The bound procedure value of this routine is specified in the
    item list built by the callback routine. This routine is called
    to perform file operations. Instead of using your own file I/O
    routine, you can call TPU$FILEIO and pass it the parameters for
    any file operation you do not want to handle. Note, however, that
    TPU$FILEIO must handle all I/O requests for any file it opens.
    Also, if it does not open the file, it cannot handle any I/O
    requests for the file. In other words, you cannot mix the file
    operations between your own file I/O routine and the one supplied
    by DECTPU.