Segmentation Fault when shutting down Linux with assembly application

The following application generates a Segmentation Fault when executed:

.set __NR_reboot, 169
.set LINUX_REBOOT_CMD_POWER_OFF, 0x4321FEDC

.section .text
.globl _start
_start:
   movl $LINUX_REBOOT_CMD_POWER_OFF, %ebx
   movl $__NR_reboot, %eax
   int $0x80

It's a quite simple application and I must be missing something really obvious. Can someone help me?

It was compiled with:

as shutdown.s -o shutdown.o
ld shutdown.o -o shutdown

EDIT:

Even a simple application that just calls syscall sync() generates a Segmentation Fault:

.set __NR_sync, 36

.section .text
.globl _start
_start:
   movl 开发者_JAVA百科$__NR_sync, %eax
   int $0x80

   movl $1, %eax         #syscall exit
   movl $0, %eax
   int $0x80

WARNING: remember to sync(2) before calling reboot(2).

The reboot(2) system call takes 4 parameters.You are confusing it with the libc wrapper.

WARNING: remember to sync(2) before calling reboot(2).

(It actually takes the magic* parameters so that people have to reread the documentation and don't forget calling sync(2).)

WARNING: Did I say that you have to sync(2) before calling reboot(2)?

I'm adding the final & working source code as this question might interest somebody in the future:

                                     # For the right sys_call numbers on your arch,
                                     # check <asm/unistd_32.h> (or unistd_64.h)

.set __NR_sync, 36                   # sys_call sync()    
.set __NR_reboot, 88                 # sys_call reboot()

.set LINUX_REBOOT_MAGIC1, 0xfee1dead # flags are specified in: <linux/reboot.h>
.set LINUX_REBOOT_MAGIC2, 672274793
.set LINUX_REBOOT_CMD_POWER_OFF, 0x4321fedc
.set LINUX_REBOOT_CMD_RESTART, 0x01234567

.section .text
.globl _start
_start:
   movl $__NR_sync, %eax             # call sync()
   int $0x80

   movl $__NR_reboot, %eax
   movl $LINUX_REBOOT_MAGIC1, %ebx
   movl $LINUX_REBOOT_MAGIC2, %ecx
   movl $LINUX_REBOOT_CMD_RESTART, %edx
   #movl $0, %esi
   int $0x80                         # call reboot()

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

From linux/i386/syscall.S: The function number should be placed in %eax and any arguments in the following registers in order: %ebx, %ecx, %edx, %esi, %edi, and %ebp.

Which is why the last movl %eax,0 in the code should be changed to movl %ebx, 0.