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Disk Image

Disk image

A disk image is a computer file containing the complete contents and structure of a data storage device. The term has been generalized to cover any such file, whether taken from an actual physical storage device or not. An ordinary backup only backs up the files it can access; boot information, and files locked by an operating system or being changed at the time, may not be saved. A disk image contains all these, and faithfully replicates the data, so it is commonly used for backing up disks with operating systems, or bootable CD/DVDs. The image is saved as a file, which for a full CD or system disk may be quite large (~10 MB to several GB). This file can be saved onto a hard drive, CD, DVD or other media, for later use. Disk images can either be compressed using some type of compression algorithm like LZW, or uncompressed ("raw"). Images of CD-ROMs most often carry the file name extension .iso, referring to the ISO 9660 file system commonly used on such discs. The .iso format is the most common format for Linux distributions and other online images. Other common CD image formats are .nrg (Nero Burning ROM's proprietary format), and .bin/.cue.

CD Sector Size FAQ

# A standard 74 min CD is made by 333,000 sectors. # Each sector is 2352 bytes, and contains 2048 bytes of PC (MODE1) Data, 2336 bytes of PSX/VCD (MODE2) Data or 2352 bytes of AUDIO. # The difference between sector size and data content are the Headers info and the Error Correction Codes, that are big for Data (high precision required), small for VCD (standard for video) and none for audio. # If you extract data in RAW format (standard for creating images) you always extract 2352 bytes per sector, not 2048/2336/2352 bytes depending on data type (basically, you extract the whole sector). This fact has two main consequences: ## You can record data at very high speed (40x) without losing information, but if you try to do the same with PSX or Audio you get unredable CD (for PSX) or audio CD with lots of clicks because there are no error correction codes (and error are more likely to occur if you record at high speed.) ## On a 74 min CD you can fit very large RAW images,up to 333,000 x 2352 = 783,216,000 bytes (747 Mb). This should be the upper limit for a RAW image created from a 74 min CD. Remember that if you store standard data (backup files), you can burn only 333,000 x 2048 = 681,984,000 bytes (the well known 650 MB limit). # Please note that an image size is ALWAYS a multiple of 2352 bytes (you extract SECTORS), if extracted in RAW mode.

.CUE/.BIN

The .cue / .bin format developed by Jeff Arnold for CDRWIN can encode CD Image formats in either 2048 or 2324 bytes per sector. The BIN file is a binary copy of an entire CD/DVD disc. The BIN file contains all the data stored on the original disc, not only its files and folders but also its system-specific information such as, boot loaders, volume attributes and any other data. BIN files are usually bigger than .iso files because in contrast to .iso files they are not a bit for bit copy of the entire CD/DVD. .bin files (or .RAW files) are images extracted in pure RAW format. That is 2352 bytes sectors, the full CD sector content: user data, sector header, error correction codes (ECC) and error detection codes (EDC). Once again, each sector is converted to digital data in the .BIN file, but more stuff is copied and the resulting file will be bigger. The .BIN file should be 251,000 x 2352 = 590,352,000 bytes big. This process will copy ANYTHING on the disc, so it is useful for exotic discs (multiple tracks, mixed track type Audio+Data or Data+Audio) and for non-PC CDs (PSX, VCD, MAC). The .cue file is a cue sheet that describes the data stored in the .bin file. The .cue file is in fact a plain text file. A typical .cue file is as follows: FILE "IMAGE.BIN" BINARY TRACK 01 MODE1/2352 INDEX 01 00:00:00 The file would be saved as IMAGE.CUE

.ISO

The .iso file is a digital copy of CD contents made this way: the ripper searches for the sectors of the CD that have been used, say 251,000 for instance (there are 330,000 sectors on a 74 min CD and 360,000 sectors on a 80 min CD). Each sector is copied on the .ISO file, one by one, and only 2048 bytes for each sector (only the ones containing the user data) are copied. The .ISO file should then be of size 251,000 x 2048 = 514,408,000 bytes. (It will be slightly bigger if the extractor puts a header on the file, like Nero .NRG files that are .ISO files plus a small file header)

.IMG

.img is the filename extension usually use for the disk image of floppy disks, and sometimes, hard disks. First populated by DOS-based software HD-Copy DiskDupe and now WinRaWrite, is a handy way to archive a floppy disk completely, including bootable ones. In fact there is no "format" in it, just a raw dump of the content of the disk. Some newer software like WinImage supports zipped version of the format and the extension is .imz.

Apple Disk Image

dump For Macintosh computers, disk images have been available as a feature of the operating system before System 7. Mounting a compatible disk image is achieved by double-clicking (opening) the image file. The archived volume is then available as a normal disk volume, indistinguishable at first glance from the physical volume. Modern Apple Disk Images for Mac OS X typically end in .dmg. Using NetBoot, client computers can start up over a network from a server-based disk image that contains system software.

Usage

A common use of disk images is for remote distribution of software such as Linux distributions: installation floppy disks or CD-ROMs can be recorded as disk image files, transferred over the Internet, and the contents of the original disk(s) duplicated exactly by end users with their own floppy or CD-R drives. So, user can burn the Isos to convert them in LiveCDs, to try an Operating System, without installing it in the hard disk . Disk images are also routinely used in connection with software piracy, creating an image of a software package and subsequently distributing it. Another common use is to provide virtual disk drive space to be used by emulators (i.e. QEMU) and virtual machines. This can prevent the CD from getting damaged. It can also reduce bulk when one wishes to carry the contents of the CD along with oneself: one can store disk images to a relatively lightweight and bootable storage device which has a higher storage capacity than that of a CD (i.e. a USB keydrive).

References

http://forums.afterdawn.com/thread_view.cfm/19244 http://isorecorder.alexfeinman.com/isorecorder.htm http://www.petri.co.il/how_to_write_iso_files_to_cd.htm

Computer file

A file in a computer system is a stream (sequence) of bits stored as a single unit, typically in a file system on disk or magnetic tape. While a file is usually presented as a single stream, it most often is stored as multiple fragments of data at different places on a disk (or even multiple disks). One of the services operating systems usually perform for applications is that of organization of files in a file system. Files are created by software and usually conform to a particular file format. They are almost always assigned file names by the file system on which they are stored, so that they can be referred to at a later time. Some operating systems allow the contents of a file to be segmented into fixed and variable length records. For example, OpenVMS allows any arbitrary set of characters to be defined as the terminators to variable length record within a file. Others, like Microsoft Windows, have only one specialised subclass of file, called a text file, where a sequence of characters separate the data into lines of text (a specialised variable length record). Some operating systems, such as Unix, do not handle file records at the operating system level, instead it is done at the application level. See record-oriented filesystem. A special file is a file system object which is accessed as though it was a file, but the sequence of bits is supplied or consumed by another process (or by the operating system itself) such as a device driver or network interface. Indeed, the philosophy that "everything is a file" is one of the best known design decisions in Unix and Unix-like operating systems (such as Linux). Files are often organized hierarchically by the operating system, placing them in directories.

