[ Previous | Next Page | Table of Contents | Library Home | Legal | Search ]



DAC. See digital-to-analog converter.

daemon. A program that runs unattended to perform a standard service. Some daemons are triggered automatically to perform their task; others operate periodically. An example is the cron daemon, which periodically performs the tasks listed in the /var/spool/cron/crontabs directory. Synonym for demon.

daemon process. A process begun by the root user or the root shell that can be stopped only by the root user. Daemon processes generally provide services that must be available at all times, such as sending data to a printer.

DAF. Destination Address Field.

dangling else. A condition arising as a result of nesting an IF statement in the IF part of an IF-ELSE statement. The ELSE statement is associated with the closest IF statement, in this case, the inner one. Placing an empty ELSE statement in the nested statement prevents misinterpretation by forcing the outer ELSE statement to associate with the outer IF statement.

DAS. Dual-attachment station. A station that connects to both the primary and secondary FDDI rings.

DASD. Direct access storage device. A device in which access time is effectively independent of the location of the data. Information is entered and retrieved without reference to previously accessed data. DASDs include both fixed and removable storage devices.

data. A representation of facts or instructions in a form suitable for communication, interpretation, or processing by human or automatic means. Data includes constants, variables, arrays, and character strings.

data area. An area of memory that contains specific control variables that are normally predefined in structures or vectors.

data block. See block.

data cache. A cache for providing data to the processor faster than it can be obtained from RAM.

data cache unit (DCU). See cache.

data circuit. A pair of associated transmit and receive lines that provide a means of two-way data communications.

data-circuit-terminating equipment (DCE). In a data station, the equipment installed at the user's premises that provides all the functions required to establish, maintain, and end a connection, and the signal conversion and coding between the data terminal equipment (DTE) and the line.

data communications. See communications.

data consumer. In Performance Toolbox, a description of a program that receives statistics over the network from the xmservd daemon and prints, post-processes, or otherwise manipulates the raw statistics. Synonymous with client. Contrast with data supplier.

data definition. A program statement that describes the features of, specifies relationships of, or establishes the context of, data. A data definition can also provide an initial value. Definitions appear outside a function or at the beginning of a block statement.

data dependency. A situation in which a source operand for a computation is the result of a preceding computation.

data description. For data objects that are not self-describing, components of the data object that describe the data so that it may be processed.

Data Encryption Standard (DES). A data encryption algorithm widely used in the United States.

data item. A unit of data to be processed that includes constants, variables, array elements, and character substrings.

data link. (1.) The assembly of parts for two data workstations that are controlled by a link protocol and the interconnecting data circuit, which enables data to be transferred from a data source to a data sink. (2.) The interconnecting data circuit and the link protocol between two or more workstations, not including the data source or data sink. (3.) The physical connection and the connection protocols between units that exchange data over a telecommunications line. See also X.25 link.

data link control layer. In SNA and X.25, the layer that consists of the link stations that schedule data transfer over a link between two nodes and perform error control for the link.

data link control (DLC) protocol. In SNA, the set of rules used by two nodes on a data link to accomplish an orderly exchange of information.

data link escape (DLE) character. In BSC, a transmission control character usually used in transparent text mode to indicate that the next character is a transmission control character.

data-link level. In the hierarchical structure of a data station, the conceptual level of control or processing logic between high level logic and the data link that maintains control of the data link. The data link level performs such functions as inserting transmit bits and deleting receive bits; interpreting address and control fields; generating, transmitting, and interpreting commands and responses; and computing and interpreting frame check sequences. Synonym for frame level. See also packet level, physical level.

data lock. (1.) The insurance of data availability to a single application program as a protection against conflicting updates to a data record. (2.) The system lock that locks data segment into memory.

data object. A collection of data referred to by a single name. See also object.

data packet. In X.25 communications, a packet used for the transmission of user data on a virtual circuit at the DTE/DCE interface. See also packet.

data storage interrupt. An interrupt posted when a fault is encountered accessing storage or I/O space. A typical data storage interrupt is a page fault or protection violation.

data stream. (1.) All information (data and control information) transmitted over a data channel in a single read or write operation. (2.) A continuous stream of data elements being transmitted, or intended for transmission, in character or binary-digit form using a defined format. (3.) All information sent to the terminal device driver with a write subroutine. Synonymous with stream.

data supplier. In Performance Toolbox, a program that supplies statistics across a network. Synonymous with server. Contrast with data consumer.

data terminal equipment (DTE). (1.) The part of data processing unit that serves as a data source, data sink, or both. (2.) The user of the network.

data terminal ready (DTR). A signal to the modem used with EIA-232 protocol.

data token. In SOM, a value that identifies a specific instance variable within an object whose class inherits the instance variable (as a result of being derived, directly or indirectly, from the class that introduces the instance variable). An object and a data token are passed to the SOM run-time procedure, somDataResolve, which returns is a pointer to the specific instance variable corresponding to the data token. See also instance token.

