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 will use OpenFirmware when you need to boot your iMac from an external booting device, reset your iMac OpenFirmware settings or if you want to boot up by using a Linux CD. OpenFirmware is easy to access.
Turn on your iMac by pressing the power button.
Open OpenFirmware by holding down the "Command," "Option," "O" and "F" keys simultaneously.
Two chimes will sound. Wait for the next command.
Momentarily, you will see "OK," then the command line will change to "0>."
Type in any commands in Open OpenFirmware after the "0>" prompt.