.section __DATA, __data
    str:
        .asciz "Hello, World!\n"

    len:
        .quad 0x0123456789ABCDEF

.section __TEXT, __text
    .globl _main

_main:
    lea str(%rip), %rdi
    call strlen
    mov %rdi, len(%rip)

    mov $0x2000004, %rax
    mov  $1, %rdi
    lea str(%rip), %rsi

    mov len(%rip), %rdx
    syscall

    mov $0, %ebx
    mov $0x2000001, %eax
    syscall

strlen:
    xor %rcx, %rcx
    xor %al, %al
    not %rcx
    cld
    repne scasb
    not %rcx
    dec %rcx
    ret