Description

   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 TPU.