DA from FOLDOC

/day'mn/ or /dee'mn/ (From the mythological meaning, later rationalised as the acronym "Disk And Execution MONitor") A program that is not invoked explicitly, but lies dormant waiting for some condition(s) to occur. The idea is that the perpetrator of the condition need not be aware that a daemon is lurking (though often a program will commit an action only because it knows that it will implicitly invoke a daemon).

For example, under ITS writing a file on the LPT spooler's directory would invoke the spooling daemon, which would then print the file. The advantage is that programs wanting files printed need neither compete for access to, nor understand any idiosyncrasies of, the LPT. They simply enter their implicit requests and let the daemon decide what to do with them. Daemons are usually spawned automatically by the system, and may either live forever or be regenerated at intervals.

Unix systems run many daemons, chiefly to handle requests for services from other hosts on a network. Most of these are now started as required by a single real daemon, inetd, rather than running continuously. Examples are cron (local timed command execution), rshd (remote command execution), rlogind and telnetd (remote login), ftpd, nfsd (file transfer), lpd (printing).

Daemon and demon are often used interchangeably, but seem to have distinct connotations (see demon). The term "daemon" was introduced to computing by CTSS people (who pronounced it /dee'mon/) and used it to refer to what ITS called a dragon.

Last updated: 1995-05-11

DAG

1. Data Address Generator.

<mathematics>

2. directed acyclic graph.

Last updated: 1997-08-30

Daisy

A functional language.

["Daisy Programming Manual", S.D. Johnson, CS Dept TR, Indiana U, 1988].

DAISY 201

An early system on G-15.

[Listed in CACM 2(5):16 (May 1959)].

daisy chain

A bus wiring scheme in which, for example, device A is wired to device B, device B is wired to device C, etc. The last device is normally wired to a resistor or terminator. All devices may receive identical signals or, in contrast to a simple bus, each device in the chain may modify one or more signals before passing them on.

Characteristic of RS-485, of Apple's LocalTalk, and of various industrial control networks; also often used to describe Thinwire Ethernet (10base2).

Last updated: 1997-01-07

daisywheel printer

<printer>

A kind of impact printer where the characters are arranged on the ends of the spokes of a wheel (resembling the petals on a daisy).

The wheel (usually made of plastic) is rotated to select the character to print and then an electrically operated hammer mechanism bends the selected spoke forward slightly, sandwiching an ink ribbon between the character and the paper, as in a typewriter.

One advantage of this arrangement over that of a typewriter is that different wheels may be inserted to produce different typefaces.

Last updated: 1998-04-28

dancing frog

<programming, humour>

A bug or glitch that only occurs for a particular user; never when the user tries to show it to anyone else.

The term is derived from a Warner Brothers cartoon in which a man discovers a frog which can sing and dance; he believes this will make his fortune but the frog never performs in front of anyone else.

Last updated: 2004-10-16

dangling pointer

<programming>

A reference that doesn't actually lead anywhere. In C and some other languages, a pointer that doesn't actually point at anything valid. Usually this happens because it formerly pointed to something that has moved or disappeared, e.g. a heap-allocated block which has been freed and reused.

Used as jargon in a generalisation of its technical meaning; for example, a local phone number for a person who has since moved is a dangling pointer.

This dictionary contains many dangling pointers - cross-references to non-existent entries, as explained in the Help page.

Last updated: 2014-09-20

DANTE

A company established by the national research networks in Europe to provide international network services.

DAP Fortran

["Efficient High Speed Computing with the Distributed Array Processor", P.M. Flanders et al, pp.113-127 (1977)].

[Same as Fortran- Plus?]

DAPLEX

["The Functional Data Model and the Data Language DAPLEX", D.W. Shipman, ACM Trans Database Sys, 6(1):140-173 (Mar 1981)].

DARE

Differential Analyzer REplacement. A family of simulation languages for continuous systems.

["Digital Continuous System Simulation", G.A. Korn et al, P-H 1978].

dark-side hacker

<jargon, legal>

A criminal or malicious hacker; a cracker. From George Lucas's Darth Vader, "seduced by the dark side of the Force". The implication that hackers form a sort of elite of technological Jedi Knights is intended.

Opposite: samurai.

Defense Advanced Research Projects Agency

Last updated: 1997-04-28

Darms

<language, music>

A music language.

["The Darms Project: A Status Report", R.F. Erickson, Computers and the Humanities 9(6):291-298 (June 1975)].

Last updated: 1995-05-12

DARPA

Dartmouth BASIC

<language>

The original BASIC language, designed by John G. Kemeny and Thomas E. Kurtz at Dartmouth College in 1963. Dartmouth BASIC first ran on a GE 235 [date?] and on an IBM 704 on 1964-05-01. It was designed for quick and easy programming by students and beginners using Dartmouth's experimental time-sharing system. Unlike most later BASIC dialects, Dartmouth BASIC was compiled.

Last updated: 2003-07-02

Darwin

Direct-Access Storage Device

1. An operating system based on the FreeBSD version of Unix, running on top of a microkernel (Mach 3.0 with darwin 1.02) that offers advanced networking, services such as the Apache web server, and support for both Macintosh and Unix file systems. Darwin was originally released in March 1999. It currently runs on PowerPC based Macintosh computers, and, in October 2000, was being ported to Intel processor-based computers and compatible systems by the Darwin community.

