Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

Vol. 2B 4-363
INSTRUCTION SET REFERENCE, N-Z
6. Clears the VM flag in the EFLAGS register, if the flag is set.
7. Begins executing the selected system procedure.
The processor does not save a return IP or other state information for the calling
procedure.
The SYSENTER instruction always transfers program control to a protected-mode
code segment with a DPL of 0. The instruction requires that the following conditions
are met by the operating system:
• The segment descriptor for the selected system code segment selects a flat,
32-bit code segment of up to 4 GBytes, with execute, read, accessed, and non-
conforming permissions.
• The segment descriptor for selected system stack segment selects a flat 32-bit
stack segment of up to 4 GBytes, with read, write, accessed, and expand-up
permissions.
The SYSENTER can be invoked from all operating modes except real-address mode.
The SYSENTER and SYSEXIT instructions are companion instructions, but they do not
constitute a call/return pair. When executing a SYSENTER instruction, the processor
does not save state information for the user code, and neither the SYSENTER nor the
SYSEXIT instruction supports passing parameters on the stack.
To use the SYSENTER and SYSEXIT instructions as companion instructions for transi-
tions between privilege level 3 code and privilege level 0 operating system proce-
dures, the following conventions must be followed:
• The segment descriptors for the privilege level 0 code and stack segments and
for the privilege level 3 code and stack segments must be contiguous in the
global descriptor table. This convention allows the processor to compute the
segment selectors from the value entered in the SYSENTER_CS_MSR MSR.
• The fast system call “stub” routines executed by user code (typically in shared
libraries or DLLs) must save the required return IP and processor state
information if a return to the calling procedure is required. Likewise, the
operating system or executive procedures called with SYSENTER instructions
must have access to and use this saved return and state information when
returning to the user code.
The SYSENTER and SYSEXIT instructions were introduced into the IA-32 architecture
in the Pentium II processor. The availability of these instructions on a processor is
indicated with the SYSENTER/SYSEXIT present (SEP) feature flag returned to the
EDX register by the CPUID instruction. An operating system that qualifies the SEP
flag must also qualify the processor family and model to ensure that the
SYSENTER/SYSEXIT instructions are actually present. For example:
IF CPUID SEP bit is set
THEN IF (Family
= 6) and (Model < 3) and (Stepping < 3)
THEN
SYSENTER/SYSEXIT_Not_Supported; FI;
ELSE