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 or of an
optical disc. 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. Raw disk images of optical
media contain a raw image of all the tracks in a disc. In the case of CD-ROMs
and DVDs, these images usually include not only the data from each sector, but
the control headers and error correction fields for each sector as well.
Since IMG files hold no additional data beyond the disk contents, these files
can only be automatically handled by programs that can detect their file
systems. For instance, a typical raw disk image of a floppy disk begins with
a FAT boot sector, which can be used to identify its file system. Disc images of
optical media are usually accompanied by a descriptor file which describes the
layout of the disc, and includes information such as track limits which
are not stored in the raw image file. = Filename extensions and variants =
The .img file extension was originally used for floppy disk raw disk images
only. A similar file extension, .ima, is also used to refer to floppy disk image
files by some programs. A variant of IMG, called IMZ, consists of a gzipped
version of a raw floppy disk image. These files use the .imz file extension,
and are commonly found in compressed images of floppy disks created by
WinImage. QEMU uses the .img file extension for
raw images of hard drive disks, calling the format simply "raw".
CloneCD stores optical disc images in .img files and generates additional
CloneCD Control Files for each image to hold the necessary metadata. The CUE/BIN
format stores disc images in .bin files, which are functionally equivalent to
.img image files, and uses .cue files as descriptor files.
= Size = The file size of a raw disk image is
always a multiple of the sector size. For floppy disks and hard drives this
size is typically 512 bytes. More precisely, the file size of a raw disk
image of a magnetic disk corresponds to: Cylinders × Heads × ×
E.g. for 80 cylinders and 2 heads with 18 sectors per track:
80 × 2 × 18 × 512 = 1,474,560 bytes or 1440 KB
For optical discs such as CDs and DVDs, the raw sector size is usually 2,352,
making the size of a raw disc image a multiple of this value.
Comparison to ISO images ISO images are another type of optical
disc image files, which 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. They can not contain multiple tracks, nor audio or video tracks. They
also do not contain the control headers and error correction fields of CD-ROM or
DVD sectors that raw disc images usually store. Their internal format follows the
structure of an optical disc file system, commonly ISO 9660 or UDF. The
CUE/BIN and CCD/IMG formats, which usually contain raw disc images, can
also store ISO images instead. 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 will convert between different raw disk image
formats. dd can be used in Unix to create raw
disk image files of disks. QEMU uses IMG files as its default
format for hard drive disk images. References