Friday, August 07, 2009

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

section     .text
global _start ;must be declared for linker (ld)

int 0x80 ;system call

_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