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| The concept of '''privilege level''' refers to protecting resources on a ]. Different execution threads can have different privilege levels that grant access to resources such as memory regions, I/O ports, and special instructions. On the ] architecture, there are 4 privilege levels ranging from 0 which is the most privileged, to 3 which is least privleged. Most modern operating systems use level 0 for the kernel/executive, and use level 3 for application programs. On ], any resource available to level n is also available to level 0..n, hence it is called a "ring model". Several terms are used in ] including the following: | |||
| *Descriptors contain a field called the descriptor privilege level (DPL). This is the numerically highest level that can access the resource. | |||
| *Selectors contain a field called the requested privilege level (RPL). This allows a program to request a resource at a lower privilege level than it would otherwise use. | |||
| *The Current Privilege Level (CPL) is the level the processor is currently operating at. It is stored in the lowest 2 bits of the ] selector (CS). | |||
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| The processor automatically evaluates the right of a procedure to access another segment by comparing the CPL to the numeric max of the RPL and DPL. If the CPL>MAX(RPL,RPL), then a ] is generated. See ] for more details. | |||
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| It is not necessary to use all four privilege levels. Existing software that was designed to use only one or two levels of privilege can simply | |||
| ignore the other levels offered by the 80386. A one-level system should use privilege level zero; a two-level system should use privilege levels zero | |||
| and three. | |||
| ==See also== | |||
| *] | |||
| *] | |||
| ==References== | |||
| *Intel 80386 Programmer's Reference | |||
| ⚫ | ] | ||
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