Intel 64 and IA-32 Architectures Software Developers Manual Volume 3A, System Programming Guide, Part 1

Vol. 3A 4-15
PROTECTION
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 5, “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.
• 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 6.3, “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 4.8.7, “Performing Fast Calls to System Procedures
with the SYSENTER and SYSEXIT Instructions.”
4.8.1 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 4-6):
• 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.)