Intel ARCHITECTURE IA-32 User Manual download pdf (Page 146)

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4-16 Vol. 3A

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 4-7, 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.

4.8.1.2 Accessing Conforming Code Segments

When accessing conforming code segments, the CPL of the calling procedure may be numeri-

cally 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 4-7. 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

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

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