ds, data, or programs for processing by the CPU. Computer keyboards, which are much like typewriter keyboards, are the most common input devices. Informationtyped at the keyboard is translated into a series of binary numbers that the CPU can manipulate. Another common input device, the mouse, is a mechanical or optomechanical device with buttons on the top and a rolling ball in its base. To move the cursor on the display screen, the user moves the mouse around on a flat surface. The user selects operations, activates commands, or creates or changes images on the screen by pressing buttons on the mouse. Other input devices include joysticks and trackballs. Light pens can be used to draw or to point to items or areas on the display screen. A sensitized digitizer pad translates images drawn on it with an electronic stylus or pen into acorresponding image on the display screen. Touch-sensitive display screens allow users to point to items or areas on the screen and to activate commands. Optical scanners "read" characters on a printedpage and translate them into binary numbers that the CPU can use. Voice-recognition circuitry digitizes spoken words and enters them into the computer. Memory-storage devices. Most digital computers store data both internally, in what is called main memory, and externally, on auxiliary storage units. As a computer processes data and instructions,it temporarily stores information internally, usually on silicon random-access memory, or RAM, chips--often called semiconductor memory. Usually mounted on the main circuit board inside the computer or on peripheral cards that plug into the board, each RAM chip may consist of as many as 16 million switches, called flip-flop switches, that respond to changes in electric current. Each switch can hold one bit of data: high voltage applied to a switch causes it to hold a 1; low voltage causes it to hold a 0. This kind of internal memory is also call...
