A computer is a device capable of performing computations and making logical decicions at speeds millions, even billions of times faster that 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. (Points to ponder: How would you know wether the person added the numbers correctly? How would you know wether the computer added the numbers correctly?) Today's fastest super-computers can perform hundreds of billions of addditions per second about as many calculations as hundreds of thousands of people could perform in one year! And trillion instructions per second, computers are already functioning in research laboraties.

Computers process data under the control of sets of instructions called computer programs. These computer programs guide the computer through orderly seets of actions specified by people called computer programmers. The various devices (such as the keybaord, monitor, disks, memory, ands 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 development costs have been rising steadily as programmers develope even more powerful and complex applications,without being able to improve the technology of software development. programming that reduce software devlopment costs would feature a structured style of programming with top-down step wise refinment, functionalization and object-oriented programming; C++ is such a language. Regardless of differences in physical appearace, virtually every computer may be envisioned as being divided into six logical units or sections.

These are:
1.Input unit. This is the "reciving" 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 the computers today through typwriter-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 prosessed by the computer and places it on various output devices to make the information available for the use outside the computer. Most informationn output from the computers today is displayed on sreens, printed of paper, or use to control other devices.
3.Memory unit.This is a 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 it 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.Arithmatic and logic unit. 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 wether or not thay are equal.
5.Central processing unit. This is the "administrative" 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 should be utilized 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 th elong-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 thatn information in primary memory. The cost per unit of secondary storag is much less than the cost per unit of primary memory.
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