Text 2. some features of a digital computer
it should be noticed that even in a large-scale digital system, such as in a computer, or in a data-processing, control or digital-communication system, there are only a few basic operations which must be performed. these operations may be operated many times. the four circuits most commonly employed in such systems are known as the or, and, not and flip-flop. they are called logic gates or circuits.
an electronic digital computer is a system which processes and stores very large amount of data and which solves scientific problems of numerical computations of such complexity and with such speed that solution by human calculation is not feasible. so the computer as a system can perform numerical computations and follow instructions with extreme speed but it cannot program itself.
we know that the numbers and the instructions which form the program, the computer is to follow, are stored in an essential part of the computer called the memory. the second important unit of the computer is the control whose function is to interpret orders. the control must convert the command into an appropriate set of voltages to operate switches and carry out the instructions conveyed by the order. the third basic element of a computer is the arithmetic device, which contains the circuits performing the arithmetic computations: addition, subtraction, etc. the control and arithmetic components are called the central processor. finally a computer requires appropriate input-output devices for inserting numbers and orders into the memory and for reading the final result.
suppose a command to perform an addition or division has been transmitted to the central processor. in response to this order the control must select the correct operands from the memory, transmit them to the arithmetic unit and return to the memory the result of the computation. the memory serves for storing not only the original input data, but also the partial results which will have to be used again as the computation proceeds.
lastly, if the computation doesn't stop with the execution of this instruction and the storage of the partial result, the control unit must automatically pass on to the next instruction. the connection of the control unit back to the input permits insertion of more data when there is room in the memory.
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1. what are the most commonly used circuits in any computer? 2. how are they called? 3. what kind of a system is a digital computer? 4. is there anything that a computer cannot do itself? what is it? 5. where are the instructions and digits stored? 6. what is the function of the control? 7. what does the arithmetic device serve for? 8. what components form the central processor? 9. what other devices in addition to the above-mentioned ones does a computer require? 10. how are computations performed in a computer?
There are four seasons in a year: winter, spring, summer and autumn. All seasons are good in their own way. Winter, for example, is full of celebrations, presents and snow. Spring is the time of nature awakening and flower blooming. Autumn can be rainy and yet beautiful, full of multi-coloured leaves. However, my favourite season is summer. I choose summer not only because my birthday is in June, but also because it’s the time of holidays. Summer is the season when the sun shines brightly, the sky is rarely cloudy and the water in the sea becomes warm enough to swim. Even if it becomes too hot, you can always go for a walk in the shady woods. I also like summer because there are many berries, fruit and vegetables, not to mention numerous flowers on the fields. Among them roses, dandelions, daisies, and other flowers. Sometimes the air in summer becomes stuffy, but occasional thunderstorms bring relief. After the thunderstorms the air becomes remarkably fresh and very often we can see the rainbow in the sky. Summer is also the time when I and my family go on holiday to the seaside. It’s a wonderful time, when we can have a rest from the intense academic year and relax. I think all children love summer because of long school holidays and I’m no exception. I also enjoy my free time, playing with my friends, watching TV, and doing other fun activities. Nevertheless, I don’t forget to do some useful reading from time to time.