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Personal Computing   Glossary of Terms
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(text, product) A product from Adobe Systems, Inc., for manipulating documents stored in Portable Document Format. Acrobat provides a platform-independent means of creating, viewing, and printing documents.

Active Server Pages

(World-Wide Web, programming) (ASP) A scripting environment for Microsoft Internet Information Server in which you can combine HTML, scripts and reusable ActiveX server components to create dynamic web pages.

IIS 4.0 includes scripting engines for Microsoft Visual Basic Scripting Edition (VBScript) and Microsoft JScript. ActiveX scripting engines for Perl and REXX are available through third-party developers.


(programming) A type of COM component that can self-register, also known as an "ActiveX control". All COM objects implement the "IUnknown" interface but an ActiveX control usually also implements some of the standard interfaces for embedding, user interface, methods, properties, events, and persistence.

ActiveX controls were originally called "OLE Controls", and were required to provide all of these interfaces but that requirement was dropped, and the name changed, to make ActiveX controls lean enough to be downloaded as part of a web page.

Because ActiveX components can support the OLE embedding interfaces, they can be included in web pages. Because they are COM objects, they can be used from languages such as Visual Basic, Visual C++, Java, VBScript.

Accelerated Graphics Port

(hardware) (AGP) A bus specification by Intel which gives low-cost 3D graphics cards faster access to main memory on personal computers than the usual PCI bus.

AGP dynamically allocates the PC's normal RAM to store the screen image and to support texture mapping, z-buffering and alpha blending.

Intel has built AGP into a chipset for its Pentium II microprocessor. AGP cards are slightly longer than a PCI card.

AGP operates at 66 MHz, doubled to 133 MHz, compared with PCI's 33 Mhz. AGP allows for efficient use of frame buffer memory, thereby helping 2D graphics performance as well.

AGP provides a coherent memory management design which allows scattered data in system memory to be read in rapid bursts. AGP reduces the overall cost of creating high-end graphics subsystems by using existing system memory.

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Bad command or file name

(operating system) The error message printed by MS DOS when it can't find a program or command to execute due to a typing error, incorrect PATH variable, or misplaced or missing executable.

Blue Screen of Death

(humour) (BSOD) The infamous white-on-blue text screen which appears when Microsoft Windows crashes. BSOD is mostly seen on the 16-bit systems such as Windows 3.1, but also on Windows 95 and apparently even under Windows NT 4. It is most likely to be caused by a GPF, although Windows 95 can do it if you've removed a required CD-ROM from the drive. It is often impossible to recover cleanly from a BSOD.

boot disk

(operating system) The magnetic disk (usually a hard disk) from which an operating system kernel is loaded (or "bootstrapped"). This second phase in system start-up is performed by a simple bootstrap loader program held in ROM, possibly configured by data stored in some form of writable non-volatile storage.

MS-DOS and Microsoft Windows can be configured (in the BIOS) to try to boot off either floppy disk or hard disk, in either order. By default they first check for the presence of a floppy disk in the drive at start-up and try to use that as a boot disk if present. If no disk is in the drive they then try to boot off the hard disk.

Some operating systems, notably SunOS and Solaris, can be configured to boot from a network rather than from disk. Such a system can thus run as a diskless workstation.


(programming) An unwanted and unintended property of a program or piece of hardware, especially one that causes it to malfunction. Antonym of feature. E.g. "There's a bug in the editor: it writes things out backward." The identification and removal of bugs in a program is called "debugging".

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cable modem

(communications, hardware) A type of modem that allows people to access the Internet via their cable television service.

A cable modem can transfer data at 500 kbps or higher, compared with 28.8 kbps for common telephone line modems, but the actual transfer rates may be lower depending on the number of other simultaneous users on the same cable.

Industry pundits often point out that the cable system still does not have the bandwidth or service level in many areas to make this feasible. For example, it has to be capable of two-way communication.


(memory management) A small fast memory holding recently accessed data, designed to speed up subsequent access to the same data. Most often applied to processor-memory access but also used for a local copy of data accessible over a network etc.

central processing unit

(architecture, processor) (CPU, processor) The part of a computer which controls all the other parts. Designs vary widely but, in general, the CPU consists of the control unit, the arithmetic and logic unit (ALU) and memory (registers, cache, RAM and ROM) as well as various temporary buffers and other logic.

The control unit fetches instructions from memory and decodes them to produce signals which control the other part of the computer. This may cause it to transfer data between memory and ALU or to activate peripherals to perform input or output.

A parallel computer has several CPUs which may share other resources such as memory and peripherals.

