Add a initial port of the MC6809 processor to the .Net collection

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

EightBit/Processor.cs

@@ -35,6 +35,15 @@ namespace EightBit
 
         protected byte OpCode { get; set; }
 
+        // http://graphics.stanford.edu/~seander/bithacks.html#FixedSignExtend
+        public static sbyte SignExtend(int b, byte x)
+        {
+            var m = (byte)(1 << (b - 1)); // mask can be pre-computed if b is fixed
+            x = (byte)(x & ((1 << b) - 1));  // (Skip this if bits in x above position b are already zero.)
+            var result = (x ^ m) - m;
+            return (sbyte)result;
+        }
+
         public ref PinLevel RESET() => ref this.resetLine;
 
         public ref PinLevel INT() => ref this.intLine;
@@ -178,12 +187,16 @@ namespace EightBit
             return this.GetWord();
         }
 
+        protected Register16 GetWord(Register16 address) => this.GetWord(address.Word);
+
         protected void SetWord(ushort address, Register16 value)
         {
             this.Bus.Address.Word = address;
             this.SetWord(value);
         }
 
+        protected void SetWord(Register16 address, Register16 value) => this.SetWord(address.Word, value);
+
         protected void Jump(ushort destination) => this.PC.Word = destination;
 
         protected void Call(ushort destination)

EightBit/Register16.cs

@@ -5,11 +5,13 @@
 namespace EightBit
 {
     using System.Diagnostics;
-    using System.Runtime.InteropServices;
 
     [DebuggerDisplay("Word = {Word}")]
     public class Register16
     {
+        private byte low;
+        private byte high;
+
         public Register16(byte low, byte high)
         {
             this.Low = low;
@@ -32,6 +34,11 @@ namespace EightBit
         {
         }
 
+        public Register16(uint value)
+        : this((ushort)value)
+        {
+        }
+
         public Register16(byte low)
         : this(low, 0)
         {
@@ -54,9 +61,9 @@ namespace EightBit
             }
         }
 
-        public byte Low { get; set; }
+        public ref byte Low => ref this.low;
 
-        public byte High { get; set; }
+        public ref byte High => ref this.high;
 
         public static Register16 operator ++(Register16 value) => Increment(value);