In general, laptop components are not intended to be replaceable or upgradable, with the exception of components which can be detached, such as a battery or CD/CDR/DVD drive. This restriction is one of the major differences between laptops and desktop computers, because the large "tower" cases used in desktop computers are designed so that new motherboards, hard disks, sound cards, RAM, and other components can be added. In a very compact laptop, such as laplets, there may be no upgradeable components at all.