8mm Tape Backup

8mm data tape storage systems appear similar to the common 8mm tape cartridges used by many video cameras, although the media is of a higher quality. A total of 14GB of compressed data can be stored on one tape (160 metres of media). See also: Zip Drives, Traven Drive, Digital Audio Tape and Tape Drives.

AC (Alternating Current)

Electricity is supplied with an alternating current. When the electricity is produced by a generator at the electrical power station, the rotational movement of the magnetic coils causes the current produced to oscillate like a sine wave - hence it is said to alternate. Computers require direct current, i.e. non-alternating current, and to achieve this transformers are used to convert the electric power supplied from power outlets into the voltage and current required by the computers electrical circuits. See also: Direct Current and Uninterruptible Power Supplies.

Access Time

Access time is the term used to describe the speed at with memory (or computer files) can be addressed and utilised – either opened, read from or written to.

Address Lines

Personal computers use binary information as the basis for their operation. A single address line is therefore capable of accessing two memory locations - i.e. 0 or 1. If the number of address lines increases to two, the processor can reference 4 memory locations (e.g. 00, 01, 10, and 11 - representing the decimal numbers 0 through to 3). Mathematically this relationship can be represented as X2 (X to the power of 2), where X is the number of address lines. The processor requires 20 address lines to access 1 megabyte of memory. See also: CPU, Intel Processors and Pentium Processor Series.

AGP (Accelerated Graphics Port)

Although the Pentium processor chips were designed to work with PCI expansion buses, these can be a limiting factor in high-end graphics processing applications. The AGP bus was developed by Intel for use with the PII processor and it provides a dedicated video bus for graphics requirements. Accelerated graphics ports have a speed in excess of 66MHz and can transfer data rapidly (e.g. 256MB per second for the original design increasing to 4GB per second for the most modern port). AGP buses also support a direct link to the CPU and they can use motherboard RAM, as well as being able to write and read data simultaneously from video memory. See also: Display Adapter and Video Standards.

Asynchronous Data Transmission

This emerging technology offers a highly versatile transmission mode for both LAN and WAN systems. It can support a maximum bandwidth of 622Mbps and has consequently been included as an Internet backbone by many Internet Service Providers. Other slower formats are available, such as the 155Mbps and 25Mbps in order to accommodate Ethernet transmission. Video, audio and data are all supported, with mechanisms for prioritising time-sensitive material. ATM divides information into 53-byte cells containing 48 bytes of data and 5 bytes of header data. This consistency of data size allows for rapid processing and bandwidth can be readily determined. See also: MODEM and Synchronous Transmission.

Audio Port

A number of different audio ports exist on modern computer motherboards or on specialist sound cards. Commonly audio ports may be marked as: audio out, audio in, speaker out, microphone input or ‘mic’, and headphones. These allow users to divert the sound that is produced by their machine to other audio equipment. Audio in ports can be used to input sound into software packages such as samplers or sound recorders. See also: I/O Ports.

Baud Rate

Baud rate is an out-dated term that is used to describe the maximum speed at which a modem can receive and transmit data. Baud rate was originally indicated the ‘number of transitions in the modem signal per second’ in the case of the old teletype systems. Since modern modems can carry multiple information bits per transition the term ‘bits per second’ or bps is preferred. See also: Bits per Second.

Beep Codes

During the series of power-on self test diagnostics that are performed during start-up, errors in hardware or the ROM data can be brought to the attention of the user by beep noises. Each beep code is able to draw attention to a particular fault with the hardware. Following a successful boot operation, the computer may beep to show that this operation has been successful. See also: Power On Self Test Sequence and BIOS.

BIOS (Basic Input/Output System)

The BIOS is a ROM chip that contains programs and information relating to the basic operation of PC components such as drives, keyboard, video display and ports. It also contains specific routines to allow set-up configuration to be viewed and edited and it contains the self-diagnostic power-on self test program used to detect fundamental faults in PC components. See also: Beep Codes and Power on Self Test Sequence.

bps (Bits per Second)

The term ‘bits per second’ is used to describe data transfer speed – the higher the number, the higher the transmission speed. Due to the extremely fast data processing components that are now available, it may be more appropriate in some circumstances to talk of ‘bytes or megabytes per second’.


A bridge provides a communications link between two or more network segments to form one logical network. Bridging can be advantageous in circumstances where particular sections of the network have to carry a lot of data traffic. Communication between the segments occurs via the bridge although data that does not need to access a workstation on the other side of the bridge is not forwarded. See also: Gateway, Routers, Network Topologies.


A brownout occurs when the power that is supplied by the electrical wall socket is insufficient to allow the PC to function correctly. Brownouts are long sags in power output that are often caused by overloaded or faulty mains distribution circuits or by a failure in the supply route from electrical power station to a building. See also: Power Sags, Power Spikes and Uninterruptible Power Supplies.

BEDO (Burst Extended Data-Out)

This type of RAM is supported by only a few motherboards and is similar in performance to SDRAM. See also: DRAM, SRAM and RAM.


A bus is a data connection that exists between the internal circuitry of the computer motherboard and a peripheral component such as a hard drive or sound card. Buses are available in industry standard formats, each with its own advantages and disadvantages. The standard functions of the bus are to provide data sharing, memory addressing, power supply and timing. Common bus types include SCSI, ISA and PCI. See also: PCI bus, SCSI Interfaces, ISA Bus and Motherboard.


A cache is an area of memory that has been assigned to the task of controlling input and output operations from drives or from a processor. Cache memory acts as an intermediate buffer where data can be stored before it is required. Using memory caches generally allows increased performance and data handling rates. See also: CPU, Intel Processors and Pentium Processor Series.

CD-ROM (Compact Disk – Read Only Memory)

A CD-ROM drive is normally provided with almost every new PC that is purchased. CD-ROM drives can read compact discs at high speeds and can take advantage of this high capacity storage medium to retrieve both data files and audio tracks. CD-ROM discs can normally hold 650MB of data (equivalent to over 400 floppy disks) or 74 minutes of audio data. CDs store data in the form of minute ‘pits’ on a metal sheet that is sandwiched between two transparent plastic circles. When a laser reads the data, the deformations alter the reflection pattern of the beam. CDs are a highly stable storage medium and are not affected by temperatures, magnets or dust as floppy discs are. One drawback to CDs is that they can only be written to once and production can be expensive due to the ultra-sterile manufacturing conditions. A solution to this problem has been provided by recordable CDs that use a slightly different media type to that present in normal CDs – this medium can be written to using a CD writer. See also: DVD, Worm Drives and Recordable CD-ROM.


The clock is responsible for synchronising the operation and function of all of the components within a PC. It also provides the basic timing signal for the processor - this has, in the past, been exploited to obtain faster processing times by boosting the clock speed - this process, termed ‘clocking’ can however produce an unstable configuration. See also: CPU.

