assembly language in os x

Question

I used assembly language step by step to learn assembly language programming on linux. I recently got a Mac, on which int 0x80 doesn\'t seem to work (illegal in

Answer 1

For practical purposes, this answer shows how to compile a hello world application using nasm on OSX.

This code can be compiled for linux as is, but the cmd-line command to compile it would probably differ:

section .text

global mystart                ; make the main function externally visible

mystart:

; 1 print "hello, world"

    ; 1a prepare the arguments for the system call to write
    push dword mylen          ; message length                           
    push dword mymsg          ; message to write
    push dword 1              ; file descriptor value

    ; 1b make the system call to write
    mov eax, 0x4              ; system call number for write
    sub esp, 4                ; OS X (and BSD) system calls needs "extra space" on stack
    int 0x80                  ; make the actual system call

    ; 1c clean up the stack
    add esp, 16               ; 3 args * 4 bytes/arg + 4 bytes extra space = 16 bytes

; 2 exit the program

    ; 2a prepare the argument for the sys call to exit
    push dword 0              ; exit status returned to the operating system

    ; 2b make the call to sys call to exit
    mov eax, 0x1              ; system call number for exit
    sub esp, 4                ; OS X (and BSD) system calls needs "extra space" on stack
    int 0x80                  ; make the system call

    ; 2c no need to clean up the stack because no code here would executed: already exited

section .data

  mymsg db "hello, world", 0xa  ; string with a carriage-return
  mylen equ $-mymsg             ; string length in bytes

Assemble the source (hello.nasm) to an object file:

nasm -f macho hello.nasm

Link to produce the executable:

ld -o hello -e mystart hello.o