GetKeyState and Return Value

Hello,

The GetKeyState function retrieves the status of the specified virtual key. MSDN

http://msdn.microsoft.com/en-us/library/windows/desktop/ms646301%28v=vs.85%29.aspx

says: "If the high-order bit is 1, the key is down; otherwise, it is up ..."
Why is the contents of register EAX: 0xFFFFFF80 or 0xFFFFFF81, when the SHIFT-Key is pressed? Why isn't 0x000008000? Can anybody explain to me please.

Code:

format PE GUI 4.0
include 'win32a.inc'

; -----------------------------------------------
section '.code' code readable executable
; -----------------------------------------------

start:
       invoke Sleep,100
       invoke GetKeyState,VK_SHIFT
       cmp    eax, 0
       jnl    start
       cinvoke wsprintf,buff,wsformat,eax
       invoke MessageBoxA,0,buff,title,0
       invoke ExitProcess,0

; -----------------------------------------------
section '.data' data readable writeable
; -----------------------------------------------

buff     rb 100
title    db 'Ausgabe ...',0
wsformat db "EAX -->> %08X",0

;-------------------------------------------------
section '.idata' import data readable writeable   
;-------------------------------------------------

library kernel32,'KERNEL32.DLL',\
        user32,'USER32.DLL'

import  kernel32,\
        ExitProcess,'ExitProcess',\
        Sleep,'Sleep'

import  user32,\
        MessageBoxA,'MessageBoxA',\
        GetKeyState,'GetKeyState',\
        wsprintf,'wsprintfA'

Sorry, my English is not so good ...