Encyclopedia of Computer Science and Technology

scheduling and prioritization 417Besides considerations of resolution and color depth, thequality of a scanned image depends on the quality of thescanner’s optics as well as on how the page or other objectreflects light. As anyone who has browsed eBay listingsknows, the quality of scans can vary considerably. Mostscanners come with software that allows for the scanner tobe controlled and adjusted from the PC, and image-editingsoftware can be used to further adjust the scanned image.Even if the input is a sheet of text, the scanner’s outputis simply a graphical image. Special software must be usedto interpret scanned images of text and identify which charactersand other features are present (see optical characterrecognition). Since such software is not 100 percentaccurate, human proofreaders may have to inspect theresulting documents.Like printers, scanners have become quite inexpensivein recent years. Quite serviceable units are available foraround $100 or so. (Popular multifunction devices ofteninclude scanner, copier, fax, and printer capabilities. Ascanner can be used as a copier or fax by sending its outputto the appropriate mechanism.)Many home users now use scanners to digitize imagesfor use in personal Web pages, online auctions, and othervenues. Since sheet-fed scanners can only process individualsheets (not books, magazines, or objects) they are now lesspopular. Handheld scanners are somewhat tedious to useand require a steady hand, so they are generally used only inspecial circumstances where a flatbed scanner is not available.For capturing images of three-dimensional objects it isoften easier to use a digital camera than a scanner.Specialized scanners are also available. For example,although many flatbed scanners have a holder for scanningfilm negatives, a dedicated film scanner (costing perhaps$500) is a better choice if one wants to scan and possiblyretouch or restore photographs. There are also high-enddrum-type scanners that can scan at resolutions of 10,000dpi or more.Further ReadingBusch, David D. Mastering Digital Scanning with Slides, Film, andTransparencies. Boston: Muska & Lipman, 2004.Chambers, Mark L. Scanners for Dummies. Hoboken, N.J.: Wiley,2004.PC Tech. Guide: Scanners. Available online. URL: http://www.pctechguide.com/55Scanners.htm. Accessed August 20, 2007.scheduling and prioritizationOften in computing, a fixed resource must be parceled outamong a number of competing users. The most obviousexample is the operating system’s scheduling the runningof programs. Most computers have a single central processor(CPU) to execute programs. However, today virtuallyall operating systems (except for certain dedicated applications—seeembedded system) are expected to have manyprograms available simultaneously. For example, a MicrosoftWindows user might have a word processor, spreadsheet,e-mail program, and Web browser all open at thesame time. Not only might all of these programs be carryingout tasks or waiting for the user’s input, but dozens of“hidden” system programs are also running in the background,providing services such as network support, virusprotection, and printing services (see multitasking).In this environment each executing program (or “process”)will be in one of three possible states. It may beactively executing (that is, its code is being run by theCPU). It may be ready to execute—that is, “wanting” toperform some activity but needing access to the CPU. Or,the program may be “blocked”—that is, not executing andunable to execute until some external condition is met.Blockage is usually caused by an input/output (I/O) operation.An example would be a program that’s waiting for datato finish loading from a file.In this sort of single-processor multiple-program system,the simplest arrangement is to have the operatingsystem dole out fixed amounts of execution time to eachprogram. Each program that indicates that it’s ready to rungets placed in a list (see queue) and given its turn. Whenthe amount of time fixed for a turn has passed, the operatingsystem saves the program’s “state” in the processor—thecontents of the registers, address pointed to by the pointerto the next instruction to be executed, and so on. Thisstored information can be considered to be a “virtual processor.”When the program’s turn comes around again, theprocessor is reloaded with the contents of the virtual processorand execution continues where it had left off.Use of PriorityThe above scheme assumes that all programs should haveequal priority. In other words, that the timely completionof one program is not more important than that of another,or that no program should be “bumped up” in the queue forsome reason. In reality, however, most operating systems dogive some programs preference over others.For example, suppose the word processor has justreceived a user’s mouse click on a menu. The next programin the queue for execution, however, is an antivirus programthat’s checking all the files on the hard drive for possibleviruses. The latter program is important, but since theuser is not waiting for it to finish, a delay in its executionwon’t cause a significant problem. The user, on the otherhand, is expecting the menu just clicked on to open almostinstantly, and will become irritated with even a short delay.Therefore, it makes sense for the operating system to givea program that’s responding to immediate user activity ahigher priority than a program that’s carrying out tasksthat don’t require user intervention.There are other times when a program must (or should)be given a higher priority. A program may be required tocomplete a task within a guaranteed time frame (for example,to dispatch emergency services personnel). The operatingsystem must therefore provide a way that the programcan request priority execution.In general, an operating system that supports real-timeapplications or that requires great attention to efficiency inusing valuable devices may need a much more sophisticatedscheduling algorithm that factors in the availability of keydevices or services and adjusts program priorities in orderto minimize bottlenecks and guarantee that the system’s