Communications System, which was superficially a large terminal that, depending
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The slow path: hardware page walks
The 386 supports four privilege rings (0 through 3), though in practice nearly all operating systems use just two: ring 0 for the kernel and ring 3 for user programs. Three privilege levels interact on every segment access: CPL (Current Privilege Level), DPL (Descriptor Privilege Level), and RPL (Requested Privilege Level).