Plus 3A implemented but not running

Core/Cpu/Z80.cs

@@ -120,7 +120,7 @@ namespace Core.Cpu
             Push(PC);
 
             // --- Interrupt Mode Dispatch ---
-            if (InterruptMode == 1)
+            if (InterruptMode == 0 || InterruptMode == 1)
             {
                 // IM 1: Hardcoded jump to ROM address 0x0038
                 PC = 0x0038;
@@ -143,7 +143,6 @@ namespace Core.Cpu
             }
             else
             {
-                // (IM 0 is theoretically possible but essentially unused on the standard Spectrum)
                 throw new NotImplementedException($"Interrupt Mode {InterruptMode} not implemented!");
             }
         }

Core/Io/IO_Bus.cs

@@ -1,5 +1,6 @@
-using Core.Interfaces;
+using System.Diagnostics;
-using System.Diagnostics;
+using Core.Interfaces;
+using Core.Memory;
 using static System.Runtime.InteropServices.JavaScript.JSType;
 
 namespace Core.Io
@@ -11,10 +12,12 @@ namespace Core.Io
         public bool BeeperState { get; private set; } = false;
         public byte[] KeyboardRows = new byte[8] { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
         TapManager _tapManager = new TapManager();
+        MemoryBus _memory;
 
-        public IO_Bus(TapManager tapManager)
+        public IO_Bus(TapManager tapManager, MemoryBus memoryBus)
         {
             _tapManager = tapManager;
+            _memory = memoryBus;
         }
 
         public byte ReadPort(ushort portAddress)
@@ -64,6 +67,18 @@ namespace Core.Io
 
                 // Bit 3 handles the cassette MIC output
             }
+
+            // 128K Standard Paging Port (0x7FFD)
+            if ((portAddress & 0x8002) == 0)
+            {
+                _memory.HandlePaging(0x7FFD, portValue);
+            }
+
+            // +2A/+3 Extended Paging Port (0x1FFD)
+            if ((portAddress & 0xF002) == 0x1000)
+            {
+                _memory.HandlePaging(0x1FFD, portValue);
+            }
         }
     }
 }

Core/Memory/MemoryBus.cs

@@ -5,61 +5,116 @@ namespace Core.Memory
 {
     public class MemoryBus : IMemory
     {
-        // The flat 64KB memory space
+        // 8 Banks of 16KB RAM (Banks 0 through 7)
-        private readonly byte[] _memory = new byte[0x10000];
+        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:
+            if (address < 0x4000) return; // Cannot write to ROM
-             * 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)
+            int slot = address >> 14;
+            int offset = address & 0x3FFF;
+
+            switch (slot)
             {
-                // Cannot write to ROM - Do nothing - maybe throw an exception
+                case 1: _ramBanks[5][offset] = value; break;
-                return;
+                case 2: _ramBanks[2][offset] = value; break;
+                case 3: _ramBanks[_currentRamBankSlot3][offset] = value; break;
+            }
         }
 
-            _memory[address] = value;
+        public void LoadRom(byte[] romData, int bankIndex)
-        }
-
-        //Put some initial random data into RAM for authenticity
-        public void CrapRAMData()
         {
-            Random random = new Random();
+            Array.Copy(romData, 0, _romBanks[bankIndex], 0, Math.Min(romData.Length, 0x4000));
-            for (int i = 0x4000; i < 0xFFFF; i++)
-            {
-                _memory[i] = (byte)random.Next(0x00, 0xFF);
-            }
         }
 
         public void CleanRAMData()
         {
-            for (int i = 0x4000; i < 0xFFFF; i++)
+            for (int bank = 0; bank < 8; bank++)
             {
-                _memory[i] = 0x00;
+                for (int i = 0; i < 0x4000; i++) _ramBanks[bank][i] = 0x00;
             }
         }
 
-        // Load the ROM file
+        public void CrapRAMData()
-        public void LoadRom(byte[] romData)
         {
-            if (romData.Length > 0x4000)
+            Random random = new Random();
+            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] = (byte)random.Next(0x00, 0xFF);
             }
-
-            // Copy the ROM
-            Array.Copy(romData, 0, _memory, 0, romData.Length);
         }
-        
     }
 }

Core/SpectrumMachine.cs

@@ -64,7 +64,7 @@ namespace Core
 
             Memory = new MemoryBus();
             TapeDeck = new TapManager();
-            IoBus = new IO_Bus(TapeDeck);
+            IoBus = new IO_Bus(TapeDeck, Memory);
             Ula = new ULA(Memory, IoBus);
 
             Memory.CrapRAMData();
@@ -75,9 +75,9 @@ namespace Core
             Cpu.WaitStateCallback = Ula.GetContentionDelay;
         }
 
-        public void LoadRom(byte[] romData)
+        public void LoadRom(byte[] romData, int bankIndex) // <-- Add the bank parameter
         {
-            Memory.LoadRom(romData);
+            Memory.LoadRom(romData, bankIndex);
         }
 
         public void Start()
@@ -115,6 +115,7 @@ namespace Core
                     {
                         Cpu.Reset();
                         Memory.CrapRAMData();
+                        Memory.ResetPaging(CurrentModel);
                         TotalFrameCount = 0;
                         scanlineCount = 0;
                         audioSampleCount = 0;

Desktop/Form1.cs

@@ -69,9 +69,11 @@ namespace Desktop
                 fastLoadToolStripMenuItem.Checked = _machine.EnableFastLoad;
                 HighSpeedToolStripMenuItem.Checked = _machine.HighSpeedMode;
 
-                // Load the ROM and start the engine
+                _machine.LoadRom(RomLoader.Load("plus3-0.rom"), 0);
-                byte[] romData = RomLoader.Load("48.rom");
+                _machine.LoadRom(RomLoader.Load("plus3-1.rom"), 1);
-                _machine.LoadRom(romData);
+                _machine.LoadRom(RomLoader.Load("plus3-2.rom"), 2);
+                _machine.LoadRom(RomLoader.Load("plus3-3.rom"), 3);
+                _machine.LoadRom(RomLoader.Load("48.rom"), 4);
 
                 _machine.Start();
             }