To conform with Microsoft's vision of a Universal Data Access (UDA) model, programmers are being encouraged to move from DAO , although still widely used, to ActiveX Data Objects (ADO) and its low-level interface with databases, OLE DB. ADO and OLE DB offer a faster interface that is also easier to program.

As an archival medium, DAT is an alternative to consider along with:

• Digital Data Storage (DDS1 through DDS3)
• Optical disk
• VHS tape

   Databases contain aggregations of data records or files, such as sales transactions, product catalogs and inventories, and customer profiles. Typically, a database manager provides users the capabilities of controlling read/write access, specifying report generation, and analyzing usage. Databases and database managers are prevalent in large mainframe systems, but are also present in smaller distributed workstation and mid-range systems such as the AS/400 and on personal computers. SQL is a standard language for making interactive queries from and updating a database such as IBM's DB2, Microsoft's Access, and database products from Oracle, Sybase, and Computer Associates.

   In telecommunications, data transfer is usually measured in bits per second. For example, a typical low-speed connection to the Internet may be 33.6 kilobits per second (Kbps). On Ethernet local area networks, data transfer can be as fast as 10 megabits per second. Network switches are planned that will transfer data in the terabit range. In earlier telecommunication systems, data transfer was sometimes measured in characters or blocks (of a certain size) per second.

In computers, data transfer is often measured in bytes per second.

   Thousands of applications use DDE, including Microsoft's Excel, Word, Lotus 1-2-3, AmiPro, Quattro Pro, and Visual Basic.

   Another facility, NetDDE, allows progams to converse across networks. For example, a Superbase program on one network node could be updated whenever an Excel program in network node was updated. Both nodes must have NetDDE installed.

   Debugging is a necessary process in almost any new software or hardware development process, whether a commercial product or an enterprise or personal application program. For complex products, debugging is done as the result of the unit test for the smallest unit of a system, again at component test when parts are brought together, again at system test when the product is used with other existing products, and again during customer beta testing, when users try the product out in a real world situation. Because most computer programs and many programmed hardware devices contain thousands of lines of code, almost any new product is likely to contain a few bugs. Invariably, the bugs in the functions that get most use are found and fixed first. An early version of a program that has lots of bugs is referred to as "buggy."

   Debugging tools help identify coding errors at various development stages. Some programming language packages include a facility for checking the code for errors as it is being written.

   The DirectX Software Development Kit (SDK) includes tools that let a developer create or integrate graphic images, overlays, sprites, and other game elements, including sound. There is also a Driver Development Kit (DDK) that lets developers create drivers for display, audio, and other I/O devices.

   DirectX is part of Microsoft's larger vision of an object-oriented development environment. For example, a sprite is created as an instance of a sprite class. Developers can also take advantage of Microsoft's set of prepackaged routines or small programs that are part of the Microsoft Foundation Class (MFC).

   DirectX is designed so that some functions can be performed on a graphics accelerator card, freeing the microprocessor for other work. The accelerator manufacturer provides a driver especially for DirectX.

   DirectX consists of five components:

• DirectDraw, an interface that lets you define two-dimensional images, specify textures, and manage double buffers (a technique for changing images)
• Direct3D, an interface for creating three-dimensional images
• DirectSound, an interface for integrating and coordinating sound with the images
• DirectPlay, a plug-in for end users, is also used by developers to test their application
• DirectInput, an interface for input from I/O devices

   Displays for personal computers have steadily improved since the days of the monochrome monitors that were used in word processors and text-based computer systems in the 1970s. In 1981, IBM introduced the Color Graphics Adapter (CGA). This display system was capable of rendering four colors, and had a maximum resolution of 320 pixels horizontally by 200 pixels vertically. While CGA was all right for simple computer games such as solitaire and checkers, it did not offer sufficient image resolution for extended sessions of word processing, desktop publishing, or sophisticated graphics applications.

   In 1984, IBM introduced the Enhanced Graphics Adapter (EGA) display. It allowed up to 16 different colors and improved the resolution to 640 pixels horizontally by 350 pixels vertically. This improved the appearance of the display and made it possible to read text more easily than with CGA. Nevertheless, EGA did not offer sufficient image resolution for high-level applications such as graphic design and desktop publishing.

   In 1987, IBM introduced the Video Graphics Array (VGA) display system. This has become the accepted minimum standard for PC clones. Many VGA monitors are still in use today. The maximum resolution depends on the number of colors displayed. You can choose between 16 colors at 640 x 480 pixels, or 256 colors at 320 x 200 pixels. All IBM-compatible computers support the VGA standard.

   In 1990, IBM intoduced the Extended Graphics Array (XGA) display as a successor to its 8514/A display. A later version, XGA-2 offers 800 by 600 pixel resolution in true color (16 million colors) and 1,024 by 768 resolution in 65,536 colors.