<programming, tool>

2. A general purpose structuring tool of use in building complex distributed systems from diverse components and diverse component interaction mechanisms. Darwin is being developed by the Distributed Software Engineering Section of the Department of Computing at Imperial College. It is in essence a declarative binding language which can be used to define hierarchic compositions of interconnected components. Distribution is dealt with orthogonally to system structuring. The language allows the specification of both static structures and dynamic structures which evolve during execution. The central abstractions managed by Darwin are components and services. Bindings are formed by manipulating references to services.

The operational semantics of Darwin is described in terms of the Pi-calculus, Milner's calculus of mobile processes. The correspondence between the treatment of names in the Pi-calculus and the management of service references in Darwin leads to an elegant and concise Pi-calculus model of Darwin's operational semantics. The model has proved useful in arguing the correctness of Darwin implementations and in designing extensions to Darwin and reasoning about their behaviour.

Distributed Software Engineering Section. Darwin publications.

E-mail: Jeff Magee <[email protected]>, Naranker Dulay <[email protected]>.

3. Core War.

Last updated: 2003-08-08

DAS

Digital Analog Simulator.

Represents analog computer design.

Last updated: 1994-11-08

DASD

DASE

Distributed Application Support Environment

DASL

Datapoint's Advanced System Language.

A cross between C and Pascal by Gene Hughes with custom features for Datapoint hardware (no stack). It is used internally by Datapoint.

Last updated: 1994-11-08

DAT

1. Digital Audio Tape.

2. Dynamic Address Translation.

data

<data, data processing, jargon>

/day't*/ (Or "raw data") Numbers, characters, images, or other method of recording, in a form which can be assessed by a human or (especially) input into a computer, stored and processed there, or transmitted on some digital channel. Computers nearly always represent data in binary.

Data on its own has no meaning, only when interpreted by some kind of data processing system does it take on meaning and become information.

For example, the binary data 01110101 might represent the integer 117 or the ASCII lower case U character or the blue component of a pixel in some video. Which of these it represents is determined by the way it is processed (added, printed, displayed, etc.). Even these numbers, characters or pixels however are still not really information until their context is known, e.g. my bank balance is £117, there are two Us in "vacuum", you have blue eyes.

Last updated: 2007-09-10

data abstraction

<data>

Any representation of data in which the implementation details are hidden (abstracted). Abstract data types and objects are the two primary forms of data abstraction.

[Other forms?].

Last updated: 2003-07-03

data acquisition

data logging

Data Address Generator

(DAG) The mechanism which generates temporary memory addresses for data that is transferred between memory and registers in a Digital Signal Processor.

Certain DSP architectures incorporate more than one DAG to simplify the programming needed to move blocks of data between buffers.

For instance, certain Fast Fourier Transform algorithms requiring bit reversing, can use the DAG for that purpose, or they can use two DAGS, one for Program Memory Data (PMD), and the other for Data Memory Data (DMD).

Last updated: 1997-08-12

database

<database>

1. One or more large structured sets of persistent data, usually associated with software to update and query the data. A simple database might be a single file containing many records, each of which contains the same set of fields where each field is a certain fixed width.

A database is one component of a database management system.

Carol E. Brown's tutorial.

<hypertext>

2. A collection of nodes managed and stored in one place and all accessible via the same server. Links outside this are "external", and those inside are "internal".

On the World-Wide Web this is called a website.

3. All the facts and rules comprising a logic programming program.

Last updated: 2005-11-17

database administrator

<job>

A person responsible for the design and management of one or more databases and for the evaluation, selection and implementation of database management systems. In smaller organisations, the data administrator and database administrator are often one in the same; however, when they are different, the database administrator's function is more technical. The database administrator would implement the database software that meets the requirements outlined by the organisation's data administrator and systems analysts.

Tasks might include controling an organisation's data resources, using data dictionary software to ensure data integrity and security, recovering corrupted data and eliminating data redundancy and uses tuning tools to improve database performance.

Last updated: 2004-03-11

database analyst

<job>

A person who uses data modeling to analyse and specify data use within an application area. A database analyst defines both logical views and physical data structures. In a client/server environment, he defines the database part of the back end system.

Last updated: 2004-03-11

database machine

<hardware>

A computer or special hardware that stores and retrieves data from a database. It is specially designed for database access and is coupled to the main (front-end) computer(s) by a high-speed channel. This contrasts with a database server, which is a computer in a local area network that holds a database. The database machine is tightly coupled to the main CPU, whereas the database server is loosely coupled via the network.

[Example?]

Last updated: 2004-03-11

database management system

<database>

(DBMS) A suite of programs which typically manage large structured sets of persistent data, offering ad hoc query facilities to many users. They are widely used in business applications.

A database management system (DBMS) can be an extremely complex set of software programs that controls the organisation, storage and retrieval of data (fields, records and files) in a database. It also controls the security and integrity of the database. The DBMS accepts requests for data from the application program and instructs the operating system to transfer the appropriate data.

When a DBMS is used, information systems can be changed much more easily as the organisation's information requirements change. New categories of data can be added to the database without disruption to the existing system.

Data security prevents unauthorised users from viewing or updating the database. Using passwords, users are allowed access to the entire database or subsets of the database, called subschemas (pronounced "sub-skeema"). For example, an employee database can contain all the data about an individual employee, but one group of users may be authorised to view only payroll data, while others are allowed access to only work history and medical data.

The DBMS can maintain the integrity of the database by not allowing more than one user to update the same record at the same time. The DBMS can keep duplicate records out of the database; for example, no two customers with the same customer numbers (key fields) can be entered into the database.

Query languages and report writers allow users to interactively interrogate the database and analyse its data.

