Lowercase to upper

.model small
.stack 100h
.data

msg1 db 'Enter a lowercase letter $'
msg2 db 10,13,'In upper case $'

.code
main proc
    mov ax,@data
    mov ds,ax

    mov ah,9
    lea dx,msg1
    int 21h

    mov ah,01
    int 21h
    mov bl,al

    sub bl,32

    mov ah,9
    lea dx,msg2
    int 21h

    mov ah,02
    mov dl,bl
    int 21h

    exit:
    mov ah,4ch
    int 21h
    main endp
end main