Add Z80 processor (untested, but complete)

Signed-off-by: Adrian Conlon <Adrian.conlon@gmail.com>

EightBit/Chip.cs

@@ -51,5 +51,23 @@ namespace EightBit
         public static int PromoteNibble(byte value) => LowByte(value << 4);
 
         public static int DemoteNibble(byte value) => HighNibble(value);
+
+        public static int CountBits(int value)
+        {
+            int count = 0;
+            while (value != 0)
+            {
+                ++count;
+                value &= value - 1;
+            }
+
+            return count;
+        }
+
+        public static int CountBits(byte value) => CountBits((int)value);
+
+        public static bool EvenParity(int value) => CountBits(value) % 2 == 0;
+
+        public static bool EvenParity(byte value) => EvenParity((int)value);
     }
 }

EightBit/IntelOpCodeDecoded.cs

@@ -6,23 +6,27 @@ namespace EightBit
 {
     public class IntelOpCodeDecoded
     {
-        private readonly int x;
-        private readonly int y;
-        private readonly int z;
-        private readonly int p;
-        private readonly int q;
-
         public IntelOpCodeDecoded()
         {
         }
 
         public IntelOpCodeDecoded(byte opCode)
         {
-            this.x = (opCode & 0b11000000) >> 6;    // 0 - 3
-            this.y = (opCode & 0b00111000) >> 3;    // 0 - 7
-            this.z = opCode & 0b00000111;           // 0 - 7
-            this.p = (this.y & 0b110) >> 1;         // 0 - 3
-            this.q = this.y & 1;                    // 0 - 1
+            this.X = (opCode & 0b11000000) >> 6;    // 0 - 3
+            this.Y = (opCode & 0b00111000) >> 3;    // 0 - 7
+            this.Z = opCode & 0b00000111;           // 0 - 7
+            this.P = (this.Y & 0b110) >> 1;         // 0 - 3
+            this.Q = this.Y & 1;                    // 0 - 1
         }
+
+        public int X { get; }
+
+        public int Y { get; }
+
+        public int Z { get; }
+
+        public int P { get; }
+
+        public int Q { get; }
     }
 }

EightBit/IntelProcessor.cs

@@ -8,6 +8,9 @@ namespace EightBit
 
     public abstract class IntelProcessor : LittleEndianProcessor
     {
+        private static readonly int[] HalfCarryTableAdd = new int[8] { 0, 0, 1, 0, 1, 0, 1, 1 };
+        private static readonly int[] HalfCarryTableSub = new int[8] { 0, 1, 1, 1, 0, 0, 0, 1 };
+
         private readonly IntelOpCodeDecoded[] decodedOpCodes = new IntelOpCodeDecoded[0x100];
 
         private Register16 sp = new Register16((ushort)Mask.Mask16);
@@ -86,6 +89,23 @@ namespace EightBit
             this.OnLoweredHALT();
         }
 
+        protected static int BuildHalfCarryIndex(byte before, byte value, int calculation)
+        {
+            return ((before & 0x88) >> 1) | ((value & 0x88) >> 2) | ((calculation & 0x88) >> 3);
+        }
+
+        protected static int CalculateHalfCarryAdd(byte before, byte value, int calculation)
+        {
+            var index = BuildHalfCarryIndex(before, value, calculation);
+            return HalfCarryTableAdd[index & (int)Mask.Mask3];
+        }
+
+        protected static int CalculateHalfCarrySub(byte before, byte value, int calculation)
+        {
+            var index = BuildHalfCarryIndex(before, value, calculation);
+            return HalfCarryTableSub[index & (int)Mask.Mask3];
+        }
+
         protected virtual void OnRaisingHALT() => this.RaisingHALT?.Invoke(this, EventArgs.Empty);
 
         protected virtual void OnRaisedHALT() => this.RaisedHALT?.Invoke(this, EventArgs.Empty);
@@ -192,5 +212,7 @@ namespace EightBit
             ++this.PC();
             this.RaiseHALT();
         }
+
+        protected IntelOpCodeDecoded GetDecodedOpCode(byte opCode) => this.decodedOpCodes[opCode];
     }
 }

EightBit/Mask.cs

@@ -4,8 +4,6 @@
 
 namespace EightBit
 {
-    using System;
-
     public enum Mask
     {
         None = 0,

EightBit/Register16.cs

@@ -76,7 +76,7 @@ namespace EightBit
                 return false;
             }
 
-            Register16 rhs = (Register16)obj;
+            var rhs = (Register16)obj;
             return rhs.Word == this.Word;
         }