Simple code clears the screen. It directly wirtes to the video memory to clear whats in that memory. ideally write '' nothing. 

clrscr proc near
   mov ax,0b800h
   mov es,ax
   mov di,0
   mov al,' '
   mov ah,07d
   loop_clear_12:
        mov word ptr es:[di],ax
        inc di
        inc di
        cmp di,4000
        jle loop_clear_12
        ret
endp