; strlen
bits 64

    SECTION .data
mystr:  db 't','e','s','t',0

    SECTION .text

global _start
_start:
    %include "header.asm.inc"

    mov rcx, 0
    xor rbx, rbx
    repnz sub rbx, 1

    ; get reference to string
    mov rdi, mystr

    ; move MAXINT to rcx
    xor rcx, rcx
    not rcx

    ; zero al
    xor al, al
    
    cld ; clear direction flag -->
        ; increment addresses during string search

    ; repne: perform <instr> (scasb) and repeat if <cond> (ne) is met
    ;        decrements rcx and aborts if rcx == 0
    ; scasb: compare byte at rdi with al and set flags; increment rdi
    repne scasb

    ; option 1: current string index minus string start minus one
    mov rax, rdi
    sub rax, mystr
    dec rax
    
    ; option 2: look at counter modified by repne (rcx)
    not rcx
    dec rcx

    %include "sysexit.asm.inc"