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The OS protects certain areas of memory. If you were allowed entry to them you could easily destroy the system. We used to do this all the time (destroy things, I mean), but we were the only users, and thus the only one punished. It's actually worse than just protecting. Memory is mapped. The physical address is not the address you see in (for example) your debugger. When the next process switches in, it might see the exact same address, but it wouldn't be the same memory. The uP has differing levels of protection. You have to get low to do anything you like. Again, the uP merely makes those capabilities available. The OS (of whatever flavor) uses them to whatever extent the OS designer saw fit to use. Including not at all.
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