data transfer. The movement, or copying, of data from one location and the storage of the data at another location.

data type. (1.) In programming languages, a set of values together with a set of permitted operations. (2.) The mathematical properties and internal representation of data and functions. (3.) An attribute used for defining data as numeric or character. (4.) The type, format, or classification of a data object. (5.) In CDE, a mechanism that associates particular data files with the appropriate applications and actions. Data types can determine the type of a file based on file-naming conventions, such as a particular extension name, or on the contents of the file. See also character type and type.

database. A collection of facts and instructions comprising at least one file that is sufficient for a given purpose.

datagram. (1.) In packet switching, a self-contained packet, independent of other packets, that carries information sufficient for routing from the originating data terminal equipment (DTE) to the destination DTE without relying on earlier exchanges between the DTEs and the network. (2.) In DCE, an unreliable network data packet that is independent of all other packets and lacks any guarantees of delivery or sequentiality.

datagram protocol. A connectionless, datagram-based transport protocol, such as UDP; an RPC protocol that runs over a connectionless transport protocol.

dataless. A workstation without local file systems or local boot images that accesses some of its resources remotely. Dataless clients use a local disk used for paging and dump devices.

dB. See decibel.

DB. Database.

DBA. See direct-bus attached.

DBCS. See Double-Byte Character Set.

D-bit. In X.25 communications, the bit in a data packet or call-request packet that is set to 1 if end-to-end acknowledgment (delivery confirmation) is required from the recipient.

DC. See DCU.

DCD. Data carrier detect used with EIA-232 protocol. See also CD and decode.

DCE. See Distributed Computing Environment and data-circuit-terminating equipment.

DCN. Distributed Computer Network.

DCU. Data cache unit. See also cache.

DD. See device driver.

DDN. Department of Defense Network.

dead code elimination. A compiler optimization that removes code that is never referenced, or that is always branched over. A compiler optimization that removes store instructions for data entities whose final values are not used.

dead letter file. A file containing mail messages that could not be sent to a proper destination file.

dead variable. In Ada language, a variable that is initialized, but is not used within the context of the program. Like unreachable code, dead variables are detected and removed by the optimizer.

dead zone. An area of a tablet from which no input reports are generated. Each virtual terminal can set its own dead zones. Synonymous with no-input zone.

deadlock. (1.) An error condition in which processing cannot continue because each of two elements of the process is waiting for an action by or a response from the other. (2.) Unresolved contention for the use of a resource. (3.) An impasse that occurs when multiple processes are waiting for the availability of a resource that does not become available because it is being held by another process that is in a similar wait state.

deallocate. To release a resource assigned to a specific task. Contrast with allocate.

DEALLOCATE. A request to remove the allocation of the specified conversation from the local transaction program.

deallocation. An operation that removes a client's permission to use a resource.

debug. To detect, locate, and correct errors in the configuration of a computer system or a software program.

debugger. A program or programs used to detect, trace, and eliminate errors in computer programs or software.

debugging. Acting to detect and correct errors in software or system configuration.

debugging mode. A special mode in which a program provides detailed output about its activities to aid a user in detecting and correcting errors in the program itself or in the configuration of the program or system.

decibel (dB). (1.) One tenth of a bel. (2.) A unit of signal strength, such as the signal on a data communications channel. (3.) A unit for measuring relative power. The number of decibels is 10 times the logarithm (base 10) of the ratio of the measured power levels.

decimal. (1.) Pertaining to a system of numbers to the base 10. The decimal digits range from 0 through 9. (2.) Characterized by a selection, choice, or condition that has 10 possible different values or states.

decimal constant. A number containing any digits 0 through 9.

declaration. (1.) A description that makes a defined object available to a function or a block. (2.) In programming languages, the mechanism for establishing a language object. A declaration normally involves attaching an identifier and allocating attributes to the language object concerned. (3.) In a programming language, a meaningful expression that affects the interpretation of other expressions in that language. (4.) In Ada language, a declaration associates an identifier (or some other notation) with an entity. This association is in effect within a region of text called the scope of the declaration. Within the scope of a declaration, there are places where it is possible to use the identifier to refer to the associated declared entity. At such places the identifier is said to be a simple name of the entity; the name is said to denote the associated entity. See also declare, simple name, scope, and name.

declarative part. In Ada language, a sequence of declarations. It may also contain related information such as subprogram bodies and representation clauses.

declarator. An identifier and optional symbols that describe the data type.

declare. A selection available from the Interpreter's Interpret menu that allows a user to assign variable names and structure definitions. See also declaration.

decode. (1.) To convert data by reversing the effect of some previous encoding. (2.) To interpret a code. See also DCD.

default. A value, attribute, or option that is assumed when no alternative is specified by the user. See also default value.

default accelerators. See accelerator.

