An Operating Systems Vade Mecum

340 GlossaryLocal. (1) A local page replacement method is one that selects pages to swap out fromthe virtual space of the process whose page needs the space. (See Global and Pool.) (2)A local file name is the name the file has in its directory.Local-area network. A set of computers at a single installation connected by communicationdevices.Locality. The phenomenon that programs actively use only a limited set of pages duringany particular time period of execution. This set of pages is called the locality of the programduring that time.Lock. (1) A security device, used with keys. (See Key.) (2) A note on an object (like afile or a page) indicating that it is in use by some transaction for reading or writing. (3) Adevice for mutual exclusion that uses an atomic test-and-set or test-and-add instruction.Log. A record of changes made to files during a transaction so that the changes can bereversed or reapplied.Logical names. Names expanded by the file manager according to tables.Logoff. The end of an interactive session.Logon. The start of an interactive session.Long-haul network. A network of computers that spans a large geographic region.Long-term scheduling. A policy that decides when to let new processes enter the readylist.Main store. The fast random-access memory of a computer, typically between 64K and16M bytes. Also called primary memory or core.Mapped access. A file access method in which the entire file is mapped into the virtualspace of the process.Mechanism. A technique by which an activity is performed. (See Policy.)Medium-term scheduling. A policy that removes processes from the ready list in orderto reduce contention for resources, particularly space and time.Memoryless. The exponential distribution is called memoryless because the expectedtime to the next event does not depend on how long it has been since the previous event.Menu. A list of commands that are legal in the current situation, typically presented on abit-mapped display.Migration. Moving a process from one machine to another to improve throughput andto balance load.Mirrored tables. The kernel duplicates information that is also stored in a hardwaretable. The kernel table represents the truth, possibly with fields that the hardware tabledoes not possess. This technique is used especially for address translation tables. Alsocalled shadow tables.Miss. When a cache does not have the information needed to complete an operation.Missed time. The elapsed time between the instant a process arrives in the ready queueand the time it leaves that queue minus the required time for the process.