An ISO image is an archive file of an optical disc, a type of disk image, composed of the data contents of every written sector of an optical disc, including the optical disc file system.[1] 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, may be written on, or "burned" to, a CD or DVD with any capable software. ISO image files often have a file extension of .iso. The name ISO is taken from the ISO 9660 file system used with CD-ROM media, but what is known as an ISO image might also contain a UDF (ISO/IEC 13346) file system or a DVD or Blu-ray Disc (BD) image.
Properties
ISO images are stored in an uncompressed format. Any CD-ROM (with no audio tracks, and only one data track) 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.
An ISO image can be opened with almost every file archiver. ISO burning has limited native support in computer operating systems, hence there are many standalone products on the market.
Hybrid disc formats include the ability to be read by different devices, operating systems, or hardware. In the past, one example of this use was for a disc that supported both Microsoft Windows and Macintosh installations from a single disk image. A more recent example is the release of hybrid ISO files that can be booted or started from both BD or DVD and USB flash drive devices when the image is written to any of these storage devices. This file format can be used to create Live USB environments through a low-level copy (for example with the Unix dd command) to a USB key.
An ISO can be "mounted" with suitable driver software, i.e. treated by the operating system as if it were a physical optical disc. Most Unix-based operating systems, including GNU/Linux and Mac OS X, have built-in capability to mount an ISO. Windows 8 also has such capability.[2] For other operating systems software drivers can be installed to achieve the same objective.
Limitations
ISO images of CD-Audio discs cannot be made for reasons that are not well understood outside the IT community. It must be understood that the CD-Audio standard does not have a conventional file system that can be easily represented by ordinary file system file formats like ISO. File system representation systems like ISO are primarily aimed at representing Hard Disk (and CD-ROM, DVD-ROM and Blu-ray) file systems.
- CD-Audio discs do not use a computer file system, but are formatted in tracks with track numbers, index points and a CD time code that are encoded into the lead-in of the CD-Audio disc.
- These reference points are found throughout the CD-Audio sub-channel, a practice not used in Hard Disk file systems.
- To store an accurate copy of a CD-Audio disc, a different image format must be used. The DDP or BIN/CUE are the equivalent of the ISO file format for CD-Audio use.
Multi-track file systems
- ISO images are also unable to store multi-track disc images.
- The ISO multi-track limitation has its origin with earlier technology, that was not capable of creating such images.
- This technical limitation in a way is a safety feature, as no solid multi-track file system representation format has emerged.
Flash memory cards and USB dongles
- Most software that is capable of writing ISO images to Hard Disks or writable (CD / DVD / BD) ROMS is generally not able to write ISO disk images to flash drives; One of the exceptions to that is PowerISO. It can write raw image files to USB flash drives starting at version 4.8 (since Jun 16, 2011).
- Most Unix and Unix based systems can use the "dd" program for writing ISO images to flash media and hard drives.
- Writing ISO disk images to Flash Drives is in many ways less complex than writing to Hard Disks, with respect to image verification.[citation needed]
References
