120 lines
4.0 KiB
C#
120 lines
4.0 KiB
C#
using System;
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using Core.Interfaces;
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namespace Core.Memory
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{
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public class MemoryBus : IMemory
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{
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// 8 Banks of 16KB RAM (Banks 0 through 7)
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private readonly byte[][] _ramBanks = new byte[8][];
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// 5 Banks of 16KB ROM (0-3 for the +2A, and 4 for the pure 48K)
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private readonly byte[][] _romBanks = new byte[5][];
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// --- Hardware State ---
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private int _currentRomBank = 4; // Defaults to 48K ROM
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private int _currentRamBankSlot3 = 0;
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private byte _port7FFD = 0;
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private byte _port1FFD = 0;
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private bool _pagingDisabled = false;
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public MemoryBus()
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{
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// Initialize the physical silicon chips!
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for (int i = 0; i < 8; i++) _ramBanks[i] = new byte[0x4000];
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for (int i = 0; i < 5; i++) _romBanks[i] = new byte[0x4000];
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}
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// Called when the machine is turned on or reset
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public void ResetPaging(MachineModel model)
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{
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_pagingDisabled = false;
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_port7FFD = 0;
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_port1FFD = 0;
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_currentRamBankSlot3 = 0;
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if (model == MachineModel.Spectrum48K)
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{
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_currentRomBank = 4; // Lock to the pure 48.rom
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}
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else
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{
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_currentRomBank = 0; // Boot into the +3 DOS menu!
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}
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}
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// Called by the IO Bus when the CPU writes to a paging port
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public void HandlePaging(ushort port, byte value)
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{
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if (_pagingDisabled || _currentRomBank == 4) return; // Ignore if locked or in 48K mode
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if (port == 0x7FFD)
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{
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_port7FFD = value;
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_currentRamBankSlot3 = value & 0x07; // Bottom 3 bits select the RAM bank
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if ((value & 0x20) != 0) _pagingDisabled = true; // Bit 5 permanently disables paging
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}
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else if (port == 0x1FFD)
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{
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_port1FFD = value;
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}
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// Calculate which of the 4 Amstrad ROMs is active based on the ports
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int romLow = (_port7FFD >> 4) & 0x01;
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int romHigh = (_port1FFD >> 2) & 0x01;
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_currentRomBank = (romHigh << 1) | romLow;
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}
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public byte Read(ushort address)
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{
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int slot = address >> 14; // Divide by 16384 to find Slot 0, 1, 2, or 3
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int offset = address & 0x3FFF; // Find the exact byte inside that 16KB slot
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switch (slot)
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{
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case 0: return _romBanks[_currentRomBank][offset]; // 0x0000 - 0x3FFF (ROM)
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case 1: return _ramBanks[5][offset]; // 0x4000 - 0x7FFF (Always RAM 5)
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case 2: return _ramBanks[2][offset]; // 0x8000 - 0xBFFF (Always RAM 2)
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case 3: return _ramBanks[_currentRamBankSlot3][offset]; // 0xC000 - 0xFFFF (Switchable RAM)
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default: return 0xFF;
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}
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}
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public void Write(ushort address, byte value)
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{
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if (address < 0x4000) return; // Cannot write to ROM
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int slot = address >> 14;
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int offset = address & 0x3FFF;
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switch (slot)
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{
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case 1: _ramBanks[5][offset] = value; break;
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case 2: _ramBanks[2][offset] = value; break;
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case 3: _ramBanks[_currentRamBankSlot3][offset] = value; break;
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}
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}
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public void LoadRom(byte[] romData, int bankIndex)
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{
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Array.Copy(romData, 0, _romBanks[bankIndex], 0, Math.Min(romData.Length, 0x4000));
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}
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public void CleanRAMData()
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{
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for (int bank = 0; bank < 8; bank++)
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{
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for (int i = 0; i < 0x4000; i++) _ramBanks[bank][i] = 0x00;
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}
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}
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public void CrapRAMData()
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{
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Random random = new Random();
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for (int bank = 0; bank < 8; bank++)
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{
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for (int i = 0; i < 0x4000; i++) _ramBanks[bank][i] = (byte)random.Next(0x00, 0xFF);
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}
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}
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}
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} |