An Operating Systems Vade Mecum

332 GlossaryCiphertext. The result of encrypting data.Circular buffer pool. A set of data buffers used for transput. As soon as one is finished,the next one in circular order may be used.Claim. In the banker’s algorithm of resource allocation, a limit specified by each processthat restricts the number of resources it can request.Cleartext. Data that have not been encrypted or that have been restored from encryptedform.Clock. A device that interrupts at the end of an interval. Also called a timer.Close. To finalize data structures or hardware when a data object or device is no longerto be used.Clustering. (1) In paging, bringing in a set of pages adjacent in virtual space when anyone of them needs to be swapped in. (2) In physical file layout, placing the blocks thatrepresent a file on the same cylinder or within a small contiguous group of cylinders.Cold start. (1) In simulations, beginning measurements from a bare initial state. Forexample, cold-start page-replacement analysis counts page faults required to start up aprocess. (2) In initializing operating systems, a cold start means that file storage is notformatted or may be corrupted and must be checked.Command. (1) An instruction in a channel program, typically specifying the transput ofa single block of information between main store and some particular device. (2) Aninteractive request given by a user to a program, particularly to the command interpreterof the operating system.Command chaining. Allowing a channel program to contain more than one command.Command interpreter. The software (part of either the kernel or a utility process) thatinterprets requests from the user. Also called monitor or shell.Command language. A language the user employs to describe what programs are to berun and what their environment should be. Also called job-control language.Command script. A command-language program given as a single interactive commandto a command interpreter.Commit. The point in a transaction after which the transaction is guaranteed to terminatewithout failing.Communication-kernel approach. A design of operating system in which a small kernelprovides mechanisms for processes to communicate with each other, but most policydecisions are outside the kernel.Communication line. A device used for transferring information between computers orbetween a computer and a user.Component. A file name is divided into components, each of which may follow a conventionthat indicates what the file contains.Composite address-translation table. An address-translation table that composes themappings specified by two or more address-translation tables at adjacent levels of avirtual-machine hierarchy.