diff --git a/EightBit/BigEndianProcessor.cs b/EightBit/BigEndianProcessor.cs
index 2a00b7d..be5f638 100644
--- a/EightBit/BigEndianProcessor.cs
+++ b/EightBit/BigEndianProcessor.cs
@@ -1,6 +1,9 @@
 // <copyright file="BigEndianProcessor.cs" company="Adrian Conlon">
 // Copyright (c) Adrian Conlon. All rights reserved.
 // </copyright>
+
+using System.Diagnostics;
+
 namespace EightBit
 {
     public abstract class BigEndianProcessor(Bus memory) : Processor(memory)
@@ -14,14 +17,14 @@ namespace EightBit
 
         public override void PokeShort(ushort address, Register16 value)
         {
-            //ArgumentNullException.ThrowIfNull(value);
+            Debug.Assert(value is not null, "value cannot be null");
             this.Bus.Poke(address, value.High);
             this.Bus.Poke(++address, value.Low);
         }
 
         protected override void FetchInto(Register16 into)
         {
-            //ArgumentNullException.ThrowIfNull(into);
+            Debug.Assert(into is not null, "into cannot be null");
             this.FetchByte();
             into.High = this.Bus.Data;
             this.FetchByte();
@@ -30,7 +33,7 @@ namespace EightBit
 
         protected override void GetInto(Register16 into)
         {
-            //ArgumentNullException.ThrowIfNull(into);
+            Debug.Assert(into is not null, "into cannot be null");
             this.MemoryRead();
             into.High = this.Bus.Data;
             this.Bus.Address.Increment();
@@ -40,7 +43,7 @@ namespace EightBit
 
         protected override void GetPagedInto(Register16 into)
         {
-            //ArgumentNullException.ThrowIfNull(into);
+            Debug.Assert(into is not null, "into cannot be null");
             this.MemoryRead();
             into.High = this.Bus.Data;
             ++this.Bus.Address.Low;
@@ -50,7 +53,7 @@ namespace EightBit
 
         protected override void PopInto(Register16 into)
         {
-            //ArgumentNullException.ThrowIfNull(into);
+            Debug.Assert(into is not null, "into cannot be null");
             this.Pop();
             into.High = this.Bus.Data;
             this.Pop();
@@ -59,14 +62,14 @@ namespace EightBit
 
         protected override void PushShort(Register16 value)
         {
-            //ArgumentNullException.ThrowIfNull(value);
+            Debug.Assert(value is not null, "value cannot be null");
             this.Push(value.Low);
             this.Push(value.High);
         }
 
         protected override void SetShort(Register16 value)
         {
-            //ArgumentNullException.ThrowIfNull(value);
+            Debug.Assert(value is not null, "value cannot be null");
             this.MemoryWrite(value.High);
             this.Bus.Address.Increment();
             this.MemoryWrite(value.Low);
@@ -74,7 +77,7 @@ namespace EightBit
 
         protected override void SetPaged(Register16 value)
         {
-            //ArgumentNullException.ThrowIfNull(value);
+            Debug.Assert(value is not null, "value cannot be null");
             this.MemoryWrite(value.High);
             ++this.Bus.Address.Low;
             this.MemoryWrite(value.Low);
diff --git a/EightBit/Bus.cs b/EightBit/Bus.cs
index a6d4677..5832b44 100644
--- a/EightBit/Bus.cs
+++ b/EightBit/Bus.cs
@@ -31,7 +31,7 @@ namespace EightBit
 
         public byte Peek(Register16 absolute)
         {
-            ArgumentNullException.ThrowIfNull(absolute);
+            Debug.Assert(absolute is not null, "absolute cannot be null");
             return this.Peek(absolute.Joined);
         }
 
@@ -41,7 +41,7 @@ namespace EightBit
 
         public void Poke(Register16 absolute, byte value)
         {
-            ArgumentNullException.ThrowIfNull(absolute);
+            Debug.Assert(absolute is not null, "absolute cannot be null");
             this.Poke(absolute.Joined, value);
         }
 
diff --git a/EightBit/InputOutput.cs b/EightBit/InputOutput.cs
index 8b73279..e8e92bc 100644
--- a/EightBit/InputOutput.cs
+++ b/EightBit/InputOutput.cs
@@ -2,6 +2,8 @@
 // Copyright (c) Adrian Conlon. All rights reserved.
 // </copyright>
 
