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IMG file format may refer to several different and incompatible file formats, which usually just have the use of the .img file extension in common:

• Binary files used to store raw disk images of floppy disks or hard drives.
• The Apple Disk Image proprietary disk image file format, used by Apple Disk Copy for Mac OS.
• Garmin's map file format, used by Garmin GPS devices.
• The GEM Raster file format, an image file format used to store bitmap digital images on the Graphical Environment Manager operating environment. Used by the GEM Paint software.
• Other digital image file formats used by different graphics software packages (such as ERDAS IMAGINE).

This article will focus on the first format indicated above, raw disk image files that use the .img extension.

IMG raw disk image format

The .img file extension is used by files called disk images, 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). 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 used by some tools to refer to the same type of raw disk image files. A variant of the format is called IMZ, and 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.

The file size of a raw disk image will always be a multiple of the sector size — generally 512 bytes, 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., 1440KB=80×2×18×512 for 80 cylinders (tracks) and 2 heads (sides) with 18 sectors per track. A typical raw disk image of a disk begins with a FAT boot sector.

Raw disk images of optical media (such as CDs and DVDs) are usually referred to as ISO images, and use the .iso file extension. Functionally, however, they are equivalent to IMG files, though their internal format follows the structure of an optical disc file system such as ISO 9660 (for CDs) or UDF (for DVDs).

Tools

The raw IMG file format is used by several tools:

• Tools such as RaWrite and WinImage use the IMG disk image format to read and write floppy disk images.
• Programs such as 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 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. GEM Raster File Format description
2. LibDsk suite of tools for accessing discs and disc image files

• Computer file formats
• Archive formats