Mac OS X Internals: A Systems Approach

2.1. Firmware

The firmware is not technically a part of Mac OS X, but it plays an important role in the operation of an Apple computer. Whereas PowerPC-based Apple computers use Open Firmware, x86-based Apple computers use the Extensible Firmware Interface (EFI).

Open Firmware is a nonproprietary, platform-independent boot firmware that lives in the boot ROM^[4] of a PowerPC-based Apple computer. Its role in booting is somewhat analogous to that of the PC BIOS for x86-based computers. However, Open Firmware includes various other capabilities: It can be used for customized booting, diagnosing, debugging, and even programming. In fact, Open Firmware is a user-accessible runtime and programming environment in its own right. EFI is conceptually very similar to Open Firmware. Chapter 4 discusses Open Firmware and EFI.

^[4] In modern Apple computers, the boot ROM is an onboard flash EEPROM.