IBM SDK/DDK Glossary

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Reprint Courtesy of International Business Machines Corporation, © International Business Machines Corporation

This is the standard IBM SDK/DDK Glossary that is used by multiple documents including the Virtual Device Driver Reference for OS/2, MMPM/2 Device Driver Reference, Display Device Driver Reference for OS/2, Virtual Device Driver Reference for OS/2, Printer Device Driver Reference for OS/2 and others.


This glossary contains terms and definitions that are, for the most part, used for OS/2 products. This is not a complete dictionary of computer terms.


This glossary defines many of the terms used in this book. It includes terms and definitions from the IBM Dictionary of Computing, as well as terms specific to the Presentation Manager, but it is not a complete glossary for OS/2.

Other primary sources for these definitions are:

  • The American National Standard Dictionary for Information Systems, ANSI X3 .172-1990, copyrighted 1990 by the American National Standards Institute, 11 West 42nd Street, New York, New York 10036. These definitions are identified by the symbol (A) after the definition.
  • The Information Technology Vocabulary, developed by Subcommittee 1, Joint Technical Committee 1, of the International Organization for Standardization and the International Electrotechnical Commission (ISO/IEC JTC1/SC1). Definitions of published parts of this vocabulary are identified by the symbol (I) after the definition; definitions taken from draft international standards, committee drafts, and working papers being developed by ISO/IEC JTC1/SC1 are identified by the symbol (T) after the definition, indicating that final agreement has not yet been reached among the participating National Bodies of SC1.



ABIOS - Advanced BIOS. See BIOS.

accumulator - (1) A register in which one operand of an operation can be stored and subsequently replaced by the result of that operation. (T) (2) In the IBM 3800 Printing Subsystem Models 3 and 8, a feature that supplies a separate storage that can hold data in raster form. It can be used either for composing a sheet of data that combines a large amount of variable and constant data, or for storing an electronic overlay in raster form that will be merged with variable data as the sheet is printed.

access permission - All access rights a user has regarding an object. (I)

adapter - A piece of hardware that modifies the system unit to allow it to operate in a particular way, often by connecting the system unit to an external device such as a video monitor.

adapter device driver - A device driver that provides hardware-dependent services for an OEMadapter.

address space - (1) The range of addresses available to a program. (A) ( 2) The area of virtual storage available for a particular job.

all points addressable (APA) - In computer graphics, pertaining to the ability to address and display or not display each picture element (pel) on a display surface.

anchor block - An area of the internal resources of OS/2 Presentation Manager which is allocated to a process or thread that calls WinInitialize.

anchor point - The position or choice from which selection or deselection is extended.

APA - All points addressable.

API - Application programming interface.

application programming interface (API) - A functional interface supplied by the operating system, or by a separately-orderable licensed program, that allows an application program written in a high-level language to use specific data or functions of the operating system or the licensed program.

archive flag - In the OS/2 operating system, a flag of files and directories that the operating system uses to determine which files are new or modified. Files with this flag are included when a backup copy is made or when all the files are restored on a hard disk. See flag.

area - In computer graphics, a filled shape such as a solid rectangle.

ASCIIZ - A string of ASCII characters that is terminated with a byte containing the value 0.

aspect ratio - (1) The ratio of the height of a rectangle to its width. A rectangle of width 10 inches and height 5 inches has an aspect ratio of 10/ 5 or 2. (2) On a display screen, the ratio of the maximum length of a display line to the maximum length of a display column.

asynchronous (ASYNC) - (1) Pertaining to two or more processes that do not depend upon the occurrence of specific events such as common timing signals . (T) (2) Without regular time relationship; unexpected or unpredictable with respect to the execution of program instructions.

atom - A constant that represents a string. Once a string has been defined as an atom, the atom can be used in place of the string to save space. Strings are associated with their respective atoms in an atom table. See integer atom.

atom table - A table used to associate atoms with the strings that they represent. This table contains the mechanism by which the presence of a string can be verified.

AVIO - Advanced Video Input/Output


background color - The color assigned to a background image.

background mix - An attribute that determines how the background of a graphic primitive is combined with the existing color of the graphics presentation space.

base device driver - An OS/2 device driver that performs I/O during the OS/ 2 kernel boot sequence to provide IPL support. Base device drivers are loaded by way of the CONFIG.SYS BASEDEV keyword, rather than the DEVICE keyword. See BASEDEV keyword, adapter device driver, and device manager.

