diff --git a/src/Chip8.Replay/ConformanceRunner.cs b/src/Chip8.Replay/ConformanceRunner.cs
new file mode 100644
index 0000000..8964b20
--- /dev/null
+++ b/src/Chip8.Replay/ConformanceRunner.cs
@@ -0,0 +1,108 @@
+using System.Collections.Generic;
+using Chip8.Core;
+
+namespace Chip8.Replay;
+
+public sealed class ConformanceRunner
+{
+    readonly RomLibrary library;
+
+    public ConformanceRunner(RomLibrary library)
+    {
+        this.library = library;
+    }
+
+    public static ConformanceRunner locate() => new ConformanceRunner(RomLibrary.locate());
+
+    public RomLibrary roms => library;
+
+    public Machine execute(RomExpectation expectation, out int cyclesExecuted)
+    {
+        var machine = new Machine(QuirksCodec.clone(expectation.quirks), expectation.randomSeed, expectation.platform);
+        machine.loadRom(library.load(expectation.romName));
+
+        int cycle = 0;
+        for (; cycle < expectation.cycles && !machine.exited; cycle++)
+        {
+            machine.step();
+            if (cycle % expectation.instructionsPerFrame == expectation.instructionsPerFrame - 1)
+            {
+                machine.tickTimers();
+            }
+        }
+
+        cyclesExecuted = cycle;
+        return machine;
+    }
+
+    public TestRomResult run(RomExpectation expectation)
+    {
+        Machine machine = execute(expectation, out int cyclesExecuted);
+        FramebufferSnapshot snapshot = FramebufferSnapshot.capture(machine);
+        int litPixels = snapshot.litCount;
+        ulong framebufferHash = FramebufferHash.compute(snapshot);
+        ulong registerHash = FramebufferHash.computeWithRegisters(machine);
+
+        bool rangeOk = expectation.litPixelsInRange(litPixels);
+        bool hashOk = !expectation.framebufferHash.HasValue || expectation.framebufferHash.Value == framebufferHash;
+        bool passed = rangeOk && hashOk;
+
+        string detail = passed
+            ? expectation.description
+            : buildFailureDetail(expectation, litPixels, framebufferHash, rangeOk, hashOk);
+
+        return new TestRomResult(
+            expectation.romName,
+            passed,
+            litPixels,
+            expectation.minLitPixels,
+            expectation.maxLitPixels,
+            framebufferHash,
+            registerHash,
+            cyclesExecuted,
+            machine.exited,
+            detail);
+    }
+
+    public IReadOnlyList<TestRomResult> runAll(IEnumerable<RomExpectation> expectations)
+    {
+        var results = new List<TestRomResult>();
+        foreach (RomExpectation expectation in expectations)
+        {
+            results.Add(run(expectation));
+        }
+        return results;
+    }
+
+    public IReadOnlyList<TestRomResult> runBuiltIns() => runAll(builtInExpectations());
+
+    static string buildFailureDetail(RomExpectation expectation, int litPixels, ulong framebufferHash, bool rangeOk, bool hashOk)
+    {
+        if (!rangeOk)
+        {
+            return "lit " + litPixels + " outside [" + expectation.minLitPixels + ".." + expectation.maxLitPixels + "]";
+        }
+        if (!hashOk)
+        {
+            return "hash " + FramebufferHash.format(framebufferHash)
+                + " != expected " + FramebufferHash.format(expectation.framebufferHash!.Value);
+        }
+        return "unexpected failure";
+    }
+
+    public static IReadOnlyList<RomExpectation> builtInExpectations() => new List<RomExpectation>
+    {
+        new RomExpectation("2-ibm-logo.ch8", Platform.Chip8, Quirks.modern(),
+            cycles: 400, minLitPixels: 300, maxLitPixels: 340, description: "IBM logo"),
+        new RomExpectation("1-chip8-logo.ch8", Platform.Chip8, Quirks.modern(),
+            cycles: 400, minLitPixels: 400, maxLitPixels: 620, description: "CHIP-8 splash logo"),
+        new RomExpectation("3-corax+.ch8", Platform.Chip8, Quirks.modern(),
+            cycles: 4000, minLitPixels: 400, maxLitPixels: 1200, description: "corax+ opcode suite"),
+        new RomExpectation("4-flags.ch8", Platform.Chip8, Quirks.modern(),
+            cycles: 6000, minLitPixels: 400, maxLitPixels: 1400, description: "flags suite"),
+        new RomExpectation("test_opcode.ch8", Platform.Chip8, Quirks.modern(),
+            cycles: 4000, minLitPixels: 200, maxLitPixels: 1000, description: "opcode test grid"),
+        new RomExpectation("BC_test.ch8", Platform.Chip8, Quirks.cosmacVip(),
+            cycles: 4000, minLitPixels: 80, maxLitPixels: 900, description: "BestCoder test"),
+    };
+}
diff --git a/src/Chip8.Replay/ReplayOutcome.cs b/src/Chip8.Replay/ReplayOutcome.cs
new file mode 100644
index 0000000..4fffea8
--- /dev/null
+++ b/src/Chip8.Replay/ReplayOutcome.cs
@@ -0,0 +1,46 @@
+using Chip8.Core;
+
+namespace Chip8.Replay;
+
+public readonly struct ReplayOutcome
+{
+    public int frames { get; }
+    public ulong framebufferHash { get; }
+    public ulong registerHash { get; }
+    public int litPixels { get; }
+    public bool exited { get; }
+
+    public ReplayOutcome(int frames, ulong framebufferHash, ulong registerHash, int litPixels, bool exited)
+    {
+        this.frames = frames;
+        this.framebufferHash = framebufferHash;
+        this.registerHash = registerHash;
+        this.litPixels = litPixels;
+        this.exited = exited;
+    }
+
+    public static ReplayOutcome capture(Machine machine, int frames)
+    {
+        var snapshot = FramebufferSnapshot.capture(machine);
+        return new ReplayOutcome(
+            frames,
+            FramebufferHash.compute(snapshot),
+            FramebufferHash.computeWithRegisters(machine),
+            snapshot.litCount,
+            machine.exited);
+    }
+
+    public bool sameAs(ReplayOutcome other) =>
+        frames == other.frames
+        && framebufferHash == other.framebufferHash
+        && registerHash == other.registerHash
+        && litPixels == other.litPixels
+        && exited == other.exited;
+
+    public override string ToString() =>
+        "frames=" + frames
+        + " fb=" + FramebufferHash.format(framebufferHash)
+        + " reg=" + FramebufferHash.format(registerHash)
+        + " lit=" + litPixels
+        + " exited=" + exited;
+}