dos 调用显示覆盖了输入回显？新手

data segmentstring db 255 dup(?),'$'data endscode segment    assume cs:code,ds:datastart:    mov ax,data    mov ds,ax    lea dx,string    mov di,dx    mov byte ptr [di],0ffh    mov ah,0ah    int 21h    mov ax,[di+1]    and ax,0ffh    mov si,ax    mov byte ptr [3+si],'$'    add dx,2    mov ah,09h    int 21h     mov ah,4ch    int 21hcode ends    end start