BASEDEV keyword - New CONFIG.SYS keyword; loads a base device driver into the operating system.

Basic Input/Output System (BIOS) - Code that controls basic hardware operations, such as interactions with diskette drives, hard disk drives, and the keyboard.

Bezier curve - A mathematical technique of specifying a smooth, continuous line or surface, requiring a starting point and an ending point, with several intermediate points that influence or control the path of the linking curve.

BIOS - Basic Input/Output System.

bit-block transfer (bitblt) - Transfer of a rectangular array of bit-map data.

bitblt - Bit-block transfer.

bit map - A representation of an image by an array of bits.

block - (1) In programming languages, a compound statement that coincides with the scope of at least one of the declarations contained within it. A block may also specify storage allocation or segment programs for other purposes. (I) (2) A string of data elements recorded or transmitted as a unit. The elements may be characters, words or physical records. (T) (3) A collection of contiguous records recorded as a unit. Blocks are separated by interblock gaps and each block may contain one or more records. (A)

Bit block transfer (bitblt) - The process of transferring one or more blocks of data.

border - A visual indicator of a window's boundaries.

BPB - BIOS Parameter Block.

breakpoint - (1) A point in a computer program where execution may be halted. A breakpoint is usually at the beginning of an instruction where halts, caused by external intervention, are convenient for resuming execution. (T) (2) An instruction in a program for halting execution. Breakpoints are usually established at positions in a program where halts, caused by external intervention, are convenient for restarting. (T) (3) A place in a program, specified by a command or a condition, where the system halts execution and gives control to the workstation user or to a specified program.

Bus Master adapter - An adapter capable of performing Reads and Writes to physical storage by communicating directly with the storage subsystem ( memory) rather than depending on a host DMA channel or host CPU. Synonymous with first-party DMA adapter.


cached micro presentation space - A presentation space from a Presentation Manager owned store of micro presentation spaces. It can be used for drawing to a window only, and must be returned to the store when the task is complete.

CDB - Command Descriptor Block.

cell - See character cell.

character box - (1) An imaginary parallelogram on a display surface that contains all parts of one graphic character. Synonymous with bounding box. (T) (2) The maximum area in which a symbol and all associated elements, such as a cursor, an underline, or space surrounding the symbol to separate it from other symbols, can be printed or displayed. Synonymous with character cell. (3) The imaginary parallelogram whose boundaries govern the size, orientation, and spacing of individual characters to be displayed on a graphics display device.

character cell - (1) An addressable location on a display surface or printing medium. (2) The physical width and height in pels of a font. See also bounding box. (3) The imaginary box whose boundaries govern the size , orientation, and spacing of individual characters to be displayed on a workstation.

character mode - A mode that, in conjunction with the font type, determines the extent to which graphics characters are affected by the character box, shear, and angle attributes.

clipping - In computer graphics, removing those parts of display elements that lie outside of given boundary.

clip limits - The area of the paper that can be reached by a printer or plotter.

clipping path - A clipping boundary in world-coordinate space.

code page - An assignment of graphic characters and control function meanings to all code points; for example, assignment of characters and meanings to 256 code points for an 8-bit code, assignment of characters and meanings to 128 code points for a 7-bit code.

code point - A 1-byte code representing one of 256 potential characters.

code segment - An executable section of programming code within a load module.

color conversion - Changing one color format to another. Required, for example, when the source color format is different from the destination color format. When going from the monochrome color format to the color format, 1 (one) bits are converted to the image foreground color, and 0 ( zero) bits are converted to the image background color.

When going from color to monochrome, all pels that match the passed background color are converted to the image background color of the destination.

All other pels are converted to the image foreground color of the destination. The color conversion takes place prior to any mix mode.

color dithering - See dithering.

command code - In this specification, refers to a group of related commands that an adapter device driver can receive.

All command codes have a prefix of "IOCC_". For example, common I/O requests (such as Read, Write, etc.) are grouped under the command code IOCC_EXECUTE_IO.

command data block - A data structure defined by the Small Computer System Interface standard to send commands to devices that conform to SCSI standards.

command descriptor block (CDB) - The structure used to communicate commands from a source to a destination.

command modifier - In this specification, a specific operation that an adapter device driver is to perform.

