ISO + Apple Extensions

Summary: Third working mode of MacImage, the mode ISO + Apple extensions. Description of the features.


To allow storing Macintosh files (composed of two forks) in the ISO 9660 file system, Apple published an "extension" to the ISO 9660 standard to be able to keep the forks together and to manage the signatures and Finder flags. (See Apple Technote FL-36).
MacImage (from version 6.4) implements this extension and allows the user to produce ISO 9660 CD-ROMs with Apple Extensions from the PC. This working mode contrasts to the Project Mode (default working mode of the program) and to the Partition Mode.

Advantages of ISO CD-ROMs with Apple Extensions

By comparison with a HFS/HFS+ CD-ROM, the storage available on the CD-ROM is used more effectively. The minimal allocation block is the sector (2048 bytes). Compare this with a HFS CD-ROM of 600MB, which has allocation blocks of 10,240 bytes. This can make a big difference when your project contains hundreds of files.
The second big advantage is a result of the fact that the Finder doesn't use a Desktop database to manage the accesses to the files. Therefore, there is no Desktop to rebuild. Since the medium is read-only, the Desktop datyabase can't be stored on the disk and it must be rebuilt each time the disk is loaded.
Since MacImage doesn't build (yet) a Desktop database for the HFS partitions, the ISO + Apple Extensions mode should be a good solution for projects containing hunderds or thousands of files.

Drawbacks of ISO CD-ROMs with Apple Extensions

The result on the target Macintosh depends on the configuration. Before Mac OS X, only short filenames (8+3) are shown if the user didn't install the Joliet Extension proposed by Thomas Tempelmann. Under Mac OS X, you get Joliet long filenames.
All files are visible. That means that the PC user can explore the folders holding Macintosh files.
Depending on your situation, each of those aspects can be a little problem or a big no-no.

Technical principle of Apple Extensions

As usual, the main problem is to store together both forks and the so-called Finder information. Apple chose to use the system entry in the file and folder records (a storage place coming at the end of a normal record, after the XA codes if those are present) for the Finder information (signature and some flags). Apple chose to use the "Associated File" feature (bit 2 of the flag byte in the file record) to identify the resource fork. This resource fork always comes before the data fork (whose bit 2 is not set). That way, the "associated file" is not displayed by programs used to explore the CD-ROM contents.

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