'Hello world!' in x86_64 assembly without using any external libs.

example3-helloworld.s

## 'Hello world!' in x86_64 AT&T gas assembly in OS X.
## To assemble $> as -arch x86_64 example3-helloworld.s -o example3-helloworld.o
## To link: $> ld -arch x86_64 example3-helloworld.o -e _main -o example3-helloworld -lSystem
.section __TEXT,__text
.macosx_version_min 10, 12
.globl _main
_main:
  movq $0x2000004, %rax   # write (system call $4 with $0x2000000 offset)
  movq $1, %rdi           # write to stdout (fid 1)
  leaq L_.str(%rip), %rsi # set the register address of the start of the string
  movq $13, %rdx          # print out the first 12 characters of the string.
  syscall                 # take the shot.
  movq $0x2000001, %rax   # exit (system call $1 with $0x2000000 offset)
  movq $0, %rdi           # use exit code 0.
  syscall                 # take the shot.

.section	__TEXT,__cstring,cstring_literals
L_.str:                                 ## @.str
	.asciz	"Hello World!\n"