Lower case to Upper case in assembly

.model small
.stack 100h
.data
msg db "Enter Lower Letter : $"
msg2 db 10,13,"Upper Letter: $"
.code
main proc
    mov ax,@data
    mov ds,ax

    mov ah,9
    lea dx,msg
    int 21h

    mov ah,1
    int 21h
    mov bl,al

    mov ah,9
    lea dx,msg2
    int 21h

    mov ah,2
    sub bl,32
    mov dl,bl
    int 21h

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