What Is a Computer?

A computer is a machine that performs calculations and processes information. A computer works under the control of a computer program , a set of instructions that tell a computer what to do. Hardware  refers to the electronic and mechanical components of a computer. Software  refers to the programs that control the hardware.

A general-purpose computer  of the sort that we will be programming stores its control programs in its memory and is capable of changing its control program and hence changing what it does. This is in contrast to a special-purpose computer , such as the one that resides in your microwave oven or the one that controls your digital watch or calculator. These types of computers contain control programs that are fixed and cannot be changed.

A computer's hardware is organized into several main subsystems or components.

• The output devices provide a means by which information held in the computer can be displayed in some human-sensible form. Common output devices include printers, monitors, and audio speakers.
• The input devices bring data and information into the computer. Some of the more common input devices are the keyboard, mouse, microphone, and scanner.
• The primary memory  or main memory of a computer is used to store both data and programs. This type of memory is built entirely out of electronic components -- integrated circuit chips -- which makes it extremely fast. A computer's main memory is volatile, which means that any information stored in it is lost when the computer's power is turned off. In a sense, main memory acts as the computer's scratch pad, storing both programs and data temporarily while a program is running.
• The secondary storage  devices are used for long-term or permanent storage of relatively large amounts of information. These devices include magnetic disks, Compact Disks (CDs), and magnetic tapes. All of these devices are nonvolatile, meaning that they retain information when the computer's power is turned off. Compared to a computer's primary memory, these devices are relatively slow, because their access devices all require some form of mechanical motion to retrieve and store data.
• The central processing unit  (CPU ) is the computer's main engine. The CPU is the computer's microprocessor, such as the Intel Pentium processor, which serves as the foundation for most Windows PCs, or the PowerPC processor, which serves as the foundation for Macintosh computers. Under the direction of computer programs (software), the CPU issues signals that control the other components that make up the computer system. One portion of the CPU, known as the arithmetic-logic unit (ALU), performs all calculations, such as addition and subtraction, and all logical comparisons, such as when one piece of data is compared to another to determine if they are equal.

The computer's software  can be distinguished into two main types:

• Application software refers to programs designed to provide a particular task or service, such as word processors, computer games, spreadsheet programs, and Web browsers.
• System software includes programs that perform the basic operations that make a computer usable at all. For example, an important piece of system software is the operating system, which contains programs that manage the data stored on the computer's disks.

Another important thing that the operating system does is to serve as an interface between the user and the hardware. The operating system determines how the user will interact with the system, or conversely, how the system will look and feel to the user. For example, in command-line systems, such as Unix and DOS (short for Disk Operating System), a program is run by typing its name on the command line. By contrast, in graphically based systems, such as Windows and Macintosh, a program is run by clicking on its icon with the mouse. Thus, this ``point and click'' interface has a totally different ``look and feel,'' but does the same thing.