Notes

# A collection of bytes in RAM isn't usually known as a file, unless it's stored in a RAM disk. # Historically it was common for files to be defined as sequences of records. However this is now uncommon except on certain mainframe operating systems. On most systems, the application or a library creates the "record" abstraction from the byte stream according to the file format. #Some operating systems use an extension (or "suffix", although the extension does not have to be placed at the end of the filename -- some systems may place it before the filename, for example) to differentiate between files whose contents or data are organized in different formats. The operating system, as well as the underlying file system, may impose restrictions on the length of file extensions. For example, MS DOS limits file extensions to three characters or less. Examples of common file extensions on MS-DOS systems include .EXE for executable programs, .TXT for plain text files, and .ZIP for archives encoded and usually compressed using methods compatible with PKWARE's PKZIP archiver product. File extensions are commonly used to simplify file management tasks for users. For example, suppose you have a directory containing files of different media formats. Some are still images, some are animations, and some are digitally sampled audio waveforms. If you know that all the samples have an extension of .PCM, you could easily issue a copy command to copy all these files to a different directory, whether it be within the same file system or on a separate physical disk. Many application software programs use the presence of a header in the file formats with which they work to determine the actual type of a given file. For example, an executable program on DOS may have an extension of .COM even though the .COM extension is usually reserved for a different, more simple executable format. DOS knows that the file is an executable because of the presence of some signature bytes at the start of the file. 'Unix like' operating systems do not have any fixed extension for files, although .tar, .gz, .sh, and others are standard (see also magic numbers). In many cases, standardised file extensions are useful because they make it easier for users of different computer systems and operating systems to share files, since the format of a file's content is more clear.

External Links and References

- [http://www.dotwhat.net/ Dotwhat? - File Extension Listing] - Listing of file extensions and the programs that use them.
- [http://filext.com/ FILExt - The File Extension Source] - Site for looking up file extensions.
- [http://www.filename.info/ Filename.info] - Information about Microsoft Windows filenames.
- [http://www.2-spyware.com/files.php Exploit Files] - List of files associated with spyware and adware. Category:Computer data File Category: Inter-process communication ko:파일

Booting

: This article is about booting, in the sense of starting a computer. For other meanings, see Booting (disambiguation). Booting (disambiguation)] In computing, booting is a bootstrapping process that starts operating systems when the user turns on a computer system. A boot sequence is the set of operations the computer performs when it is switched on which load an operating system.

Boot loader

Most computer systems can only execute code found in the memory (ROM or RAM); modern operating systems are mostly stored on hard disks, LiveCDs and USB flash drives. Just after a computer has been turned on, it doesn't have an operating system in memory. The computer's hardware alone cannot perform complicated actions of the operating system, such as loading a program from disk; so a seemingly irresolvable paradox is created: to load the operating system into memory, one appears to need to have an operating system already installed. The solution to the paradox involves using a special small program, called a bootstrap loader or boot loader. This program doesn't have the full functionality of an operating system, but is tailor-made to load enough other software for the operating system to start. Often, multiple-stage boot loaders are used, in which several small programs summon each other, until the last of them loads the operating system. The name bootstrap loader comes from the image of one pulling oneself up by one's bootstraps (see bootstrapping). Early programmable computers had toggle switches on the front panel to allow the operator to place the bootloader into the program store before starting the CPU. This would then read the operating system in from an outside storage medium such as paper tape or an old fixed head disk drive. Pseudo-assembly code for the bootloader might be as simple as the following eight instructions: 0: set the P register to 8 1: check paper tape reader ready 2: if not ready, jump to 1 3: read a byte from paper tape reader to accumulator 4: if end of tape, jump to 8 5: store accumulator to address in P register 6: increment the P register 7: jump to 1 A related example is based on a loader for a 1970's Nicolet Instrument Corporation minicomputer. Note that the bytes of the second-stage loader are read from paper tape in reverse order. 0: set the P register to 106 1: check paper tape reader ready 2: if not ready, jump to 1 3: read a byte from paper tape reader to accumulator 4: store accumulator to address in P register 5: decrement the P register 6: jump to 1 The length of the second stage loader is such that the final byte overwrites location 6. After the instruction in location 5 executes, location 6 starts the second stage loader executing. The second stage loader then waits for the much longer tape containing the operating system to be placed in the tape reader. The difference between the boot loader and second stage loader is the addition of checking code to trap paper tape read errors, a frequent occurrence with the hardware of the time, which in this case was an ASR-33 teletype. In modern computers the bootstrapping process begins with the CPU executing software contained in ROM (for example, the BIOS of an IBM PC) at a predefined address (the CPU is designed to execute this software after reset without outside help). This software contains rudimentary functionality to search for devices eligible to participate in booting, and load a small program from a special section (most commonly the boot sector) of the most promising device. Boot loaders may face peculiar constraints, especially in size; for instance, on the IBM PC and compatibles, the first stage of boot loaders must fit into the first 446 bytes of the Master Boot Record, in order to leave room for the 64-byte partition table and the 2-byte AA55h 'signature', which the BIOS requires for a proper boot loader.

Second-stage boot loader

partition table The small program is most often not itself an operating system, but only a second-stage boot loader, such as NTLDR, LILO or GRUB. It will then be able to load the operating system proper, and finally transfer execution to it. The system will initialize itself, and may load device drivers and other programs that are needed for the normal operation of the OS. The boot process is considered complete when the computer is ready to interact with the user or the operating system is capable of running ordinary applications. Typical modern PCs boot in about a minute (of which about 15 seconds are taken by the preliminary boot loaders, and the rest by loading the operating system), while large servers may take several minutes to boot and to start all services - to ensure high availability, they bring up some services before others. Most embedded systems must boot almost instantly -- for instance, waiting a minute for the television to come up is not acceptable. Therefore they have their whole operating system in ROM or flash memory, so it can be executed directly.

BIOS boot devices

A boot device is any device that must be initialized prior to loading the operating system. This includes the primary input device (keyboard), the primary output device (display), and the initial program load device (floppy drive, hard drive, CD-ROM, keydrive, etc.). (An IPL device is any device in the system that can boot and load an operating system, a stand alone utility (i.e. memtest86+) or even a boot loader; in old AT machines, this is the floppy drive or hard drive.) In a modern BIOS, the user can select one of several devices from which to boot, for example hard disk, floppy, SCSI, CDROM, ZIP, LS-120 or USB (USB-FDD, USB-ZIP, USB-CDROM, USB-HDD). For example, one can install Microsoft Windows on the first hard disk and Linux on the second. By changing the BIOS boot device, the user can select the operating system to load.

