This is my first assembly code:

;program name - first.asm
section .data ;stores initialised var

section .bss  ;stores uninit var

section .text  ;actual code
global _start

_start:
  mov eax,1
  mov ebx,256
  int 0x80

ebx is a 32-bit register, and from what I understood, eax=1 is syscall for program termination, and it returns the value stored in ebx. But when I ran the program, it’s returning 0. Why is there an overflow?

#run.bash
#!/bin/bash

nasm -felf64 $1.asm
ld $1.o -o $1
./$1
echo $?
rm -rf $1.o $1

in the terminal I ran sh run.bash first, and it returned 0 instead of 256.