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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 new PowerBook computer, 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 hardware channels: UltraDMA66, FireWire, and USB.
The default operating system, the Mac OS ROM image, is loaded from the current startup device. The user may interrupt Open Firmware's boot device selection by holding down the Option key while booting. This invokes the OS Picker, an Open Firmware application that lets the user select an alternate operating system or boot device.
Alternatively, 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.