An Operating Systems Vade Mecum

350 GlossaryTrap-door encryption. An encryption algorithm that has no associated decryption algorithm,or whose decryption algorithm is very difficult to discover.Used field. A field in the translation table set by the hardware when a page or segment isaccessed.User. The human being who interacts with a computer and who has personal interestthat the computer interpret instructions properly.User identifier. A number associated with all the processes and files created for a particularuser. This number is used for accounting and protection purposes. (See Groupidentifier.)User interface. The interface between the user and the operating system, especially thecommand language.User Principle. Operations that are performed frequently should be especially easy toinvoke.Utilization. The fraction of time the computer spends performing computation, asopposed to transput, resource management, and other kernel work.Version. A file version holds the contents of a file at one point when it was written.Later modifications to the file create new versions.Virtual. The result of an abstraction, typically as seen by a process. (Opposite of Physical.)Virtual device. A device that is provided not by hardware but by the kernel, often withphysical devices used for actual data storage.Virtual interrupt. The kernel interrupts a process by modifying its program counter tothe address of its virtual-interrupt handler.Virtualizing kernel. The kernel of a virtual-machine operating system. Also called avirtual-machine monitor.Virtual machine. A process interface that looks just like the bare hardware.Virtual space. The address space of the computer as seen from a process.Virtual store. The memory associated with the virtual space.Virtual time. The amount of time used by a process, advanced only when the process isrunning.Wait. A service call used to block the calling process until a previously requested asynchronoustransput operation has completed.Wait list. A list of processes that are not currently runnable because of a schedulingdecision or because they require resources that have not yet been granted.Warm start. (1) In simulations, waiting until measurements have reached a steady state.For example, warm start page replacement analysis does not start counting page faultsrequired to start up a process. (2) In initializing operating systems, assuming that filestorage is uncorrupted (or at least present).