diff --git a/core/tests/test_protobuf.py b/core/tests/test_protobuf.py
index c6de12331..524b28069 100644
--- a/core/tests/test_protobuf.py
+++ b/core/tests/test_protobuf.py
@@ -1,9 +1,7 @@
 from common import *
 
 import protobuf
-
-if False:
-    from typing import Awaitable, Dict
+from trezor.utils import BufferIO
 
 
 class Message(protobuf.MessageType):
@@ -12,47 +10,22 @@ class Message(protobuf.MessageType):
         self.enum_field = enum_field
 
     @classmethod
-    def get_fields(cls) -> Dict:
+    def get_fields(cls):
         return {
             1: ("sint_field", protobuf.SVarintType, 0),
             2: ("enum_field", protobuf.EnumType("t", (0, 5, 25)), 0),
         }
 
 
-class ByteReader:
-    def __init__(self, data: bytes) -> None:
-        self.data = data
-        self.pos = 0
-
-    async def areadinto(self, buf: bytearray) -> int:
-        remaining = len(self.data) - self.pos
-        limit = len(buf)
-        if remaining < limit:
-            raise EOFError
-
-        buf[:] = self.data[self.pos : self.pos + limit]
-        self.pos += limit
-        return limit
-
-
-class ByteArrayWriter:
-    def __init__(self) -> None:
-        self.buf = bytearray(0)
-
-    async def awrite(self, buf: bytes) -> int:
-        self.buf.extend(buf)
-        return len(buf)
-
-
 def load_uvarint(data: bytes) -> int:
-    reader = ByteReader(data)
-    return await_result(protobuf.load_uvarint(reader))
+    reader = BufferIO(data)
+    return protobuf.load_uvarint(reader)
 
 
 def dump_uvarint(value: int) -> bytearray:
-    writer = ByteArrayWriter()
-    await_result(protobuf.dump_uvarint(writer, value))
-    return writer.buf
+    writer = BufferIO(bytearray(16))
+    protobuf.dump_uvarint(writer, value)
+    return memoryview(writer.buffer)[:writer.offset]
 
 
 class TestProtobuf(unittest.TestCase):
@@ -91,21 +64,23 @@ class TestProtobuf(unittest.TestCase):
     def test_validate_enum(self):
         # ok message:
         msg = Message(-42, 5)
-        writer = ByteArrayWriter()
-        await_result(protobuf.dump_message(writer, msg))
-        reader = ByteReader(bytes(writer.buf))
-        nmsg = await_result(protobuf.load_message(reader, Message))
+        length = protobuf.count_message(msg)
+        buffer_io = BufferIO(bytearray(length))
+        protobuf.dump_message(buffer_io, msg)
+        buffer_io.seek(0)
+        nmsg = protobuf.load_message(buffer_io, Message)
 
         self.assertEqual(msg.sint_field, nmsg.sint_field)
         self.assertEqual(msg.enum_field, nmsg.enum_field)
 
         # bad enum value:
+        buffer_io.seek(0)
         msg = Message(-42, 42)
-        writer = ByteArrayWriter()
-        await_result(protobuf.dump_message(writer, msg))
-        reader = ByteReader(bytes(writer.buf))
+        # XXX this assumes the message will have equal size
+        protobuf.dump_message(buffer_io, msg)
+        buffer_io.seek(0)
         with self.assertRaises(TypeError):
-            await_result(protobuf.load_message(reader, Message))
+            protobuf.load_message(buffer_io, Message)
 
 
 if __name__ == "__main__":
diff --git a/core/tests/test_trezor.wire.codec_v1.py b/core/tests/test_trezor.wire.codec_v1.py
index 931c0d85e..0785c2f92 100644
--- a/core/tests/test_trezor.wire.codec_v1.py
+++ b/core/tests/test_trezor.wire.codec_v1.py
@@ -1,14 +1,14 @@
 from common import *
 from ubinascii import unhexlify
+import ustruct
 
 from trezor import io
 from trezor.loop import wait
-from trezor.utils import chunks
+from trezor.utils import chunks, BufferIO
 from trezor.wire import codec_v1
 
 
 class MockHID:
-
     def __init__(self, num):
         self.num = num
         self.data = []
@@ -20,150 +20,162 @@ class MockHID:
         self.data.append(bytearray(msg))
         return len(msg)
 
+    def wait_object(self, mode):
+        return wait(mode | self.num)
+
+
+MESSAGE_TYPE = 0x4242
 
-class TestWireCodecV1(unittest.TestCase):
 
