Encyclopedia of Computer Science and Technology

468 TclHaylor, Phil. Computer Storage: A Manager’s Guide. Victoria, B.C.,Canada: Trafford, 2005.Scapicchio, Mark. “How Tape Drives Work: Tape Backup Still aGood Option.” Smart Computing, October 2002, pp. 69–72.Available online. URL: http://www.smartcomputing.com/editorial/article.asp?article=articles/archive/r0608/13 r08/13r08.asp&guid=. Accessed August 22, 2007.White, Ron, and Timothy Edward Downs. How Computers Work.8th ed. Indianapolis: Que, 2005.TclDeveloped by John Ousterhout in 1988, Tcl (Tool commandlanguage) is used for scripting, prototyping, testing interfaces,and embedding in applications (see scripting languages).Tcl has an unusually simple and consistent syntax. Ascript is simply a series of commands (either built in or userdefined) and their arguments (parameters). A commanditself can be an argument to another command, creating theequivalent of a function call in other languages.For example, setting the value of a variable uses the setcommand:set total 0The value of the variable can now be referenced as$total.Control structures are simply commands that run othercommands. A while loop, for example, consists of a commandor expression that performs a comparison, followedby a series of commands to be executed each time it returns“true”:while { MoreInFile } {GetDataDisplayData}In practice, many of the commands used are utilitiesfrom the operating system, usually UNIX or Linux. Tclalso includes a number of useful data structures such asassociative arrays, which consist of pairs of data itemssuch as:set abbr (California)Extensions and ApplicationsTcl includes a number of extensions that, for example,provide access to popular database formats such as MySQLand can interface with other programming languages suchas C++ and Java. The most widely used extension is Tk,which provides a library for creating user interfaces for avariety of operating systems and languages such as Perl,Python, and Ruby.Tcl has been described as a “glue” to connect existingapplications. It is relatively easy to write and test a scriptinteractively (often at the command line), and then insert itinto the code of an application. When the application runs,the Tcl interpreter runs the script, whose output can thenbe used by the main application (see interpreter).CAFurther ReadingFoster-Johnson, Eric. Graphical Applications with Tcl & Tk. 2nd ed.New York: Hungry Minds, 1997.Wall, Kurt. Tcl and Tk Programming for the Absolute Beginner. Boston:Course Technology, 2007.Welch, Brent B., and Ken Jones. Practical Programming in Tcl andTk. 4th ed. Upper Saddle River, N.J.: Prentice Hall, 2003.TCP/IPContrary to popular perception, the Internet is not e-mail,chat rooms, or even the World Wide Web. It is a system bywhich computers connected to various kinds of networksand with different kinds of hardware can exchange dataaccording to agreed rules, or protocols. All the applicationsmentioned (and many others) then use this infrastructureto communicate.TCP/IP (Transmission Control Protocol/Internet Protocol)provides the rules for transmitting data on the Internet.It consists of two parts. The IP (Internet Protocol) routespackets of data. The header information also includes:• The total length of the packet. In theory packets canbe as large as 65 kbytes; in practice they are limited toa smaller maximum.• An identification number that can be used if a packetis broken into smaller pieces for efficiency in transmission.This allows the packet to be reassembled atthe destination.• A “time to live” value that specifies how many hops(movements from one intermediate host to another)the packet will be allowed to take. This is reduced by1 for each hop. If it reaches 0, the packet is assumedto have gotten “lost” or stale, and is discarded.• A protocol number (the protocol is usually TCP, seebelow).• A checksum for checking the integrity of the headeritself (not the data in the packet).• The source and destination addresses.The source and destination are given as IP addresses,which are 32 bits long and typically written as four sets ofup to three numbers each—for example, 208.162.106.17A Network of NetworksAs the name implies, the Internet is a network that connectsmany local networks. The IP address includes an IDfor each network (called a subnet) and each host computeron the network. The arrangement and meaning of thesefields differs somewhat among five classes of IP addresses.The first three classes are designed for different sizes ofnetworks, and the latter two are used for special purposessuch as “multicasting” where the same data packet is sentto multiple hosts.Many Internet users (at home as well as in offices) arepart of a local network (see local area network). Typically,all users on the local network share a single Internetconnection, such as a DSL or cable line. This sharing