Optical disc image

Last updated
ISO 9660 image
Filename extensions
.iso, .udf
Internet media type
application/vnd.efi.iso [1]
Uniform Type Identifier (UTI) public.iso-image
Magic number Volume descriptor: CD001 at 32769. NSR0 at 38913 or 32769 for UDF. [2]
Type of format Disk image
Standard ISO 9660, UDF

An optical disc image (or ISO image, from the ISO 9660 file system used with CD-ROM media) is a disk image that contains everything that would be written to an optical disc, disk sector by disc sector, including the optical disc file system. [3] ISO images contain the binary image of an optical media file system (usually ISO 9660 and its extensions or UDF), including the data in its files in binary format, copied exactly as they were stored on the disc. The data inside the ISO image will be structured according to the file system that was used on the optical disc from which it was created.

Contents

ISO images can be created from optical discs by disk imaging software, or from a collection of files by optical disc authoring software, or from a different disk image file by means of conversion. Software distributed on bootable discs is often available for download in ISO image format. And like any other ISO image, it may be written to an optical disc such as CD, DVD and Blu-Ray.

Description

Optical-disc images are uncompressed and do not use a particular container format; they are a sector-by-sector copy of the data on an optical disc, stored inside a binary file. Other than ISO 9660 media, an ISO image might also contain a UDF (ISO/IEC 13346) file system (commonly used by DVDs and Blu-ray Discs), including the data in its files in binary format, copied exactly as they were stored on the disc. The data inside the ISO image will be structured according to the file system that was used on the optical disc from which it was created.

The .iso file extension is the one most commonly used for this type of disc images. The .img extension can also be found on some ISO image files, such as in some images from Microsoft DreamSpark; however, IMG files, which also use the .img extension, tend to have slightly different contents. The .udf file extension is sometimes used to indicate that the file system inside the ISO image is actually UDF and not ISO 9660.

ISO files store only the user data from each sector on an optical disc, ignoring the control headers and error correction data, and are therefore slightly smaller than a raw disc image of optical media. Since the size of the user-data portion of a sector (logical sector) in data optical discs is 2,048 bytes, the size of an ISO image will be a multiple of 2,048.

Any single-track CD-ROM, DVD or Blu-ray disc can be archived in ISO format as a true digital copy of the original. Unlike a physical optical disc, an image can be transferred over any data link or removable storage medium. An ISO image can be opened with almost every multi-format file archiver. Native support for handling ISO images varies from operating system to operating system.

With a suitable driver software, an ISO can be "mounted" – allowing the operating system to interface with it, just as if the ISO were a physical optical disc. Most Unix-based operating systems, including Linux and macOS, have this built-in capability to mount an ISO. Versions of Windows, beginning with Windows 8, also have such a capability. [4] For other operating systems, separately available software drivers can be installed to achieve the same objective.

Multiple-track images

A CD can have multiple tracks, which can contain computer data, audio, or video. File systems such as ISO 9660 are stored inside one of these tracks. Since ISO images are expected to contain a binary copy of the file system and its contents, there is no concept of a "track" inside an ISO image, since a track is a container for the contents of an ISO image. This means that CDs with multiple tracks can not be stored inside a single ISO image; at most, an ISO image will contain the data inside one of those multiple tracks, and only if it is stored inside a standard file system.

This also means that audio CDs, which are usually composed of multiple tracks, can not be stored inside an ISO image. Furthermore, not even a single track of an audio CD can be stored as an ISO image, since audio tracks do not contain a file system inside them, but only a continuous stream of encoded audio data. This audio is stored on sectors of 2352 bytes different from those that store a file system and it is not stored inside files; it is addressed with track numbers, index points and a CD time code that are encoded into the lead-in of each session of the CD-Audio disc.

Video CDs and Super Video CDs require at least two tracks on a CD, so it is also not possible to store an image of one of these discs inside an ISO image file, however an .IMG file can achieve this.

Formats such as CUE/BIN, CCD/IMG and MDS/MDF formats can be used to store multi-track disc images, including audio CDs. These formats store a raw disc image of the complete disc, including information from all tracks, along with a companion file describing the multiple tracks and the characteristics of each of those tracks. This would allow an optical media burning tool to have all the information required to correctly burn the image on a new disc. For audio CDs, one can also transfer the audio data into uncompressed audio files like WAV or AIFF, optionally reserving the metadata (see CD ripping).

