Implemented a load more Z80 OpCodes. Added a breakpoint to the debugger

Core/Cpu/Z80.cs

@@ -160,6 +160,37 @@ namespace Core.Cpu
             if (result < 0) AF.Low |= 0x01;
         }
 
+        private byte Dec8(byte value)
+        {
+            byte result = (byte)(value - 1);
+
+            // Store the existing Carry flag so we can preserve it
+            byte carry = (byte)(AF.Low & 0x01);
+
+            // Clear all flags
+            AF.Low = 0;
+
+            // Sign Flag (Bit 7)
+            if ((result & 0x80) != 0) AF.Low |= 0x80;
+
+            // Zero Flag (Bit 6)
+            if (result == 0) AF.Low |= 0x40;
+
+            // Half-Carry Flag (Bit 4) - Set if borrow from bit 4 (happens if the lower nibble was 0)
+            if ((value & 0x0F) == 0) AF.Low |= 0x10;
+
+            // Parity/Overflow Flag (Bit 2) - Set if the original value was 0x80 (maximum negative)
+            if (value == 0x80) AF.Low |= 0x04;
+
+            // Subtract Flag (Bit 1) - ALWAYS SET for decrements
+            AF.Low |= 0x02;
+
+            // Restore the original Carry Flag (Bit 0)
+            AF.Low |= carry;
+
+            return result;
+        }
+
         private void Cp(byte value)
         {
             byte a = AF.High;
@@ -262,7 +293,17 @@ namespace Core.Cpu
                     return 7;
                 case 0x23: // INC HL
                     HL.Word++;
-                    return 6; 
+                    return 6;
+                case 0x28: // JR Z, e
+                    offset = (sbyte)FetchByte();
+
+                    // Check if the Zero Flag (Bit 6) IS set
+                    if ((AF.Low & 0x40) != 0)
+                    {
+                        PC = (ushort)(PC + offset);
+                        return 12; // Jump taken
+                    }
+                    return 7; // Jump not taken
                 case 0x2B: // DEC HL
                     HL.Word--;
                     return 6;
@@ -276,6 +317,17 @@ namespace Core.Cpu
                         return 12; // Jump taken
                     }
                     return 7; // Jump not taken
+                case 0x35: // DEC (HL)
+                           // Read the current byte from memory
+                    byte memValue = _memory.Read(HL.Word);
+
+                    // Decrement it and update flags
+                    byte decremented = Dec8(memValue);
+
+                    // Write the new value back to memory
+                    _memory.Write(HL.Word, decremented);
+
+                    return 11; // Takes 11 T-States
                 case 0x36: // LD (HL), n
                     byte nValue = FetchByte();
                     _memory.Write(HL.Word, nValue);
@@ -310,6 +362,20 @@ namespace Core.Cpu
                     _ioBus.Write(portAddress, AF.High);
 
                     return 11; // Takes 11 T-States
+                case 0xD9: // EXX
+                    ushort tempBC = BC.Word;
+                    BC.Word = BC_Prime.Word;
+                    BC_Prime.Word = tempBC;
+
+                    ushort tempDE = DE.Word;
+                    DE.Word = DE_Prime.Word;
+                    DE_Prime.Word = tempDE;
+
+                    ushort tempHL = HL.Word;
+                    HL.Word = HL_Prime.Word;
+                    HL_Prime.Word = tempHL;
+
+                    return 4; // Takes 4 T-States
                 case 0xDE: // SBC A, n
                     Sbc(FetchByte());
                     return 7;

Desktop/DebuggerForm.Designer.cs

@@ -44,6 +44,10 @@
             lstDisassembly = new ListBox();
             lstStack = new ListBox();
             btnExit = new Button();
+            saveFileDialog1 = new SaveFileDialog();
+            label1 = new Label();
+            txtBreakpoint = new TextBox();
+            label2 = new Label();
             SuspendLayout();
             // 
             // lblAF
@@ -128,7 +132,7 @@
             // 
             // txtMemoryStart
             // 
-            txtMemoryStart.Location = new Point(238, 10);
+            txtMemoryStart.Location = new Point(300, 21);
             txtMemoryStart.Margin = new Padding(2);
             txtMemoryStart.Name = "txtMemoryStart";
             txtMemoryStart.Size = new Size(121, 27);
@@ -161,7 +165,7 @@
             // 
             // btnRefreshMemory
             // 
-            btnRefreshMemory.Location = new Point(376, 7);
+            btnRefreshMemory.Location = new Point(425, 21);
             btnRefreshMemory.Margin = new Padding(2);
             btnRefreshMemory.Name = "btnRefreshMemory";
             btnRefreshMemory.Size = new Size(90, 27);
@@ -208,11 +212,39 @@
             btnExit.UseVisualStyleBackColor = true;
             btnExit.Click += btnExit_Click;
             // 
+            // label1
+            // 
+            label1.AutoSize = true;
+            label1.Location = new Point(289, 288);
+            label1.Name = "label1";
+            label1.Size = new Size(81, 20);
+            label1.TabIndex = 19;
+            label1.Text = "Breakpoint";
+            // 
+            // txtBreakpoint
+            // 
+            txtBreakpoint.Location = new Point(376, 281);
+            txtBreakpoint.Name = "txtBreakpoint";
+            txtBreakpoint.Size = new Size(125, 27);
+            txtBreakpoint.TabIndex = 20;
+            // 
+            // label2
+            // 
+            label2.AutoSize = true;
+            label2.Location = new Point(233, 21);
+            label2.Name = "label2";
+            label2.Size = new Size(62, 20);
+            label2.TabIndex = 21;
+            label2.Text = "Address";
+            // 
             // DebuggerForm
             // 
             AutoScaleDimensions = new SizeF(8F, 20F);
             AutoScaleMode = AutoScaleMode.Font;
             ClientSize = new Size(928, 350);
+            Controls.Add(label2);
+            Controls.Add(txtBreakpoint);
+            Controls.Add(label1);
             Controls.Add(btnExit);
             Controls.Add(lstStack);
             Controls.Add(lstDisassembly);
@@ -254,6 +286,10 @@
         private ListBox lstStack;
         private Button btnExit;
         public ListBox lstDisassembly;
+        private SaveFileDialog saveFileDialog1;
+        private Label label1;
+        private TextBox txtBreakpoint;
+        private Label label2;
         //private TextBox textBox4;
     }
 }

