BizHawk/BizHawk.Emulation/Consoles/Sega/SMS/MemoryMap.Sega.cs

namespace BizHawk.Emulation.Consoles.Sega
{
    public partial class SMS
    {
        // The Sega memory mapper layout looks like so:
        // $0000-$03FF - ROM (unpaged)
        // $0400-$3FFF - ROM mapper slot 0
        // $4000-$7FFF - ROM mapper slot 1
        // $8000-$BFFF - ROM mapper slot 2 - OR - SaveRAM
        // $C000-$DFFF - System RAM
        // $E000-$FFFF - System RAM (mirror)
        // $FFFC - SaveRAM mapper control
        // $FFFD - Mapper slot 0 control
        // $FFFE - Mapper slot 1 control
        // $FFFF - Mapper slot 2 control

        private const ushort BankSizeMask = 0x3FFF;
        private const ushort RamSizeMask = 0x1FFF;

        private byte ReadMemory(ushort address)
        {
            if (address < 1024)
                return RomData[address];
            if (address < BankSize)
                return RomData[(RomBank0*BankSize) + address];
            if (address < BankSize * 2)
                return RomData[(RomBank1*BankSize) + (address & BankSizeMask)];
            if (address < BankSize * 3)
            {
                switch (SaveRamBank)
                {
                    case 0: return RomData[(RomBank2*BankSize) + (address & BankSizeMask)];
                    case 1: return SaveRAM[address & BankSizeMask];
                    case 2: return SaveRAM[BankSize + (address & BankSizeMask)];
                }
            }
            return SystemRam[address & RamSizeMask];
        }

        private void WriteMemory(ushort address, byte value)
        {
            if (address >= 0xC000)
                SystemRam[address & RamSizeMask] = value;

            else if (address >= 0x8000)
            {
                SaveRamModified = true;
                switch (SaveRamBank)
                {
                    case 1: SaveRAM[address & BankSizeMask] = value; return;
                    case 2: SaveRAM[BankSize + (address & BankSizeMask)] = value; return;
                }
            }

            if (address >= 0xFFFC)
            {
                if (address == 0xFFFC)
                {
                    if ((value & 8) != 0)
                        SaveRamBank = (byte)((value & 4) == 0 ? 1 : 2); // SaveRAM selected
                    else
                        SaveRamBank = 0; // ROM bank selected
                }
                else if (address == 0xFFFD) RomBank0 = (byte)(value % RomBanks);
                else if (address == 0xFFFE) RomBank1 = (byte)(value % RomBanks);
                else if (address == 0xFFFF) RomBank2 = (byte)(value % RomBanks);
                return;
            }
        }

        private void InitSegaMapper()
        {
            Cpu.ReadMemory = ReadMemory;
            Cpu.WriteMemory = WriteMemory;
            WriteMemory(0xFFFC, 0);
            WriteMemory(0xFFFD, 0);
            WriteMemory(0xFFFE, 1);
            WriteMemory(0xFFFF, 2);
        }

        // Mapper when loading a BIOS as a ROM

        private bool BiosMapped { get { return (Port3E & 0x08) == 0; } }

        private byte ReadMemoryBIOS(ushort address)
        {
            if (BiosMapped == false && address < BankSize * 3) 
                return 0x00;

            if (address < 1024)
                return RomData[address];
            if (address < BankSize)
                return RomData[(RomBank0 * BankSize) + address];
            if (address < BankSize * 2)
                return RomData[(RomBank1 * BankSize) + (address & BankSizeMask)];
            if (address < BankSize * 3)
                return RomData[(RomBank2 * BankSize) + (address & BankSizeMask)];
            
            return SystemRam[address & RamSizeMask];
        }

        private void WriteMemoryBIOS(ushort address, byte value)
        {
            if (address >= 0xC000)
                SystemRam[address & RamSizeMask] = value;

            if (address >= 0xFFFC)
            {
                     if (address == 0xFFFD) RomBank0 = (byte)(value % RomBanks);
                else if (address == 0xFFFE) RomBank1 = (byte)(value % RomBanks);
                else if (address == 0xFFFF) RomBank2 = (byte)(value % RomBanks);
                return;
            }
        }

        private void InitBiosMapper()
        {
            Cpu.ReadMemory = ReadMemoryBIOS;
            Cpu.WriteMemory = WriteMemoryBIOS;
            WriteMemory(0xFFFC, 0);
            WriteMemory(0xFFFD, 0);
            WriteMemory(0xFFFE, 1);
            WriteMemory(0xFFFF, 2);
        }
    }
}