Apple Disk Image is a proprietary disk image format commonly used with the Mac OS X operating system. When opened, an Apple Disk Image is "mounted" as a volume within the Finder.
The format allows secure password protection as well as file compression and hence serves both security and file distribution functions; it is most commonly used to distribute software over the Internet. Universal Disk Image (UDIF) is a flat file format, and is the native image format for Mac OS X.
Newly created Apple Disk Images typically have a .dmg extension, although legacy Apple Disk Image files intended for Mac OS 9 and earlier generally have .smi or .img file extensions (.smi files are actually applications that mount an embedded disk image, thus a "Self Mounting Image"). Apple Disk Image files are published with a MIME type of application/x-apple-diskimage.
Apple originally created their disk image formats because the resource fork used by Mac applications could not easily be transferred over heterogeneous networks such as the Internet. Even as the use of resource forks declined with Mac OS X, disk images remained the standard software distribution format. Disk Images allow the distributor to control the Finder's presentation of the window, which is commonly used to instruct the user to copy the application to the correct folder.
Apple Disk Images can be created using utilities bundled with Mac OS X, specifically Disk Copy in Mac OS X v10.2 and earlier and Disk Utility in Mac OS X v10.3 and later. These utilities can also use Apple Disk Image files as images for burning CDs and DVDs. Disk Image files may also be managed via the command line using the hdiutil utility. The format can also be opened by MagicISO and the freeware MagicDisc on Microsoft Windows, along with several free software implementations.
Read more about Apple Disk Image: Data Format
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