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| A '''privilege level''' in the ] ] controls the access of the program currently running on the processor to resources such as memory regions, I/O ports, and special instructions. There are 4 privilege levels ranging from 0 which is the most privileged, to 3 which is least privileged. Most modern operating systems use level 0 for the kernel/executive, and use level 3 for application programs. Any resource available to level n is also available to level 0..n, so the privilege levels are "]". Privilege levels appear in several places in the x86 architecture, including: | |||
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| * 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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| * Every ] has a privilege level called '''Descriptor Privilege Level (DPL)'''. Segment descriptors contain a field corresponding to the descriptor privilege level (DPL). This is the numerically highest level that can access the resource. Thus, if the current privilege level (CPL) is 0 then the task can access all the privilege descriptors and if the current privilege level (CPL) is 3 then only the level 3 descriptors can be accessed. | |||
| * ]s 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. This is called as the '''Effective Privilege Level'''. RPL may weaken the privilege level of a task. | |||
| 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>](RPL, DPL), 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 and later processors. 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 | |||
| ⚫ | ] | ||
| {{Compu-hardware-stub}} | |||
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