CODEWITHSUNDEEP
assemblyx86nasm
Hamed
Hamed

Reputation: 313

NASM: The loop turns into an infinite loop


I am writing an assembly code to get a number and print some text as many times as that number.

for example when the input is 4, I want to write "Hello!" 4 times.

my code:

section .data
msg db 'Hello!',0xA
len equ $-msg

section .bss
    n resb 1

section .text
    global _start

_start:
    mov edx, 1
    mov ecx, n
    mov ebx, 0
    mov eax, 3
int 0x80

mov ecx, n
loop1:
    push ecx
    mov edx, len
    mov ecx, msg
    mov ebx, 1
    mov eax, 4
    int 0x80
    pop ecx
loop loop1

mov eax, 1
int 0x80

I run it with this codes in terminal

nasm -f elf32 test.asm
ld -m elf_i386 -o test test.o
./test

But i get an infinity "Hello!"

Upvotes: 2

Views: 689

Answers (1)

Fifoernik
Fifoernik

Reputation: 9899

mov ecx, n

In NASM an instruction like this loads the address of the variable in ECX.
(MASM would have complained about the variable not being a dword!)

You knew this since you used it correctly for the input of the single character.

However the instruction that initializes the loop counter should have dereferenced to obtain the actual input. You need to use square brackets for this.

Now by itself this is not enough! The input that you got represents a digit character where you need the actual value of that digit.
e.g. If you input the character "4", the variable n will hold 52.

  movzx ecx, byte [n]   ; Load 1 byte and store in dword register
  sub   cl, '0'         ; Convert from character "4" to value 4 (e.g.)
loop1:

Upvotes: 3

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