reverse.asm

; 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