%macro prolog 1
  push rbp
  mov rbp, rsp
  sub rsp, %1
%endmacro

%macro epilog 1
  add rsp, %1
  pop rbp
%endmacro

%macro xcall 1
  call [rel %1 wrt ..got]
%endmacro

%macro xlea 2
  lea %1, [rel %2]
%endmacro

extern printf
extern exit

section .text
    global xstrlen
    xstrlen:
        push rbp
        mov rbp, rsp

        mov rax, rdi               ;; Change this to RCX and it works on windoze ABI.
        mov rsi, rax
        xor rbx, rbx

        __xlen_count:
            cmp byte [rsi+rbx], 0
            je __xlen_exit
            inc rbx
            jne __xlen_count

        __xlen_exit:
            mov rax, rbx
            pop rbp
        ret

     ;; This will not work on Windoze. Change the macro LEA and CALL maybe.
    global main
    main:
        prolog 4*8
        xlea rdi, s1
        call xstrlen
        xlea rsi, rax         ;;RDX - windoze ABI
        xlea rdi, p_tpl       ;;RCX - windoze ABI
        xcall printf
        xor edi, edi
        xcall exit
        epilog 4*8            ;;Never gets here but w/e.
        ret

section .rodata
    s1              db "Good luck getting the stuff below this to assemble.",13,10,0
    p_tpl           db "len: %i",13,10,0