yaboot prompt options. How to load initrd?

[Jose.ALISTE]

Hi,
My question may be trivial, but I have "googled" a lot and I can't find 
a solution, so I try here. 
I have a non-working kernel and initrd in my ibook configured in my 
yaboot.conf(/boot/vmlinux and /boot/initrd.img). I know that the 
previous initrd and kernel worked and  they are in /boot/vmlinux.old and 
/boot/initrd.img.old in the same partition(/dev/hda3) . At yaboot prompt 
I try to do:
boot:/boot/vmlinux.old root=/dev/hda3 ro
but this ends with a  KERNEL panic because I don't load the initrd(it's  
the standard kernel and initrd shipped with ubuntu). I have tried to 
find how to tell yaboot in the prompt about the initrd, But I can't find 
the info. I'm using yaboot 1.3.13 and  I tried adding:
initrd=/boot/initrd.img.old or initrd /boot/initrd.img.old to the 
previous prompt line but it does not load the initrd.  Is there a way to 
do it? or I definitely have to rescue with a CD?

Greetings,

Jose