Intel ARCHITECTURE IA-32 User Manual download pdf (Page 158)

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PROTECTION

new CPL (excluding conforming code segments), the segment register is loaded with a null

segment selector.

See the description of the RET instruction in Chapter 3, Instruction Set Reference, of the IA-32

Intel Architecture Software Developer’s Manual, Volume 2, for a detailed description of the priv-

ilege level checks and other protection checks that the processor performs on a far return.

4.8.7 Performing Fast Calls to System Procedures with the

SYSENTER and SYSEXIT Instructions

The SYSENTER and SYSEXIT instructions were introduced into the IA-32 architecture in the

Pentium II processors for the purpose of providing a fast (low overhead) mechanism for calling

operating system or executive procedures. SYSENTER is intended for use by user code running

at privilege level 3 to access operating system or executive procedures running at privilege level

0. SYSEXIT is intended for use by privilege level 0 operating system or executive procedures

for fast returns to privilege level 3 user code. SYSENTER can be executed from privilege levels

3, 2, 1, or 0; SYSEXIT can only be executed from privilege level 0.

The SYSENTER and SYSEXIT instructions are companion instructions, but they do not consti-

tute a call/return pair. This is because SYSENTER does not save any state information for use

by SYSEXIT on a return.

The target instruction and stack pointer for these instructions are not specified through instruc-

tion operands. Instead, they are specified through parameters entered in MSRs and general-

purpose registers.

For SYSENTER, target fields are generated using the following sources:

• Target code segment — Reads this from IA32_SYSENTER_CS.

• Target instruction — Reads this from IA32_SYSENTER_EIP.

• Stack segment — Computed by adding 8 to the value in IA32_SYSENTER_CS.

• Stack pointer — Reads this from the IA32_SYSENTER_ESP.

For SYSEXIT, target fields are generated using the following sources:

• Target code segment — Computed by adding 16 to the value in the

IA32_SYSENTER_CS.

• Target instruction — Reads this from EDX.

• Stack segment — Computed by adding 24 to the value in IA32_SYSENTER_CS.

• Stack pointer — Reads this from ECX.

The SYSENTER and SYSEXIT instructions preform “fast” calls and returns because they force

the processor into a predefined privilege level 0 state when SYSENTER is executed and into a

predefined privilege level 3 state when SYSEXIT is executed. By forcing predefined and consis-

tent processor states, the number of privilege checks ordinarily required to perform a far call to

another privilege levels are greatly reduced. Also, by predefining the target context state in

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