Data Dictionary - D

D

  • Data Addressing
  • Daemon
  • DASD
  • Data
  • Database
  • Data Bit
  • Data Chaining
  • Data Manipulation Language
  • Data Stream
  • DCD
  • Debug
  • Demodulation
  • Disk Buffer
  • Dots Per Inch
  • DVD
  • DVD+R & DVD-R
  • DVD+RW & DVD-RW
  • DVD-5
  • DVD-9

Data Addressing
There are 2 main methods of addressing the data located in a hard disk drive: CHS (cylinder head sector) and LBA (logical block address). CHS locates data by specifying the cylinder, head, and sector. While, LBA uses sequential number to locate sector of the drive.

Daemon
Pronounced “demon.” A program usually associated with UNIX systems that performs a utility (housekeeping or maintenance) function without begin requested or even known of by the user. A daemon sits in the background and is called into play only when needed – for example, to help correct an error from which another program cannot recover.

DASD
Sometimes pronounced “dazz-dee.” Acronym for direct access storage device, a data storage device on which information can be accessed directly, rather than by starting at the beginning of the data and passing sequentially over all intervening storage areas. Thus, a disk drive is a DASD unit. A tape is not a DASD unit because the data is stored as a linear block.

Data
Plural of the Latin datum, meaning an item of information. Following classical usage, one item of information should be called a datum, and more than one item should be called data: “The datum is,” but “the data are.” In practice, however, data is frequently used for the singular as well as the plural form of the noun.

Database
Loosely, any aggregation of data; a file consisting of a number of records (or tables), each of which is constructed of fields (columns) of a particular type, together with a collection of operations that facilitate searching, sorting, recombination, and similar activities.

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Data Bit
In asynchronous communications, a group of bits – typically 5,6,7, or 8 bits- used to represent a single character of data for transmission. The number of data bits used in a transmission must be agreed upon by sending and receiving parties. Each group of data bits in a transmission is preceded by a start bit and followed by an optional parity bit as well as one or more stop.

Data Chaining
The process of storing segments of data in discontinuous areas of a disk, a tape, or memory, while retaining the ability to reconnect the parts in proper order to re-create the whole – for example, continuing a file onto another tape or disk, or storing segments of one file on different parts of a disk.

Data Manipulation Language
Abbreviated DML. A language, usually part of a database management system, that is used to move data into (insert), change data in (update), or read data from (query) a database. The DML often contains features that ease report generation, including the ability to perform simple arithmetic, financial, and statistical calculations.

Data Stream
An undifferentiated, byte-by-byte flow of data. A data stream can be distinguished in practice from a block transfer, although the moving of blocks could itself be considered a “stream” (of coarser granularity).

DCD
Abbreviation for Data Carrier Detected, a signal used in serial communications; sent by a modem to its computer to indicate that the modem is on line and ready for transmission. DCD is a hardware signal sent over line 8 in RS-232 connections. DCD is also called RLSD (Received Line Signal Detect).

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Debug
With software, to detect, locate, and correct logical or syntactical errors in a computer program. With hardware, to detect, locate, and correct a malfunction or to fix an inoperable system. The term troubleshoot is more commonly used in hardware contexts – especially if the problem is suspected to be a major one.

Demodulation
The process of recovering information from a previously modulated carrier frequency. A modulated carrier is a wave, like sound, that has been modified in such a way that variations in such features as amplitude (wave height) and frequency (timing) represent meaningful information. In computer communications, demodulation is the means by which a modem converts data transmitted in audio form over a telephone line to the digital form that is needed by a computer, with as little distortion as possible.

Disk Buffer
A small amount of memory set aside for the purpose of storing data read from, or about to be written to, disk. Because disk devices are slow compared with the CPU, it is not efficient to access the disk for only one or two bytes of data. Instead, during a read, a large chunk of data is read and stored in the disk buffer. When the program wants information, it is copied from the buffer. Many requests for data can be satisfied by a single disk access. The same technique can be applied to disk writes. When the program has information to store, it writes it into the disk buffer area in memory. When the buffer are written to disk in a single operation.

Dots Per Inch
Abbreviated dpi. A measure of screen and printer resolution that is expressed as the number of dots that a device can print or display per linear inch.

DVD
Stands for Digital Video Disc or Digital Versatile Disc. A high-density optical compact disc for data and movie storage. The DVD disc uses the same diameter plate as a CD disc (4.75” diameter/ 12cm), but it is contains a larger memory space compared to a CD disc.

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DVD+R & DVD-R
A recordable (write) once DVD disk for storage of both data and movies.

DVD+RW & DVD-RW
A re-recordable (rewritable) DVD disk for storage of both data and movies.

DVD-5
A single sided and single layer DVD disk that holds 4.7GB.

DVD-9
A single sided and double layer DVD disk that holds 8.5GB.

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