;计算字符串的长度送len单元。
data segment
s1 db 'This is a string.$'
len db ?
data ends
code segment
assume cs:code,ds:data
start: mov ax,data
       mov ds,ax
       lea si,s1
       sub bl,bl
l1:    mov al,[si]
       cmp al,'$'
       je stop
       inc bl
       inc si
       jmp l1
stop:  mov len,bl
       mov ah,4ch
       int 21h
code   ends
       end start