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Open Firmware is the process that controls the microprocessor after hardware initialization and diagnostics are performed, but before the main operating system is passed control. Open Firmware is responsible for, among other things, building the device tree and probing the expansion slots for I/O devices. Open Firmware queries PCI devices for address space requirements and dynamically assigns the needed address space to each device. It is during this probing process that each device and ASIC on the logic board is given a node in the device tree. Hardware and software engineers can use the Open Firmware user interface to debug their device and driver, respectively. See Technote 1044, Understanding PCI Expansion Choices for Mac OS 8, Part III in the Open Firmware Technote Series, for details about properties and methods for various devices. You must be able to traverse the device tree to get to your device node and then to edit and debug that node.
The Macintosh implementation of Open Firmware includes the user interface described in IEEE Standard 1275. The user interface provides an interactive terminal environment that is useful in viewing and manipulating Open Firmware data structures and other system-level resources, such as memory and device registers, in the absence of a running operating system. On Power Macintosh PCI computers that don't support the NewWorld architecture, the default implementation operates from a remote terminal connected by a serial communication link to the modem port of the target PCI-based Power Macintosh computer. On Power Macintosh computers that support the NewWorld architecture, the default implementation operates in a one machine mode, and can be set to remote serial mode. The serial link's default settings are as follows:
Open Firmware version 1.0 and 2.0 use 38400 baud Open Firmware version 3.0 uses 57600 baud No parity 8 data bits 1 stop bit XON/XOFF handshake ANSI/VT102 terminal protocol
In addition to the remote terminal method of accessing Open Firmware, Power Macintosh G3 computers support invocation of the user interface on the current machine without the need for a remote serial connection. See Invoking the User Interface On the Current Machine, for additional information.