VMS Help MACRO, /MIGRATION, Macros for 64-Bit Addressing, $IS DESC64, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    $IS_DESC64 tests the fields which distinguish a 64-bit descriptor
    from a 32-bit descriptor. If it is in 64-bit form, a branch is
    taken to the specified target. The address to be tested is read
    as a longword, unless SIZE=QUAD is specified.