main.s

        .globl _start

        .section .rodata
msg:    .ascii "Hello World!\n"
        .set MSG_LEN, . - msg

        .section .text
_start:
        ##  write (STDOUT_FILENO, msg, MSG_LEN)
        movl $4, %eax           # sys_write
        movl $1, %ebx           # STDOUT_FILENO
        movl $msg, %ecx
        movl $MSG_LEN, %edx
        int $0x80

        ##  exit (0)
        movl $1, %eax           # sys_exit
        movl $0, %ebx
        int $0x80

build instructions

as --32 -o main.o main.s
ld -m elf_i386 -o main main.o