Passing parameter in command line, Assembly Programming

Question

Let me explain it briefly :).

I approach the argument 10, by using EBP register with ebx register because of the stack structure of which is containing EBP(base register), return address, # of parameter, parameter 1, parameter 2... since I don't use a local register. I could see the parameter was appropriately input as I could print it using call print_string. But, since < while: > code line, it seems like the string 10 doesn't seem to be read for the commend line does nothing when it comes to it. I would gently ask where to start with the code. thanks for reading.

Input : ./atoi 10
Result : 10

%include "asm_io.inc"

    segment .data

    segment .bss
    input   resw 1

    segment .text
    global  main
main:
    enter   0,0
    pusha
    mov ebx, [ebp+12]
    mov eax, [ebx+4]
     ; call print_string
    dump_stack 1,2,4
    mov ebx, 0      
    mov ecx, 10
while:
    cmp al, 0x0a
    je print
    sub eax, 0x30
    mov [input], eax
    mov eax, ebx
    mul ecx
    add eax, [input]
    mov ebx, eax
    jmp while
print:
    mov eax, ebx
    call print_int
    call print_nl

    popa
    mov     eax, 0 
    leave
    ret

eax is a pointer. You need to 1) dereference and 2) increment it. Also make sure you use the correct operand sizes (a char is 1 byte). — Jester
– Jester, Commented Jun 5, 2016 at 12:05
Replacing eax to %eax and resw to resb are the solution you mean? — jay
– jay, Commented Jun 5, 2016 at 12:30
%eax is at&t syntax and even there it doesn't dereference. I even missed input, that's defined as word which is doubly wrong. Yeah resb would be better but then of course you can't store 32 bits into it. Not that I understand why you need that at all. — Jester
– Jester, Commented Jun 5, 2016 at 12:37
Input is used for the space to sum integer numbers out of string which is created by subtraction like 'A'-'a'. ( you can think of atoi function. ). — jay
– jay, Commented Jun 5, 2016 at 12:51
Okay then use whatever is appropriate for the range of numbers you want to process. But make sure you use the proper size to access it. — Jester
– Jester, Commented Jun 5, 2016 at 13:14

Sep Roland · Accepted Answer · 2016-06-05 21:48:04Z

2

Your while loop doesn't read any characters! You retrieve these using mov dl,[eax].
As you can see from the code below, there's no need to use a temporary input variable.

  xor   ebx, ebx            ;Result
while:
  movzx edx, byte ptr [eax] ;Read 1 character
  test  dl, dl              ;Test for end of string
  jz    print               ;End found
  sub   dl, 0x30            ;Go from character to value [0,9]
  imul  ebx, 10             ;Result x10
  add   ebx, edx            ;Add new digit
  inc   eax                 ;To next character
  jmp   while

edited Jun 5, 2016 at 21:48

answered Jun 5, 2016 at 13:50

Sep Roland

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5 Comments

jay Over a year ago

xor ebx, ebx ;Result

jay Over a year ago

sorry bro I don't get it, can you explain a little more? BTW These code don't work

Sep Roland Over a year ago

xor ebx,ebx is meant to clear the EBX register. Just like mov ebx,0 does but the instruction is better.

Peter Cordes Over a year ago

@jay: did you try single-stepping your code in a debugger? That usually helps.

Sep Roland Over a year ago

Late catch. Aren't these strings supposed to end with a terminating zero? Then instead of comparing with 0x0a use cmp dl,0.

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