DISABLE_UPCALLS is primarily a debugging aid. It allows the
    system manager to disable threads upcalls of specific types
    for the entire system. The value is a bit mask, with the bits
    corresponding to the upcall types. The upcall types are defined
    in the definition macro $TMCDEF.

    This special parameter is use by VSI and is subject to change. Do
    not change this parameter unless VSI recommends that you do so.

    DISABLE_UPCALLS is a DYNAMIC parameter.