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<p>Most CPUs has a concept of more rings that what most modern operating systems use. This has historical reasons, which is also why <code>Ring 0</code> and <code>Ring 3</code> are used (and not 1, 2).</p>
<p>Now every entry in the page table has additional information about it, amongst that information is the information about what ring it belongs to. This information is set up when your OS boots up.</p>
<p>Code executed in <code>Ring 0</code> has almost unrestricted access to external de