Open Firmware basics

pSeries CHRP systems use a firmware that conforms to the open firmware IEEE Std. 1275-1994. Open Firmware is primarily a boot firmware that does not specify a particular system or processor. It also provides a machine independent devices tree interface that lists the properties of each IO device which can be displayed and altered using a command line interface.
There are Open Firmware bindings for a variety of systems architectures. The one that pertains to pSeries is described in: http://playground.sun.com/1275/bindings/chrp/chrp1_8a.ps
The home of the Open Firmware working group is located at: http://playground.sun.com/1275.

Open Firmware prompt

pSeries systems can be booted to SMS or to the open firmware prompt, sometimes referred to as the ok prompt. You can reach the open firmware prompt by hitting the 8 key while booting. If under AIX, system administrators rarely have to use this.
For installing Linux, however, we sometimes had to boot to the OK prompt to pass arguments to the kernel, as described in Example 2-29 on page 75. The Open Firmware prompt is also used extensively by the netboot command in CSM to boot/install from the network. SMS itself is an Open Firmware application.
Next, we give a very brief introduction to the Open Firmware prompt command line interface. A quick reference guide is available from: http://www.firmworks.com/QuickRef.html
A thorough description can be found at these sites:
http://developer.apple.com/technotes/tn/tn1061.html
http://developer.apple.com/technotes/tn/tn1062.html

Open Firmware Commands

Booting the system in Open Firmware gives you the screen shown in

Example 2-32 Open Firmware prompt
1 = SMS Menu 5 = Default Boot List
6 = Stored Boot List 8 = Open Firmware Prompt
memory keyboard network scsi speaker ok
0 >

You can navigate inside the device tree. Upon startup, you are at the top of the device tree. Use the ls command to display the device tree, a walk along all the buses and adapters that constitute the system under examination.