Lisa's scores

From: A.R. Duell <>
Date: Thu Jun 26 19:56:34 1997

> In using my own Lisa, it really isn't that sluggish. Especially if you
> compare it to a Mac 128 or 512. The filesystem on the Lisa is VERY
> advanced and allows for recovery from errors that'd hork other
> filesystems. It has memory protection which keeps it from being as

What did they do in that filesystem? I am curious because the PERQ, which
was another machine that came from the Xerox PARC work also had a very
interesting filesystem under POS (the first OS that was written for it).
In particular :
Files are linked lists of blocks. The sector header of each sector on the
hard disk contains pointers to the previous and next blocks in the file,
along with some other info that I've forgotten
Files may be sparse - the fact that block n exists does not mean that
block n-1 does
Block 0 of each file (I think, maybe block -1) contains the 'file
descriptor' - bascially an i-node.
Negative block numbers are the file allocation map. You can use this to
quickly find any block in the file without following the links.
Directories are sparse files. When you access a file in a directory, the
filename is hashed in some way (the algorithm is not in the manual), and
the hash value is used as the first directory block to look in.
If the 'file descriptor' gets mangled it can be recreated from the header
pointers. If the header pointers get mangled they can be recreated from
the file descriptor. Ditto about other important structures on the disk -
loss of any single one is generally recoverable using standard utilities.

Were any of those things done on the Lisa?

> *ling

The gates in my computer are AND,OR and NOT, not Bill
Received on Thu Jun 26 1997 - 19:56:34 BST

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