diff --git a/EightBit/Processor.cs b/EightBit/Processor.cs
index c8fe80d..d3bb3b9 100644
--- a/EightBit/Processor.cs
+++ b/EightBit/Processor.cs
@@ -92,10 +92,7 @@ namespace EightBit
             return current;
         }
 
-        protected virtual byte FetchInstruction()
-        {
-            return this.FetchByte();
-        }
+        protected virtual byte FetchInstruction() => this.FetchByte();
 
         public void Execute(byte value)
         {
@@ -219,19 +216,13 @@ namespace EightBit
 
         protected abstract void GetInto(Register16 into);
 
-        protected virtual void GetWord()
-        {
-            this.GetInto(this.Intermediate);
-        }
+        protected virtual void GetWord() => this.GetInto(this.Intermediate);
 
         protected abstract void SetWord(Register16 value);
 
         protected abstract void GetPagedInto(Register16 into);
 
-        protected virtual void GetWordPaged()
-        {
-            this.GetPagedInto(this.Intermediate);
-        }
+        protected virtual void GetWordPaged() => this.GetPagedInto(this.Intermediate);
 
         protected void GetWordPaged(byte page, byte offset)
         {
@@ -262,10 +253,7 @@ namespace EightBit
 
         protected abstract void FetchInto(Register16 into);
 
-        protected void FetchWord()
-        {
-            this.FetchInto(this.Intermediate);
-        }
+        protected void FetchWord() => this.FetchInto(this.Intermediate);
 
         protected void FetchWordAddress()
         {
diff --git a/M6502/Core.cs b/M6502/Core.cs
index fc20c97..6bdde2c 100644
--- a/M6502/Core.cs
+++ b/M6502/Core.cs
@@ -554,14 +554,19 @@ namespace M6502
 
         protected sealed override void BusWrite()
         {
-            this.Tick();
-            this.WriteToBus();
+            this.OnWritingMemory();
+                this.Tick();
+                this.WriteToBus();
+            this.OnWrittenMemory();
         }
 
         protected sealed override byte BusRead()
         {
-            this.Tick();
-            return this.ReadFromBus();
+            this.OnReadingMemory();
+                this.Tick();
+                this.ReadFromBus();
+            this.OnReadMemory();
+            return this.Bus.Data;
         }
 
         private byte ReadFromBus()
@@ -1060,7 +1065,7 @@ namespace M6502
 
         #region Increment/decrement
 
-        private void DEX() =>this.X = this.DEC(this.X);
+        private void DEX() => this.X = this.DEC(this.X);
 
         private void DEY() => this.Y = this.DEC(this.Y);
 
diff --git a/MC6809/MC6809.cs b/MC6809/MC6809.cs
index f38b16a..0c0ee52 100644
--- a/MC6809/MC6809.cs
+++ b/MC6809/MC6809.cs
@@ -573,16 +573,21 @@ namespace MC6809
 
         protected override void BusWrite()
         {
-            this.Tick();
-            this.LowerRW();
-            base.BusWrite();
+            this.OnWritingMemory();
+                this.Tick();
+                this.LowerRW();
+                base.BusWrite();
+            this.OnWrittenMemory();
         }
 
         protected override byte BusRead()
         {
-            this.Tick();
-            this.RaiseRW();
-            return base.BusRead();
+            this.OnReadingMemory();
+                this.Tick();
+                this.RaiseRW();
+                base.BusRead();
+            this.OnReadMemory();
+            return this.Bus.Data;
         }
 
         #endregion
@@ -1262,7 +1267,7 @@ namespace MC6809
             }
             else
             {
-                this.Execute(this.FetchByte());
+                this.Execute(this.FetchInstruction());
             }
         }
 
@@ -2017,7 +2022,7 @@ namespace MC6809
         private byte Negate(byte operand)
         {
             this.CC = SetBit(this.CC, StatusBits.VF, operand == (byte)Bits.Bit7);
-            this.Intermediate.Word = (ushort)(0 - operand);
+            this.Intermediate.Word = (ushort)(~operand + 1);
             operand = this.Intermediate.Low;
             this.CC = this.AdjustNZ(operand);
             this.CC = this.AdjustCarry(this.Intermediate);
@@ -2126,8 +2131,8 @@ namespace MC6809
         private void SWI()
         {
             this.SaveEntireRegisterState();
-            this.CC = SetBit(this.CC, StatusBits.IF);  // Disable IRQ
-            this.CC = SetBit(this.CC, StatusBits.FF);  // Disable FIRQ
+            this.CC = SetBit(this.CC, StatusBits.IF | StatusBits.FF);  // Disable IRQ / FIRQ
+            Debug.Assert(this.FastInterruptMasked && this.InterruptMasked);
             this.SwallowRead();
             this.GetPagedInto(0xff, SWI_vector, this.EA);
             this.Jump(this.EA);
@@ -2202,13 +2207,13 @@ namespace MC6809
         private void Prefix10()
         {
             this._prefix10 = true;
-            this.Execute(this.FetchByte());
+            this.Execute(this.FetchInstruction());
         }
 
         private void Prefix11()
         {
             this._prefix11 = true;
-            this.Execute(this.FetchByte());
+            this.Execute(this.FetchInstruction());
         }
 
         #endregion