;exportar label de inicio do programa
global _start

section .data		;seccao data - dados inicializados 

; variaveis de 32 bits
x:	dd 0
n:	dd 4

section .text		;seccao text - codigo

_start: nop

; x = n! - (n-1)!
	
	push	dword [n]
	call	calcula
	add	esp, 4
	mov	[x], eax

; chamada ao sistema para terminar o programa
	mov 	eax, 1		;codigo de "exit"
	mov	ebx, 0		;valor retornado ao sistema
	int	0x80		;chamda ao sitema


calcula:
	push	ebp
   	mov 	ebp, esp	
	push	ebx
	
	sub	esp, 4		;ebp - 4 = n!
	push	dword	[ebp + 8] 
	call	factorial
	add	esp, 4
	mov	[ebp - 4], eax	;save n;
	mov	eax, [n]
	dec	eax
	push	eax
	call	factorial
	add	esp, 4
	mov	ebx, eax	;ebx = (n - 1)!
	mov	eax, [ebp - 4]	;eax = n!
	sub	eax, ebx

	add	esp, 4
	pop	ebx
   	pop	ebp
	ret


factorial:
	push	ebp
   	mov 	ebp, esp
	push	ebx

   	mov 	ebx, [ebp + 8]	
	mov	eax, ebx
  loop: 
	dec	ebx
	cmp	ebx, 1
	jle	endfact
	imul	ebx
	jmp 	loop
   endfact:
	pop	ebx
   	pop	ebp
 	ret