yaboot boot variables

[telkis]

I am trying to boot an iBook with a bootable Debian PowerPC CD (image
date 5-30-00). When the CD stars, the yaboot message comes up fine. Then
as the operating systems starts to come up (at about where I think the
installation screen should come up), the screen freaks out with black
and blue vertical bands. I think all I need to do is pass the kernel
some sort of video parameter through yaboot. How does one pass
parameters to the kernel at the yaboot screen?
Thanks,