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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:

• One of several file formats 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.
• Other digital image file formats used by different graphics software packages.

This article will focus on raw disk image file formats that use the extension .img.

IMG raw disk image format

The .img file extension is used by files which contain raw dumps of a disk, commonly called disk images. There is no single standard for raw disk IMG files, and several different disk image formats share the same .img file extension. The actual format of the file contents will depend on the filesystem from which the raw disk image was created. However, the .img file format is usually associated to floppy disk raw disk images.

A similar file extension, .ima, is used to refer to the same type of disk image files by some tools. A variant of the format is called IMZ, and uses a gzipped version of a raw disk image along with the .imz file extension. Both these extensions are almost exclusively used when referring to raw disk images of floppy disks only.

A typical raw floppy disk image begins with a FAT boot sector, where the first byte is normally hexadecimal EB (code for a 8bit short jump, offset in the 2nd byte) with third byte 90 (code for a NOP), or rarely EA (code for a 16bit jump, offset in 2nd and 3rd byte). The file size will always be a multiple of the sector size — generally 512 bytes, but other sizes such as 128 and 1024 existed. More precisely the file size corresponds to Cylinders×Heads×(Sectors per track), e.g., 1440KB=80×2×18×512 for 80 cylinders (tracks) and 2 heads (sides) with 18 sectors per track.

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 Windows.
• mtools allows manipulation of MS-DOS floppy disk images in Unix systems.
• Nero Burning ROM supports reading IMG files for creating bootable CDs.
• Qemu uses it as its default format for hard drive disk images.
• Programs such as dsktrans from the LibDsk suite of command-line tools will convert between raw disk image formats, for Linux, MS-DOS, and Microsoft Windows.
• dd can be used in Linux to create raw disk image files of disks.

References

1. GEM Raster File Format description
2. LibDsk suite of tools for accessing discs and disc image files

• Computer file formats
• Archive formats