diff --git a/src/Chip8.OctoAsm/OctoConstEvaluator.cs b/src/Chip8.OctoAsm/OctoConstEvaluator.cs
new file mode 100644
index 0000000..de8a85a
--- /dev/null
+++ b/src/Chip8.OctoAsm/OctoConstEvaluator.cs
@@ -0,0 +1,362 @@
+using System;
+using System.Collections.Generic;
+
+namespace Chip8.OctoAsm;
+
+public sealed class OctoConstEvaluator
+{
+    readonly IReadOnlyList<OctoToken> tokens;
+    readonly int end;
+    readonly Func<OctoToken, double> resolveName;
+    int cursor;
+
+    OctoConstEvaluator(IReadOnlyList<OctoToken> tokens, int start, int end, Func<OctoToken, double> resolveName)
+    {
+        this.tokens = tokens;
+        this.cursor = start;
+        this.end = end;
+        this.resolveName = resolveName;
+    }
+
+    public static double evaluate(
+        IReadOnlyList<OctoToken> tokens, int start, int end, Func<OctoToken, double> resolveName, SourcePosition where)
+    {
+        if (start >= end)
+        {
+            throw error("expected an expression", where);
+        }
+
+        var evaluator = new OctoConstEvaluator(tokens, start, end, resolveName);
+        double value = evaluator.parseLogicalOr();
+        if (evaluator.cursor != end)
+        {
+            OctoToken leftover = evaluator.tokens[evaluator.cursor];
+            throw error("unexpected token " + leftover + " in expression", leftover.position);
+        }
+        return value;
+    }
+
+    double parseLogicalOr()
+    {
+        double value = parseLogicalAnd();
+        while (matchWord("||"))
+        {
+            double right = parseLogicalAnd();
+            value = truthy(value) || truthy(right) ? 1 : 0;
+        }
+        return value;
+    }
+
+    double parseLogicalAnd()
+    {
+        double value = parseEquality();
+        while (matchWord("&&"))
+        {
+            double right = parseEquality();
+            value = truthy(value) && truthy(right) ? 1 : 0;
+        }
+        return value;
+    }
+
+    double parseEquality()
+    {
+        double value = parseRelational();
+        while (true)
+        {
+            if (matchWord("=="))
+            {
+                value = value == parseRelational() ? 1 : 0;
+            }
+            else if (matchWord("!="))
+            {
+                value = value != parseRelational() ? 1 : 0;
+            }
+            else
+            {
+                return value;
+            }
+        }
+    }
+
+    double parseRelational()
+    {
+        double value = parseBitOr();
+        while (true)
+        {
+            if (matchWord("<="))
+            {
+                value = value <= parseBitOr() ? 1 : 0;
+            }
+            else if (matchWord(">="))
+            {
+                value = value >= parseBitOr() ? 1 : 0;
+            }
+            else if (matchWord("<"))
+            {
+                value = value < parseBitOr() ? 1 : 0;
+            }
+            else if (matchWord(">"))
+            {
+                value = value > parseBitOr() ? 1 : 0;
+            }
+            else
+            {
+                return value;
+            }
+        }
+    }
+
+    double parseBitOr()
+    {
+        double value = parseBitXor();
+        while (matchWord("|"))
+        {
+            value = toInt(value) | toInt(parseBitXor());
+        }
+        return value;
+    }
+
+    double parseBitXor()
+    {
+        double value = parseBitAnd();
+        while (matchWord("^"))
+        {
+            value = toInt(value) ^ toInt(parseBitAnd());
+        }
+        return value;
+    }
+
+    double parseBitAnd()
+    {
+        double value = parseShift();
+        while (matchWord("&"))
+        {
+            value = toInt(value) & toInt(parseShift());
+        }
+        return value;
+    }
+
+    double parseShift()
+    {
+        double value = parseAdditive();
+        while (true)
+        {
+            if (matchWord("<<"))
+            {
+                value = toInt(value) << (int)toInt(parseAdditive());
+            }
+            else if (matchWord(">>"))
+            {
+                value = toInt(value) >> (int)toInt(parseAdditive());
+            }
+            else
+            {
+                return value;
+            }
+        }
+    }
+
+    double parseAdditive()
+    {
+        double value = parseMultiplicative();
+        while (true)
+        {
+            if (matchWord("+"))
+            {
+                value += parseMultiplicative();
+            }
+            else if (matchWord("-"))
+            {
+                value -= parseMultiplicative();
+            }
+            else
+            {
+                return value;
+            }
+        }
+    }
+
+    double parseMultiplicative()
+    {
+        double value = parseMinMax();
+        while (true)
+        {
+            if (matchWord("*"))
+            {
