Apple Disk Image

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As Apple Disk Image ( filename extension .dmg , .sparseimage or .sparsebundle ) are various file formats for memory dumps referred to Apple, Inc. under the operating systems Mac OS Classic (pre-1996 "System" or " Macintosh System Software") and macOS (of 1998 to 2012 known under the name "Mac OS X", from 2012 to 2016 "OS X"). The files are each an image of a data carrier and can be formatted like a hard disk drive, for examplesubjected and divided into partitions .

Universal disk image format

The Universal Disk Image Format (UDIF) is the most widely used file format for memory images on Mac OS X / OS X / macOS. The data in it can be both password protected and compressed , which is why UDIF is mainly used to protect sensitive data and distribute software over the Internet. When the UDIF file is opened, the memory image is mounted and appears as a " Volume " in the Finder or its mount point as a directory with the name of the file system below /Volumes. A license agreement may appear before opening .

Memory images in the Universal Disk Image Format can be writable or write-protected and always have a fixed size.

Growing memory images

Memory dumps, which grow automatically when they are filled with data, as "sparse disk image" (English sparse image ) denotes ( English sparse  = sparse, see also sparse file ). Due to a translation error, the German text of Mac OS X Panther (10.3, 2003) turned it into a "simple image". This lapse has been corrected in newer operating system versions. "Growing images" can initially only grow, but not become smaller, if files are deleted in them. However, they can be reduced to the minimum size via the command line (see hdiutil ).

The “growing bundle image” ( “sparse bundle” ) was introduced with Mac OS X Leopard (10.5, 2007) and broken down, transparent for the user, into smaller files of 8  MiB each . This has the advantage over a large individual file that with an incremental data backup (e.g. via " Time Machine ") only the changed parts of the image need to be backed up, instead of the entire image file for every change, no matter how small. Furthermore, large images, in particular those with more than 4 GiB, can be stored on FAT32 volumes.

Disk image files can be with or without encryption ( AES 128-bit key length, from Mac OS X Leopard, 10.5, 2007 with 256 bits key length) with the Disk Utility ( English Disk Utility ), or in earlier versions of Disk Copy creates . These programs also use this file format to burn CDs and DVDs. The hdiutil program is on the command line . Other programs are FileStorm and FreeDMG .

access

Since with each new version of Mac OS X, OS X and macOS, new functions have been introduced and existing functions have been expanded both for the memory images that grow with the child and for UDIF files, images created on newer operating system versions can often not be used with older Mac OS X versions are integrated (opened), or vice versa. For example, AES encryption is only available from Mac OS X Leopard (2007), so an image that uses this encryption will not work on earlier versions. It is the same with the file systems in the image. From macOS High Sierra , 10.13, 2017, the Apple File System (APFS) is used by default on SSDs and can also be represented in images. Such images cannot be used with older versions of macOS, as they do not yet support APFS - with the exception of macOS Sierra (10.12, 2016), where APFS can be activated manually and experimentally.

Access from other operating systems

Windows

There are several commercial and free programs available to access disk image files under Windows, such as: B. TransMac and the free program HFSExplorer . Various archiving programs such as 7-Zip can also read the format. However, this only applies to images that use the HFS + file system. Support for the Apple File System (APFS, from 2016) is not yet available.

Linux

Under Linux , disk image files can in principle be integrated directly into the system . However, this only applies to uncompressed files; compressed disk images must be converted beforehand. The versions cannot be identified directly, but the Unix file command can be used to determine the MIME type of the file. A distinction can then be made via this. Both the partition table and the file system must be supported by the respective Linux distribution. The Apple File System (APFS) is currently (as of December 2017) not yet supported.

Conversion on Mac OS X

Disk image files can be converted to an ISO image on macOS . The program in the command line that is called via the Terminal utility is used for this. The ISO image created afterwards can be burned onto a CD or DVD . hdiutil

Images of the type “sparse image” or “sparse bundle” can be reduced in size by hdiutil compactdeleting or deallocating unused areas with the command .

Under Mac OS X Leopard (10.5, 2007), disk images can be converted into another format not only via the command line, but also with the disk utility. For example, a “sparse image” can be converted into a “sparse bundle” . It can also be used to change the size of an image. For OS X El Capitan (11/10/2015) the disk utility has been massively redesigned.

Historical formats

Since most formats had a data fork and a resource fork for the original Macintosh system software (up to System 7 ) and after the renaming in Mac OS up to Mac OS 9 , the distribution to non-Macintosh systems (e.g. PC Diskettes with FAT file system) and the respective images are packed into an archive via the Internet . On the Macintosh , StuffIt , filename extension or , from Aladdin Systems, was prevalent until recently . .sit.sitx

Mac OS X supported the historical formats "Disk Copy 4.2" ( DC42) and NDIF with hdiutilup to Mac OS X Snow Leopard (10.6, 2009).

Disk Copy 4.2

The oldest format for diskette images is that of Disk Copy, as introduced in System 7 in version 4.2 . Disk Copy 4.2 also works on System 6 . This means that images of u. a. Macintosh-typical 400k and 800k-3.5 ″ floppy disks in a standardized data format possible. The filename extension was typically .imageor .img.

Another format was DART, which compressed the data and used the file name extension .dart.

Untreated 1: 1 copies ( English raw ) typically had the extension under classic Mac OS.dsk .

Self-mounting image

Filename extension .smi.

New Disk Image Format

With Disk Copy 6.0, Mac OS 9 introduced the " New Disk Image Format " (NDIF).

Individual evidence

  1. ↑ hdiutil (1) Mac OS X Manual Page . Apple. April 22, 2009. Retrieved January 21, 2012.
  2. ↑ http://www.chip.de/downloads/c1_downloads_12994267.html

Web links