diff --git a/src/Chip8.OctoAsm/Chip8.OctoAsm.csproj b/src/Chip8.OctoAsm/Chip8.OctoAsm.csproj
new file mode 100644
index 0000000..1ded6a1
--- /dev/null
+++ b/src/Chip8.OctoAsm/Chip8.OctoAsm.csproj
@@ -0,0 +1,16 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net6.0</TargetFramework>
+    <ImplicitUsings>disable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <RootNamespace>Chip8.OctoAsm</RootNamespace>
+    <AssemblyName>Chip8.OctoAsm</AssemblyName>
+    <LangVersion>10</LangVersion>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Chip8.Core\Chip8.Core.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/src/Chip8.OctoAsm/OctoDiagnostic.cs b/src/Chip8.OctoAsm/OctoDiagnostic.cs
new file mode 100644
index 0000000..9cc8951
--- /dev/null
+++ b/src/Chip8.OctoAsm/OctoDiagnostic.cs
@@ -0,0 +1,57 @@
+using System;
+using System.Collections.Generic;
+
+namespace Chip8.OctoAsm;
+
+public enum OctoSeverity
+{
+    Error,
+    Warning,
+}
+
+public readonly struct OctoDiagnostic
+{
+    public OctoDiagnostic(OctoSeverity severity, string message, SourcePosition position)
+    {
+        this.severity = severity;
+        this.message = message;
+        this.position = position;
+    }
+
+    public OctoSeverity severity { get; }
+    public string message { get; }
+    public SourcePosition position { get; }
+
+    public int line => position.line;
+    public int column => position.column;
+
+    public override string ToString() =>
+        severity.ToString().ToLowerInvariant() + " at " + position + ": " + message;
+}
+
+public sealed class OctoCompileException : Exception
+{
+    public OctoCompileException(OctoDiagnostic diagnostic)
+        : this(new[] { diagnostic })
+    {
+    }
+
+    public OctoCompileException(IReadOnlyList<OctoDiagnostic> diagnostics)
+        : base(describe(diagnostics))
+    {
+        this.diagnostics = diagnostics;
+    }
+
+    public IReadOnlyList<OctoDiagnostic> diagnostics { get; }
+
+    public OctoDiagnostic diagnostic => diagnostics[0];
+
+    static string describe(IReadOnlyList<OctoDiagnostic> diagnostics)
+    {
+        if (diagnostics.Count == 1)
+        {
+            return diagnostics[0].ToString();
+        }
+        return diagnostics.Count + " Octo compile errors; first: " + diagnostics[0];
+    }
+}
diff --git a/src/Chip8.OctoAsm/OctoToken.cs b/src/Chip8.OctoAsm/OctoToken.cs
new file mode 100644
index 0000000..990eb13
--- /dev/null
+++ b/src/Chip8.OctoAsm/OctoToken.cs
@@ -0,0 +1,51 @@
+namespace Chip8.OctoAsm;
+
+public enum OctoTokenKind
+{
+    Word,
+    Number,
+    OpenBrace,
+    CloseBrace,
+    OpenParen,
+    CloseParen,
+    EndOfFile,
+}
+
+public readonly struct SourcePosition
+{
+    public SourcePosition(int line, int column)
+    {
+        this.line = line;
+        this.column = column;
+    }
+
+    public int line { get; }
+    public int column { get; }
+
+    public override string ToString() => "line " + line + ", col " + column;
+}
+
+public readonly struct OctoToken
+{
+    public OctoToken(OctoTokenKind kind, string text, int value, SourcePosition position)
+    {
+        this.kind = kind;
+        this.text = text;
+        this.value = value;
+        this.position = position;
+    }
+
+    public OctoTokenKind kind { get; }
+    public string text { get; }
+    public int value { get; }
+    public SourcePosition position { get; }
+
+    public bool isWord(string word) => kind == OctoTokenKind.Word && text == word;
+
+    public override string ToString() => kind switch
+    {
+        OctoTokenKind.EndOfFile => "<eof>",
+        OctoTokenKind.Number => "number(" + value + ")",
+        _ => "'" + text + "'",
+    };
+}