Upper case letter to Lower case letter conversion in Assembly

.MODEL SMALL
.STACK 100H

.DATA
 MSG1 DB 'ENTER AN UPPERCASE LETTER:$'
 MSG2 DB 0DH,0AH,'IN LOWERCASE IT IS: '
 CHAR DB ?,'$'
 
.CODE
 MAIN PROC
 
 MOV AX,@DATA
 MOV DS,AX
 
 LEA DX,MSG1
 MOV AH,9
 INT 21H
 
 MOV AH,1
 INT 21H
 ADD AL,20H
 MOV CHAR,AL
 
 
 LEA DX,MSG2
 MOV AH,9
 INT 21H
 
 MOV AH,4CH
 INT 21H
 
 MAIN ENDP
END MAIN