CMOS (Complimentary Metal Oxide Semiconductor)

The CMOS RAM contains essential information concerning the set-up and configuration of the PC. CMOS devices can store data and remain stable using only a small amount of power - normally in the form of a rechargeable battery. See also: BIOS and Power On Self Test Sequence.

Coaxial Cable

This type of cable is formed from two separate conductors that share a common axis (hence the term co-axial). The outer conductor, a wire mesh, is isolated from the inner conductor, a copper wire, by plastic insulation - electromagnetic interference is limited, in part, by the wire mesh. Two categories of coax cable are available: thick and thin. These are classified using the radio grade rating scale. Twisted-pair cabling has now largely superceded coaxial cable. The technology that underlies the use of coaxial cable is highly developed and can support greater bandwidths than TP cables. Installation is simple, although coax cable can be sensitive to EMI and reconfiguration of an installed coax network is potentially difficult. See also: UTP Cable and Fiber Optic Cable.

CPU (Central Processing Unit)

The CPU or processor acts as the main target for program code. Within the processor, program code is interpreted and executed, memory is allocated, and mathematical and logical calculations are performed. Modern CPUs contain both a microprocessor and a maths co-processor, which has been optimised to execute mathematical functions - leaving the central processor free to deal with other code issues. See also: Maths Co-processor, Intel Processors and Pentium Processor Sequence.

CRC (Cyclical Redundancy Check)

A CRC algorithm treats a block of transmitted data as a single large binary number and divides this by a 16- or 32-bit number (called the polynomial). The remainder of this division is termed the checksum. This is transmitted with the data and is compared to the checksum generated by the receiving modem. If the two cyclic redundancy checks are not the same, the data block is rejected and a request for data re-transmission is sent to the source.

DAT (Digital Audio Tape)

Digital audio tapes can be used as a high-capacity storage medium for PC data storage. DAT data tapes are similar in appearance and size to the DAT audio tapes that were originally released by the Sony Corporation. Data tapes all conform to Digital Data Storage standards and can support a variety of capacities from 1.3GB to 12GB (uncompressed), although the different formats are occasionally incompatible. DAT drives use 2 magnetic heads to access data – one head reads the information, the other head writes data. See also: Tape Drives, DLT Technology and 8mm Tape Drives.

Data Caching

Data caching is a technique used for speeding up performance during file access and processing. Caching involves either reading part or the whole of the file into a memory buffer from which it can be accessed more quickly than from its original storage medium (e.g. a hard disk). In some elaborate caching systems files are accessed and the next relevant section of the file is pre-loaded in anticipation of file access thus increasing performance in the situation where a whole file is required.

Data Clusters

When data is written to a medium, it is often written discontinuously and so some means must exist in order to track the position of data chunks. Filing systems can only cope with a finite number of locations – in the case of FAT16 this is 65536 and in the case of FAT32, this is 232 (2 raised to the power 32). Since these values are enormous, a method for reducing this number is made available by grouping the data locations together in clusters of 2, 4, 6, and 8, etc. However, a drawback to this system occurs when a piece of data does not completely fill a cluster – if a cluster is equivalent to 4KB and the file is 512 bytes – some of the cluster is wasted as more that one piece of data cannot occupy the same cluster as another. Thus, the smaller the cluster size the smaller the data overhead in terms of wasted space. The FAT32 filing system can track more locations than the FAT16 system and so clusters are smaller – thus switching from FAT16 to FAT32 can provide useful extra space (this option is available in Windows95 and later revisions). See also: Filing Systems, File Allocation Table, Track and Sector.

Data Rate

Data rate is the speed at which data can be sent or received from two or more computer components. These components may be part of the same computer, or may be connected across a network. The type of link that exists, the bus or port to which it is connected and the rate at which the data can be understood and processed are all factors that influence data rate. See also: Bits per Second.

Data Registers

Data registers are temporary storage areas that can hold data prior to processing by the Arithmetic Logic Unit (ALU) and other components of the processor. Data registers can be described in terms of number (data path size) and the size of information that they can contain (register size). The Pentium II processor can handle 64-bit data. See also: Intel Processors and CPU.

Data Skew

When data is transmitted in parallel format, i.e. where there is one physical wire per data bit, minute differences between each conductor can act so as to delay the arrival of the data at the other end of the connection. This delay in arrival of data is termed data skew and it is exacerbated by increasing connection length. Modern, well controlled, production methods have limited this effect to some extent, but it can still affect both parallel and SCSI ports. Data skew does not occur with serial connections. See also: Serial Port.

DC (Direct Current)

PCs and most computer components function by using power supplied in the form of direct current. This is normally at low voltage and is produced by ‘stepping down’ the voltage from the mains supply. Direct current, unlike alternating current is consistently positive and it does not oscillate between positive and negative states. See also: Alternating Current and Uninterruptible Power Supplies.

DCE (Data Circuit-Terminating Equipment)

DCE equipment provides an interface between data terminating equipment (DTE) and communications systems or networks such as modems. The RS232-C serial port standard allows DTE hardware to be linked to DCE equipment with a cable, where pin 1 attaches to pin 1 and pin 2 attaches to pin 2, etc. This form relies on the property of DTE equipment to send data using pin 2 and DCE equipment to receive on pin 2. This cable therefore links the two devices correctly.

DIMM (Dual In-line Memory Module)

Dual in-line memory modules resemble large SIMM cards in appearance although they are larger and normally have either 144 or 168 pins. DIMM packages normally require 3.3v or 5v direct current and they are not compatible with the SIMM slots on a PC, despite initial appearances. Laptop computers use a specialised format DIMM package termed a small outline DIMM - which are greatly downsized versions of the former card. See also: SIMMs, DIPPs and SIPPs.

DIPP (Dual In-line Pin Package)

Dual in-line packages resemble the classic computer chip with two rows of pin connectors either side of the silicon chip. These are no longer used in modern PCs although some older graphics cards and printers may contain this type of chip. See also: DIMMs and SIMMs.

Display Adapter

A display adapter provides the interface between the graphics components of the computer and the display device to which the computer is attached. Display adapters receive information from the microprocessor and it stores this data in video RAM. A digital to analogue converter and a colour processing circuit transform the data into a signal that the monitor can use. The display adapter may also be referred to as the graphics or video card and it is generally in the form of an expansion card. See also: AGP Port.

DLT (Digital Linear Tape)

DLT is a popular backup system used for both mini-computers and large servers and it was originally developed for use with high-speed, LAN-based systems. DLT systems are very fast and have high data transfer rates. Capacities vary from 15 to 20GB per tape (uncompressed data). See also: 8mm Tape Drive, Traven Drive, Digital Audio Tape and Tape Drives.

DMA (Direct Memory Access)

In many older computers, devices where only capable of accessing system memory by first accessing the processor. Direct memory access is used today as a method for accessing memory without the need for CPU intervention - typically hard disk controllers, tape streamers and network cards employ this type of access method. See also: Interrupts and CPU.

