$ objdump -S program

program:     file format elf64-x86-64


Disassembly of section .text:

00000000004000b0 <_start>:
SECTION .text
global  _start

_start:
    ; Print "Hello world!".
    mov     RAX, 1      ; RAX is the kernel op code (1 is sys_write).
  4000b0:       b8 01 00 00 00          mov    $0x1,%eax
    mov     RDI, 1      ; RDI is the file descriptor (0 is stdout, 1 is stdin, 2 is stderr).
  4000b5:       bf 01 00 00 00          mov    $0x1,%edi
    mov     RSI, message; RSI is the address of the buffer.
  4000ba:       48 be d8 00 60 00 00    movabs $0x6000d8,%rsi
  4000c1:       00 00 00
    mov     RDX, 13     ; RDX is is length of the buffer.
  4000c4:       ba 0d 00 00 00          mov    $0xd,%edx
    syscall
  4000c9:       0f 05                   syscall

    ; Exit the origram
    mov     RAX, 60     ; RAX is the kernel op code (60 is sys_exit).
  4000cb:       b8 3c 00 00 00          mov    $0x3c,%eax
    mov     RDI, 0      ; RDI is return code (0 for success).
  4000d0:       bf 00 00 00 00          mov    $0x0,%edi
    syscall
  4000d5:       0f 05                   syscall