From an ISL prompt, you can pass arguments to the HPUX boot loader. This is very useful in a couple of not-so-common situations. First of all, if you need to change the root password, and you can't log in as root for some reason (like you forgot the root password), you can tell ISL to boot to single user, and log you in as root without a password. From the ISL prompt, type hpux -is (-is means force init level s). Note that this only works if you to have console access.

If you rebuilt the kernel, and the 'new improved' kernel doesn't work, you can go back to the old one like this: hpux -is /stand/vmunix.prev. This only works if you remembered to name the old kernel with this name (this is the HP-UX standard name for the old kernel).

Let's say you were silly, and you named the old kernel a funky name, and you forgot what it was, you could get a directory listing of /stand like this: hpux ls.

Another reason you might want to use ISL to pass HPUX arguments is if you are using MirrorDiskU/X (mirroring) to protect the boot drive. HP-UX uses the idea of a quorum. Basically, at boot time, the operating system looks at all the boot slices that are mirrored together, and holds an election. The slices that are identical, and that have a majority, win the election and are booted from. So let's say you have a boot slice with 2 mirrors. The boot slice dies. HP-UX has an election at boot, and the 2 mirror slices are identical, and the boot slice is old or dead. The boot slice is ignored (since there is a majority or quorum of mirror slices) and the system boots from the mirror slices. That makes sense. But what happens if the boot slice has only one mirror (which is the way most places protect their boot slice)? Uh, oh. No one wins the election (there is no quorum) and the machine won't boot. You can override quorum with a -lq (i.e., hpux -lq or hpux -is -lq).