On a PC, there is a basic input/output system, or BIOS, that stores configuration information to allow your software and hardware to function together. The BIOS can be entered at start-up to allow the user to manipulate setting using a graphical interface. On a Mac with OS X, most of the hardware settings must be altered within the operating system, though there is an Open Firmware which is similar to BIOS that can be accessed at boot. It is a command line interface that allows the user to change some settings, including selecting the boot device.
Press the power button on your Mac system.
Pres and hold CMD, OPT and "F" with your left hand. The CMD key has an apple on it and the OPT key is in the same position as the ALT key on a PC keyboard.
Press "O" with your right hand.
Enter commands when the prompt "0 >" loads on the screen.
Type "Mac-boot" and press "Enter" to exit the firmware and continue booting the operating system.
Type "Shut-down" and press "Enter" to exit the firmware and shut down the computer.
Be very cautious when working in the Mac firmware. Mistakes can render the system unstable and can even void your warranty.