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Open Firmware now bears responsibility for locating a startup device. This is very different from previous Mac OS systems where the Mac OS ROM had responsibility for locating the startup device. On the Power Macintosh G3 computer, the Mac OS ROM image itself comes from the startup disk, so decisions regarding startup device must be made earlier in the startup process. Open Firmware recreates as much as possible the user experience of earlier systems but the implementation is very different.
Previous systems stored the user's selected startup device in PRAM. The startup device was set in NVRAM when the user selected a device in the Startup Disk control panel. This device was honored by the Mac OS ROM unless the selected device was unavailable or was overridden by the user.
The startup disk routine for the NewWorld computers sets an Open Firmware configuration variable called boot-device, rather than setting Mac OS PRAM. This setting is honored by Open Firmware unless the selected device was unavailable or was overridden by the user.
The following keys can be used to override the selected startup device.
Once Open Firmware locates a startup device and successfully loads a Mac OS ROM image, it passes information about the chosen device in the bootpath variable. This information, rather than that previously set in PRAM, is subsequently used by the Mac OS ROM to locate the device containing the startup System Folder.
The previous API for controlling the startup device selection, using _GetDefaultStartup and _SetDefaultStartup, is not effective on computers that support the NewWorld architecture.