Divide by 10 in assembly not giving correct result

Question

I was trying to implement division by 10 in arm assembly. I followed the method mentioned in the first answer to the question at ARM DIVISION HOW TO DO IT?

This is my code

MOV     r2, #10         
LDR     r3,=0x1999999A   ; 1999999A == (2^32 / 10) + 1
UMULL   r9,r3,r5,r3    ;divide an integer value in r5 by 10   
MOV     r6, r9,LSR r2  ; r6 has the quotient

But this code is not giving the correct result.For example, If I give 0x0000000B in r5, after these steps value in r6 is 0x00066666.

Am I doing anything wrong?

Also, `(2^32 / 10) + 1` is `0x1999999A` according to my calculations ;) — Jester, Apr 14 '14 at 17:02
sorry that was a typo. it is 0x1999999A itself. will change it. — User1234321232, Apr 14 '14 at 17:17
`r9` will contain the low 32 bits of the 64-bit product from `UMULL` (i.e. the fractional part of the approximated quotient). The integer part would be in `r3`. Also, why are you shifting right by `r2`? Can you explain what you're actually trying to compute? — Michael, Apr 14 '14 at 18:07

score 3 · Answer 1 · answered Apr 14 '14 at 21:14

3

uint32(x / 10) = uint64(x * 0xcccccccd) >> 35

E.g. the following code will divide unsigned r5 by 10 with the quotient in r0:

ldr    r2,=0xcccccccd
umull  r0,r1,r5,r2
mov    r0,r1,lsr #3

answered Apr 14 '14 at 21:14

apangin

92,924
10
193
247

score 1 · Answer 2 · answered Nov 12 '15 at 19:13

There is a rather good writeup of division by constant http://thinkingeek.com/2013/08/11/arm-assembler-raspberry-pi-chapter-15/ It has a python script to generate the magic numbers and the code for any constant. For 10 it's

/* r0 contains the argument to be divided by 10 */
ldr r1, .Lu_magic_number_10 /* r1 ← magic_number */
umull r1, r2, r1, r0   /* r1 ← Lower32Bits(r1*r0). r2 ← Upper32Bits(r1*r0) */
mov r0, r2, LSR #3     /* r0 ← r2 >> 3 */
bx lr                  /* leave function */
.align 4
  .Lu_magic_number_10: .word 0xcccccccd

And the arm website has the code on it, but i found it here now http://www.sciencezero.org/index.php?title=ARM:_Division_by_10

Note that this even manages without the later introduced 64bit multiplication.

Divide by 10 in assembly not giving correct result

2 Answers2