Digit sum in assembly

.model small
.data




msg1 db 0ah, 0dh,'The sum of '
num1 db ?
msg2 db ' and '
num2 db ?
msg3 db ' is: '
sum db ?
msg4 db '$'


.code

main proc
    mov ax, @data
    mov ds, ax

    mov ah,2
    mov dl,'?'
    int 21h

    mov ah,1
    int 21h
    mov num1,al
    mov bl, al
    int 21h
    mov num2,al

    add bl, al
    sub bl, 30h
    mov sum, bl


    mov ah, 9
    lea dx, msg1
    int 21h


    mov ah, 4ch
    int 21h


    main endp
end main