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Open Firmware is a boot environment developed using the Forth programming language. The purpose of Open Firmware is to provide a machine independent mechanism for loading operating systems from a variety of boot devices. Open Firmware probes the PCI bus looking for devices and possible Open Firmware drivers for those devices. These drivers can either be built into the Open Firmware module or located in the external device, thus providing plug and play capabilities for new boot devices. Open Firmware is capable of using these drivers to load an operating system from the device.
Functions of the Open Firmware code include
On the iBook, the Open Firmware code has been extended so that the device tree describes the new hardware features of the computer. The Open Firmware code also includes FCode drivers for the new UltraDMA66 and USB hardware channels.
Open Firmware normally starts up using the default device and operating systems. The user can interrupt Open Firmware by holding down the Command, Option, O, and F keys. Open Firmware responds by providing a command line interface using the keyboard and built-in display. Using this interface, users can change the stored parameters used by Open Firmware.