section .text
    global _start           ;must be declared for linker (ld)

_syscall:
    int 0x80        ;system call
    ret

_start:             ;tell linker entry point

    push    dword len   ;message length
    push    dword msg   ;message to write
    push    dword 1     ;file descriptor (stdout)
    mov eax,0x4     ;system call number (sys_write)
    call    _syscall    ;call kernel

                ;the alternate way to call kernel:
                ;push   eax
                ;call   7:0

    add esp,12      ;clean stack (3 arguments * 4)

    push    dword 0     ;exit code
    mov eax,0x1     ;system call number (sys_exit)
    call    _syscall    ;call kernel

                ;we do not return from sys_exit,
                ;there's no need to clean stack
section .data

msg db  "Hello, world!",0xa ;our dear string
len equ $ - msg         ;length of our dear string