Architecture (Несколько текстов для зачёта), страница 10

Common features found in browsers include the ability to automatically designate a Web site to which the browser opens with each use, the option to create directories of favorite or useful Web sites, access to search engines (programs that permit the use of key words to locate information on the Internet, an internet or an intranet), and the ability to screen out certain types of information by blocking access to certain categories of sites.

A browser’s performance depends upon the speed and efficiency of the user’s computer, the type of modem being used, and the bandwidth of the data-transmission medium (the amount of information that can be transmitted per second). Low bandwidth results in slow movement of data between source and recipient, leading to longer transmission times for documents. Browsers may also have difficulty reaching a site during times of heavy traffic on the network or because of high use of the site.

The most commonly used browsers for the Web are available for free or for a small charge and can be downloaded from the Internet. Browsers have become one of the most important tools—ranking with e-mail—for computer network users. They have provided tens of millions of people with a gateway to information and communication through the Internet.

Hypermedia

Hypermedia, in computer science, the integration of graphics, sound, video, and animation into documents or files that are linked in an associative system of information storage and retrieval. Hypermedia files contain cross references called hyperlinks that connect to other files with related information, allowing users to easily move, or navigate, from one document to another through these associations.

Hypermedia is structured around the idea of offering a working and learning environment that parallels human thinking—that is, an environment that allows the user to make associations between topics rather than move sequentially from one to the next, as in an alphabetical list. Hypermedia topics are thus linked in a manner that allows the user to jump from subject to related subject in searching for information. For example, a hypermedia presentation on navigation might include links to such topics as astronomy, bird migration, geography, satellites, and radar. If the information is primarily in text form, the document or file is called hypertext. If video, music, animation, or other elements are included, the document is called a hypermedia document.

The World Wide Web (WWW) isa well-known hypermedia environment that users can access through the Internet. Other forms of hypermedia applications include CD-ROM encyclopedias and games. To view hypermedia documents, a user’s computer must have hardware and software that support multimedia. This usually consists of sound and video cards, speakers, and a graphical operating system such as Windows 95 or the Apple operating system. To view hypermedia documents on the Internet, a program called a browser is necessary. Browsers provide varying levels of support for the graphics, sound and video available on the Internet. Examples of browsers are Netscape, Internet Explorer, and Mosaic.

A wide variety of computer programs are used to create different media applications on the WWW. To run these applications, a browser must be supplemented by programs called plug-ins. Plug-ins are tools that allow a computer to display or interpret the various file formats in which multimedia files exist. Plug-ins are usually available for free and can be downloaded to and stored on a computer’s hard drive.

The number of people experiencing hypermedia is growing rapidly as a result of increased exposure to the WWW, better computer and modem performance, and increases in data transmission rates.

Web Site

Web Site, in computer science, file of information located on a server connected to the World Wide Web (WWW). The WWW is a set of protocols and software that allows the global computer network called the Internet to display multimedia documents. Web sites may include text, photographs, illustrations, video, music, or computer programs. They also often include links to other sites in the form of hypertext, highlighted or colored text that the user can click on with their mouse, instructing their computer to jump to the new site.

Every web site has a specific address on the WWW, called a Uniform Resource Locator (URL). These addresses end in extensions that indicate the type of organization sponsoring the web site, for example, .gov for government agencies, .edu for academic institutions, and .com for commercial enterprises. The user’s computer must be connected to the Internet and have a special software program called a browser to retrieve and read information from a web site. Examples of browsers include Navigator from the Netscape Communications Corporation and Explorer from the Microsoft Corporation.

The content presented on a web site usually contains hypertext and icons, pictures that also serve as links to other sites. By clicking on the hypertext or icons with their mouse, users instruct their browser program to connect to the web site specified by the URL contained in the hypertext link. These links are embedded in the web site through the use of Hypertext Markup Language (HTML), a special language that encodes the links with the correct URL.

Web sites generally offer an appearance that resembles the graphical user interfaces (GUI) of Microsoft’s Windows operating system, Apple’s Macintosh operating system, and other graphics based operating systems. They may include scroll bars, menus, buttons, icons, and toolbars, all of which can be activated by a mouse or other input device.

To find a web site, a user can consult an Internet reference guide or directory, or use one of the many freely available search engines, such as WebCrawler from America Online Incorporated. These engines are search and retrieval programs, of varying sophistication, that ask the user to fill out a form before executing a search of the WWW for the requested information. The user can also create a list of the URLs of frequently visited web sites. Such a list helps a user recall a URL and easily access the desired web site. Web sites are easily modified and updated, so the content of many sites changes frequently.

Computer Program

Computer Program, set of instructions that directs a computer to perform some processing function or combination of functions. For the instructions to be carried out, a computer must execute a program, that is, the computer reads the program, and then follows the steps encoded in the program in a precise order until completion. A program can be executed many different times, with each execution yielding a potentially different result depending upon the options and data that the user gives the computer.

