VMS Help System Services, $FREE USER CAPABILITY (Alpha Only), Arguments  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 cap_num

    OpenVMS usage:longword
    type:         longword (unsigned)
    access:       read only
    mechanism:    by 32- or 64-bit reference
    Capability number to be released by the calling Kernel thread.
    This number can range from 1 to 16. The cap_num argument is
    the 32- or 64-bit address of the longword containing the user
    capability number.

 prev_mask

    OpenVMS usage:mask_quadword
    type:         quadword (unsigned)
    access:       write only
    mechanism:    by 32- or 64-bit reference
    The previous user capability reservation mask before execution
    of this service call. The prev_mask argument is the 32- or 64-bit
    address of a quadword into which the service writes a quadword
    bit mask specifying the previously reserved user capabilities.

 flags

    OpenVMS usage:mask_quadword
    type:         quadword (unsigned)
    access:       read only
    mechanism:    by 32- or 64-bit reference
    Options selected for the user capability reservation. The flags
    argument is a quadword bit vector wherein a bit corresponds to an
    option.

    Each option (bit) has a symbolic name, which the $CAPDEF macro
    defines. The flags argument is constructed by performing a
    logical OR operation using the symbolic names of each desired
    option.

    At this time, all bits are reserved to Compaq and must be 0.