Glossary - D

 D    

 DASD - Direct-access storage device. 

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

 DBCS - Double-byte character set. 

 DC - Device context. 

 DDB - Device-dependent bit map. 

 deinstantiation - See instantiation. 

 DevHlp - Device helper. 

 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, 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 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. 

 direct access storage device (DASD) - A device in which access time is 
    effectively independent of the location of the data. 

 direct memory access (DMA) -  (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. 

 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. 

 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 - A technique for interleaving dark and light pels so that the 
    resulting image looks smoothly shaded from a distance. 

 DLL - Dynamic link library. 

 DMA - Direct memory access. 

 double-byte character set (DBCS) - 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. 

 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. 

 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.