All command modifiers have a prefix of "IOCM_". For example, an adapter device driver might receive an IOCC_EXECUTE_IO command with a command modifier of IOCM_READ.

compatibility kernel - The portion of the OS/2 kernel that exists to support DOS INT 20, 21, 25, 26, and 27 functions. It acts as an interface to common kernel functionality such as the file system.

CON - Character-device name reserved for the console keyboard and screen.

conditional compilation -Processing by the preprocessor of certain specified code in the file, depending on the evaluation of a specified condition.

context hook - Similar to a "force flag" in earlier versions of OS/2. These are events, signaled by a virtual device driver, that are processed at task time. Forcing an IRET, and simulating an NMI, can fall into this category.

control program - A computer program designed to schedule and to supervise the execution of programs of a computer system.

controller sector buffer - One or more buffers, managed by a hardware adapter, to improve I/O transfer rates by helping to match a device and software timing requirements.


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.


entry point - (1) In a database, the record that is first accessed upon entry into a database, caused by a user's command. (T) (2) The address or label of the first instruction executed on entering a computer program, routine, or subroutine. A computer program, routine, or subroutine may have a number of different entry points, each perhaps corresponding to a different function or purpose. (I) (A) Synonymous with entrance, entry. (3) In a routine, any place to which control can be passed. (A) (4) In the C, FORTRAN, and Pascal languages, the address or label of the first instruction processed or entered in a program, routine, or subroutine. A program, routine, or subroutine can have a number of different entry points , each corresponding to a different function or purpose.

EOI - End Of Interrupt


Far call - Code that calls from one segment into another segment.

fillet - An arc that is tangential to the end points of two adjacent lines. See also polyfillet.

filter adapter device driver - A special class of adapter device drivers that do not manage the hardware directly, but monitor the stream of commands between a device manager and an adapter device driver. See Device Managerand adapter device driver.

first-party DMA adapter - See bus master adapter.

flag - A characteristic of a file or directory that enables it to be used in certain ways. See also archive flag, hidden flag, and read-only flag.

flat address - See linear address.

frame styles - Standard window layouts provided by the Presentation Manager .

freeze and thaw services - Functions that prevent a DOS session from executing (VDHFreezeVDM) until the matching thaw function (VDHThawVDM) is called. The freeze occurs when the specified DOS session leaves kernel mode .


GDT - Global descriptor table.

Global Descriptor Table (GDT) - A table that defines code and data segments available to all tasks in an application.

glyph - A graphic symbol whose appearance conveys information; for example, the vertical and horizontal arrows on cursor keys that indicate the directions in which they control cursor movement, the sunburst symbol on the screen illumination control of a display device.

GPI - Graphics programming interface

graphic primitive - In computer graphics, a basic element, such as an arc or a line, that is not made up of smaller parts and that is used to create diagrams and pictures.

graphics attributes - The attributes that apply to graphics primitives. Examples are color selection, line type, and shading pattern definition. Contrast with segment attributes.

Graphics programming interface (GPI) - The formally-defined programming language that lies between an IBM graphics program and the user of the program.

graphics segment - A sequence of related graphic primitives and graphics attributes. See also graphic primitive.

GRE - Graphics engine.


handle - (1) An identifier that represents an object, such as a device or a window, to the Presentation Interface. (2) In the Advanced DOS and OS/2 operating systems, a binary value created by the system that identifies a drive, directory, and file so that the file can be found and opened.

handshaking - A method by which two pieces of hardware, such as a personal computer and a plotter, can communicate. Depending upon the devices communicating, handshaking occurs either as a hardware function or through software, such as a device driver.

hard error - An error condition on a network that requires that the network be reconfigured or that the source of the error be removed before the network can resume reliable operation.

hardware palette - The array of RGBs that the physical device is displaying .

heap - An area of free storage available for dynamic allocation by an application. Its size varies depending on the storage requirements of the application.

hex - See hexadecimal

hexadecimal - Pertaining to a system of numbers to the base 16; hexadecimal digits range from 0 through 9 and A through F, where A represents 10 and F represents 15.

hook - A point in a system-defined function where an application can supply additional code that the system processes as though it were part of the function.

hook chain - A sequence of hook procedures that are "chained" together so that each event is passed in turn to each procedure in the chain.


