A computer is a device capable of performing computations and making logicaldecisions at speeds millions, and even billions times faster than human beings can. For example, many of today's personal computers can perform tens of millions of additions per second. A person operating a desk calculator might require decades to complete the same number of calculations a powerful personal computer can perform in one second.How would you know if 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 calculators as hundredes of thousands of peoplecould perform in one year! And trillion instruction per second computers are already functioning in research labratories.

Computers process dataunder the control of sets of instructions called computer programs. These computer programs guide the computer thruogh orderly sets of actions specified by people called computer programmers. The various devices that compromise a computer system are reffered to as hardware. The computer programs that run on a computer are reffered to as software. Hardware costs have been declining dramatically in recent years, to the point that a personal computers have become a comodity. Unfortunatley, software development costs have been rising steadily as programmers develope even more complex and powerful applications, without being able to improve the technology of software developement. Programming that reduce software development costs would feature a structured style of programming with top-down step wise refinement, functionalization and object-oriented programming; C++ is such a language. Regardless of physical appearance, virtually every computer may be envissioned as being divided into six logical units or sections.

1. Input UnitThis is the "recieving" section of the computer. It obtains information from various imput devices and places this information at the disposal of the other units so that the information may be processed. Most informationis entered into computers today through typewritter-like keyboards and "mouse" devices. In the future, perhapsmost 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 rapld, relatively low capacity "warehouse" section of the computer. It retains information that has been enered through the input unit so that the information may be made immediately available for prosses when it is needed. The memmory unit also retains information that has allready been prossesed until that information can be placed on output devices by the output unit. The memory unit is often called memory or primary memory.
4. Arithmatic and Logic Unit.This is the "manufacturing" section of the computer. It is responsable for performing calculations such as addition, subtraction, multiplication, and division. It contains the desician mechanisms that allow the computer, for example, to compare two items from the memmory unit to determine wheather or not they are equal.
5. Central Processing Unit.This is the "adminastrative" section of the computer. It is the computer's coordinater and is responsable 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 informatio from the memory unit should be utalized in calculations, and tells the output unit when to send information from 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 until they are again needed, possibly hours, days, months, or even years latter. 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.