Implemented a load of Z80 OpCodes. Added SimpleIOBus.

Core/Cpu/Z80.cs

@@ -8,6 +8,10 @@ namespace Core.Cpu
         //T-State counter
         public long TotalTStates { get; set; }
 
+        // Interrupt Flip-Flops
+        public bool IFF1;
+        public bool IFF2;
+
         // Main Register Set
         public RegisterPair AF;
         public RegisterPair BC;
@@ -32,11 +36,13 @@ namespace Core.Cpu
 
         // The Memory Bus
         private readonly IMemory _memory;
+        private readonly IIoBus _ioBus;
 
-        public Z80(IMemory memory)
+        public Z80(IMemory memory, IIoBus ioBus)
         {
             _memory = memory;
-            Reset();
+            _ioBus = ioBus;
+            Reset();            
         }
 
         public void Reset()
@@ -62,6 +68,19 @@ namespace Core.Cpu
             return tStates;
         }
 
+        // Reads a 16-bit word from the current PC (Little-Endian) and advances PC by 2
+        private ushort FetchWord()
+        {
+            byte low = _memory.Read(PC++);
+            byte high = _memory.Read(PC++);
+            return (ushort)((high << 8) | low);
+        }
+
+        private byte FetchByte()
+        {
+            return _memory.Read(PC++);
+        }
+
         public string GetFlagsString()
         {
             byte f = AF.Low;
@@ -75,18 +94,110 @@ namespace Core.Cpu
                    $"C:{f & 1}";
         }
 
+        private void Sbc(byte value)
+        {
+            byte a = AF.High;
+            byte carry = (byte)(AF.Low & 0x01); // Get the current Carry flag (Bit 0)
+
+            // Calculate the raw integer result to check for borrows/underflows
+            int result = a - value - carry;
+
+            // Update the Accumulator
+            AF.High = (byte)result;
+
+            // --- Update Flags (F Register) ---
+            AF.Low = 0; // Clear all flags
+
+            // Sign Flag (Bit 7)
+            if ((result & 0x80) != 0) AF.Low |= 0x80;
+
+            // Zero Flag (Bit 6)
+            if ((byte)result == 0) AF.Low |= 0x40;
+
+            // Half-Carry Flag (Bit 4) - Set if borrow from bit 4
+            if (((a & 0x0F) - (value & 0x0F) - carry) < 0) AF.Low |= 0x10;
+
+            // Overflow Flag (Bit 2) - Set if operands have different signs and result sign changes
+            if ((((a ^ value) & 0x80) != 0) && (((a ^ result) & 0x80) != 0)) AF.Low |= 0x04;
+
+            // Subtract Flag (Bit 1) - ALWAYS set for subtraction
+            AF.Low |= 0x02;
+
+            // Carry Flag (Bit 0) - Set if the overall result dropped below 0
+            if (result < 0) AF.Low |= 0x01;
+        }
+
         private int ExecuteOpcode(byte opcode)
         {
             switch (opcode)
             {
-                case 0x00: // NOP (No Operation)
+                case 0x00: // NOP
-                    return 4; // Takes 4 T-states
+                    return 4;
+                case 0x11: //LD DE, nn
+                    DE.Word = FetchWord();
+                    return 10;
+                case 0x2B: // DEC HL
+                    HL.Word--;
+                    return 6;
+                case 0x36: // LD (HL), n
+                    byte nValue = FetchByte();
+                    _memory.Write(HL.Word, nValue);
+                    return 10;
+                case 0x3E: //LD A, n
+                    AF.High = FetchByte();
+                    return 7;
+                case 0x47: // LD B, A
+                    BC.High = AF.High;
+                    return 4;
+                case 0x62: // LD H, D
+                    HL.High = DE.High;
+                    return 4;
+                case 0x6B: // LD L, E
+                    HL.Low = DE.Low;
+                    return 4;
+                case 0xC3:
+                    PC = FetchWord();
+                    return 10;
+                case 0xD3: // OUT (n), A
+                    byte portOffset = FetchByte();
 
