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IMG, in computing, refers to binary files with .img filename extension that store raw disk images of floppy disks, hard drives, or optical discs.

Overview

The .img filename extension is used by disk image files, which contain raw dumps of a magnetic disk. Since a raw image consists of a sector-by-sector binary copy of the source medium, the actual format of the file contents will depend on the file system of the disk from which the image was created (such as a version of FAT). Since IMG files hold no additional data beyond the disk contents, their contents is only understood by programs that can detect its file systems. For instance, a typical raw disk image of a floppy disk begins with a FAT boot sector.

Filename extensions

The .img file extension was originally use for floppy disk raw disk images only, though it is currently used to refer to hard drive disk images as well. A similar file extension, .ima, is also used to refer to the same type of raw disk image files. CloneCD stores disk images in .img files but also generate additional CloneCD Control Files (with .ccd extension) for each image to hold necessary metadata.

A variant of IMG is the CUE/BIN format which store disk images in .bin files. This images usually include not only the data from each sector in an optical disc, but the control headers and error correction fields for each sector as well. The exact contents of the image files are described in the accompanying .ccd or .cue descriptor files.

Another variant of IMG, called IMZ, consists of a gzipped version of a raw disk image. These files use the .imz file extension, and are commonly found in compressed images of floppy disks created by WinImage.

Size

The file size of a raw disk image is always be a multiple of the sector size, which is generally 512 bytes for floppy disks and hard drives, but other sizes such as 128 and 1024 exist. More precisely the file size corresponds to:

Cylinders × Heads × (Sectors per track) × (Sector size)

e.g. for 80 cylinders (tracks) and 2 heads (sides) with 18 sectors per track:

80 × 2 × 18 × 512 = 1,474,560 bytes or 1440 kB

Raw disk images of optical media (such as CDs and DVDs) contain a raw image of all the tracks in a disc (which can include audio, data and video tracks).

Comparison of ISO images

ISO images, commonly use the .iso file extension, but sometimes use the .img file extension as well. They are similar to the raw optical disc images, but contain only one track with computer data obtained from an optical disc (not multiple tracks, nor audio or video tracks). They also do not contain the control headers and error correction fields that raw disc images usually store. Their internal format follows the structure of an optical disc file system such as ISO 9660 (for CDs) or UDF (for DVDs). The CUE/BIN and CCD/IMG formats support using ISO images instead of raw disc images as well, though this is not their default structure.

Tools

The raw IMG file format is used by several tools:

• RaWrite and WinImage use the IMG disk image format to read and write floppy disk images.
• ImDisk and Virtual Floppy Drive can mount a raw image of a floppy disk to emulate a floppy drive under Microsoft Windows.
• Nero Burning ROM supports reading IMG files for creating bootable CDs.
• mtools allows manipulation of MS-DOS floppy disk images in Unix systems.
• Programs such as dsktrans from the LibDsk suite of command-line tools (available for Linux, MS-DOS, and Microsoft Windows) will convert between different raw disk image formats.
• dd can be used in Linux to create raw disk image files of disks.
• Qemu uses IMG files as its default format for hard drive disk images, calling the format simply "raw".

References

1. LibDsk suite of tools for accessing discs and disc image files

• Computer file formats
• Archive formats