An ultraportable IBM X31 with 12" screen on an IBM T43 Thin & Light laptop with a 14" screen A laptop computer or laptop (also notebook computer, notebook and notepad) is a small mobile computer, typically weighing 3 to 12 pounds (1.4 to 5.4 kg), although older laptops may weigh more. Laptops usually run on a single main battery or from an external AC/DC adapter that charges the battery while it also supplies power to the computer itself, even in the event of a power failure. This very powerful main battery should not be confused with the much smaller battery nearly all computers use to run the real-time clock and backup BIOS configuration into the CMOS memory when the computer is without power. Laptops contain components that are similar to their desktop counterparts and perform the same functions, but are miniaturized and optimized for mobile use and efficient power consumption, although typically less powerful for the same price. Laptops usually have liquid crystal displays and most of them use different memory modules for their random access memory (RAM), for instance, SO-DIMM in lieu of the larger DIMMs. In addition to a built-in keyboard, they may utilize a touchpad (also known as a trackpad) or a pointing stick for input, though an external keyboard or mouse can usually be attached.