"Yaboot is an OpenFirmware ELF executable that bootstraps the Linux kernel." (man yaboot)
Yaboot is used to gain fine grained control over the linux boot process. OF is capable of executing a linux kernel image directly without the use of yaboot. This requires that the kernel image has the correct boot time settings precompiled into it as I am not aware of any way to this from OF. Yaboot however, will look for a text configuration file when it is executed. If yaboot was retrieved using tftp, it will look for its config file from the same tftp source. If a boot time option needs to be changed, just change it on the spot; no recompilation needed. Thus yaboot and its yaboot.conf is very much like lilo and its lilo.conf.
I provide several different premade yaboot.conf files for booting Linux on the cluster.
Yaboot acts as a "second" stage boottime switch. The "first" stage being the retrieved boot file. In the IFI-G4COW system, any yaboot.conf file that adheres to the naming policy will be automatically added to the "Master Switch" CGI web page's options.
When I first started work with yaboot, it was not able to retrieve its config file via tftp, even though it had been transferred to OF via tftp itself. I had to debug it, and performa a one line hack to make that work. I submitted my patch and a description of the problem to the maintainer of yaboot, and he promptly reworked the problem sections of code into the very next release. Gentle reader, you don't have to apply a yaboot patch. Just grab the latest release (apt-get install yaboot). I feel it is important to tell this story to encourage users of free software to dig in and most importantly, provide constructive feedback to the authors of the software they use.
See tftp.html for info on yaboot.conf naming policy when used in conjunction with apache/cgi.
See partition.html for info on the local partition policy.