-                // We will expand this massive list soon!
+                    // The Z80 puts 'A' on the top 8 bits, and 'n' on the bottom 8 bits of the port address
+                    ushort portAddress = (ushort)((AF.High << 8) | portOffset);
+
+                    _ioBus.Write(portAddress, AF.High);
+
+                    return 11; // Takes 11 T-States
+                case 0xDE: // SBC A, n
+                    Sbc(FetchByte());
+                    return 7;
+                case 0xED:
+                    return ExecuteExtendedPrefix();
+                case 0xF3: // DI (Disable Interrupts)
+                    IFF1 = false;
+                    IFF2 = false;
+                    return 4;
+                case 0xAF: // XOR A
+                    AF.High = 0;
+                    AF.Low = 0x44;
+                    return 4;
 
                 default:
                     throw new NotImplementedException($"Opcode 0x{opcode:X2} at PC 0x{(PC - 1):X4} is not implemented.");
             }
         }
+        private int ExecuteExtendedPrefix()
+        {
+            // Fetch the actual extended instruction
+            byte extendedOpcode = _memory.Read(PC++);
+
+            switch (extendedOpcode)
+            {
+                case 0x47: // LD I, A
+                    I = AF.High;
+                    return 9;
+                default:
+                    throw new NotImplementedException($"Extended ED Opcode 0x{extendedOpcode:X2} at PC 0x{(PC - 1):X4} is not implemented.");
+            }
+        }
     }
 }

Core/Interfaces/IIoBus.cs

@@ -0,0 +1,8 @@
+namespace Core.Interfaces
+{
+    public interface IIoBus
+    {
+        byte Read(ushort port);
+        void Write(ushort port, byte value);
+    }
+}

Core/Io/SimpleIoBus.cs

@@ -0,0 +1,20 @@
+using System.Diagnostics;
+using Core.Interfaces;
+
+namespace Core.Io
+{
+    public class SimpleIoBus : IIoBus
+    {
+        public byte Read(ushort port)
+        {
+            // If the CPU reads an unconnected port, the Z80 usually sees 0xFF
+            return 0xFF;
+        }
+
+        public void Write(ushort port, byte value)
+        {
+            // For now, let's just log it to the Visual Studio Output window
+            Debug.WriteLine($"Hardware I/O Write -> Port: 0x{port:X4}, Value: 0x{value:X2}");
+        }
+    }
+}

Core/Memory/MemoryBus.cs

@@ -24,15 +24,14 @@ namespace Core.Memory
 
             if (address < 0x4000)
             {
-                // Attempted to write to the ROM area. 
+                // Cannot write to ROM - Do nothing - maybe throw an exception
-                // We simply ignore the write command, just like real hardware.
                 return;
             }
 
             _memory[address] = value;
         }
 
-        // Helper method to load the original Sinclair ROM file
+        // Load the ROM file
         public void LoadRom(byte[] romData)
         {
             if (romData.Length > 0x4000)
@@ -40,7 +39,7 @@ namespace Core.Memory
                 throw new ArgumentException("ROM file exceeds the 16KB capacity of Bank 0.");
             }
 
-            // Copy the ROM data into the very beginning of the memory array
+            // Copy the ROM
             Array.Copy(romData, 0, _memory, 0, romData.Length);
         }
     }

