If you do not need to intercept any file I/O, you can use the
    entry point EDT$FILEIO for this argument or you can omit it. If
    you need to intercept only some file I/O, call the EDT$FILEIO
    routine for the other cases.

    When you use EDT$FILEIO as a value for the fileio argument, files
    are opened as follows:

    o  The record argument is always the RMS file name.

    o  The rhb argument is always the RMS default file name.

    o  There is no related name for the input file.

    o  The related name for the output file is the input file with
       OFP (output file parse). EDT passes the input file name, the
       output file name, or the name from the EXIT command in the
       record argument.

    o  The related name for the journal file is the input file name
       with the OFP RMS bit set.

    o  The related name for the INCLUDE file is the input file name
       with the OFP set. This is unusual because the file is being
       opened for input.

    EDT contains support for VFC files. Normally, EDT will zero the
    length of the RHB field if the file is not a VFC file. However,
    when the user supplies the FILEIO routines, they are responsible
    for performing this operation.

    EDT checks for a VFC file with the following algorithm:

    IF FAB$B_RFM = FAB$C_VFC
    AND FAB$B_RAT <> FAB$M_PRN
    THEN
            VFC file
    ELSE
            not VFC file, zero out RHB descriptor length field.