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5-10 Vol. 3
PROTECTION
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 gener
-
ates a general-protection exception (#GP).
To carry out privilege-level checks between code segments and data segments, the
processor recognizes the following three types of privilege levels:
• Current privilege level (CPL) — The CPL is the privilege level of the currently
executing program or task. It is stored in bits 0 and 1 of the CS and SS segment
registers. Normally, the CPL is equal to the privilege level of the code segment
from which instructions are being fetched. The processor changes the CPL when
program control is transferred to a code segment with a different privilege level.
The CPL is treated slightly differently when accessing conforming code segments.
Conforming code segments can be accessed from any privilege level that is equal
to or numerically greater (less privileged) than the DPL of the conforming code
segment. Also, the CPL is not changed when the processor accesses a conforming
code segment that has a different privilege level than the CPL.
• Descriptor privilege level (DPL) — The DPL is the privilege level of a segment
or gate. It is stored in the DPL field of the segment or gate descriptor for the
segment or gate. When the currently executing code segment attempts to access
a segment or gate, the DPL of the segment or gate is compared to the CPL and
RPL of the segment or gate selector (as described later in this section). The DPL
is interpreted differently, depending on the type of segment or gate being
accessed:
— Data segment — The DPL indicates the numerically highest privilege level
that a program or task can have to be allowed to access the segment. For
Figure 5-3. Protection Rings
Level 0
Level 1
Level 2
Level 3
Protection Rings
Operating
Operating System
Services
System
Kernel
Applications