Programs fall into two major classes: application programs and operating systems. An application program is one that carries out some function directly for a user, such as word processing or game-playing. An operating system is a program that manages the computer and the various resources and devices connected to it, such as RAM (random access memory), hard drives, monitors, keyboards, printers, and modems, so that they may be used by other programs. Examples of operating systems are DOS, Windows 95, OS/2, and UNIX.

Software designers create new programs by using special applications programs, often called utilityprograms or development programs. A programmer uses another type of program called a text editor to write the new program in a special notation called a programming language. With the text editor, the programmer creates a text file, which is an ordered list of instructions, also called the program source file. The individual instructions that make up the program source file are called source code. At this point, a special applications program translates the source code into machine language, or object code—a format that the operating system will recognize as a proper program and be able to execute.

Three types of applications programs translate from source code to object code: compilers, interpreters, and assemblers. The three operate differently and on different types of programming languages, but they serve the same purpose of translating from a programming language into machine language.

A compiler translates text files written in a high-level programming language—such as Fortran, C, or Pascal—from the source code to the object code all at once. This differs from the approach taken by interpreted languages such as BASIC, APL and LISP, in which a program is translated into object code statement by statement as each instruction is executed. The advantage to interpreted languages is that they can begin executing the program immediately instead of having to wait for all of the source code to be compiled. Changes can also be made to the program fairly quickly without having to wait for it to be compiled again. The disadvantage of interpreted languages is that they are slow to execute, since the entire program must be translated one instruction at a time, each time the program is run. On the other hand, compiled languages are compiled only once and thus can be executed by the computer much more quickly than interpreted languages. For this reason, compiled languages are more common and are almost always used in professional and scientific applications.

Another type of translator is the assembler, which is used for programs or parts of programs written in assembly language. Assembly language is another programming language, but it is much more similar to machine language than other types of high-level languages. In assembly language, a single statement can usually be translated into a single instruction of machine language. Today, assembly language is rarely used to write an entire program, but is instead most often used when the programmer needs to directly control some aspect of the computer’s function.

Programs are often written as a set of smaller pieces, with each piece representing some aspect of the overall application program. After each piece has been compiled separately, a program called a linker combines all of the translated pieces into a single executable program.

Programs seldom work correctly the first time, so a program called a debugger is often used to help find problems called bugs. Debugging programs usually detect an event in the executing program and point the programmer back to the origin of the event in the program code.

Recent programming systems, such as Java, use a combination of approaches to create and execute programs. A compiler takes a Java source program and translates it into an intermediate form. Such intermediate programs are then transferred over the Internet into computers where an interpreter program then executes the intermediate form as an application program.

Most programs are built from just a few kinds of steps that are repeated many times in different contexts and in different combinations throughout the program. The most common step performs some computation, and then proceeds to the next step in the program, in the order specified by the programmer.

Programs often need to repeat a short series of steps many times, for instance in looking through a list of game scores and finding the highest score. Such repetitive sequences of code are called loops.

One of the capabilities that makes computers so useful is their ability to make conditional decisions and perform different instructions based on the values of data being processed. If-then-else statements implement this function by testing some piece of data and then selecting one of two sequences of instructions on the basis of the result. One of the instructions in these alternatives may be a goto statement that directs the computer to select its next instruction from a different part of the program. For example, a program might compare two numbers and branch to a different part of the program depending on the result of the comparison:

If x is greater than y
then
goto instruction #10
else continue

Programs often use a specific sequence of steps more than once. Such a sequence of steps can be grouped together into a subroutine, which can then be called, or accessed, as needed in different parts of the main program. Each time a subroutine is called, the computer remembers where it was in the program when the call was made, so that it can return there upon completion of the subroutine. Preceding each call, a program can specify that different data be used by the subroutine, allowing a very general piece of code to be written once and used in multiple ways.

Most programs use several varieties of subroutines. The most common of these are functions, procedures, library routines, system routines, and device drivers. Functions are short subroutines that compute some value, such as computations of angles, which the computer cannot compute with a single basic instruction. Procedures perform a more complex function, such as sorting a set of names. Library routines are subroutines that are written for use by many different programs. System routines are similar to library routines but are actually found in the operating system. They provide some service for the application programs, such as printing a line of text. Device drivers are system routines that are added to an operating system to allow the computer to communicate with a new device, such as a scanner, modem, or printer. Device drivers often have features that can be executed directly as applications programs. This allows the user to directly control the device, which is useful if, for instance, a color printer needs to be realigned to attain the best printing quality after changing an ink cartridge.

Modern computers usually store programs on some form of magnetic storage media that can be accessed randomly by the computer, such as the hard drive disk permanently located in the computer, or a portable floppy disk. Additional information on such disks, called directories, indicate the names of the various programs on the disk, when they were written to the disk, and where the program begins on the disk media. When a user directs the computer to execute a particular application program, the operating system looks through these directories, locates the program, and reads a copy into RAM. The operating system then directs the CPU (central processing unit) to start executing the instructions at the beginning of the program. Instructions at the beginning of the program prepare the computer to process information by locating free memory locations in RAM to hold working data, retrieving copies of the standard options and defaults the user has indicated from a disk, and drawing initial displays on the monitor.