Dot Pitch

Monitors, like televisions, represent images by selectively illuminating clusters of red, green and blue phosphor dots present on the surface of the tube. Dot pitch is the measurement between the adjacent centres of this group of three dots (a triad). The smaller the dot pitch, the higher the image quality – standard monitors have a dot pitch of 0.28mm, cheaper monitors have a dot pitch of 0.29-0.31mm and high quality monitors used for graphics-intensive applications, 0.26mm. See also: Display Adapters, Resolution and Interlacing.

DRAM (Dynamic RAM)

Dynamic RAM is a type of volatile memory that stores data in the form of electronic charges within transistors. Due to the effects of leakage and the subsequent loss of electrical charge, DRAM has to be refreshed at regular intervals. Memory refreshing can be performed when the data bits are accessed regularly, but this periodic access slows down the operation of this memory type. Standard DRAM is the lowest common denominator of the DRAM types and although it is highly compatible, modern PCs normally use a DRAM derivative to store data. See also: RAM.

Drive Architecture

Drive architecture is a term that describes the arrangement of tracks, sectors, and data clusters on a hard or floppy disk drive. It is also used (in a loose sense) to refer to the number and size of any hard disk partitions that may have been created. See also: Filing System, File Allocation Table, Track and Sector.

DVD (Digital Video/Versatile Disk)

DVD discs are now being heralded as a revolutionary new medium that will alter the world of home entertainment. They offer high capacities (4.7GB per layer per side) and provide greater quality in terms of sound and image than videocassettes. DVD drives can be used to play normal CD-ROM discs and this has offset (to some extent) the higher initial cost. See also: CD-ROM.

ECP (Extended Capabilities Port)

Hewlett-Packard and Microsoft jointly created the ECP to include fast data transfer rates (more than 2MB per second), bi-directional operation (8-bit), transmission of data or commands to control peripheral operation, support for high-speed printers and CD-ROMS, etc. DMA support was also added to reduce processor operation overheads. The computer’s BIOS and CMOS settings must be able to support this standard before it can be used. See also: Data Skew, Extended Capabilities Port and Parallel Port.

Electrostatic Discharge

Metal and plastic objects can become charged if they are in close proximity to electrical sources or if they are ‘brushed against’. When this happens, charge builds up on the surface (which becomes more positive) to the surroundings. This potential difference can discharge if a potential difference is formed between the charged object and a neutrally charged object. This electrical discharge - which often occurs at pointed regions of the object - can damage silicon chips and computer components if they are exposed to it. Care should be taken when working on exposed computer components so that they do not become damaged, or that you do not experience ‘static shocks’.

EIDE (Enhanced IDE)

Western Digital produced the EIDE technology to improve upon the restrictions of the original IDE design. Enhanced IDE supports higher capacity devices (up to 8GB – if the BIOS is compatible) and higher data transfer rates (over double that of the IDE system). This system also supports CD-ROM and other ATAPI devices and can control up to four drives with the addition of a second 40-pin connector on a second controller. See also: IDE Device and CD-ROM.

EISA (Enhanced ISA)

The Enhanced ISA standard was developed as an extension the original ISA format. EISA provides support for both 16- and 32-bit communication and fast data transfer rates. ISA cards are compatible with this newer standard and ISA cards can be used with EISA bus slots on a computer. See also: PCI Bus, SCSI Bus and MCA Bus.

ESDI (Enhanced Small Device Interface)

Maxtor released this interface technology in 1983 and this type of interface integrated some of the drive control systems onto the drive itself. The effect of this layout was to remove noise problems to which earlier non-integrated designs were prone – this allowed support for much higher capacities and data transfer rates of 1MB per second and above. See also: IDE Device and EIDE Device.

EPP (Enhanced Parallel Port)

Enhanced parallel ports, as their name would suggest, develop on the basic properties of the parallel port and include 2MB per second data transfer rates and bi-directional 8-bit operation. There is also added support for multiple peripherals to be connected to a single port by ‘daisy-chaining’ them together. Intel, Xircom and Zenith Data Systems jointly developed the EPP standard. The computer’s BIOS and CMOS settings must be able to support this standard before it can be used. See also: Data Skew, Extended Capabilities Port and Parallel Port.

EDO (Extended Data-Out)

EDO RAM is an extension of the original DRAM chip structure that improves read access times. These performance gains, sometimes as much as 30% are brought about by the ability of the chip to continually output data from a particular address whilst simultaneously configuring an access request for the next address. See also: Burst Extended Data-Out DRAM, SRAM and RAM.

FAT (File Allocation Table)

When a disk is formatted a file allocation table or FAT is written to the disk in a particular track or sector of the medium. The FAT contains information relating to the position of file data chunks on the disk – this is useful since data is not always written to one area of the disk, but may instead be spread over several tracks. See also: Filing System, Track, Sector and Data Clusters.

Filing Systems

When data is stored on a hard or floppy disk, it is located on that medium in a particular, standardised format. This allows the drive and the computer to be able to extract the information from the disk using similar functions and thus data can be accessed in a predictable manner. Currently, several filing systems are in existence – each with its own merits and shortcomings, however, several features are central to a filing system. These are a file allocation table and numerous data clusters that contain the information. Examples of filing systems include FAT16, FAT32, VFAT and NTFS. See also: File Allocation Table, Track, Sector and Data Clusters.

Firewire (IEEE 1394 Standard)

Firewire is the terminology that has been assigned to the IEEE standard 1394. This serial bus standard supports high data rates (up to 400Mbps) and this in turn, makes it attractive for applications requiring intensive data transfer - e.g. video cameras, satellite receivers, etc. Although only a few devices currently support this type of hardware, manufacturers such as Apple are being to use this port as a standard on their ‘iMac’ range of computers. This type of port is also referred to as ‘serial SCSI type 3’. See also: Parallel Port and Serial Port.

Flash RAM

Flash RAM is similar to a ROM chip in that it retains information even when power is removed, but it adds flexibility in that it can be reprogrammed with new contents at any time. Flash RAM is often used to store BIOS information thus allowing motherboard manufacturers to produce BIOS updates on disk rather than by replacing the chip. In previous years, flash RAM has been suggested as providing virus programmers with a useful, stable storage area in which to store virus code. See also: RAM and ROM.

Floppy Disk

The floppy disk is the most common form of removable media used by computers. Floppy disks have generally been supplied in two formats – a 5.25-inch thin floppy diskette that was used by earlier computers and a 3.5-inch semi-rigid floppy disk that is standard on new computers. Floppy disks have been able to support a range of capacities from 320KB to 1.44MB, and some newer disks can support 2.88MB. Before being used on a computer system, floppy disks must be formatted to provide a basic file structure and write track information to the medium. Some disks can be pre-etched with track information and can support high data capacities – an example of this is the LS-120 disk produced by the Imation Corporation. See also: High Density, Zip Disk and Hard Disk.

Flow Control

Flow control is a term that describes the passage of data through a modem link with the presence of handshaking. This latter feature also the two connected devices to determine when they can send or receive information from the other device. See also: MODEM and Handshaking.

