i had a sub routine for printing a string dynamically, without manually listing the length of the string in x86-64 intel style through following code (with stack) :-

section .data
output1 db "hello world",10,0
output2 db "hello world again",10,0

section .bss

section .text
    global _start

_start:
    mov rax,output1
    call _output    

    mov rax,output2
    call _output

    call _exit  

_output:
_output_setup:
    push rax
    mov rbx, 0

_output_length_loop:
    inc rax
    inc rbx
    
    mov cl,[rax]
    
    cmp cl, 0
    jne _output_length_loop

_output_output:
    mov rax, 1
    mov rdi, 1
    pop rsi
    mov rdx, rbx
    syscall
    ret

_exit:
    mov rax, 60
    mov rdi, 0
    syscall
    ret

but i wanted the sub routine without stack so i tried writing the sub routine to do the same without using the stack so i did through following:-

section .data
output1 db "print this string",10,0
output2 db "print this string again",10,0

section .text
    global _start

_start:
    mov rcx,output1
    call _output
    mov rcx,output2
    call _output
    call _exit
    
_output: 
_output_setup:
    mov rbx,0

_output_length_loop:
    inc rbx
    inc rcx

    mov dl,[rcx]

    cmp dl,0
    jne _output_length_loop

_output_output:
    mov rax,0
    mov rdi,0
    mov rsi,rcx
    mov rdx,rbx
    syscall
    ret
    
_exit:
    mov rax,60
    mov rdi,0 
    syscall
    ret

it complied without error but when i ran the executable it, for some reason got stuck presumably it created a infinite loop for some reason so i had heard about a tool called GDB for debugging but being honest it was a mess for me but i did a line by line register analysis but i was not able to deduce reason for above

my line by line analysis:-

rcx     rbx     dl_
                             _start
b1                                                   - mov rcx, output1
                             - call _output

                             _output
    0                        - mov rbx,0

                             _output_loop_length -0
    1                        - inc rbx
ib1                          - inc rcx
        p                    - mov dl,[rcx]
                             - cmp dl,0 (p != 0)
                             - jne _output_length_loop

                             _output_loop_length -1
    2                        - inc rbx 
ib1                          - inc rcx
        r                    - mov dl,[rcx]
                             - cmp dl,0 (p != 0)
                             - jne _output_length_loop

                                                         ...

                             _output_loop_length -2
    3                        - inc rbx
ib1                          - inc rcx
            0                            - mov dl,[rcx]
                             - cmp dl,0 (p == 0)
                             - jne _output_length_loop

                                                          ...

b1 stands for init byte
i in front of b1 stands for increased position of the init byte

i suspect "inc rcx" to be creating a infinite loop some how but i am not sure how in addition i have some ambiguity related the respective line.