A computer is a device capable of preforming computations and making logal decisions at speeds millions,and even billions of times faster than human beings can. For example,many of today's presonal computers can preform tens of millions of additions pre second. a preson operating a desk calcutor might require decades to complet the same number of calcuations a powerful personal computer can preform in one secon.(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 faster super-computers can preform hundreds of billions of additions per second about as many calcutions as hundreds of thousands of people could preform in one year!And trillion insturuction per second computers are already functioning in research labtaties.

Computers process data under the control of the sets sof instructions called computer programs.These computer programs guide the computer throufh ordely sets of action specified by people called computer programmers.The various devices (such as the keyborad ,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 reffred to as sofwear.Hardware costs have been declining dramatically in recent years,to the point that personal computers have become a commodity.Unfortunately, softweare development costs have been risisng steadily as programming that reduce software development costs would feature a structured stlye of programming with top-down step wise refinement, functionaliztion and object-oriented programming;c++ is such a language.Regardless of differences in physical apperance, virtually every computer may be envisioned as being divied into six logical units or sections.

1.Input unitThis is the "receving"section of the computer.it obtains information (data and computer programs) from various input devices and placs this information at the desposal of the other units so that the infromation may be processed. Most infromation is entered into computers today through typwriter-like keyborads and "mouse"devices. In the future,prehaps most infromation will be entered by speaking to your computer.

2.output unit It takes infromation that has been procesed by the computer and places it on various output devices to make the information avaible for use outside the computer. Most infromation output from computers today through prited paper, or used to control other devices.

3.Memory unitThis is the rapid access, relatively low-capacity "warehouse" section of the computer. It retains infromation that has been entered through the input unit so that the infromation may be immediately available for processing when it is needed. The memory unit also retains infromation can be placed that infromation can be placed on output devices by the ouput unit. The memory unit is often called either memory or primary memory.

4.Arithmetic unit This is the "manufacturing" section of the computer. It is responssible for preforming calculations such as additon,subractin,multiplication, and division. It contains the decision mechanisms that allow the computer, for example, to compare two items from the memory unit to determine wheather or not they are equal.

5.Central processing unit This is the "administrative" section of the computer. It is the computer's soordinator and is responsible for supervising the operation of the other sections. The CPU tells the input unit when infromation should be read into the memory unit, tells the ALU when infromation from teh memory unit should be ultilzed in calculations, and tels the output unit when to send infromation 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 (such as disks) until they are needed, possibly hours, days, mounths, or even years later. Infromation in secondary storage takes much longer to access than infromation in primary memory. The cost of secondary storage is mech less then the cost per unit of primary memory.