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.
For processors supporting Intel® EM64T, the SYSENTER and SYSEXIT instructions are enhanced to allow fast system calls from user code running at privilege level 3 (in compatibility mode or 64-bit mode) to 64-bit executive procedures running at privilege level 0.