Encyclopedia of Computer Science and Technology

478 touchscreencode. This would mean that the AT&T copyright wouldnot apply, and the functional equivalent of UNIX could begiven away for free. Stallman and the FSF had already providedkey tools such as the C compiler and the Emacs programeditor. However, they had not yet created the heart ofthe operating system (see kernel). The kernel contains theessential functions needed for the operating system to controlthe computer’s hardware, such as creating and managingfiles on the hard drive.In 1991, Torvalds wrote his own kernel and put ittogether with the various GNU utilities to create what soonbecame known as Linux. Torvalds adopted the open sourcelicense (GPL) pioneered by Stallman and the FSF, allowingLinux to be distributed freely. The software soon spreadthrough ftp sites on the Internet, where hundreds of enthusiasticusers (mainly at universities) helped to improveLinux, adding features and writing drivers to enable it towork with more kinds of hardware.By the mid-1990s, the free and reliable Linux hadbecome the operating system of choice for many Web sitedevelopers. Torvalds, who still worked at the University ofHelsinki as a researcher, faced an ever-increasing burdenof coordinating Linux development and deciding when torelease successive versions. As companies sprang up to marketsoftware for Linux, they offered Torvalds very attractivesalaries, but he did not want to be locked into one particularLinux package (distribution).Instead, in 1997 Torvalds moved to California’s SiliconValley, where he became a key software engineer at Transmeta,a company that makes Crusoe, a processor designedfor mobile computing.In 2003 Torvalds left Transmeta. In 2004 he movedto Portland, Oregon, where the Linux Foundation, a nonprofitconsortium dedicated to promoting the growth ofLinux, supports his work. There he concentrates on guidingthe continuing development of the Linux core, or kernel.Although he strongly supports open-source software, Torvaldshas been criticized by some advocates for his pragmaticapproach of using proprietary software when it seemsto be more suitable to a given task.Further ReadingDibona, Chris. Open Sources: Voices from the Open Source Revolution.Sebastapol, Calif.: O’Reilly, 2001.“FM Interview with Linus Torvalds: What Motivates Free SoftwareDevelopers?” First Monday, vol. 3 (1998). Available online.URL: http://www.firstmonday.org/issues/issue3_3/torvalds/.Accessed August 23, 2007.Linux Foundation. Available online. URL: http://www.linuxfoundation.org/en/Main_Page.Accessed August 23, 2007.Richardson, Marjorie. “Interview: Linus Torvalds.” Linux Journal,November 1, 1999. Available online. URL: http://www.linuxjournal.com/article/3655.Accessed August 23, 2007.Torvalds, Linus, and David Diamond. Just for Fun: The Story of anAccidental Revolutionary. New York: HarperCollins, 2001.touchscreenAs the name implies, a touchscreen is a screen display thatcan respond to various areas being touched or pressed.Invented in 1971, the first form of touchscreens to becomepart of daily life were found on automatic teller machines(ATMs) and point-of-sale credit card processors.Touchscreens can detect the pressure of a finger or stylusin several ways: A “resistive” touchscreen uses two layersof electrically conductive metallic material separated bya space. When an area is touched, the two layers are electricallyconnected, and the change in electrical current is registeredand converted to a code that identifies the locationtouched. Surface acoustic wave (SAW) touchscreens usean ultrasonic wave that is interrupted by a touch; capacitivetouchscreens respond to the change in electron storage(capacitance) caused by contact with a human body. Variousother acoustic, mechanical (strain-based), or opticalsystems can also be used, with the latter being particularlypopular.Touchscreens can have drawbacks ranging from problemswith long fingernails to screen “keys” placed too closetogether for normal fingers. Responsiveness is also considerablyslower than with a keyboard. Depending on the technologyused, dirt or grease can also become a problem.Other ApplicationsIn addition to dedicated uses such as banking and retailing,touchscreens are a common form of input for mobilephones, PDAs, and similar devices (see pda and smartphone)where a “virtual” on-screen keyboard is often usedfor entering text. Particularly versatile systems such asApple’s iPhone combine proximity sensors with touchscreentechnology in order to be able to recognize gestures suchas pinching and flicking. Another example is Microsoft’s“Surface” interface. (For related technologies, see graphicstablet and tablet pc.)Further ReadingBaig, Edward C. iPhone for Dummies. New York: Wiley, 2007.Buxton, Bill. “Multi-Touch Systems That I Have Known andLoved.” Available online. URL: http://www.billbuxton.com/multitouchOverview.html. Accessed November 29, 2007.Microsoft Surface. Available online. URL: http://www.microsoft.com/surface/. Accessed November 29, 2007.Plaisant, Catherine. “High-Precision Touchscreens: MuseumKiosks, Home Automation and Touchscreen Keyboards.”University of Maryland Human-Computer Interaction Lab,January 31, 1999. Available online. URL: http://www.cs.umd.edu/hcil/touchscreens/. Accessed November 29, 2007.Woyke, Elizabeth. “Reach Out and Touch—A Phone Screen.”Forbes.com, November 28, 2007. Available online. URL:h t t p: // w w w. f o r b e s . c o m / t e c h n o l o g y / 2 0 0 7/ 11 / 2 8 /touch-screen s-phone s -tech-hol id ay tech07- c x _ e w_1128touch.html. Accessed November 29, 2007.transaction processingMany computer applications involve the arrival of a set ofdata that must be processed in a specified way. For example,a bank’s ATM system receives a customer’s request todeposit money together with identification of the accountand the amount to be deposited. The system must accept thedeposit, update the account balance, and return a receipt tothe customer. This is an example of real-time transactionprocessing.