A digital computer is machine that stores data in a numerical format and performs operations on that data using mathematical manipulation. This type of computer typically includes some sort of device to store information, some method for input and output of data, and components that allow mathematical operations to be performed on stored data. Digital computers are almost always electronic but do not necessarily need to be so.
There are two main methods of modeling the world with a computing machine. Analog computers use some physical phenomenon, such as electrical voltage, to model a different phenomenon, and perform operations by directly modifying the stored data. A digital computer, however, stores all data as numbers and performs operations on that data arithmetically. Most computers use binary numbers to store data, as the ones and zeros that make up these numbers are easily represented with simple on-off electrical states.
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Computers based on analog principles have advantages in some specialized areas, such as their ability to continuously model an equation. A digital computer, however, has the advantage of being easily programmable. This means that they can process many different sets of instructions without being physically reconfigured.
The earliest digital computers date back to the 19th century. An early example is the analytical engine theorized by Charles Babbage. This machine would have stored and processed data mechanically. That data, however, would not have been stored mechanically but rather as a series of digits represented by discrete physical states. This computer would have been programmable, a first in computing.
Digital computing came into widespread use during the 20th century. The pressures of war led to great advances in the field, and electronic computers emerged from the Second World War. This sort of digital computer generally used arrays of vacuum tubes to store information for active use in computation. Paper or punch cards were used for longer-term storage. Keyboard input and monitors emerged later in the century.
In the early 21st century, computers rely on integrated circuits rather than vacuum tubes. They still employ active memory, long-term storage, and central processing units. Input and output devices have multiplied greatly but still serve the same basic functions.
In 2011, computers are beginning to push the limits of conventional circuitry. Circuit pathways in a digital computer can now be printed so close together that effects like electron tunneling must be taken into consideration. Work on digital optical computers, which process and store data using light and lenses, may help in overcoming this limitation.
Nanotechnology may lead to a whole new variety of mechanical computing. Data might be stored and processed digitally at the level of single molecules or small groups of molecules. An astonishing number of molecular computing elements would fit into a comparatively tiny space. This could greatly increase the speed and power of digital computers.