syscall write not printing correctly the given string

Question

I'm simply trying to read a number from 0 up to 9 and subtract 4 from it, store the value in a table, then print it to the terminal. It kinda works, but for some reason when I print "bye!" at the end of the program it prints "eye!". Also, for some reason I need to set the pointer 2 bytes backwards to "work". The code is the fallowing:

segment .data
val:                                ; defines a new array of 2 bytes long
    db 0
    db 0

num db 0
msg db 'bye!', 0ah

segment .bss

segment .text
    global _start                   ; defines the entry point

_start:
    mov  ecx ,  num
    mov  edx ,  01h                 ; sets the reading length
    mov  ebx ,  00h                 ; tells that it's an input call
    mov  eax ,  03h                 ; system call (read)
    int  80h                        ; calls it

    mov  eax , [num]                ; moves number to register
    sub  eax ,  04h                 ; subtracts 4 from it

    mov  ecx ,  val                 ; sets ecx to point to val

    mov [ecx],  eax                 ; adds the result to val, pointed by ecx
    inc  ecx                        ; moves the pointer to the next address
    mov [ecx],  byte 0ah            ; appends \n to the end of it
    sub  ecx ,  02h                 ; retreats pointer (is over here that I need to retreat 2 for some reason

    mov  edx ,  03h                 ; sets the string length
    mov  ebx ,  01h                 ; tells that it's an output call
    mov  eax ,  04h                 ; system call (write)
    int  80h                        ; calls it

    mov  ecx ,  msg                 ; passes the string to the function
    mov  edx ,  05h                 ; sets the string length
    mov  ebx ,  01h                 ; tells that it's an output call
    mov  eax ,  04h                 ; system call (write)
    int  80h                        ; calls it

    mov  eax ,  01h                 ; system call (exit)
    int  80h                        ; calls it

output:

[mmd@pxdev asm]$ ./main
9
5
eye!
[mmd@pxdev asm]$ 
[mmd@pxdev asm]$

for some reason it's exiting twice (?) too. If I set the line 29 to retreat only one byte, the output is yeye! in the last line.

I'm at the very beginning of assembly and, honestly, I don't feel that tutorials out there are explained enough. So sorry if I'm missing something obvious or I'm doing something really stupid.

Edit: I found one of my errors at line 31, because I was telling the system that I'm printing 3 bytes, when actually I was above to print only 2. So no need anymore to retreat two bytes from ecx. But the main error keeps happening.

Your `num` is a byte but you overwrite 4 bytes hence you destroy the `bye` which follows. — Jester, Aug 29 '21 at 14:29
`mov [ecx], eax ` is writing 4 bytes because `eax` is 32 bits. Also `mov eax , [num] ` is reading 4 bytes but that is not that harmful in this case. — Jester, Aug 29 '21 at 14:45
Do not post your answer within your question. Post it as an answer. — Rob, Aug 29 '21 at 14:57
Alright, sorry. Where do I find these rules about how to behave? — mateus.md, Aug 29 '21 at 20:26

score 2 · Answer 1 · answered Aug 29 '21 at 14:58

Note: This is the copied solution by the questioner who did not properly post it as as an answer.

Solution: I was moving a 32-bit value (eax) to my single byte variable num (in mov [ecx], eax) so hence I was messing with the next addresses in stack (msg). Once fixed, not it works (except that it keeps exiting twice)

syscall write not printing correctly the given string

1 Answers1