汇编语言实验疑惑，多谢解答

assume cs:codedata segment    db 'Welcome to Masm!';要显示的文字data endscode segment    start:mov ax, 0B800h                mov es, ax                mov bx, 0                                                mov ax, data                mov ds, ax                            s:call cls;循环显示                mov bp, 0                mov di, 0                mov cx, 16                                            s1:mov al, [bp];显示文字                     mov es:[bx+di],al                     mov al, 202                     mov es:[bx+di+1],al                     inc bp                     add di, 2                      loop s1              add bx, 2              mov cx, bx              sub cx, 3968              jcxz s3            jmp far ptr s                            mov ax, 4c00h                int 21h                         s3:mov bx, 0                 jmp s                       cls:push cx;清理屏幕              push ax              push bx              push ds                            mov ax, 0B800h              mov ds, ax              mov bx, 0                            mov cx, 1999                     sn:mov al, 32               mov [bx], al               mov al, 7               mov 1[bx], al               add bx, 2               loop sn                              pop ds               pop bx               pop ax               pop cx              retcode endsend start