Optical disc image facts for kids

An optical disc image (often called an ISO image) is like a perfect digital copy of an entire optical disc, such as a CD-ROM, DVD, or Blu-ray Disc. It contains all the information that was on the original disc, copied exactly, including the way the files are organized (called the file system).

Think of an ISO image as a "snapshot" of a disc. It's a single file that holds everything from the disc, just as it was stored. This includes the actual data files and the special structure that helps your computer read them.

You can create ISO images from physical discs using special software. You can also make them from a group of computer files, or even convert them from other types of disc image files. Many software programs, especially those that come on discs, are available to download as ISO images. Just like a real disc, an ISO image can be "burned" onto a blank CD, DVD, or Blu-ray.

What is an ISO Image?

An ISO image is a complete copy of an optical disc. It's not compressed, meaning it keeps all the original data without making it smaller. It's like taking a direct copy of every tiny piece of data (called a disk sector) from the disc and saving it into one file.

Most ISO images use a file system called ISO 9660. This is the standard way data is organized on many CDs. However, some ISO images might use UDF, which is common for DVDs and Blu-ray Discs. The way the data is arranged inside the ISO image will match how it was on the original disc.

Common File Extensions

The most common file extension for these images is .iso. Sometimes, you might see .img used, especially for some older Microsoft files. There are also other .img files that are a bit different. If an ISO image uses the UDF file system, it might sometimes have the .udf extension.

How Big are ISO Files?

ISO files only store the main data from each part of an optical disc. They don't include extra information like error correction data. Because of this, an ISO image is usually a tiny bit smaller than a perfect copy of the entire disc. Since the main data part of a disc sector is 2,048 bytes, the size of an ISO image will always be a multiple of 2,048 bytes.

How Do We Use ISO Images?

Any single-track CD-ROM, DVD, or Blu-ray disc can be saved as an ISO file. This creates an exact digital copy of the original disc. Unlike a physical disc, you can easily send an ISO image over the internet or store it on a USB flash drive. Many programs that open different file types can also open ISO images.

Mounting an ISO Image

One cool thing you can do with an ISO image is "mount" it. This means your computer treats the ISO file as if it were a real physical disc inserted into a drive. Many operating systems, like Linux, macOS, and even Windows 8 and newer versions, can do this without needing extra software. For older systems, you might need to install a special program.

Why Some Discs Don't Become ISOs

A CD can have different sections called "tracks." These tracks can hold computer data, audio, or video. File systems like ISO 9660 are stored inside one of these tracks. However, an ISO image is designed to copy a single file system and its contents. It doesn't have a way to store multiple tracks from a disc.

This means that discs with many tracks, like most audio CDs, cannot be stored as a single ISO image. An ISO image can only hold the data from one of those tracks, and only if that track contains a standard file system.

Audio CDs and ISOs

Audio CDs are usually made up of many separate audio tracks. These tracks don't contain a file system. Instead, they have a continuous stream of audio data. Because of this, you can't save an audio CD, or even a single audio track, as an ISO image. The audio data is stored differently and is organized by track numbers and time codes, not by files.

Similarly, Video CDs and Super Video CDs often need at least two tracks. So, you can't store a full image of these discs in an ISO file either. Other formats, like CUE/BIN or MDS/MDF, are used to store images of multi-track discs, including audio CDs. These formats save a complete copy of the disc, along with extra files that describe all the tracks. This information helps burning software create a perfect copy of the original disc.

More Ways to Use ISO Files

ISO files are very useful in emulators. Emulators are programs that let you run software designed for one system (like a game console) on another system (like your computer). For example, Dolphin uses ISO files to play Wii and GameCube games, and PCSX2 uses them for PlayStation 2 games.

ISO files can also act as virtual CD-ROMs for programs like VMware Workstation or VirtualBox. These programs create "virtual machines" that act like separate computers inside your main computer. You can use an ISO file to install an operating system on a virtual machine, just as if you were using a physical disc. Another common use is burning operating system images onto physical discs for installation.

See Also

In Spanish: Imagen ISO para niños

• Comparison of disc image software
• Live USB
• No-disc crack