USING DISKS AND DISK DRIVES
A disk is a data storage device consisting of a flat, circular, magnetic surface on which information can be recorded. You will receive several disks with your computer. These already contain information and programs, and are ready to use.
A disk, like a cassette tape, is reusable storage device that holds information, such as software and data, in files. As with random access memory, the amount of space on a disk is measured in bytes. Unlike information stored in random-access memory, however, information stored on a disk is not deleted when you turn off your computer. If you choose to, you can delete the information on a disk, and with proper care, you can use the disk over and over again. Your computer’s disk drives move information stored on the disks into and out of random-access memory. For example, your computer can read software instructions from a disk into random-access memory, and write your data to a disk for safekeeping.
A hard disk drive contains a non-removable disk that is built in your system. With a hard disk drive, you can store large amounts of information in one convenient place.
The hard disk drive can also write and read information to and from your system much faster than a floppy disk drive can write and read information to and from a disk. When your system writes or reads information to or from a disk, the indicator light on the drive goes on.
Each disk drive has a letter assigned to it so you can tell your system where to find instructions and information. For example on many systems the floppy disk drive is called the A drive, and the hard disk drive is called the C drive.
The default drive is the one which MS-DOS uses unless you tell it to do otherwise. It is shown by the prompt that MS-DOS displays.
The elements of programming
Most programs are designed to solve a problem. They solve problems by manipulating information or data. As a programmer you do the following:
· get the information into the program — input. This means reading values in from the keyboard, from a disk, or from an I/O port.
· have a place to keep it — data. These are constants, variables, and structures, that contain numbers (integer and real), text (characters and strings), or addresses (of variables and structures).
· give the right instructions to manipulate the data— operations. These assign one value to another, combine values (add, divide, and so forth), and compare values (equal, not equal, and so on).
· be able to get the data back out of the program to the user (you, usually) — output. This means writing information to the screen, to a disk, or to an I/O port.
You can organize your instructions so that
· some are executed only when a specific condition (or set of conditions) is True — conditional execution. This refers to executing a set of instructions if a specified condition is True (and skipping them or executing a different set if it is False) or if a data item has a specified value or range of values.
· others are repeated a number of times — loops. These execute a set of instructions some fixed number of times, while some condition is True or until some condition is True.
· others are broken off into chunks that can be executed at different locations in your program — subroutines. These are separately named sets of instructions that can be executed anywhere in the program just by referencing the name.
These are the seven basic elements of programming: input, data, operations, output, conditional execution, loops, and subroutines. This list is not comprehensive, but it does describe those elements that programs (and programming languages) usually have in common. Many programming languages, including Pascal, have additional features. And when you want to learn a new language quickly, you can find out how that language implements these seven elements, and then build from there.
HIGH LEVEL PROGRAMMING LANGUAGE PASCAL
PASCAL is a general-purpose high level programming language. It is named after the famous French mathematician, Blaise Pascal, who in 1642 designed and built the first mechanical calculator, the “Pascaline”. PASCAL is noted for its simplicity and structured programming design. It is available as both a compiler and an interpreter.
PASCAL was proposed and defined in 1971, and gained popularity in universities and colleges in Europe and the United States. It was later revised and appeared as standard PASCAL in 1975. Its principal features are on teaching programming and on the efficient implementation of the language.
PASCAL may be considered a successor to ALGOL-60, from which it inherits syntactic appearances. The novelties of PASCAL lie mainly in extensive data structuring facilities such as record, set and file structures. It also affords more sophisticated control structures suitable to structured programming.
An algorithm of a computer program consists of two essential parts: a description of actions which are to be performed, and a description of the data, which are manipulated by these actions. Actions are described by statements, and data are described by declarations and definitions.
The program is divided into a heading and a body, called a block. The heading gives the program a name and lists its parameters. These are file variables and represent the arguments and results of the computation. The file output is a compulsory parameter. The block consists of six sections. They are: label declaration part, constant definition part, type definition part, variable declaration part, procedure and function declaration part, and statement part.
The first section lists all labels defined in this block. The second section introduces identifiers for constants. The third section contains type declarations, and the fourth - variable definitions. The fifth section defines procedures and functions. And the last, the sixth, gives the statements which specify the actions to be taken
The WORLD-WIDE WEB
People have dreamt of a universal information database since late nineteen forties. In this database, not only would the data be accessible to people around the world, but it would also easily link to other pieces of information, so that only the most important data would be quickly found by a user. Only recently the new technologies have made such systems possible. The most popular system currently in use is the World-Wide Web (WWW) which began in March 1989. The Web is an Internet-based computer network that allows users on one computer to access information stored on another through the world-wide network.
