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Another major difference introduced with the NewWorld architecture is that logical RAM and physical RAM are no longer mapped one-to-one, as they have been in previous PCI-based Macintosh computers. This means that well-behaved software must call the LogicalToPhysical or PrepareMemoryForIO functions, as described in Memory Services Used During I/O Operations. Software logic that assumes the logical and physical addresses are the same, even when virtual memory is not turned on, will fail to operate properly.