 +7,37 @@ namespace BizHawk.Emulation.Consoles.Atari._2600
 {
 	class mF4 :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 == 0x1FF4) toggle = 0;
+			if (addr == 0x1FF5) toggle = 1;
+			if (addr == 0x1FF6) toggle = 2;
+			if (addr == 0x1FF7) toggle = 3;
+			if (addr == 0x1FF8) toggle = 4;
+			if (addr == 0x1FF9) toggle = 5;
+			if (addr == 0x1FF9) toggle = 5;
+			if (addr == 0x1FFA) toggle = 6;
+			if (addr == 0x1FFB) toggle = 7;
+		}
 	}
 }