Boot sequence on standard PC (IBM-PC compatible)

operating system Upon starting, a personal computer's CPU runs the instruction located at the memory location FFFF0h of the BIOS. This memory location is close to the end of system memory. It contains a jump instruction that transfers execution to the location of the BIOS start-up program. This program runs a Power-On Self Test (POST) to check that devices the computer will rely on are functioning; it also initializes these devices. Then, the BIOS goes through a preconfigured list of devices until it finds one that is bootable. If it finds no such device, an error is given and the boot process stops. If the BIOS finds a bootable device, it loads and executes its boot sector. In the case of a hard drive, this is referred to as the master boot record (MBR) and is often not operating system specific. Usually, the MBR code checks the partition table for an active partition. If one is found, the MBR code loads that partition's boot sector and executes it. The boot sector is often operating system specific, however in most operating systems its main function is to load and execute a kernel, which continues startup. If there is no active partition or the active partition's boot sector is invalid, the MBR may load a secondary boot loader and pass control to it and this secondary boot loader will select a partition (often via user input) and load its boot sector, which usually loads the corresponding operating system kernel.

Other kinds of boot sequence

Some other processors have other kinds of boot modes; most digital signal processors have the following boot modes:
- Serial mode boot
- Parallel mode boot
- HPI boot
- Warm boot or soft reboot (as opposed to hard reboot) refers to an abridged start up which does not require that power be removed and reapplied.

Random reboot

A Random reboot is a random and unintended occurrence of computer reboot. This may be caused by:
- software issues, including
- incompatible software
- improper software or driver configuration
- buggy code
- computer virus or malicious software
- hardware issues, including
- incompatible hardware
- improper hardware configuration
- loose connections
- improper memory timing
- defective/damaged hardware
- failing hard drive (especially in TiVos)
- overheating
- CPU overheating
- other issues, including
- electrical problems.
- power surge

External links

- [http://homepages.tesco.net./~J.deBoynePollard/FGA/dos-windows-boot-process.html The DOS and DOS/Windows boot processes]
- [http://www.pcnineoneone.com/howto/multiboot1.html Multi-booting, running different versions of Windows on your PC]
- [http://am.xs4all.nl/phpwiki/index.php/jumping%20bootfloppy Jumping bootfloppy]
- [http://www.kernelthread.com/mac/osx/arch_boot.html Mac OS X Boot Process]
- [http://www.pycs.net/lateral/stories/23.html The Linux boot process unveiled]
- [http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html From power up to Linux bash prompt HOWTO]
- [http://www.khmere.com/freebsd_book/html/ch02.html Bootstrapping FreeBSD]
- Vea, Matthew (2005): [http://www.omninerd.com/2005/11/05/articles/40 "How To Program a Bootstrap Loader" Bootstrap source code tutorial in x86 assembly language.] Category:Operating system technology ja:ブート

Operating system

In computing, an operating system (OS) is the system software responsible for the direct control and management of hardware and basic system operations. Additionally, it provides a foundation upon which to run application software such as word processing programs, web browsers and others. Network operating system and firmware are other types of operating systems.

Introduction

Early computers lacked operating systems. A human operator would manually load and run programs. When programs were developed to load and run other programs, it was natural to draw their name from the human job they replaced. Most current usage of the term "operating system" today, by both popular and professional sources, refers to all the software that is required in order for the user to manage the system and to run third-party application software for that system. That is, the common understanding includes not only the low-level "kernel" that interacts directly with the hardware, but also libraries required by applications as well as basic programs to manipulate files and configure the system. The exact delineation between the operating system and application software is not precise, however, and is occasionally subject to controversy. For example, one of the key questions in the United States v. Microsoft antitrust trial was whether Microsoft's Internet Explorer web browser was part of its Windows operating system or if it was a separable piece of application software. As another example, the GNU/Linux naming controversy is, in part, due to disagreement about the relationship between the Linux kernel and the Linux operating system. The lowest level of any operating system is its kernel, the first layer of software loaded into computer memory when it starts up. As the first software layer, all other software that gets loaded after it depends on this software to provide them with various common core services. These common core services include, but are not limited to: disk access, memory management, task scheduling, and access to other hardware devices. Like the term "operating system" itself, the question of what exactly should form the "kernel" is subject to some controversy—with various camps advocating "microkernels", "monolithic kernels", and so on—with debates over whether things like file systems should be included in the kernel.

System Calls

System calls are operations/services that are requested by applications from the operating system. As noted on the System Call page, "System calls often use a special machine code instruction which causes the processor to change mode (e.g. to "supervisor mode" or "protected mode")."

Common core services

As operating systems evolve, ever more services are expected to be common core. Since the 1990s, OS's have often been required to provide network and Internet connectivity. They may be required to protect the computer's other software from damage by malicious programs, such as viruses. The list of common core services is ever expanding. Programs communicate with each other through application programming interfaces, similar to how humans interact with programs through user interfaces. This is especially true between application programs and the OS. The OS's common core services are accessed by application programs through the OS's APIs. Thus an OS enables the communication between hardware and software. CPU scheduling is also a main function of the operating system. See also: POSIX

Today's operating systems

Firstly, there is a distinction between console OS's, using only the keyboard for input, such as DOS, and the modern visual OS's, focusing on the mouse and using a GUI (sometimes implemented as a shell around the former). Secondly, which OS can be used often depends on the hardware architecture, most specifically the CPU that is used, with only Linux and BSD running on almost any architecture. In the past there have been many types of OS's, but starting in the 1990's the choice for personal computers has come to be largely restricted to the Microsoft Windows family and the Unix-like family, of which Linux is becoming the major representative. Mainframe computers and embedded systems use a variety of different operating systems, many with no direct connection to Windows or Unix, but mostly closer to Unix than Windows.
- Personal computers
- IBM PC compatible - smaller Unix-variants, like Linux and BSD, and Microsoft Windows
- Apple Macintosh - Mac OS X, Linux and BSD
- Mainframes - Unix variants, Microsoft Windows and a score of other OS's, mostly related to Unix
- Embedded systems - a variety of dedicated OS's, and limited versions of Linux or other OS's

Unix-like systems

Embedded system.]] The Unix-like family is a diverse group of operating systems, with several major sub-categories including System V, BSD, and Linux. The name "Unix" is a trademark of The Open Group which licenses it for use to any operating system that has been shown to conform to the definitions that they have cooperatively developed. The name is commonly used to refer to the large set of operating systems which resemble the original Unix. Unix systems run on a wide variety of machine architectures. They are used heavily as server systems in business, as well as workstations in academic and engineering environments. Free software Unix variants, such as Linux and BSD, are increasingly popular. They have made inroads on the desktop market as well, particularly with "user-friendly" Linux distributions such as Ubuntu Linux. Some proprietary Unix variants like HP's HP-UX and IBM's AIX are designed to run only on that vendor's proprietary hardware. Others, such as Solaris, can run on both proprietary hardware and on commodity x86 PCs. Apple's Mac OS X, a BSD variant derived from NeXTSTEP and FreeBSD, has replaced Apple's earlier (non-Unix) Mac OS in a small but dedicated market, in the process becoming the most popular proprietary Unix system. Over the past several years, free Unix systems have supplanted proprietary ones in many markets. For instance, scientific modeling and computer animation were once the province of SGI's IRIX. Today, they are dominated by Linux-based clusters.

