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The system software in the new PowerBook computer uses the ROM-in-RAM approach also used in the PowerBook G3 Series 1999 computer and other current Macintosh computers. With the ROM-in-RAM approach, also called the NewWorld software architecture, a small ROM contains the code needed to initialize the hardware and load an operating system. The rest of the system code that formerly resided in the Mac OS ROM is loaded into RAM from disk or from the network.
The small ROM that is needed for the computer's start-up activities, called the boot ROM, is 1 MB in size. It includes the hardware-specific code and tables needed to start up the computer, to run Open Firmware, to provide common hardware access services, and to load the Mac OS ROM image.
High-level software resides in an image called the Mac OS ROM that is read into RAM during startup. Once the Mac OS begins operation, the Mac OS ROM image in RAM behaves in the same way that the corresponding code in ROM formerly did. Most of the changes are completely transparent to the Mac OS.
For more information about the ROM-in-RAM approach, see Technote 1167, NewWorld Architecture, available on the Technote website at
http://developer.apple.com/technotes/tn/tn1167.html