Intel 253668-032US User Manual

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