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Vol. 3 5-15
PROTECTION
• The target operand points to a TSS, which contains the segment selector for the
target code segment.
• The target operand points to a task gate, which points to a TSS, which in turn
contains the segment selector for the target code segment.
The following sections describe first two types of references. See Section 7.3, “Task
Switching,” for information on transferring program control through a task gate
and/or TSS.
The SYSENTER and SYSEXIT instructions are special instructions for making fast calls
to and returns from operating system or executive procedures. These instructions
are discussed briefly in
Section 5.8.7, “Performing Fast Calls to System Procedures
with the SYSENTER and SYSEXIT Instructions.”
5.8.1 Direct Calls or Jumps to Code Segments
The near forms of the JMP, CALL, and RET instructions transfer program control
within the current code segment, so privilege-level checks are not performed. The far
forms of the JMP, CALL, and RET instructions transfer control to other code segments,
so the processor does perform privilege-level checks.
When transferring program control to another code segment without going through a
call gate, the processor examines four kinds of privilege level and type information
(see
Figure 5-6):
• The CPL. (Here, the CPL is the privilege level of the calling code segment; that is,
the code segment that contains the procedure that is making the call or jump.)
• The DPL of the segment descriptor for the destination code segment that
contains the called procedure.
Figure 5-6. Privilege Check for Control Transfer Without Using a Gate
CPL
RPL
DPL
Privilege
Check
CS Register
Segment Selector
For Code Segment
Destination Code
Segment Descriptor
C