Desktop/DebuggerForm.Designer.cs

@@ -129,7 +129,7 @@
             // txtMemoryStart
             // 
             txtMemoryStart.Location = new Point(238, 10);
-            txtMemoryStart.Margin = new Padding(2, 2, 2, 2);
+            txtMemoryStart.Margin = new Padding(2);
             txtMemoryStart.Name = "txtMemoryStart";
             txtMemoryStart.Size = new Size(121, 27);
             txtMemoryStart.TabIndex = 9;
@@ -140,7 +140,7 @@
             // btnStep
             // 
             btnStep.Location = new Point(9, 315);
-            btnStep.Margin = new Padding(2, 2, 2, 2);
+            btnStep.Margin = new Padding(2);
             btnStep.Name = "btnStep";
             btnStep.Size = new Size(90, 27);
             btnStep.TabIndex = 12;
@@ -151,17 +151,18 @@
             // btnRun
             // 
             btnRun.Location = new Point(122, 315);
-            btnRun.Margin = new Padding(2, 2, 2, 2);
+            btnRun.Margin = new Padding(2);
             btnRun.Name = "btnRun";
             btnRun.Size = new Size(90, 27);
             btnRun.TabIndex = 13;
             btnRun.Text = "Run";
             btnRun.UseVisualStyleBackColor = true;
+            btnRun.Click += btnRun_Click;
             // 
             // btnRefreshMemory
             // 
             btnRefreshMemory.Location = new Point(376, 7);
-            btnRefreshMemory.Margin = new Padding(2, 2, 2, 2);
+            btnRefreshMemory.Margin = new Padding(2);
             btnRefreshMemory.Name = "btnRefreshMemory";
             btnRefreshMemory.Size = new Size(90, 27);
             btnRefreshMemory.TabIndex = 14;
@@ -172,7 +173,7 @@
             // txtMemoryView
             // 
             txtMemoryView.Location = new Point(88, 80);
-            txtMemoryView.Margin = new Padding(2, 2, 2, 2);
+            txtMemoryView.Margin = new Padding(2);
             txtMemoryView.Name = "txtMemoryView";
             txtMemoryView.Size = new Size(416, 195);
             txtMemoryView.TabIndex = 15;
@@ -182,7 +183,7 @@
             // 
             lstDisassembly.FormattingEnabled = true;
             lstDisassembly.Location = new Point(533, 11);
-            lstDisassembly.Margin = new Padding(2, 2, 2, 2);
+            lstDisassembly.Margin = new Padding(2);
             lstDisassembly.Name = "lstDisassembly";
             lstDisassembly.Size = new Size(186, 264);
             lstDisassembly.TabIndex = 16;
@@ -191,7 +192,7 @@
             // 
             lstStack.FormattingEnabled = true;
             lstStack.Location = new Point(723, 11);
-            lstStack.Margin = new Padding(2, 2, 2, 2);
+            lstStack.Margin = new Padding(2);
             lstStack.Name = "lstStack";
             lstStack.Size = new Size(186, 264);
             lstStack.TabIndex = 17;
@@ -199,7 +200,7 @@
             // btnExit
             // 
             btnExit.Location = new Point(818, 313);
-            btnExit.Margin = new Padding(2, 2, 2, 2);
+            btnExit.Margin = new Padding(2);
             btnExit.Name = "btnExit";
             btnExit.Size = new Size(90, 27);
             btnExit.TabIndex = 18;
@@ -228,9 +229,9 @@
             Controls.Add(lblDE);
             Controls.Add(lblBC);
             Controls.Add(lblAF);
-            Margin = new Padding(2, 2, 2, 2);
+            Margin = new Padding(2);
             Name = "DebuggerForm";
-            Text = "DebuggerForm";
+            Text = "Debugger";
             ResumeLayout(false);
             PerformLayout();
         }
@@ -250,9 +251,9 @@
         private Button btnRun;
         private Button btnRefreshMemory;
         private RichTextBox txtMemoryView;
-        private ListBox lstDisassembly;
         private ListBox lstStack;
         private Button btnExit;
+        public ListBox lstDisassembly;
         //private TextBox textBox4;
     }
 }

Desktop/DebuggerForm.cs

@@ -10,6 +10,7 @@ namespace Desktop
     {
         private readonly Z80 _cpu;
         private readonly MemoryBus _memoryBus;
+        private bool _isRunning = false;
 
         public DebuggerForm(Z80 cpu, MemoryBus memoryBus)
         {
@@ -46,6 +47,53 @@ namespace Desktop
             UpdateDisplay();
         }
 