As the popularity of the Internet increases, people become more aware of its colossal potential. The World-Wide Web is a product of the continuous search for innovative ways of sharing information resources. The WWW project is based on the principle of universal readership: “if information is available, then any person should be able to access it from anywhere in the world.” The Web’s implementation follows a standard client-server model. In this model, a user relies on a program (the client) to connect to a remote machine (the server), where the data is stored. The architecture of the WWW is the one of clients, such as Netscape, Mosaic, or Lynx, “which know how to present data but not what its origin is, and servers, which know how to extract data” but are ignorant of how it will be presented to the user.
One of the main features of the WWW documents is their hypertext structure. On a graphic terminal, for instance, a particular reference can be represented by underlined text, or an icon. “The user clicks on it with the mouse, and the referenced document appears.” This method makes copying of information unnecessary: data needs only to be stored once, and all referenced to it can be linked to the original document. You move from place to place, from site to site on the Web by using a mouse to click on a portion of text, icon or region of a map. These items are called hyperlinks or links. Each link you select represents a document, an image, a video clip or an audio file somewhere on the Internet. The user doesn't need to know where it is, the browser follows the link.
All sorts of things are available on the WWW. One can use Internet for recreational purposes. Many TV and radio stations broadcast live on the WWW. Essentially, if something can be put into digital format and stored in a computer, then it's available on the WWW. You can even visit museums, gardens, cities throughout the world, learn foreign languages and meet new friends. Moreover, of course, you can play computer games through WWW, competing with partners from other countries and continents.
Just a little bit of exploring the World Wide Web will show you what a lot of use and fun it
Computers in Modern Communication
Computers are used by scientists to exchange ideas, to collect, manipulate, and analyse new theories and designs. Computers are also widely used in education. Personal computer users use educational means on magnetic or optical disks or get the necessary information through various telecommunication networks.
Local area networks link the computers in different offices and educational institutions.
Modems are telecommunication devices that transmit data through telephone lines. Modems link individual computers to other computers anywhere in the country or in the world.
Word processorsare used by journalists and writers for their literary activity, for writing books and articles. Their works are presented on magnetic disks or transmitted through telephone lines.
Invention of computers belongs to the greatest developments of mankind. It can be compared with the invention of steam-engine at the end of the 18th century or the utilization of atomic energy.
Optical Computing Used for Communication
Optical computing used for communication is a method of computing that uses photons. Photons are packets of light that process information. According to the definition, a photon is a quantum of electromagnetic radiation, whose main characteristic is that it has a zero rest mass.
Traditional microprocessors include tiny electronic switches, or logic circuits, for processing. Theoretically, an unlimited number of electronic switches can be added to a microprocessor. But it is only theoretically, since practically the number of electronic switches is limited to the millions because of the space limitations in wiring. As to photons, they, unlike electrons, are able to pass through one another unchanged, which makes wiring unnecessary. Besides, photons travel faster than electrons.
What is the history of designing optical computers? In 1989 a Bell Laboratory team produced a device consisting of lasers, lenses, and mirrors. A few years later a general-purpose device was produced in which data was not stored but circulates as light pulse. Thus, optical computers, thousands of times faster than the most powerful electronic computers, are believed to be constructed. The biggest difficulty in designing them is that of miniaturization.
Babbage, Charles (1792-1871)
Although he was a 19th-century mathematician, Charles Babbage is credited with inventing the modern computer. He also designed a type of speedometer and the cowcatcher (a sloping frame on the front of a locomotive that tosses obstacles off the railroad tracks).
Charles Babbage was born on Dec. 26, 1792, in Teignmouth, Devon England. At the age of 19 he helped found the Analytical Society, whose purpose introduce developments from Europe into English mathematics At about the same time Babbage first got his idea for mechanically calculating mathematical tables. Later he made a small calculator that could perform certain mathematical computations to eight decimal places. In 1816 he was elected a fellow of the Royal Society of London, the oldest scientific society in Great Britain. Then, in 1823, he received government support for the design of a projected calculator with a 20-decimal capacity. While he was developing this machine he also served (1828—39) as a professor of mathematics at the University of Cambridge.
In the mid-1830's Babbage invented the principle of the analytical engine, the forerunner of the modern electronic computer. The government refused Babbage further support, however, and the device was never completed. A calculator based on his ideas was made in 1855 by a Swedish firm, but the computer was not developed until the electronic age.