FPM (Fast Page Mode)

Fast Page Mode DRAM provides users with faster access that the standard DRAM chip structure. In the standard form, DRAM requires that each a row and column-type reference is used to access a particular memory cell. Fast page mode sends the row address once for many accesses to memory areas that are located in close proximity or are contiguous. Since the row information is sent only once, operational time is improved. See also: RAM and SRAM.


A gateway is a computer or other device that acts as a translator between two completely dissimilar computer systems. For example, a connection from a PC LAN to an IBM mainframe would require a gateway – this provides translation between the networks. Gateways tend to be slower than bridges or routers. Occasionally, the term gateway (or ‘default gateway’) is used to denote a router on a network that uses the TCP/IP protocol, this however is not a true gateway as described above, since both systems use the common TCP/IP protocol to facilitate communication. See also: Router.


When data is transferred from device to device, each must be able to contact the other so as to establish a rate for data flow. Handshaking allows computers and peripherals to communicate in such as way so that the receiving device can tell the source component to ‘send’, ‘stop’, ‘pause transmission’, etc. Hardware and software methods exist to provide handshaking, with each playing a slightly different role in the regulation of transmission. See also: Parallel Port and Serial Port.

Hard Disk

A hard disk drive is manufactured in glass or metal and is contained within a sealed unit (to prevent dust contamination) that is usually fixed within the system unit. Hard disks have large data capacities (due to data packaging on the disk or by the use of multiple disks on a single drive spindle) and they are also able to access data more quickly. Drive activity is normally halted in cases where the drive is not used for several minutes – this prevents wear and tear to the drive arm. See also: Floppy Disk.

High Density

Disk drives are generally classified as either low or high density. The term density refers to the property of the medium to store data – a medium with a high density has more space to store data (i.e. more tracks can exist per area of the medium) than the low-density media does. New technology has lead to the release of more high-density drives as demand for storage space has increased. An example of a high-density drive is the LS-120 SuperDisk that is marketed by the Imation Corporation. See also: Floppy Disk, Hard Disk, Track, Sector and Zip Disk.

IDE (Integrated Drive Electronics)

IDE systems (also called ATA or AT Attachment) integrate the hard disk controller with the hard drive itself. The IDE device is connected to the motherboard using a single 40-way ribbon cable and this can be used to connect one or two drives in serial when jumpers are used. The IDE standard has now been enhanced and most modern systems use a format termed ‘Enhanced IDE’. See also: EIDE Device.

IEEE1284 Standards Group

The IEEE 1284 standard describes the various implementations of the parallel port and classifies them as ‘Compatible’, ‘4-bit’ or ‘8-bit’. Two further standards have also been developed - the enhanced capabilities port and the extended parallel port. Compatibility mode was the original structure that included no DMA support, had a transfer rate of 100-200KB per second and had only an 8-bit output. The 4-bit system (Bi-Tronics) featured 40-60KB per second input (for peripherals to signify problems such as printer jams, etc.) and a 100-200KB per second data transmission rate. 8-Bit mode provided the bi-directional input and output and had data rates of 80-300KB per second. None of these modes had the capability to access memory without processor intervention (direct memory access). See also: Parallel Port, Extended Capabilities Port and Enhanced Parallel Port.

IrDA (Infra-red Device Association)

The IrDA has defined a set of standards describing a fast, serial infra-red communications link between computers and devices such as printers. Infra-red ports of this type can be internally fixed or added externally. See also: I/O Ports.

Intel Processors

Intel processors were first included in the first IBM PCs. The 8088 processor was a 40-pin DIPP with a speed of 4.77MHz. This sixteen-bit processor was limited in terms of performance due to its connection to an eight-bit external data bus. The 8088 could access up to 1MB of memory and turbo versions could operate at 9.54MHz. The 8088 processor was succeeded by the Intel 8068 and the first major upgrade - the 80286, which was able to function in real and protected modes. 80386 processors were first issued in 1986 and were available in SX and DX forms. The 80386DX was a 32-bit processor and had a 32-bit data path, compared to the 24-bit data path of the 386SX, and could access 4GB of memory. The 80486 was released in 1989 and was essentially an upgraded 386, and 80387 maths co-processor, cache controller and an 8K internal data cache. Subsequent ‘overdrive’ versions of the 486 were released that included ‘clock multiplier circuits’, a new heat-sink design and some variations contained voltage-stepping circuits to reduce the power supply from the motherboard. See also: Pentium Processor Series, Clock, Cache and Motherboard.


Low quality monitors display high-resolution images in two passes – this feature is termed interlacing. These monitors are cheaper to produce, but image quality suffers due to noticeable flickering as resolution is increased. See also: Dot Pitch and Display Adapter.


When hard drives where first produced, sectors were arranged in a logical contiguous pattern. One drawback to this layout however was that as the drive head read the data it would have sped past the next sector before the previous data could have been processed. The process of interleaving provided a novel solution to this problem: by arranging the sectors around uniform gaps, the motion of the drive head could be used advantageously. One the end of the track had been reached the sector numbering would be arranged around the previous sectors, such that an example with 15 sectors would appear – 1, 6, 11, 2, 7, 12, 3, 8, 13, 4, 9, 14, 5, 10, 15. See also: Hard Disk and Sector.

I/O (Input/Output) Addresses

Input/output peripherals have a special area of memory set aside to allow data reading and writing functions. This 64K space contains the address information for these devices and electronic components. Typically the I/O port is assigned a hexadecimal number in the range 0000-FFFF and these can be edited using a CMOS configuration program. See also: Memory Addresses and I/O Ports.

I/O (Input/Output) Ports

An input-output port essentially describes a device connection through which data can be sent and received. The serial port on modern computers is a good example of this type.

IRQ (Interrupt)

An interrupt is a communications channel that is formed between a hardware device and the system processor (an Interrupt Request Line - IRQ). When the device requires the attention of the processor it sends an alert signal along this channel and this improves processor efficiency since it does not have to continuously scan for the status of every device. When hardware is added to the computer it must be configured with a unique interrupt number (although some tape and hard disk controllers can share an IRQ). If two devices attempt to use the same interrupt, this may cause the computer to crash or the system to hang. See also: DMA.

ISA (Industry Standard Architecture)

The ISA bus was originally an 8-bit connection used on the IBM PC and XT motherboards. IBM AT and other compatible machines used an improved 16-bit design. Despite continual improvements in design, the ISA bus offers limited performance (8MHz) and alternative bus designs have been implemented. See also: EISA Bus, PCI Bus and MCA Bus.

LBA (Logical Block Addressing) Mode

One way of accessing information on a disk drive is to use the cylinder, head and track data to describe a specific location. Another method, which avoids these limitations, is to use a ‘logical block addressing’ system that would indicate the data by means of reference to the number of physical storage locations it has. See also: File Allocation Table, Filing Systems, Sector, Track and Data Cluster.

