mov eax,3
mov ebx,0
mov ecx,X
mov edx,1
int 80h

mov ax,[X]
xor ax,1
mov [X],ax

mov eax,4
mov ebx,1
mov ecx,X
mov edx,1
int 80h

mov eax,1
mov ebx,0
int 80h

section .data
X db 0