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

Answer 1

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