If the DBMS provides a way to interactively enter and update the database, as well as interrogate it, this capability allows for managing personal databases. However, it may not leave an audit trail of actions or provide the kinds of controls necessary in a multi-user organisation. These controls are only available when a set of application programs are customised for each data entry and updating function.

A business information system is made up of subjects (customers, employees, vendors, etc.) and activities (orders, payments, purchases, etc.). Database design is the process of deciding how to organize this data into record types and how the record types will relate to each other. The DBMS should mirror the organisation's data structure and process transactions efficiently.

Organisations may use one kind of DBMS for daily transaction processing and then move the detail onto another computer that uses another DBMS better suited for random inquiries and analysis. Overall systems design decisions are performed by data administrators and systems analysts. Detailed database design is performed by database administrators.

The three most common organisations are the hierarchical database, network database and relational database. A database management system may provide one, two or all three methods. Inverted lists and other methods are also used. The most suitable structure depends on the application and on the transaction rate and the number of inquiries that will be made.

Database machines are specially designed computers that hold the actual databases and run only the DBMS and related software. Connected to one or more mainframes via a high-speed channel, database machines are used in large volume transaction processing environments. Database machines have a large number of DBMS functions built into the hardware and also provide special techniques for accessing the disks containing the databases, such as using multiple processors concurrently for high-speed searches.

The world of information is made up of data, text, pictures and voice. Many DBMSs manage text as well as data, but very few manage both with equal proficiency. Throughout the 1990s, as storage capacities continue to increase, DBMSs will begin to integrate all forms of information. Eventually, it will be common for a database to handle data, text, graphics, voice and video with the same ease as today's systems handle data.

database manager

The part of the database management system (DBMS) that handles the organisation, storage and retrieval of the data. A database manager may work with traditional programming languages, such as COBOL and BASIC, or may work only with its proprietary programming language. The terms database manager and database management system are used interchangeably.

A database manager links two or more files together and is the foundation for developing routine business systems. Contrast with file manager, which works with only one file at a time and is typically used interactively on a personal computer for managing personal, independent files, such as name and address lists.

database normalisation

<database>

A series of steps followed to obtain a database design that allows for efficient access and storage of data in a relational database. These steps reduce data redundancy and the chances of data becoming inconsistent.

A table in a relational database is said to be in normal form if it satisfies certain constraints. Codd's original work defined three such forms but there are now five generally accepted steps of normalisation. The output of the first step is called First Normal Form (1NF), the output of the second step is Second Normal Form (2NF), etc.

First Normal Form eliminates repeating groups by putting each value of a multi-valued attribute into a new row.

Second Normal Form eliminates functional dependencies on a partial key by putting the fields in a separate table from those that are dependent on the whole key.

Third Normal Form eliminates functional dependencies on non-key fields by putting them in a separate table. At this stage, all non-key fields are dependent on the key, the whole key and nothing but the key.

Fourth Normal Form separates independent multi-valued facts stored in one table into separate tables.

Fifth Normal Form breaks out data redundancy that is not covered by any of the previous normal forms.

http://bkent.net/Doc/simple5.htm.

[What about non-relational databases?]

Last updated: 2005-07-28

database query language

<database>

A language in which users of a database can (interactively) formulate requests and generate reports. The best known is SQL.

Last updated: 1998-04-15

database server

A stand-alone computer in a local area network that holds and manages the database. It implies that database management functions, such as locating the actual record being requested, is performed in the server computer. Contrast with file server, which acts as a remote disk drive and requires that large parts of the database, for example, entire indexes, be transmitted to the user's computer where the real database management tasks are performed.

First-generation personal computer database software was not designed for a network; thus, modified versions of the software released by the vendors employed the file server concept. Second-generation products, designed for local area networks, perform the management tasks in the server where they should be done, and consequently are turning the file server into a database server.

database transaction

<database>

A set of related changes applied to a database. The term typically implies that either all of the changes should be applied or, in the event of an error, none of them, i.e. the transaction should be atomic. Atomicity is one of the ACID properties a transaction can have, another is isolation - preventing interference between processes trying to access the database cocurrently. This is usually achieved by some form of locking - where one process takes exclusive control of a database table or row for the duration of the transaction, preventing other processes from accessing the locked data.

The canonical example of a transaction is transferring money between two bank accounts by subtracting it from one and adding it to the other.

Some relational database management systems require the user to explicitly start a transaction and then either commit it (if all the individual steps are successful) or roll it back (if there are any errors).

Last updated: 2013-06-03

Data/BASIC

<language>

(Or "Pick BASIC") A BASIC-like language with database capabilities, the main programming language on the Pick OS.

["The Data/BASIC Language - A Data Processing Language for Non-Professional Programmers", P.C. Dressen, Proc SJCC 36, AFIPS, Spring 1970].

Last updated: 2001-04-30

DATABUS

DATApoint BUSiness Language.

A language like an interpreted assembly language, used for custom applications on Datapoint computers.

Last updated: 1995-01-16

data bus

The bus (connections between and within the CPU, memory, and peripherals) used to carry data. Other connections are the address bus and control signals.

The width and clock rate of the data bus determine its data rate (the number of bytes per second it can carry), which is one of the main factors determining the processing power of a computer. Most current processor designs use a 32-bit bus, meaning that 32 bits of data can be transferred at once. Some processors have an internal data bus which is wider than their external bus in order to make external connections cheaper while retaining some of the benefits in processing power of a wider bus.

datacenter manager

<job>

A person who plans and directs all computer and peripheral operations, data entry, data control scheduling and quality control.