default arguments. Arguments that are declared with default values in a C++ function prototype or declaration. If a call to the function omits these arguments, default values are used. Arguments with default values must be the trailing arguments in a function prototype argument list.

default button labels. In AIXwindows, XmLabel widgets or gadgets that are used when no other button label has been specified.

default cell (global access). An NCS cell that allows access from any node in the network. This is the most common cell used in License Use Management configurations because it allows all other License Use Management servers in the default cell to communicate freely.

default clause. In a C switch statement, the keyword default followed by a colon and one or more C statements. When the conditions of the specified case labels in the switch statement do not hold, the default clause is chosen.

default constructor. A C++ constructor that takes no arguments, or if it takes any arguments, all its arguments have default values.

default device. The device attached to your computer (such as a printer or disk drive) that is used when no alternative is specified by the operator.

default directory. The directory name supplied by the operating system if none is specified.

default drive. The drive name supplied by the operating system if none is specified.

default files. Data files in which resource default values are stored in ASCII form to permit the assignment of alternative resource values at run time without need for rewriting or recompiling source code.

default initialization. The initial value assigned to a data object by the compiler if no initial value is specified by the programmer. In C language, external and static variables receive a default initialization of zero, while the default initialization for auto and register variables is undefined. See also default value.

default label. See label.

default printer. A printer that accepts all the printed output from a display station assigned to it.

default shell. In AIXwindows, the shell that is used when no other shell properties have been specified.

default value. A value stored in the system that is used when no other value is specified. See also default and default initialization.

defaults file. See default files.

define. Creates an entry in the ODM Customized Devices Database and establishes the parent device and connection location.

define method. Used to create a device instance in the ODM Customized Database. It takes a device from the undefined or nonexistent state to the defined state.

define statement. A preprocessor statement that causes the preprocessor to replace an identifier or macro call with specified code.

defined state. The state a device is put into when its defined method is run or when an available device's unconfigure method is run. The device is not a usable device at this point.

DEL. See delete character.

delayed port. A port that is enabled like a shared port except that the login herald is not displayed until you type one or more characters (usually carriage returns). A port directly connected to a remote system or intelligent modem is usually enabled as a delayed port.

delete. (1.) To remove. For example, to delete a file. See also erase. (2.) The C++ keyword delete identifies a free store deallocation operator. In C++, the delete operator is used to destroy objects created by new.

delete character (DEL). (1.) A control character used primarily to obliterate an erroneous or unwanted character. (2.) A character that identifies a record to be removed from a file.

delimiter. (1.) A character or sequence of characters that marks the beginning or end of character string or unit of data. (2.) A character that groups or separates words or values in a line of input.

deliver (callback). Delivering a callback or upcall means to cause its invocation.

delivery-confirmation bit. See D-bit.

delta. The finite increment of a variable.

demangling. The conversion of mangled C++ names back to their original source code names. During compilation, identifiers such as function and static class member names are mangled (encoded) with type and scoping information to ensure type-safe linkage. These mangled names appear in the object file and the final executable file. Demangling converts these names back to their original names to make program debugging easier.

demon. Synonym for daemon.

denote. See declaration.

density. In printing, refers to the number of characters per inch horizontally.

dependency line. The first line of an entry in a description file. It contains a list of target files followed by a colon and an optional list of prerequisite files or dependencies.

dependent. A software product that requires another product or update to be installed before or at the same time it is installed. Contrast with prerequisite.

dependent workstation. A workstation having little or no stand-alone capability that must be connected to a host or server in order to provide any meaningful capability to the user.

dependents. Ada-language compilation units that would have to be recompiled if another unit were to be recompiled because of the compilation order imposed by the Ada language.

depth. (1.) In Enhanced X-Windows, the number of bits per pixel for a window or pixmap. (2.) In a three-dimensional context, the second dimension.

depth-cueing. In 3D computer graphics, varying the intensity of a line with depth. Typically, the points on the line further from the eye are darker, so the line seems to fade into the distance.

dequeue. To remove items from a queue. Contrast with enqueue.

dereferenced pointer. In Pascal, an expression using the -> or @ operator used to locate a dynamic variable from a pointer.

derivation. The process of deriving a C++ class from an existing class, called a base class.

derived class. A C++ class that inherits the properties of a base class. You can add additional data members and member functions to the derived class. A derived class object can be manipulated as if it were a base class object. The derived class can override virtual functions of the base class.

derived metaclass. (Or SOM-derived metaclass.) A metaclass that SOM creates automatically (often even when the class implementor specifies an explicit metaclass) as needed to ensure that, for any code that executes without method-resolution error on an instance of a given class, the code will similarly execute without method-resolution error on instances of any subclass of the given class. SOM's ability to derive such metaclasses is a fundamental necessity in order to ensure binary compatibility for client programs despite any subsequent changes in class implementations.

derived type. In Ada language, a type whose operations and values are replicas of those of an existing type. The existing type is called the parent type of the derived type. See also parent type.

