Technical information

CHAPTER 5
System Software
The New Approach 79
comparable CPUs and speeds due to the improved interrupt handling with the
NewWorld approach. In addition, performance is improved due to executing
code that normally exists in ROM in RAM, because the RAM devices operate
faster than the ROM devices normally used.
RAM Footprint 5
The 1999 PowerBook G3 Series computer has its Mac OS ROM image stored in
RAM. This removes approximately 3 megabytes of RAM from availability for
other uses. In effect, a system with 64 megabytes of RAM appears to have only
61 megabytes available. Some portion of the missing 3 megabytes is offset by
having fewer patches in RAM. Other mechanisms are being explored in an
attempt to minimize the impact of ROM-in-RAM.
User Experience 5
Setting the startup device from the Startup Disk control panel makes all the
changes to the boot process that are necessary to operate with a Mac OS ROM
image in RAM. The control panel user interface remains unchanged for this
release.
Data Structures and Files 5
The Mac OS ROM image is contained in a new file, named “Mac OS ROM”, that
is kept in the System Folder. The Mac OS ROM image is exactly the same as it
would be if it were an actual Mac OS ROM, containing the high-level software,
the kernel software, and the 68K emulator.
The Startup Disk control panel sets the Open Firmware’s boot-device
configuration variable by modifying the Open Firmware NV-RAM partition
that contains the Open Firmware’s configuration variables. The format of the
NV-RAM partition is defined in the Open Firmware CHRP Binding. The
partition is accessed using RTAS.
Compatibility 5
A Mac OS ROM image that is in write-protected RAM will appear to be a ROM
to all MacOS software and applications. Because the image of the Mac OS ROM
in RAM appears to be a ROM, the ROM-in-RAM approach is completely
compatible with all application and system software.