Atari 2600 - implement mapper F4SC

BizHawk.Emulation.Cores/Consoles/Atari/2600/Mappers/mF4SC.cs

@@ -1,4 +1,4 @@
-using System;
+using BizHawk.Common;
 
 namespace BizHawk.Emulation.Cores.Atari.Atari2600
 {
@@ -8,9 +8,75 @@ namespace BizHawk.Emulation.Cores.Atari.Atari2600
 	*/
 	internal class mF4SC : MapperBase
 	{
-		public mF4SC()
+		private int _bank4k;
+		private ByteBuffer _ram = new ByteBuffer(128);
+
+		private byte ReadMem(ushort addr, bool peek)
 		{
-			throw new NotImplementedException();
+			if (!peek)
+			{
+				Address(addr);
+			}
+
+			if (addr < 0x1000)
+			{
+				return base.ReadMemory(addr);
+			}
+
+			if (addr < 0x1080)
+			{
+				_ram[(addr & 0x7F)] = 0xFF; // Reading from the write port triggers an unwanted write of open bus
+				return 0xFF; // 0xFF is used for deterministic emulation, in reality it would be a random value based on pins being high or low
+			}
+			else if (addr < 0x1100)
+			{
+				return _ram[(addr & 0x7F)];
+			}
+
+			return core.rom[(_bank4k << 12) + (addr & 0xFFF)];
+		}
+
+		public override byte ReadMemory(ushort addr)
+		{
+			return ReadMem(addr, false);
+		}
+
+		public override byte PeekMemory(ushort addr)
+		{
+			return ReadMem(addr, true);
+		}
+
+		public override void WriteMemory(ushort addr, byte value)
+		{
+			Address(addr);
+			if (addr < 0x1000)
+			{
+				base.WriteMemory(addr, value);
+			}
+			else if ((addr & 0x0FFF) < 0x80)
+			{
+				_ram[addr & 0x7F] = value;
+			}
+		}
+
+		public override void SyncState(Serializer ser)
+		{
+			base.SyncState(ser);
+			ser.Sync("toggle", ref _bank4k);
+			ser.Sync("auxRam", ref _ram);
+		}
+
+		private void Address(ushort addr)
+		{
+			if (addr == 0x1FF4) _bank4k = 0;
+			if (addr == 0x1FF5) _bank4k = 1;
+			if (addr == 0x1FF6) _bank4k = 2;
+			if (addr == 0x1FF7) _bank4k = 3;
+			if (addr == 0x1FF8) _bank4k = 4;
+			if (addr == 0x1FF9) _bank4k = 5;
+			if (addr == 0x1FF9) _bank4k = 5;
+			if (addr == 0x1FFA) _bank4k = 6;
+			if (addr == 0x1FFB) _bank4k = 7;
 		}
 	}
 }