Loopback Testing

This type of test determines the viability of the connection between source and host computers. Three common forms are used: analogue, local digital and remote digital. In the analogue loopback test data is sent from the host to the modem, where it is converted into analogue form and looped back, converted into digital once again and received back at the computer. This is used to test the circuitry of the local modem (i.e. the connection between the modem and the computer that it is slotted into). Local digital loopback tests involve sending data from the destination computer, through the modem (where it is converted), across a network connection and to another modem. The data is received, decoded and converted back into analogue before being sent back to the remote computer. The remote digital loopback test is the exact opposite of this principle. See also: MODEM.

Maths Co-processors

Computer processors are capable of operating at great speeds and performing millions of mathematical calculations every second. However, standard processors cannot effectively handle trigonometric or other complex calculations. Maths co-processors are specifically designed to fulfil this role and can be used to speed up time when using computer-aided design or graphics software. Thus, with well-designed software, labour can be divided on the basis of processor ability and this can improve performance markedly. Maths co-processors are usually in-built with the Intel processor range that followed the introduction of the 486DX. See also: CPU, Intel Processors and Pentium Processor Series.

MCA (Micro Channel Architecture)

IBM developed the MCA bus in order to improve upon the performance offered by other bus architectures such as the ISA bus. Micro channel architecture buses run at 10Mhz (compared to the 8MHz offered by the ISA bus). This type of bus can support both 16- and 32-bit data although it has not been a widely implemented format due to the large royalty payments demanded by IBM and its incompatibility with the older ISA standard. See also: PCI Bus, ISA Bus and EISA Bus.

Memory Addresses

Memory addresses describe a particular region of memory in hexadecimal notation - e.g. an area of memory that is used by a particular device may be assigned to the region C000-C800.

MMX (Multimedia Extension) Instruction Set

The Multimedia Extension processors where introduced at the start of 1997. MMX adds a total of 57 new multi-media-oriented instructions to the basic Intel Pentium instruction set. These extra commands are designed to speed up the processing of graphics, audio and video functions in cases where software has been designed to use these extra processing operations. This type of technology is based upon the concept of single instruction, multiple data processing, thus many processing elements will perform the same operation on different data, with a central broadcast being passed to all elements to select the function. See also: Pentium Processor Series.

MODEM (Modulator/Demodulator)

Modems are devices that are used to convert the digital signals from a computer into the appropriate analogue signal that is required for transmission over public phone lines – this is called modulation. The reverse process, demodulation, occurs at the receiving computer. Modems are available in internal and external forms for different computer expansion slots and vary in terms of speed and data handling capabilities. See also: Flow Control, Handshaking, Loopback Testing, UART, Asynchronous Transmission and Synchronous Transmission.


Multiprocessing can be used in systems where two or more processors are used on a single motherboard and this can allow operations to be shared thereby increasing performance. In order to use multiprocessing arrangements, the PC must have a compatible motherboard, an operating system that is able to use multiple processors and well-written software that does not intensively use one processor above another. Multiprocessing may be described as either symmetric or asymmetric. In the latter case, some or one of the processors perform only system tasks whilst the other processor(s) are used by applications. In symmetric multiprocessing, both the system and the applications can run on any of the processors and thus this option provides more flexibility. See also: Multitasking.


Multitasking occurs where one or more application can be run at any one time. Pre-emptive multitasking describes the process in which the system controls and manages processor times and allocates it to applications according to requirements. The other type of multitasking is termed co-operative and it occurs where applications themselves control processor time - this however relies on the use of well-designed applications. See also: Multiprocessing.


The computer motherboard, also called the system board, provides the basic foundation for all of the computer’s hardware including the processor, RAM, BIOS and expansion cards. Several motherboard standards are available each with a different layout and associated advantages. Common motherboard types include the AT (and baby-AT), the ATX and Mini-ATX, LPX and Mini-LPX and the NLX. The most common layout (called a form factor) in newer PCs are the ATX and NLX derivatives. See also: CPU, Clock, ROM and BIOS.


In its most simple form, a network consists of two or more computers connected to each other by an appropriate transmission medium that allows them to share data. More complex networks can be developed from this basic principle – networks can be interconnected in different ways and even dissimilar networks can be linked. See also: Network Interface Cards and Network Topologies.

Network Topologies

The shape or structure of a network is commonly described as its’ topology. Topologies may be either physical, i.e. the actual appearance of the network layout, or logical, where the topology refers to the flow of data across the network. Star, bus, ring and hybrid topologies are the more everyday designs – with each kind of arrangement having individual merits and drawbacks. See also: Network Interface Cards and Network.

NIC (Network Interface Card)

The network interface card allows a physical connection between the computer and the transmission media. The NIC is variously known as the network card, the network adapter card or the transceiver. The network card moves data from the computer to the network and vice versa – to do this the card must convert the data into the necessary format for the computer and the network to understand. Network interface cards can address other cards and can recognise data that is destined for it. It does this by using a unique address known as the media access control address – the card also performs error checking. Network cards are designed for specific types of networks and do not work on different network structures, e.g. Ethernet and token ring. Network cards are available for several different expansion ports on the computer, including the ISA and PCI ports. See also: Network Topologies, Gateway, Routers and Network.

NTFS (New Technology Filing System)

The NT filing system is incompatible with both the FAT16 and the FAT32 filing systems used by other types of Windows and DOS software. NTFS does not have problems relating to cluster size and is able to provide extra features such as file-by-file compression and RAID support as well as advanced file attribute management tools. See also: File Allocation Table and Filing Systems.

Null Modem

Where two items of direct terminating equipment are to be linked, i.e. two computers, a special cable is required to compensate for the similar nature of the ports at either end of the link. In serial link example, the serial ports could not be directly linked for the purposes of data transmission and reception because the pins would be aligned - thus the data send pin would associate with the data send pin of the other connector, etc. To overcome this problem a null modem is used. This is a cross-wired cable or connector that compensates for this alignment problem.

Optical Fibre Cable

Fiber optic cable employs light signals as the basis for data transmission as opposed to the electrical signals that are used by the other main cable types. The light ‘pulses’ are produced by light emitting diodes or by laser diodes – these pulses travel down the glass core of the fiber (known as the waveguide) and the cladding that surrounds this core reflects light back to ensure transmission efficiency. Two main categories of fiber are available; monomode, which uses a single light path through the waveguide, or multimode, which as its name suggests, can allow multiple pathways. At the receiving end of the cable, light-sensitive diodes re-convert the light pulse into an electrical signal. Fiber optic networks are expensive to install and configure although rewards in terms of high bandwidths and immunity to both interference and eavesdropping have justified this expenditure in many cases. Repeaters are required at 2km intervals to boost the transmission signal. See also: UTP Cable and Repeaters.


Parallel Port

The Centronics Corporation originally invented the Centronics Parallel interface to connect external peripherals such as printers to the computer. The port itself utilises a 25-pin female ‘D’ connector. Hardware handshaking controls the transmission of each data byte (one bit per physical wire using byte-by-byte transmission) and the original parallel connection was unidirectional although some hardware hacks could manipulate the handshaking protocols to make the transmission two-way. Modern PCs can support improved versions of the parallel port such as the ECP or EPP ports. See also: Extended Capabilities Port, Enhanced Parallel Port, Serial Port and IEEE1284 Standards Group.


