Disk images are used in many places and for many purposes:emulation, CD/DVD mastering, backup, disk duplication, andsoftware distribution.Many image formats are just plain sector-by-sector copies ofthe disk's data, and disktype can handle them without anyspecial precautions. This section lists the formats that needspecial handling, because they omit unused space (creating aso-called sparse image), use compression,or have a special sector encoding.
The CD format has many layers. The uppermost layer provides 2048 bytesectors for storing computer data. Some CD mastering or duplicationprograms work one level below that (usually called 'raw mode'), wherea sector has 2352 bytes and includes the last level of error correction codes.The popular .bin/.cue CD image format often uses this raw-sector format.
Apple wireless alphanumeric keyboard. disktype recognizes the raw-sector format using the syncronization bytesand mode information at the start of each sector. It automaticallyextracts the 2048 actual data bytes from each sector for filesystem analysis. disktype doesn't know or care about multipletracks or sessions, but the current code seems to be sufficienteven for (S)VCD disks.
Apple has been using disk images for quite some time. Over that time,a wealth of different formats has accumulated. The manual page for thehdiutil(1) utility on Mac OS X lists the following:
- UDIF supports ADC (an old proprietary compression format by Apple), zlib, bzip2 (as of Mac OS X v10.4), and LZFSE (as of Mac OS X v10.11) compression internally.
- UDIF supports ADC (an old proprietary compression format by Apple), zlib, bzip2 (as of Mac OS X v10.4), and LZFSE (as of Mac OS X v10.11) compression internally. The UDIF metadata is found at the end of the disk image following the data. This trailer can be described using the following C structure.
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Unfortunately, Apple is very secretive and doesn't publish the formatspecifications. Apparently, they fear a degradation in user experience,were third parties allowed to write alternative utilities for handlingdisk images. Some older formats use proprietary compression algorithms,although the latest compressed format (UDZO) uses zlib.Mac OS X has a library (DiskImages.framework
)that handles the various formats using a nice plug-in architecture,but it is marked private and neither headers nor documentation are available.
The 'Disk Copy 4.2' format is the oldest one. It is quite straightforward(no sparse blocks, no compression) and Apple provides sample code toread it. However, the format is long obsolete and (to my knowledge) wasonly used for floppies.
The NDIF ('New Disk Image Format') format was introduced with Disk Copy 6.0.NDIF is a dual-fork format, meaning that all meta-data is stored in theresource fork. This makes them fragile for cross-system transport.Various variants of the format allow for sparse images (only actually used sectors are present), compression, and self-mounting images.Some more information on the introduction of NDIF is availablein TidBITS #339.
Later, Apple introduced the UDIF format, which is basically the sameas an NDIF, but in a robust single-fork format. Actually, it is simplythe concatenation of the data fork, the resource fork,and a 512 byte header, without any padding.disktype recognizes the magic of that header at the end of the file,but that's all it can do for now until the meta-data format is published.Detection may still work for the read-write and uncompressed read-onlyformats.
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The cloop format was designed for Linux Live CDs. A standard Linux file systemis compressed in chunks, so that random read access is still possible withoutdecompressing the whole image.
disktype currently only recognizes the signature of the cloop image.The contents of the image are not made accessible; this would require thatdisktype links with zlib.
The Virtual PC emulator / virtual machine has its own image formatfor the hard disks of the virtual PCs. There is a basicstatically-sized format and a dynamic format that expands asdata is written. Other types of disk images include differentialimages (for undoable drives), and pointers to existing disks or partitions.
I was able to deduce the format of the static and dynamic imagesfrom examples. disktype recognizes them and properly analyzesthe contents. The other formats are recognized, but cannot be analyzedfurther because they don't contain a complete image. To my knowledge,all versions of Virtual PC (Mac or Windows) starting with version 4use this image format.