Essgee.Unity/Assets/Plugins/Essgee/Emulation/Cartridges/Sega/SegaMapperCartridge.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;

using Essgee.Exceptions;
using Essgee.Utilities;

using static Essgee.Emulation.Utilities;

namespace Essgee.Emulation.Cartridges.Sega
{
	public class SegaMapperCartridge : ICartridge
	{
		byte[] romData;

		[StateRequired]
		byte[] ramData;

		[StateRequired]
		readonly byte[] pagingRegisters;

		[StateRequired]
		byte romBankMask;
		[StateRequired]
		bool hasCartRam;

		bool isRamEnabled { get { return IsBitSet(pagingRegisters[0], 3); } }
		bool isRomWriteEnable { get { return IsBitSet(pagingRegisters[0], 7); } }
		int ramBank { get { return ((pagingRegisters[0] >> 2) & 0x01); } }
		int romBank0 { get { return pagingRegisters[1]; } }
		int romBank1 { get { return pagingRegisters[2]; } }
		int romBank2 { get { return pagingRegisters[3]; } }

		public SegaMapperCartridge(int romSize, int ramSize)
		{
			pagingRegisters = new byte[0x04];
			pagingRegisters[0] = 0x00;  /* Mapper control */
			pagingRegisters[1] = 0x00;  /* Page 0 ROM bank */
			pagingRegisters[2] = 0x01;  /* Page 1 ROM bank */
			pagingRegisters[3] = 0x02;  /* Page 2 ROM bank */

			romSize = Math.Max(romSize, 0xC000);

			romData = new byte[romSize];
			ramData = new byte[ramSize];

			romBankMask = 0xFF;
			hasCartRam = false;
		}

		public void LoadRom(byte[] data)
		{
			Buffer.BlockCopy(data, 0, romData, 0, Math.Min(data.Length, romData.Length));

			var romSizeRounded = 1;
			while (romSizeRounded < romData.Length) romSizeRounded <<= 1;

			romBankMask = (byte)((romSizeRounded >> 14) - 1);

			/* Ensure startup banks are within ROM size */
			pagingRegisters[1] &= romBankMask;
			pagingRegisters[2] &= romBankMask;
			pagingRegisters[3] &= romBankMask;
		}

		public void LoadRam(byte[] data)
		{
			Buffer.BlockCopy(data, 0, ramData, 0, Math.Min(data.Length, ramData.Length));
		}

		public byte[] GetRomData()
		{
			return romData;
		}

		public byte[] GetRamData()
		{
			return ramData;
		}

		public bool IsRamSaveNeeded()
		{
			return hasCartRam;
		}

		public ushort GetLowerBound()
		{
			return 0x0000;
		}

		public ushort GetUpperBound()
		{
			return 0xBFFF;
		}

		public void Step(int clockCyclesInStep)
		{
			/* Nothing to do */
		}

		public byte Read(ushort address)
		{
			switch (address & 0xC000)
			{
				case 0x0000:
					if (address < 0x400)
						/* First 1kb is constant to preserve interrupt vectors */
						return romData[address];
					else
						return romData[((romBank0 << 14) | (address & 0x3FFF))];

				case 0x4000:
					return romData[((romBank1 << 14) | (address & 0x3FFF))];

				case 0x8000:
					if (isRamEnabled)
						return ramData[((ramBank << 14) | (address & 0x3FFF))];
					else
						return romData[((romBank2 << 14) | (address & 0x3FFF))];

				default:
					throw new EmulationException(string.Format("Sega mapper: Cannot read from cartridge address 0x{0:X4}", address));
			}
		}

		public void Write(ushort address, byte value)
		{
			if (address >= 0xFFFC && address <= 0xFFFF)
			{
				/* Write to paging register */
				if ((address & 0x0003) != 0x00) value &= romBankMask;
				pagingRegisters[address & 0x0003] = value;

				/* Check if RAM ever gets enabled; if it is, indicate that we'll need to save the RAM */
				if (!hasCartRam && isRamEnabled && (address & 0x0003) == 0x0000)
					hasCartRam = true;
			}
			if (isRamEnabled && (address & 0xC000) == 0x8000)
			{
				/* Cartridge RAM */
				ramData[((ramBank << 14) | (address & 0x3FFF))] = value;
			}
			else if (isRomWriteEnable)
			{
				/* ROM write enabled...? */
			}

			/* Otherwise ignore writes to ROM, as some games seem to be doing that? (ex. Gunstar Heroes GG to 0000) */
		}
	}
}