read the hardware manual for the H80 class machines.  When you boot up and
see the rs/6000 logo or a ton of "rs6000" words across the screen  press
either 1,3 or 7 (I can't remember exactly, memory going bad with age) but
this allows you to go into the nvram and select the boot devices.

or as someone mentioned the bootlist command so that on next reboot it hits
the cd first, example

bootlist -m normal cd0 hdisk0

again check man paging in case I forgotten some flags but will try a cd
boot and then to hdisk0 (assuming that's where rootvg or one of the rootvg
disks)

hope that helps

rick