The following comes from the "IA-32 Intel Architecture Software Developer’s Manual Volume 3: System Programming Guide", chapter 4, page 7:
"The processor’s segment-protection mechanism recognizes 4 privilege levels, numbered from 0 to 3. The greater numbers mean lesser privileges. Figure 4-2 shows how these levels of privilege can be interpreted as rings of protection. The center (reserved for the most privileged code, data, and stacks) is used for the segments containing the critical software, usually the kernel of an operating system. Outer rings are used for less critical software. (Systems that use only 2 of the 4 possible privilege levels should use levels 0 and 3.)
"The processor uses privilege levels to prevent a program or task operating at a lesser privilege level from accessing a segment with a greater privilege, except under controlled situations. When the processor detects a privilege level violation, it generates a general-protection exception..."
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explanation of "rings"
Posted by: gus3 on July 24, 2005 12:57 PM"The processor’s segment-protection mechanism recognizes 4 privilege levels, numbered from 0
to 3. The greater numbers mean lesser privileges. Figure 4-2 shows how these levels of privilege
can be interpreted as rings of protection. The center (reserved for the most privileged code, data,
and stacks) is used for the segments containing the critical software, usually the kernel of an
operating system. Outer rings are used for less critical software. (Systems that use only 2 of the
4 possible privilege levels should use levels 0 and 3.)
"The processor uses privilege levels to prevent a program or task operating at a lesser privilege
level from accessing a segment with a greater privilege, except under controlled situations.
When the processor detects a privilege level violation, it generates a general-protection exception..."
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