Re: Ewen Wannop's ProDOS Directory Info

[mjmahon@aol.com (Michael J. Mahon)]

John B. Matthews wrote:



>In article <4033D78A.931F3ECB@swbell.net>,
> Rubywand <rubywand@swbell.net> wrote:
>
>> Bill Garber writes ...
>>>[perils of extending top level directory]
>>      Interesting info. Wonder if Ewen tried moving the Volume Bit Map so
>that
>> he would have contiguous Volume Directory blocks. If he did, then the
>failures
>> may have been due to Appleworks's assumption that the VBM would start at
>Block
>> 6. It would be writing file entries to Block 6 _and_ marking the VBM at
>Block
>> 6-- sure to mess up the directory.
>
>It's possible, but the location of the pointer to the first VBM 
>block was well documented in the ProDOS technical reference 
>manual. For example, see procedure FreeCount in the code at
>
>http://www.wright.edu/~john.matthews/apple2.html#df
>
>I always speculated that the limit was a compromise to 
>accomodate 140K floppies.

But why not fixed-allocate just the _first_ block of the
Volume Directory, and let it be automatically extended
as needed like any subdirectory?

This would provide an even lower limit on Volume Directory
size (1 block) for small disks and accommodate any size
appropriate for large ones.

Perhaps the designers were simply hinting at the maximum
size they expected for _any_ directory, by enforcing it for
the Volume Directory.  Linked-list searches on disk are not
a very efficient data organization for directory lookups.

A fixed-size directory of four contiguous blocks allows for
a lookup routine to read the entire directory in as a single I/0
(or at least, without having to seek), then search in the buffer.

On the other hand, the links allow for flexible directory size.

Perhaps the choice of a fixed-size, contiguous, but linked
Volume Directory was just the designer's way to say, "You
are free to make subdirectories of any size, but I think that
a reasonable practical limit is four blocks worth."

This hybrid approach is actually more complex than simply
making the Volume Directory the "root" subdirectory, so
there must have been some motivation for it.

Whatever the reason, the assumption of fixed Volume Directory
size has been made for so long that it would be impractical to
change it now, except under special circumstances (like the
Appleworks 5.25" disk  ;-).

-michael

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