ZXSpectrum48K/Core/Memory/MemoryBus.cs

using Core.Interfaces;
using System;
using System.Diagnostics;

namespace Core.Memory
{
    public class MemoryBus : IMemory
    {
        // 8 Banks of 16KB RAM (Banks 0 through 7)
        private readonly byte[][] _ramBanks = new byte[8][];

        // 5 Banks of 16KB ROM (0-3 for the +2A, and 4 for the pure 48K)
        private readonly byte[][] _romBanks = new byte[5][];

        // --- Hardware State ---
        private int _currentRomBank = 4; // Defaults to 48K ROM
        private int _currentRamBankSlot3 = 0;
        private byte _port7FFD = 0;
        private byte _port1FFD = 0;
        private bool _pagingDisabled = false;

        public MemoryBus()
        {
            // Initialize the physical silicon chips!
            for (int i = 0; i < 8; i++) _ramBanks[i] = new byte[0x4000];
            for (int i = 0; i < 5; i++) _romBanks[i] = new byte[0x4000];
        }

        // Called when the machine is turned on or reset
        public void ResetPaging(MachineModel model)
        {
            _pagingDisabled = false;
            _port7FFD = 0;
            _port1FFD = 0;
            _currentRamBankSlot3 = 0;

            if (model == MachineModel.Spectrum48K)
            {
                _currentRomBank = 4; // Lock to the pure 48.rom
            }
            else
            {
                _currentRomBank = 0; // Boot into the +3 DOS menu!
            }
        }

        // Called by the IO Bus when the CPU writes to a paging port
        public void HandlePaging(ushort port, byte value)
        {
            //Debug.WriteLine($"[PAGING] 0x7FFD Triggered! Hex: 0x{value:X2} | Bank: {value & 0x07}");
            if (_pagingDisabled || _currentRomBank == 4) return; // Ignore if locked or in 48K mode

            if (port == 0x7FFD)
            {
                _port7FFD = value;
                _currentRamBankSlot3 = value & 0x07; // Bottom 3 bits select the RAM bank
                if ((value & 0x20) != 0) _pagingDisabled = true; // Bit 5 permanently disables paging
            }
            else if (port == 0x1FFD)
            {
                _port1FFD = value;
            }

            // Calculate which of the 4 Amstrad ROMs is active based on the ports
            int romLow = (_port7FFD >> 4) & 0x01;
            int romHigh = (_port1FFD >> 2) & 0x01;
            _currentRomBank = (romHigh << 1) | romLow;
        }

        private int GetBankForSlot(int slot)
        {
            // Amstrad +2A / +3 Special Paging Mode
            bool specialPaging = (_port1FFD & 0x01) != 0;

            if (specialPaging)
            {
                // Bits 1 and 2 dictate the specific Special RAM layout
                int config = (_port1FFD >> 1) & 0x03;
                switch (config)
                {
                    case 0: return slot == 0 ? 0 : slot == 1 ? 1 : slot == 2 ? 2 : 3;
                    case 1: return slot == 0 ? 4 : slot == 1 ? 5 : slot == 2 ? 6 : 7;
                    case 2: return slot == 0 ? 4 : slot == 1 ? 5 : slot == 2 ? 6 : 3;
                    case 3: return slot == 0 ? 4 : slot == 1 ? 7 : slot == 2 ? 6 : 3;
                }
            }

            // Standard 128K Paging
            switch (slot)
            {
                case 1: return 5;
                case 2: return 2;
                case 3: return _currentRamBankSlot3;
                default: return -1; // -1 means it is pointing to a ROM chip!
            }
        }
        public byte Read(ushort address)
        {
            int slot = address >> 14; // Find the 16KB Slot (0, 1, 2, or 3)
            int offset = address & 0x3FFF;

            int bank = GetBankForSlot(slot);

            if (bank == -1)
            {
                return _romBanks[_currentRomBank][offset];
            }
            else
            {
                return _ramBanks[bank][offset];
            }
        }

        public void Write(ushort address, byte value)
        {
            int slot = address >> 14;
            int offset = address & 0x3FFF;

            int bank = GetBankForSlot(slot);

            // If the bank is NOT a ROM chip, we are allowed to write to it!
            // (This allows the BIOS to write to 0x0000 during the self-test)
            if (bank != -1)
            {
                _ramBanks[bank][offset] = value;
            }
        }

        public byte[] GetVideoRam()
        {
            // Bit 3 of 7FFD dictates the active screen. 0 = Bank 5 (Standard), 1 = Bank 7 (Shadow).
            return (_port7FFD & 0x08) != 0 ? _ramBanks[7] : _ramBanks[5];
        }
        public void LoadRom(byte[] romData, int bankIndex)
        {
            Array.Copy(romData, 0, _romBanks[bankIndex], 0, Math.Min(romData.Length, 0x4000));
        }

        public void CleanRAMData()
        {
            for (int bank = 0; bank < 8; bank++)
            {
                for (int i = 0; i < 0x4000; i++) _ramBanks[bank][i] = 0x00;
            }
        }

        public void CrapRAMData()
        {
            Random random = new Random();
            for (int bank = 0; bank < 8; bank++)
            {
                for (int i = 0; i < 0x4000; i++) _ramBanks[bank][i] = (byte)random.Next(0x00, 0xFF);
            }
        }
    }
}