Videodisc, also known as laserdisc, thin circular platter that contains video and audio information similar to, but with greater storage capacity than, a compact disc. Videodiscs are used primarily in the entertainment industry to store and play movies and in education to contain information such as an encyclopedia.
The videodisc is made of clear plastic and is coated with a thin photosensitive material. Its diameter ranges from 12 to 30 cm (5 to 12 in). A videodisc contains a series of microscopic pits burned into it by a laser. Each pit represents the number 1, and each smooth area (untouched by the laser) represents the number 0. These 1s and 0s, called binary numbers, are used to store the information in a format compatible with the electronic circuitry of a computer. The sequence of microscopic pits is arranged in tracks that spiral into the center of the disc. These tracks are small enough to fit nearly 500 of them in the period at the end of this sentence. For mass duplication of a particular disc, a mold is made from a master disc, and subsequent discs are made from imprints of the master.
A finished videodisc can be placed in a laserdisc player designed to read the information encoded upon it. A low-power laser illuminates the disc as it spins, and the resulting reflected patterns made by the series of pits and smooth areas are focused onto a detector. The detector decodes the patterns into binary numbers, which are then translated by electronic circuitry into the original video and audio signals.
Schemes for encoding and compressing video signals, such as motion joint photographic expert group (JPEG) or motion picture expert group (MPEG and MPEG2), have enabled large amounts of video information to be stored on a single disc. Compression techniques identify a recurring series of numbers that represent a particular piece of information and replace that series with a single number to save space.
Compression technology, along with techniques that correct information when it is improperly stored or transferred from the disc, have made optical storage systems, such as videodiscs and compacts discs, useful for personal computer systems. This technology will allow the production of a new generation of optical storage media known as Digital Video Discs (DVD).
A DVD is a high-density compact disc encoded with MPEG2 video and audio information. These discs will initially contain about 136 minutes of video and audio information. The DVD will offer superior resolution for video information and will support both standard and high-definition television formats. Eventually, a higher-density version of the DVD will contain about nine hours of video and audio information.
The main drawback with optical storage media such as videodiscs is that new information may not overwrite information that has been burned into the disc. Magnetic tape, such as the kind used in VCRs, has the distinct advantage of being able to record and rerecord, but it lacks the resolution and durability of videodiscs. The videodisc is more resistant to heat and scratches, and produces higher-resolution images with better quality sound.

Compact discs (CDs), which may be found in over 25 million American homes, not to mention backpacks and automobiles, first entered popular culture in the 1980s. But their history goes back to the 1960s, when an inventor named James Russell decided to create an alternative to his scratched and warped phonograph records—a system that could record, store, and replay music without ever wearing out. The result was the compact disc (CD). Made from 1.2 mm of polycarbonate plastic, the disc is coated with a much thinner aluminum layer that is then protected with a film of lacquer. The lacquer layer can be printed with a label. CDs are typically 120 mm in diameter, and can store about 74 minutes of music. There are also discs that can store 80, 90, 99, and 100 minutes of music, but they are not as compatible with various stereos and computers as the 74–minute size. The information on a standard CD is contained on the polycarbonate layer, as a single spiral track of pits, starting at the inside of the disk and circling its way to the outside. This information is read by shining light from a 780 nm wavelength semiconductor laser through the bottom of the polycarbonate layer. The light from the laser follows the spiral track of pits, and is then reflected off either the pit or the aluminum layer. Because the CD is read through the bottom of the disc, each pit looks like a bump to the laser. Information is read as the laser moves over the bumps (where no light will be reflected) and the areas that have no bumps, also known as land (where the laser light will be reflected off the aluminum). The changes in reflectivity are interpreted by a part of the compact disc player known as the detector. It is the job of the detector to convert the information collected by the laser into the music that was originally recorded onto the disc. This invention brought 22 patents to James Russell, who today says he working on an even better system for recording and playing back music.

In computer science, acronym for compact disc read-only memory, a rigid plastic disk that stores a large amount of data through the use of laser optics technology. Because they store data optically, CD-ROMs have a much higher memory capacity than computer disks that store data magnetically. However, CD-ROM drives, the devices used to access information on CD-ROMs, can only read information from the disc, not write to it.