+using System.Diagnostics;
+
 namespace EightBit
 {
     public sealed class InputOutput
@@ -22,7 +24,7 @@ namespace EightBit
 
         public byte ReadInputPort(Register16 port)
         {
-            ArgumentNullException.ThrowIfNull(port);
+            Debug.Assert(port is not null, "port cannot be null");
             ReadingPort?.Invoke(this, new PortEventArgs(port));
             var value = this._input[port.Joined];
             ReadPort?.Invoke(this, new PortEventArgs(port));
@@ -33,7 +35,7 @@ namespace EightBit
 
         public void WriteInputPort(Register16 port, byte value)
         {
-            ArgumentNullException.ThrowIfNull(port);
+            Debug.Assert(port is not null, "port cannot be null");
             this.WriteInputPort(port.Joined, value);
         }
 
@@ -41,7 +43,7 @@ namespace EightBit
 
         public void WriteOutputPort(Register16 port, byte value)
         {
-            ArgumentNullException.ThrowIfNull(port);
+            Debug.Assert(port is not null, "port cannot be null");
             WritingPort?.Invoke(this, new PortEventArgs(port));
             this._output[port.Joined] = value;
             WrittenPort?.Invoke(this, new PortEventArgs(port));
@@ -51,7 +53,7 @@ namespace EightBit
 
         public byte ReadOutputPort(Register16 port)
         {
-            ArgumentNullException.ThrowIfNull(port);
+            Debug.Assert(port is not null, "port cannot be null");
             return this.ReadOutputPort(port.Joined);
         }
     }
diff --git a/EightBit/LittleEndianProcessor.cs b/EightBit/LittleEndianProcessor.cs
index 4ad6497..a49e7b3 100644
--- a/EightBit/LittleEndianProcessor.cs
+++ b/EightBit/LittleEndianProcessor.cs
@@ -2,6 +2,8 @@
 // Copyright (c) Adrian Conlon. All rights reserved.
 // </copyright>
 
+using System.Diagnostics;
+
 namespace EightBit
 {
     public abstract class LittleEndianProcessor(Bus memory) : Processor(memory)
@@ -15,14 +17,14 @@ namespace EightBit
 
         public override void PokeShort(ushort address, Register16 value)
         {
-            //ArgumentNullException.ThrowIfNull(value);
+            Debug.Assert(value is not null, "value cannot be null");
             this.Bus.Poke(address, value.Low);
             this.Bus.Poke(++address, value.High);
         }
 
         protected override void FetchInto(Register16 into)
         {
-            //ArgumentNullException.ThrowIfNull(into);
+            Debug.Assert(into is not null, "into cannot be null");
             this.FetchByte();
             into.Low = this.Bus.Data;
             this.FetchByte();
@@ -31,7 +33,7 @@ namespace EightBit
 
         protected override void GetInto(Register16 into)
         {
-            //ArgumentNullException.ThrowIfNull(into);
+            Debug.Assert(into is not null, "into cannot be null");
             this.MemoryRead();
             into.Low = this.Bus.Data;
             this.Bus.Address.Increment();
@@ -41,7 +43,7 @@ namespace EightBit
 
         protected override void GetPagedInto(Register16 into)
         {
-            //ArgumentNullException.ThrowIfNull(into);
+            Debug.Assert(into is not null, "into cannot be null");
             this.MemoryRead();
             into.Low = this.Bus.Data;
             ++this.Bus.Address.Low;
@@ -51,7 +53,7 @@ namespace EightBit
 
         protected override void PopInto(Register16 into)
         {
-            //ArgumentNullException.ThrowIfNull(into);
+            Debug.Assert(into is not null, "into cannot be null");
             this.Pop();
             into.Low = this.Bus.Data;
             this.Pop();
@@ -60,14 +62,14 @@ namespace EightBit
 
         protected override void PushShort(Register16 value)
         {
-            //ArgumentNullException.ThrowIfNull(value);
+            Debug.Assert(value is not null, "value cannot be null");
             this.Push(value.High);
             this.Push(value.Low);
         }
 
         protected override void SetShort(Register16 value)
         {
-            //ArgumentNullException.ThrowIfNull(value);
+            Debug.Assert(value is not null, "value cannot be null");
             this.MemoryWrite(value.Low);
             this.Bus.Address.Increment();
             this.MemoryWrite(value.High);
@@ -75,7 +77,7 @@ namespace EightBit
 
         protected override void SetPaged(Register16 value)
         {
-            //ArgumentNullException.ThrowIfNull(value);
+            Debug.Assert(value is not null, "value cannot be null");
             this.MemoryWrite(value.Low);
             ++this.Bus.Address.Low;
             this.MemoryWrite(value.High);
diff --git a/EightBit/Processor.cs b/EightBit/Processor.cs
index e6795d1..2f725bc 100644
--- a/EightBit/Processor.cs
+++ b/EightBit/Processor.cs
@@ -2,6 +2,8 @@
 // Copyright (c) Adrian Conlon. All rights reserved.
 // </copyright>
 
