## Input/Output Device Driver Reference

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### 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.