88 lines
1.8 KiB
C#
88 lines
1.8 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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This is another 8K bankswitching method with two 4K banks. The rationale is that it's
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cheap and easy to implement with only a single 74HC153 or 253 dual 4:1 multiplexer.
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This multiplexer can act as a 1 bit latch AND the inverter for A12.
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To bankswitch, the following mask it used:
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A13 A0
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----------------
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0 1xxx xBxx xxxx
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Each bit corresponds to one of the 13 address lines. a 0 or 1 means that bit must be
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0 or 1 to trigger the bankswitch. x is a bit that is not concidered (it can be either
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0 or 1 and is thus a "don't care" bit).
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B is the bank we will select. sooo, accessing 0800 will select bank 0, and 0840
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will select bank 1.
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*/
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internal class m0840 : MapperBase
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{
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private int _bank4K;
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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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base.HardReset();
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}
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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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public override byte ReadMemory(ushort addr)
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{
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return ReadMem(addr, false);
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}
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public override byte PeekMemory(ushort addr)
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{
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return ReadMem(addr, true);
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}
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public override void WriteMemory(ushort addr, byte value)
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{
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Address(addr);
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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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switch (addr & 0x1840)
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{
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case 0x0800:
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_bank4K = 0;
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break;
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case 0x0840:
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_bank4K = 1;
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break;
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}
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}
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}
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}
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