DES. See Data Encryption Standard.

descendant. See child.

descending key sequence. The arrangement of data in order from the highest value of the key field to the lowest value of the key field.

descriptor. (1.) In ODM, a named and typed variable that defines a single characteristic of an object. See also terminal descriptor, link descriptor, and method descriptor. (2.) In information retrieval, a parameter word used to categorize or index information. (3.) In XOM, the means by which the client and service exchange an attribute value and the integers that denote its representation, type, and syntax. (4.) In XDS, a defined data structure that is used to represent an OM attribute type and a single value. (5.) In SOM, an ID representing the identifier of a methoddefinition or an attribute definition in the Interface Repository. The IR definition contains information about the method's return type and the type of its arguments.

deselect. To cancel the selection of a button. With a mouse, you deselect a highlighted area with the Select (left) button. Otherwise, you can use the Select key on the keyboard. To deselect a default button, select an alternate button in the selection list.

deserialize. (1.) To change from serial-by-bit to parallel-by-byte. (2.) In XDR, to change from XDR format to a particular machine representation.

desktop. A visual representation of a group of objects in your system, brought together to help you organize your work.

destination cursor. A point or location marked by the cursor to which data is to be pasted or inserted.

destination disk. The disk to which you are installing.

destructor. A special member function of a class with the same name as the class with a ~ (tilde) preceding the name. You cannot specify arguments or a return type for this function. A destructor "cleans up" after an object by doing such things as freeing any storage that was dynamically allocated when the object was created.

device. (1.) A mechanical, electrical, or electronic machine that is designed for a specific purpose and that attaches to your computer, such as a printer, plotter, or disk drive. (2.) A valuator, button, or the keyboard. Buttons have values of 0 or 1 (up or down); valuators (mouse, dials) return values in a range, and the keyboard returns ASCII values.

device class. Functional grouping of devices. The generic name for a group of device types, for example, all display stations belong to the same device class. Contrast with device type.

Device Configuration Database. Stores all information relevant to support the device configuration process. It consists of a Predefined Database and a Customized Database. See also Predefined Database and Customized Database.

device definition. Information about a device that is in the Customized Database including attributes and connection locations.

device description. Text used to give a short description of the device. For example, the device description for the token-ring adapter might be "Token-Ring High-Performance Adapter."

device driver (DD). (1.) A program that operates a specific device, such as a printer, disk drive, or display. (2.) A collection of subroutines that control the interface between I/O device adapters and the processor.

device handler. The component of a device driver that communicates directly with the hardware. Synonymous with virtual device driver.

device head. The component of a device driver that implements the application program interface to a device.

device instance. When a device is defined, a Customized Devices Object Class entry is created. This entry is considered a device instance. There is a device instance for each device defined in the system.

device location. Indicates the location path of a device. This is a field in the Customized Devices Object Class.

device manager. For complex interfaces, a collection of routines that acts as an intermediary between drivers and virtual machines. For example, supervisor calls from a virtual machine are examined by a device manager and routed to the appropriate subordinate device drivers.

device name. (1) The logical or symbolic name reserved by the system to refer to a specific device. (2) SNA uses the operating-system device name of the network adapter to get information that defines the interface. See also logical name.

device number. The reference number assigned to any external device.

device stanza. Defines a device attached to a queue in the print spooling system. A device stanza contains all information pertaining to the device (usually a printer) and is found in the /etc/qconfig file.

device state. Indicates the current configuration status of a device instance. Possible values are defined, available, and stopped. This is a field in the Customized Devices Object Class.

device subclass. Distinguishes devices within the same functional class. It is used to indicate different interfaces. For example, the printer class has three subclasses: rs232, rs422, and parallel.

device switch table. (1) A table that is used as an interface to the device drivers. (2) A table that contains a pointer to the entry points for each device head.

device type. The general name for a kind of device sharing the same model number; for example, 2311, 2400, 2400-1. Contrast with device class.

DFT. See distributed function terminal.

DHCP. Dynamic Host Configuration Protocol. An application-layer protocol that allows a machine on the network, the client, to get an IP address and other configuration parameters from the server.

diacritic. A diacritic is a mark added to a letter to indicate a special phonetic value. Diacritics are implemented by a nonspacing character sequence, a two-key sequence consisting of one of 13 diacritics followed by an alphabetic character or a space. The terminal device driver converts this two-key sequence into a single code point.

diagnostic. Pertaining to the detection and isolation of errors in programs and faults in equipment.

diagnostic aid. A tool (procedure, program, or reference manual) used to detect and isolate a device or program malfunction or error.

diagnostic code. In X.25 communications, a 1-byte code included in clear- and reset-indication packets that gives information about the reason for sending the packet. See also cause code.

