ASSEMBLY Calculate String Length

calculate_string_length.asm

section .data
    msg db 'This is a test message.',0Ah
    
SECTION .text
global  _start

_start:
    mov ebx, msg
    mov eax, ebx
    
gotonextchar:
    
    cmp byte [eax], 0
    jz calculatelength
    inc eax
    jmp gotonextchar
    
calculatelength:
    sub     eax, ebx
    mov     edx, eax       
    
writemessage:    
    mov     ecx, msg        
    mov     ebx, 1
    mov     eax, 4
    int     80h
    mov     ebx, 0
    mov     eax, 1
    int     80h