VMS Help MACRO, /MIGRATION, Compiler Directives, .DEFINE PAL, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    By default, the compiler defines many OpenVMS Alpha PALcode
    instructions as built-ins. If you need to use an OpenVMS Alpha
    PALcode instruction that is not available as a compiler built-
    in, you must define the built-in yourself using the .DEFINE_PAL
    directive.