Encyclopedia of Computer Science and Technology

Jobs, Steven Paul 257of Web services (such as databases and search engines)and create custom pages in response to user actions (seealso Ajax). JavaScript can also be embedded in applicationsother than Web browsers: for example, the Adobe Acrobatand Reader and even operating-system scripting (such asMicrosoft’s JScript and JScript.NET). Although attention inrecent years seems to have shifted more to languages suchas PHP, JavaScript remains a widely used and powerful Webdesign tool.Further ReadingFlanagan, David. JavaScript: The Definitive Guide. 5th ed. Sebastapol,Calif.: O’Reilly, 2006.Heilmann, Christian. Beginning JavaScript with DOM Scripting andAjax. Berkeley, Calif.: APress, 2005.JavaScript at Webdeveloper.com. Available online. URL: http://www.webdeveloper.com/javascript/. Accessed September 24,2007.JavaScript.com: The Definitive JavaScript Resource. Availableonline. URL: http://www.javascript.com/. Accessed September24, 2007.Wilton, Paul, and Jeremy McPeak. Beginning JavaScript. 3rd ed.Indianapolis: Wiley, 2007.job control languageIn the early days of computing, data processing generallyhad to be done in batches. By modern standards the memorycapacity of the computer was very limited (see mainframe).Typically, programs had to be loaded one at a time frompunch cards or tape. The data to be processed by each programalso had to be made available by being mounted on atape drive or inserted as a stack of cards into the card reader(see punched cards and paper tape). After the programran, its output would consist of more data cards or tape,which might in turn be used as input for the next program.For example, a series of programs might be used toread employee time cards and calculate the payments dueafter various items of withholding. That data might in turnbe input into a program to print the payroll checks andanother program to print a summary report.In order for all this to work, the computer’s operatingsystem must be told which files (on which devices) are tobe used by the program, the memory partition in whichthe program is to be run, the device to which output willbe written or saved, and so on. This is done by giving thecomputer instructions in job control language (JCL). (Inthe punch card days, the JCL cards were put at the top ofthe deck before the cards with the instructions for the programitself.)For a simple example, we will use some elements of IBMMVS JCL. In this version of job control language the generalform for all statements is//name operation operands commentwhere name is a label that can be used to reference thestatement from elsewhere, operation indicates one of a setof defined JCL language commands, operand is a series ofvalues to be passed to the system, and comment is optionalexplanatory text.The three basic types of statement found in most jobcontrol languages are JOB, EXEC, and DD. The JOB statementidentifies the job and the user running it and sets upsome parameters to specify the handling of the job.//JOB,CLASSPROJ1,GROUP=J999996,USER=P999995,//PASSWORD=?This statement passes information to the system thatidentifies the job name, group as assigned by the facility,and user ID. The PASSWORD parameter is given a questionmark to indicate that it will be prompted for at the terminal.Other parameters can be used to specify such matters as theamount of computer time to be allocated to the job and theway in which any error messages will be displayed.The EXEC statement identifies the program to be run.Some systems can also have a library of stored JCL proceduresthat can also be specified in the EXEC statement.This means that frequently run jobs can be run withouthaving to specify all the details each time. An exampleEXEC statement is://Datasort EXEC BINSORT,BUFFER=256KHere the statement is labeled Datasort so it can be referencedfrom another part of the program. The procedure tobe executed is named BINSORT, and it is passed a parametercalled BUFFER with a value of 256K (presumably this is theamount of memory to be used to hold data to be sorted).One or more DD (Data Definition) statements are usedto specify sets (sources) of data to be used by the program.This includes a specification of the type (such as disk ortape) and format of the data. It also includes instructionsspecifying what is to happen to the data set. For example,the data set might be old (existing) or newly created by theprogram. It may also be temporary (possibly to be passedon to the next program) or permanent (“cataloged”).Since interactive, multitasking operating systems suchas Windows and UNIX are now the norm in most computing,JCL is used less frequently today. However, it is stillneeded in large computer installations running operatingsystems such as IBM MVS (see mainframe) and for somebatch processing of scientific or statistical programs (suchas in FORTRAN or SAS).Further ReadingBrown, Gary Deward. System 390 Job Control Language. 4th ed.New York: John Wiley, 1998.Malaga, Ernie, and Ted Holt. Complete CL: The Definitive ControlLanguage Programming Guide. 3rd ed. Carlsbad, Calif.: MidrangeComputing, 1999.Jobs, Steven Paul(1955– )AmericanEntrepreneurSteve Jobs was cofounder of Apple Computer and shapedthe development and marketing of its distinctive Macintoshpersonal computer (see Apple Corporation). Jobs showed