convert lowercase letter to uppercase

.model small
 .stack 100h
 .data
 .code
 main proc
    mov ah,1
    int 21h

    mov bl,al

    mov ah,2
    mov dl,0dh
    int 21h

    mov dl,0ah
    int 21h

    sub bl,20h

    mov ah,2
    mov dl,bl
    int 21h

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