Microsoft Windows

IRIX The Microsoft Windows family of operating systems originated as a graphical layer on top of the older MS-DOS environment for the IBM PC. Modern versions are based on the newer Windows NT core that first took shape in OS/2. Windows runs on 32- and 64-bit Intel and AMD computers, although earlier versions also ran on the DEC Alpha, MIPS and PowerPC architectures (and there was work in progress to make it work also on the SPARC architecture). Today, Windows is a popular desktop operating system, enjoying a near-monopoly of around 90% of the worldwide desktop market share. It is also widely used on low-end and mid-range servers, supporting applications such as web servers and database servers. In recent years, Microsoft has spent significant marketing and R&D money to demonstrate that Windows is capable of running any enterprise application (see the TPC article).

Other operating systems

Mainframe operating systems, such as IBM's z/OS, and embedded operating systems such as QNX, eCos, and PalmOS, are usually unrelated to Unix and Windows, except Windows CE, Windows NT Embedded 4.0 and Windows XP Embedded which are related to Windows and several
- BSDs and Linux distributions tailored for the requirements of an embedded system. Older operating systems which are still used in niche markets include the Windows-like OS/2 from IBM; VMS from Hewlett-Packard (formerly DEC); Mac OS, the non-Unix precursor to Apple's Mac OS X; RISC OS, which is specifically designed to run on ARM processor architectures; and AmigaOS, the first graphical user interface (GUI) based operating system with advanced multimedia capabilities available to the general public. Research and development of new kinds of operating systems is an active subfield of computer science. Microsoft Singularity is a research project to develop an operating system with better memory protection.

Examples of operating systems

- Mac OS
- Microsoft Windows
- Unix (including BSD and its derivatives, and "unix-like" OSes such as Linux and GNU)
- yellowTAB Zeta, based on BeOS
- DOS (and its fore-runner CP/M) For more examples, see the list of operating systems.

Classifications and terminology

: An operating system is conceptually broken into three sets of components: a user interface (which may consist of a GUI and/or a command line interpreter or "shell"), low-level system utilities, and a kernel--which is the heart of the operating system. As the name implies, the shell is an outer wrapper to the kernel, which in turn talks directly to the hardware. Hardware <-> Kernel <-> Shell <-> Applications | | +----------+ 1 2 3 In some operating systems the shell and the kernel are completely separate entities, allowing you to run varying combinations of shell and kernel (e.g. UNIX), in others their separation is only conceptual. Kernel design ideologies include those of the monolithic kernel, microkernel, hybrid kernel(modified micro kernel) and exokernel. Many of the major commercial systems such as UNIX, Windows (dos based), and Linux use a monolithic approach, some systems use a microkernel (such as in AmigaOS, QNX, and BeOS) and others like Apple's Mac OS X and Microsoft Windows NT line use the hybrid approach. The microkernel approach is also very popular among research operating systems. Both approaches have produced successful systems and have their advantages. Many embedded systems use ad hoc exokernels.

Bootable

Boot loader

Second-stage boot loader

BIOS boot devices

Boot sequence on standard PC (IBM-PC compatible)

Other kinds of boot sequence

Random reboot

External links

DVD

DVD (sometimes called "Digital Versatile Disc" or "Digital Video Disc") is an optical disc storage media format that can be used for data storage, including movies with high video and sound quality. DVDs resemble compact discs as their physical dimensions are the same (12cm in diameter) but they are encoded in a different format and at a much higher density, using the Universal Disk Format (UDF) file system. The official DVD specification is maintained by the DVD Forum.

History

In the early 1990s two high density optical storage standards were being developed: one was the MultiMedia Compact Disc (MMCD), backed by Philips and Sony, and the other was the Super Density Disc (SD), supported by Toshiba, Time-Warner, Matsushita Electric, Hitachi, Mitsubishi Electric, Pioneer, Thomson, and JVC. IBM's president, Lou Gerstner, acting as a matchmaker, led an effort to unite the two camps behind a single standard, anticipating a repeat of the costly format war between VHS and Betamax in the 1980s. Philips and Sony abandoned their MMCD format and agreed upon Toshiba's SD format with two modifications that are both related to the servo tracking technology. The first one was the adoption of a pit geometry that allows "push-pull" tracking, a proprietary Philips/Sony technology. The second modification was the adoption of Philips' EFMPlus. EFMPlus, created by Kees Immink, who also designed EFM, is 6% less efficient than Toshiba's SD code, which resulted in a capacity of 4.7 Gbyte instead of SD's original 5 Gbyte. The great advantage of EFMPlus is its great resilience against disc damage such as scratches and fingerprints. The result was the DVD specification Version 1.0, announced in 1995 and finalized in September 1996. The first DVD players and discs were available in November 1996 in Japan, March 1997 in the United States, 1998 in Europe and in 1999 in Australia. The first pressed DVD was the movie Twister in 1996. The movie had the first test for 2.1 surround sound. In 1999 Independence Day was the first movie to introduce 5.1 surround sound. By the spring of 1999 the price of a DVD player had dropped below the US$300 mark. At that point Wal-Mart began to offer DVD players for sale in its stores. When Wal-Mart began selling DVDs in their stores, DVDs represented only a small part of their video inventory; VHS tapes of movies made up the remainder. As of 2005 the situation is now completely reversed; DVD sales make up the bulk of gross sales and VHS is a slim minority. The price of a DVD player has dropped to below the level of a typical VCR; a low-end player with reasonable quality can be purchased for under US$50 in many retail stores and many modern computers are sold with DVD-ROM drives stock. Most, but not all, movie "sets" or series have been released in box sets, as have some entire seasons or selected episode volumes of older and newer television programs. DVD rentals first topped those of VHS during the week of June 15, 2003 (27.7M rentals DVD vs. 27.3M rentals VHS). Major U.S. retailers Circuit City and Best Buy stopped selling VHS tapes in 2002 and 2003, respectively. In June 2005, Wal-Mart and several other retailers announced plans to phase out the VHS format entirely, in favor of the more popular DVD format. According to the Digital Entertainment Group (DEG), all DVD sales and rentals (films, television series, special interests, etc) totaled $21.2 billion in 2004. The sales portion of that was $15.5 billion. In comparison, the total 2004 US box office for theatrical rentals was $9.53 billion (per the National Association of Theater Owners or NATO). While the growth of theatrical films on DVD has cooled recently, that of television programs and music video has increased dramatically. In 2000, Sony released its PlayStation 2 console in Japan. In addition to playing video games developed for the system it was also able to play DVD movies. This proved to be a huge selling point because the PS2 cost about the same as DVD player but could do a whole lot more. As a result, many electronic stores that normally did not carry video game consoles carried PS2s. In keeping with this tradition, Sony has announced that it will implement one of DVD's possible successors, Blu-ray, into its next PlayStation console currently known as the PlayStation 3. Microsoft's Xbox, released a year after the PlayStation 2, also had the capability to play DVD discs with an add-on kit, cementing the DVD's place in video game consoles. "DVD" was originally an initialism for "Digital Video Disc." Some members of the DVD Forum believe that it should stand for "Digital Versatile Disc" to reflect its widespread use for non-video applications. Toshiba, which maintains the official DVD Forum site [http://www.dvdforum.org/faq-dvdprimer.htm#1], adheres to the interpretation of "Digital Versatile Disc." The DVD Forum never reached a consensus on the matter, however, and so today the official name of the format is simply "DVD"; the letters do not officially stand for anything.[http://www.dvddemystified.com/dvdfaq.html]

