diff --git a/adafruit_pioasm.py b/adafruit_pioasm.py
index 6471db7..8ed258a 100644
--- a/adafruit_pioasm.py
+++ b/adafruit_pioasm.py
@@ -147,7 +147,11 @@ def __init__(self, text_program: str, *, build_debuginfo=False) -> None:
             elif instruction[0] == "in":
                 #                instr delay src count
                 assembled.append(0b010_00000_000_00000)
-                assembled[-1] |= IN_SOURCES.index(instruction[1]) << 5
+                source = instruction[1]
+                try:
+                    assembled[-1] |= IN_SOURCES.index(source) << 5
+                except ValueError as exc:
+                    raise ValueError(f"Invalid in source '{source}'") from exc
                 count = int(instruction[-1], 0)
                 if not 1 <= count <= 32:
                     raise RuntimeError("Count out of range")
@@ -155,7 +159,13 @@ def __init__(self, text_program: str, *, build_debuginfo=False) -> None:
             elif instruction[0] == "out":
                 #                instr delay dst count
                 assembled.append(0b011_00000_000_00000)
-                assembled[-1] |= OUT_DESTINATIONS.index(instruction[1]) << 5
+                destination = instruction[1]
+                try:
+                    assembled[-1] |= OUT_DESTINATIONS.index(destination) << 5
+                except ValueError as exc:
+                    raise ValueError(
+                        f"Invalid out destination '{destination}'"
+                    ) from exc
                 count = int(instruction[-1], 0)
                 if not 1 <= count <= 32:
                     raise RuntimeError("Count out of range")
diff --git a/tests/test_in.py b/tests/test_in.py
new file mode 100644
index 0000000..d2b76dd
--- /dev/null
+++ b/tests/test_in.py
@@ -0,0 +1,69 @@
+# SPDX-FileCopyrightText: KB Sriram
+#
+# SPDX-License-Identifier: MIT
+
+"""
+Tests in
+"""
+
+import pytest
+from pytest_helpers import assert_assembles_to, assert_assembly_fails
+
+
+@pytest.mark.parametrize(
+    "source,expected",
+    [
+        ("pins", 0b000),
+        ("x", 0b001),
+        ("y", 0b010),
+        ("null", 0b011),
+        ("isr", 0b110),
+        ("osr", 0b111),
+    ],
+)
+def test_in_sources(source: str, expected: int) -> None:
+    #                delay src bitcount
+    encoding = 0b010_00000_000_10001
+    # add in the expected source
+    encoding |= expected << 5
+    assert_assembles_to(f"in {source}, 17", [encoding])
+
+
+@pytest.mark.parametrize("delay", [0, 1, 9, 17, 31])
+def test_in_delay(delay: int) -> None:
+    #                delay src bitcount
+    encoding = 0b010_00000_000_10001
+    # add in the expected delay
+    encoding |= delay << 8
+    assert_assembles_to(f"in pins, 17 [{delay}]", [encoding])
+
+
+@pytest.mark.parametrize("bitcount", [1, 9, 17, 32])
+def test_in_bitcount(bitcount: int) -> None:
+    #                delay dst bitcount
+    encoding = 0b010_00000_000_00000
+    # add in the expected bitcount. Note that
+    # 32 should be encoded as 0, which we do by
+    # masking the bitcount with 0x1f
+    encoding |= bitcount & 0x1F
+    assert_assembles_to(f"in pins, {bitcount}", [encoding])
+
+
+def test_in_delay_with_sideset() -> None:
+    source = [
+        ".side_set 2",
+        "in pins 17 side 2 [5]",
+    ]
+    assert_assembles_to("\n".join(source), [0b010_10_101_000_10001])
+
+
+def test_in_bad_source():
+    assert_assembly_fails(
+        "in bad, 17", match="Invalid in source 'bad'", errtype=ValueError
+    )
+
+
+def test_in_bad_bitcount():
+    assert_assembly_fails(
+        "in pins, 0", match="Count out of range", errtype=RuntimeError
+    )
diff --git a/tests/test_out.py b/tests/test_out.py
new file mode 100644
index 0000000..5956fce
--- /dev/null
+++ b/tests/test_out.py
@@ -0,0 +1,71 @@
+# SPDX-FileCopyrightText: KB Sriram
+#
+# SPDX-License-Identifier: MIT
+
+"""
+Tests out
+"""
+
+import pytest
+from pytest_helpers import assert_assembles_to, assert_assembly_fails
+
+
+@pytest.mark.parametrize(
+    "destination,expected",
+    [
+        ("pins", 0b000),
+        ("x", 0b001),
+        ("y", 0b010),
+        ("null", 0b011),
+        ("pindirs", 0b100),
+        ("pc", 0b101),
+        ("isr", 0b110),
+        ("exec", 0b111),
+    ],
+)
+def test_out_destinations(destination: str, expected: int) -> None:
+    #                delay dst bitcount
+    encoding = 0b011_00000_000_10001
+    # add in the expected destination
+    encoding |= expected << 5
+    assert_assembles_to(f"out {destination}, 17", [encoding])
+
+
+@pytest.mark.parametrize("delay", [0, 1, 9, 17, 31])
+def test_out_delay(delay: int) -> None:
+    #                delay dst bitcount
+    encoding = 0b011_00000_000_10001
+    # add in the expected delay
+    encoding |= delay << 8
+    assert_assembles_to(f"out pins, 17 [{delay}]", [encoding])
+
+
+@pytest.mark.parametrize("bitcount", [1, 9, 17, 32])
+def test_out_bitcount(bitcount: int) -> None:
+    #                delay dst bitcount
+    encoding = 0b011_00000_000_00000
+    # add in the expected bitcount. Note that
+    # 32 should be encoded as 0, which we do by
+    # masking the bitcount with 0x1f
+    encoding |= bitcount & 0x1F
+    assert_assembles_to(f"out pins, {bitcount}", [encoding])
+
+
+def test_out_delay_with_sideset() -> None:
+    source = [
+        ".side_set 2",
+        "out pins 17 side 2 [5]",
+    ]
+    assert_assembles_to("\n".join(source), [0b011_10_101_000_10001])
+
+
+def test_out_bad_destination():
+    assert_assembly_fails(
+        "out bad, 17", match="Invalid out destination 'bad'", errtype=ValueError
+    )
+
+
+def test_out_bad_bitcount():
+    assert_assembly_fails(
+        "out pins, 0", match="Count out of range", errtype=RuntimeError
+    )