Last updated: 2004-03-11

data channel

Data Communication Equipment

A channel (on a BRI or PRI line) used to carry control information, to set up connections on the associated bearer channels. The name wasn't too bad back when users were sending voice (not data) over the bearer channels, but in 1997 it's quite a misnomer.

Last updated: 1997-03-10

DATACODE I

<language>

An early system used on the Datatron 200 series.

[Listed in CACM 2(5):16, May 1959].

Last updated: 1994-12-06

Datacom

A DBMS from Computer Associates International.

Last updated: 1994-12-06

Data Communication Equipment

<communications, hardware>

(DCE) The devices and connections of a communications network that connect the communication circuit between the data source and destination (the Data Terminal Equipment or DTE). A modem is the most common kind of DCE.

Before data can be transmited over a modem, the DTR (Data Terminal Ready) signal must be active. DTR tells the DCE that the DTE is ready to transmit and receive data.

DCE and DTE are usually connected by an EIA-232 serial line. It is necessary to distinguish these two types of device because their connectors must be wired differently if a "straight-through" cable (pin 1 to pin 1, pin 2 to pin 2 etc.) is to be used. DCE should have a female connector and should transmit on pin two and receive on pin three. It is a curious fact that many modems are "DTE" according to the original standard.

Last updated: 1995-02-28

data communications analyst

<job>

A person who installs, maintains, and troubleshoots data networks. A data communications analyst may have knowledge of T1 lines, TCP/IP, fiber optics, SNA, frame relay. He assists users with problems related to connectivity, analyses data flow, configures modems, DSUs, multiplexors, and routers, and uses network tools such as NetView or Netspy.

Last updated: 2004-03-11

Data Communications Equipment

data compression

<algorithm>

compression. Probably to distinguish it from (electronic) signal compression.

Last updated: 1995-04-02

Data definition language

<language, database>

(DDL)

1. A language enabling the structure and instances of a database to be defined in a human-, and machine-readable form.

SQL contains DDL commands that can be used either interactively, or within programming language source code, to define databases and their components, e.g. CREATE and DROP.

See also Data manipulation language (DML).

2. A specification language for databases, based on the entity-relationship model. It is used in the Eli compiler-compiler to manage type definitions.

["DDL Reference Manual", ECE Dept U Colorado, 1991].

Last updated: 1999-04-26

data dictionary

<database>

A data structure that stores metadata, i.e. data about data. The term "data dictionary" has several uses.

Most generally it is a set of data descriptions that can be shared by several applications.

Usually it means a table in a database that stores the names, field types, length, and other characteristics of the fields in the database tables.

An active data dictionary is automatically updated as changes occur in the database. A passive data dictionary must be manually updated.

In a DBMS, this functionality is performed by the system catalog. The data dictionary is a more general software utility used by designers, users, and administrators for information resource management.

The data dictionary may maintain information on system hardware, software, documentation, users, and other aspects.

Data dictionaries are also used to document the database design process itself and can accumulate metadata ready to feed into the system catalog.

[Does anybody call them "codebooks"?]

Last updated: 2001-04-24

data dictionary file

<database>

(DDF) A set of files describing the structure of a database file. DDFs define database tables and include information about file locations, field layouts and indexes. DDFs are the standard method for defining field and index characteristics for Btrieve files.

Last updated: 1997-06-03

data driven

A data driven architecture/language performs computations in an order dictated by data dependencies. Two kinds of data driven computation are dataflow and demand driven.

From about 1970 research in parallel data driven computation increased. Centres of excellence emerged at MIT, CERT-ONERA in France, NTT and ETL in Japan and Manchester University.

Data Driven Machine

<language>

(DDM) A dataflow language.

["The Architecture and System Method of DDM-1: A Recursively Structured Data Driven Machine", A. Davis, Proc 5th Ann Symp Comp Arch, IEEE 1978].

Last updated: 1999-04-26

Data Encryption Algorithm

(DEA) An ANSI standard defined in ANSI X3.92-1981. It is identical to the Data Encryption Standard (DES).

Last updated: 1994-12-06

Data Encryption Key

(DEK) Used for the encryption of message text and for the computation of message integrity checks (signatures).

See cryptography.

Last updated: 1994-12-06

Data Encryption Standard

(DES) The NBS's popular, standard encryption algorithm. It is a product cipher that operates on 64-bit blocks of data, using a 56-bit key. It is defined in FIPS 46-1 (1988) (which supersedes FIPS 46 (1977)). DES is identical to the ANSI standard Data Encryption Algorithm (DEA) defined in ANSI X3.92-1981.

DES has been implemented in VLSI. SunOS provides a des command which can make use of DES hardware if fitted. Neither the software nor the hardware are supposed to be distributed outside the USA.

Unix manual pages: des(1), des(3), des(4).

Last updated: 1994-12-06

data feed

<data, architecture>

Some process for transferring data from one system to another in a predetermined form.

Last updated: 2009-05-17

data flow

Data General MicroNova 601

A data flow architecture or language performs a computation when all the operands are available. Data flow is one kind of data driven architecture, the other is demand driven. It is a technique for specifying fine-grain concurrency, usually in the form of two-dimensional graphs in which instructions that are available for concurrent execution are written alongside each other while those that must be executed in sequence are written one under the other. Data dependencies between instructions are indicated by directed arcs. Instructions do not reference memory since the data dependence arcs allow data to be transmitted directly from the producing instruction to the consuming one.

