By default, TPU$FILEIO creates variable-length files with
    carriage-return record attributes (FAB$B_RFM = VAR, FAB$B_RAT
    FORM item, that item is used instead.

    The following combinations of formats and attributes are
    acceptable:

    Format           Attributes

    STM,STMLF,STMCR  0,BLK,CR,BLK+CR
    VAR              0,BLK,FTN,CR,BLK+FTN,BLK+CR

    All other combinations are converted to VAR format with CR
    attributes.

    This routine always puts values greater than 511 in the first
    longword of the stream data structure. Because a user-written
    file I/O routine is restricted to the values 0 through 511, you
    can easily distinguish the file control blocks (FCB) this routine
    fills in from the ones you created.

                                   NOTE

       TPU uses TPU$FILEIO by default when you use the
       simplified callable interface. When you use the full
       callable interface, you must explicitly invoke TPU$FILEIO
       or provide your own file I/O routine.