Technical information

DVDs are made from a 0.6 mm thick disc of polycarbonate plastic coated with a much thinner (reflective) aluminium layer. Two such discs are glued together to form a 1.2 mm double-sided disc. The substrates are half as thick as a CD to make it possible to use a lens with a higher numerical aperture and therefore use smaller pits and narrower tracks. A single-layer DVD can store 4.7 Gbyte, which is around seven times as much a standard CD-ROM. By employing a red laser at 650 nm (was 780 nm) wavelength and a numerical aperture of 0.6 (was 0.45), the read-out resolution is increased by a factor 1.65. This holds for two dimensions, so that the actual physical data density increases by a factor of 3.5. DVD uses a more efficient coding method in the physical layer. CD's error correction, CIRC, is replaced by a powerful Reed-Solomon product code, RS-PC; Eight-to-Fourteen Modulation (EFM) is replaced by a more efficient version, EFMPlus, which has the same characteristics as classic EFM. The CD subcode is removed. As a result, the DVD format is 47 percent more efficient with respect to CD-ROM, which uses a "third" error correction layer. After separating the DVD by its contents, we can distinguish following types of DVDs :
- DVD-Video (containing movies (video and sound))
- DVD-Audio (containing high-definition sound)
- SACD (containing high-definition sound)
- DVD-Data (containing data) The disc medium can be:
- DVD-ROM (read only, manufactured by a press)
- DVD-R/RW (R = Recordable once, RW = ReWritable)
- DVD-RAM (random access rewritable)
- DVD+R/RW (R = Recordable once, RW = ReWritable)
- DVD-R DL (dual layer)
- DVD+R DL (double layer) DVD+R DL The disc may have one or two sides, and one or two layers of data per side; the number of sides and layers determines the disc capacity.
- DVD-5: single sided, single layer, 4.7 gigabytes (GB), or 4.38 gibibytes (GiB)
- DVD-9: single sided, double layer, 8.5 GB (7.92 GiB)
- DVD-10: double sided, single layer on both sides, 9.4 GB (8.75 GiB)
- DVD-14: double sided, double layer on one side, single layer on other, 13.3 GB (12.3 GiB)
- DVD-18: double sided, double layer on both sides, 17.1 GB (15.9 GiB) The capacity of a DVD-ROM can be visually determined by noting the number of data sides, and looking at the data side(s) of the disc. Double-layered sides are sometimes gold-colored, while single-layered sides are silver-colored, like a CD. One additional way to tell if a DVD contains one or two layers is to look at the center ring on the underside of the disc. If there are two barcodes, it is a dual layer disc. If there is one barcode, there is only one layer. Each medium can contain any of the above content and can be any layer type. Double layer DVD+R discs are already on the market. right The DVD Forum created the official DVD-ROM/R/RW/RAM standards and the DVD+RW Alliance created the DVD+R/RW standards. Since DVD+R/RW discs are not technically DVDs as per the DVD Forum standards, they are not allowed to display the DVD logo; instead, they display an "RW" logo (even if it is not re-writeable, something some consider deceptive advertising). However, they are readable by most DVD drives, so they are referred to as DVD+R and DVD+RW. The "+" (plus) and "-" (dash) are similar technical standards and are partially compatible. As of 2004, both formats are equally popular, with about half of the industry supporting "+", and the other half "-". All DVD readers are supposed to read both formats, though real-world compatibility is around 90% for both formats, with DVD-R having the best overall compatibility in independent tests. Most new DVD writers can write both formats and carry both the RW and DVD logos. Unlike compact discs, where sound (CDDA, Red Book) is stored in a fundamentally different fashion than data (Yellow book et al.), a properly authored DVD will always contain data in the UDF filesystem. UDF The data transfer rate of a DVD drive is given in multiples of 1350 kB/s, which means that a drive with 16x speed designation allows a data transfer rate of 16 × 1350 = 21600 kB/s (21.09 MB/s). As CD drive speeds are given in multiples of 150 kB/s, one DVD "speed" equals nine CD "speeds," so an 8x DVD drive should have a data transfer rate similar to that of a 72x CD drive. In physical rotation terms (spins per second), one DVD "speed" equals three CD "speeds," so an 8x DVD drive has the same rotational speed as 24x CD drive. Early CD and DVD drives read data at a constant rate. The data on the disc is passed under the read head at a constant rate (Constant Linear Velocity, or CLV). As linear (meters per second) track speed grows at outer parts of the disc proportionally to the radius, the rotational speed of the disc was adjusted according to which portion of the disc was being read. Most current CD and DVD drives have a constant rotation speed (Constant Angular Velocity, or CAV). The maximum data rate specified for the drive/disc is achieved only at the end of the disc's track (discs are written from inside). The average speed of the drive therefore equals to only about 50–70% of the maximum nominated speed. While this seems a disadvantage, such drives have a lower seek time as they do not have to change the disc's speed of rotation.

