It times and regulates the operations of the entire computer system; its instruction decoder reads the patterns of data in a designated register and translates the pattern into an activity, such as adding or comparing; and its interrupt unit indicates the order in which individual operations use the CPU, and regulates the amount of CPU time that each operation may consume.

The last segment of a CPU chip or microprocessor is its internal bus, a network of communication lines that connects the internal elements of the processor and also leads to external connectors that link the processor to the other elements of the computer system. The three types of CPU buses are: (1) a control bus consisting of a line that senses input signals and another line that generates control signals from within the CPU; (2) the address bus, a one-way line from the processor that handles the location of data in memory addresses; and (3) the data bus, a two-way transfer line that both reads data from memory and writes new data into memory.

Input Devices

These devices enable a computer user to enter data, commands, and programs into the CPU. The most common input device is the keyboard. Information typed at the typewriter-like keyboard is translated by the computer into recognizable patterns. Other input devices include the mouse, which translates physical motion into motion on a computer video display screen; the joystick, which performs the same function, and is favoured for computer games; the trackball, which replaces the mouse on laptops; scanners, which "read" words or symbols on a printed page and translate them into electronic patterns that the computer can manipulate and store; light pens, which can be used to "write" directly on the monitor screen; and voice recognition systems, which take spoken words and translate them into digital signals for the computer. Storage devices can also be used to input data into the processing unit.

Storage Devices

Computer systems can store data internally (in memory) and externally (on storage devices). Internally, instructions or data can be temporarily stored in silicon RAM (Random Access Memory) chips that are mounted directly on the computer's main circuit board, or in chips mounted on peripheral cards that plug into the computer's main circuit board. These RAM chips consist of millions of switches that are sensitive to changes in electric current. So-called static RAM chips hold their data as long as current flows through the circuit, whereas dynamic RAM (DRAM) chips need high or low voltages applied at regular intervals-every two milliseconds or so-if they are not to lose their information.

Another type of internal memory consists of silicon chips on which all switches are already set. The patterns on these ROM (Read-Only Memory) chips form commands, data, or programs that the computer needs to function correctly. RAM chips are like pieces of paper that can be written on, erased, and used again; ROM chips are like a book, with its words already set on each page.