An Operating Systems Vade Mecum

Glossary 333Compute-bound. A process is compute bound if its running time depends mostly on thespeed of the central processing unit, not on the speed of the transput devices. (Oppositeof Transput-bound.)Concurrency control. Synchronization and mutual exclusion to assure that activitiesthat share data have predictable behavior.Conditional critical region. A programming language construct used to enforce synchronizationand mutual exclusion. (See Synchronization, 2, and Mutual exclusion.)Conflict. Two regions conflict if execution of one must exclude execution of the other.Conservative. A resource allocation policy is conservative if it avoids allocatingresources unless stringent requirements are met. These requirements are intended toprevent deadlock and starvation. (Opposite of Liberal.)Context block. The data used by the operating system to describe each process. Thecontext block typically includes information about current use of main and backing store,the relationship between this process and others, the status of the process, and accountinginformation.Context switch. The events involved in switching the hardware between execution of aprocess and execution of the kernel or in switching in the other direction. Switchingfrom the kernel to the process is also called dispatching. (See Process switch.)Controller. Hardware that connects a device either to the cpu or to a channel. The controllerkeeps track of the status of its devices and governs their actions.Convoy phenomenon. A situation caused by long busy waiting in which many activitiesend up waiting for the same region and then proceed as a group through subsequentregions.Copy on write. A form of lazy evaluation in which a segment is shared by twoprocesses until either one tries to modify its data, at which point a separate copy is made.Crash. (1) A calamity in which a disk head touches the disk surface and destroys it. (2)An unplanned halt in the operating system.Critical region. A programming language construct used to enforce mutual exclusion.(See Mutual exclusion.)Crowd monitor. An extension to a monitor that distinguishes guard procedures fromordinary procedures within the monitor and imposes run-time restrictions on when ordinaryprocedures may be invoked. (See Monitor.)Cursor. A visual indicator of the current location of interest on a display.Customization. Modifying the interface between the operating system and the useraccording to the personal desires of the user. Line-editing characters, command syntax,and default settings for programs are useful to customize.Cylinder. A collection of tracks on a disk pack that can be simultaneously accessed bythe read/write heads on the different disk surfaces.Data abstraction. A programming-language tool that separates operations on data fromtheir implementation, thus hiding details of lower-level structures and introducing newfacilities. (See Abstraction.)