If you are installing onto a NewWorld PowerMac you must create a special bootstrap partition to hold the boot loader. The size of this partition must be 800KB and its partition type must be Apple_Bootstrap. If the bootstrap partition is not created with the Apple_Bootstrap type your machine cannot be made bootable from the hard disk. This partition can easily be created by creating a new partition in partman and telling it to use it as a “NewWorld boot partition”, or in mac-fdisk using the b command.
Partitioning Newer PowerMacs
The special partition type Apple_Bootstrap is required to prevent MacOS from mounting and damaging the bootstrap partition, as there are special modifications made to it in order for OpenFirmware to boot it automatically.
Note that the bootstrap partition is only meant to hold 3 very small files: the yaboot binary, its configuration yaboot.conf, and a first stage OpenFirmware loader ofboot.b. It need not and must not be mounted on your file system nor have kernels or anything else copied to it. The ybin and mkofboot utilities are used to manipulate this partition.
In order for OpenFirmware to automatically boot Ubuntu the bootstrap partition should appear before other boot partitions on the disk, especially MacOS boot partitions. The bootstrap partition should be the first one you create. However, if you add a bootstrap partition later, you can use mac-fdisk's r command to reorder the partition map so the bootstrap partition comes right after the map (which is always partition 1). It's the logical map order, not the physical address order, that counts.
Apple disks normally have several small driver partitions. If you intend to dual boot your machine with MacOSX, you should retain these partitions and a small HFS partition (800k is the minimum size). That is because MacOSX, on every boot, offers to initialize any disks which do not have active MacOS partitions and driver partitions.