C H A P T E R 5
Software
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.
IMPORTANT
The previous API for controlling the startup device selection, using _GetDefaultStartup and _SetDefaultStartup, is not effective on the iMac. ▲
Open Firmware and the Device Tree
NewWorld relies heavily on a functioning Open Firmware with a complete device tree.
Interrupt Layout
The interrupt layout is determined by information in the device tree. An interrupt tree overlays the other information in the device tree to describe how the interrupts are configured. The Trampoline code traverses this device tree interrupt tree and builds data structures that are used to dispatch interrupts. The device tree interrupt tree is defined in the Open Firmware Recommended Practice: Interrupt Mapping. It is not necessary to change any of the interrupt dispatching code, either 68K or native. All the necessary information is retrieved from the device tree.
This interrupt dispatch code has drastically reduced latency times as compared to all previous PCI Macintosh computers.
Machine Identification
Applications can find out which machine they are running on by calling the Gestalt Manager. The gestaltMachineType value returned for the iMac is 406 (196 hexadecimal).
Because NewWorld uses the same Universal and ProductInfo tables for all computer models that it runs on, those computers all have the same Box Flag. All those computers use the same enablers, and no patches are made to the
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