Diagnostic Control Program. The top-level control program and configuration manager for diagnostics. It traverses the configuration database, testing resources and their interdependencies. It analyzes conclusions from diagnostic applications and generates a problem report.

diagnostic output. Error or status messages produced by processes, in addition to those messages produced by standard output. Synonymous with error output.

dial. (1) A computer input device that allows a user to set parameter values. Dials are a type of valuator. See also valuator.(2) An I/O device used to input variables by way of thumbwheels.

dial and switch box. An I/O device with 8 dials (valuators) and 32 switches. The switch box is also called a "button box" or the "lighted programmable function keys (LPFKs)."

dial-code. In BNU, a code representing a telephone number or portion of a telephone number.

dialing directory. In ATE, a list of telephone numbers that can be called with Asynchronous Terminal Emulation (ATE). It is similar to a page in a telephone directory.

dialog. In an interactive system, a series of related inquiries and responses similar to a conversation between two people.

Dialog. In AIXwindows, a two-way text interface between an application and its user. The interface takes the form of a collection of widgets and gadgets, including a DialogShell widget, a BulletinBoard widget (or a subclass of a BulletinBoard widget or some other container widget), plus various children, including Label, PushButton, and Text widgets.

dialog box. A window that is displayed when further information is needed from the user, or when the system needs to display information. A line enclosure on a structure, such as a widget or shell, that contains a specific, two-way text interface.

dialog shell. A subclass of transient shell, this shell is used for dialog boxes. See also dialog widget.

dialog widget. Any of a class of widgets chosen through the Create menu's Dialog submenu; dialog widgets always have the dialog shell as their implicit shell. See also dialog shell.

digest. Data that has been organized into a format that provides for quick access to each piece of data.

digit. (1) A character that represents a nonnegative integer. Synonymous with numeric character. (2) A symbol that represents one of the nonnegative integers smaller than the radix. (3) Any of the numerals from 0 through 9.

digital-to-analog converter (DAC). (1) A highly specialized chip that converts the digital values coming out of the frame buffer into the rapidly varying analog voltage levels that are required by the monitor. (2) That portion of the display subsystem that converts pixels into colors or grayscale.

dimension. The attribute of size given to arrays and tables.

direct addressing. (1) An addressing method that uses an expression as an operand entry to represent an instruction address. (2) A method of addressing in which the address part of an instruction contains a direct address.

direct-bus attached. Used in relation to the fixed disk drives attached directly to the system board rather then through a SCSI adapter card.

direct color. (1.) In Enhanced X-Windows, a class of color map in which a pixel value is decomposed into three separate subfields for indexing. One subfield indexes an array to produce red intensity values, the second indexes another array for blue intensity values, and the third for green intensity values. The RGB values can be changed dynamically. This is mutually exclusive to the Pseudocolor color map color. (2.) Also DirectColor, a value.

direct connection. The attachment of a system, workstation, or other I/O device through a selected communication interface and a limited-length cable. No modem is required.

direct-mapped cache. A cache in which exactly one line corresponds to each possible value of the virtual-address field that identifies the line to be interrogated.

direct memory access (DMA). (1.) The transfer of data between memory and an input/output device without processor intervention. (2.) General term given to the system facility that allows a device on the Micro Channel bus to get direct access to the system or bus memory without the intervention of the system processor. The system-supplied resource that provides this facility and mediates the data transfers between the memory and devices is the DMA controller.

direct visibility. See visibility.

directional component. In AIXwindows, a portion of a compound string that specifies a direction with a given value. The directional component is created with the XmStringDirectionCreate function.

directive. In SOM, a message (a pre-defined character constant) received by a replica from the Replication Framework. Indicates a potential failure situation.

directory. (1.) A type of file containing the names and controlling information for other files or other directories. (2.) A table of identifiers and references to the corresponding items of data. (3.) An index used by a control program to locate blocks of data that are stored in separate areas of a data set in direct access storage. (4.) Contrast with special file. (5.) A listing of related files arranged in a useful hierarchy. (6.) In CDS, a logical unit for storing entries under one name (the directory name) in a CDS namespace. In addition to object entries, a directory can contain soft links and child pointers. You can copy, delete, and control access to a directory. Each physical instance of a directory is called a replica. (7.) In XDS, a collection of open systems that cooperate to hold a logical database of information about a set of objects in the real world. (8.) In CDE, a collection of files and other subdirectories. In graphical user interface applications, may be called a folder.

directory ID. See directory identifier.

directory identifier (directory ID). An identifier for distinguishing several configurations of the directory service within an installation.

directory mask. A pattern of characters that controls which portions of a directory will be retained and which portions will not be retained.

dirty object. In SOM, a persistent object that has been modified since it was last written to persistent storage.

disable. (1.) To make nonfunctional. In interactive communications, to disconnect or stop a subsystem. Contrast with enable. (2.) To bring a queue or a device attached to a queue off line so that no print jobs get sent to it.

disabled port. In Asynchronous Terminal Emulation (ATE), a port configuration indicating that a port is ready to call out.