IDC - Inter-device-driver communication.

in-memory buffer - A block of memory in the address space of the host machine, used for data transfer.

init time - See initialization time, device driver.

initialization time, device driver - After the OS/2 loads a device driver, it sends it an OS/2 request packet to initialize. During this initialization, certain DevHlp functions are not permitted. Also called init time.

Input/Output Control (IOCtl) - A system service that provides a way for an application to send device-specific control commands to a device driver.

Input/Output Privilege Level (IOPL) - Allows part of a Ring 3 application or device driver to execute at Ring 0.

input router - OS/2 internal process that removes messages from the system queue.

inter-device-driver communication (IDC) - A mechanism that enables a physical device driver to communicate with another physical device driver.

interprocess communication - In the OS/2 operating system, the exchange of information between processes or threads through semaphores, queues, and shared memory.

interrupt - An instruction that directs the microprocessor to suspend what it is doing and run a specified routine. When the routine is complete, the microprocessor resumes its original work. See also routine.

interrupt request (IR) - Broadly, an "interrupt request level", referring to pending or in-service interrupt requests, or to a specific level (for example, IR 4).

interrupt request flag - A bit in the 8259 PIC controller that indicates an interrupt is pending on particular level. The VPIC also maintains a virtual interrupt request flag for each interrupt level for each DOS session.

interrupt service flag - A bit in the 8259 PIC controller that indicates an interrupt request is being serviced. It is cleared when the PIC is sent EOI . The VPIC maintains a virtual interrupt service flag indicating that a simulated interrupt is in-progress in a DOS session.

interrupt time - When a device driver is run because of an interrupt rather than because of an application request. OS/2 device drivers receive interrupts either from the hardware they manage or from the system real- time clock.

During interrupt time, certain DevHlp functions are not permitted. Also, addresses received directly from OS/2 applications might not be valid unless they are converted system addresses.

IOCtl - Input/Output Control.

IOPL - Input/Output Privilege Level.

IORB - Input/Output Request Block.

Input/Output Request Block (IORB) - A data structure defined by this specification that is passed as a parameter on all calls to an adapter device driver. It contains a fixed section, followed by a command-dependent section.

IORBH - Input/Output Request Block Header

IRET - Interrupt return.

IRQ - Interrupt Request.


journal - A special-purpose file or data set that can be used to provide an audit trail of operator and system actions, or as a means of recovering superseded data.


kanji - A graphic character set consisting of symbols used in Japanese ideographic alphabets. Each character is represented by 2 bytes.

kernel - (1) The part of an operating system that performs basic functions such as allocating hardware resources. (2) A program that can run under different operating system environments.

kerning - The design of graphic characters so that their character boxes overlap. The toned picture elements (pels) of the character appear outside the character cell.

Note: Kerning allows character boxes to overlap and characters to run together, so that characters can be designed for cursive languages, ligatures, or any other kind of character that requires more than one character box. It also allows for design of proportional-spaced fonts. By overlapping character boxes, characters can be placed closer together, or they can be placed farther apart by using overlapped blank character boxes.


LCT - logical color table.

LDT - Local descriptor table.

LIFO stack - A data structure from which data is retrieved in "Last-In, First-Out" order.

linked list - A list in which the data elements may be dispersed, but in which each data element contains information for locating the next. Synonym for chained list.

linear address - A unique value that identifies the memory object.

Local Descriptor Table (LDT) - A table that defines code and data segments specific to a single task.

logical palette - An array of RGB and mapping index pairs, created by the device driver when defining a palette (as a result of a GpqCreatePalette call).

