IMG (file format)

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

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 (such as a version of FAT). 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). 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.

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.

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