To covert a lowercase letter to uppercase letter

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Objectives
-to learn how to get input from keyboard
-to learn how to input a letter
-to learn how to store a letter into a variable
-to learn how to covert a lowercase letter to uppercase

.MODEL SMALL
.STACK 100H
.DATA
CR EQU 0DH
LF EQU 0AH

MSG1 DB 'Enter a lower case letter: $'
MSG2 DB 0DH, 0AH,'In upper case it is: '
CHAR DB ?,'$'

.CODE
MAIN PROC

;initializing ds
MOV AX,@DATA
MOV DS,AX

;print user prompt
LEA DX,MSG1
MOV AH,9
INT 21H

;input a charater and convert to upper case
MOV AH,1
INT 21H
SUB AL,20H
MOV CHAR,AL

;display on the next line
LEA DX,MSG2
MOV AH,9
INT 21H

;dos exit
MOV AH,4CH
INT 21H

MAIN ENDP
END MAIN

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