An Operating Systems Vade Mecum

346 GlossaryRotational latency. The delay in accessing a disk block while the disk rotates into thecorrect position.Run-length encoding. A data compression technique that replaces a run of similar characterswith a run-length indication.Safe. A resource allocation state is safe if the processes form a sequence, each elementof which can acquire its resources and terminate if all the previous ones do.Safe sequence. A sequence of processes used to show that a resource allocation state issafe.Salvage. To examine the data on a garbaged disk and recover the original structure.Saturation. The ratio of arrival rate to service rate, usually denoted by ρ. Also calledTraffic intensity.Scatter-gather. See Data chaining.Scheduling. (1) Managing the time resource by deciding when each process should beallowed to run. (2) Orchestrating the starting times for any set of operations, such as diskreads or transaction operations.Secondary store. Long-term storage used for files, typically disk.Secrecy. Preventing unauthorized inspection of data. (See Security.)Sector. A region of a track on a disk pack that holds one block of information.Security. Preventing unwanted accesses to objects. (See Protection, Secrecy, andAuthentication.)Seek. (1) Motion of the disk read/write head from one track to another. (2) File operationthat moves to a given point in the file.Seek latency. The delay in accessing a disk block necessitated by a disk seek operation.Segment. A region of virtual space whose addresses all specify that segment and whichis allocated as a single unit.Segment fault. A trap generated by the hardware when a segment absent from mainstore is accessed by a program.Segment table. The name of the translation table when segmentation is used.Semaphore. A mutual-exclusion and synchronization device that contains a lock, acount of concurrent activities, and a queue of waiting activities.Sequencer. A counter used to assign an arbitrary order to unordered events.Sequential access. A file access method in which the read/write mark is incrementedafter each data transfer by the amount of data transferred.Serializability. A property of the simultaneous execution of a number of transactionsthat has those transactions produce the same result they would have had they been runsequentially in some order.Serialization. Running one process or transaction at a time, or only allowing one at atime to acquire resources.