i need help..

im having a prOblem with my code..

its dOne actually but there's missing..

its a conversion of an uppercase letter into a lowercase..

it is required to display an 'INVALID' whenever i entered a lowercase..

and that is my problem..

here's my code..

please help me..

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.MODEL SMALL
.STACK 100h
.DATA

UpperPrompt DB "Enter an Uppercase letter: $"
DisplayLower DB 13,10, "Lowercase: $"

.CODE

start:
mov ax, @data
mov ds, ax
mov dx, OFFSET UpperPrompt
mov ah, 9h
int 21h

mov ah, 1h
int 21h

mov bl, al

add bl, 32d


mov dx, OFFSET DisplayLower
mov ah, 9h
int 21h

mov dl, bl
mov ah, 2h
int 21h

mov ax, 4c00h
int 21h

end start

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thanks in advance..