Apple Desktop Computer manual NanoKernel, Startup Disk Control Panel

C H A P T E R 5

Software

variable-sized partitions that are used by Mac OS, Open Firmware, and any other client. PRAM resides in the Mac OS partition. The partitioning scheme is part of the CHRP specification.

NanoKernel

The previous version of the NanoKernel has code that is processor-specific to create data structures. Under NewWorld, the Trampoline code creates these data structures from information in the Open Firmware Device Tree.

NanoKernel is typically no longer changed to support a new CPU. Support for new processors has moved to POST, which is responsible for configuring all processor-specific registers. Run-time cache control is part of RTAS.

Startup Disk Control Panel

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 iMac 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 implemention is very different.

Previous systems stored the user's selected startup device in PRAM. The startup device was set in PRAM 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 iMac, rather than setting Mac OS PRAM, sets an Open Firmware config variable called boot-device. 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.

■Key combination Command-Option-Shift-Delete: ignore the boot-device setting and scan for alternate devices.

■C key: force the internal CD-ROM drive to be the startup device

■D key: force the internal hard disk to be the startup device

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