Data parity is an integral feature of network transmission. When a data packet is sent, it contains a specific number of bits – either an odd or an even number of bits are present. If parity is set to odd, the computer that is sending the data adds the required number of bits so that the number of data bits becomes odd. This is the same principle if parity is set to even. Parity provides a simple error checking system through which transmission faults can be overcome. If two computers have been set up to transmit using the same parity, any fault that causes the data to be disrupted may lead to an altering of parity. This discrepancy in parity is used to signify data error. See also: CRC Checking and MODEM.

PCI (Peripheral Component Interconnect)

Peripheral Component Interconnect was introduced in 1995 with the Pentium processor. It is a local bus, like the Enhanced Industry Standard Architecture (EISA) bus and it connects the CPU, memory and peripherals to a wider, faster data pathway. PCI supports Plug and Play and it supports 64-bit data, which in turn leads to performance improvements in graphics and audio intensive operations. See also: ISA Bus, EISA Bus, SCSI Bus and MCA Bus.

PCL (Printer Control Language)

Hewlett-Packard developed PCL for the purposes of page description and it works like a form of elaborate printer instruction set. Many non-postscript printers have also adopted this language as a standard and in this situation are described as in ‘HP-emulation’ mode. See also: PostScript, Printer and Printer Control Codes.

PCMCIA (Personal Computer Memory Card Industry Association)

The Personal Computer Memory Card Industry Association was founded to provide a standard bus for laptop computers. Although this type of bus was originally designed to allow memory cards to be used, it has been extended to included modems and network cards.

Peer to Peer

In peer-to-peer networks there is no dedicated server, but instead, each computer connected to the network acts as both a server and client – i.e. each computer is a ‘peer’ of the other computers. These types of networks were originally developed as a low-cost alternative to server-based systems for use in smaller companies and organizations where there are approximately ten to fifteen users. A major drawback to this type of network is a comparative lack of security whereby each user must control access to resources on his/her machine. See also: Network.

Pentium Processor Series

The Intel Pentium Processor series was introduced in the 1993 and was specifically designed to work synergistically with the PCI bus standard. Pentiums use 32-bit address and data registers and a 64-bit data path to improve speed. Although the initial Pentium chip design was dogged by heat, reliability and power problems first releases ran at 60MHz speeds. Many of the first generation Pentium chips also suffered from a floating-point bug which affected precision calculations, however this was rectified by the second and third generation chips that were released in 1994 and 1995 respectively. The Pentium chip differs from the 486 predecessor in that it features superscalar architecture. This arrangement provides the chip with tow instruction lines, called U and V that can be used to handle sections of program code. The V-line has a reduced instruction set to the U-line and so can execute a limited range of instructions at a faster rate. Subsequent Pentium chip releases included the Intel Pentium Overdrive, the Pentium Pro, the MMX Pentium and the Pentium II, that have all added extra features on top of the original design. Modern Pentium chips contain branch prediction routines, internal data caches and optimisation routines to enhance processing operation. See also: Intel Processors, Clock, Cache, Multitasking and CPU.


Adobe Systems created PostScript as a standard document description language for use in high-end printing systems. A PostScript printer has a complete on-board computer that interprets the PostScript codes precisely so as to obtain an exact replica of the original document as it appears on screen. Some, more expensive, PostScript printers have on-board memory in which font definitions are stored and this can also be used as a memory buffer for documents. See also: PCL Language, Printer Control Codes and Printer.

POST (Power-on Self Test)

The POST procedure is a hardware checking system that is built into the PCs BIOS operating system. This test sequentially monitors the state of the memory chips, the processor, system clock, display and BIOS itself. Errors that occur within vital components such as these are signified by beep codes emitted by the internal speaker of the computer. Additional interpreter boards can be purchased that can supply information concerning boot failure and are termed ‘logic boards’. See also: BIOS, ROM and Beep Codes.

Power Sag

A power sag can occur when the power supply entering a computers components dips briefly below that which is required. Sags are commonly caused when certain types of device are switched on - especially those that require a large initial voltage - thus draining the local supply of power. See also: Power Spike and Uninterruptible Power Supplies.

Power Spike

A power spike is the logical opposite of a power sag and can occur when electrical devices are turned off or when electrical storms are happening. Normally, spikes and surges are not sufficient to cause problems but occasionally big fluctuations may lead to the system crashing or hanging. See also: Power Sag and Uninterruptible Power Supplies.


A printer is a device that is used to produce a hard copy of a computer file - usually a document or picture. Several types of printer are available and they vary in terms of resolution, output quality, speed and method of printing ink onto the medium. Printers also differ in their abilities to reproduce colour, fonts, in running costs and ‘price per page’. See also: Printer Control Codes, Printer Memory Buffer and PostScript.

Printer Control Codes

Printer control codes are a series of short commands that are set to the printer from the computer in order to regulate the appearance of certain pages, paragraphs, sentences or words. Control codes were first developed by Epson and featured an ‘escape key sequence’ followed by a key combination. Although sophisticated results can be obtained occasionally, they do not generally offer the finesse of the PostScript or PCL languages. See also: Printer, Printer Memory Buffer and PostScript.


Printer Memory Buffer

The printer memory buffer is an area of memory within the printer itself where document descriptions and complete pages can be stored prior to printing. This frees up the memory of the computer to perform other tasks. See also: Printer, Printer Control Codes and PostScript.

QIC (Quarter Inch Cartridge)

This type of storage media has been available for many years and can be purchased in two formats: the original DC6x and DC3x, or the smaller mini cartridges. Capacities vary between 60MB and 25GB although the capacity depends upon the tape length of the particular variant that is used. Tape storage systems are generally a popular form of backup strategy as they are available at a low ‘cost per byte’ price. QIC devices can be connected to SCSI, IDE or Enhanced IDE ports. See also: SCSI Device, IDE Device and EIDE Device.

RAID (Redundant Array of Inexpensive Devices)

Using RAID technology, users can use multiple hard disks to provide a backup measure for network servers and workstations. Several levels of backup are suggested by this system, ranging from level 0 to level 6, each level representing a particular type of fault tolerance. See also: Hard Disks and NT Filing System.

RAM (Random Access Memory)

Random Access Memory is the principal storage space for computer data and program instructions. RAM is generally described as being volatile in the sense that once power has been removed or the computer has been re-booted, data is lost. In modern PCs, two principle forms of RAM are used: dynamic and static RAM. See also: SRAM, DRAM, BEDO DRAM, EDO DRAM, ROM, FPM RAM and ROM Shadowing.

