An Operating Systems Vade Mecum

Glossary 335Device interface. The interface between the kernel and devices, consisting of controlrequests and interrupts.Device register. A means of communication between devices and the cpu used to carrydata, instructions from the cpu to the device, and the status of the device.Difference encoding. A data compression technique that converts values into differencesbetween one value and the next.Direct access. A file access method in which the read/write mark can be positionedexplicitly. Also called random access.Direct memory access (DMA). A hardware technique that allows a device to transferlarge quantities of data to or from main store.Directory. A list of pairs that can be searched by the first element, called the key, toderive the second.Dirty field. A field in the translation table set by the hardware when a page or segment iswritten into.Discrete. A resource is discrete if it can be granted entirely or not at all but not partially.Disk. A device used for secondary and backing store.Disk descriptor. Information recorded in a well-known place on a disk that describesthe layout of data on the disk.Disk-head scheduling. Ordering the outstanding disk-access requests to minimize seeklatency.Disk pack. The part of the disk on which information is stored.Distributed computation. Co-ordinated work on several computers simultaneously.(See Multicomputer and Multiprocessor.)Driver. See Device driver.Dump. (1) A file dump is a backup copy of one or more files to protect against accidentalloss. (2) A main-store dump is a file copy of the address space of a process that hasencountered a run-time error.Dynamic. Something that changes over time. For example, the number of processescompeting for service is a dynamic property of an operating system.Eager evaluation. Doing work that may prove useful to save time later if the work isrequired. (Example: Read-ahead. Opposite of Lazy evaluation.)Echo. Repeating information that has been heard back to the sender is called echoing.In particular, the terminal driver of an operating system may echo all characters typedinto keyboards back out to the associated display.Editing. Entry or modification of textual information, usually interactive. Files areedited with the assistance of programs called text editors. Command lines and other linesinput to programs may be edited with the direct assistance of a line editor, which is partof the terminal driver in the operating system.