Plus 3A implemented but not running

Core/Memory/MemoryBus.cs

@@ -5,61 +5,116 @@ namespace Core.Memory
 {
     public class MemoryBus : IMemory
     {
-        // The flat 64KB memory space
-        private readonly byte[] _memory = new byte[0x10000];
+        // 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)
+        {
+            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;
+        }
 
         public byte Read(ushort address)
         {
-            return _memory[address];
+            int slot = address >> 14; // Divide by 16384 to find Slot 0, 1, 2, or 3
+            int offset = address & 0x3FFF; // Find the exact byte inside that 16KB slot
+
+            switch (slot)
+            {
+                case 0: return _romBanks[_currentRomBank][offset];      // 0x0000 - 0x3FFF (ROM)
+                case 1: return _ramBanks[5][offset];                    // 0x4000 - 0x7FFF (Always RAM 5)
+                case 2: return _ramBanks[2][offset];                    // 0x8000 - 0xBFFF (Always RAM 2)
+                case 3: return _ramBanks[_currentRamBankSlot3][offset]; // 0xC000 - 0xFFFF (Switchable RAM)
+                default: return 0xFF;
+            }
         }
 
         public void Write(ushort address, byte value)
         {
-            /* ZX Spectrum 48K Memory Map:
-             * 0x0000 - 0x3FFF: ROM (16KB) -> Cannot write here!
-             * 0x4000 - 0x57FF: Display File (Screen Pixels)
-             * 0x5800 - 0x5AFF: Color Attributes
-             * 0x5B00 - 0xFFFF: General Purpose RAM
-             */
+            if (address < 0x4000) return; // Cannot write to ROM
 
-            if (address < 0x4000)
+            int slot = address >> 14;
+            int offset = address & 0x3FFF;
+
+            switch (slot)
             {
-                // Cannot write to ROM - Do nothing - maybe throw an exception
-                return;
+                case 1: _ramBanks[5][offset] = value; break;
+                case 2: _ramBanks[2][offset] = value; break;
+                case 3: _ramBanks[_currentRamBankSlot3][offset] = value; break;
             }
-
-            _memory[address] = value;
         }
 
-        //Put some initial random data into RAM for authenticity
-        public void CrapRAMData()
+        public void LoadRom(byte[] romData, int bankIndex)
         {
-            Random random = new Random();
-            for (int i = 0x4000; i < 0xFFFF; i++)
-            {
-                _memory[i] = (byte)random.Next(0x00, 0xFF);
-            }
+            Array.Copy(romData, 0, _romBanks[bankIndex], 0, Math.Min(romData.Length, 0x4000));
         }
 
         public void CleanRAMData()
-        {            
-            for (int i = 0x4000; i < 0xFFFF; i++)
-            {
-                _memory[i] = 0x00;
-            }
-        }
-
-        // Load the ROM file
-        public void LoadRom(byte[] romData)
         {
-            if (romData.Length > 0x4000)
+            for (int bank = 0; bank < 8; bank++)
             {
-                throw new ArgumentException("ROM file exceeds the 16KB capacity of Bank 0.");
+                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);
             }
-
-            // Copy the ROM
-            Array.Copy(romData, 0, _memory, 0, romData.Length);
         }
-        
     }
 }