; reverse string 

name "reverse"

org 100h

jmp start

; when reversed it will be a readable string, 
; '$' marks the end of the string: 
string1 db '!gnirts a si siht$'

start:      lea bx, string1

            mov si, bx

next_byte:  cmp [si], '$'
            je found_the_end
            inc si
            jmp next_byte

found_the_end:  dec si

; now bx points to beginning, 
; and si points to the end of string. 


; do the swapping: 

do_reverse: cmp bx, si
            jae done

            mov al, [bx]
            mov ah, [si]

            mov [si], al
            mov [bx], ah

            inc bx
            dec si
jmp do_reverse



; reverse complete, print out: 
done:       lea dx, string1
            mov ah, 09h
            int 21h

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

ret