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What is an ISO image?

In the context of files and programs, an “image”, whether an ISO, floppy, or other media image, is simply a file that can be used as a virtually identical copy of the original media. This file not only contains individual data files; it also contains track and sector information and arranges all this information in a file system, just like disk media. Image files, unlike normal files, are usually not opened; rather, they are mounted.

An ISO image is an archive file (also known as a disc image) of an optical disc in a format defined by the International Organization for Standardization (ISO). An ISO image is simply a CD-ROM or DVD image saved in ISO-9660 format. This format is supported by many software vendors. ISO image files typically have a file extension of .iso. The name ISO is taken from the ISO 9660 file system used with CD-ROM media, but an ISO image can also contain a UDF file system since UDF is backward-compatible with ISO 9660.

As with any other archive, an ISO image includes all the data of files contained on the archived CD/DVD, or any other disc format. They are stored in an uncompressed format. In addition to data of the files it also contains all the file system metadata, including boot code, structures, and attributes. ISO images do not support multi-track, thus they cannot be used for audio CDs, VCD, and hybrid audio CDs, which are usually ripped as audio files.

However, for disks that contain a single track of data followed by tracks of audio, such as video game disks, the first track can be ripped as an ISO, and the rest as audio files. A valid ISO image is an uncompressed collection of various files merged into one single resulting file, according to definite and standard formatting.

The most important feature of an ISO image is that it can be easily rendered or “burned” to a DVD or CD by using media “authoring” or disc “burning” software. It can also be opened using archival applications such as 7-Zip file manager or the WinRAR shareware archiver.

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

Hybrid 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 Windows and Macintosh from one image. One recent example is the release of hybrid ISO files which can be “booted” or started from both CD/DVD and USB flash drive devices when the image is written to either of these storage devices.