Apple Disk Image
The icon represents an internal hard drive within a generic file icon.
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| Filename extension | .dmg, .smi, .img |
|---|---|
| Internet media type | application/x-apple-diskimage |
| Uniform Type Identifier (UTI) | com.apple.disk-image, |
| Developed by | Apple Inc. |
| Type of format | disk image |
| Website | apple.com |
An Apple Disk Image is a disk image commonly used by the macOS operating system. When opened, an Apple disk image is "mounted" as a volume within the Finder. An Apple disk image can be structured according to one of several proprietary disk image formats, including the Universal Disk Image Format (UDIF) and the New Disk Image Format (NDIF). An Apple disk image file's name usually has ".dmg" as its extension.
An Apple disk image allows secure password protection as well as file compression, and hence serves both security and file distribution functions; such a disk image is most commonly used to distribute software over the Internet.
Apple originally created its disk image formats because the resource fork used by Mac applications could not easily be transferred over mixed networks such as those that make up 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.
Universal Disk Image Format (UDIF) is the native disk image format for Mac OS X. Disk images in this format typically have a .dmg extension. New Disk Image Format (NDIF) was the previous default disk image format in Mac OS 9, and disk images with this format generally have a .img (not to be confused with raw .img disk image files) or .smi file extension. Files with the .smi extension are actually applications that mount an embedded disk image, thus a "Self Mounting Image", and are intended only for Mac OS 9 and earlier. A previous version of the format, intended only for floppy disk images, is usually referred to as "Disk Copy 4.2" format, after the version of the Disk Copy utility that was used to handle these images. A similar format that supported compression of floppy disk images is called DART. Apple disk image files are published with a MIME type of application/x-apple-diskimage.
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