An Operating Systems Vade Mecum

334 GlossaryDatabase. A collection of structured, interrelated data, usually managed by utility programsthat perform queries and updates.Data chaining. A channel’s ability to gather data from different places in main store orto scatter data to different places in main store.Debugging. Discovering the sources of errors in a program and fixing them.Decryption. A transformation used to convert ciphertext back to the original cleartext.Deadlock. A situation in which each process in a cycle is waiting for resources held bythe next process in the cycle. Also called deadly embrace. (See Livelock and Starvation.)Deadlock detection. Noticing deadlock situations when they arise instead of applyingdeadlock prevention.Deadlock-free. A resource allocation state is deadlock-free if the processes form asequence, each element of which might finish if all the previous ones do.Deadlock-free sequence. A sequence of processes used to show that a resource allocationstate is deadlock-free.Deadlock prevention. Avoiding a deadlock situation by using an appropriate resourceallocation policy.Deadlock recovery. Terminating one or more members of a deadlock for the good ofthe community.Default setting. An assumption that may be overridden if necessary.Delay. The amount of time between the moment a writer sends information on a communicationline and the moment a reader gets that information.Delete. To remove a file from secondary store. Also called remove or destroy. (SeePartial deletion.)Delimiting. A property of a communication link that data written to the link areseparated into discrete messages.Demand paging. A page is brought in when a page fault requires it.Departure. When a process disappears from the view of the short-term scheduler, eitherbecause it has terminated, because it must wait for transput or some other resource, orbecause the medium-term scheduler is discriminating against it. (Opposite of Arrival.)Descriptor. A small number given by the kernel to a process to allow it to refer to anopened resource.Device. A piece of hardware connected to the computer. Devices are principally usedfor transput.Device descriptor. A small integer returned by the kernel when a device is opened by aprocess. The process then uses the descriptor to refer to the device in other service calls.Device driver. A device-specific program in the kernel to control a particular type ofdevice.