Convert from ascii to integer in AT&T Assembly

Question

How can I convert from ascii to integer?

    data

asd: 
    .int 32

.text

.globl _start

_start:

    movl $4, %eax
    movl $1, %ebx
    movl $1,(asd)
    add $48,(asd)
    movl $asd, %ecx
    movl $4, %edx
    int $0x80

    # Exit the program
    movl $1, %eax
    movl $0, %ebx
    int $0x80

The code is writing an ascii value, and I think that if I can add 48 to value. I can write, but I can't print, a two-stage number "for example 53 or 156". How can I print?

You have to do it one digit at a time. Divide by 10 to isolate the next digit. — Bo Persson
– Bo Persson, Commented Aug 16, 2015 at 11:01
NASM version of this: NASM Assembly convert input to integer? — Peter Cordes
– Peter Cordes, Commented Dec 31, 2021 at 8:12

Peter Cordes · Accepted Answer · 2022-01-01 06:12:37Z

2

It's usually best to think about a high level language version first. If the number has n digits and is stored in array a, then we'd want:

char *p = a;
unsigned val = 0;
while (n > 0) {
  n--;
  val = 10 * val + (*p++ - '0');
}

So let's say %esi is p above %eax is val, and %ecx is n. Translate line by line to keep it as simplistic and literal as possible:

  movl $n, %ecx
  movl $a, %esi
  xorl %eax, %eax     # val = 0
eval_while_cond:
  testl %ecx, %ecx    # if (n <= 0) 
  jle done            #   goto done
  subl $1, %ecx       # n--
  movl %eax, %ebx     # tmp1 = val
  imul $10, %ebx      # tmp1 = 10 * tmp1
  movzbl (%esi), %eax # tmp2 = *p
  addl $1, %esi       # p++
  subl $'0, %eax      # tmp2 = tmp2 - '0'
  addl %eax, %ebx     # val = tmp2 + tmp1
  jmp eval_while_cond
done:
  # result is now in %eax

Note this is just a general methodology for getting functional assembly language. This isn't optimized in any way. The tricks needed to get optimal performance are a whole other topic.

The other options is to use a compiler (e.g. https://godbolt.org/z/WzGYjePzx) with light optimization like -O1, -Og, or -Os to get somewhat-readable asm (although in this case they use not-totally-obvious tricks with LEA to multiply by 10 and subtract '0' (48), or they make a mess with weird loop conditions.) See also How to remove "noise" from GCC/clang assembly output?

edited Jan 1, 2022 at 6:12

Peter Cordes

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answered Sep 6, 2015 at 21:18

Gene

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1 Comment

Peter Cordes Over a year ago

mul doesn't work with immediates, you want imul, and the comment character in AT&T syntax is #. Also, prefer test %ecx, %ecx instead of or. See NASM Assembly convert input to integer? for an efficient version of this loop. (Except it stops on *p not being a digit, rather than using a counted loop.)

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