The SYS_LOADABLE ADD command adds an entry to the system images
    file SYS$UPDATE:VMS$SYSTEM_IMAGES.IDX. You can then process
    this file using the command procedure SYS$UPDATE:VMS$SYSTEM_
    IMAGES.COM. Processing the file with VMS$SYSTEM_IMAGES.COM
    generates a new system images data file that the system uses
    when it boots.

    If the file SYS$UPDATE:VMS$SYSTEM_IMAGES.IDX does not exist, the
    SYS_LOADABLE ADD command creates a new one.