VMS Help LINK, Qualifiers, /THREADS_ENABLE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /THREADS_ENABLE[=(MULTIPLE_KERNEL_THREADS,UPCALLS)]
       /NOTHREADS_ENABLE (default)

    Kernel threads allow a multithreaded application to have a
    thread executing on every CPU in a multiprocessor system. The
    /THREADS_ENABLE qualifier allows you to turn kernel threads
    on and off on a per-image basis.

    When you specify this qualifier, the OpenVMS linker sets the
    appropriate bits in the image header of the image being linked.
    These bits control the following:

    o Whether the image is allowed to enter a multiple kernel threads
      environment
    o Whether the image can receive upcalls from the OpenVMS scheduler

    On Alpha systems, the MULTIPLE_KERNEL_THREADS keyword sets the
    MULTIPLE_KERNEL_THREADS bit in the image header of the image being
    built.  This bit indicates to the image activator that the image
    can be run in a multiple kernel threads environment.

    If you specify this keyword for OpenVMS VAX links, it is ignored.

    The UPCALLS keyword sets the UPCALLS bit in the OpenVMS image
    header of the image being built. This bit indicates to the image
    activator that the process can receive upcalls from the OpenVMS
    scheduler.