The term "processor" has to some extent replaced "CPU", though RAM and ROM are not normally considered as part of a processor. This is particularly true of common modern microprocessors though there have been microprocessors which include RAM and/or ROM on the same integrated circuit.

command line interface

(operating system) A means of communication between a program and its user, based solely on textual input and output. Commands are input with the help of a keyboard or similar device and are interpreted and executed by the program. Results are output as text or graphics to the terminal.

Command line interfaces usually provide greater flexibility than graphical user interfaces, at the cost of being harder for the novice to use.


(architecture) Central processing unit. Also known as processor.

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(database) One or more large structured sets of persistent data, usually associated with software to update and query the data. A simple database might be a single file containing many records, each of which contains the same set of fields where each field is a certain fixed width.


(tool, file system) /dee-frag(-ment)'/ (Or "defrag") To coalesce files and free space on a file system.

As modern file systems are used and files are deleted and created, the total free space becomes split into smaller non-contiguous blocks (composed of "clusters" or "sectors" or some other unit of allocation). Eventually new files being created, and old files being extended, cannot be stored each in a single contiguous block but become scattered across the file system. This degrades performance as multiple seek operations are required to access a single fragmented file.

Defragmenting consolidates each existing file and the free space into a continuous group of sectors. Access speed will be improved due to reduced seeking.

The rate of fragmentation depends on the algorithm used to allocate space and the number and position of free sectors. A nearly-full file system will fragment more quickly.

MSDOS and Microsoft Windows use the simplest algorithm to allocate free clusters and so fragmentaton occurs quickly. A disk should be defragmented before fragmentation reaches 10%.

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electronic funds transfer

(application, communications) (EFT, EFTS, - system) Transfer of money initiated through electronic terminal, automated teller machine, computer, telephone, or magnetic tape. In the late 1990s, this increasingly includes transfer initiated via the World-Wide Web. The term also applies to credit card and automated bill payments.


(networking) A local area network first described by Metcalfe & Boggs of Xerox PARC in 1976. Specified by DEC, Intel and XEROX (DIX) as IEEE 802.3 and now recognised as the industry standard.

Data is broken into packets and each one is transmitted using the CSMA/CD algorithm until it arrives at the destination without colliding with any other packet. The first contention slot after a transmission is reserved for an acknowledge packet. A node is either transmitting or receiving at any instant. The bandwidth is about 10 Mbit/s. Disk-Ethernet-Disk transfer rate with TCP/IP is typically 30 kilobyte per second.

Version 2 specifies that collision detect of the transceiver must be activated during the inter-packet gap and that when transmission finishes, the differential transmit lines are driven to 0V (half step). It also specifies some network management functions such as reporting collisions, retries and deferrals.

Ethernet cables are classified as "XbaseY", e.g. 10base5, where X is the data rate in Mbps, "base" means "baseband" (as opposed to radio frequency) and Y is the category of cabling. The original cable was 10base5 ("full spec"), others are 10base2 ("thinnet") and 10baseT ("twisted pair") which is now (1998) very common. 100baseT ("Fast Ethernet") is also increasingly common.

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file server

Hardware and software that together provide file-handling and storage functions for multiple users on a local area network. The most common choices for file server software are Sun Microsystems' Network File System for Unix and Novell Netware for IBM PC compatibles. There is also a version of NFS for PCs called PC-NFS. Storing files on a file server saves having multiple copies stored on individual computers, thus economising on disk space and also makes administrating and updating the files easier.

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(unit) 2^30 = 1,073,741,824 bytes = 1024 megabytes. Roughly the amount of data required to encode a human gene sequence (including all the redundant codons).

graphics adaptor

(hardware, graphics) (Or "graphics adapter", "graphics card", "video adaptor", etc.) A circuit board fitted to a computer, especially an IBM PC, containing the necessary video memory and other electronics to provide a bitmap display.

Adaptors vary in the resolution (number of pixels) and number of colours they can display, and in the refresh rate they support. These parameters are also limited by the monitor to which the adaptor is connected. A number of such display standards, e.g. SVGA, have become common and different software requires or supports different sets.

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hard disk

(storage) (In contrast to floppy disk). One or more rigid magnetic disks rotating about a central axle with associated read/write heads and electronics, used to store data. Most hard disks are permanently connected to the drive (fixed disks) though there are also removable disks.

High speed disks have an access time of 28 milliseconds or less, and low-speed disks run 65 milliseconds or more. The higher speed disks also transfer their data faster than the slower speed units.

Each surface of each disk is divided into a number of evenly spaced concentric circular tracks. The set of all tracks at a given radius on all surfaces (the tracks which can be accessed without moving the heads) are known as a cylinder. Each track is divided into sectors.

Disk drives are commonly characterised by the kind of interface used to connect to the computer, e.g. ATA, IDE, SCSI.