An Operating Systems Vade Mecum

336 GlossaryEncryption. A data transformation intended to provide secrecy (and possibly authentication).Entry. In the communication-kernel approach, the recipient of a message, much like aprocedure.Error-correcting code. A data encoding method that is capable of detecting andcorrecting some recording or transmission errors.Escape convention. A character that means that the next character is to be treated asdata, not as a control character.Essential utility. A part of the operating system that every user must use but which doesnot need special privileges. (See Optional utility and Kernel.)Event count. A counter that is used for synchronization without mutual exclusion. Itsoperations are Read, Advance, and Await.Execution. When a process is running, we say it is in the execution phase, as distinctfrom the loading phase.Explicit. Explicit process information is provided by the user before the process is firstscheduled.Exponential average. An averaging technique that updates a statistic by this formula:NewValue := w . OldValue + (1−w ) . NewMeasurementExtent. A group of contiguous disk blocks that constitute part of a file.External waste. See Waste.Extrinsic. Extrinsic properties of a process are characteristics associated with the userwho owns the process, such as how urgent the process is.Failure atomicity. A property of transactions that either all or none of the operations inthe transaction take effect. (See Synchronization atomicity.)Fault. A trap generated by the hardware when address translation cannot continuebecause of an absent page or segment. These situations are known as page faults andsegment faults, respectively.Fault-rate graph. A graph showing the behavior of a page-replacement policy for agiven page-reference string across a range of main-store sizes.File. A named collection of data. Files may be written by a process and later read by thesame or another process. Files are usually represented by a logically connected set ofbytes on a disk or magnetic tape.File descriptor. A structure on a disk that stores data associated with a file, such as itsaccess rights, physical arrangement, and usage statistics.File descriptor area. A region on disk dedicated to file descriptors.File manager. The portion of the operating system that deals with file structures.File server. A computer dedicated to providing files to a collection of computers.