PATH
Devices connected to third-party expansion cards that typically expect to participate in the boot process must provide Open Firmware boot drivers in the expansion ROM on the card. These drivers are written in FCode, the forth programming language. Without the FCode driver, devices are not recognized early in the startup process. Expansion cards without FCode are recognized only after the Mac OS has started and the associated runtime drivers are located on the disk. For example, a disk drive attached to a SCSI expansion card that does not include proper FCode in the expansion ROM is not able to boot, but the drive shows up on the desktop in the Finder after the SCSI card runtime driver is loaded from disk by the Mac OS.
Devices connected to Macintosh built-in I/O, such as ATA storage devices and on-board Ethernet ports, are supported by FCode in the Macintosh bootROM on the main logic board. Third-party PCI cards on the PCI bus, such as SCSI mass storage device controllers, Ethernet cards, and graphics display controllers, require Open Firmware FCode drivers in the PCI card expansion ROM in order for the devices connected to those cards to be recognized early in the boot process, before startup control switches from Open Firmware to the Mac OS.
Open Firmware provides boot mechanisms and a description of the system hardware, in the form of a device tree . The IEEE Std 1275-1994: Standard for Boot (Initialization, Configuration) Firmware: Core Requirements and Practices documentation, along with associated bindings, provides the specification for Open Firmware. The current bindings are available at
http://playground.sun.com/1275/home.html
The Macintosh implementation of Open Firmware is discussed in Startup and System Configuration NewWorld startup disk behavior is described in Startup Disk Control Panel. An introduction to Open Firmware drivers and the Open Firmware environment on the Macintosh is provided in PCI Open Firmware Drivers.