NASM Hello World for x86 and x86_64 Intel Mac OS X(get yourself an updated nasm with brew)

32.asm

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

global start

section .text
start:
    push    dword msg.len
    push    dword msg
    push    dword 1
    mov     eax, 4
    sub     esp, 4
    int     0x80
    add     esp, 16

    push    dword 0
    mov     eax, 1
    sub     esp, 12
    int     0x80

section .data

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

64.asm

; To convert this assembly code to an executable file (machine language) and execute it, run:
; /usr/local/bin/nasm -f macho64 64.asm && ld -macosx_version_min 10.7.0 -lSystem -o 64 64.o && ./64

global start

section .text

start:
    mov     rax, 0x2000004 ; write
    mov     rdi, 1 ; stdout
    lea     rsi, [rel msg]
    mov     rdx, msg.len
    syscall

    mov     rax, 0x2000001 ; exit
    mov     rdi, 0
    syscall

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

msg:    db      "Hello, world!", 10
; Length of `msg`. `$` refers to the address of this constant, so `$ - msg` is the length of message
.len:   equ     $ - msg

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