Printing an integer with x86 32-bit Linux sys_write (NASM)

Question

I\'m new to this forum. I have a little experience with high-level languages (really little). Nearly one month ago I thought it would be a good idea to see how assembly work

Answer 1

Your numbers will quickly grow larger than just a single digit. What you ought to be doing is have an integer in num rather than a character, and then convert that integer into a string that you can print with sys_write.

Here's one way of doing the conversion: repeated division by 10, getting the lowest digit first as the remainder:

; Input:
; eax = integer value to convert
; esi = pointer to buffer to store the string in (must have room for at least 10 bytes)
; Output:
; eax = pointer to the first character of the generated string
; ecx = length of the generated string
int_to_string:
  add esi,9
  mov byte [esi],0    ; String terminator

  mov ebx,10
.next_digit:
  xor edx,edx         ; Clear edx prior to dividing edx:eax by ebx
  div ebx             ; eax /= 10
  add dl,'0'          ; Convert the remainder to ASCII 
  dec esi             ; store characters in reverse order
  mov [esi],dl
  test eax,eax            
  jnz .next_digit     ; Repeat until eax==0

  ; return a pointer to the first digit (not necessarily the start of the provided buffer)
  mov eax,esi
  ret

Which you can use like this:

    mov    dword [num],1
    ...
    mov    eax,[num]       ; function args using our own private calling convention
    mov    esi,buffer
    call   int_to_string
; eax now holds the address that you pass to sys_write
    ...

section .bss
    num    resd 1
    buffer resb 10

Your number-doubling can be simplified to shl dword [num],1. Or better, double it at some point while it's still in a register with add eax,eax.