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

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

Collectives™ on Stack Overflow

Assembly Language Using Nasm

1 Answer 1

Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Related