关于王爽 汇编语言16章 实验16 的问题

Mr_Liu淸淮

以下是我的int 7ch安装程序
assume cs:code
code segment
start:
mov ax,cs
mov ds,ax
mov si,offset setscreen
mov ax,0
mov es,ax
mov di,200h
mov cx,offset setscreenend-offset setscreen
cld
rep movsb


mov ax,0
mov es,ax
    mov word ptr es:[7ch*4],200h
    mov word ptr es:[7ch*4+2],0
   
    mov ax,4c00h
    int 21h

setscreen:
    jmp short set

table dw sub1,sub2,sub3,sub4
set:
push bx

cmp ah,3
ja sret
mov bl,ah
mov bh,0
add bx,bx

call word ptr table[bx]

sret:
pop bx
iret
  
sub1:
push bx
push cx
push es
mov bx,0b800h
mov es,bx
mov bx,0
mov cx,2000
sub1s:
mov byte ptr es:[bx],' '
add bx,2
loop sub1s
pop es
pop cx
pop bx
ret

sub2:
push bx
push cx
push es

mov bx,0b800h
mov es,bx
mov bx,1
mov cx,2000
sub2s:
and byte ptr es:[bx],11111000b
or es:[bx],al
add bx,2
loop sub2s

pop es
pop cx
pop bx
ret

sub3:
push bx
push cx
push es
mov cl,4
shl al,cl
mov bx,0b800h
mov es,bx
mov bx,1
mov cx,2000
sub3s:
and byte ptr es:[bx],10001111b
or es:[bx],al
add bx,2
loop sub3s
pop es
pop cx
pop bx
ret

sub4:
push cx
push si
push di
push es
push ds

mov si,0b800h
mov es,si
mov ds,si
mov si,160
mov di,0
cld
mov cx,24
sub4s:
push cx
mov cx,160
rep movsb
pop cx
loop sub4s

mov cx,80
mov si,0
sub4s1:
mov byte ptr [160*24+si],' '
add si,2
loop sub4s1

pop ds
pop es
pop di
pop si
pop cx
ret

setscreenend:
    nop
   
code ends
end start


以下为调用程序
assume cs:code
code segment
start:
mov ah,0
int 16h

mov ah,al
int 7ch

mov ax,4c00h
int 21h

code ends
end start

运行调用程序没有反映,debug显示程序有进入中断...
大神帮忙啊！！
小弟万分感谢啊！！

冲啊大笼包