The underside of the plastic CD-ROM disk is coated with a very thin layer of aluminum that reflects light. Data is written to the CD-ROM by burning microscopic pits into the reflective surface of the disk with a powerful laser. The data is in digital form, with pits representing a value of 1 and flat spots, called land, representing a value of 0. Once data is written to a CD-ROM, it cannot be erased or changed, and this is the reason it is termed read-only memory. Data is read from a CD-ROM with a low power laser contained in the drive that bounces light—usually infrared—off of the reflective surface of the disk and back to a photodetector. The pits in the reflective layer of the disk scatter light, while the land portions of the disk reflect the laser light efficiently to the photodetector. The photodetector then converts these light and dark spots to electrical impulses corresponding to 1s and 0s. Electronics and software interpret this data and accurately access the information contained on the CD-ROM.
CD-ROMs can store large amounts of data and so are popular for storing databases and multimedia material. The most common format of CD-ROM holds approximately 630 megabytes (see Byte). By comparison, a regular floppy disk holds approximately 1.44 megabytes.
CD-ROMs and Audio CDs are almost exactly alike in structure and data format. The difference between the two lies in the device used to read the data—either a CD-ROM player or a compact disc (CD) player. CD-ROM players are used almost exclusively as computer components or peripherals. They may be either internal (indicating they fit into a computer’s housing) or external (indicating they have their own housing and are connected to the computer via an external port).
Both types of players spin the discs to access data as they read the data with a laser device. CD-ROM players only spin the disc to access a sector of data and copy it into main memory for use by the computer, while audio CDs spin throughout the time that the audio recording is read out, directly feeding the signal to an audio amplifier.
The most important distinguishing feature among CD-ROM players is their speed, which indicates how fast they can read data from the disc. A single-speed CD-ROM player reads 150,000 bytes of data per second. Double-speed (2X), triple-speed (3X), quadruple-speed (4X), six-times speed (6X), and eight-times speed (8x) CD-ROM players are also widely available.
Other important characteristics of CD-ROM players are seek time and data transfer rate. The seek time (also called the access time) measures how long it takes for the laser to access a particular segment of data. A typical CD-ROM takes about a third of a second to access data, as compared to a typical hard drive, which takes about 10 milliseconds (thousandths of a second) to access data. The data transfer rate measures how quickly data is transferred from the disk media to the computer’s main memory.
The computer industry also manufactures blank, recordable compact discs, called CD-Rs (compact disc-recordables), that users can record data onto for one-time, permanent storage using CD-R drives. Compact disc-rewriteables (CD-RWs) are similar to CD-Rs, but can be erased and rewritten multiple times. Another technology that allows the user to write to a compact disc is the magneto-optical (MO) disk, which combines magnetic and optical data storage. Users can record, erase, and save data to these disks any number of times using special MO drives.

Even after decades of evolution in computing, the hard disk drive remains the primary means of fast data storage and retrieval in computer systems of all sizes. The disk itself consists of a rigid aluminum alloy platter coated with a magnetic oxide material. The platter can be rotated at speeds of more than 10,000 rpm. A typical drive consists of a stack of such platters mounted on a rotating spindle, with a read/write head mounted above each platter. Early hard drive heads were controlled by a stepper motor, which positioned the head in response to a series of electrical pulses. (This system is still used for floppy drives.) Today’s hard drives, however, are controlled by a voice-coil actuator, similar in structure to an audio speaker. The coil surrounds a magnet. When a current enters the coil, it generates a magnetic field that interacts with that of the permanent magnet, moving the coil and thus the disk head. Unlike the stepper motor, the voice coil is continuously variable and its greater precision allows data tracks to be packed more tightly on the platter surface, increasing disk capacity. The storage capacity of a drive is determined by the number of platters and the spacing (and thus number) of tracks that can be laid down on each platter. Capacities have steadily increased while prices have plummeted: In 1980, for example, a hard drive for an Apple II microcomputer cost more than $1,000 and held only 5 MB of data. As of 2007 internal hard drives with a capacity of 500 GB or more cost around a $150.00.

