Hello World - Assembly Language (x86)

This is a "Hello World" program written in pure assembly language on Linux/X86 platform.

  1 /* as helloworld.s -o helloworld.o; ld helloworld.o -o helloworld   */
  2 /* gcc helloworld.s -nostartfiles -nostdlib -static -o helloworld     */
  3 
  4 .text
  5 .globl _start
  6 
  7 _start:
  8     mov     $13,  %edx
  9     mov     $msg, %ecx
 10     mov     $1,   %ebx
 11     mov     $4,   %eax
 12     int     $0x80
 13 
 14     xorl    %ebx, %ebx         /* exit code */
 15     movl    $1,   %eax         /* exit function */
 16     int     $0x80
 17 
 18    msg:
 19     .string "Hello world\n"
 20     

The result is...

$ gcc helloworld.s -nostartfiles -nostdlib -static -o helloworld
$
$ uname -a
Linux lungswu-mobile 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17 01:57:59 UTC 2009 i686 GNU/Linux
$
$ ./helloworld
Hello world
$ 
$

Of Linux on X86 CPU...

The Linux kernel entry, from user mode application into kernel mode, is triggled by INT 0x80 instrusion.

The system call number is got from the CPU register EAX.

The system call parameter is placed in the sequence of EBX, ECX, EDX...

In the example of the WRITE() system call

ssize_t write (int __fd, __const void *__buf, size_t __n);
 The system call index (EAX) is 0x04
 __fd  is EBX
 __buf is ECX
 __n   is EDX