;; Hello World Program #1
    ;; Compile with: nasm -f elf64 hello.s
    ;; Link with: ld -o hello hello.o
    ;; Run with: ./hello

    ;; sys/unistd_64.h
%define     SYS_write	1
%define     SYS_exit	60

    ;; unistd.h
%define STDOUT		1

section .data
    msg     db  "Hello You Beautiful Human, You're Looking Mighty Fine!", 0Ah, 00h

section .text    
    global _start

_start:
    mov     rsi, msg            ; Move the address of the message into rsi
    mov     rax, rsi            ; Move the address of the message into rax
                                ; (Register-to-register copying is faster that a constant!) 

nextchar:
    cmp     byte [rax], 0       ; Compare the byte pointed to by eax with zero
    

    ;; Small detail: cmp and sub use the same internal architecture,
    ;; except cmp doesn't copy the results into the first operand and
    ;; sub does.  cmp sets flags; does sub?  This is why 'jz' works,
    ;; because if they're equal the result of subtraction is zero.
    jz      counted             ; Jump if the zero flag set
    inc     rax
    jmp nextchar

counted:    
    sub     rax, rsi            ; Substract source from endpointer, leaving counter
    mov     rdx, rax            ; Length of the message
    mov     rsi, msg            ; Address of the message
    mov     rdi, STDOUT         ; using STDOUT (see definition above)
    mov     rax, SYS_write      ; Using WRITE in 32-bit mode?
    syscall

    mov     rdi, 0
    mov     rax, SYS_exit
    syscall