Data flow schemes differ chiefly in the way that they handle re-entrant code. Static schemes disallow it, dynamic schemes use either "code copying" or "tagging" at every point of reentry.

An example of a data flow architecture is MIT's VAL machine.

data flow analysis

<programming>

A process to discover the dependencies between different data items manipulated by a program. The order of execution in a data driven language is determined solely by the data dependencies. For example, given the equations

 1. X = A + B
 2. B = 2 + 2
 3. A = 3 + 4

a data-flow analysis would find that 2 and 3 must be evaluated before 1. Since there are no data dependencies between 2 and 3, they may be evaluated in any order, including in parallel.

This technique is implemented in hardware in some pipelined processors with multiple functional units. It allows instructions to be executed as soon as their inputs are available, independent of the original program order.

Last updated: 1996-05-13

Data Flow Diagram

<programming>

A graphical notation used to describe how data flows between processes in a system. Data flow diagrams are an important tool of most structured analysis techniques.

http://smartdraw.com/resources/centers/software/dfd.htm.

Last updated: 2003-05-17

data fork

Macintosh file system

data frame

activation record

Data General

<company>

A US computer manufacturer. Responsible for the Nova minicomputer.

Quarterly sales $284M, profits -$12M (Aug 1994).

Last updated: 1994-09-26

Data General mN601

data glove

<hardware, virtual reality>

An input device for virtual reality in the form of a glove which measures the movements of the wearer's fingers and transmits them to the computer. Sophisticated data gloves also measure movement of the wrist and elbow. A data glove may also contain control buttons or act as an output device, e.g. vibrating under control of the computer. The user usually sees a virtual image of the data glove and can point or grip and push objects.

Examples are Fifth Dimension Technologies (5DT)'s 5th Glove, and Virtual Technologies' CyberGlove. A cheaper alternative is InWorld VR's CyberWand.

["Full freedom plus input", PC Magazine, Mar 14 1995, pp. 168-190].

[Inventor?]

Last updated: 1995-04-04

datagram

A self-contained, independent entity of data carrying sufficient information to be routed from the source to the destination computer without reliance on earlier exchanges between this source and destination computer and the transporting network.

data hierarchy

The system of data objects which provide the methods for information storage and retrieval. Broadly, a data hierarchy may be considered to be either natural, which arises from the alphabet or syntax of the language in which the information is expressed, or machine, which reflects the facilities of the computer, both hardware and software.

A natural data hierarchy might consist of bits, characters, words, phrases, sentences, paragraphs, and chapters. One might use components bound to an application, such as field, record, and file, and these would ordinarily be further specified by having data descriptors such as name field, address field, etc. On the other hand, a machine or software system might use bit, byte, word, block, partition, channel, and port.

Programming languages often provide types or objects which can create data hierarchies of arbitrary complexity, thus allowing software system designers to model language structures described by the linguist to greater or lesser degree.

The distinction between the natural form of data and the facilities provided by the machine may be obscure, because users force their needs into the molds provided, and programmers change machine designs. As an example, the natural data type "character" and the machine type "byte" are often used interchangeably, because the latter has evolved to meet the need of representing the former.

Last updated: 1995-11-03

data integrity

<data>

The absence of unintended changes or errors in some data. Integrity implies that the data is an exact copy of some original version, e.g. that it has not been corrupted in the process of being written to, and read back from, a hard disk or during transmission via some communications channel.

Integrity may further imply that the information represented by the data has been validated, i.e. verified to conform to certain constraints, e.g. a date's year, month and day parts are within the appropriate ranges and the date actually exists.

Last updated: 2009-06-03

Data Interchange Standards Association

<standard>

(DISA) A not-for-profit corporation that acts as the secretariat for ANSI's EDI standards committee, ASC X12 that works on ANSI X12. DISA manages ASC X12's membership, balloting, standards development and maintenance, publications, and communications with ANSI.

Last updated: 1999-09-18

Data Jack

<hardware>

A wall-mounted or desk-mounted connector (frequently a wide telephone-style 8-pin RJ-45) for connecting to data cabling in a building.

Last updated: 1997-01-07

Datakit

A circuit-switched digital network, similar to X.25. Datakit supports host-to-host connections and EIA-232 connections for terminals, printers, and hosts.

Most of Bell Laboratories is trunked together on Datakit. On top of DK transport service, people run UUCP for electronic mail and dkcu for remote login.

ISN is the version of Datakit supported by AT&T Information Systems. Bell Laboratories in Holmdel, New Jersey, uses ISN for internal data communication.

http://fc.net:80/phrack/files/p18/p18-9.html.

["Towards a universal data transport system", A. G. Fraser, IEEE Journal on Selected Areas in Communications, SAC-1(5) pp. 803-16, 1983].

Last updated: 1996-10-20

Dataless Management Services

(DMS)

http://cs.arizona.edu/computer.help/policy/DIGITAL_unix/AA-PS3LE-TE_html/sharing10.html.

Last updated: 2005-09-15

dataless management utility

(DMU) A Dataless Management Services (DMS) utility for managing the sharing of installed operating software between DMS servers and clients. It allows users to install, configure, show and delete DMS environments and add, list, modify and remove DMS clients.

Last updated: 2005-09-15

Data Link Connection Identifier

(DLCI) A channel number which is attached to data frames to tell a Frame Relay network how to route the data. In Frame Relay, multiple logical channels are multiplexed over a single physical channel. The DLCI says which of these logical channels a particular data frame belongs to.

http://etinc.com/frmain.htm#whatsadlci.

