Technical information

CHAPTER 5
System Software
86 What Is Different
NV-RAM 5
Instead of using hard-coded offsets to locations in NV-RAM for Mac OS
NV-RAM and other information, the Trampoline code breaks NV-RAM into
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. With the NewWorld architecture, 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. Runtime 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 1999 PowerBook G3 Series
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 1999 PowerBook G3 Series computer, 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.
■ Command-Option-Shift-Delete: ignore the boot-device setting and scan for
alternate devices.