DVD-Video

seek time seek time DVD-Video discs require a DVD-drive with a MPEG-2 decoder (e.g. a DVD-player or a DVD computer drive with a software DVD player). Commercial DVD movies are encoded using a combination of MPEG-2 compressed video and audio of varying formats (often multi-channel formats as described below). Typical data rates for DVD movies range from 3–10 Mbit/s, and the bit rate is usually adaptive. The video resolution on NTSC discs is 720 × 480 and on PAL discs is 720 × 576. A high number of audio tracks and/or lots of extra material on the disc will often result in a lower bit rate (and image quality) for the main feature. The audio data on a DVD movie can be of the format PCM, DTS, MP2, or Dolby Digital (AC-3). In countries using the NTSC standard any movie should contain a sound track in (at least) either PCM or Dolby AC-3 formats, and any NTSC player must support these two; all the others are optional. This ensures any standard compatible disc can be played on any standard compatible player. The vast majority of commercial NTSC releases today employ AC-3 audio. Initially, in countries using the PAL standard (e.g. most of Europe) the sound of DVD was supposed to be standardized on PCM and MP2, but apparently against the wishes of Philips, under public pressure on December 5, 1997, the DVD Forum accepted the addition of Dolby AC-3 to the optional formats on discs and mandatory formats in players. The vast majority of commercial PAL releases employ AC-3 audio by now. DVDs can contain more than one channel of audio to go together with the video content. In many cases, sound tracks in more than one language track are present (for example the film's original language as well as a dubbed track in the language of the country where the disc is being sold). With several channels of audio from the DVD, the cabling needed to carry the signal to an amplifier or TV can occasionally be somewhat frustrating. Most systems include an optional digital connector for this task, which is then paired with a similar input on the amplifier. The selected audio signal is sent over the connection, typically over RCA connectors or TOSLINK, in its original format to be decoded by the audio equipment. When playing compact discs, the signal is sent in S/PDIF format instead. Video is another issue which continues to present problems. Current players typically output analog video only, both composite video on an RCA jack, as well as S-Video in the standard connector. However neither of these connectors were intended to be used for progressive video, so yet another set of connectors has started to appear, to carry a form of component video, which keeps the three components of the video, one luminance signal and two color difference signal, as stored on the DVD itself, on fully separate wires (whereas S-Video uses two wires, uniting and degrading the two color signals, and composite only one, uniting and degrading all three signals). The connectors are further confused by using a number of different physical connectors on different player models, RCA or BNC, as well as using VGA cables in a non-standard way (VGA is normally analog RGB—a different, incompatible form of component video). Even worse, there are often two sets of component outputs, one carrying interlaced video, and the other progressive. In Europe and other PAL areas, SCART connectors are typically used, which carry both composite and analog RGB interlaced video signals, as well as analog two-channel sound on a single multiwire cable, and which offer a reasonable compromise between video quality—which is superior to S-Video though inferior to progressive component video —and cost. HDMI is a new connection similar to SCART, but it carries High Definition, Enhanced Definition and Standard Definition video. Along with video HDMI also supports up to eight-channel digital audio. Some HDMI-equipped DVD players can upconvert the video to higher definition formats such as 720p and, more rarely, 1080p. DVD Video may also include one or more subtitle tracks in various languages, including those made especially for the hearing impaired. They are stored as images with transparent background which are overlaid over the video during playback. Subtitles are restricted to four colors (including transparency) and thus tend to look cruder than permanent subtitles on film. DVD Video may contain Chapters for easy navigation (and continuation of a partially watched film). If space permits, it is also possible to include several versions (called "angles") of certain scenes, though today this feature is mostly used—if at all—not to show different angles of the action, but as part of internationalization to e.g. show different language versions of images containing written text, if subtitles will not do. A major selling point of DVD Video is that its storage capacity allows for a wide variety of extra features in addition to the feature film itself. This can include audio commentary that is timed to the film sequence, documentary features, unused footage, trivia text commentary, simple games and film shorts.

Restrictions

DVD-Video has four complementary systems designed to restrict the DVD user in various ways: Macrovision, Content Scrambling System (CSS), region codes, and disabled user operations (UOPs).

Content-scrambling system

Many DVD-Video titles use content-scrambling system (CSS) encryption, which is intended to discourage people from bypassing the region control mechanism (see below). Usually, users need to install software provided on the DVD or downloaded from the Internet such as WinDVD, PowerDVD, or VLC to be able to view the disc in a computer system. The CSS has caused major problems for the inclusion of DVD players in any open source operating systems, since open source player implementations are not officially given access to the decryption keys or license the patents involved in the CSS. Proprietary software players were also difficult to find on some platforms. However, a successful effort has been made to write a decoder by reverse engineering, resulting in DeCSS. This has led to long-running legal battles and the arrest of some of those involved in creating or distributing the DeCSS code, through the use of the controversial U.S. Digital Millennium Copyright Act, on the grounds that such software could also be used to facilitate unauthorized copying of the data on the discs. But as U.S. law stops at the border of the United States, the rest of the world can enjoy de-scrambling software to bypass the DVD restrictions. A number of software programs have since appeared on the Web to view DVDs on a number of platforms. The restrictions also prevent people from copying DVDs. In the past few years a large amount of software has been created to make copies such as DVD Shrink, DVD Decrypter and [http://www.shrinkto5.com/ ShrinkTo5].

Region codes

Each DVD-Video disc contains one or more region codes, denoting the area(s) of the world in which distribution and playback are intended. The commercial DVD-Video player specification dictates that a player must only play discs that contain its region code. In theory, this allows the motion picture studios to control the various aspects of a release (including content, date and price) on a region-by-region basis. In practice, many DVD players allow playback of any disc, or can be modified to do so. Entirely independent of encryption, region coding pertains to regional lockout, which originated in the video game industry. video game See a [http://www.robertsdvd.com/world.gif world map showing region codes] European Region 2 DVDs may be sub-coded "D1" through "D4." "D1" identifies a UK-only release. "D2" and "D3" identify European DVDs that are not sold in the UK and the Republic of Ireland. "D4" identifies DVDs that are distributed throughout Europe. Any combination of regions can be applied to a single disc. For example, a DVD designated Region 2/4 is suitable for playback in Western Europe, Oceania and any other Region 2 or Region 4 area. Often labeled "all" or "all regions," a so-called "Region 0" disc (actually coded Region 1/2/3/4/5/6) is meant to be playable worldwide. The term "Region 0" also describes DVD players that were designed or modified to incorporate Regions 1–6 simultaneously, thereby providing compatibility with virtually any disc, irrespective of region[s]. This apparent solution was popular in the early days of the DVD format, but studios quickly responded by adjusting discs to refuse to play in such machines. This system is known as "Regional Coding Enhancement" or RCE. Nowadays, many "multi-region" DVD players defeat regional lockout and RCE by automatically identifying and matching a disc's region code and/or allowing the user to manually select a particular region. Others simply bypass the region code check entirely. Some manufacturers of DVD players now freely supply information on how to disable regional lockout, and on some recent models, it appears to be disabled by default. Many view region code enforcement as a violation of WTO free trade agreements; however, no legal rulings have yet been made in this area.

User operations

DVD-Video allows the disc to specify whether or not the user may perform any operation, such as selecting a menu, skipping chapters, forwarding or rewinding—essentially any function on the remote control. This is known as User Operation Prohibitions, or UOPs for short. Most DVD players respect these commands (e.g. by preventing fast-forwarding through a copyright message at the beginning of a disc), although some can be configured to ignore them. Many grey-market players ignore UOPs.

DVD-Audio

DVD-Audio is a format for delivering high-fidelity audio content on a DVD. It offers many channels (from mono to 5.1 surround sound) at various sampling frequencies and sample rates. Compared to the CD format, the much higher capacity DVD format enables the inclusion of either considerably more music (with respect to total running time and quantity of songs) or far higher audio quality (reflected by higher linear sampling rates and higher vertical bit-rates, and/or additional channels for spatial sound reproduction). Despite DVD-Audio's superior technical specifications, there is debate as to whether or not the resulting audio enhancements are distinguishable to typical human ears. DVD-Audio currently forms a niche market, probably due to its dependency upon new and relatively expensive equipment.

Security

DVD-Audio discs employ a robust copy prevention mechanism, called Content Protection for Prerecorded Media (CPRM / CPPM) supported by the DVD+RW Alliance (IBM, Intel, Matsushita, and Toshiba). Unlike DVD-Video's CSS, as of 2005, CPPM has not yet been cracked. All above formats are also available as 8 cm (3 inch) sized DVD mini discs (not mini-DVD, which describes DVD data on a CD) with a disc capacity of 1.5 GB.

