A computer is a device capable of performing comutations and making logical decisiion at speeds millions, and eveens of times faster than human bengs can. For example, many of today's personal computers can peform tens of millions of additions per second. A person operating a desk calculator might require decades to complete the same number of calcutersa powerful personal computer san perform in one second. (Points to ponder: How would you know whether the person added the number correctly? How would you know whether the computer added the number 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 prform in one year! And trillion inatruction per second computer are already functioning in research laboraties.

Computers process data under the control of sets of instructions called computer progams. These computers progams guide the computer throuth orderly sets of actions specified by prople called computer programmers. The various devices (such as the keyboard, moniter, disks, memory, and processing unit) that comprise a computer system are referred to as hardware. the computer progams that run on a compter 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 developmrnt costshve been rising stesdily as prgrmmers develop ever more powerful and comlex applications, without being able to improve the technology of software development. Programing that reduce software development coosts would feature a structured style of programing with top-down step wise refinement, functionalization and object-oriented programing; C++ is such alanguage. 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 devuces and this information at the disposal of the other units so that the information may be processed. Most information is entered into comnputers today through typewriter-like keyboards and "mouse" devices. In the futer, perhaps most information will be entered by spesking into your compter.
2.Output unit. This is the "shipping" section of the computer. It takes information that hasben processed by the computer and places it onvarious output devices to make the information available for the use ooutside the computer. Most information output from computers today is dislayed on screens, printed on paper, or used to control other devices.
3.Memory unit. This is the rapid acces, relativey low-capacity "warehouse" section of the computer. It retains information that been entered through the input unit so that the information may be made immediatly available for processing when it is needed. The memory unit also retaindinformation thet has already been processed until that information can be placed onto output devices by the output unit. The memory unit is often called either memory or primary memory.
4.Aruthmectic and logic unit. This is the "manufacturing" section of the computer. It is responsible for perfroming calculations such as addition, subration, multipling, and divison. It contans the decision mechaniams 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 "administrative" section of the computer. It is the computer's coordinator and is responal for supervising the operation of theother 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 utilized in calculations, and tells the output unit when to send information from hte memory unit to certain output devices.
6.Secondary storge unit. This is the long-trem, high-capacity "warehouse" section of the computer. Progams 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, possiby hours, days, 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 much less than the cost per unit of primary memory.