Open Firmware is a type of firmware that some computer systems use when they boot up. It controls the processor and performs system diagnostics before the operating system is loaded. Open Firmware also builds the "device tree," which locates internal and external devices connected to the computer. Each device is then assigned a unique address so it can be used once the computer starts up.
Several types of computers use Open Firmware, including PowerPC-based Macintosh systems, Sun Microsystems SPARC-based workstations, and IBM POWER systems. (Most Windows-based PCs use the BIOS for the same purpose.) Because Open Firmware is an "open" standard, devices that support Open Firmware can be typically be used in multiple Open Firmware-based systems. For example, identical PCI cards could be used in both Sun and Macintosh-based computer systems.
To access the Open Firmware interface on a PowerPC-based Macintosh, press and hold "Command-Option-O-F" during startup. On Sun systems, the Open Firmware interface is displayed at startup and can be accessed afterwards by pressing "L1-A" (or Stop-A) while the computer is running.