Misplaced Pages

This is an old revision of this page, as edited by David.kaplan (talk | contribs) at 22:56, 4 October 2006 (catagorized). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

The concept of privilege level refers to protecting resources on a CPU. 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 x86 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 x86, 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 x86 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 code segment selector (CS).

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 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. 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

References

• Intel 80386 Programmer's Reference

• X86 Architecture