reverse of string

//reverse
data segment
string db "hello world$"
reverse db ?
data ends

code segment
assume ds:data,cs:code
start: mov ax,ds
       mov ds,ax
       mov cl,00h
       lea si,string

   up: mov bl,[si]
       cmp bl,'$'
       jz down
       inc cl
       inc si
       jmp up

 down: dec si
       lea di,reverse

  up1: mov al,[si]
       mov [di],al
       dec si
       inc di
       dec cl
       jnz up1
       int 03h
code ends
end start