Recordable CD Drives

CD-ROM writers can be purchased at reasonable costs and can be used to write information to a compact disc. Recordable CDs are available in two general forms: those that can be written to once and read many times (WORM discs) and those that can be written to and erased (Re-writeable CDs). CD writers use a laser to disrupt the medium of the disk - either by heating a dye, by altering the magnetic properties of the metal disk or by changing disc structure through ‘phase-change’ techniques. Discs vary in the method by which they are written to, quality medium and in the speed at which data can be written to them. See also: WORM Drives, CD-ROM and DVD.


Resistance ® describes the property of every material to prevent electrical flow through itself. Metals have little electrical resistance whereas plastics and rubber have very high resistance and in most cases will not allow electrical current to pass through them. The resistance of a body to electrical current is measured in Ohms (() and is related to potential difference (V) and current (I) by the equation V=IR.

Resource Conflicts

A resource conflict occurs when one or more devices have been configured to use the same set of system resources such as interrupts or memory areas. Typical indications of these conflicts are system failures such as device failure, system crashes and the generation of ‘General Protection Faults’. See also: Memory Addresses, Interrupts and DMA.

Removable Media

In order to share files and programs, computers can either be connected to each other (i.e. across a direct link or via a network) or must be able store and retrieve files from an interim storage medium. The most common types of removable media are floppy disks and CD-ROM discs. However the term ‘removable media’ also covers tape drives, high capacity disks (e.g. Jaz and Zip disks) and removable hard drives. See also: CD-ROM, Floppy Disk, ZIP Disk and Tape Drives.


Routing is similar in principle to bridging although it allows a greater degree of flexibility. Routers are able to link dissimilar networks and can support multiple alternate paths between locations based upon the parameters of speed, traffic loads and cost. Routers form the basic connections of the Internet and can allow data to take multiple paths (hence reducing the likelihood of transmission failure) to reach a destination. Routers function at the network layer and can therefore access source and destination addresses within packets and can keep track of multiple active paths within a given source and destination network. This allows more fault-tolerance than bridges where multiple concurrent paths are not allowed. Routing may be either ‘static’ or ‘dynamic’. See also: Repeaters, Bridges, Gateways


A repeater is a device that takes a signal and repeats it to the devices that are connected to it. Repeaters can be used to maintain signal integrity and amplitude across a connection or a network. See also: Routers, Bridges, Gateways and Optical Fibre Cable.


When an image is displayed on a monitor, it is broken up into many thousands of small dots. If the image is broken up into a very high number of dots, the image is generally of higher quality as lines and edges appear more clearly. Resolution is the term that describes the number of dots that a monitor has in which to separate an image – the higher the resolution, the better the quality. In order to produce a stable picture, the display adapter and the monitor must be compatible and able to create and display images at the same resolution. See also: Display Adapter, Dot Pitch, Interlacing and Video Standards.

ROM (Read Only Memory)

A ROM chip is a special form of memory that has data written to it during the manufacturing process and thus is not amenable to alteration. The code stored on this type of chip, often called firmware, can only be updated by replacing the chip itself. Some specialised ROM chips are available that allow data to be written and erased - these EPROM or EEPROM chips are generally more expensive although they can be useful where continual ROM updates are required. See also: RAM.

ROM (Read Only Memory) Shadowing

ROM shadowing is a technique used to enhance the performance of a computer when it accesses information from device ROM chips. Using this method, ROM code is copied into a block of RAM (which has generally faster access times than ROM chips) - this is called shadowing. Shadowing is normally used for video chip code and BIOS code. See also: ROM and BIOS.

SCSI (Small Computer Systems Interface)

The SCSI port is an industry-recognised standard that is used by many computer manufacturers in the home and business environments. Many varieties of hardware devices can use this technology and be daisy-chained to allow even greater expansion capabilities. Several manufacturers include SCSI ports as standard, although on many PCs, SCSI expansion cards are required. SCSI buses can share data amongst themselves and can also interact with the other computer components through the main data bus. Currently, 3 main SCSI systems are available: SCSI 1 (the original 8-bit bus, with a transfer rate of 5MB per second), SCSI 2 (approved in 1990 and features many enhancements to the SCSI standard) and SCSI 3. SCSI 2 includes improvements to the data bus size - called wide SCSI and to the transfer rate (fast SCSI) whilst still maintaining backward compatibility with most of the original devices. The SCSI 3 format has yet to be officially recognised and several of the key features are conflicting - the significant overlap between SCSI 3 and SCSI 2 technologies permits compatibility in many cases. See also: PCI Bus, ISA Bus, EISA Bus and MCA Bus.


A sector is the term given to the regularly sized subdivision of a drive track. During formatting, the size and position of the sectors is written to the disk so that the data can be placed into uniform ‘spots’ that the drive head can easily access. See also: Track.

Serial Port

Modern PCs usually have two serial ports that can be used to connect devices such as modems and mice to the system unit. Serial ports differ from parallel ports in that they transmit data bit-by-bit using a single data line. Two data lines within the serial link allow simultaneous input and output of information, although due to the sequential bit transmission method, flow rates can be slower than those for parallel ports. Although the original serial port used a 25-pin male D connector, most PCs today use the male DB9 (9-pin) connection method - this is suitable for most simple devices that require data control and handshaking signals. See also: Parallel Ports and I/O Ports.

SIMM (Single In-line Memory Module)

SIMMs are generally available in two formats - 32 and 72-pin and comprise a series of memory chips mounted to a small circuit board. At the bottom edge of the board, tin or gold connectors are used to form the contact between the SIMM and the motherboard although it should be noted that the connectors are usually specific for a particular connector. SIMMs are available in varying capacities and keying notches are present on the board to allow correct alignment. In the 30-pin format, SIMMs supply memory in the form of 8-bit chunks and therefore must be installed in sets of four to work with a 32-bit processor. As a corollary, 70-pin SIMMs supply memory in 32-bit chunks and so only one board is required for this processor type. See also: DIPPs, DIMMs and RAM.

SIPP (Single In-line Pin Package)

Single inline pin packages are computer memory boards that have a small, long memory circuit board attached to the motherboard of the computer using a single row of pins. These are no longer used for production motherboards and have been superseded by SIMMs and DIMMs. See also: SIMMs and DIMMs.

SRAM (Static RAM)

Static RAM is a fast form of RAM that can operate at approximately four to five times faster than DRAM since continual refreshment of data is not required. One problem with SRAM however, is that since each ‘cell’ within the chip requires six or more transistors to function, chip sizes are larger than the ‘1 transistor per cell’ DRAM chips. Static RAM is normally reserved for speed critical operations such as the system cache. See also: DRAM and RAM.

SDRAM (Synchronous DRAM)

Synchronous DRAM is a variant on the DRAM chip structure that has been specifically designed to run at the speed of the system clock thus accelerating the periodic refresh cycle times. SDRAM can run at much higher clock speeds than other types of DRAM and it provides the possibility of supporting faster motherboard designs, as they become available. See also: DRAM, SRAM and RAM.