+using System.Diagnostics;
+
 namespace EightBit
 {
     public abstract class Processor(Bus memory) : ClockedChip
@@ -236,7 +238,7 @@ namespace EightBit
 
         protected void GetPagedInto(byte page, byte offset, Register16 into)
         {
-            ArgumentNullException.ThrowIfNull(into);
+            Debug.Assert(into is not null, "into cannot be null");
             this.Bus.Address.Assign(offset, page);
             this.GetPagedInto(into);
         }
diff --git a/EightBit/Register16.cs b/EightBit/Register16.cs
index 2eafc81..d4d2763 100644
--- a/EightBit/Register16.cs
+++ b/EightBit/Register16.cs
@@ -2,6 +2,8 @@
 // Copyright (c) Adrian Conlon. All rights reserved.
 // </copyright>
 
+using System.Diagnostics;
+
 namespace EightBit
 {
     public sealed class Register16
@@ -38,7 +40,7 @@ namespace EightBit
 
         public Register16(Register16 rhs)
         {
-            //ArgumentNullException.ThrowIfNull(rhs);
+            Debug.Assert(rhs is not null, "rhs cannot be null");
             this.Low = rhs.Low;
             this.High = rhs.High;
         }
@@ -59,7 +61,7 @@ namespace EightBit
 
         public static bool operator ==(Register16 left, Register16 right)
         {
-            //ArgumentNullException.ThrowIfNull(left);
+            Debug.Assert(left is not null, "left cannot be null");
             return left.Equals(right);
         }
 
@@ -83,7 +85,7 @@ namespace EightBit
 
         public void Assign(Register16 from)
         {
-            //ArgumentNullException.ThrowIfNull(from);
+            Debug.Assert(from is not null, "from cannot be null");
             this.Low = from.Low;
             this.High = from.High;
         }
diff --git a/LR35902/ObjectAttribute.cs b/LR35902/ObjectAttribute.cs
index 5dd9468..a901570 100644
--- a/LR35902/ObjectAttribute.cs
+++ b/LR35902/ObjectAttribute.cs
@@ -5,6 +5,7 @@
 namespace LR35902
 {
     using EightBit;
+    using System.Diagnostics;
 
     public class ObjectAttribute
     {
@@ -14,7 +15,7 @@ namespace LR35902
 
         public ObjectAttribute(Ram ram, ushort address)
         {
-            ArgumentNullException.ThrowIfNull(ram);
+            Debug.Assert(ram is not null, "ram cannot be null");
             this.PositionY = ram.Peek(address);
             this.PositionX = ram.Peek(++address);
             this.Pattern = ram.Peek(++address);
diff --git a/LR35902/SM83.HarteTest/Checker.cs b/LR35902/SM83.HarteTest/Checker.cs
index 67b18db..cb95f75 100644
--- a/LR35902/SM83.HarteTest/Checker.cs
+++ b/LR35902/SM83.HarteTest/Checker.cs
@@ -119,8 +119,8 @@
 
         private bool Check(string what, string? expected, string? actual)
         {
-            ArgumentNullException.ThrowIfNull(expected);
-            ArgumentNullException.ThrowIfNull(actual);
+            Debug.Assert(expected is not null, "expected cannot be null");
+            Debug.Assert(actual is not null, "actual cannot be null");
             var success = actual == expected;
             if (!success)
             {
@@ -341,7 +341,7 @@
 
         private void DumpCycle(ushort address, byte? value, string? action)
         {
-            ArgumentNullException.ThrowIfNull(action);
+            Debug.Assert(action is not null, "action cannot be null");
             this.Messages.Add($"Address: {address:X4}, value: {value:X2}, action: {action}");
         }
 
@@ -349,7 +349,7 @@
 
         private void DumpCycles(IEnumerable<Cycle>? cycles)
         {
-            ArgumentNullException.ThrowIfNull(cycles);
+            Debug.Assert(cycles is not null, "cycles cannot be null");
             foreach (var cycle in cycles)
             {
                 this.DumpCycle(cycle);
diff --git a/LR35902/SM83.HarteTest/Cycle.cs b/LR35902/SM83.HarteTest/Cycle.cs
index c1cbb4e..4fc56be 100644
--- a/LR35902/SM83.HarteTest/Cycle.cs
+++ b/LR35902/SM83.HarteTest/Cycle.cs
@@ -19,7 +19,7 @@ namespace SM83.HarteTest
 
         public Cycle(List<object> input)
         {
-            ArgumentNullException.ThrowIfNull(input);
+            Debug.Assert(input is not null, "input cannot be null");
 