Last updated: 2000-02-13

data link layer

Layer two, the second lowest layer in the OSI seven layer model. The data link layer splits data into frames (see fragmentation) for sending on the physical layer and receives acknowledgement frames. It performs error checking and re-transmits frames not received correctly. It provides an error-free virtual channel to the network layer. The data link layer is split into an upper sublayer, Logical Link Control (LLC), and a lower sublayer, Media Access Control (MAC).

Example protocols at this layer are ABP, Go Back N, SRP.

Last updated: 1995-02-14

data link level

data link layer

Data Link Provider Interface

(DLPI) The interface that a network driver presents to the (higher level) logical link layer for driving the network at the datagram level in a Unix STREAMS environment and possibly elsewhere.

DLPI corresponds to ISO 8802/2 (LLC) which covers both connection-oriented and connectionless protocols.

[Is this correct? Better explanation?]

Last updated: 1996-01-29

Data Link Switching

(DLSw) A standard for transporting IBM Systems Network Architecture (SNA) and network basic input/output system (NetBIOS) traffic over an Internet protocol network.

Initially, in 1992, DLSw was proprietary to IBM. It was submitted to the IETF as RFC 1434 in 1993, later updated by RFC 1795.

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/dlsw.htm.

Last updated: 2008-01-11

data logger

data logging

<data>

(data acquisition) Storing a series of measurements over time, usually from a sensor that converts a physical quantity such as temperature, pressure, relative humidity, light, resistance, current, power, speed, vibration into a voltage that is then converted by a digital to analog converter (DAC) into a binary number. Data logging hardware may have several DACs for multiple simultaneous measurements. The hardware usually connects to a parallel port, serial port or USB port on a PC.

Last updated: 2004-11-15

Data Management Language

<language>

(DML)

1. Any language for manipulating data or files, e.g. IBM's Distributed Data Management (DDM).

2. An early ALGOL-like language with lists and graphics, that ran on the Honeywell 635.

["DML: A Data Management Language", D.W. Bray et al, GE, Syracuse NY].

Last updated: 1999-06-07

Data Manipulation Language

<language, database>

(DML, or Data Management Language) A language for the manipulation of data in a database by applications and/or directly by end-users.

SQL contains DML commands such as INSERT, UPDATE, and DELETE.

See also Data Definition Language (DDL).

Last updated: 1999-04-26

data mart

<database>

A type of data warehouse designed primarily to address a specific function or department's needs, as opposed to a data warehouse which is traditionally meant to address the needs of the organisation from an enterprise perspective. In addition, a data mart often uses aggregation or summarisation of the data to enhance query performance. However, it is important to maintain the ability to access the underlying base data to enable drill-down analysis as necessary.

Last updated: 1998-04-24

Datamatic Corporation

Honeywell

Datamation

/day"t*-may"sh*n/ A magazine that many hackers assume all suits read. Used to question an unbelieved quote, as in "Did you read that in "Datamation?"" It used to publish something hackishly funny every once in a while, like the original paper on COME FROM in 1973, and Ed Post's "Real Programmers Don't Use Pascal" ten years later, but it has since become much more exclusively suit-oriented and boring.

Data Over Cable Service Interface Specification

data mining

<database>

Analysis of data in a database using tools which look for trends or anomalies without knowledge of the meaning of the data. Data mining was invented by IBM who hold some related patents.

Data mining may well be done on a data warehouse.

ShowCase STRATEGY is an example of a data mining tool.

Last updated: 2001-02-08

data model

<database>

The product of the database design process which aims to identify and organize the required data logically and physically.

A data model says what information is to be contained in a database, how the information will be used, and how the items in the database will be related to each other.

For example, a data model might specify that a customer is represented by a customer name and credit card number and a product as a product code and price, and that there is a one-to-many relation between a customer and a product.

It can be difficult to change a database layout once code has been written and data inserted. A well thought-out data model reduces the need for such changes. Data modelling enhances application maintainability and future systems may re-use parts of existing models, which should lower development costs.

A data modelling language is a mathematical formalism with a notation for describing data structures and a set of operations used to manipulate and validate that data.

One of the most widely used methods for developing data models is the entity-relationship model. The relational model is the most widely used type of data model. Another example is NIAM.

["Principles of Database and Knowledge-Base Systems", J.D. Ullman, Volume I, Computer Science Press, 1988, p. 32].

Last updated: 2000-06-24

data modeling

<spelling>

US spelling of "data modelling".

Last updated: 2000-06-24

data modelling

data model

Data Over Cable Service Interface Specification

<communications, networking>

(DOCSIS) ITU-approved interface requirements for cable modems involved in high-speed data distribution over a cable television network. DOCSIS compatible equipment uses a 6 MHz carrier band for downstream, using 64 and 256 QAM (ITU Annex B), and QPSK and 16 QAM for upstream, allowing up to 36 and 10 Mb/s, respectively for downstream and upstream channels.

CableLabs FAQ.

Last updated: 2001-07-10

Data Over Cable Systems Interface Specifications

data packet

packet

Dataparallel-C

<language, parallel>

C with parallel extensions by Hatcher and Quinn of the University of New Hampshire. Dataparallel-C was based on an early version of C* and runs on the Intel iPSC-2 and nCube.

Data Parallel Haskell

<language, parallel>

Adds Parallel Objects with arbitrary Dimension (PODs) and POD comprehensions to Haskell.

ftp://redstar.dcs.qmw.ac.uk/cpc/jon_hill/dpGlue.ps.Z.

["Data Parallel Haskell: Mixing Old and New Glue", J. Hill].

