How to sort characters in string by ascii codes in assembly

I am new in assembly and I don't understand, how to sort characters in string. I can only input and output string. I find bubble sort in assembly for arrays, but how to use it for strings?

 mov bx, offset array
 mov cx, n
 for_i:
   dec cx
   xor dx, dx
 for_j:
   cmp dx, cx
   jae exit_for_j
   jbe no_swap
   mov ah, byte ptr bx[di]
   mov byte ptr bx[di], al
   mov byte ptr bx[si], ah
 no_swap:
  inc dx
  jmp for_j
 exit_for_j:
  loop    for_i

I need your help. Thanks