Section 1.2. What Is a Computer?

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1.2. What Is a Computer?

A computer is an electronic device capable of performing computations and making logical decisions at speeds millions, billions and even trillions of times faster than human being. For example, many of today's personal computers can perform a billion additions per second. A person operating a desk calculator could spend an entire lifetime performing calculations and still not complete as many calculations as even today's more modest personal computers can perform in one second. (Points to ponder: How would you know whether the person added the numbers correctly? How would you know whether the computer added the numbers correctly?) The most powerful computers are called supercomputers; some of these are already performing trillions of additions per second!

Computers process data under the control of sets of instructions called computer programs. These programs guide computers through orderly sets of actions that are specified by people known as computer programmers.

A computer consists of various devices referred to as hardware (e.g., the keyboard, screen, mouse, hard drive, memory, DVDs and processing units). The programs that run on a computer are referred to as software (e.g., word processing programs, e-mail and games). Hardware costs have been declining dramatically in recent years, to the point that personal computers have become a commodity. Historically, however, software development costs have risen steadily as programmers develop ever more powerful and complex applications without being able to significantly improve the software development process. In this book, you will learn object-oriented programminga technology that is dramatically reducing software development costs.

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1.3. Computer Organization

Regardless of differences in physical appearance, virtually every computer may be envisioned as being divided into six logical units or sections:

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  1. Input unit. This is the "receiving" section of the computer. It obtains information (data and computer programs) from input devices (e.g., the keyboard and the mouse) and places this information at the disposal of the other units so that it can be processed. Information also can be entered in many other ways, including by speaking to your computer, scanning images and having your computer receive information from a network, such as the Internet.

  2. Output unit. This is the "shipping" section of the computer. It takes information that the computer has processed and places it on various output devices to make the information available for use outside the computer. Most information output from computers today is displayed on screens, printed on paper or used to control other devices. Computers also can output their information to networks, such as the Internet.

  3. Memory unit. This is the rapid-access, relatively low-capacity "warehouse" section of the computer. The memory unit retains information entered through the input unit so that it will be immediately available for processing when needed. The memory unit also retains processed information until it can be placed on output devices by the output unit. Information in the memory unit is typically lost when the computer's power is turned off. The memory unit is often called either memory or primary memory. (Historically, this unit has been called "core memory," but that term is fading from use today.)

  4. Arithmetic and logic unit (ALU). This is the "manufacturing" section of the computer. It is responsible for performing calculations, such as addition, subtraction, multiplication and division. It contains the decision mechanisms that allow the computer, for example, to compare two items from the memory unit to determine whether they are equal.

  5. Central processing unit (CPU). This is the "administrative" section of the computer. It coordinates and supervises the operation of the other sections. The CPU tells the input unit when information should be read into the memory unit, tells the ALU when information from the memory unit should be used in calculations and tells the output unit when to send information from the memory unit to certain output devices. Many of today's computers have multiple CPUs and thus can perform many operations simultaneouslysuch computers are called multiprocessors.

  6. Secondary storage unit. This is the long-term, high-capacity "warehousing" section of the computer. Programs or data not actively being used by the other units normally are placed on secondary storage devices, such as your hard drive, until they are again needed, hours, days, months or even years later. Information in secondary storage takes much longer to access than information in primary memory, but the cost per unit of secondary storage is much less than that of primary memory. Other secondary storage devices include CDs and DVDs, which can hold up to hundreds of millions of characters and billions of characters, respectively.