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What Has Changed

Hardware-specific code that performs the computer's start-up activities resides in firmware (ROM). That code fits into one ROM called the boot ROM. The boot ROM includes the hardware specific code and tables needed to start up the computer, to load an operating system, and to provide common hardware access services.

All higher level software resides somewhere else. For now, think of it residing in what has been historically known as the Mac OS ROM, but with much of the old hardware-specific code moved into the boot ROM. As before, the Mac OS ROM can still be augmented by enablers, the System file, and extensions.

Prior to the iMac, all Macintosh computers required a ROM component that contained manyh components of the Mac OS software. The NewWorld approach sidesteps this requirement by copying an image of the Mac OS ROM into RAM before the Mac OS begins operation. The area of RAM that contains the Mac OS ROM image is excluded from the available memory space in RAM and is marked as read-only. Once the Mac OS begins operation, a Mac OS ROM image in RAM and an actual Mac OS ROM behave in the same way.

No new or different software interfaces are directly accessible from the Mac OS. During the boot process, software contained in the Mac OS ROM file communicates with Open Firmware to collect information about the hardware, using the Open Firmware Client Interface.

Note

Open Firmware is a central component of the NewWorld architecture. For information on how to get reference material about Open Firmware, see Open Firmware .

Most of the changes are completely transparent to the Mac OS. Only the Startup Disk control panel is affected: it includes added code to modify the Open Firmware's configuration variables in the NV-RAM.