Lisa filesystem

https://github.com/claunia/DiscImageChef/releases/tag/v3.1.0

Here you have a pre-release of DiscImageChef with full support for Lisa filesystem, V1, V2 and V3.

The code also contains a fully documented, heavily commented, list of all the structures that make the filesystem.

I have reverse engineered it looking not a single confidential file (which I do not have) or source file (which I neither have) but purely experimenting with disks. Indeed I'm writing a book on how I made it.

Some field names I took from the Lisa OS public programming interfaces while some others I invented they myself.

It has read any single disk and its contents I have been able to throw to it with the following caveats:

• Disks must be in DiskCopy 4.2, DART is not supported at all.
• Disks must contain sector tags. While my code can be changed to work without them, Lisa OS doesn't, so not so much of a problem.
• Corrupt disks. However it is able to recover information from disks so corrupt that crash Lisa OS scavenging.
• Lisa Office folders are ignored. They are a lie, all files are stored in the root directory, and the folders name and structure are stored in a file calle {!CATALOG}. This is the same as Mac OS' Finder does on MFS disks.
• Lisa OS 3 supports subdirectories. They are currently ignored. You mostly never used them (unless you created a file with path -#11-subdirectory-File.TEXT, you didn't use it :p). However if you did, just tell me, and I'll add support.
• Lisa Office filenames are ignored. They are a lie also, stored in the LisaInfo structure, that's extracted by my tool, so just check it there.
• LisaTest, LisaMonitor, Lyrix and LisaXENIX disks. They do not use Lisa filesystem, so their filesystem needs to be added separately (thankfully it is documented, so if you need it just tell me and I'll make it a priority).

It wholly supports Twiggies, Profiles, Widgest, IDEFiles, etc, anything you can throw in DiskCopy 4.2. I have not tested Priam disks, but if you got a DiskCopy 4.2 image of them, send me!

In a nutshell, to debunk all misinformation about the filesystem:

Every disk starts with the boot file (shown as $Boot by DiscImageChef) that contains code to load the OS loader (shown as $Loader). The OS Loader is next block in floppies but 8 blocks farther away in all hard disk images. Next to the $Loader resides the MDDF (shown as $MDDF), like the Lisa filesystem superblock, it contains information describing the volume. Next to the MDDF resides the volume usage bitmap (shown as $Bitmap), and pointed by it (usually next to it) the S-Records file (shown as $S-Records).

The $S-Records contain a hashtable of entries, where the hash is the file id, and the entry contains a pointer to the Extents File to that file id, as well as the file size in bytes.

The Extents File (2 sectors in V1, 1 sector in V2 and V3) contains all information, including the LisaInfo structure, for a file, plus the extents to read them (extents = pointer + length), and dates (epoch is 1 January 1901, 3 years before Mac OS epoch).

In V1 and V2, the root (and only) catalog is a file whose Extents File is pointed by the S-Records file, containing the filenames and file IDs only.

In V3 the catalogs are 4-sector blocks that connect using a double-linked (before and after pointers) structure.

While seems that the only public information until today about Lisa FS has veen that v1 uses a simple table catalog, v2 a hash table one and v3 a B-Tree one, for me doesn't seem the case. But note, that looking at the raw data, as I've done it, I can't be sure that v3 is not a B-Tree (it may be a simple B-Tree that's readable as a double-linked list), and for me, v2 and v1 catalogs are identically structured (and read flawlessly with the same code). It may also be referring to an on-memory structure for the OS.

So, that's all, thanks for waiting for me all this years :p

Regards,
Natalia Portillo

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