Data is organized on the disk by dividing the tracks into segments called sectors. When the disk is prepared to receive data (a process called formatting), each sector is tested by writing and reading sample data. If an error occurs, the operating system marks the sector as unusable (virtually any hard disk will have at least a few such bad sectors). The set of vertical corresponding tracks on the stack of platters that make up the drive is called a cylinder. Since the drive heads are connected vertically, if a head is currently reading or writing for example sector 89 on one platter, it is positioned over that same sector on all the others. Therefore, the operating system normally stores files by filling the full cylinder before going to a new sector number.
Another way to improve data flow is to use sector interleaving. Because many disk drives can read data faster than the operating system can read it from the disk’s memory buffer, data is often stored by skipping over adjacent sectors. Thus, instead of storing a file on sectors 1, 2, and 3, it might be stored on sectors 1, 3, and 5 (this is called a 2:1 interleave). Moving the head from sector 1 to sector 3 gives the system enough time to process the data. (Otherwise, by the time the system was ready to read sector 2, the disk would have rotated past it and the system would have to wait through a complete rotation of the disk.) Newer CPUs are often fast enough to keep up with contiguous sectors, avoiding the need for interleaving. Data throughput tends to decrease as a hard drive is used. This is due to fragmentation. The operating system runs out of sufficient contiguous space to store new files and has to write new files to many sectors widely scattered on the disk. This means the head has to be moved more often, slowing data access. Using an operating system (or Parts of a typical hard disk drive. Many hard drives have multiple heads and platters to allow for storage of larger amounts of data. Users can periodically reorganize their hard drive so that files are again stored in contiguous sectors. Files can also be reorganized to optimize space rather than access time. If an operating system has a minimum cluster size c4K, a single file with only 32 bytes of data will still consume 4,096 bytes. However, if all the files are written together as one huge file (with an index that specifies where each file begins) that waste of space would be avoided. This is the principle of disk compression. Disk compression does slow access somewhat (due to the need to look up and position to the actual data location for a file) and the system becomes more fragile (since garbling the giant file would prevent access to the data in perhaps thousands of originally separate files). The low cost of high capacity drives today has made compression less necessary.

Computer designers must deal with the way different parts of a computer system process data at different speeds. For example, text or graphical data can be stored in main memory (RAM) much more quickly than it can be sent to a printer, and in turn data can be sent to the printer faster than the printer is able to print the data. The solution to this problem is the use of a buffer (sometimes called a spool), or memory area set aside for the temporary storage of data. Buffers are also typically used to store data to be displayed (video buffer), to collect data to be transmitted to (or received from) a modem, for transmitting audio or video content (see streaming) and for many other devices (see input/output). Buffers can also be used for data that must be reorganized in some way before it can be further processed. For example, character data is stored in a communications buffer so it can be serialized for transmission.

Buffering Techniques
The two common arrangements for buffering data are the pooled buffer and the circular buffer. In the pool buffer, multiple buffers are allocated, with the buffer size being equal to the size of one data record. As each data record is received, it is copied to a free buffer from the pool. When it is time to remove data from the buffer for processing, data is read from the buffers in the order in which it had been stored (first in, first out, or FIFO). As a buffer is read, it is marked as free so it can be used for more incoming data. In the circular buffer there is only a single buffer, large enough to hold a number of data records. The buffer is set up as a queue (see queue) to which incoming data records are written and from which they are read as needed for processing. Because the queue is circular, there is no “first” or “last” record. Rather, two pointers (called In and Out) are maintained. As data is stored in the buffer, the In pointer is incremented. As data is read back from the buffer, the Out pointer is incremented. If either pointer reaches around back to the beginning, it begins to wrap around. The software managing the buffer must make sure that if the In pointer goes past the Out pointer, then the Out pointer must not go past In. Similarly, if Out goes past In, then In must not go past Out. The fact that programmers sometimes fail to check for buffer overflows has resulted in a seemingly endless series of security vulnerabilities, such as in earlier versions of the UNIX sendmail program. In one technique, attackers can use a too-long value to write data, or worse, commands into the areas that control the program’s execution, possibly taking over the program (see also computer crime and security). Buffering is conceptually related to a variety of other techniques for managing data. A disk cache is essentially a special buffer that stores additional data read from a disk in anticipation that the consuming program may soon request it. A processor cache stores instructions and data in anticipation of the needs of the CPU. Streaming of multimedia (video or sound) buffers a portion of the content so it can be played smoothly while additional content is being received from the source. Depending on the application, the buffer can be a part of the system’s main memory (RAM) or it can be a separate memory chip or chips onboard the printer or other device. Decreasing prices for RAM have led to increases in the typical size of buffers. Moving data from main memory to a peripheral buffer also facilitates the multitasking feature found in most modern operating systems, by allowing applications to buffer their output and continue processing.