DISC. Disconnect.

discipline. Ordering method used to line up jobs for printing, FCFS (first-come-first-served) or SJN (shortest-job-next). See also first-come-first-served and shortest-job-next.

disconnect. In X.25 communications, to disconnect a port from the X.25 network.

disconnected mode (DM). In SDLC, a response from a secondary station indicating that it is disconnected and wants to be online.

discrete type. In Ada language, a type that has an ordered set of distinct values. The discrete types are the enumeration and integer types. Discrete types are used for indexing and iteration, and for choices in case statements and record variants.

discretionary access control. A security mechanism that protects information from unauthorized disclosure or modification through owner-controlled access to files. See also access control list, base permission, and extended permission.

discriminant. In Ada language, a distinguished component of an object or value of a record type. The subtypes of other components, or even their presence or absence, may depend on the value of the discriminant.

discriminant constraint. In Ada language, on a record type or private type, specifies a value for each discriminant of the type.

discriminated union. In XDR, a C language union that holds several data types, with one arm of the union being an enumeration value, or discriminant, which holds a specific object to be processed over the system first.

disk. A storage device made of one or more flat, circular plates with magnetic surfaces on which information can be stored.

disk adapter. The hardware used by a computer to access and control disk drives. On POWERstations and POWERservers, disk adapters each fit into a single Micro Channel slot, and each are controlled by software device drivers.

disk drive. The mechanism used to read and write information on a disk.

disk I/O. Fixed-disk input and output.

Disk Operating System. See DOS.

disk-usage accounting. The record of the number of disk blocks occupied by a user's files. Disk-usage accounting is performed by the acctdisk command.

diskette. A thin, flexible magnetic plate that is permanently sealed in a protective cover. It can be used to store information copies from the disk or another diskette.

diskette drive. The mechanism used to read and write information on diskettes.

diskless. A workstation without local file systems or local boot images that accesses some of its resources remotely. Diskless clients boot remotely from a diskless server and use the server for remote paging.

Diskless Workstation Manager (DWM). Operating-system software that initializes and maintains resources for diskless clients and diskless servers. It is a group of commands, awk command scripts, and source code.

dispatch. To allocate processing time on a specific device for a job that is ready to run.

dispatch-function resolution. Dispatch-function resolution is the slowest, but most flexible, of the three method-resolution techniques SOM offers. Dispatch functions permit method resolution to be based on arbitrary rules associated with an object's class. Thus, a class implementor has complete freedom in determining how methods invoked on its instances are resolved. See also dispatch method and dynamic dispatching.

dispatch method. In SOM, a method(such as somDispatch or somClassDispatch) that is invoked (and passed an argument list and the ID of another method) in order to determine the appropriate method procedure to execute. The use of dispatch methods facilitates dispatch-function resolution in SOM applications and enables method invocation on remote objects in DSOM applications. See also Dynamic Invocation Interface, dynamic dispatching, and dispatch-function resolution.

dispatcher. In XOM, the software that implements the service interface functions using workspace interface functions.

displacement. (1.) A positive or negative number that can be added to the contents of a base register to calculate an effective address. (2.) The distance from the beginning of a record, block, or segment to the beginning of a field.

display. (1.) A visual presentation of data. (2.) To present data visually. (3.) A computer output screen on which visual information is displayed. (4.) In Enhanced X-Windows, a set of one or more screens and input devices that are driven by a single X Server. Synonym for monitor.

display attribute. In XGSL, the following are some of the attributes associated with displays: (1.) APA (all points addressable) or character. An APA display can address each pixel, while a character display addresses character-sized blocks of pixels, (2.) blink supported or not supported, (3.) color or monochrome, and (4.) changeable color palette or fixed color palette.

display device. See display.

display list (object). In GL, also called an object. It is a sequence of drawing commands that have been compiled into a unit. Conceptually, a display list is like a macro: it can be invoked multiple times simply by referring to its name. The object can be instantiated at different locations, sizes, and orientations by appropriate use of the transformation matrices. For instance, series of polygons arranged in the shape of a bolt can be compiled into an object. The bolt can then be drawn multiple times by invoking its display list.

Display PostScript (DPS). Extension to X server created by Adobe.

display screen. The part of the display device that displays information visually. Synonymous with terminal screen. See also screen.

display session. A 3270 Host Connection Program 2.1 and 1.3.2 (HCON) mode of operation during interaction with a host computer that emulates a 3278/79 terminal display.

display station. An input/output device that includes a keyboard from which an operator can send information to the system and a display screen on which an operator can see also the information sent to or received from the computer.

display symbol. A predefined printable graphics symbol (such as, characters, numbers, math symbols, Greek letters, and so on) that can be displayed on a graphics display.

display symbol set. A set of display symbols placed in a table. There are up to 1024 display symbols. Display symbols 0 through 31 represent control functions and have no graphic representation.

distinguished encoding. The restrictions to the Basic Encoding Rules designed to ensure a unique encoding of each ASN.1 value, defined in the X.500 Directory Standards (CCITT X.509).

distortion. In data communications, an undesirable change in a wave form that can occur between two points in a transmission system. The six major forms of distortion are bias, characteristic, delay, end, fortuitous, and harmonic.

distributed. The programs and computerized sources of information that make up a computing environment can be physically located on different computer systems, while still working together as a single logical unit. Transaction processing systems are easily moved to a distributed computing environment, because these systems traditionally involve modifying a centralized source of information by submitting modification or information requests from remote terminals.