Last updated: 1995-03-30

data path

A CPU's internal data bus and functional units. The width of the data path in bits is a major determiner of the processor's performance.

Last updated: 1997-07-09

Dataphone Digital Service

<communications, product>

(DDS) The first private-line digital service offered by AT&T, with data rates typically at 2.4, 4.8, 9.6 and 56 kilobits per second. DDS is now part of AT&T's Accunet family of services. Most LEC (local exchange carriers) and IXC (IntereXchange Carriers) offer similar services.

Last updated: 1995-02-28

DataPoint

<company>

An early minicomputer manufacturer which also developed ARCnet.

Last updated: 2004-08-25

data processing

<data processing, history>

An antiquated term for the input, verification, organisation, storage, retrieval and transformation of data and the extraction of information. The term was associated with commercial applications such as stock control or payroll. The term "electronic data processing" (EDP) means using computers for these tasks.

Last updated: 2019-01-26

Data Protection Act

<legal>

(DPA) A UK law guaranteeing rights to individuals in relation to personal data that others hold on them. For example, under the DPA, you have the right to see what data a company holds on you.

Last updated: 2007-06-17

data rate

data transfer rate

data redundancy

<data, communications, storage>

Any technique that stores or transmits extra, derived data that can be used to detect or repair errors, either in hardware or software. Examples are parity bits and the cyclic redundancy check.

If the cost of errors is high enough, e.g. in a safety-critical system, redundancy may be used in both hardware AND software with three separate computers programmed by three separate teams ("triple redundancy") and some system to check that they all produce the same answer, or some kind of majority voting system.

The term is not typically used for other, less beneficial, duplication of data.

2. The proportion of a message's gross information content that can be eliminated without losing essential information.

Technically, redundancy is one minus the ratio of the actual uncertainty to the maximum uncertainty. This is the fraction of the structure of the message which is determined not by the choice of the sender, but rather by the accepted statistical rules governing the choice of the symbols in question.

[Shannon and Weaver, 1948, p. l3]

Last updated: 2010-02-04

data segment

<memory>

The range of memory locations where the initialised data of a program produced by a Unix linker is located.

Executable code is located in the code segment and uninitialised data in the bss segment.

Last updated: 2004-02-24

data service unit

(DSU or "data service unit") A device used in digital transmission for connecting a CSU (Channel Service Unit) to Data Terminal Equipment (a terminal or computer), in the same way that a modem is used for connection to an analogue medium.

A DSU provides a standard interface to a user's terminal which is compatible with modems and handles such functions as signal translation, regeneration, reformatting, and timing. The transmitting portion of the DSU processeses the customers' signal into bipolar pulses suitable for transmission over the digital facility. The receiving portion of the DSU is used both to extract timing information and to regenerate mark and space information from the received bipolar signal.

Last updated: 1995-01-30

data set

<operating system, storage>

An IBM term for a file.

Last updated: 1997-04-15

data set organization

<operating system, storage>

(DSORG) An IBM term for file structure. These include PS physical sequential, DA direct access, IS indexed sequential, PO partitioned (a library). This system dates from OS/360, and breaks down beginning with VSAM and VTAM, where it is no longer applied.

Sequential and indexed data sets can be accessed using either a "basic" or a "queued" "access method." For example a DSORG=PS file can use either BSAM (basic sequential access method) or QSAM (queued sequential access method). It can also be processed as a direct file using BDAM. Likewise a library can be processed using BPAM (basic partitioned access method), BSAM, QSAM, or BDAM. DSORG and access method are somewhat, but not completely, orthogonal.

The "basic" access method deals with physical blocks rather than records, and usually provides more control over the specific device. Each I/O operation using the "basic" access method reads or writes a single block. A "basic" read or write starts an asynchronous I/O operation, and the programmer is responsible for waiting for completion and checking for errors.

The "queued" access method deals with logical records and provides blocking and deblocking services. It is "queued" because it provides read-ahead and write-behind services. While a program is processing records in one input block, for example, QSAM may be reading one or more blocks ahead. Queued "get" or "put" operations are synchronous as far as the programmer is concerned. The operation is complete when the next logical record has been successfully processed.

EXCP (Execute Channel Program) is a lower-level method of accessing data.

IBM manuals usually named "Data Administration Guide", e.g. SC26-4505-1 for MVS/ESA DFP 3.1, provide more detail about data set organizations and access methods.

Last updated: 2005-08-08

DataStage

<database, tool>

A tool set for designing, developing, and running applications that populate one or more tables in a data warehouse or data mart.

[Reference]?

Last updated: 2004-06-23

data storage

<storage>

(Or "memory") A device or medium into which data can be entered, in which it can be held, and from which it can be retrieved at a later time.

The distinguishing characteristics of a device are its capacity (the number of bytes it can hold), its access speed, whether it is volatile (loses data when the power is turned off), whether it is removeable or fixed and whether it is writeable or read-only.

Some examples are DRAM, hard disk, CD-ROM, Flash memory.

Storage timeline by https://www.frontierinternet.com/.

Last updated: 2018-04-11

Datastorm Technologies, Inc.

<company>

The original suppliers of Procomm.

Address: Columbia MO, USA.

Last updated: 2004-06-29

data striping

<storage>

Segmentation of logically sequential data, such as a single file, so that segments can be written to multiple physical devices (usually disk drives) in a round-robin fashion. This technique is useful if the processor is capable of reading or writing data faster than a single disk can supply or accept it. While data is being transferred from the first disk, the second disk can locate the next segment.

