VMS Help MACRO, /MIGRATION, Macros for 64-Bit Addressing, $SETUP CALL64, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    This macro initializes the state for a 64-bit call. It must be
    used before using $PUSH_ARG64 and $CALL64.

    If there are six or fewer arguments, the code is always in line.

    By default, if there are more than six arguments, this macro
    creates a JSB routine that is invoked to perform the actual call.
    However, if the inline option is specified as INLINE=TRUE, the
    code is generated in line. This option should be enabled only
    if the code in which it appears has a fixed stack depth. A fixed
    stack depth can be assumed if no RUNTIMSTK or VARSIZSTK messages
    have been reported. Otherwise, if the stack alignment is not
    at least quadword, there might be many alignment faults in the
    called routine and in anything the called routine calls. The
    default behavior (INLINE=FALSE) does not have this problem.

    If there are more than six arguments, there can be no references
    to AP or SP between a $SETUP_CALL64 and the matching $CALL64,
    because the $CALL64 code may be in a separate JSB routine. In
    addition, temporary registers (R16 and above) may not survive the
    $SETUP_CALL64. However, they can be used within the range, except
    where R16 through R21 interfere with the argument registers
    already set up. In such cases, higher temporary registers should
    be used instead.

                                   NOTE

       The $SETUP_CALL64, $PUSH_ARG64, and $CALL64 macros are
       intended to be used in an inline sequence. That is, you
       cannot branch into the middle of a $SETUP_CALL64/$PUSH_
       ARG64/$CALL64 sequence, nor can you branch around $PUSH_
       ARG64 macros or branch out of the sequence to avoid the
       $CALL64.