Thanks to those who help....
I was following the wrong convention on the previous post.
Now I am following the C calling convention,
and I got a new question... =)

This is what I have now ,
----------------------------------------------------------------------------
-
        .data
msg:    .string "Hello\n"
        len = . - msg - 1

        .text
        .extern exit
        .global main

do_sys:
        int     $0x80
        ret
main:
        pushl   $len
        pushl   $msg
        pushl   $1
        movl    $0x4, %eax
        call   do_sys
        addl   $12, %esp
        pushl   $0
        call    exit
----------------------------------------------------------------------------
-

My newbie question is that "why" the call/ret combination (call do_sys) can
be eliminated by pushing an extra dword ?

----------------------------------------------------------------------------
-
      .data
msg:    .string "Hello\n"
        len = . - msg - 1

        .text
        .extern write
        .extern exit
        .global main
main:
        pushl   $len
        pushl   $msg
        pushl   $1
        movl    $0x4, %eax
        pushl   %eax    # Or any other word
        int     $0x80
        addl    $16, %esp
        pushl   $0
        call    exit


Thanks all for the patient and suggestions