Babbage published papers on mathematics, statistics, physics, and geology. He also assisted in establishing England's modem postal system. Babbage died in London on Oct. 18, 1871.
The Internet, a global computer network which embraces millions of users all over the world began in the United States in 1969 as a military experiment. It was designed to survive a nuclear war. Information sent over the Internet takes the shortest path available from one computer to another. Because of this, any two computers on the Internet will be able to stay in touch with each other as long as there is a single route between them. This technology is called packet- switching. Owing to this technology, if some computers on the network are knocked out (by a nuclear explosion, for example), information will just route around them. One such packet- switching network which has already survived a war is the Iraqi computer network which was not knocked out during the Gulf War.
Most of the Internet host computers (more than 50 %) are in the United States, while the rest are located in more than 100 other countries. Although the number of host computers can be counted fairly accurately, nobody knows exactly how many people use the Internet, there are millions worldwide, and their number is growing by thousands each month.
The most popular Internet service is e-mail. Most of the people, who have access to the Internet, use the network only for sending and receiving e-mail messages. However, other popular services are available on the Internet: reading USENET News, using the World-Wide Web, telnet, FTP, and Gopher.
In many developing countries, the Internet may provide executives with a reliable alternative to the expensive and unreliable telecommunications systems of these countries. Commercial users can communicate cheaply over the Internet with the rest of the world. When they send e-mail messages, they only have to pay for phone calls to their local service providers, not for calls across their countries or around the world. But who actually pays for sending e-mail messages over the Internet long distances around the world? The answer is very simple: users pay their service provider a monthly or hourly fee. Part of this fee goes towards its costs to connect to a larger service provider and part of the fee received by the larger provider goes to cover its cost of running a worldwide network of wires and wireless stations.
But saving money is only the first step. If people see that they can make money from the Internet, commercial use of this network will drastically increase. For example, some western architecture companies and garment centers already transmit their basic designs and concepts over the Internet into China, where they are reworked and refined by skilled - but inexpensive - Chinese computer-aided-design specialists.
However, some problems remain. The most important is security. When you send an e- mail message to somebody, this message can travel through many different networks and computers. The data is constantly being directed towards its destination by special computers called routers. However, because of this, it is possible to get into any of the computers along the route, intercept and even change the data being sent over the Internet. In spite of the fact that there are many good encoding programs available, nearly all the information being sent over the Internet is transmitted without any form of encoding, i. e. "in the clear". But when it becomes necessary to send important information over the network, these encoding programs may be useful. Sonic American banks and companies even conduct transactions over the Internet. However there are still both commercial and 1 problems which will take time to be resolved.
MODERN MEANS OF COMMUNICATION AND ELECTRONIC COMMERCE
Connecting many computer networks and using common addressing system, the Internet has been growing rapidly since its creation in 1983, radio, telephone and cable television wires, satellites being used to deliver Internet services. By the mid-1990s the Internet linked millions of computers throughout the world and it is sure to be the most important commercial and popular means of communication nowadays. The original uses of the Internet were electronic mail, file transfer, bulletin boards and remote computer access. Having expanded considerably during the 1990s, the World Wide Web enables users easily to examine the Internet sites and now it is likely to have become the leading informational service of the Internet.
The first electronic transactions are supposed to have been handled in the 1950s due to telex, radio teletype and telephone. In the following decades various industries elaborated upon the system of electronic data interchange before a simple and independent of any particular machine standard was created. Since the mid- 1990s electronic commerce has become one of the most rapidly growing retail sectors involving the use of computer telecommunication networks for maintaining business relationships and selling information, services and commodities. Although e-commerce usually refers only to the trading of goods and services over the Internet, it actually includes broader economic activity such as business-to-consumer and business-to-business commerce as well as internal organizational transactions that support these activities.
A large part of e-commerce was transferred to the Internet after the first graphical "browser" software for the access to the World Wide Web had been introduced in 1993 and when the number of companies and individuals using "on-line" had greatly increased. In some fields new Internet retailers such as the Amazon bookseller company seem to have grown up overnight and begun successfully competing with traditional retailers. Most of recently established companies are known to include the electronic commerce in their business as well. For example, the Intel. Corporation sold almost half of its chips in the annual computer sales directly through its Web site in 1999 and is planning to move all of its sales to the Web by the end of2000.
The further development of secure electronic transfer of sensitive information,1 such as credit card numbers and electronic funds transfer orders, is certainly to be essential to the continued growth of e-commerce. It is often necessary to ensure the encrypting of Web purchase forms, many individuals also usually encrypting their e-mail.