Most software that is capable of writing from ISO images to hard disks or recordable media (CD / DVD / BD) is generally not able to write from ISO disk images to flash drives. This limitation is more related to the availability of software tools able to perform this task, than to problems in the format itself. However, since 2011, various software has existed to write raw image files to USB flash drives. [5] [6]

Uses

.ISO files are commonly used in emulators to replicate a CD image. Emulators such as Dolphin and PCSX2 use .iso files to emulate Wii and GameCube games, and PlayStation 2 games, respectively. [7] [8] They can also be used as virtual CD-ROMs for hypervisors such as VMware Workstation or VirtualBox. Other uses are burning disk images of operating systems to physical install media.

See also

Related Research Articles

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<span class="mw-page-title-main">ISO 9660</span> File system for CD-R and CD-ROM optical discs

ISO 9660 is a file system for optical disc media. The file system is an international standard available from the International Organization for Standardization (ISO). Since the specification is available for anybody to purchase, implementations have been written for many operating systems.

<span class="mw-page-title-main">Optical disc</span> Flat, usually circular disc that encodes binary data

An optical disc is a flat, usually disc-shaped object that stores information in the form of physical variations on its surface that can be read with the aid of a beam of light. Optical discs can be reflective, where the light source and detector are on the same side of the disc, or transmissive, where light shines through the disc to be detected on the other side.

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<span class="mw-page-title-main">ISO 13490</span> Successor to ISO 9660, an optical disc recording standard

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<span class="mw-page-title-main">Mount Rainier (packet writing)</span> Method used to write to recordable optical discs

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<span class="mw-page-title-main">Live CD</span> Complete, bootable computer installation that runs directly from a CD-ROM

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<span class="mw-page-title-main">Optical disc authoring</span> Content publishing on optical disks

Optical disc authoring, including CD, 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 ("burned") onto an optical disc. This act is sometimes done illegally, by pirating copyrighted material without permission from the original artists.

<span class="mw-page-title-main">K3b</span> Optical disc authoring software

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<span class="mw-page-title-main">CDBurnerXP</span>

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An NRG file is a proprietary optical disc image file format originally created by Nero AG for the Nero Burning ROM utility. It is used to store disc images. Other than Nero Burning ROM, however, a variety of software titles can use these image files. For example, Alcohol 120%, or Daemon Tools can mount NRG files onto virtual drives for reading.

Media Descriptor File (MDF) is a proprietary disc image file format developed for Alcohol 120%, an optical disc authoring program. Daemon Tools, CDemu, MagicISO, PowerDVD, and WinCDEmu can also read the MDF format. A disc image is a computer file replica of the computer files and file system of an optical disc.

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<span class="mw-page-title-main">CD-ROM</span> Pre-pressed compact disc containing computer data

A CD-ROM is a type of read-only memory consisting of a pre-pressed optical compact disc that contains data computers can read, but not write or erase. Some CDs, called enhanced CDs, hold both computer data and audio with the latter capable of being played on a CD player, while data is only usable on a computer.

Notable software applications that can access or manipulate disk image files are as follows, comparing their disk image handling features.

Live File System is the term Microsoft uses to describe the packet writing method of creating discs in Windows Vista and later, which allows writeable optical media to act like mass storage by replicating its file operations. Live File System lets users manage files on recordable and rewriteable optical discs inside the file manager with the familiar workflow known from mass storage media such as USB flash drives and external hard disk drives.

References

  1. ↑ Siyuan, Fu (24 January 2017). "application/vnd.efi.iso". IANA . Retrieved 18 July 2023.
  2. ↑ "File Signatures". www.garykessler.net. Archived from the original on 2020-07-03. Retrieved 2020-07-01.
  3. ↑ Fisher, Tim (24 April 2018). "What Is an ISO File?". Lifewire. Archived from the original on 23 June 2018. Retrieved 23 June 2018.
  4. ↑ "Windows 8 Explorer will support native mounting of ISO and VHD". ExtremeTech. Archived from the original on 2012-05-31. Retrieved 2012-05-21.
  5. ↑ "ISO image to USB conversion". ISO to USB burning tool. Archived from the original on 15 February 2015. Retrieved 15 February 2015.
  6. ↑ "How to Setup Windows 7 or Windows 8 from USB drive?". PowerISO. Archived from the original on 20 June 2016. Retrieved 15 February 2015.
  7. ↑ "What dump formats are supported by Dolphin?". Dolphin Emulator Project. Archived from the original on 4 March 2017. Retrieved 11 March 2021.
  8. ↑ "So how do I use it? - PCSX2". PCSX2 Team. Archived from the original on 9 March 2021. Retrieved 11 March 2021.
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