Open Firmware
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Open Firmware (also, OpenBoot) is a hardware-independent firmware (computer software which loads the operating system), developed by Sun Microsystems, and used in PowerPC-based "New World" Apple Macintosh computers, Sun Microsystems SPARC based workstations and servers, IBM POWER systems, and PegasosPPC systems, among others. On those computers, Open Firmware fulfills the same tasks as BIOS does on PC computers.
It is accessed by a user by a Forth-based language shell interface. Forth is a powerful high level language. For example it is possible to program Open Firmware to solve the Tower of Hanoi problem. [1] (http://www.kernelthread.com/publications/firmware/src/ofhanoi.txt)
It is described by IEEE standard IEEE-1275.
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Advantages of Open Firmware
Because the Open Firmware Forth code is compiled into Fcode (a bytecode) and not into the machine language of any particular computer architecture, Open Firmware code included in, say, an I/O card can be executed by any system that uses Open Firmware. In this way, an I/O card can provide boot-time diagnostics, configuration code, and device drivers that will be usable on any system running Open Firmware, allowing many of the same I/O cards to be used on Sun systems and Macintoshes.
Accessing Open Firmware
Open Firmware displays "ok" as its prompt.
On Sun systems, the Open Firmware interface is displayed on the console terminal prior to the bootstrapping of the system software. If a keyboard is connected, the main video display will be used as the console terminal and Open Firmware can be re-entered at any time by pressing Stop-A (L1-A) on the keyboard. If no keyboard is connected, then the first serial line on the system is usually used as the console and Open Firmware is re-entered by sending a "Break" on the serial line. While the system software is running, various Open Firmware settings can be read or written using the "eeprom" command.
On Macintosh, the Open Firmware interface can be accessed by pressing the keys Cmd-Option-O-F at startup. This functionality is generally only used by developers; for common users, the Mac OS X operating system provides a high level graphical user interface to set up Open Firmware. For instance, it is possible to specify the boot disk or partition without directly using the Open Firmware interface.