Somewhat off topic from the main thread of the article, but I have always wondered about the multiple privilege levels. What's the expected/intended use for them? The only thing I think of is separating out hardware drivers (assuming ring 1 can still directly read/write I/O ports or memory addresses mapped to hardware) so they can't crash the kernel should the drivers or hardware turn out to be faulty. But I don't think I've ever heard of such a design being used in practice. It seems everyone throws their driver code into ring 0 with the rest of the kernel, and if the driver or hardware faults and takes the kernel with it, too bad, so sad. Ground R̅E̅S̅E̅T̅ and start over.

What I find myself wondering is why? It seems like a good idea on paper, at least. Is it just a hangover from other CPU architectures that only had privileged/unprivileged modes, and programmers just ended up sticking with what they were already familiar and comfortable with? Was there some painful gotcha about multiple privilege modes that made them impractical to use, like the time overhead of switching privilege levels made it impossible to meet some hardware deadline? Silicon-level bugs? Something else?