The system software in all current Macintosh computers uses a ROM-in-RAM approach, also called the New World architecture. For more information about this architecture, see Technote 1167, NewWorld Architecture, available on Apple's technote website at
http://developer.apple.com/technotes/tn/tn1167.html
With the ROM-in-RAM approach, memory is not mapped one-to-one as it was for earlier PCI-based Macintosh computers. This could be a compatibility issue with some software. For more information see Technical Q&A DV 33, PrepareMemoryForIO for the New World, available on Apple's Q&A website at
http://developer.apple.com/qa/dv/dv33.html