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CHAPTER 5
System Software
What Is Different 85
Interrupt Handling 5
Interrupt handling is very different with the NewWorld approach. The
interrupt code has been rewritten to allow for dynamic creation of the interrupt
layout. The new code has two features that did not exist in the old code. One is
that interrupt latency has been reduced to such an extent as to make it
negligible. The other is that the interrupt handling code no longer requires
changes to support a new machine unless it has new interrupt controller
hardware. The description of the interrupt layout is now part of an Open
Firmware interrupt tree that is interlaced within the Open Firmware device tree.
The Trampoline code uses this interrupt tree to build the Mac OS native
interrupt tree.
Outmoded Resources 5
Prior to the NewWorld architecture, many resources in the ROM existed in the
System Folder as well, often as replacements that fix or enhance those in the
ROM, but sometimes merely because the ROM resources have not yet been
removed from the ROM. With the NewWorld approach, any resources that are
not needed early in the boot sequence are no longer in the Mac OS ROM Image,
and only the resources from the System Folder are in use.
RAM Footprint 5
The NewWorld architecture puts the Mac OS ROM Image in RAM, and marks it
read-only. Although the image is 4 megabytes in size, not all of it is in use. The
portion that is not used is returned to the Mac OS for use as part of system
RAM. At the time this document was written, less than 3 megabytes of the 4
megabyte Mac OS ROM Image are in use, allowing more than 1 megabyte to be
returned to the Mac OS.
RTAS 5
Certain hardware devices differ from machine to machine, but provide similar
functions. RTAS ( Run-time Abstraction Services) provides such
hardware-specific functions, including functions for accessing the real-time
clock, nonvolatile RAM (NV-RAM), restart, shutdown, and PCI configuration
cycles. The I/O primitives for these functions in the Mac OS ROM Image for
NewWorld use RTAS.