-    def test_reader(self):
-        rep_len = 64
-        interface_num = 0xdeadbeef
-        message_type = 0x4321
-        message_len = 250
-        interface = MockHID(interface_num)
-        reader = codec_v1.Reader(interface)
-
-        message = bytearray(range(message_len))
-        report_header = bytearray(unhexlify('3f23234321000000fa'))
-
-        # open, expected one read
-        first_report = report_header + message[:rep_len - len(report_header)]
-        self.assertAsync(reader.aopen(), [(None, wait(io.POLL_READ | interface_num)), (first_report, StopIteration()), ])
-        self.assertEqual(reader.type, message_type)
-        self.assertEqual(reader.size, message_len)
-
-        # empty read
-        empty_buffer = bytearray()
-        self.assertAsync(reader.areadinto(empty_buffer), [(None, StopIteration()), ])
-        self.assertEqual(len(empty_buffer), 0)
-        self.assertEqual(reader.size, message_len)
-
-        # short read, expected no read
-        short_buffer = bytearray(32)
-        self.assertAsync(reader.areadinto(short_buffer), [(None, StopIteration()), ])
-        self.assertEqual(len(short_buffer), 32)
-        self.assertEqual(short_buffer, message[:len(short_buffer)])
-        self.assertEqual(reader.size, message_len - len(short_buffer))
-
-        # aligned read, expected no read
-        aligned_buffer = bytearray(rep_len - len(report_header) - len(short_buffer))
-        self.assertAsync(reader.areadinto(aligned_buffer), [(None, StopIteration()), ])
-        self.assertEqual(aligned_buffer, message[len(short_buffer):][:len(aligned_buffer)])
-        self.assertEqual(reader.size, message_len - len(short_buffer) - len(aligned_buffer))
-
-        # one byte read, expected one read
-        next_report_header = bytearray(unhexlify('3f'))
-        next_report = next_report_header + message[rep_len - len(report_header):][:rep_len - len(next_report_header)]
-        onebyte_buffer = bytearray(1)
-        self.assertAsync(reader.areadinto(onebyte_buffer), [(None, wait(io.POLL_READ | interface_num)), (next_report, StopIteration()), ])
-        self.assertEqual(onebyte_buffer, message[len(short_buffer):][len(aligned_buffer):][:len(onebyte_buffer)])
-        self.assertEqual(reader.size, message_len - len(short_buffer) - len(aligned_buffer) - len(onebyte_buffer))
-
-        # too long read, raises eof
-        self.assertAsync(reader.areadinto(bytearray(reader.size + 1)), [(None, EOFError()), ])
-
-        # long read, expect multiple reads
-        start_size = reader.size
-        long_buffer = bytearray(start_size)
-        report_payload = message[rep_len - len(report_header) + rep_len - len(next_report_header):]
-        report_payload_head = report_payload[:rep_len - len(next_report_header) - len(onebyte_buffer)]
-        report_payload_rest = report_payload[len(report_payload_head):]
-        report_payload_rest = list(chunks(report_payload_rest, rep_len - len(next_report_header)))
-        report_payloads = [report_payload_head] + report_payload_rest
-        next_reports = [next_report_header + r for r in report_payloads]
-        expected_syscalls = []
-        for i, _ in enumerate(next_reports):
-            prev_report = next_reports[i - 1] if i > 0 else None
-            expected_syscalls.append((prev_report, wait(io.POLL_READ | interface_num)))
-        expected_syscalls.append((next_reports[-1], StopIteration()))
-        self.assertAsync(reader.areadinto(long_buffer), expected_syscalls)
-        self.assertEqual(long_buffer, message[-start_size:])
-        self.assertEqual(reader.size, 0)
-
-        # one byte read, raises eof
-        self.assertAsync(reader.areadinto(onebyte_buffer), [(None, EOFError()), ])
-
-
-    def test_writer(self):
-        rep_len = 64
-        interface_num = 0xdeadbeef
-        message_type = 0x87654321
-        message_len = 1024
-        interface = MockHID(interface_num)
-        writer = codec_v1.Writer(interface)
-        writer.setheader(message_type, message_len)
-
-        # init header corresponding to the data above
-        report_header = bytearray(unhexlify('3f2323432100000400'))
-
-        self.assertEqual(writer.data, report_header + bytearray(rep_len - len(report_header)))
-
-        # empty write
-        start_size = writer.size
-        self.assertAsync(writer.awrite(bytearray()), [(None, StopIteration()), ])
-        self.assertEqual(writer.data, report_header + bytearray(rep_len - len(report_header)))
-        self.assertEqual(writer.size, start_size)
-
-        # short write, expected no report
-        start_size = writer.size
-        short_payload = bytearray(range(4))
-        self.assertAsync(writer.awrite(short_payload), [(None, StopIteration()), ])
-        self.assertEqual(writer.size, start_size - len(short_payload))
