Apple iMac - Manual (Page 66)

C H A P T E R 5
Software
66 What Is Different
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 5
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 5
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