A computer is a device capable of performing computations and making logical decisions at speed millions, and even billions of times faster than human beings can. For example, many of today's personal computers can perform tens of millions of additions per second. A nperson opersting a desk calculator might require decades to complete the same numberof calculations a powerful personal computer 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?) Today's fastest super-computers can perform hundreds of billions of additions per second about as many calculations as hundreds of thousands of people could perform in one year! And trillion instruction per second computers are already functioning in research laboratories.

C omputers process data under the control of sets of instructions called computer programs. These computer programs guide the computer through orderly sets of actions specified by people called computer programmers. The various devices (such as the keyboard, monitor, disks, memory, and processing unit) that comprise a computer system are referred to as hardware. The computer programs that run on a computer are referred to as software. Hardware costs have been declining dramatically in recent years, to the point that personal computers have become a commodity. Unfortunately, software developing costs have been rising steadily as programmers develop ever more powerful and complex applications, without being able to improve the technology of software development. Programming that reduce software developmentcosts would feature a structured style of programming with top-down step wise refinement, funtionalization and object-oriented programming; C++ is such a language. Regardless of differences in physical appearance, virtually every computer may be envisioned as being divided into six logical units or sections.

These are:
1. Input unit. This is the "receiving" section of the computer. It obtains information (data and computer programs) from various input devices and places this information at the disposal of the other units so that the information may be processed. Most information is entered into computers today through typewriter-like keyboards and "mouse" devices. In the future, perhaps most information will be entered by speaking to your computer.
2. Output unit. This is the "shipping" section of the computer. It takes information that has been processed by the computer 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.
3. Memory unit. This is the rapid access, relatively low-capacity "warehouse" section of the computer. It retains information that has been entered through the input unit so that the information may be made immediately available for processing when is is needed. The memory unit also retains information that has already been processed until that information can be placed on output devices by the output unit. The memory unit is often called either memory or primary memory.
4. Arithmetic and logic unit. This is the "manufacturing" section of the computer. It is reponsible 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 or not they are equal.
5. Central processing unit. This is the "adminstrative" section of the computer. It is the computer's coordinator and is responsible for supervising 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 shouls be utilized in calculations, and tells the output unit when to send informationfrom the memory unit to certain output devices.
6. Secondary storage unit. This is the long-term, high-capacity "warehouse" section of the computer. Programs or data not actively being used by the other units are normally placed on secondary storage devices (such as disks) until they are again needed, possibly hours, days, months, or even years later. Information in secondary storage takes much longer to access than information in primary memory. The cost per unit of secondary storage is less than the cost of primary memory.