编写多功能中断例程

这是王爽汇编上的一道实验题，安装一个包含四个功能子程序的中断例程。
没什么特别这处，主要是第一次没用org指令（王爽在书中好像没提到），结果总是出错。

assume cs:code,ss:stack
stack segment
db 128 dup (0)
stack ends

code segment
start: mov ax,stack
mov ss,ax
mov sp,128

push cs
pop ds
mov si,offset int7c

mov ax,0
mov es,ax
mov di,200h

mov cx,offset int7cend-offset int7c
cld
rep movsb

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

mov ah,1
mov al,2
int 7ch

mov ax,4c00h
int 21h
org 200h ;如果没有这句，会导致下边的call word ptr table[bx]找不到正确的地址
int7c: 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 es
push cx
mov bx,0b800h
mov es,bx
mov bx,0
mov cx,2000
sub1s: mov byte ptr es:[bx],’ ‘
add bx,2
loop sub1s
pop cx
pop es
pop bx
ret

sub2:push cx
push bx
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 bx
pop cx
ret

sub3:push bx
push cx
push es
mov bx,0b800h
mov es,bx
mov cl,4
shl al,cl
mov bx,1
mov cx,2000
sub3c: and byte ptr es:[bx],10001111b
or es:[bx],al
add bx,2
loop sub3c
pop es
pop cx
pop bx
ret

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

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 cx
pop di
pop ds
pop es
pop si
ret

int7cend:nop

code ends
end start