.MODEL SMALL
.STACK 100h
.DATA

MAXIMUM_STRING_LENGTH  EQU  1000

StringToReverse       DB      'Type a string: $'    ;MAXIMUM_STRING_LENGTH DUP(?)

ReverseString        DB      13,10,'Result: $'    ;MAXIMUM_STRING_LENGTH DUP(?)

.CODE
       mov  ax,@data
       mov  ds,ax                     ;set DS to point to the data segment
       mov  ah,3fh                     ;DOS read from handle function #
       mov  bx,0                                ;standard input handle
       mov  dx,OFFSET StringToReverse      ;store the string here
       mov  cx,MAXIMUM_STRING_LENGTH   ;read up to maximum number of characters
    
       int  21h                            ;get the string
       and  ax,ax                               ;were any characters read?
       jz   Done                             ;no, so you're done
       mov  cx,ax                                      ;put string length in CX, where
                                                     ; you can use it as a counter
       push cx                               ;save the string length
       mov  bx,OFFSET StringToReverse
       mov  si,OFFSET ReverseString
       add  si,cx
       dec  si                                       ;point to the end of the
                                                    ; reverse string buffer
ReverseLoop:

       mov  al,[bx]                        ;get the next character
       mov  [si],al                                   ;store the characters in reverse order
       inc  bx                                         ;point to next character
       dec  si                                         ;point to previous location in reverse buffer
       loop ReverseLoop                                ;move next character, if any
       pop  cx                                              ;get back the string length
       mov  ah,40h                                         ;DOS write from handle function #
       mov  bx,1                                            ;standard output handle
      
     mov  dx,OFFSET ReverseString                  ;print this string
       int  21h                                             ;print the reversed string

Done:
       mov  ah,4ch                                     ;DOS terminate program function #
       int  21h                                      ;terminate the program

END