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A privilege level in the x86 instruction set 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 "rings". Privilege levels appear in several places in the x86 architecture, including:

• The Current Privilege Level (CPL) is the level the processor is currently operating at. It is stored in the lowest 2 bits of the code segment selector (CS).
• Every segment descriptor 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.
• Segment 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. 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>MAX(RPL, DPL), then a general protection fault is generated. See memory segment for more details.

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

• Protected mode
• Memory segment
• Segment descriptors

References

• Intel 80386 Programmer's Reference

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