LVB - Logical Video Buffer.


memory device context - A logical description of a data destination that is a memory bit map. See also device context.

metafile - A file containing a series of attributes that set color, shape, and size, usually of a picture or a drawing. Using a program that can interpret these attributes, a user can view the assembled image.

metafile device context - A logical description of a data destination that is a metafile which is used for graphics interchange. See also device context.

mickey - A unit of measurement for physical mouse motion whose value depends on the mouse device driver that is currently loaded.

mixed character string - A string containing a mixture of one-byte and kanji or Hangeul (two-byte) characters.

mutex semaphore - (Mutual exclusion semaphore). A semaphore that enables threads to serialize their access to resources. Only the thread that currently owns the mutex semaphore can gain access to the resource, thus preventing one thread from interrupting operations being performed by another.


named pipe - A named buffer that provides client-to-server, server-to- client or duplex communication between unrelated processes. Contrast with unnamed pipe.

notification callout - The feature that provides for a routine to be called on completion of an input/output request. See also notification routine.

notification routine - The routine indicated in an input/output request block to be called on completion of that request. See also notification callout.

null-terminated string - A string of (n+1) characters where the (n+1)th character is the "null" character (X'00') and is used to represent an n- character string with implicit length. Also called a "zero-terminated" string or an "ASCIIZ". string.


palette - A list of colors assigned to various areas on a panel. A user can change the color of these areas.

PDD - Physical Device Driver.

PDE - PageDirectoryEntry.

pel - Picture element.

permissible action - In a conceptual schema language, an action conforming to specified rules or constraints that changes a presumably consistent collection of sentences into a consistent one or makes known a consistent one present in the information base or conceptual schema.

phase alignment - Aligning source bits with destination bits. Often required in a Bitblt function move operation where byte blocks are moved on bit boundaries.

physical address - A 32-bit byte address giving the actual address in physical storage for a data item.

physical device driver (PDD) - A system interface that handles hardware interrupts and supports a set of input and output functions.

pipe - See named pipe, unnamed pipe.

picture element (pel, pixel) - (1) In computer graphics, the smallest element of a display surface that can be independently assigned color and intensity. (T) . (2) The area of the finest detail that can be reproduced effectively on the recording medium. (3) An element of a raster pattern about which a toned area on a photoconductor can appear.

PIO - Programmed I/O.

pixel - Picture element.

polyfillet - A curve based on a sequence of lines. The curve is tangential to the end points of the first and last lines, and tangential also to the midpoints of all other lines.

polyline - In computer graphics, a sequence of adjoining lines.

pop - To remove an item from the top of a pushdown list. Contrast with push .

prefetch - To locate and load a quantity of data in anticipation of a request.

presence-check function - A Ring 3 (non-privileged) .EXE program that determines whether a given hardware interface is present on a workstation.

PRESENCECHECK - A keyword, interpreted by the DDINSTAL utility, to determine whether to process the device driver profile file, based on the return code from PRESENCECHECK.

printer driver - A file that describes the physical characteristics of a printer, plotter, or other peripheral device, and is used to convert graphics and text into device-specific data at the time of printing or plotting.

Print Manager - In the Presentation Manager, the part of the spooler that manages the spooling process. It also allows the user to view print queues and to manipulate print jobs.

privilege level - A method of protection that allows only certain program instructions to be used by certain programs.

program group - Several programs that can be acted upon as a single entity.

protect mode - A method of program operation that limits or prevents access to certain instructions or areas of storage. Contrast with real mode.

push - To add an item to the top of a pushdown list. Contrast with pop.


queued device context - A logical description of a data destination (for example, a printer or plotter) where the output is to go through the spooler. See also device context.


read-only memory basic input/output system (ROM-BIOS) - Microcode in read- only memory that controls basic input/output operations such as interactions with cassettes, diskette drives, hard disk drives, and the keyboard. See also BIOS, NetBIOS.

Note: ROM BIOS allows the user to write programs and add or remove devices without concern for characteristics such as device addresses.

real mode - In the OS/2 operating system, a method of program operation that does not limit or prevent access to any instructions or areas of storage. The operating system loads the entire program into storage and gives the program access to all system resources.

reentrant - The attribute of a program or routine that allows the same copy of the program or routine to be used concurrently by two or more tasks.

removable-media indicator - A flag (bit) indicating that a device permits media removal.

resource - The means of providing extra information used in the definition of a window. A resource can contain definitions of fonts, templates, accelerators and mnemonics; the definitions are held in a resource file.

resurrection - The Presentation Manager event that occurs when switched back from a full-screen DOS or WIN-OS/2 session.

RETF - Return far.

reverse video - A form of highlighting a character, field, or cursor by reversing the color of the character, field, or cursor with its background; for example, changing a red character on a black background to a black character on a red background.

