How Does a Mac Not Have a BIOS?
A PC's Basic Input/Output System is the first program to run at start-up. It initiates the boot process, loads the operating system and provides an interface between the operating system and the hardware. It also provides a user interface, separate from the operating system, that allows you to view and alter your hardware preferences. Although Macs don't have a BIOS, they still need a system that performs similar functions. Some Macs achieve this with a system known as Open Firmware, while others use a program called Extensible Firmware Interface.
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Open Firmware
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All Macs with Power PC processors use Open Firmware rather than a BIOS. Open Firmware is similar to a BIOS in that it performs the Power On Self Test every time you start up your computer. The POST ensures that all your hardware devices are operational and able to communicate with each other. Open Firmware also allows you to access its settings, like a BIOS. According to MacTech magazine, it's possible to lock Open Firmware, preventing an unauthorized source from booting your computer.
Extensible Firmware Interface
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In 2006 Apple replaced Open Firmware with an Intel technology known as the Extensible Firmware Interface. At that time, Macs started using Intel processors. Like BIOS and Open Firmware, EFI performs the POST at start-up, boots up the computer and mediates communication between the operating system and the hardware. However, EFI Macs don't include a separate interface that allows you to access EFI settings.
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Access
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You can access the interface or "shell" for Open Firmware on a Mac by entering a specific key command at start-up. To do so, turn your computer on, and press and hold the Option, Command, O and F keys before it boots up. This only works with Macs with Power PC processors. To determine if you have one of these processors, go to the Apple menu in the upper-left corner of the screen and select "About This Mac." This opens a window that displays your processor model. If you have an Intel processor, you have EFI rather than Open Firmware.
Alternatives
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Even though Intel Macs don't provide access to their EFI, they allow you to view and alter some of the hardware settings through the computer's graphical user interface. For example, choosing Startup Disk from System Preferences opens a window that allows you to choose the system that boots your computer. You can also find further hardware options in the Utilities section, located in the Applications folder.
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References
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