Digital computers use different kinds of memory, each of which is either volatile or nonvolatile. On most computers only the hard drive memory is nonvolatile, i.e., it retains all information stored on it when the power is off. When a computer is turned on, an operating system stored on the hard drive is loaded into the computer's memory cache and system memory. Cache and system memory are volatile, i.e., data in them are lost when the computer is powered down. A more ideal computer would have nonvolatile memory for both cache and system memory to allow for "instant-on" capabilities. Of course any such memory would have to be as fast and as durable as existing volatile devices to be considered an improvement over current systems.