Encyclopedia of Computer Science and Technology

354 operators and expressionsOperating systems, particularly those designed for multipleusers, must also manage and secure user accounts.The administrator (or sometimes, ultimately, the “superuser” or “root”) can assign users varying levels of access toprograms and files. The owners of files can in turn specifywhether and how the files can be read or changed by otherusers (see data security).In today’s highly networked world most operating systemsprovide basic support for networking protocols suchas TCP/IP. Applications can use this facility to establishnetwork connections and transfer data over the local orremote network (see network).The operating system’s functions are made available toprogrammers in the form of program libraries or an applicationprogramming interface (API). (See library, programand application programming interface.)The user can also interact directly with the operatingsystem. This is done through a program called a shell thataccepts and responds to user commands. Operating systemssuch as MS-DOS and early versions of UNIX acceptedonly typed-in text commands. Systems such as MicrosoftWindows and UNIX (through facilities such as XWindows)allow the user to interact with the operating system throughicons, menus, and mouse movements. Application programmerscan also provide these interface facilities through theAPI. This means that programs from different developerscan have a similar “look and feel,” easing the learning curvefor users.Issues and TrendsAs the tasks demanded of an operating system have becomemore complex, designers have debated the best overall formof architecture to use. One popular approach, typified byUNIX, is to use a relatively small kernel for the core functions.A community of programmers can then write theutilities needed to manage the system, performing taskssuch as listing file directories, editing text, or sending e-mail. New releases of the operating system then incorporatethe most useful of these utilities. The user also has a varietyof shells (and thus interfaces) available.The kernel approach makes it relatively easy to port theoperating system to a different computer platform and thendevelop versions of the utilities. (Kernels were also a necessitywhen system memory was limited and precious, butthis consideration is much less important today.)Designers of modern operating systems face a numberof continuing challenges:• security, in a world where nearly all computers arenetworked, often continuously (see computer crimeand security and firewall)• the tradeoff between powerful, attractive functionssuch as scripting and the security vulnerabilities theytend to present• the need to provide support for new applications suchas streaming audio and video (see streaming)• ease of use in installing new devices (see devicedriver and plug and play)• The continuing development of new user-interfaceconcepts, including alternative interfaces for the disabledand for special applications (see user interfaceand disabled persons and computing)• the growing use of multiprocessing and multiprogramming,requiring coordination of processors sharingmemory and communicating with one another (seemultiprocessing and concurrent programming)• distributed systems where server programs, clientprograms, and data objects can be allocated amongmany networked computers, and allocations continuallyadjusted or balanced to reflect demand on thesystem (see distributed computing)• the spread of portable, mobile, and handheld computersand computers embedded in devices such asengine control systems (see laptop computer, PDA,and embedded system). (Sometimes the choice isbetween devising a scaled-down version of an existingoperating system and designing a new OS that isoptimized for devices that may have limited memoryand storage capacity.)Further ReadingBach, Maurice J. The Design of the UNIX Operating System. EnglewoodCliffs, N.J.: Prentice Hall, 1986.Ritchie, Dennis M. “The Evolution of the UNIX Time-Sharing System.”Lecture Notes in Computer Science #79: Language Designand Programming Methodology, New York: Springer-Verlag,1980. Available online. URL: http://cm.bell-labs.com/cm/cs/who/dmr/hist.html. Accessed August 14, 2007.Silberschatz, Abraham, Peter Baer Galvin, and Greg Gagne. OperatingSystem Concepts. 7th ed. New York: Wiley, 2004.operators and expressionsAll programming languages provide operators to specifyarithmetic functions. Some of them, such as addition +,subtraction -, multiplication ×, and division ÷, are familiarfrom elementary school arithmetic (although the asteriskrather than the traditional x is used for multiplication inprogram code, to avoid confusion with the letter x). Additionaloperators found in languages such as C, C++, andJava include % (modulus, or remainder after division), ++(adds one and stores the result back into the operand), and-- (decrement; subtracts one and stores the result back intothe operand).Operands are data items such as variables, constants, orliterals (actual numbers) that are operated on by the operator.An operator is called unary if it takes just one operand(the increment operator ++ is an example). An operator thattakes two operands is considered to be binary, and this istrue of most arithmetic operations such as addition, multiplication,subtraction, and division.A combination of operands and operators constitutes anarithmetic expression that evaluates to a particular valuewhen the program runs. Thus in the C statement:Total = SubTotal + SubTotal Tax × Tax_Rate;