Handle special cases for CP0 register writes.

- The timer interrupt is cleared on writes to the Compare register (see: #6.3.4).
- Only software interrupt bits are writable into the Cause register (see: #6.3.6).

DotN64/CPU/VR4300/CP0/VR4300.SystemControlUnit.CauseRegister.cs

@@ -21,6 +21,7 @@ namespace DotN64.CPU
                                                             ce = BitVector32.CreateSection((1 << 2) - 1, constant2),
                                                             constant3 = BitVector32.CreateSection(1, ce),
                                                             bd = BitVector32.CreateSection(1, constant3);
+                public static readonly ulong WriteMask = (ulong)(InterruptPending.WriteMask << ip.Offset);
                 #endregion
 
                 #region Properties
@@ -80,9 +81,10 @@ namespace DotN64.CPU
                     #region Fields
                     private BitVector32 bits;
 
-                    private static BitVector32.Section softwareInterrupts = BitVector32.CreateSection((1 << 2) - 1),
+                    private static readonly BitVector32.Section softwareInterrupts = BitVector32.CreateSection((1 << 2) - 1),
                     externalNormalInterrupts = BitVector32.CreateSection((1 << 5) - 1, softwareInterrupts),
                     timerInterrupt = BitVector32.CreateSection(1, externalNormalInterrupts);
+                    public static readonly byte WriteMask = (byte)(softwareInterrupts.Mask << softwareInterrupts.Offset);
                     #endregion
 
                     #region Properties

DotN64/CPU/VR4300/CP0/VR4300.SystemControlUnit.cs

@@ -34,7 +34,26 @@ namespace DotN64.CPU
                 Cause = new CauseRegister(this);
                 operations = new Dictionary<Instruction, Action<Instruction>>
                 {
-                    [From(OpCode.MT)] = i => Registers[i.RD] = cpu.GPR[i.RT],
+                    [From(OpCode.MT)] = i =>
+                    {
+                        var destination = i.RD;
+                        var data = cpu.GPR[i.RT];
+
+                        switch ((RegisterIndex)destination)
+                        {
+                            case RegisterIndex.Cause:
+                                Registers[destination] &= ~CauseRegister.WriteMask;
+                                Registers[destination] |= data & CauseRegister.WriteMask;
+                                return;
+                            case RegisterIndex.Compare:
+                                var ip = Cause.IP;
+                                ip.TimerInterrupt = false;
+                                Cause.IP = ip;
+                                break;
+                        }
+
+                        Registers[destination] = data;
+                    },
                     [From(OpCode.MF)] = i => cpu.GPR[i.RT] = (ulong)(int)Registers[i.RD],
                     [From(FunctOpCode.TLBWI)] = i => { /* TODO. */ },
                     [From(FunctOpCode.ERET)] = i =>