Re: please help--tracks, sectors, VTOC, and disk geometry

[cranem@[nospam]aol.com]

On 24 Feb 2005 05:07:17 -0800, jutland1916@hotmail.com wrote:

Let me start with the caveat that it's been around 15 years since I
last looked at the structure of a Dos 3.3 disk.

>I have a disk image whose VTOC indicates that the last allocated track
>was 22, moving inward ($16, $01), yet there is good data beyond that
>(at tracks 33-34). How can that happen?

At a guess, I'd say there was a file (or files) there that were
deleted.  When a file is deleted, it is not pysically overwritten -
just marked as deleted in the catalog (change the first character of
the name to $FF if I recall correctly) and the space is released in
the VTOC.  Thus the information contained in the file is still present
on the disk, which is probably what you saw.

>Does the VTOC retain a record of "erased" data on the disk that has not
>yet been overwritten?

No - so if you ever want to recover a file that you've erased, the
best chance is to do so immediately after you've deleted it.  Any
further writes to the disk reduces your chance of ever recovering that
file intact, especially if the new file overwrites the former files
information in the catalog.  The actual mechanics of recovering an
erased file are another topic.

>Why are the VTOC and Catalog sectors always at track 17? Is there a
>mechanical reason for this, something related to the position of the
>read/write head, given that it's the middle track of the drive?

This was done simply to optimize disk access times.  Since there are
usually 35 tracks on an Apple II disk, 17 is a good central point to
minimize the distance the read/write head has to move to reach any
requested track when loading a file.  There is nothing preventing it
from being changed to a different location on the disk, but most disk
editor programs expect it to be in that location and will choke on a
non-standard location.

Hope this helps.