             if (input.Count != 3)
             {
diff --git a/M6502/M6502.HarteTest/Checker.cs b/M6502/M6502.HarteTest/Checker.cs
index e1db043..53696b6 100644
--- a/M6502/M6502.HarteTest/Checker.cs
+++ b/M6502/M6502.HarteTest/Checker.cs
@@ -113,8 +113,8 @@
 
         private bool Check(string what, string? expected, string? actual)
         {
-            ArgumentNullException.ThrowIfNull(expected);
-            ArgumentNullException.ThrowIfNull(actual);
+            Debug.Assert(expected is not null, "expected cannot be null");
+            Debug.Assert(actual is not null, "actual cannot be null");
             var success = actual == expected;
             if (!success)
             {
@@ -292,7 +292,7 @@
 
         private void DumpCycle(ushort address, byte value, string? action)
         {
-            ArgumentNullException.ThrowIfNull(action);
+            Debug.Assert(action is not null, "action cannot be null");
             this.Messages.Add($"Address: {address:X4}, value: {value:X2}, action: {action}");
         }
 
@@ -300,7 +300,7 @@
 
         private void DumpCycles(IEnumerable<Cycle>? cycles)
         {
-            ArgumentNullException.ThrowIfNull(cycles);
+            Debug.Assert(cycles is not null, "cycles cannot be null");
             foreach (var cycle in cycles)
             {
                 this.DumpCycle(cycle);
diff --git a/M6502/M6502.HarteTest/Cycle.cs b/M6502/M6502.HarteTest/Cycle.cs
index 8b1cd4b..8a8ed9f 100644
--- a/M6502/M6502.HarteTest/Cycle.cs
+++ b/M6502/M6502.HarteTest/Cycle.cs
@@ -1,4 +1,6 @@
-namespace M6502.HarteTest
+using System.Diagnostics;
+
+namespace M6502.HarteTest
 {
     internal sealed class Cycle
     {
@@ -17,7 +19,7 @@
 
         public Cycle(List<object> input)
         {
-            ArgumentNullException.ThrowIfNull(input);
+            Debug.Assert(input is not null, "input cannot be null");
 
             if (input.Count != 3)
             {
diff --git a/Z80/Z80.HarteTest/Checker.cs b/Z80/Z80.HarteTest/Checker.cs
index 9ad2a5a..3541bdd 100644
--- a/Z80/Z80.HarteTest/Checker.cs
+++ b/Z80/Z80.HarteTest/Checker.cs
@@ -142,8 +142,8 @@
 
         private bool Check(string what, string? expected, string? actual)
         {
-            ArgumentNullException.ThrowIfNull(expected);
-            ArgumentNullException.ThrowIfNull(actual);
+            Debug.Assert(expected is not null, "expected cannot be null");
+            Debug.Assert(actual is not null, "actual cannot be null");
             var success = actual == expected;
             if (!success)
             {
@@ -414,7 +414,7 @@
 
         private static string ExpandCycle(string prefix, ushort address, byte? value, string? action)
         {
-            ArgumentNullException.ThrowIfNull(action);
+            Debug.Assert(action is not null, "action cannot be null");
             return value is null
                 ? $"{prefix}: Address: {address:X4},            action: {action}"
                 : $"{prefix}: Address: {address:X4}, value: {value:X2}, action: {action}";
diff --git a/Z80/Z80.HarteTest/Cycle.cs b/Z80/Z80.HarteTest/Cycle.cs
index 13722d0..9b11c92 100644
--- a/Z80/Z80.HarteTest/Cycle.cs
+++ b/Z80/Z80.HarteTest/Cycle.cs
@@ -22,7 +22,7 @@
 
         public Cycle(List<object> input)
         {
-            ArgumentNullException.ThrowIfNull(input);
+            Debug.Assert(input is not null, "input cannot be null");
 
             if (input.Count != 3)
             {
diff --git a/Z80/Z80.HarteTest/Port.cs b/Z80/Z80.HarteTest/Port.cs
index 2561862..1e89b87 100644
--- a/Z80/Z80.HarteTest/Port.cs
+++ b/Z80/Z80.HarteTest/Port.cs
@@ -1,4 +1,6 @@
-namespace Z80.HarteTest
+using System.Diagnostics;
+
+namespace Z80.HarteTest
 {
     // Cycle-by-cycle breakdown of bus activity
     internal sealed class Port
@@ -19,7 +21,7 @@
 
         public Port(List<object> input)
         {
-            ArgumentNullException.ThrowIfNull(input);
+            Debug.Assert(input is not null, "input cannot be null");
 
             if (input.Count != 3)
             {