大写字母换成小写字母源码

将大写字母换成小写字母和将小写字母换成大学字母的程序
code   SEGMENT
assume cs:code,DS:code
start:
push cs
pop ds
again:
mov ah,1
int 21h
cmp al,0dh;回车退出程序
jz exit
cmp al,'A'
jb done
cmp al,'Z'
jbe r5
cmp al,'a'
jb done
cmp al,'z'
ja done
sub al,20h
jmp done
r5:add al,20h
done:mov ah,0eh
int 10h
jmp again
exit:
mov ah,4ch
int 21h  

code ends