;Copy a string backwards
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
         .model small
	 .586
         .stack 100h    ;Set maximum stack size to 256 bytes

;; declare externally used subroutines
         extrn Writestring:proc, Crlf:proc, Clrscr:proc, Writeint:proc
         extrn Random_range:proc, Randomize:proc, Readint:proc

          .data 
msg1       db "The devil speaks like this!", 0h
msg2       db "Yet another string",0h
msg3       db "madam Im adam",0h
dest       db 80 dup (?)         ;


          .code
START:    mov ax, @data        ;ax <-- data segment start address
          mov ds, ax                  ;ds <-- initialize data segment register
	 
	  mov	dx,offset msg1
	  mov   si, offset dest
	  call  pback
	  mov   dx, offset msg2
	  mov   si, offset dest
	  call  pback
	  mov   dx, offset msg3
	  mov   si, offset dest
	  call  pback
EXIT:     mov ax, 4c00h               ;ax <-- 4c DOS 21h program halt function
          int 21h                     ;DOS service interrupt

;; copy string in dx to si backwards, print both strings
pback	  proc
	  push	si	  ;; save SI
	  call	Writestring
	  call  Crlf
	  mov bx,dx     
;; find end of first string 
	  xor cx,cx       ;; keep count of # of characters         
lp1:
	  mov al, [bx]
	  inc bx
	  inc cx
	  cmp al,0
	  jne lp1
;; bx points at 1 location past end of string
;; point at last character by decrementing twice
	  dec bx
	  dec bx
	  dec cx
;; copy string to where SI points
lp2:
	  mov  al,[bx]
	  mov  [si], al
	  dec  bx
	  inc  si
	  loop lp2
          ;; mark end of copied string
	  xor al,al
	  mov [si],0
	  pop  si	 ;; restore SI
	  mov dx,si      ;; print copied string
	  call  Writestring
	  call  Crlf
	  ret
pback     endp

END       START


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