DDK Glossary - D

Glossary

DASD: Direct-access storage device. A device in which access time is effectively independent of the location of the data.

data bus: A bus used to communicate data internally and externally to and from a processing unit, storage, and peripheral devices. (A) See bus.

data segment: A nonexecutable section of a program module; that is, a section of a program that contains data definitions.

data structure: The syntactic structure of symbolic expressions and their storage-allocation characteristics. (T)

data transfer: The movement of data from one object to another by way of the clipboard or by direct manipulation.

DBCS: Double-byte character set. A set of characters in which each character is represented by two bytes. Languages such as Japanese, Chinese, and Korean, which contain more characters than can be represented by 256 code points, require double-byte character sets. Because each character requires 2 bytes, the typing, display, and printing of DBCS characters requires hardware and programs that support DBCS. Contrast with single-byte character set.

DDB: Device-dependent bit map.

DDE: Dynamic data exchange. A message protocol used to communicate between applications that share data. The protocol uses shared memory as the means of exchanging data between applications.

deadlock: (1) Unresolved contention for the use of a resource. (2) 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. (3) An impasse that occurs when multiple processes are waiting for the availability of a resource that will not become available because it is being held by another process that is in a similar wait state.

debug: To detect, diagnose, and eliminate errors in programs. (T)

decipoint: In printing, one tenth of a point. There are 72 points in an inch.

default procedure: A function provided by the Presentation Manager Interface that may be used to process standard messages from dialogs or windows.

default value: A value assumed when no value has been specified. Synonymous with assumed value. For example, in the graphics programming interface, the default line-type is 'solid'.

definition list: A type of list that pairs a term and its description.

deinstantiation: See instantiation.

delta: An application-defined threshold, or number of container items, from either end of the list.

Deselect all: A choice that cancels the selection of all of the objects that have been selected in that window.

Desktop Manager: In the Presentation Manager, a window that displays a list of groups of programs, each of which can be started or stopped.

desktop window: The window, corresponding to the physical device, against which all other types of windows are established.

detached process: A background process that runs independent of the parent process.

detent: A point on a slider that represents an exact value to which a user can move the slider arm.

device context (DC): A logical description of a data destination such as memory, metafile, display, printer, or plotter. See also direct device context, information device context, memory device context, metafile device context, queued device context, and screen device context.

device driver: A file that contains the code needed to attach and use a device such as a display, printer, or plotter.

device driver initialization (init) time: See initialization (init) time, device driver.

device driver profile: A file with a "DDP" extension, containing a script that is interpreted by the OS/2 DDINSTAL utility. Among other things, it defines which files to copy from installation diskettes to target directories and specifies how the CONFIG.SYS file will be updated.

device helper (DevHlp): (1) A kernel service (memory, hardware interrupt, software interrupt, queuing, semaphore, and so forth) provided to physical device drivers. (2) A callable C-language or assembler-language routine that provides an operating system service for an OS/2 device driver.

device object: A device that provides a means of communication between a computer and the outside world. A printer is an example of a device object.

device space: (1) Coordinate space in which graphics are assembled after all GPI transformations have been applied. Device space is defined in device-specific units. (2) (D of C) In computer graphics, a space defined by the complete set of addressable points of a display device. (A)

device table: A data structure containing a summary of the adapters an adapter device driver supports and a list of the I/O devices attached to each adapter. This data structure is built by the adapter device driver in response to an IOCC_CONFIGURATION IOCM_GET_DEVICE_TABLE request.

dialog: The interchange of information between a computer and its user through a sequence of requests by the user and the presentation of responses by the computer.

dialog box: In SAA Advanced Common User Access architecture, a movable window, fixed in size, containing controls that a user uses to provide information required by an application so that it can continue to process a user request. See also message box, primary window, secondary window. Also known as a pop-up window.

Dialog Box Editor: A WYSIWYG editor that creates dialog boxes for communicating with the application user.

dialog item: A component (for example, a menu or a button) of a dialog box. Dialog items are also used when creating dialog templates.

dialog procedure: A dialog window that is controlled by a window procedure. It is responsible for responding to all messages sent to the dialog window.

dialog tag language: A markup language used by the DTL compiler to create dialog objects. Synonymous with DTL.

dialog template: The definition of a dialog box, which contains details of its position, appearance, and window ID, and the window ID of each of its child windows.

direct device context: A logical description of a data destination that is a device other than the screen (for example, a printer or plotter), and where the output is not to go through the spooler. Its purpose is to satisfy queries. See also device context.

direct manipulation: The user's ability to interact with an object by using the mouse, typically by dragging an object around on the Desktop and dropping it on other objects.

directory: A type of file containing the names and controlling information for other files or other directories.

display frame: (1) In computer graphics, an area in storage in which a display image can be recorded. (2) In computer micrographics, an area on a microform in which a display image can be recorded.

display point: Synonym for pel.

dispatch table: (1) A block of memory, allocated by the graphics engine, for the containment of entry points for use by a display driver. (2) An array of pointers to function-handling routines.

dithering: (1) The process used in color displays whereby every other pel is set to one color, and the intermediate pels are set to another. Together they produce the effect of a third color at normal viewing distances. This process can only be used on solid areas of color; it does not work, for example, on narrow lines. (2) In computer graphics, a technique of interleaving dark and light pixels so that the resulting image looks smoothly shaded when viewed from a distance.

DMA: Direct memory access. (1) A technique for moving data directly between main storage and peripheral equipment without requiring processing of the data by the processing unit. (2) The transfer of data between memory and input/output units without processor intervention.

DOS Protect Mode Interface (DPMI): An interface between protect mode and real mode programs.

doubleword: A contiguous sequence of bits or characters that comprises two computer words and is capable of being addressed as a unit. (A)

drag: In SAA Common User Access, to use a pointing device to move an object; for example, clicking on a window border, and dragging it to make the window larger.

dragging: (1) In computer graphics, moving an object on the display screen as if it were attached to the pointer. (2) In computer graphics, moving one or more segments on a display surface by translating. (I) (A)

driver: (1) A program (and possibly data files) that contain information needed to run a particular unit, such as a plotter, printer, port, or mouse. See also device driver and printer driver. (2) A system or device that enables a functional unit to operate.

drop: To fix the position of an object that is being dragged, by releasing the select button of the pointing device. See also drag.

DTL: Dialog tag language. See dialog tag language.

dual-boot function: A feature of the OS/2 operating system that allows the user to start DOS from within the operating system, or an OS/2 session from within DOS.

duplex: Pertaining to communication in which data can be sent and received at the same time. Synonymous with full duplex.

dynamic data formatting: A formatting procedure that enables you to incorporate text, bit maps, or metafiles in an IPF window at execution time.

dynamic link library (DLL): A file containing executable code and data bound to a program at load time or run time, rather than during linking. The code and data in a dynamic link library can be shared by several applications simultaneously.

dynamic linking: The process of resolving external references in a program module at load time or run time rather than during linking.

dynamic segments: Graphics segments drawn in exclusive-OR mix mode so that they can be moved from one screen position to another without affecting the rest of the displayed picture.

dynamic storage: (1) A device that stores data in a manner that permits the data to move or vary with time such that the specified data is not always available for recovery. (A) (2) A storage in which the cells require repetitive application of control signals in order to retain stored data. Such repetitive application of the control signals is called a refresh operation. A dynamic storage may use static addressing or sensing circuits. (A) (3) See also static storage.

dynamic time slicing: Varies the size of the time slice depending on system load and paging activity.

dynamic-link module: A module that is linked at load time or run time.