how to write fibonacci function in arm assembly code example

Example: Arm assembly fibonacci

/*
 * fibonacci.s
 * Computes the nth fibonacci number.
 *
 */

	.syntax unified
	.global _start


_start:
	mov r0, 0 @ answer
	mov r1, 9 @ n = 9 
	mov r2, 0 @ first value in sequence
	mov r3, 1 @ second value in sequence
	mov r4, 0 @ i = 0

loop:
	add r4, r4, 1
	cmp r1, r4
	ble _exit
	mov r5, r0
	add r0, r2, r3
	mov r2, r0
	mov r3, r5
	bal loop
_exit:
	mov r7, 1
	swi 0