Save characters from the screen

Good afternoon, I want to save the characters to a file. The size is 20 rows and 40 columns. I developed code, but it's only saving 14 lines. Can someone help me?

Code:

mov bx,0
mov si,0
mov ax,0
mov di,si
mov cx, 80
lab_ciclo: 
       mov al, es:[bx]
       mov buffer[si], al
       inc si
       add bx,2
    loop lab_ciclo
       add di,82
       mov si,di
       mov cx,82
       cmp si, 2050
       jne lab_ciclo