VMS Help MACRO, /MIGRATION, Locking Pages into a Working Set, On-the-Fly Lockdown, $LOCK PAGE, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    This macro is placed inline in executable code and must be
    followed by the $UNLOCK_PAGE macro. The $LOCK_PAGE/$UNLOCK_PAGE
    macro pair creates a separate routine in a separate psect. $LOCK_
    PAGE locks the pages and linkage section of this separate routine
    into the working set and JSRs to it. All code between this macro
    and the matching $UNLOCK_PAGE macro is included in the locked
    routine and is locked down.

    All registers are preserved by this macro unless the error
    address parameter is present and one of the calls fail. If that
    happens, R0 reflects the status of the failed call. R1 then
    contains 0 if the call to lock the code failed or 1 if that call
    succeeded but the call to lock the linkage section failed.

    If the ERROR parameter is used, the ERROR label must be placed
    outside the scope of the $LOCK_PAGE and $UNLOCK_PAGE pair. This
    is because the error routine is branched to before calling the
    subroutine that the $LOCK_PAGE and $UNLOCK_PAGE routines create.

    Note that since the locked code is made into a separate routine,
    any references to local stack storage within the routine will
    have to be changed, as the stack context is no longer the same.
    Also, you cannot branch into or out of the locked code from the
    rest of the routine.