Among other innovations that have contributed to the growth of e-commerce are electronic directories and search systems for finding information on the Web; software agents that act autonomously to allocate goods and services; and special identifying services over the Internet. These intermediary services facilitate the sale of goods (actually delivering the goods in case of information), the rendering of services such as banking, ticket reservations, and stock market transactions, and even die delivery of remote education and entertainment. Specialists consider electronic auction sales and markets to be other rapidly developing parts of e-commerce. The former offer a large variety of goods from computers and electronics to books, recordings, automobiles and real estate, while the latter allow a buyer to choose offers from many sellers. It is interesting that from its establishing in 1995, the world's largest on-line open auction site, eBay, grew to more than 5 million members by 1999.
Businesses often develop private intranets for sharing information and collaborating within the company, these networks usually being isolated from the surrounding Internet by special computer-security systems. Businesses also often rely on extranets which are extensions2 of a company's intranet. Such extranets allow portions of company's internal network to be accessible to collaborating businesses, access to the ones being generally restricted through passwords.
One should mention some more important benefits of e-commerce. Due to its development the role of geographic distance in forming business relationships is being reduced. If you were interested in the beginning of a retail business, it would be relatively inexpensive to start a retail Web site. Some traditional businesses are being replaced by their electronic equivalents or are being made entirely useless. Having published fare information and enabled ticketing directly over the Internet, airlines have greatly decreased the role of traditional agencies. Prices of commodity products are generally lower on the Web and it results not only from the lower costs of doing electronic business but also from the ease of comparison shopping in cyberspace. A new form of collaboration known as a virtual company is flourishing now. This type of company is actually a network of firms, each performing some of the processes needed to manufacture a product or deliver a service.
A Spanish teacher was explaining to her class that in Spanish, unlike English, nouns are designated as either masculine or feminine. A student asked, “What gender is ‘computer’?” Instead of giving the answer, the teacher split the class into two groups, male and female, and asked them to decide for themselves whether “computer” should be a masculine or a feminine noun. Each group was asked to give four reasons for its recommendation. The men’s group decided that “computer” should definitely be of the feminine gender (“la computadora”), because:
1. No one but their creator understands their internal logic;
2. The native language they use to communicate with other computers is incomprehensible to everyone else;
3. Even the smallest mistakes are stored in long term memory for possible later retrieval; and...
4. As soon as you make a commitment to one, you find yourself spending half your paycheck on accessories for it.
The women’s group, however, concluded that computers should be Masculine (“el computador”), because:
1. In order to do anything with them, you have to turn them on;
2. They have a lot of data but still can’t think for themselves;
3. They are supposed to help you solve problems, but half the time they ARE the problem; and ...
4. As soon as you commit to one, you realize that if you had waited a little longer, you could have gotten a better model. The women won.
The First Computers
In 1930 the first analog computer was built by American named Vannevar Bush. This device was used in World War II to help aim guns.
Many technical developments of electronic digital computers took place in the 1940s and 1950s. Mark I, the name given to the first digital computer, was completed in 1944. The man responsible for this invention was Professor Howard Aiken. This was the first machine that could figure out long lists of mathematical problems at a very fast rate.
In 1946 two engineers at the University of Pennsilvania, J. Eckert and J. Maushly, built their digital computer with vacuum tubes. They named their new invention ENIAC (the Electronic Numerical Integrator and Calculator).
Another important achievement in developing computers came in 1947, when John von Neumann developed the idea of keeping instructions for the computer inside the computer's memory. The contribution of John von Neumann was particularly significant. As contrasted with Babbage's analytical engine, which was designed to store only data, von Neumann's machine, called the Electronic Discrete Variable Computer, or EDVAC, was able to store both data and instructions. He also contributed to the idea of storing data and instructions in a binary code that uses only ones and zeros. This simplified computer design. Thus computers use two conditions, high voltage, and low voltage, to translate the symbols by which we communicate into unique combinations of electrical pulses. We refer to these combinations as codes.
Neumann's stored program computer as well as other machines of that time were made possible by the invention of the vacuum tube that could control and amplify electronic signals. Early computers, using vacuum tubes, could perform computations in thousandths of seconds, called milliseconds, instead seconds required by mechanical devices.
©2015- 2021 megalektsii.ru Все материалы представленные на сайте исключительно с целью ознакомления читателями и не преследуют коммерческих целей или нарушение авторских прав.