DISABLE_UPCALLS

   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.