; counts the number of characters of a zero terminated string. 

name "counter"

org 100h

jmp start

str1 db 'abcdefg hijklmnop qrstvuwxyz', 0

start: lea bx, str1 ; load address of string. 
       mov ax, 0 ; reset counter. 

compare: cmp [bx], 0 ; is it end of string? 
         je done  ; if zero, then it's the end. 
         inc ax   ; count char. 
         inc bx   ; next memory position in string. 
         jmp compare

; print result in binary: 
done:
mov bx, ax
mov cx, 8
print: mov ah, 2   ; print function. 
       mov dl, '0'
       test bl, 10000000b  ; test first bit. 
       jz zero
       mov dl, '1'
zero:  int 21h
       shl bl, 1
loop print

; print binary suffix: 
mov dl, 'b'
int 21h


; wait for any key press.... 
mov ah, 0
int 16h


ret