+        private async void btnRun_Click(object sender, EventArgs e)
+        {
+            // If it is already running, this button acts as a STOP button
+            if (_isRunning)
+            {
+                _isRunning = false;
+                btnRun.Text = "Run";
+                return;
+            }
+
+            // Start the run state
+            _isRunning = true;
+            btnRun.Text = "Stop";
+
+            // Fire up a background thread so the Windows UI doesn't freeze
+            await Task.Run(() =>
+            {
+                try
+                {
+                    // Free-run the CPU until the flag is flipped or it crashes!
+                    while (_isRunning)
+                    {
+                        _cpu.Step();
+                    }
+                }
+                catch (Exception ex)
+                {
+                    _isRunning = false;
+
+                    // We are on a background thread. We MUST use Invoke to tell the 
+                    // main UI thread to update the labels and show the message box!
+                    this.Invoke((MethodInvoker)delegate
+                    {
+                        btnRun.Text = "Run";
+                        UpdateDisplay();
+                        MessageBox.Show(ex.Message, "CPU Break", MessageBoxButtons.OK, MessageBoxIcon.Information);
+                    });
+                }
+            });
+
+            // If the user clicked Stop manually (no exception thrown), we still want to update the UI
+            if (!_isRunning)
+            {
+                UpdateDisplay();
+            }
+        }
+
         private void btnExit_Click(object sender, EventArgs e)
         {
             Environment.Exit(0);
@@ -68,6 +116,8 @@ namespace Desktop
 
             // 3. Update Memory Viewer
             UpdateMemoryView();
+            UpdateStackView();
+            UpdateDisassemblyView();
         }
 
         private void UpdateMemoryView()
@@ -108,9 +158,6 @@ namespace Desktop
         private void UpdateStackView()
         {
             lstStack.Items.Clear();
-
-            // The Z80 stack starts at 0xFFFF and grows downwards.
-            // If SP is at the very top (e.g., 0xFFFF), we don't want to read past the end of memory and crash!
             int itemsToShow = 5;
             ushort currentSp = _cpu.SP;
 
@@ -139,36 +186,103 @@ namespace Desktop
         {
             lstDisassembly.Items.Clear();
 
+            // THIS is the critical link! It forces the top of the list 
+            // to always be exactly where the CPU currently is.
             ushort currentPc = _cpu.PC;
+
             int instructionsToShow = 8;
 
             for (int i = 0; i < instructionsToShow; i++)
             {
                 byte opcode = _memoryBus.Read(currentPc);
                 string mnemonic;
-                int instructionLength = 1; // Default to 1 byte long
+                int instructionLength = 1; // Default to 1
 
