Apple iMac - Manual (Page 65)

C H A P T E R 5
Software
What Is Different 65
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 NewWorld, many resources in the ROM exist in the System Folder as
well, often as replacements that fix or enhance those in the ROM, but sometimes
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 ToolBox ROM Image, and only the resources from
the System Folder are in use.
RAM Footprint 5
NewWorld puts the ToolBox 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 Mac OS for use as part of system RAM. At the
time this document was written, less than 3 megabytes of the 4 megabyte
ToolBox ROM Image are in use, allowing more than 1 megabyte to be returned
to 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, non-volatile RAM (NV-RAM), restart, shutdown, and PCI configuration
cycles. The I/O primitives for these functions in the ToolBox ROM Image for
NewWorld use RTAS.
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