Distributed Computing Environment (DCE). A computing environment in which the resources and data may be located on different processors.

distributed file system. A file system composed of files or directories that physically reside on more than one computer in a communications network.

distributed free space. Synonym for free space.

Distributed Function Terminal (DFT). A workstation that performs operations previously accomplished by the processing unit, such as managing data links, controlling devices, and formatting data.

distributed transaction. A transaction which can update data in many user processes on many machines.

distribution medium. The medium on which the operating system software, a licensed program, or an application program is distributed to the user. The distribution medium can be any of several different media supported by the hardware, such as streaming cartridge tape, 9-track tape, or 3.5-inch diskette.

dithering. In computer graphics, a technique of interleaving dark and light pixels so that the resulting image looks smoothly shaded when viewed from a distance.

diversion. In text formatting, a command used to save text for printing later in a document, such as for footnotes.

DIX connector. A device that connects an Ethernet network adapter to a standard ("thick") Ethernet local area network (LAN). Its name is derived from the names of the principal developers of Ethernet (Digital Equipment Corporation, Intel, and Xerox).

DLC. See data link control protocol.

DLE. See data link escape character.

DLPI. Data Link Provider Interface.

DLS. Data link service.

DLS provider. In X.25 communications, the data link layer protocol that provides the services of the Data Link Provider Interface (DLPI).

DLS user. In X.25 communications, the user-level application or user-level or kernel-level protocol that accesses the services of the data link layer.

DLSAP (data-link-service access point). In X.25 communications, a point at which a data link service (DLS) user attaches itself to a DLS provider to access data link services.

DLSAP address. In X.25 communications, an identifier used to differentiate and locate specific DLS user access points to a DLS provider.

DM. See disconnected mode.

DMA. See direct memory access.

DMA slave. A device on the Micro Channel bus that uses the system-provided DMA facilities instead of having a built-in controller. See also Bus Master.

DO loop. In FORTRAN, a range of statements called repetitively by a DO statement.

do statement. (1.) In C language, a looping statement that contains the keyword do followed by a statement (the action), the keyword while, and an expression in parentheses (the condition). (2.) A statement used to group a number of statements in a procedure.

DO variable. In FORTRAN, a variable, specified in a DO statement, that is incremented or decremented on each iteration of the relative DO loop and controls the number of iterations of the loop.

domain. (1.) That part of a network in which the data processing resources are under common control. (2.) In a database, all the possible values of an attribute or a data element. (3.) In TCP/IP, the naming system used in hierarchical networks. The domain naming system uses the DOMAIN protocol and the named daemon. In a domain system, groups of hosts are administered separately within a tree-structured hierarchy of domains and subdomains.

domain name. A level in the hierarchy of names used throughout the Internet.

DOS (Disk Operating System). A disk operating system used on personal computers.

dot. A symbol (.) that indicates the current directory in a relative path name.

dot dot. A symbol (..) in a relative path name that indicates the parent directory.

dot matrix. A printer with wires or other means that uses a matrix of dots for printing characters.

dotted decimal. A common notation for Internet host addresses that divides the 32-bit address into four 8-bit fields. The value of each field is specified as a decimal number and the fields are separated by periods (for example, or

double buffer mode. In GL, a mode in which two buffers are alternately displayed and updated. A new image can be drawn into the back buffer while the front buffer (containing the previous image) is displayed. See also single buffer mode.

Double-Byte Character Set (DBCS). A set of characters in which each character is represented in 2 bytes of storage. Languages such as Japanese, Chinese, and Korean, which contain more symbols than can be expressed in a single byte, are represented in this character set.

double-click. To click twice in rapid succession without moving the mouse pointer.

double precision. (1.) Pertaining to the use of two computer words to represent a number in accordance with the required precision. (2.) A specification that causes a floating-point value to be stored internally in the long format. See also precision.

double-strike. A process of printing a character twice to create the appearance of bold type, used frequently with impact printers. A more flexible form of double-strike is emphasized printing. See also boldface and emphasized.

double-wide print. A print format in which characters are twice as wide as they normally are.

download. To transfer data from one computer for use on another one. Typically, users download from a larger computer to a diskette or fixed disk on a smaller computer or from a system unit to an adapter.

downstream. The direction from stream head to driver.

downward jump. For the bfs command, the act of moving from the current location in a file toward the bottom or end of the file.