With any computer system a fundamental design problem is how to provide for the basic communication between the processor and the devices used to obtain or display data, such as the video screen, keyboard, and parallel and serial ports. In personal computers, the BIOS (Basic Input-Output System) solves this problem by providing a set of routines for direct control of key system hardware such as disk drives, the keyboard, video interface, and serial and parallel ports.
 In PCs based on the IBM PC architecture, the BIOS is divided into two components. The fixed code is stored on a PROM (programmable read-only memory) chip commonly called the “ROM BIOS” or “BIOS chip.” This code handles interrupts (requests for attention) from the peripheral devices (which can include their own specialized BIOS chips). During the boot sequence the BIOS code runs the POST (power-on self test) and queries various devices to make sure they are functional. (At this time the PC’s screen will display a message giving the BIOS manufacturer, model, and other information.) Once DOS is running, routines in the operating system kernel can access the hardware by making calls to the BIOS routines. In turn, application programs can call the operating system, which passes requests on to the BIOS routines.
The BIOS scheme has some flexibility in that part of the BIOS is stored in system files (in IBM PCs, IO.SYS and IBMIO.COM). Since this code is stored in files, it can be upgraded with each new version of DOS. In addition, separate device drivers can be loaded from files during system startup as directed by DEVICE commands in CONFIG.SYS, a text file containing various system settings. For further flexibility in dealing with evolving device capabilities, PCs also began to include CMOS (complementary metal oxide semiconductor) chips that allow for the storage of additional parameters, such as for the configuration of memory and disk drive layouts.
In modern PCs the BIOS setup screen also allows users to specify the order of devices to be used for loading system startup code. This, for example, might allow a potentially corrupted hard drive to be bypassed in favor of a bootable CD or DVD with disk repair tools. Another scenario would allow users to boot from a USB memory stick and use a preferred operating system and working files without disturbing the PC’s main setup. The data on these chips is maintained by a small onboard battery so settings are not lost when the main system power is turned off. Additionally, modern PC BIOS chips use “flash memory” (EEPROM or “electrically erasable programmable read-only memory”) to store the code. These chips can be “flashed” or reprogrammed with newer versions of the BIOS, enabling the support of newer devices without having to replace any chips.

It is widely believed that viruses are incapable of causing permanent damage to computer hardware. This belief, however, is incorrect, since viruses can cause damage to computer components in a number of different ways.
 Both Magistr and Kriz, for instance, are capable of effectively destroying a computer by overwriting the BIOS, the special software needed to control the keyboard, display screen, disk drives, serial communications, and a number of other functions. 20 Without a functioning BIOS, a computer is unable to start up. To repair the machine, it may be necessary to replace the computer’s main circuit board (motherboard), which is a time-consuming and expensive operation.

All materials associated with computer operations other than the actual machinery (the hardware). Essentially, software is a set of instructions for a computer, installed via a variety of media, including CD-ROMs, discs or the Internet, or created and installed manually in the computer, using the keyboard. Most software is grouped into two categories of programs— system software and application software.
 System software controls basic programs that permit the computer to operate, while application software instructs the computer to perform specific, practical tasks such as word processing or accessing the Internet. Computer owners can purchase or copy an almost limitless variety of software, although the amount that any given computer can store is limited by computer capacity, and it can be more convenient to access software programs over the Internet just long enough to perform a specific operation without installing the software onto the user computer.

The number of dedicated Internet cafés in the United States and many other highly developed countries has been declining in recent years. This is largely due to the growing number of people who connect to the Internet through their own laptops and other mobile devices. Thus many locations, including coffee chains such as Starbucks, do not provide machines, but simply offer wireless Internet access.

