.MODEL SMALL
.STACK 100H
.DATA
MSG1 DB 'ENTER A STRING$'
MSG2 DB 'IN REVERSE ORDER:$'
.CODE
MAIN PROC
    MOV AX,@DATA
    MOV DS,AX
    LEA DX, MSG1
    MOV AH,9
    INT 21H
    MOV CX,0
    MOV AH,1

INPUT:
    INT 21H
    CMP AL,0DH ;check for carriage return
    JE OUTPUT
LOWER:
    CMP AL,5AH ;if greater,it is lowercase
    JG UPPER   ;convert to upper case
    CMP AL,41H
    JL _PUSH
    ADD AL,20H ;if not, convert to lowercase
    JMP _PUSH

UPPER:
    CMP AL,61H
    JL _PUSH
    CMP AL, 7AH
     JG _PUSH
    SUB AL,20H ;convert to upper
_PUSH:
    PUSH AX ;push the converted character to the stack
    INC CX
    JMP INPUT

OUTPUT:

NEW_LINE:
    MOV AH,2
    MOV DL,0DH
    INT 21H
    MOV DL,0AH
    INT 21H

TOP:         ;print the converted string in reverse order until cx becomes 0
    POP DX
    INT 21H
    LOOP TOP

EXIT:
MOV AH,4CH
INT 21H
MAIN ENDP
END MAIN