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

    1.$is_desc64 r9, 10$

      In this example, the descriptor pointed to by R9 is tested, and
      if it is in 64-bit form, a branch to 10$ is taken.

    2.$is_desc64 8(r0), 20$, size=quad

      In this example, the quadword at 8(R0) is read, and the
      descriptor it points to is tested. If it is in 64-bit form,
      a branch to 20$ is taken.