diff --git a/EightBit/MemoryMapping.cs b/EightBit/MemoryMapping.cs
index da5fd5e..0a92995 100644
--- a/EightBit/MemoryMapping.cs
+++ b/EightBit/MemoryMapping.cs
@@ -4,21 +4,43 @@
 
 namespace EightBit
 {
-    public sealed class MemoryMapping(Memory memory, ushort begin, ushort mask, AccessLevel access)
+    public sealed class MemoryMapping
     {
+        public MemoryMapping(Memory memory, ushort begin, ushort mask, AccessLevel access)
+        {
+            this._memory = memory;
+            this._begin = begin;
+            this._mask = mask;
+            this._access = access;
+            for (int i = ushort.MinValue; i < ushort.MaxValue + 1; ++i)
+            {
+                System.Diagnostics.Debug.Assert(i >= ushort.MinValue);
+                System.Diagnostics.Debug.Assert(i <= ushort.MaxValue);
+                this._offsets[i] = this.CalculateOffset((ushort)i);
+            }
+        }
+
         public MemoryMapping(Memory memory, ushort begin, Mask mask, AccessLevel access)
         : this(memory, begin, (ushort)mask, access)
         {
         }
 
-        public Memory Memory { get; set; } = memory;
+        private readonly int[] _offsets = new int[ushort.MaxValue + 1];
+        private readonly Memory _memory;
+        private readonly ushort _begin;
+        private readonly ushort _mask;
+        private readonly AccessLevel _access;
 
-        public ushort Begin { get; set; } = begin;
+        public Memory Memory => this._memory;
 
-        public ushort Mask { get; set; } = mask;
+        public ushort Begin => this._begin;
 
-        public AccessLevel Access { get; set; } = access;
+        public ushort Mask => this._mask;
 
-        public int Offset(ushort absolute) => (absolute - this.Begin) & this.Mask;
+        public AccessLevel Access => this._access;
+
+        private int CalculateOffset(ushort absolute) => (absolute - this.Begin) & this.Mask;
+
+        public int Offset(ushort absolute) => this._offsets[absolute];
     }
 }
diff --git a/EightBit/Register16.cs b/EightBit/Register16.cs
index 098fb08..0c31602 100644
--- a/EightBit/Register16.cs
+++ b/EightBit/Register16.cs
@@ -89,7 +89,8 @@ namespace EightBit
 
         public void Assign(Register16 from)
         {
-            this.Assign(from.Low, from.High);
+            this.Low = from.Low;
+            this.High = from.High;
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
diff --git a/M6502/Core.cs b/M6502/Core.cs
index 80aa131..b05be1a 100644
--- a/M6502/Core.cs
+++ b/M6502/Core.cs
@@ -972,11 +972,11 @@ namespace M6502
 
         private void JSR()
         {
-            this.Intermediate.Low = this.FetchByte();
+            var low = this.FetchByte();
             this.SwallowPop();
             this.PushWord(this.PC);
-            this.PC.High = this.FetchByte();
-            this.PC.Low = this.Intermediate.Low;
+            var high = this.FetchByte();
+            this.PC.Assign(low, high);
         }
 
         private void PHP() => this.Push(SetBit(this.P, StatusBits.BF));