nasm in Mac OS X

I was trying to compile a simple assembly program in Mac OS X (Leopard).

I was getting an error message

ld: could not find entry point "_start" (perhaps missing crt1.o) for inferred architecture i386

A simple asm program for FreeBSD and MacOS
(we have to use the stack for system call, from http://asm.sourceforge.net//howto/Assembly-HOWTO.rtf)

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

In Mac OS X we should use format macho

nasm -f macho hello.asm

and for the linker (we need to specify the entry point)

ld -e _start -o hello hello.o

resources:
http://asm.sourceforge.net//howto/Assembly-HOWTO.rtf
http://zathras.de/angelweb/blog-intel-assembler-on-mac-os-x.htm