Desktop/DebuggerForm.cs

@@ -11,6 +11,7 @@ namespace Desktop
         private readonly Z80 _cpu;
         private readonly MemoryBus _memoryBus;
         private bool _isRunning = false;
+        private ushort? _breakpoint = null;
 
         public DebuggerForm(Z80 cpu, MemoryBus memoryBus)
         {
@@ -57,18 +58,45 @@ namespace Desktop
                 return;
             }
 
+            // --- NEW: Parse the Breakpoint ---
+            if (!string.IsNullOrWhiteSpace(txtBreakpoint.Text))
+            {
+                if (ushort.TryParse(txtBreakpoint.Text, System.Globalization.NumberStyles.HexNumber, null, out ushort parsedBp))
+                {
+                    _breakpoint = parsedBp;
+                }
+                else
+                {
+                    MessageBox.Show("Invalid hex address in breakpoint!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Warning);
+                    return;
+                }
+            }
+            else
+            {
+                _breakpoint = null; // No text means no breakpoint
+            }
+            // ---------------------------------
+
             // Start the run state
             _isRunning = true;
             btnRun.Text = "Stop";
 
-            // Fire up a background thread so the Windows UI doesn't freeze
+            // Fire up a background thread
             await Task.Run(() =>
             {
                 try
                 {
-                    // Free-run the CPU until the flag is flipped or it crashes!
                     while (_isRunning)
                     {
+                        // --- NEW: Breakpoint Check ---
+                        // We check BEFORE stepping so it stops exactly on the instruction
+                        if (_breakpoint.HasValue && _cpu.PC == _breakpoint.Value)
+                        {
+                            _isRunning = false;
+                            break; // Cleanly exit the while loop
+                        }
+                        // -----------------------------
+
                         _cpu.Step();
                     }
                 }
@@ -76,22 +104,17 @@ namespace Desktop
                 {
                     _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
+            // Whether it stopped because of a breakpoint, a crash, or the user clicking "Stop",
-            if (!_isRunning)
+            // we MUST update the UI when the background thread finishes!
-            {
+            btnRun.Text = "Run";
-                UpdateDisplay();
+            UpdateDisplay();
-            }
         }
 
         private void btnExit_Click(object sender, EventArgs e)
@@ -186,8 +209,6 @@ 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;
@@ -220,6 +241,12 @@ namespace Desktop
                     case 0x23:
                         mnemonic = "INC HL";
                         break;
+                    case 0x28:
+                        sbyte jrZOffset = (sbyte)_memoryBus.Read((ushort)(currentPc + 1));
+                        ushort jrZDest = (ushort)(currentPc + 2 + jrZOffset);
+                        mnemonic = $"JR Z, 0x{jrZDest:X4}";
+                        instructionLength = 2;
+                        break;
                     case 0x2B:
                         mnemonic = "DEC HL";
                         break;
@@ -229,6 +256,9 @@ namespace Desktop
                         mnemonic = $"JR NC, 0x{dest:X4}";
                         instructionLength = 2;
                         break;
+                    case 0x35:
+                        mnemonic = "DEC (HL)";
+                        break;
                     case 0x36:
                         byte memValue = _memoryBus.Read((ushort)(currentPc + 1));
                         mnemonic = $"LD (HL), 0x{memValue:X2}";
@@ -268,6 +298,9 @@ namespace Desktop
                         mnemonic = $"OUT (0x{outPort:X2}), A";
                         instructionLength = 2;
                         break;
+                    case 0xD9:
+                        mnemonic = "EXX";
+                        break;
                     case 0xDE:
                         byte sbcValue = _memoryBus.Read((ushort)(currentPc + 1));
                         mnemonic = $"SBC A, 0x{sbcValue:X2}";

Desktop/DebuggerForm.resx

@@ -117,4 +117,7 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
+  <metadata name="saveFileDialog1.TrayLocation" type="System.Drawing.Point, System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a">
+    <value>17, 17</value>
+  </metadata>
 </root>