DPS. See Display PostScript.

DR1I. Definite response 1 indicator.

DR2I. Definite response 2 indicator.

drag. To point to an object with the mouse pointer, hold down the mouse button, move the mouse, and then release the mouse button. A method of "dragging" an object to a desired point.

DRAM. See dynamic random access memory.

drawable. In Enhanced X-Windows, a collective term for both windows and pixmaps when used as destinations in graphics operations. However, an InputOnly window cannot be used as a source or destination drawable in a graphics operation.

drawn button. In AIXwindows, a graphic object that simulates a real-world button with a symbol or other image drawn on its face.

driver. The end of a stream closest to an external interface. The principal functions of the driver are handling any associated device and transforming data and information between the external device and stream. It can also be a pseudo-driver, not directly associated with a device, which performs functions internal to a stream, such as a multiplexer or log driver.

driving table. A table that describes all the printer-specific information for the nroff command.

drop. In CDE, after starting the drag of an object, the act of releasing the mouse button. If the object is dropped in an appropriate area, an action is initiated.

drop-in. Some applications can detect that an icon has been dropped on to their window to perform an appropriate action, such as opening a document.

drop target. In CDE, a rectangular graphic that represents the drop zone in an application.

drop zone. In CDE, an area of the workspace, including the Trash Can, Print Manager, and Mailer Front Panel controls, that accepts a dropped icon. Icons can be dropped on the workspace for quick access.

dropped folio. A page numbering style in which the page number is printed at the foot of the page. See also folio, blind folio, and expressed folio.

dropping locks. Releasing the locks that a transaction holds on data.

DSI. See data storage interrupt.

DSR. Data set ready; used with EIA-232 protocol. See your modem manual for more information.

DTE. See data terminal equipment.

DTR. See data terminal ready.

dummy argument. In FORTRAN, a variable within a subprogram or statement function definition with which actual arguments from the calling program or function reference are positionally associated. See also formal parameter.

dump. (1.) To copy the contents of all or part of storage onto another data medium or to an output device. (2.) Data that has been dumped.

dump data. The data collected by the kernel dump program. It is obtained from memory locations used by kernel components.

dump table entry. A record in the master dump table that identifies the location of a component dump table. All kernel components that need to have special data collected by the dump program need to generate a dump table entry.

duplex. Pertains to communications data that can be sent and received at the same time. Synonymous with full duplex and FDX. Contrast with half duplex.

duplex connector. In a fiber channel link environment, the component that terminates both jumper cable fibers in one housing and provides physical keying for attachment to a duplex receptacle.

duplexed output. Output that uses both the front and back of each sheet of paper for printing.

DWM. See Diskless Workstation Manager.

dyadic operator. Synonym for binary operator.

dynamic. A style of creating pop-up menus.

dynamic block header. A data structure used by a compiler to link dynamic variables that are in the same heap.

dynamic connection. A connection that is established when needed rather than being predetermined or fixed.

dynamic dispatching. In SOM, method dispatching using dispatch-function resolution; the use of dynamic method resolution at run time. See also dispatch method, dispatch-function resolution, and dynamic method.

Dynamic Invocation Interface (DII). In SOM, the CORBA-specified interface, implemented in DSOM, that is used to dynamically build requests on remote objects. Note that DSOM applications can also use the somDispatch method for dynamic methodcalls when the object is remote. See also dispatch method.

dynamic license. A license that specifies multiple nodelocked licenses. The license is installed at a license server, and then the license server derives license passwords (each of which specifies a single nodelocked license) from the dynamic license. The server automatically installs the individual licenses at the user nodes from which the licensed product is invoked.

dynamic linking. Linking of a program in which library procedures are not incorporated into the load module but are dynamically loaded from their library each time the program is loaded.

dynamic method. A SOM method that is not declared in the IDL interface statement for a class of objects, but is added to the interface at run time, after which instances of the class (or of its subclasses) will respond to the registered dynamic method. Because dynamic methods are not declared, usage bindings for SOM classes cannot support their use; thus, offset method resolution is not available. Instead, name-lookup or dispatch-function method resolution must be used to invoke dynamic methods. (There are currently no known uses of dynamic methods by any SOM applications.) See also dynamic dispatching, method, and static method.

dynamic random access memory. A storage in which the cells require repetitive application of control signals to retain stored data. Such repetitive application of the control signals is normally called a refresh operation.

dynamic routing. A method of setting paths between hosts, networks, or both by using daemons that update the routing table as needed.

dynamic string. See string.

dynamic variable. A variable allocated only when needed. Explicit allocations and deallocations are required. In Pascal, the predefined procedures NEW and DISPOSE are provided for this purpose.

dynamic window. A variable that can change dynamically within a certain window or range of values.

[ Previous | Next Page | Table of Contents | Library Home | Legal | Search ]