Players and recorders

Modern recorders often support additional formats, including DVD+/-R/RW, CD-R/RW, MP3, WMA, SVCD, JPEG, PNG, SVG, KAR and MPEG-4 (DivX/XviD). Some also include USB ports or flash memory readers. Many players are priced from under $/€ 50 and recorders from $/€ 200. DVD drives for computers usually come with one of two kinds of Regional Playback Control (RPC), either RPC-1 or RPC-2; This is used to enforce the publisher's restrictions on what regions of the world the DVD can be played. See Regional lockout.

Competitors and successors

There are several possible successors to DVD being developed by different consortiums: Sony's Blu-ray Disc, Hitachi/Toshiba's HD DVD, the Holographic Versatile Disc, and Maxell's holographic disc. The first generation of holographic media with 300 GB of storage capacity and a 160 Mbit/s transfer rate is scheduled for release in late 2006 by Maxell and its partner, InPhase. On November 18, 2003, the Chinese news agency Xinhua reported the final standard of the Chinese government-sponsored Enhanced Versatile Disc (EVD), and several patents for it. On November 19, 2003, the DVD Forum decided by a vote of eight to six that HD DVD will be the HDTV successor to DVD. On April 15, 2004, in a co-op project with TOPPAN Printing Co., the electronic giant Sony Corp. successfully developed the paper disc, a storage medium that is made out of 51% paper and offers up to 25 GB of storage, about five times more than the standard 4.7 GB DVD. The disc can be easily cut with scissors and recycled, offering foolproof data security and an environment-friendly storage media. As reported in a summer, 2005, issue of Popular Mechanics, it is not yet clear which technology will win the format war over DVD. HD DVD discs contain less information than Blu-ray discs (15GB vs. 25GB for single layer, 30GB–50GB for dual layer), but Blu-ray requires changes in manufacturing machinery and techniques. This situation—two new formats fighting as the successor to a format rapidly approaching obsoletion—previously appeared as the "war of the speeds" in the record industry of the 1950s; see gramophone record for details of that situation. It was also, of course, the driver behind the VHS/Betamax war in consumer video recorders in the late 1970s.

References

- DVD Demystified, Jim Taylor; McGraw-Hill Professional; ISBN 0071350268 (2nd edition, December 22 2000)
- DVD Authoring and Production, Ralph Labarge; CMP Books; ISBN 1578200822 (August 2001)
- Bennett, Hugh. Understanding Recordable & Rewritable DVD. Cupertino: Optical Storage Technology Association, Apr. 2004. [http://www.osta.org/technology/dvdqa/]

External links

Official

- http://www.dvdforum.org DVD Forum
- http://www.dvdrw.com/ DVD+RW Alliance
- [http://www.dvdcca.org/ DVD Copy Control Association and the Content Scramble System (CSS)]

Technology

- [http://www.osta.org/technology/dvdqa/ Understanding Recordable & Rewritable DVD] by Hugh Bennett
- [http://www.dvdrecorderworld.com/formatlist.php DVD recorder formats explained]

DVD collection

- [http://www.ultimatedisney.com The Ultimate Guide to Disney DVD] News, reviews, and more
- [http://www.cinemademerde.com Cinema de Merde] Reviews and essays on bad and cheesy movies on video
- [http://www.dvdaficionado.com/ DVD Aficionado organizer]
- [http://dvd.ign.com/releases.html IGN | Upcoming DVD releases] at IGN
- [http://www.thedvdinsider.com DVD reviews and analysis] at The DVD Insider
- [http://dvdmg.com/ A Weekly Guide to DVD Movie Releases & Reviews] at DVD Movie Guide
- [http://www.herorealm.com/ Weekly look at DVDs coming out] at Hero Realm
- [http://www.dvdanthology.com/ DVD Anthology Collectors Community]

Other

- [http://www.dvddemystified.com/dvdfaq.html DVD Frequently Asked Questions (and Answers)]
- [http://www.rabid.oneuk.com/dvd.html Free DVD rental offers in the UK]
- [http://www.dvd-copy.com/reference/The_Challenge_of_DVD_Authoring.pdf The Challenge of DVD Authoring] a PDF report at DVD-Copy.com
- [http://www.nbc4.tv/news/2281636/detail.html "DVD Rentals Overtake VHS Cassettes For First Time,"] June 19, 2003 article from NBC4 TV
- [http://www.dvdark.co.uk Archive of Region 2 and Region 0 DVDs from 1997 onwards] provided by DVDark.co.uk
- [http://fy.chalmers.se/~appro/linux/DVD+RW/ DVD+RW/+R/-R(W) for Linux]
- [http://www.dvdtoday.com/Recordable%20DVD%20Test.htm DVD and DVD+RW compatibility testing]
- [http://www.howstuffworks.com/dvd.htm How DVD works] from HowStuffWorks.com Category:120 mm discs Category:Computer storage media Category:Audio_storage Category:Video storage Category:consumer electronics ko:DVD ms:DVD ja:DVD simple:DVD th:ดีวีดี

Megabyte

A megabyte is a unit of information or computer storage equal to approximately one million bytes. Megabyte is commonly abbreviated as MB (not to be confused with Mb, which is used for megabits), and sometimes as meg.

Definition

Because of inconsistencies in the use of SI-derived prefixes such as kilo- and mega-, the exact number can be any one of the following: # 1,000,000 bytes (1000², 10⁶): This is the definition recommended by IEC. It is used primarily in networking contexts and most storage media, particularly hard drives and DVDs. This definition of 'mega-' as a "binary prefix is consistent with the other SI prefixes, and with many other uses of the prefix in computing, such as CPU clock speeds or measures of performance. # 1,048,576 bytes (1,024², 2²⁰): This definition is used for nearly all discussions of computer memory (as computer addresses are naturally powers of two, and memory is most easily manufactured in power-of-two capacities) and file storage. As of 2005, most software uses this definition to express storage capacity (e.g. file size). # 1,024,000 bytes (1,024×1,000): This definition occurs rarely. It was used in a small number of storage contexts, most notably the "1.44 MB" (actually 1,474,560 bytes) "3.5-inch" (actually 90 mm) high-density floppy diskette. Several attempts to resolve these inconsistencies have been proposed, most notably the standard created by the IEC to redefine 1,048,576 bytes as a "mebibyte". This standard has been adopted by several organizations, such as the NIST and IEEE. See Binary prefix for more information.

Megabytes in use

As of 2005, the storage capacities of hardware devices such as Hard drives are given as 1 megabyte = 1,000,000 bytes (especially for items made in Japan, as Japan has standardized on the metric definition). Other capacities, such as for the RAM capacity of most personal computers, are given as 1 megabyte = 1,048,576 bytes (also known as a "mebibyte"). Depending on compression methods and file format, a megabyte of data can hold:
- one book,
- a hundred small images, or
- roughly a minute of compressed music. A digital photograph produced by a typical digital camera in 2005 might be between 1–4 MB depending on the camera's image resolution and level of compression used. Until the introduction of hard drives with a capacity of one gigabyte or more, the capacity of hard drives was measured in megabytes.

