Assembly Language Using Nasm

Question

I am a newbie to assembly language. I am trying to add two numbers using this code; the values are hardcoded. I am using "nasm", and I am successfully building the executable. When I run it, it gives no output. I came to know that if the value is an integer like in this case sum of two numbers, we need to convert it to string to write it using sys_write. If it's the only thing that needs to be taken care of, how should I do it? If not, why am I not able to see the output on stdout even when I can see the output in the registers using gdb?

global _start
section .text
_start:

mov eax,0x3
mov ebx,0x5
add eax,ebx
mov ecx,eax
mov eax,0x4
mov ebx,0x1
mov edx,0x1
int 0x80

mov eax,0x1
mov ebx,0x0
int 0x80

segment .data
sum db 0x00

Jester · Accepted Answer · 2014-10-03 18:27:05Z

2

You are invoking the write system call which looks like this:

ssize_t write(int fd, const void *buf, size_t count);

Instead of a pointer to a buffer containing what you want printed, you pass the resulting number. That won't work. Also, your number is in binary, not text. As a quick fix for single digit numbers you can add the ascii code of 0, and place the resulting character into memory for printing. You can use the stack for this.

global _start
section .text
_start:

mov eax,0x3
mov ebx,0x5
add eax,ebx
add al, '0'  ; convert to text
push eax     ; store on stack
mov ecx,esp  ; pass its address for printing
mov eax,0x4
mov ebx,0x1
mov edx,0x1
int 0x80
add esp, 4   ; free stack space

mov eax,0x1
mov ebx,0x0
int 0x80

segment .data
sum db 0x00

answered Oct 3, 2014 at 18:27

Jester

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