INT 2F U - DOS 3+ internal - SUM MEMORY
        AX = 121Dh
        DS:SI -> memory to add up
        CX = 0000h
        DX = limit
Return: AL = byte which exceeded limit
        CX = number of bytes before limit exceeded
        DX = remainder after adding first CX bytes
        DS:SI -> byte beyond the one which exceeded the limit
Note:   used by DOS to determine year or month given day count since 1/1/80