External links

- [http://www.numion.com/Calculators/Time.html Download time calculator]
- [http://www.numion.com/Calculators/Units.html Unit Converter]
- [http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci212542,00.html the megabyte] (established definition in Networking and Storage industries; from [http://whatis.techtarget.com/definition/0,,sid9_gci551931,00.html whatis.com])
- [http://www.iec.ch/zone/si/si_bytes.htm International Electrotechnical Commission definitions]
- [http://www.lyberty.com/encyc/articles/kb_kilobytes.html KiloBytes vs. kilobits vs. Kibibytes] (a common standard proposal) Category:Units of information th:เมกะไบต์

Gigabyte

:This article is about the unit of measurement, for the computer hardware manufacturer see Gigabyte Technology. A gigabyte (derived from the SI prefix giga-) is a unit of information or computer storage equal to one billion bytes. It is commonly abbreviated GB in writing (not to be confused with Gb, which is used for gigabit) and gig in writing or speech. There are two slightly different definitions of the size of a gigabyte in use:
- 1,000,000,000 bytes or 10⁹ bytes is the decimal definition used in telecommunications (such as network speeds) and some computer storage manufacturers (such as hard disks and flash drives). This usage is compatible with SI.
- 1,073,741,824 bytes, equal to 1024³, or 2³⁰ bytes. This is the definition used for computer memory sizes, and most often used in computer engineering, computer science, and most aspects of computer operating systems. The IEC recommends that this unit should instead be called a gibibyte (abbreviated GiB), as it conflicts with SI units used for bus speeds and the like.
Distinction between 1000 and 1024 megabytes
:Main article: Binary prefix As a result of this confusion, the unadorned term gigabyte is useful only where just one digit of precision is required. In technical specifications, the first usage is typically expanded to remove the ambiguity ("GB is one billion bytes"). The only exception is RAM, where sizes are always given in the power-of-two units natural to this domain. Thus, to convert metric gigabytes into binary gigabytes (for example a 100 GB drive contains 93 GiB when installed), follow this formula: : $\frac$ where $y$ is size of drive in metric gigabytes To clarify the distinction between decimal and binary prefixes, in 1997 the International Electrotechnical Commission (IEC), a standards body, proposed complete conformity with the International System of Units (SI) conventions (standard IEC 60027-2). New units were formed as unions of the SI prefixes with the word "binary". Thus 2³⁰ bytes would be called a gibibyte (GiB) while the use of gigabyte is deprecated in this context. Thus, gigabyte unambiguously follows the SI convention as 10⁹ bytes. This naming convention is steadily gaining acceptance, but deprecated usage is common. The convention must be inferred from context or fine print.
Gigabytes in use

- As of 2005, most consumer hard drives are defined by their gigabyte-range capacities. The true capacity is usually some number above or below the class designation. Although most hard disk manufacturers' definition of GB is 1,000,000,000 bytes (only computer memory has a natural inclination towards units that are powers of 2), most computer operating systems use the 1,073,741,824 byte definition. This distinction can cause confusion.
- As of 2005, consumer hard drive per-gigabyte costs are 0.50-0.80 USD.
- In speech, gigabyte is often informally abbreviated to gig, as in "This is a ten-gig hard drive".
- A DVD-5 format disc is capable of storing 4.7 GB. One gigabyte is roughly equal to 18 hours of MP3 music (at 128 kbit/s). Many USB flash drives have a capacity of 1 GB.
See also

- gibibyte
- gigabit
- binary prefix
- orders of magnitude (data)
External links

- http://physics.nist.gov/cuu/Units/binary.html
- http://www.iec.ch/zone/si/si_bytes.htm
- http://www.quinion.com/words/turnsofphrase/tp-kib1.htm
- http://www.nist.gov/public_affairs/techbeat/tb9903.htm Category:Units of information th:จิกะไบต์

Compression algorithm

In computer science, data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than a more obvious representation would use, through use of specific encoding schemes. For example, this article could be encoded with fewer bits if we accept the convention that the word "compression" is encoded as "comp". One popular instance of compression that many computer users are familiar with is the ZIP file format, which, as well as providing compression, acts as an archiver, storing many files in a single output file. As is the case with any form of communication, compressed data communication only works when both the sender and receiver of the information understand the encoding scheme. For example, this text makes sense only if the receiver understands that it is intended to be interpreted as characters representing the English language. Similarly, compressed data can only be understood if the decoding method is known by the receiver. Compression is possible because most real-world data are very statistically redundant. When represented in its human-interpretable form (or in the case of text to be printed on a computer screen, a simple machine-interpretable form such as ASCII), the data are represented in a non-concise way. For example, the letter 'e' is much more common in English text than the letter 'z', and the likelihood of the letter 'q' being followed by the letter 'z' is rather remote. Analysis of these statistical behaviors can allow the same information to be represented much more concisely. Further compression is possible if some loss of fidelity is allowable. For example, a person viewing a picture or television video scene might not notice if some of its finest details are removed or not represented perfectly. Similarly, two strings of samples representing an audio recording may sound the same but actually not be exactly the same under detailed computer analysis. Specialized signal processing techniques can take advantage of allowing relatively minor differences in order to enable representing the picture, video, or audio using fewer bits. Compression is important because it helps reduce the consumption of expensive resources, such as disk space or connection bandwidth. However, compression requires information processing power, which can also be expensive. The design of data compression schemes therefore involves trade-offs between various factors including compression capability, any amount of introduced distortion, computational resource requirements, and often other considerations as well. Some schemes are reversible so that the original data can be reconstructed (lossless data compression), while others accept some loss of data in order to achieve higher compression (lossy data compression).

Applications

One very simple means of compression, for example, is run-length encoding, wherein large runs of consecutive identical data values are replaced by a simple code with the data value and length of the run. This is an example of lossless data compression. It is often used to better use disk space on office computers, or better use the connection bandwidth in a computer network. For symbolic data such as spreadsheets, text, executable programs, etc., losslessness is essential because changing even a single bit cannot be tolerated (except in some limited cases). For visual and audio data, some loss of quality can be tolerated without losing the essential nature of the data. By taking advantage of limitations of the human sensory system, a great deal of space can be saved while producing output which is nearly indistinguishable from the original. These lossy data compression methods typically offer a three-way tradeoff between compression speed, compressed data size and quality loss. Lossy image compression is used in digital cameras, greatly reducing their storage requirements while hardly degrading picture quality at all. Similarly, DVDs use the lossy MPEG-2 codec for video compression. In lossy audio compression, methods of psychoacoustics are used to remove non-audible (or less audible) components of the signal. Compression of human speech is often performed with even more specialized techniques, so that "speech compression" or "voice coding" is someti

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Computer file

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Compression algorithm

Applications