Areas where one can make such a wireless connection are called “hot spots.” Today virtually all major hotels and airports provide hot spots; there is normally a fee for access as with Internet cafés. (The fee is collected by routing all access through a portal.) However, a number of venues offer free Wifi access. Users of Internet cafés or hot spots should be aware that they are sharing an ad hoc network with strangers and may be exposed to malicious software. Passwords or other sensitive data may be “sniffed” using special software. It is therefore generally a good idea not to conduct financial transactions or otherwise send sensitive information when connected to such venues, unless one has provided for encryption or can access a virtual private network. Additionally, users connecting their own machines to a hot spot should have up-to-date firewall and antivirus software.

Wireless networking allows you to connect to a network and often the Internet without using a cable. As more people purchase laptops, PDAs, and other mobile devices, wireless network access has become increasingly popular and convenient. Wireless access points must be set up properly to provide sufficient security. However, most wireless access points are set up in a manner that is extremely insecure. Without the proper precautions, wireless networking can place your privacy, your data, and your computer at significant risk. 
When connected to an insecure wireless network, anyone within range of your computer and using the right tools can easily capture your traffic as it is transmitted across the network. This type of ‘‘listening in,’’ known as sniffing, can be done with a laptop (or PDA), a wireless card, and some freely available software and is very difficult to detect.
Attackers may also use your connection for their own nefarious purposes. This could include anything from illegally downloading copyrighted files to posting child pornography on the Internet.
Follow these tips to use someone else’s wireless connection safely:
• Avoid sending sensitive information over a wireless network.
• Encrypt your traffic.
• Provide sensitive information only to ‘‘secure’’ sites, i.e., sites that display https:// in the address bar and a padlock.
• Use virtual private networking (VPN) to encrypt all network traffic to and from your computer. If you have VPN access through your company or school, use it whenever you access a wireless network.
If you have set up your own wireless router or access point, you are responsible for ensuring that you have configured the router settings to provide sufficient security. Ensure that you have configured the following:
• Disable set service identifier (SSID) broadcasting. To make it easier to connect to a wireless network, access points broadcast the name of their network so that mobile devices can easily detect them. Disabling SSID broadcasting makes your wireless network ‘‘invisible’’ unless you already know the name of the network.
• Enable media access control (MAC) address filtering. Each wireless network card has a unique identification number known as a MAC address. By allowing only your network cards to access your wireless network, you can prevent other people from using your Internet connection without your permission.
• Enable Wi-Fi protection access (WPA) encryption. Enabling encryption on your access point serves two purposes. First, it prevents attackers from sniffing your traffic, which prevents them from watching what you are doing online. Second, it forces anyone attempting to access your wireless network to enter a password to gain access to your network. Wireless routers provide a number of other ways to ensure only permitted computers are accessing the network.

The internet is a global ‘network of networks’, allowing computers around the world to communicate with each other. This network has been in existence since the late 1960s. In the early 1990s a new language for accessing information on this network was created: Hypertext Markup Language (HTML). This allowed the creation of web pages as we now know them.

A crucial feature of these pages is the ability to move from one to another very simply, by clicking on a hyperlink. This allows for easy navigation between web pages, which are linked together to collectively make up the World Wide Web. The ease of access which this enabled facilitated the rapid expansion of the internet, and its development from a military and research tool into a form of mass communication and commerce. It is hard accurately to measure the true scale of the web, but one estimate in February 2007 suggested it comprised nearly 30 billion pages, on 109 million distinct websites.
Each computer on the internet has an IP address (for example, 123.4.5.678), a numerical equivalent to a telephone number. Since IP numbers are hard to remember, the addressing system has a second element, the domain name system (DNS), which uses names such as ‘www.oft.gov.uk’. One or more domain names can be associated with a particular IP address. The association of IP addresses and domain names allows a user to easily access a website in its physical location on a computer, on a network.
The network is operated by Internet Service Providers (ISPs), who have arrangements to exchange communications between one another. The domain name system is managed by international, regional and national bodies. In the UK, Nominet is responsible for registering ‘.uk’ domain names.