32.asm

; /usr/local/bin/nasm -f macho 32.asm && ld -macosx_version_min 10.7.0 -o 32 32.o && ./32 

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; The .data section is for storing and naming constants.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
section .data

msg:    db    "Hello world!", 10
.len:   equ   $ - msg

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;The .text section is for the actual code.  
;(I assume .text refers to source code being text)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
section .text
global start ; indicate the entry point with global. 
;OSX expects this to be called "start"
        
start:
        ; we are going to syscall "write"
        ; user_ssize_t write(int fd, user_addr_t cbuf, user_size_t nbyte);
        ; The calling convention for OSX 32bit code is to have the params on 
        ; the stack
        push  dword msg.len ; push the length 
        push  dword msg     ; push the address of the message 
        push  dword 1       ; push the file descripter.  1 is stdout
        mov   eax, 4        ; put the syscall number in eax
        sub   esp, 4        ; put some space on the stack, required for syscalls on osx (and bsd)
        int   0x80          ; execute the syscall
        add   esp,16        ; cleanup the stack.  4 bytes per arg pulse the 4 bytes we sub'd is 16 bytes

        ; now we syscall "exit
        ; void exit(int rval);
        push  dword 0  ; push the rval on the stack
        mov   eax, 1   ; put the syscall number in eax
        sub   esp, 4   ; put some space on the stack, required for syscalls on osx (and bsd)
        int   0x80     ; execute the sys call.  

64.asm

; /Usr/local/bin/nasm -f macho64 64.asm && ld -macosx_version_min 10.7.0 -lSystem -o 64 64.o && ./64

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; The .data section is for storing and naming constants.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
section .data 

msg:    db      "Hello, world!", 10
.len:   equ     $ - msg             ;$ refers to the address of this constant, so $ - msg is the length of message
        
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;The .text section is for the actual code.  
;(I assume .text refers to source code being text)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
section .text  
global start   ;global is used to tell the kernel where to enter the program.  
;OSX expects this to be called "start"
        
;You can put underscores in numbers to make them easier to read.  They are ignored by nasm.
start:
        mov     rax, 0x200_0004 ;The number for the syscall "write": user_ssize_t write(int fd, user_addr_t cbuf, user_size_t nbyte);
        mov     rdi, 1          ;First param for write is the file descripter.  1 is stdout
        mov     rsi, msg        ;Second param is a pointer to the msg
        mov     rdx, msg.len    ;Third param is the length of the message
        syscall
        
        mov     rax, 0x200_0001 ;The number for the syscall "exit": void exit(int rval);
        mov     rdi, 0         ; First param is the return code
        syscall

References.markdown

• NASM Manual - for the syntax
• What are the calling conventions for UNIX & Linux system calls on x86-64 - comparison
• AMD64 ABI reference - A.2.1 - for the actual x86_64 calling convention
• X86 calling conventions - moar calling conventions
• Mac OS X syscalls reference