diff --git a/tests/Chip8.Tests/OpcodeTests.cs b/tests/Chip8.Tests/OpcodeTests.cs
new file mode 100644
index 0000000..7cc44c8
--- /dev/null
+++ b/tests/Chip8.Tests/OpcodeTests.cs
@@ -0,0 +1,168 @@
+using Chip8.Core;
+using Xunit;
+
+namespace Chip8.Tests;
+
+public class OpcodeTests
+{
+    [Fact]
+    public void clearScreenBlanksDisplay()
+    {
+        var machine = TestHarness.withOpcode(0x00E0);
+        machine.display[100] = true;
+        machine.step();
+        Assert.All(machine.display, pixel => Assert.False(pixel));
+    }
+
+    [Fact]
+    public void jumpSetsProgramCounter()
+    {
+        var machine = TestHarness.withOpcode(0x1ABC);
+        machine.step();
+        Assert.Equal(0xABC, machine.pc);
+    }
+
+    [Fact]
+    public void callPushesReturnAddressAndReturnRestoresIt()
+    {
+        var machine = new Machine(Quirks.modern(), 1);
+        machine.memory[0x200] = 0x22; machine.memory[0x201] = 0x06; // 2206 call 0x206
+        machine.memory[0x206] = 0x00; machine.memory[0x207] = 0xEE; // 00EE ret
+        machine.step();
+        Assert.Equal(0x206, machine.pc);
+        Assert.Equal(1, machine.sp);
+        Assert.Equal(0x202, machine.stack[0]);
+        machine.step();
+        Assert.Equal(0x202, machine.pc);
+        Assert.Equal(0, machine.sp);
+    }
+
+    [Fact]
+    public void skipIfEqualImmediate()
+    {
+        var taken = TestHarness.withOpcode(0x3042);
+        taken.v[0] = 0x42; taken.step();
+        Assert.Equal(0x204, taken.pc);
+
+        var skipped = TestHarness.withOpcode(0x3042);
+        skipped.v[0] = 0x00; skipped.step();
+        Assert.Equal(0x202, skipped.pc);
+    }
+
+    [Fact]
+    public void skipIfNotEqualImmediate()
+    {
+        var taken = TestHarness.withOpcode(0x4042);
+        taken.v[0] = 0x00; taken.step();
+        Assert.Equal(0x204, taken.pc);
+    }
+
+    [Fact]
+    public void skipIfRegistersEqualAndNotEqual()
+    {
+        var equal = TestHarness.withOpcode(0x5010);
+        equal.v[0] = 9; equal.v[1] = 9; equal.step();
+        Assert.Equal(0x204, equal.pc);
+
+        var notEqual = TestHarness.withOpcode(0x9010);
+        notEqual.v[0] = 9; notEqual.v[1] = 8; notEqual.step();
+        Assert.Equal(0x204, notEqual.pc);
+    }
+
+    [Fact]
+    public void loadImmediateAndSetIndex()
+    {
+        var load = TestHarness.withOpcode(0x6A55);
+        load.step();
+        Assert.Equal(0x55, load.v[0xA]);
+
+        var index = TestHarness.withOpcode(0xA123);
+        index.step();
+        Assert.Equal(0x123, index.i);
+    }
+
+    [Fact]
+    public void jumpWithOffsetHonoursQuirk()
+    {
+        var classic = TestHarness.withOpcode(0xB300, Quirks.modern());
+        classic.v[0] = 0x12; classic.step();
+        Assert.Equal(0x312, classic.pc);
+
+        var superchip = TestHarness.withOpcode(0xB300, Quirks.superChip());
+        superchip.v[3] = 0x12; superchip.step();
+        Assert.Equal(0x312, superchip.pc);
+    }
+
+    [Fact]
+    public void randomMaskedByImmediate()
+    {
+        var machine = TestHarness.withOpcode(0xC000);
+        machine.step();
+        Assert.Equal(0, machine.v[0]);
+    }
+
+    [Fact]
+    public void drawSetsPixelsAndReportsCollision()
+    {
+        var machine = new Machine(Quirks.modern(), 1);
+        machine.memory[0x200] = 0xD0; machine.memory[0x201] = 0x11; // DXYN draw 1 row at (V0,V1)
+        machine.i = 0x300;
+        machine.memory[0x300] = 0b1010_0000;
+        machine.v[0] = 0; machine.v[1] = 0;
+        machine.step();
+        Assert.True(machine.display[0]);
+        Assert.False(machine.display[1]);
+        Assert.True(machine.display[2]);
+        Assert.Equal(0, machine.v[0xF]);
+
+        machine.pc = 0x200;
+        machine.step();
+        Assert.False(machine.display[0]);
+        Assert.Equal(1, machine.v[0xF]);
+    }
+
+    [Fact]
+    public void drawClipsAtRightAndBottomEdges()
+    {
+        var machine = new Machine(Quirks.modern(), 1);
+        machine.memory[0x200] = 0xD0; machine.memory[0x201] = 0x11; // DXYN draw 1 row at (V0,V1)
+        machine.i = 0x300;
+        machine.memory[0x300] = 0b1111_1111;
+        machine.v[0] = 62; machine.v[1] = 0;
+        machine.step();
+        Assert.True(machine.display[62]);
+        Assert.True(machine.display[63]);
+        Assert.Equal(2, AsciiScreen.litPixelCount(machine.display));
+    }
+
+    [Fact]
+    public void skipOnKeyStateBothDirections()
+    {
+        var pressed = TestHarness.withOpcode(0xE09E);
+        pressed.v[0] = 0x5; pressed.pressKey(0x5); pressed.step();
+        Assert.Equal(0x204, pressed.pc);
+
+        var notPressed = TestHarness.withOpcode(0xE0A1);
+        notPressed.v[0] = 0x5; notPressed.step();
+        Assert.Equal(0x204, notPressed.pc);
+    }
+
+    [Fact]
+    public void awaitKeyBlocksUntilReleaseThenStores()
+    {
+        var machine = TestHarness.withOpcode(0xF00A);
+        machine.step();
+        Assert.Equal(0x200, machine.pc);
+        Assert.True(machine.waitingForKey);
+
+        machine.pressKey(0x9);
+        machine.step();
+        Assert.Equal(0x200, machine.pc);
+
+        machine.releaseKey(0x9);
+        machine.step();
+        Assert.Equal(0x202, machine.pc);
+        Assert.Equal(0x9, machine.v[0]);
+        Assert.False(machine.waitingForKey);
+    }
+}