+                value *= parseMinMax();
+            }
+            else if (matchWord("/"))
+            {
+                double divisor = parseMinMax();
+                if (divisor == 0)
+                {
+                    throw error("division by zero", previous().position);
+                }
+                value /= divisor;
+            }
+            else if (matchWord("%"))
+            {
+                long divisor = toInt(parseMinMax());
+                if (divisor == 0)
+                {
+                    throw error("modulo by zero", previous().position);
+                }
+                value = toInt(value) % divisor;
+            }
+            else
+            {
+                return value;
+            }
+        }
+    }
+
+    double parseMinMax()
+    {
+        double value = parsePower();
+        while (true)
+        {
+            if (matchWord("min"))
+            {
+                value = Math.Min(value, parsePower());
+            }
+            else if (matchWord("max"))
+            {
+                value = Math.Max(value, parsePower());
+            }
+            else
+            {
+                return value;
+            }
+        }
+    }
+
+    double parsePower()
+    {
+        double value = parseUnary();
+        if (matchWord("**") || matchWord("pow"))
+        {
+            return Math.Pow(value, parsePower());
+        }
+        return value;
+    }
+
+    double parseUnary()
+    {
+        if (matchWord("-"))
+        {
+            return -parseUnary();
+        }
+        if (matchWord("+"))
+        {
+            return parseUnary();
+        }
+        if (matchWord("~"))
+        {
+            return ~toInt(parseUnary());
+        }
+        if (matchWord("!"))
+        {
+            return truthy(parseUnary()) ? 0 : 1;
+        }
+        if (peekIsFunction(out string function))
+        {
+            advance();
+            return applyFunction(function, parseUnary());
+        }
+        return parsePrimary();
+    }
+
+    double parsePrimary()
+    {
+        OctoToken token = advance();
+
+        if (token.kind == OctoTokenKind.OpenParen)
+        {
+            double value = parseLogicalOr();
+            expect(OctoTokenKind.CloseParen);
+            return value;
+        }
+
+        if (token.kind == OctoTokenKind.Number)
+        {
+            return token.value;
+        }
+
+        if (token.kind == OctoTokenKind.Word)
+        {
+            return resolveName(token);
+        }
+
+        throw error("expected a value but found " + token, token.position);
+    }
+
+    static double applyFunction(string name, double argument) => name switch
+    {
+        "sqrt" => Math.Sqrt(argument),
+        "sin" => Math.Sin(argument),
+        "cos" => Math.Cos(argument),
+        "tan" => Math.Tan(argument),
+        "exp" => Math.Exp(argument),
+        "log" => Math.Log(argument),
+        "abs" => Math.Abs(argument),
+        "sign" => Math.Sign(argument),
+        "ceil" => Math.Ceiling(argument),
+        "floor" => Math.Floor(argument),
+        _ => throw new OctoCompileException(
+            new OctoDiagnostic(OctoSeverity.Error, "unknown function '" + name + "'", new SourcePosition(0, 0))),
+    };
+
+    static readonly HashSet<string> functionNames = new()
+    {
+        "sqrt", "sin", "cos", "tan", "exp", "log", "abs", "sign", "ceil", "floor",
+    };
+
+    bool peekIsFunction(out string function)
+    {
+        if (cursor < end && tokens[cursor].kind == OctoTokenKind.Word && functionNames.Contains(tokens[cursor].text))
+        {
+            function = tokens[cursor].text;
+            return true;
+        }
+        function = "";
+        return false;
+    }
+
+    bool matchWord(string word)
+    {
+        if (cursor < end && tokens[cursor].kind == OctoTokenKind.Word && tokens[cursor].text == word)
+        {
+            cursor++;
+            return true;
+        }
+        return false;
+    }
+
+    OctoToken advance()
+    {
+        if (cursor >= end)
+        {
+            throw error("unexpected end of expression", previous().position);
+        }
+        return tokens[cursor++];
+    }
+
+    void expect(OctoTokenKind kind)
+    {
+        if (cursor >= end || tokens[cursor].kind != kind)
+        {
+            SourcePosition where = cursor < end ? tokens[cursor].position : previous().position;
+            throw error("expected " + kind, where);
+        }
+        cursor++;
+    }
+
+    OctoToken previous() => tokens[cursor > 0 ? cursor - 1 : 0];
+
+    static long toInt(double value) => (long)value;
+
+    static bool truthy(double value) => value != 0;
+
+    static OctoCompileException error(string message, SourcePosition where) =>
+        new(new OctoDiagnostic(OctoSeverity.Error, message, where));
+}