PATH ADC Home > Documentation > Hardware > Device Managers and Drivers > PCI Card Services > Designing PCI Cards and Drivers for Power Macintosh Computers

  

ROM-in-RAM

The NewWorld architecture puts the Mac OS ROM image in RAM and marks it read-only. Although the image is 4 MB in size, not all of those 4 MB are 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, about 3 MB of the 4 MB Mac OS ROM image are in use, allowing about 1 MB to be returned to Mac OS as available RAM.

The fact that ROM is stitched into RAM is the reason that the logical and physical memory addresses are no longer mapped one-to-one.

© 1999 Apple Computer, Inc. – (Last Updated 26 March 99)