Direct-Access Storage Device


(DASD) IBM mainframe terminology for a disk drive, in contrast with a tape drive which is a sequential access device.

Last updated: 1995-03-01

Direct Client to Client Protocol


(DCC) An IRC protocol created to allow users to chat privately and to send and receive files directly instead of having to go thorugh the IRC servers. DCC protects users from being monitored by IRC Server operators that have enabled conversation logging. It also allows much more efficient use of available bandwidth as the data does not need to be broadcast all over the world just to reach a specific user.

The available DCC commands include DCC CHAT (direct user to user chat), DCC SEND (direct user to user file send) and DCC GET (file acknowledgement from a receiver).

Last updated: 1995-04-12

Direct Connection

A re-seller of Internet connections to the PIPEX backbone.

directed acyclic graph

(DAG) A directed graph containing no cycles. This means that if there is a route from node A to node B then there is no way back.

Last updated: 1994-12-07

directed graph

(digraph) A graph with one-way edges.

See also directed acyclic graph.

Last updated: 1994-11-11

Directed Oc


(Doc) A language related to Oc.

["Programming Language Doc and Its Self-Description, or 'X=X Is Considered Harmful'", M. Hirata, Proc 3rd Conf Japan Soc Soft Sci Tech, pp. 69-72, 1986].

Last updated: 1999-10-08

directed set


A set X is directed under some relation, <= (less than or equal), if it is non-empty and if for any two elements x and y there exists an element z such that x <= z and y <= z. I.e. all pairs have an upper bound.

Last updated: 1994-11-11

Direct Inward Dialing


(DID) A service offered by telephone companies which allows the last 3 or 4 digits of a phone number to be transmitted to the destination exchange.

For example, a company could have 10 incoming lines, all with the number 234 000. If a caller dials 234 697, the call is sent to 234 000 (the company's exchange), and the digits 697 are transmitted. The company's exchange then routes the call to extension 697. This gives the impression of 1000 direct dial lines, whereas in fact there are only 10. Obviously, only 10 at a time can be used.

This system is also used by fax servers. Instead of an exchange at the end of the 234 000 line, a computer running fax server software and fax modem cards uses the last three digits to identify the recipient of the fax. This allows 1000 people to have their own individual fax numbers, even though there is only one 'fax machine'.

Dictionary of PC Hardware and Data Communications Terms.

Last updated: 1997-06-29

directional coupler


(tap) A passive device used in cable systems to divide and combine radio frequency signals. A directional coupler has at least three ports: line in, line out, and the tap. The signal passes between line in and line out ports with loss referred to as the insertion loss. A small portion of the signal power applied to the line in port passes to the tap port. A signal applied to the tap port is passed to the line in port less the tap attenuation value. The tap signals are isolated from the line out port to prevent reflections. A signal applied to the line out port passes to the line in port and is isolated from the tap port. Some devices provide more than one tap output line (multi-taps).

Last updated: 1995-12-23

Directly Executable Test Oriented Language


(DETOL) A simple language to control a specific type of test equipment.

["Improved DETOL Programming Manual for the Series 5500 Automatic Test System", Pub. 5500-31-0-1, AAI Corporation Sep 1973].

Last updated: 1995-09-29

direct mapped cache


A cache where the cache location for a given address is determined from the middle address bits. If the cache line size is 2^n then the bottom n address bits correspond to an offset within a cache entry. If the cache can hold 2^m entries then the next m address bits give the cache location. The remaining top address bits are stored as a "tag" along with the entry.

In this scheme, there is no choice of which block to flush on a cache miss since there is only one place for any block to go. This simple scheme has the disadvantage that if the program alternately accesses different addresses which map to the same cache location then it will suffer a cache miss on every access to these locations. This kind of cache conflict is quite likely on a multi-processor. See also fully associative cache, set associative cache.

Direct Memory Access


(DMA) A facility of some architectures which allows a peripheral to read and write memory without intervention by the CPU. DMA is a limited form of bus mastering.

Last updated: 1996-08-23




<file system>

A node in a hierarchical file system which contains zero or more other nodes - generally, files or other directories.

The term "folder" is sometimes used in systems such as the Macintosh or Microsoft Windows in which directories are traditionally depicted as folders (like small briefcases).

Last updated: 2007-02-21

Directory Access Protocol

X.500 protocol used for communication between a Directory User Agent and a Directory System Agent.

directory service

<database, networking>

A structured repository of information on people and resources within an organisation, facilitating management and communication.

On a LAN or WAN the directory service identifies all aspects of the network including users, software, hardware, and the various rights and policies assigned to each. As a result applications can access information without knowing where a particular resource is physically located, and users interact oblivious to the network topology and protocols.

To allow heterogeneous networks to share directory information the ITU proposed a common structure called X.500. However, its complexity and lack of seamless Internet support led to the development of Lightweight Directory Access Protocol (LDAP) which has continued to evolve under the aegis of the IETF. Despite its name LDAP is too closely linked to X.500 to be "lightweight".

LDAP was adopted by several companies such as Netscape Communications Corporation (Netscape Directory Server) and has become a de facto standard for directory services. Other LDAP compatible offerings include Novell, Inc.'s Novell Directory Services (NDS) and Microsoft Corporation's Active Directory. The Netscape and Novell products are available for Windows NT and Unix platforms. Novell Directory Services also run on Novell platforms. Microsoft Corporation's Active Directory is an integral part of Microsoft's Windows 2000 and although it can interface with directory services running on other systems it is not available for other platforms.

Last updated: 2001-01-02

Directory System Agent

(DSA) The software that provides the X.500 Directory Service for a portion of the directory information base. Generally, each DSA is responsible for the directory information for a single organisation or organisational unit.

Last updated: 1994-12-07

Directory User Agent

(DUA) The software that accesses the X.500 Directory Service on behalf of the directory user. The directory user may be a person or another software element.

Last updated: 1994-12-07


<programming, hardware>

A Microsoft programming interface standard, first included with Windows 95. DirectX gives (games) programmers a standard way to gain direct access to enhanced hardware features under Windows 95 instead of going via the Windows 95 GDI. Some DirectX code runs faster than the equivalent under MS DOS.

DirectX promises performance improvements for graphics, sound, video, 3D, and network capabilites of games, but only where both hardware and software support DirectX.

DirectX 2 introduced the Direct3D interface. Version 5 was current at 1998-02-01. Version 8.1 is included in Windows XP.

Last updated: 2001-12-31


Do It Right the First Time


Design In Real Time


(XEROX PARC) A small, perhaps struggling outsider; not in the major or even the minor leagues. For example, "Xerox is not a dirtball company".

Outsiders often observe in the PARC culture an institutional arrogance which usage of this term exemplifies. The brilliance and scope of PARC's contributions to computer science have been such that this superior attitude is not much resented. - ESR

[Jargon File]

Last updated: 1994-12-07

dirty power

Electrical mains voltage that is unfriendly to the delicate innards of computers. Spikes, drop-outs, average voltage significantly higher or lower than nominal, or just plain noise can all cause problems of varying subtlety and severity (these are collectively known as power hits).

[Jargon File]

Nearby terms:

Direct-Access Storage DeviceDirect Client to Client Protocol

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