If the machine is already running use: init -s (or -S) At boot time do the following:

 1. reboot system
 2. when message displayed to interupt boot, hit any key within 10 seconds
 3. boot from normal path
 4. At interact with ISL enter Y
 5. At ISL prompt enter: hpux -is (;0)/stand/vmunix (although hpux -is will usually suffice)