After power on:

• Hold “c” → Boot from cdrom
• Hold “o” + “f” + “command” + “option” → Boot in OpenFirmware shell
• Hold “option” → Select boot drive between cdrom, (blessed) internal partitions, (blessed) partition on FireWire disk. NO USB SUPPORT

• printenv boot-device → Print current default boot device/elf

1. powerup your mac in openfirmware shell
2. type: boot hd:6,grub.elf

NOTE: 6 is partition contains the elf binary, grub.elf is the elf name

If you would change your default elf at the boot time type: setenv boot-device hd:6,grub.elf

• A guide to Open Firmware - The Apple BIOS