Some small consistency tidyups.

EightBit/Bus.cs

@@ -87,16 +87,15 @@ namespace EightBit
         {
             var mapped = this.Mapping(absolute);
             var offset = (ushort)mapped.Offset(absolute);
-            if (mapped.Access == AccessLevel.ReadOnly)
+            if (mapped.Access != AccessLevel.ReadOnly)
             {
-                if (!this._writing)
-                {
-                    this.Data = mapped.Memory.Peek(offset);
-                }
-                return ref this._data;
+                return ref mapped.Memory.Reference(offset);
             }
-
-            return ref mapped.Memory.Reference(offset);
+            if (!this._writing)
+            {
+                this.Data = mapped.Memory.Peek(offset);
+            }
+            return ref this._data;
         }
 
         protected ref byte Reference() => ref this.Reference(this.Address.Joined);

EightBit/Processor.cs

@@ -186,7 +186,7 @@ namespace EightBit
 
         protected virtual void MemoryWrite() => this.BusWrite();
 
-        protected virtual void BusWrite() => this.Bus.Write();   // N.B. Should be the only real call into the "Bus.Write" code.
+        protected void BusWrite() => this.Bus.Write();   // N.B. Should be the only real call into the "Bus.Write" code.
 
         protected void MemoryRead(byte low, byte high)
         {
@@ -202,7 +202,7 @@ namespace EightBit
 
         protected virtual void MemoryRead() => this.BusRead();
 
-        protected virtual void BusRead() => this.Bus.Read();   // N.B. Should be the only real call into the "Bus.Read" code.
+        protected void BusRead() => this.Bus.Read();   // N.B. Should be the only real call into the "Bus.Read" code.
 
         protected virtual void IncrementPC() => this.PC.Increment();
 

EightBit/Register16.cs

@@ -75,7 +75,11 @@ namespace EightBit
 
         public override bool Equals(object? obj) => this.Equals(obj as Register16);
 
-        public bool Equals(Register16? rhs) => ReferenceEquals(this, rhs) || (rhs is not null && rhs.Low == this.Low && rhs.High == this.High);
+        public bool Equals(Register16? rhs)
+        {
+            Debug.Assert(rhs is not null, "rhs cannot be null");
+            return this.Joined == rhs.Joined;
+        }
 
         public void Assign(byte low, byte high = 0)
         {

M6502/Core.cs

@@ -554,7 +554,7 @@ namespace M6502
 
             // Can't use "FetchByte", since that would add an extra tick.
             this.ImmediateAddress();
-            this.ReadFromBus();
+            this.ReadFromMemory();
 
             System.Diagnostics.Debug.Assert(this.Cycles == 1, "BUS read has introduced stray cycles");
             this.RaiseSYNC();
@@ -566,32 +566,32 @@ namespace M6502
 
         #region Bus/Memory access
 
-        protected sealed override void BusWrite()
+        protected sealed override void MemoryWrite()
         {
             this.OnWritingMemory();
                 this.Tick();
-                this.WriteToBus();
+                this.WriteToMemory();
             this.OnWrittenMemory();
         }
 
-        protected sealed override void BusRead()
+        protected sealed override void MemoryRead()
         {
             this.OnReadingMemory();
                 this.Tick();
-                this.ReadFromBus();
+                this.ReadFromMemory();
             this.OnReadMemory();
         }
 
-        private void ReadFromBus()
+        private void ReadFromMemory()
         {
             this.RaiseRW();
-            base.BusRead();
+            base.MemoryRead();
         }
 
-        private void WriteToBus()
+        private void WriteToMemory()
         {
             this.LowerRW();
-            base.BusWrite();
+            base.MemoryWrite();
         }
 
         protected abstract void ModifyWrite(byte data);

MC6809/MC6809.cs

@@ -571,21 +571,21 @@ namespace MC6809
 
         #region Bus control
 
-        protected override void BusWrite()
+        protected override void MemoryWrite()
         {
             this.OnWritingMemory();
                 this.Tick();
                 this.LowerRW();
-                base.BusWrite();
+                base.MemoryWrite();
             this.OnWrittenMemory();
         }
 
-        protected override void BusRead()
+        protected override void MemoryRead()
         {
             this.OnReadingMemory();
                 this.Tick();
                 this.RaiseRW();
-                base.BusRead();
+                base.MemoryRead();
             this.OnReadMemory();
         }
 
@@ -772,7 +772,7 @@ namespace MC6809
             else
             {
                 // EA = ,R + 5-bit offset
-                this.EA.Joined = (ushort)(r.Joined + SignExtend(5, (byte)(type & (byte)Mask.Five)));
+                this.EA.Joined = (ushort)(r.Joined + SignExtend(5, type & (byte)Mask.Five));
                 this.SwallowCurrent();
                 this.SwallowRead();
             }