problem about call gate to ring 0

[pker]

i read an article about adding a privilege 3 call gate specifying to a ring 0 code segment in GDT. i wrote one, but it only work in xp. i got a service starting error in 2k. here's the code in FASM:

format PE native 4.0
entry _start

_start:
push ebx
sgdt [esp-2]
pop ebx

mov ecx,3a0h

cmp dword [ebx+ecx+2],0ec0003a8h
jz _ret

mov byte [ebx],0c3h

mov eax,ebx
mov word [ebx+ecx],ax
shr eax,16
mov word [ebx+ecx+6],ax
mov dword [ebx+ecx+2],0ec0003a8h

mov dword [ebx+ecx+8],0000ffffh
mov dword [ebx+ecx+12],00cf9a00h

_ret:
xor eax, eax
ret 8

does anybody know what the problem is?