ISO image
An ISO image is an archive file
Archive file
An archive file is a file that is composed of one or more files along with metadata that can include source volume and medium information, file directory structure, error detection and recovery information, file comments, and usually employs some form of lossless compression. Archive files may be...

 (also known as a disc image) of an optical disc
Optical disc
In computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data in the form of pits and lands on a special material on one of its flat surfaces...

, composed of the data contents of every written sector of an optical disc, including the optical disc file system. ISO images can be created from optical discs or from a collection of files
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...

 by image creation software; images can be used to write optical discs. Software distributed on bootable discs is often available for download in ISO image format, and used to write a CD or DVD. ISO image files often have a file extension of .iso. The name ISO is taken from the ISO 9660
ISO 9660
ISO 9660, also referred to as CDFS by some hardware and software providers, is a file system standard published by the International Organization for Standardization for optical disc media....

 file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...

 used with CD-ROM media, but what is known as an ISO image might also contain a UDF
Universal Disk Format
Universal Disk Format is an implementation of the specification known as ISO/IEC 13346 and ECMA-167 and is an open vendor-neutral file system for computer data storage for a broad range of media. In practice, it has been most widely used for DVDs and newer optical disc formats, supplanting ISO 9660...

 (ISO/IEC 13346) file system or a DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

 or Blu-ray Disc
Blu-ray Disc
Blu-ray Disc is an optical disc storage medium designed to supersede the DVD format. The plastic disc is 120 mm in diameter and 1.2 mm thick, the same size as DVDs and CDs. Blu-ray Discs contain 25 GB per layer, with dual layer discs being the norm for feature-length video discs...

 (BD) image.


ISO images are stored in an uncompressed format. Any CD or DVD can be archived by .ISO format as a true digital copy of the original. The ISO image file is not stored in a container file. Unlike a physical optical disc, an image can be transferred over any data link or removable storage medium.

What is known as a "valid" ISO image is an uncompressed collection of various files merged into one single resulting file, according to definite and standard formatting.

An ISO image can be rendered, or "burned," to a CD, DVD, or BD by using media authoring
Optical disc authoring
Optical disc authoring, including DVD and Blu-ray Disc authoring , is the process of assembling source material—video, audio or other data—into the proper logical volume format to then be recorded onto an optical disc .-Process:To burn an optical disc, one usually first creates an...

 or disc burning software. It can also be opened using any file archiver
File archiver
A file archiver is a computer program that combines a number of files together into one archive file, or a series of archive files, for easier transportation or storage...

. ISO burning is now typically a native feature of modern home and business computer operating systems.

Hybrid disc formats include the ability to be read by different devices, operating systems, or hardware. In the past, one example of this was a disc that supported both Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

 and Macintosh
The Macintosh , or Mac, is a series of several lines of personal computers designed, developed, and marketed by Apple Inc. The first Macintosh was introduced by Apple's then-chairman Steve Jobs on January 24, 1984; it was the first commercially successful personal computer to feature a mouse and a...

 from a single disc image. A later example is the release of hybrid ISO files which can be booted
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...

 or started from both BD or DVD and USB flash drive devices when the image is written to any of these storage device
Data storage device
thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....


An ISO can be "mounted" with suitable driver
Device driver
In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....

 software, i.e. treated by the operating system as if it were a physical optical disc. Most Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

-based operating systems, including GNU/Linux and Mac OS X, have built-in capability to mount an ISO; in other cases software drivers can be installed to achieve the same objective.


ISO images of CD-Audio discs cannot be made; CD-Audio discs do not use a computer file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...

, but are formatted in tracks
Track (CD)
On an optical disc, a track or title is a subdivision of its content. Specifically, it is a consecutive set of sectors on the disc containing a block of data. One session may contain one or more tracks of the same or different types...

; track numbers, index points and CD time code are encoded into the lead-in of the CD-Audio disc, and these reference points are found throughout the CD-Audio sub-channel. To store an accurate copy of a CD-Audio disc, a different image format must be used, such as DDP
Disc Description Protocol
Disc Description Protocol is a format for specifying the content of optical discs, including CDs and DVDs. It is commonly used for delivery of disc masters for duplication. The proprietary format is the property of DCA, Inc...


ISO images are also unable to store multi-track disc images.

no "portable" programs for the Windows OS were known to exist that were capable of mounting ISOs on a USB flash ("thumb") drive or of reading them from such a drive. It is believed that this was because mounting and/or reading ISOs in Windows NT and later, network-based, versions of Windows OS required a driver, which is inaccessible to users of these operating systems who lack administrative privileges.
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