Synchronous Transmission

Synchronous transmission involves the sending of 8-bit (1 byte) blocks as a continuous data stream with no delay between the information. The data stream begins with a sync byte and terminates with an ‘end sync’ byte – to allow this type of transmission to take place, both sender and receiver must be synchronised. Specialist systems and mainframe computers generally only use this type of communications system. Synchronous transmission has fewer data transmission overheads than asynchronous transmission in cases where the data throughput is great – this is not the case for small chunks of data. See also: MODEM and Asynchronous Transmission.

Tape Drives

Tape drives are a popular form of data backup since they provide robust, high-speed, high-capacity storage systems. These drives are available for various PC ports and can be connected to the SCSI, IDE or EIDE buses. Examples of tape drives include the quarter inch cartridge, Traven drives, Digital Audio Tapes and 8mm tape cassettes. See also: Removable Media.

TFT (Thin Film Transistor) Active Matrix Display

The TFT display provides the best resolution of all of the currently available flat-panel monitor designs, although they are also the most expensive. TFT displays offer very high image clarity, contrast ratios of between 150:1 to 200:1, fast refresh rates and wide viewing angles. See also: Dot Pitch, Resolution, Display Adapter and Interlacing.


When data is written onto a drive, it is stored as magnetic changes in the structure of the disk. These alterations are written as concentric rings as the disk spins. Each of these ‘circles’ is termed a track. See also: Sector, Removable Media, Filing Systems, File Allocation Table and High Density.

Traven Drive

The Traven tape drive is an extension of the quarter inch cartridge system that offers higher storage capacities and this is generally reflected in the price difference between the two systems. Traven drives do not require that the storage medium is formatted along its whole length. See also: Tape Drives.

Twisted Paired Cable

Twisted-pair is a common type of cable that has been extensively used for telephone systems. This cable is formed from one or more twisted cable pairs (i.e. where two insulated cables are twisted about each other). The twisting of the wires (which are generally 22 or 26 American Wire Gauge copper wires) acts to reduce interference and crosstalk. Each pair of wires is twisted at a different rate to ensure that the pairs do not interfere with each other. Several drawbacks of TP cabling however are its’ sensitivity to electromagnetic interference and eavesdropping and that it can not be used for long-distance transmission. See also: UTP Cable and Optical Fibre Cable.

UART (Universal Asynchronous Receiver-Transmitter)

A UART chip is the major component within a PCs serial port. It is responsible for processing incoming and outgoing serial data into the correct format. UARTs are also associated with the handling of parity generation and checking, the addition of start/stop bits and serial to parallel conversion (and vice versa). See also: MODEM.

UPS (Uninterruptible Power Supplies)

Uninterruptible power supplies provide an alternative AC power supply in the event of power failure. Using this type of device, power surges and spikes are minimised. A UPS requires an array of batteries, a charging circuit, an inverter to convert DC to AC current, a circuit to allow the system to take over from a failing power supply and some degree of spike and surge protection. See also: Power Sags and Power Spikes.

UNIX Systems

UNIX was originally developed by the telecommunications company, AT&T during the late 60s and early 70s. UNIX is now a family of more than 20 related operating systems that are produced by various companies. It has become the operating system of choice for many high powered workstations. It is capable of supporting parallel processing and is able to be run on a wide variety of platforms. UNIX offers a multitude of file systems in addition to its native system. UNIX servers are the main types of server that form the Internet and it is able to use the TCP/IP protocol suite to provide compatibility between networks.

USB (Universal Serial Bus)

The universal serial bus permits the connection of up to 127 different peripherals, using a single standard connector type. USB allows the connection and disconnection of equipment without the need to power-down the computer system. Power is supplied to the devices via the USB. See also: PCI Bus, ISA Bus, MCA Bus, EISA Bus, SCSI Bus and I/O Ports.

UTP (Unshielded Twisted Pair) Cable

Most twisted-pair cable that is used in networks is unshielded. The primary reason for UTP’s popularity is that it is extensively used in many telephone systems (albeit at a lower specification). UTP is widely available and various specifications have been standardized in terms of the frequency and capacity of data that each can transmit. See also: TP Cable, Optical Fibre Cable.

VESA (Video Electronic Standards Association)

The Video Electronic Standards Association produced the VESA bus in an attempt to provide a recognised format for video and other similar applications. VESA buses can run at either 25 or 33 MHz and provide a 32-bit data pathway. VESA cards are large and have a additional set of connectors - ISA cards can be used with VESA slots and simply omit this latter connection. In most modern applications, this bus type has been superseded by the PCI bus. See also: PCI Bus.

Vertical Refresh Frequency

The picture that is displayed on a monitor is updated (by vertical refreshing) many times per second. The more times the image is refreshed, the more stable the picture. High vertical refresh rate monitors are more expensive that lower refresh rate monitors, but image quality is higher and they are recommended for those who sit for long periods in front of computer screens. Although the human eye cannot perceive refresh rates at normal resolutions, television cameras, which function at low rates than the human eye can detect vertical refreshes – this is commonly seen when computers are featured in television programs and the screen appears to flicker. See also: Dot Pitch, TFT Display, Interlacing and Resolution.

Video Memory

Video memory is used by display adapters as a storage area in which to process picture information – the greater the size of the memory, the more able the adapter to display high resolutions and a greater number of colours. See also: Display Adapter and Resolution.

Video Standards

Since the birth of the PC, it has undergone many changes – perhaps most dramatically in its ability to display and handle graphical images. Several video standards now exist that are limited to text, simple block characters and low-colour, low-resolution images. The most widely available video standard that is used today is the Super VGA (video graphics array) that is able to offer resolutions between 800-600 and 1280-1024 pixels in 256 - 16 million colours. This format has replaced earlier standards such as the VGA and Extended Graphics Array (XGA). See also: Video Memory and Display Adapter.

VRAM (or dual port RAM)

High-performance video adapters use this type of RAM. VRAM has two data ports - one for read functions and the other for write operations to allow the ability to read and write data at the same time if required. The abbreviation VRAM describes VRAM video memory rather than normal ‘video memory’ (DRAM) to which it is frequently and incorrectly referred. See also: RAM, SRAM and DRAM.

WRAM (Windows RAM)

Windows RAM is the Extended Data Out form of VRAM used by Windows to execute fast data transfer and access operations. See also: EDO DRAM, DRAM, SRAM and RAM.

WORM (Write Once, Read Many) Drive

A WORM drive is able to write data to a recordable CD disc only once, although the data can be read many times. Due to the unalterable nature of data that it written to the disc, WORM discs can be used as evidence within courts. See also: Removable Media, CD-ROM and Recordable CD-ROM.

ZIP Drives

Zip drives have been developed by the Iomega Corporation and the disks used with this system are not much bigger than standard floppy disks. Zip disks are capable however of holding 100MB of data and are relatively inexpensive to buy. Zip drive hardware is available in SCSI, IDE or EIDE formats. The Iomega Corporation has also produced higher capacity disks (holding up to 2GB of data) that have quick access times – these Jaz disks (which require a Jaz drive) are more expensive than the Zip disks. See also: SCSI Device, IDE Device, EIDE Device, High Density and Floppy Disk.