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5-14 Vol. 3
PROTECTION
• Load a data-segment register with a segment selector for a nonconforming,
readable, code segment.
• Load a data-segment register with a segment selector for a conforming,
readable, code segment.
• Use a code-segment override prefix (CS) to read a readable, code segment
whose selector is already loaded in the CS register.
The same rules for accessing data segments apply to method 1. Method 2 is always
valid because the privilege level of a conforming code segment is effectively the
same as the CPL, regardless of its DPL. Method 3 is always valid because the DPL of
the code segment selected by the CS register is the same as the CPL.
5.7 PRIVILEGE LEVEL CHECKING WHEN LOADING THE SS
REGISTER
Privilege level checking also occurs when the SS register is loaded with the segment
selector for a stack segment. Here all privilege levels related to the stack segment
must match the CPL; that is, the CPL, the RPL of the stack-segment selector, and the
DPL of the stack-segment descriptor must be the same. If the RPL and DPL are not
equal to the CPL, a general-protection exception (#GP) is generated.
5.8 PRIVILEGE LEVEL CHECKING WHEN TRANSFERRING
PROGRAM CONTROL BETWEEN CODE SEGMENTS
To transfer program control from one code segment to another, the segment selector
for the destination code segment must be loaded into the code-segment register
(CS). As part of this loading process, the processor examines the segment descriptor
for the destination code segment and performs various limit, type, and privilege
checks. If these checks are successful, the CS register is loaded, program control is
transferred to the new code segment, and program execution begins at the instruc
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tion pointed to by the EIP register.
Program control transfers are carried out with the JMP, CALL, RET, SYSENTER,
SYSEXIT, INT n, and IRET instructions, as well as by the exception and interrupt
mechanisms. Exceptions, interrupts, and the IRET instruction are special cases
discussed in
Chapter 6, “Interrupt and Exception Handling.” This chapter discusses
only the JMP, CALL, RET, SYSENTER, and SYSEXIT instructions.
A JMP or CALL instruction can reference another code segment in any of four ways:
• The target operand contains the segment selector for the target code segment.
• The target operand points to a call-gate descriptor, which contains the segment
selector for the target code segment.