-        self.assertEqual(writer.data,
-                  report_header +
-                  short_payload +
-                  bytearray(rep_len - len(report_header) - len(short_payload)))
-
-        # aligned write, expected one report
-        start_size = writer.size
-        aligned_payload = bytearray(range(rep_len - len(report_header) - len(short_payload)))
-        self.assertAsync(writer.awrite(aligned_payload), [(None, wait(io.POLL_WRITE | interface_num)), (None, StopIteration()), ])
-        self.assertEqual(interface.data, [report_header +
-                                   short_payload +
-                                   aligned_payload +
-                                   bytearray(rep_len - len(report_header) - len(short_payload) - len(aligned_payload)), ])
-        self.assertEqual(writer.size, start_size - len(aligned_payload))
-        interface.data.clear()
-
-        # short write, expected no report, but data starts with correct seq and cont marker
-        report_header = bytearray(unhexlify('3f'))
-        start_size = writer.size
-        self.assertAsync(writer.awrite(short_payload), [(None, StopIteration()), ])
-        self.assertEqual(writer.size, start_size - len(short_payload))
-        self.assertEqual(writer.data[:len(report_header) + len(short_payload)],
-                  report_header + short_payload)
-
-        # long write, expected multiple reports
-        start_size = writer.size
-        long_payload_head = bytearray(range(rep_len - len(report_header) - len(short_payload)))
-        long_payload_rest = bytearray(range(start_size - len(long_payload_head)))
-        long_payload = long_payload_head + long_payload_rest
-        expected_payloads = [short_payload + long_payload_head] + list(chunks(long_payload_rest, rep_len - len(report_header)))
-        expected_reports = [report_header + r for r in expected_payloads]
-        expected_reports[-1] += bytearray(bytes(1) * (rep_len - len(expected_reports[-1])))
-        # test write
-        expected_write_reports = expected_reports[:-1]
-        self.assertAsync(writer.awrite(long_payload), len(expected_write_reports) * [(None, wait(io.POLL_WRITE | interface_num))] + [(None, StopIteration())])
-        self.assertEqual(interface.data, expected_write_reports)
-        self.assertEqual(writer.size, start_size - len(long_payload))
-        interface.data.clear()
-        # test write raises eof
-        self.assertAsync(writer.awrite(bytearray(1)), [(None, EOFError())])
-        self.assertEqual(interface.data, [])
-        # test close
-        expected_close_reports = expected_reports[-1:]
-        self.assertAsync(writer.aclose(), len(expected_close_reports) * [(None, wait(io.POLL_WRITE | interface_num))] + [(None, StopIteration())])
-        self.assertEqual(interface.data, expected_close_reports)
-        self.assertEqual(writer.size, 0)
-
-
-if __name__ == '__main__':
+def make_header(mtype, length):
+    return b"?##" + ustruct.pack(">HL", mtype, length)
+
+
+class TestWireCodecV1(unittest.TestCase):
+    def setUp(self):
+        self.interface = MockHID(0xDEADBEEF)
+
+    def test_read_one_packet(self):
+        # zero length message - just a header
+        message_packet = make_header(mtype=MESSAGE_TYPE, length=0)
+        buffer = bytearray(64)
+
+        gen = codec_v1.read_message(self.interface, buffer)
+
+        query = gen.send(None)
+        self.assertObjectEqual(query, self.interface.wait_object(io.POLL_READ))
+
+        with self.assertRaises(StopIteration) as e:
+            gen.send(message_packet)
+
+        # e.value is StopIteration. e.value.value is the return value of the call
+        result = e.value.value
+        self.assertEqual(result.type, MESSAGE_TYPE)
+        self.assertEqual(result.data.buffer, b"")
+
+        # message should have been read into the buffer
+        self.assertEqual(buffer, b"\x00" * 64)
+
+    def test_read_many_packets(self):
+        message = bytes(range(256))
+
+        header = make_header(mtype=MESSAGE_TYPE, length=len(message))
+        first_len = codec_v1._REP_LEN - len(header)
+        # first packet is header + (remaining)data
+        # other packets are "?" + 63 bytes of data
+        packets = [header + message[:first_len]] + [
+            b"?" + chunk for chunk in chunks(message[first_len:], codec_v1._REP_LEN - 1)
+        ]
+
+        buffer = bytearray(256)
+        gen = codec_v1.read_message(self.interface, buffer)
+        query = gen.send(None)
+        for packet in packets[:-1]:
+            self.assertObjectEqual(query, self.interface.wait_object(io.POLL_READ))
+            query = gen.send(packet)
+
+        # last packet will stop
+        with self.assertRaises(StopIteration) as e:
+            gen.send(packets[-1])
+
