SYSENTER instruction.. Pentium II..

| sysenter instruction |

(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

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.