求解，这程序哪错了

assume cs:code
data segment
db 'Welcome to masm!',0
data ends
stack segment
db 16 dup (0)
stack ends
code segment
start:
mov dh,8
mov dl,3
mov cl,2
sub ch,ch
mov ax,data
mov ds,ax
mov ax,stack
mov ss,ax
mov sp,16
sub si,si
sub di,di
mov ax,0b800h
mov es,ax
call show_str
mov ax,4c00h
int 21h
show_str:
push cx
mov cl,[si]
jcxz ok
pop cx
mov al,[si]
mov ah,cl
mov es:[di+160*8+3],ax
add di,2
inc si
jmp short show_str
ok:
pop cx
ret
code ends
end start