+        # e.value is StopIteration. e.value.value is the return value of the call
+        result = e.value.value
+        self.assertEqual(result.type, MESSAGE_TYPE)
+        self.assertEqual(result.data.buffer, message)
+
+        # message should have been read into the buffer
+        self.assertEqual(buffer, message)
+
+    def test_read_large_message(self):
+        message = b"hello world"
+        header = make_header(mtype=MESSAGE_TYPE, length=len(message))
+
+        packet = header + message
+        # make sure we fit into one packet, to make this easier
+        self.assertTrue(len(packet) <= codec_v1._REP_LEN)
+
+        buffer = bytearray(1)
+        self.assertTrue(len(buffer) <= len(packet))
+
+        gen = codec_v1.read_message(self.interface, buffer)
+        query = gen.send(None)
+        self.assertObjectEqual(query, self.interface.wait_object(io.POLL_READ))
+        with self.assertRaises(StopIteration) as e:
+            gen.send(packet)
+
+        # e.value is StopIteration. e.value.value is the return value of the call
+        result = e.value.value
+        self.assertEqual(result.type, MESSAGE_TYPE)
+        self.assertEqual(result.data.buffer, message)
+
+        # read should have allocated its own buffer and not touch ours
+        self.assertEqual(buffer, b"\x00")
+
+    def test_write_one_packet(self):
+        gen = codec_v1.write_message(self.interface, MESSAGE_TYPE, b"")
+
+        query = gen.send(None)
+        self.assertObjectEqual(query, self.interface.wait_object(io.POLL_WRITE))
+        with self.assertRaises(StopIteration):
+            gen.send(None)
+
+        header = make_header(mtype=MESSAGE_TYPE, length=0)
+        expected_message = header + b"\x00" * (codec_v1._REP_LEN - len(header))
+        self.assertTrue(self.interface.data == [expected_message])
+
+    def test_write_multiple_packets(self):
+        message = bytes(range(256))
+        gen = codec_v1.write_message(self.interface, MESSAGE_TYPE, message)
+
+        header = make_header(mtype=MESSAGE_TYPE, length=len(message))
+        first_len = codec_v1._REP_LEN - len(header)
+        # first packet is header + (remaining)data
+        # other packets are "?" + 63 bytes of data
+        packets = [header + message[:first_len]] + [
+            b"?" + chunk for chunk in chunks(message[first_len:], codec_v1._REP_LEN - 1)
+        ]
+
+        for _ in packets:
+            # we receive as many queries as there are packets
+            query = gen.send(None)
+            self.assertObjectEqual(query, self.interface.wait_object(io.POLL_WRITE))
+
+        # the first sent None only started the generator. the len(packets)-th None
+        # will finish writing and raise StopIteration
+        with self.assertRaises(StopIteration):
+            gen.send(None)
+
+        # packets must be identical up to the last one
+        self.assertListEqual(packets[:-1], self.interface.data[:-1])
+        # last packet must be identical up to message length. remaining bytes in
+        # the 64-byte packets are garbage -- in particular, it's the bytes of the
+        # previous packet
+        last_packet = packets[-1] + packets[-2][len(packets[-1]):]
+        self.assertEqual(last_packet, self.interface.data[-1])
+
+    def test_roundtrip(self):
+        message = bytes(range(256))
+        gen = codec_v1.write_message(self.interface, MESSAGE_TYPE, message)
+
+        # exhaust the iterator:
+        # (XXX we can only do this because the iterator is only accepting None and returns None)
+        for query in gen:
+            self.assertObjectEqual(query, self.interface.wait_object(io.POLL_WRITE))
+
+        buffer = bytearray(1024)
+        gen = codec_v1.read_message(self.interface, buffer)
+        query = gen.send(None)
+        for packet in self.interface.data[:-1]:
+            self.assertObjectEqual(query, self.interface.wait_object(io.POLL_READ))
+            query = gen.send(packet)
+
+        with self.assertRaises(StopIteration) as e:
+            gen.send(self.interface.data[-1])
+
+        result = e.value.value
+        assert result.type == MESSAGE_TYPE
+        assert result.data.buffer == message
+
+
+if __name__ == "__main__":
     unittest.main()
diff --git a/core/tests/unittest.py b/core/tests/unittest.py
index a78a68d03..dcade810c 100644
--- a/core/tests/unittest.py
+++ b/core/tests/unittest.py
@@ -9,6 +9,7 @@ class AssertRaisesContext:
 
     def __init__(self, exc):
         self.expected = exc
+        self.value = None
 
     def __enter__(self):
         return self
@@ -17,6 +18,7 @@ class AssertRaisesContext:
         if exc_type is None:
             ensure(False, "%r not raised" % self.expected)
         if issubclass(exc_type, self.expected):
+            self.value = exc_value
             return True
         return False