add sega mapper mod

mod to F8 that starts in bank 1

BizHawk.Emulation.Cores/Consoles/Atari/2600/Mappers/mF8_sega.cs

@@ -0,0 +1,98 @@
+using BizHawk.Common;
+
+namespace BizHawk.Emulation.Cores.Atari.Atari2600
+{
+	/*
+	F8 (Atari style 8K)
+	-----
+
+	This is the fairly standard way 8K of cartridge ROM was implemented.  There are two
+	4K ROM banks, which get mapped into the 4K of cartridge space.  Accessing 1FF8 or
+	1FF9 selects one of the two 4K banks.  When one of these two addresses are accessed,
+	the banks switch spontaniously.
+
+	ANY kind of access will trigger the switching- reading or writing.  Usually games use
+	LDA or BIT on 1FF8/1FF9 to perform the switch.
+
+	When the switch occurs, the entire 4K ROM bank switches, including the code that is
+	reading the 1FF8/1FF9 location.  Usually, games put a small stub of code in BOTH banks
+	so when the switch occurs, the code won't crash.
+	*/
+
+	internal class mF8_sega : MapperBase
+	{
+		private int _bank4K=1;
+
+		public override void SyncState(Serializer ser)
+		{
+			base.SyncState(ser);
+			ser.Sync("bank_4k", ref _bank4K);
+		}
+
+		public override void HardReset()
+		{
+			_bank4K = 1;
+			base.HardReset();
+		}
+
+		private byte ReadMem(ushort addr, bool peek)
+		{
+			if (!peek)
+			{
+				Address(addr);
+			}
+
+			if (addr < 0x1000)
+			{
+				return base.ReadMemory(addr);
+			}
+
+			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);
+		}
+
+		private void WriteMem(ushort addr, byte value, bool poke)
+		{
+			if (!poke)
+			{
+				Address(addr);
+			}
+
+			if (addr < 0x1000)
+			{
+				base.WriteMemory(addr, value);
+			}
+		}
+
+		public override void WriteMemory(ushort addr, byte value)
+		{
+			WriteMem(addr, value, poke: false);
+		}
+
+		public override void PokeMemory(ushort addr, byte value)
+		{
+			WriteMem(addr, value, poke: true);
+		}
+
+		private void Address(ushort addr)
+		{
+			if (addr == 0x1FF8)
+			{
+				_bank4K = 0;
+			}
+			else if (addr == 0x1FF9)
+			{
+				_bank4K = 1;
+			}
+		}
+	}
+}