diff --git a/src/Chip8.Core/Assembly/AsmException.cs b/src/Chip8.Core/Assembly/AsmException.cs
new file mode 100644
index 0000000..b55d58e
--- /dev/null
+++ b/src/Chip8.Core/Assembly/AsmException.cs
@@ -0,0 +1,16 @@
+using System;
+
+namespace Chip8.Core.Assembly;
+
+public sealed class AsmException : Exception
+{
+    public AsmException(string message, SourcePosition position)
+        : base("assembly error at " + position + ": " + message)
+    {
+        this.reason = message;
+        this.position = position;
+    }
+
+    public string reason { get; }
+    public SourcePosition position { get; }
+}
diff --git a/src/Chip8.Core/Assembly/Operand.cs b/src/Chip8.Core/Assembly/Operand.cs
new file mode 100644
index 0000000..fd8d256
--- /dev/null
+++ b/src/Chip8.Core/Assembly/Operand.cs
@@ -0,0 +1,56 @@
+using System.Collections.Generic;
+
+namespace Chip8.Core.Assembly;
+
+public enum OperandKind
+{
+    Register,
+    RegisterRange,
+    Immediate,
+    IndexRegister,
+    IndexIndirect,
+    DelayTimer,
+    SoundTimer,
+    KeyReg,
+    FontReg,
+    BigFontReg,
+    BcdReg,
+    RplReg,
+}
+
+public sealed class Operand
+{
+    Operand(OperandKind kind, SourcePosition position)
+    {
+        this.kind = kind;
+        this.position = position;
+    }
+
+    public OperandKind kind { get; }
+    public SourcePosition position { get; }
+    public int registerIndex { get; private init; }
+    public int rangeEnd { get; private init; }
+    public IReadOnlyList<Token> expressionTokens { get; private init; } = System.Array.Empty<Token>();
+
+    public static Operand register(int index, SourcePosition position) =>
+        new(OperandKind.Register, position) { registerIndex = index };
+
+    public static Operand registerRange(int start, int end, SourcePosition position) =>
+        new(OperandKind.RegisterRange, position) { registerIndex = start, rangeEnd = end };
+
+    public static Operand immediate(IReadOnlyList<Token> tokens, SourcePosition position) =>
+        new(OperandKind.Immediate, position) { expressionTokens = tokens };
+
+    public static Operand special(OperandKind kind, SourcePosition position) => new(kind, position);
+
+    public bool isRegister(int index) => kind == OperandKind.Register && registerIndex == index;
+
+    public override string ToString() => kind switch
+    {
+        OperandKind.Register => "V" + registerIndex.ToString("X1"),
+        OperandKind.RegisterRange => "V" + registerIndex.ToString("X1") + "-V" + rangeEnd.ToString("X1"),
+        OperandKind.Immediate => "imm",
+        OperandKind.IndexIndirect => "[I]",
+        _ => kind.ToString(),
+    };
+}
diff --git a/src/Chip8.Core/Assembly/Statement.cs b/src/Chip8.Core/Assembly/Statement.cs
new file mode 100644
index 0000000..3405827
--- /dev/null
+++ b/src/Chip8.Core/Assembly/Statement.cs
@@ -0,0 +1,79 @@
+using System.Collections.Generic;
+
+namespace Chip8.Core.Assembly;
+
+public abstract class Statement
+{
+    protected Statement(SourcePosition position)
+    {
+        this.position = position;
+    }
+
+    public SourcePosition position { get; }
+}
+
+public sealed class LabelStatement : Statement
+{
+    public LabelStatement(string name, SourcePosition position) : base(position)
+    {
+        this.name = name;
+    }
+
+    public string name { get; }
+}
+
+public sealed class ConstantStatement : Statement
+{
+    public ConstantStatement(string name, IReadOnlyList<Token> expression, SourcePosition position) : base(position)
+    {
+        this.name = name;
+        this.expression = expression;
+    }
+
+    public string name { get; }
+    public IReadOnlyList<Token> expression { get; }
+}
+
+public sealed class OrgStatement : Statement
+{
+    public OrgStatement(IReadOnlyList<Token> expression, SourcePosition position) : base(position)
+    {
+        this.expression = expression;
+    }
+
+    public IReadOnlyList<Token> expression { get; }
+}
+
+public sealed class AlignStatement : Statement
+{
+    public AlignStatement(IReadOnlyList<Token> expression, SourcePosition position) : base(position)
+    {
+        this.expression = expression;
+    }
+
+    public IReadOnlyList<Token> expression { get; }
+}
+
+public sealed class DataStatement : Statement
+{
+    public DataStatement(bool word, IReadOnlyList<IReadOnlyList<Token>> items, SourcePosition position) : base(position)
+    {
+        this.word = word;
+        this.items = items;
+    }
+
+    public bool word { get; }
+    public IReadOnlyList<IReadOnlyList<Token>> items { get; }
+}
+
+public sealed class InstructionStatement : Statement
+{
+    public InstructionStatement(string mnemonic, IReadOnlyList<Operand> operands, SourcePosition position) : base(position)
+    {
+        this.mnemonic = mnemonic;
+        this.operands = operands;
+    }
+
+    public string mnemonic { get; }
+    public IReadOnlyList<Operand> operands { get; }
+}
diff --git a/src/Chip8.Core/Assembly/Token.cs b/src/Chip8.Core/Assembly/Token.cs
new file mode 100644
index 0000000..28365aa
--- /dev/null
+++ b/src/Chip8.Core/Assembly/Token.cs
@@ -0,0 +1,61 @@
+namespace Chip8.Core.Assembly;
+
+public enum TokenKind
+{
+    Identifier,
+    Number,
+    Comma,
+    Colon,
+    Minus,
+    Plus,
+    Star,
+    Slash,
+    Percent,
+    Ampersand,
+    Pipe,
+    Caret,
+    Tilde,
+    ShiftLeft,
+    ShiftRight,
+    LeftParen,
+    RightParen,
+    LeftBracket,
+    RightBracket,
+    EndOfLine,
+    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 Token
+{
+    public Token(TokenKind kind, string text, int value, SourcePosition position)
+    {
+        this.kind = kind;
+        this.text = text;
+        this.value = value;
+        this.position = position;
+    }
+
+    public TokenKind kind { get; }
+    public string text { get; }
+    public int value { get; }
+    public SourcePosition position { get; }
+
+    public bool isIdentifier(string name) =>
+        kind == TokenKind.Identifier && string.Equals(text, name, System.StringComparison.OrdinalIgnoreCase);
+
+    public override string ToString() => kind + "('" + text + "')";
+}