'Hello World' in MacOS X with 'as' and 'ld'

hello.s

# Usage:
#   Save this file as 'hello.s'.
#   $ as -o hello.o hello.s
#   $ ld -o hello   hello.o
#   $ ./hello
#
# How to call a system call
#   1) Set rax to the index of the system call.
#   2) Set rdi, rsi, rdx, ... to arguments in this order.
#   3) Call 'syscall'.
#
# The entry point of an executable is 'start' ('_start' in Linux)
# The index of write(2) is '0x2000004' ('0x1' in Linux)
# The index of _exit(2) is '0x2000001' ('0x3c' in Linux)

.text
  .globl start

start:
  movq $0x2000004, %rax # write(2)
  movq $1, %rdi         # fildes <- 1 == stdout
  leaq msg(%rip), %rsi  # buf <- address of msg as relative address from rip
  movq $14, %rdx        # nbyte <- 14 == strlen("Hello world!\n")
  syscall

  movq $0x2000001, %rax # _exit(2)
  movq $0, %rdi         # status <- 0
  syscall

.data
msg:
  .ascii "Hello world!\n"