/sys$common/syshlp/helplib.hlb Sys Parameters, REALTIME_SPTS  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    (VAX only) REALTIME_SPTS reserves a number of system page table
    entries for mapping connect-to-interrupt processes into system
    space. This value should normally remain at the default (0)  in
    an environment that is not real-time. Where connect-to-interrupt
    processes do use the system, this value should represent the
    maximum number of pages that all concurrent connect-to-interrupt
    processes must map into system space. See the OpenVMS VAX Device
    Support Manual

    REALTIME_SPTS has the DYNAMIC, GEN, and MAJOR attributes.