On Thursday, April 11, 2002, at 03:53 AM, Biswaroop(External) wrote:

Then in older Macs if we read a filesystem of type    HFS+ in a HFS wrapper, do that wrapper also    take care for the internal differences in Data structures.       A more summary of this wrapper concept would     satisfy my interest. Anybody can just give a more     basic understanding of this Wrapper and its functionalities would be wonderfull.

A wrapper does NOT allow files on the HFS Plus volume to be accessed as an HFS volume. It's not like a hybrid ISO/HFS disk where the same content appears in both directory structures. If we had been able to make the increased storage efficiency of HFS Plus accessible using HFS directory structures, we wouldn't have needed to invent a new volume format...

A wrapper is merely a way to store an HFS Plus volume inside an HFS volume. It is sort of like the way a DOS disk can have primary partitions that contain secondary partitions. It is an HFS volume where some contiguous but otherwise unused space has been set aside to contain an HFS Plus volume. It helps solve a couple of backwards compatibility problems:

1. Improve the user experience on systems that don't understand HFS Plus

Consider what would happen if you took a pure HFS Plus volume (no wrapper) and tried to mount it on a system that doesn't understand HFS Plus at all, but does understand HFS. The signature fields in the Volume Header won't match anything the OS knows about (not even HFS). Most consumer operating systems will display a dialog telling you that the disk has no recognizable contents. It may even offer to format the disk for you. That's just asking for the user to accidentally format the disk and lose all of their data.

If the HFS Plus volume has a wrapper, and the OS understands HFS, the wrapper mounts as an ordinary HFS volume. The user gets feedback that the computer recognized their disk, so they know it isn't blank.

But they also won't see the files they are expecting. But since the wrapper is just an HFS volume, it can contain files. When Apple's code produces wrappers, it puts a "Read Me" file in the wrapper. When the wrapper mounts on an HFS-only system, the user will hopefully see and read that "Read Me" file. It contains text that explains that the volume is really HFS Plus and that they need a newer version of system software to be able to use the files on the disk. We think that makes for a much friendlier user experience than the "Do you want to format this disk?" dialog they would otherwise get.

2. Allow booting on machines without built-in support for HFS Plus

At the time HFS Plus was first released, no Apple computers had HFS Plus support in ROM. But we wanted to be able to boot from an HFS Plus volume. The wrapper makes that possible.

Apple created wrappers contain a tiny System file (the basic part of the operating system that gets loaded by the boot code in ROM). This isn't an entire operating system. In fact, it is only enough code to find the embedded HFS Plus volume and read the System file stored there. The real System file in the embedded HFS Plus volume contains all the code to read and write HFS Plus volumes, so the rest of the boot process proceeds normally, and completely ignorant of the HFS wrapper.

-Mark