   Most PC displays sold today are described as Super Video Graphics Array (SVGA) displays. SVGA originally just meant "beyond "VGA" and was not a single standard. More recently, the Video Electronics Standards Assocation (VESA) has established a standard programming interface for SVGA displays, called the VESA BIOS Extension. Typically, an SVGA display can support a palette of up to 16,000,000 colors, although the amount of video memory in a particular computer may limit the actual number of displayed colors to something less than that. Image-resolution specifications vary. In general, the larger the diagonal screen measure of an SVGA monitor, the more pixels it can display horizontally and vertically. Small SVGA monitors (14-inch diagonal) usually display 800 pixels horizontally by 600 pixels vertically. The largest monitors (20 inches or more diagonal measure) can display 1280 x 1024, or even 1600 x 1200, pixels.

   In computers, a dynamic link library (DLL) is a collection of small programs, any of which can be called when needed by a larger program that is running in the computer. The small program that lets the larger program communicate with a specific device such as a printer or scanner is often packaged as a DLL program (usually referred to as a DLL file).

The advantage of DLL files is that, because they don't get loaded into random access memory (RAM) together with the main program, space is saved in RAM. When and if a DLL file is needed, then it is loaded and run. For example, as long as a user of Microsoft Word is editing a document, the printer DLL file does not need to be loaded into RAM. If the user decides to print the document, then the Word application causes the printer DLL file to be loaded and run.

A DLL file is often given a ".dll" file name suffix. DLL files are dynamically linked with the program that uses them during program execution rather than being compiled with the main program. The set of such files (or the DLL) is somewhat comparable to the library routines provided with programming languages such as C and C++.

 

   Direct Memory Access is a capability provided by some computer bus architectures that allows data to be sent directly from an attached device (such as a disk drive) to the memory on the computer's motherboard. The microprocessor is freed from involvement with the data transfer, thus speeding up overall computer operation.

Usually a specified portion of memory is designated as an area to be used for direct memory access. In the ISA bus standard, up to 16 megabytes of memory can be addressed for DMA. The EISA and MCA standards allow access to the full range of memory addresses (assuming they're addressable with 32 bits). PCI accomplishes DMA by using bus mastering (with the microprocessor "delegating" I/O control to the PCI controller).

An alternative to DMA is the Programmed Input/Output (PIO) interface in which all data transmitted between devices goes through the processor. A newer protocol for the ATA/IDE interface is Ultra DMA/33, which provides a burst data transfer rate up to 33 MB (megabytes) per second. Hard drives that come with Ultra DMA/33 also support PIO modes 1, 3, and 4, and multiword DMA mode 2 (at 16.6 megabytes per second).

   A certain amount of the random access memory was reserved as a place to load in the BIOS or initializing program, another space was reserved for buffer areas for display data, and another space for interrupt data, and so forth. The remaining 640 kilobytes of memory could be used by the DOS operating system (which was relatively small) and application programs.

   The reason that all of this is not academic is that, as new microprocessors and operating systems have been developed, many older programs written with the original memory addressing limitations need to continue running in the newer systems. This means that the newer systems have had to accommodate the older programs so that they can run in the original limited 640 kilobye contiguous address range. A program that is run with this constraint is described as running in real mode. (Programs running without this constraint are described as running in protected mode.)

   With the successors to the 8088, the 80286 and 80386 microprocessors, available memory increased considerably, with up to 15 megabytes possible in a 286 and up to almost 4 gigabytes of RAM possible in a 386. This extra memory above the orginal one megabyte that DOS was designed to address is called extended memory. Being able to address extended memory means running in protected mode.

   In general, DOS applications can only run in real mode since DOS itself requires the memory constraint. However, some programs have been developed called DOS extenders that can be compiled with the application and provide a built-in memory management capability. In order for multiple DOS programs with memory extenders to run concurrently, some common approach was required so that programs could share the extended memory effectively. Several standards developed to manage this sharing: XMS (Extended Memory Specification), VCPI (Virtual Control Program Interface), and DPMI (DOS protected mode interface).

   DVD-Video is the usual name for the DVD format designed for full-length movies and is a box that will work with your television set. DVD-ROM is the name of the player that will (sooner or later) replace your computer's CD-ROM. It will play regular CD-ROM disks as well as DVD-ROM disks. DVD-RAM is the writeable version. DVD-Audio is a player designed to replace your compact disc player.

   DVD uses the MPEG-2 file and compression standard. MPEG-2 images have four times the resolution of MPEG-1 images and can be delivered at 60 interlaced fields per second where two fields constitute one image frame. (MPEG-1 can deliver 30 noninterlaced frames per second.) Audio quality on DVD is comparable to that of current audio compact disks.

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