-                // This switch statement will grow as you add more opcodes to the CPU!
                 switch (opcode)
                 {
-                    case 0x00:
+                    case 0x00: mnemonic = "NOP"; break;
-                        mnemonic = "NOP";
+                    case 0x11:
+                        // LD DE, nn
+                        byte deLow = _memoryBus.Read((ushort)(currentPc + 1));
+                        byte deHigh = _memoryBus.Read((ushort)(currentPc + 2));
+                        mnemonic = $"LD DE, 0x{deHigh:X2}{deLow:X2}";
+                        instructionLength = 3;
+                        break;
+                    case 0x2B:
+                        mnemonic = "DEC HL";
+                        break;
+                    case 0x36:
+                        byte memValue = _memoryBus.Read((ushort)(currentPc + 1));
+                        mnemonic = $"LD (HL), 0x{memValue:X2}";
+                        instructionLength = 2;
                         break;
                     case 0x3E:
-                        // LD A, n (Loads the next byte into register A)
+                        mnemonic = $"LD A, 0x{_memoryBus.Read((ushort)(currentPc + 1)):X2}";
-                        byte nextByte = _memoryBus.Read((ushort)(currentPc + 1));
+                        instructionLength = 2;
-                        mnemonic = $"LD A, 0x{nextByte:X2}";
+                        break;
-                        instructionLength = 2; // This instruction takes up 2 bytes
+                    case 0x47:
+                        mnemonic = "LD B, A";
+                        break;
+                    case 0x62:
+                        mnemonic = "LD H, D";
+                        break;
+                    case 0x6B:
+                        mnemonic = "LD L, E";
+                        break;
+                    case 0xAF:
+                        mnemonic = "XOR A";
+                        break;
+                    case 0xC3:
+                        // JP nn
+                        byte jpLow = _memoryBus.Read((ushort)(currentPc + 1));
+                        byte jpHigh = _memoryBus.Read((ushort)(currentPc + 2));
+                        mnemonic = $"JP 0x{jpHigh:X2}{jpLow:X2}";
+                        instructionLength = 3;
+                        break;
+                    case 0xD3:
+                        byte outPort = _memoryBus.Read((ushort)(currentPc + 1));
+                        mnemonic = $"OUT (0x{outPort:X2}), A";
+                        instructionLength = 2;
+                        break;
+                    case 0xDE:
+                        byte sbcValue = _memoryBus.Read((ushort)(currentPc + 1));
+                        mnemonic = $"SBC A, 0x{sbcValue:X2}";
+                        instructionLength = 2;
+                        break;
+                    case 0xED:
+                        byte extendedOp = _memoryBus.Read((ushort)(currentPc + 1));
+
+                        switch (extendedOp)
+                        {
+                            // Example: ED 47 is LD I, A
+                            case 0x47:
+                                mnemonic = "LD I, A";
+                                instructionLength = 2; // 0xED + 0x47
+                                break;
+
+                            // Example: ED B0 is LDIR (a massive block copy instruction)
+                            case 0xB0:
+                                mnemonic = "LDIR";
+                                instructionLength = 2;
+                                break;
+
+                            default:
+                                mnemonic = $"EXT UNKNOWN (ED {extendedOp:X2})";
+                                instructionLength = 2; // Most ED instructions are 2 bytes, but some have operands!
+                                break;
+                        }
+                        break;
+                    case 0xF3:
+                        mnemonic = "DI";
                         break;
                     default:
                         mnemonic = $"UNKNOWN (0x{opcode:X2})";
                         break;
                 }
 
-                // Add to the list box
                 lstDisassembly.Items.Add($"{currentPc:X4}: {mnemonic}");
 
-                // Advance to the start of the next instruction
+                // Advance the fake PC just for drawing the next line in the UI
                 currentPc += (ushort)instructionLength;
             }
         }

Desktop/Form1.Designer.cs

@@ -36,8 +36,7 @@
             AutoScaleMode = AutoScaleMode.Font;
             ClientSize = new Size(800, 450);
             Name = "Form1";
-            Text = "Form1";
+            Text = "Parsons Sinclair ZX Spectrum 48K - 2026";
-            //this.Load += new System.EventHandler(this.Form1_Load);
             ResumeLayout(false);
         }
 

Desktop/Form1.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Windows.Forms;
 using Core.Cpu;
+using Core.Io;
 using Core.Memory;
 
 namespace Desktop
@@ -9,6 +10,7 @@ namespace Desktop
     {
         private Z80 _cpu = null!;
         private MemoryBus _memoryBus = null!;
+        private SimpleIoBus _simpleIoBus = null!;
 
         public Form1()
         {
@@ -22,16 +24,16 @@ namespace Desktop
             {
                 // 1. Initialize the memory bus
                 _memoryBus = new MemoryBus();
+                _simpleIoBus = new SimpleIoBus();
 
-                // 2. Load the ROM from disk
+                // 2. Load the ROM 
-                // Make sure "48.rom" matches the name of the file you added to your project
                 byte[] romData = RomLoader.Load("48.rom");
 
                 // 3. Inject the ROM into the memory bus
                 _memoryBus.LoadRom(romData);
 
                 // 4. Initialize the CPU with the populated memory
-                _cpu = new Z80(_memoryBus);
+                _cpu = new Z80(_memoryBus, _simpleIoBus);
 
                 DebuggerForm debugger = new DebuggerForm(_cpu, _memoryBus);
                 debugger.Show();