The BIOS on your computer allows the bootup sequence to begin when you turn your computer on. The BIOS on an iMac is actually called OpenFirmware. You use OpenFirmware when you need to boot your iMac from an external booting device, change or reset your iMac OpenFirmware settings or if you want to boot the comptuter using a Linux CD. OpenFirmware is easy to access.
Turn on your iMac by pressing the power button.
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Open OpenFirmware by holding down the "Command," "Option," "O" and "F" keys simultaneously. Listen for Two chimes will sound and wait for the next command. After a second or two, "OK" appears on the screen and the command line changes to "0>."
Type in any commands in Open OpenFirmware after the "0>" prompt.