使用rep movsb报段错误（Segmentation fault (core dumped)）

Question

%define SYS_EXIT_GROUP 231  ;int status
section .data
    start_text db "a"
    start_text_len equ $ - start_text
section .bss

section .text

global  _start

_start:
    mov rsi, start_text
    mov rcx, start_text_len
    inc rcx ; to get null
    cld
    rep movsb 
    jmp exit

exit:
    xor rdi, rdi 
    mov rax, SYS_EXIT_GROUP
    syscall

这段代码的错误在哪？我要怎么检查？