91 lines
2.0 KiB
C#
91 lines
2.0 KiB
C#
using BizHawk.Common;
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namespace BizHawk.Emulation.Cores.Atari.Atari2600
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{
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/*
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F8 (Atari style 8K)
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-----
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This is the fairly standard way 8K of cartridge ROM was implemented. There are two
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4K ROM banks, which get mapped into the 4K of cartridge space. Accessing 1FF8 or
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1FF9 selects one of the two 4K banks. When one of these two addresses are accessed,
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the banks switch spontaniously.
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ANY kind of access will trigger the switching- reading or writing. Usually games use
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LDA or BIT on 1FF8/1FF9 to perform the switch.
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When the switch occurs, the entire 4K ROM bank switches, including the code that is
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reading the 1FF8/1FF9 location. Usually, games put a small stub of code in BOTH banks
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so when the switch occurs, the code won't crash.
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*/
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internal sealed class mF8 : MapperBase
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{
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private int _bank4K;
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public mF8(Atari2600 core) : base(core)
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{
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}
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public override void SyncState(Serializer ser)
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{
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base.SyncState(ser);
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ser.Sync("bank_4k", ref _bank4K);
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}
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public override void HardReset()
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{
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_bank4K = 0;
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}
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public override byte ReadMemory(ushort addr) => ReadMem(addr, false);
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public override byte PeekMemory(ushort addr) => ReadMem(addr, true);
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public override void WriteMemory(ushort addr, byte value)
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=> WriteMem(addr, value, false);
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public override void PokeMemory(ushort addr, byte value)
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=> WriteMem(addr, value, true);
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private byte ReadMem(ushort addr, bool peek)
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{
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if (!peek)
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{
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Address(addr);
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}
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if (addr < 0x1000)
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{
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return base.ReadMemory(addr);
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}
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return Core.Rom[(_bank4K << 12) + (addr & 0xFFF)];
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}
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private void WriteMem(ushort addr, byte value, bool poke)
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{
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if (!poke)
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{
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Address(addr);
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}
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if (addr < 0x1000)
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{
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base.WriteMemory(addr, value);
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}
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}
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private void Address(ushort addr)
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{
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if (addr == 0x1FF8)
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{
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_bank4K = 0;
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}
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else if (addr == 0x1FF9)
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{
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_bank4K = 1;
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}
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}
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}
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} |