Double a number in ARM

double.s

.section .text
.global _start

atoi:
	push {fp, lr}
	mov fp, sp
	push {r6, r8, r9, r10}
	add r2, r0, r1
	add r8, r0, #0
	mov r10, #0
	mov r6, #10
loop_atoi:
	ldr r9, [r8]
	and r9, #0xff
	cmp r9, #0x30
	blt end_atoi
	cmp r9, #0x39
	bgt end_atoi
	sub r9, #0x30
	mla r10, r6, r10, r9
	add r8, #1
	b loop_atoi
end_atoi:
	mov r0, r10
	pop {r6, r8, r9, r10}
	pop {fp, pc}
itoa:
	push {fp, lr}
	mov fp, sp
	push {r8, r9, r10}
	mov r8, #0
	mov r9, #0
	mov r10, #0
	ldr r6, =0x1999999a
loop_itoa:
	mov r4, r0
	ldr    r2,=0xcccccccd
	umull  r0,r5,r4,r2
	mov    r0,r5,lsr #3
	mov r6, #10
	mul  r5, r0, r6
	sub  r8, r4, r5
	add r8, r8, #0x30
	lsl r10, r10, #8
	orr r10, r10, r8
	add r9, #1
	cmp r0, #0
	bne loop_itoa

end_itoa:
	str r10, [r1]
	mov r0, r9
	pop {r8, r9, r10}
	pop {fp, pc}
_start:
	push {fp, lr}
	mov fp, sp
	sub sp, sp, #16
	mov r0, #0
	mov r1, sp
	mov r2, #4
	mov r7, #3
	swi 0x0
	mov r1, r0
	mov r0, sp
	bl atoi
	lsl r0, r0, #1
	mov r1, sp
	bl itoa
	add r1, sp, r0
	mov r2, #0xa
	str r2, [r1]

	mov r1, sp
	add r2, r0, #1
	mov r0, #1
	mov r7, #4
	swi 0x0

	mov r0, #0
	mov r7, #1
	swi 0x0
	hlt