VMS Help PATCH, Qualifiers, /ABSOLUTE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Patches a file at absolute virtual addresses. The /ABSOLUTE
    function allows a user to patch any file (not just image files)
    at absolute virtual addresses relative to the beginning of the
    file. This feature allows replacement of existing data with new
    data of the same length. If the data is smaller than that of
    the original data, PATCH uses the appropriate fill character for
    the mode in use. For example, if the current mode is instruction
    mode, a NOP is used for fill; if it is data (numeric or ASCII)
    mode, a NULL is used for fill. Any PATCH operation that results
    in a data replacement longer than the length of the original
    data generates an error message and terminates the command in
    progress; either the PATCH or DCL prompt is then displayed,
    whichever is appropriate.

    Also, note that there is no default patch area, and none will be
    created, because of the tendency to corrupt a file. Patch area is
    meaningless in other than an image file.

    If you patch a file in absolute mode, remember that there are no
    symbols available to assist you in locating data locations. You
    must exercise great care to ascertain that the correct locations
    are modified.

    Most PATCH commands will work in their normal fashion. However,
    only REPLACE and DEPOSIT should be used for write operations;
    other commands are acceptable for read operations. Commands that
    attempt to expand the file, such as ALIGN and INSERT, should
    be avoided because they will probably corrupt the file. (These
    commands will be trapped by PATCH and an error message will be
    issued indicating that the replacement data must not exceed the
    length of the original data.)

    File attributes are propagated from the original input file to
    the output file. These include ALQ, TYPE, MRS, RAT, RFM, and RAC.