ROM BIOS - Read-Only Memory Basic Input/Output System.

ROP - Raster operation.

RTC - Real-Time Clock.


SBCS - Single-byte character set

SCB - See subsystem control block architecture.

screen device context - A logical description of a data destination that is a particular window on the screen. See also device context.

SCSI - Small Computer System Interface.

seamless windows - An architecture contained within OS/2 which permits one or more applications to share windowed desktop graphical space and other resources, while executing concurrently. Application session windows managed by seamless windows can share border information, and pointing device transitions from session to session are handled smoothly and transparently.

second-party DMA adapter - See DMA slave.

semaphore - (1) A variable that is used to enforce mutual exclusion. (T) (2) An indicator used to control access to a file; for example, in a multiuser application, a flag that prevents simultaneous access to a file. (3) An entity used to control access to system resources. Processes can be locked to a resource with semaphores if the processes follow certain programming conventions.

sense data - Data which describes an I/O error as defined by the ANSI SCSI specifications.

single-byte character set (SBCS) - A character set in which each character is represented by a one-byte code. Contrast with double-byte character set.

Small Computer System Interface (SCSI) - An input and output bus that provides a standard interface between the OS/2 multimedia system and peripheral devices.

spline curve - In computer graphics, a shape created when a user specifies a series of points and the computer software draws a curve that smoothly approaches those points.

spooler - A program that intercepts data going to a device driver and writes it to a disk. The data is later printed or plotted when the required device is available. A spooler prevents output from different sources from being intermixed.

synchronous - Pertaining to two or more processes that depend upon the occurrence of specific events such as common timing signals.


text window - See VIO window.

thread - The smallest unit of operation to be performed within a process.

thunk - term used to describe the process of address conversion, stack, and structure realignment that is necessary when passing control between 16-bit and 32-bit modules.

thunk layer - An interface that converts 32-bit parameters to 16-bit parameters, and maps linear addresses to segmented addresses.

time slice - (1) The period of processing time allocated for running a program. (2) An interval of time on the processing unit allocated for use in performing a task. After the interval has expired, processing unit time is allocated to another task, so a task cannot monopolize processing unit time beyond a fixed limit.

tuple - In a relational database, a part of a relation that uniquely describes an entity and its attribute.


unnamed pipe - A circular buffer created in memory; used by related processes to communicate with one another. Contrast with named pipe.


VBIOS - Virtual BIOS device driver

VCMOS - Virtual CMOS device driver

VDD - Virtual device driver

VDH - Virtual video Device Handler

VDM - Virtual DOS Machine; use DOS session.

VDMA - Virtual Direct Memory Access device driver

VDSK - Virtual hard DiSK device driver

video graphics adapter (VGA) - A computer adapter that provides high- resolution graphics and a total of 256 colors.

VIO - Virtual Input/Output

VIRR - Virtual Interrupt Request Register

Virtual Device Driver (VDD) - In the OS/2 operating system, a type of device driver used by DOS programs running in a DOS session to access devices, such as the screen or mouse, which must be shared with other processes in the system. The virtual device driver maps DOS device commands to the normal (physical) device driver under OS/2 2.0 and later versions of the operating system.

virtual DevHlp (VDH) - Kernel (linear memory, paging, hardware interrupt, event control, port control) services provided to virtual device drivers.

virtual I/O (VIO) - A facility that pages data into and out of external page storage.

virtual memory - Synonym for virtual storage.

Virtual Programmable Interrupt Controller - Virtualizes the 8259 Programmable Interrupt Controller (PIC). A special virtual device driver, in that it provides services to other virtual device drivers.

virtual storage - Addressable space that is apparent to the user as the processor storage space, from which the instructions and the data are mapped into the processor storage locations. Synonymous with virtual memory .

visible region - A window's presentation space clipped to the boundary of the window and the boundaries of any overlying window.

VPIC - Virtual Programmable Interrupt Controller device driver.

VRAM - Video Random-Access Memory.

VTIMER - Virtual TIMER device driver.

V86 mode - Virtual 8086 mode of the 80386 CPU.


window coordinates - A set of coordinates by which a window position or size is defined; measured in device units, or pels.