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

    SMP_CPUS identifies which secondary processors, if available,
    are to be booted into the multiprocessing system at boot time.
    SMP_CPUS is a 32-bit mask; if the value of a bit in the mask is
    1, the processor with the corresponding CPU ID is booted into
    the multiprocessing system (if it is available). For example, if
    you want to boot only the CPUs with CPU IDs 0 and 1, specify the
    value 3 (both bits are on).

    The default value of SMP_CPUS, -1, boots all available CPUs into
    the multiprocessing system.

    Note that although a bit in the mask corresponds to the primary
    processor's CPU ID, the primary processor is always booted.
    That is, if the mask is set to 0, the primary CPU still boots.
    Any available secondary processors are not booted into the
    multiprocessing system.

    This parameter is ignored if the MULTIPROCESSING parameter is set
    to 0.