Data striping is used in some modern databases, such as Sybase, and in certain RAID devices under hardware control, such as IBM's RAMAC array subsystem (9304/9395).

Data striping is different from, and may be used in conjunction with, mirroring.

Last updated: 1996-10-17

data structure

<data, programming>

Any method of organising a collection of data to allow it to be manipulated effectively. It may include meta data to describe the properties of the structure.

Examples data structures are: array, dictionary, graph, hash, heap, linked list, matrix, object, queue, ring, stack, tree, vector.

Last updated: 2003-09-11

Data Structure Diagram

Bachman Diagram

Data Structures Language

<language>

A dialect of MAD with extensions for lists and graphics, on Philco 212.

["A Compiler Language for Data Structures", N. Laurance, Proc ACM 23rd Natl Conf 36 (1968)].

Last updated: 1995-02-28

Data Terminal Equipment

<communications, hardware>

(DTE) A device which acts as the source and/or destination of data and which controls the communication channel. DTE includes terminals, computers, protocol converters, and multiplexors.

DTE is usually connected via an EIA-232 serial line to Data Communication Equipment (DCE), typically a modem. It is necessary to distinguish these two types of device because their connectors must be wired differently if a "straight-through" cable (pin 1 to pin 1, pin 2 to pin 2 etc.) is to be used. DTE should have a male connector and should transmit on pin three and receive on pin two. It is a curious fact that many modems are actually "DTE" according to the original standard.

Last updated: 1995-02-28

Data Terminal Ready

(DTR) The wire in a full RS-232 connection that tells the Data Communication Equipment (DCE, typically a modem) that the Data Terminal Equipment (DTE, typically a computer or terminal) is ready to transmit and receive data.

Last updated: 2000-04-05

DATA-TEXT

<tool>

A system from Harvard for numerical computations in the Social Sciences.

["DATA-TEXT Primer", D.J. Armor, Free Press 1972].

Last updated: 1994-12-06

data transfer

<data>

Copying or moving data from one place to another, typically via some kind of network (e.g. Asynchronous Transfer Mode, File Transfer Protocol) or local data connection (bus, SCSI, IDE, SATA).

Last updated: 2009-06-09

data transfer rate

(Or "throughput, data rate", "transmission rate") The amount of data transferred in one direction over a link divided by the time taken to transfer it, usually expressed in bits per second (bps), bytes per second (Bps) or baud. The link may be anything from an interface to a hard disk to a radio transmission from a satellite.

Where data transfer is not continuous throughout the given time interval, the data transfer rate is thus an average rate that will be lower than the peak rate. The peak or maximum possible rate may itself be lower than the capacity of the communication channel if the channel is shared, or part of the signal is not considered as data, e.g. checksum or routing information.

When applied to data rate, the multiplier prefixes "kilo-", "mega-", "giga-", etc. (and their abbreviations, "k", "M", "G", etc.) always denote powers of 1000. For example, 64 kbps is 64,000 bits per second. This contrasts with units of storage where they stand for powers of 1024, e.g. 1 KB = 1024 bytes.

The other important characteristic of a channel is its latency.

The bandwidth of a channel determines the data transfer rate but is a different characteristic, measured in Hertz. [Relationship?]

Last updated: 2008-02-08

DATATRIEVE

<database, language>

A query and report system for use with DEC's VMS (RMS, VAX Rdb/VMS or VAX DBMS).

Last updated: 2007-01-16

Datatron 200 series

<computer>

A family of computers produced by Burroughs that included the Datatron 204 and Datatron 220.

Last updated: 2007-01-16

data type

type

DataViews

Graphical user interface development software from V.I.Corporation, aimed at constructing platform-independent interactive views of dynamic data.

Last updated: 1994-12-07

DataVis

A dataflow language for scientific visualisation.

["Data Flow Visual Programming Languages", D. Hils, J Vis Langs and Comput, Dec 1991].

Last updated: 1994-12-06

data warehouse

<database>

(Or corporate data warehouse, CDW) Any system for storing, retrieving and managing large amounts of data. Data warehouse software often includes sophisticated compression and hashing techniques for fast searches, as well as advanced filtering. A data warehouse is often a relational database containing a recent snapshot of corporate data and optimised for searching. Planners and researchers can use this database without worrying about slowing down day-to-day operations of the production database. The latter can be optimised for transaction processing (inserts and updates).

Compare data mart.

Last updated: 2007-05-16

data warehousing

data warehouse

date

<convention, data>

A string unique to a time duration of 24 hours between 2 successive midnights defined by the local time zone. The specific representation of a date will depend on which calendar convention is in force; e.g., Gregorian, Islamic, Japanese, Chinese, Hebrew etc. as well as local ordering conventions such as UK: day/month/year, US: month/day/year.

Inputting and outputting dates on computers is greatly complicated by these localisation issues which is why they tend to operate on dates internally in some unified form such as seconds past midnight at the start of the first of January 1970.

Many software and hardware representations of dates allow only two digits for the year, leading to the year 2000 problem.

Unix manual page: date(1), ctime(3).

Last updated: 1997-07-11

DAU

/dow/ [German Fidonet] D"ummster Anzunehmender User. A German acronym for stupidest imaginable user. From the engineering-slang GAU for Gr"osster Anzunehmender Unfall (worst foreseeable accident), especially of a LNG tank farm plant or something with similarly disastrous consequences. In popular German, GAU is used only to refer to worst-case nuclear accidents such as a core meltdown.

See cretin, loser and weasel.