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Vol. 3 5-17
PROTECTION
The RPL of the segment selector that points to a nonconforming code segment has a
limited effect on the privilege check. The RPL must be numerically less than or equal
to the CPL of the calling procedure for a successful control transfer to occur. So, in the
example in
Figure 5-7, the RPLs of segment selectors C1 and C2 could legally be set
to 0, 1, or 2, but not to 3.
When the segment selector of a nonconforming code segment is loaded into the CS
register, the privilege level field is not changed; that is, it remains at the CPL (which
is the privilege level of the calling procedure). This is true, even if the RPL of the
segment selector is different from the CPL.
5.8.1.2 Accessing Conforming Code Segments
When accessing conforming code segments, the CPL of the calling procedure may be
numerically equal to or greater than (less privileged) the DPL of the destination code
segment; the processor generates a general-protection exception (#GP) only if the
CPL is less than the DPL. (The segment selector RPL for the destination code segment
is not checked if the segment is a conforming code segment.)
Figure 5-7. Examples of Accessing Conforming and Nonconforming Code Segments
From Various Privilege Levels
Code
Segment D
Code
Segment C
Code
Segment A
Lowest Privilege
Highest Privilege
CPL=3
Code
Segment B
Nonconforming
Code Segment
Conforming
Code Segment
3
2
1
0
CPL=2
DPL=2
DPL=1
Segment Sel. D1
RPL=2
Segment Sel. D2
RPL=3
Segment Sel. C2
RPL=3
Segment Sel. C1
RPL=2