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Four rings: kernel (ring 0) through user (ring 3) lower numbers are more privileged lower numbers never call/trust higher numbers higher numbers call lower numbers only through narrow interfaces (e. g. , system calls) Cpl - current privilege level (of executing code) Rpl - request privilege level; when code t high privilege level executes on behalf of code at lower level, some accesses may voluntarily lower privilege to that of caller/beneficiary. Dpl - descriptor privilege level; level necessary to execute code/data. A segment is a contiguous portion of a linear address space such as the 32-bit space of physical addresses x86 in protected mode always uses segmentation. Es still more extras (floating point + another) fs. Gdt entries can also describe local descriptor tables (ldts) Ldt originally meant to be per-task segment tables. Ldtr points to the current ldt (includes base, size, and index of ldt in gdt)

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