implemented decf Instruction (#10)

Co-authored-by: Darkress <30271678+DarkressX@users.noreply.github.com>
Reviewed-on: darkress/pic16f84-sim#10

de/darkress/pic16f84sim/Main.java

@@ -14,8 +14,8 @@ class Main
         ArrayList<Command> program = new ArrayList<>();
         int input1 = 0x27FF;
         program.add(CommandDecoder.decode(0x3011)); //Write 0x11 to W
-        Memory.setRegister(0x14, 0x14);
+        Memory.setRegister(0x14, 0x01);
-        program.add(CommandDecoder.decode(0x0994));
+        program.add(CommandDecoder.decode(0x0314));
         for(int i = 0; i < 2; i++)
         {
             program.get(ProgramCounter.getPc()).execute();

de/darkress/pic16f84sim/commands/Decf.java

@@ -0,0 +1,27 @@
+package de.darkress.pic16f84sim.commands;
+
+import de.darkress.pic16f84sim.microcontroller.Memory;
+import de.darkress.pic16f84sim.microcontroller.ProgramCounter;
+
+public class Decf extends FileRegisterCommandUtils implements Command
+{
+    private final int address;
+    private final boolean destinationBit;
+
+    public Decf(int input)
+    {
+        address = input & 0x007F;
+        destinationBit = checkDestinationBit(input);
+    }
+
+    @Override
+    public void execute()
+    {
+        int result = Memory.getRegister(address) + 255; // Allow underflow
+
+        checkZeroBit(result);
+
+        writeToDestination(destinationBit, address, result % 256); // Catch underflow
+        ProgramCounter.incPC();
+    }
+}

de/darkress/pic16f84sim/decoder/CommandDecoder.java

@@ -15,8 +15,7 @@ public class CommandDecoder
             case 0x900:
                 return new Comf(input);
             case 0x300:
-                //decf();
+                return new Decf(input);
-                break;
             case 0xB00:
                 //decfsz();
                 break;