def recv_messages():
    global ridx, widx, lastid, timestamp_cycles

    # read updated write-pointer
    widx = ddr_widx.value
    assert widx < NUM_MSGS  # sanity-check

    if ridx == widx:
        return  # no messages received

    # process messages
    while ridx != widx:
        # note: it may be faster to copy a batch of messages from shared memory
        # instead of directly accessing individual messages and their fields.
        msg = msgbuf[ ridx ]

        # sanity-check that message id increments monotonically
        lastid = ( lastid + 1 ) & 0xffffffff
        assert msg.id == lastid

        # get 32-bit timestamp (in cycles) from message and unwrap it:
        timestamp_cycles += ( msg.timestamp - timestamp_cycles ) & 0xffffffff

        # process rest of message
        position = msg.position
        force = msg.force

        # consume message and update read pointer
        del msg  # direct access to message forbidden beyond this point
        ridx += 1
        if ridx == NUM_MSGS:
            ridx = 0
        shmem.ridx = ridx

    # update user interface
    ts_ms = timestamp_cycles // 200000
    ts_s = ( ts_ms % 60000 ) / 1000
    ts_m = ts_ms // 60000
    ts_h = ts_m // 60
    ts_m = ts_m % 60
    print(f'\ridx=0x{ridx:04x} id=0x{lastid:08x} time={ts_h:02d}:{ts_m:02d}:{ts_s:06.3f} position={position:08d} force={force:08d}', end='', flush=True )