What is a SYSENTER instruction ?

ABOUT sysenter instruction

The SYSENTER instruction is part of the "Fast System Call" facility introduced on the Pentium(R) II
processor. The SYSENTER instruction is optimized to provide the maximum performance for transitions to
protection ring 0 (CPL  0). The SYSENTER instruction sets the following registers according to values
specified by the operating system in certain model-specific registers. CS register set to the value of
(SYSENTER_CS_MSR) EIP register set to the value of  (SYSENTER_EIP_MSR) SS register set to the sum of (8
plus the value in SYSENTER_CS_MSR) ESP register set to the value of (SYSENTER_ESP_MSR) The processor
does not save user stack or return address information, and does not save any registers.

(gdb) disass
Dump of assembler code for function __kernel_vsyscall:
0xb780b414 : push %ecx
0xb780b415 : push %edx
0xb780b416 : push %ebp
0xb780b417 : mov %esp,%ebp
0xb780b419 : sysenter
0xb780b41b : nop
0xb780b41c : nop
0xb780b41d : nop
0xb780b41e : nop
0xb780b41f : nop
0xb780b420 : nop
0xb780b421 : nop
0xb780b422 : jmp 0xb780b417
=> 0xb780b424 : pop %ebp
0xb780b425 : pop %edx
0xb780b426 : pop %ecx
0xb780b427 : ret
End of assembler dump.

0xb780b419 : sysenter

http://www.intel.com/software/products/documentation /vlin/mergedprojects/analyzer_ec/mergedprojects/reference _olh/mergedprojects/instructions/instruct32_hh/vc311.htm (This link is either broken or not correct)