Added CS command

The Clear And Subtract command

AGC/AGC.csproj

@@ -44,6 +44,7 @@
     <Compile Include="Base\OnesCompliment.cs" />
     <Compile Include="Instructions\BranchZeroOrMinusToFixed.cs" />
     <Compile Include="Instructions\ClearAndAdd.cs" />
+    <Compile Include="Instructions\ClearAndSubtract.cs" />
     <Compile Include="Instructions\CountCompareAndSkip.cs" />
     <Compile Include="Instructions\QuarterCode5.cs" />
     <Compile Include="Instructions\TransferToStorage.cs" />

AGC/Base/OnesCompliment.cs

@@ -44,7 +44,8 @@ public static ushort Add(this ushort left, ushort right)
             // if we have overflow, most likely from subtracting negative numbers
             if ((sum & 0x10000) > 0)
             {
-                // we need to Single Precision correct the negative number by adding 1 and taking the lower 16 bits
+                // we need to ones compliment correct the negative number by adding 1 and taking the lower 16 bits
+                // this process is called "end around carry"
                 sum = sum + 1;
                 sum = sum & 0xFFFF;
             }

AGC/InstructionSet.cs

@@ -21,6 +21,7 @@ public InstructionSet(Processor cpu)
             Add(new Add());
             Add(new ClearAndAdd());
             Add(new CountCompareAndSkip());
+            Add(new ClearAndSubtract());
         }
 
         public new IInstruction this[ushort code]

AGC/Instructions/ClearAndSubtract.cs

@@ -0,0 +1,40 @@
+using Apollo.Virtual.AGC.Base;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace Apollo.Virtual.AGC.Instructions
+{
+    /// <summary>
+    /// CS - 0100
+    /// 
+    /// The "Clear and Subtract" instruction moves the 1's-complement (i.e., the negative) of a memory location into the accumulator.
+    /// 
+    /// Also:
+    /// COM - 0100 0000 0000 0000
+    /// The "Complement the Contents of A" bitwise complements the accumulator
+    /// Assembles as CS A
+    /// </summary>
+    class ClearAndSubtract : IInstruction
+    {
+        public ushort Code
+        {
+            get { return 0x4; }
+        }
+
+        public Processor CPU { get; set; }
+
+        public void Execute(ushort K)
+        {
+            var value = CPU.Memory[K];
+
+            // write the compliment to the accumulator
+            CPU.A.Write((ushort)~value);
+
+            // if not the A register, re-write value to K
+            if (K != CPU.A.Address)
+                CPU.Memory[K] = value;
+        }
+    }
+}

AGC/Instructions/CountCompareAndSkip.cs

@@ -27,12 +27,12 @@ public void Execute(ushort K)
 
             // 1) compute the Diminished ABSolute value found at K and set in A
 
-            // if negative, XOR 1's to get ABS
+            // if negative, NOT 1's to get ABS
             var isNegative = (value & 0x8000) > 0;
-            var abs = isNegative ? value ^ 0xFFFF : value;
+            var abs = (ushort)(isNegative ? ~value : value);
 
             if (abs > 1)
-                CPU.A.Write((ushort)(abs - 1));
+                CPU.A.Write(abs.Add(OnesCompliment.NegativeOne));
             else
                 CPU.A.Write(0);
 

AGC/Processor.cs

@@ -35,7 +35,7 @@ public class Processor
         /// The lower product after MP instructions
         /// also known as L
         /// </summary>
-        internal IWord LP;
+        internal IWord L;
 
         /// <summary>
         ///  Remainder from the DV instruction, 
@@ -229,7 +229,7 @@ public Processor(MemoryMap memory)
 
             // main registers?
             A = memory.AddRegister<Accumulator>();
-            LP = memory.AddRegister<FullRegister>(0x01);
+            L = memory.AddRegister<FullRegister>(0x01);
             Q = memory.AddRegister<FullRegister>(0x02);
             EB = memory.GetWord(0x3);
             FB = memory.GetWord(0x4);

Tests/AGC.Tests/AGC.Tests.csproj

@@ -57,6 +57,7 @@
     <Compile Include="Instructions\BranchZeroOrMinusToFixed.cs" />
     <Compile Include="Instructions\BranchZeroToFixed.cs" />
     <Compile Include="Instructions\ClearAndAdd.cs" />
+    <Compile Include="Instructions\ClearAndSubtract.cs" />
     <Compile Include="Instructions\CountCompareAndSkip.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Registers\CycleRightRegister.cs" />

Tests/AGC.Tests/Instructions/ClearAndSubtract.cs

@@ -0,0 +1,48 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace AGC.Tests.Instructions
+{
+    [TestClass]
+    public class ClearAndSubtract : BaseTest
+    {
+        [TestMethod]
+        public void ClearAndSubtractAccumulator()
+        {
+            // arrange
+            Memory[0x0] = 0x0101;
+
+            Memory.LoadFixedRom(new ushort[] {
+                0x4000
+            });
+
+            // act
+            CPU.Execute();
+
+            // assert
+
+            Assert.AreEqual(0xFEFE, Memory[0x0]);
+        }
+
+        [TestMethod]
+        public void ClearAndSubtractMemory()
+        {
+            // arrange
+            Memory[0x200] = 0xF0F0;
+
+            Memory.LoadFixedRom(new ushort[] {
+                0x4000 | 0x0200
+            });
+
+            // act
+            CPU.Execute();
+
+            // assert
+            Assert.AreEqual(0x0F0F, Memory[0x0]);
+        }
+    }
+}