From: pjs@ariel.ucs.unimelb.EDU.AU (Peter John Swiatkowski)
Newsgroups: alt.cd-rom
Subject: Re: multi-session - what's different?
Date: 16 Dec 1992 18:19:20 +1100
Organization: University of Melbourne
NNTP-Posting-Host: ariel.ucs.unimelb.edu.au

In article <1992Dec14.182303.18272@eng.umd.edu> chuck@eng.umd.edu (Chuck Harris - WA3UQV) writes:
>In article <1992Dec14.090229.883@ica.philips.nl> adrie@ica.philips.nl (Adrie Koolen) writes:
>...
>
>My problem here is that you are telling me why most drives can't handle the
>multi-session "standard".  I already understand, and appreciate that problem.
>The question I would like an answer to is: Why didn't the people who made the
>multisession "standard" just store the additional adjustable pointers in a
>hidden file of some sort in the normal program area?  This would have required
>a small change to the software drivers in order to handle multisession, but
>would require no changes to the CDROM drives.
>
>For instance, they could have defined a hidden file named (for lack of a better
>name) multipch.ptr.  Made the file big enough to hold as many "modifications"
>to the disk as you are willing to allow, and just let the laser "blot" over the
>old pointers that don't apply after the new session to the CDROM.  The software
>driver would have to look at this file on startup, skip over the old pointers
>that have been "blotted" out, and make a table in memory that has the
>pointer adjustments necessary to map the directory to the new session's files.
>
>This seems to me to be plenty simple, and much more desirable than obsoleting
>everyone's CDROM drive.
>
>Chuck Harris - WA3UQV
>chuck@eng.umd.edu
>
>

I think that the part of the issue is that multisession disks were never
envisioned since the disc contents were fixed by the factory production
process. Mutisession capability has come into being because the orange book
standard permits additional data to be **written** to what is ostensibly a
**read only** medium.

An analogy in the DOS world might be partitioning a hard disk into multiple
logical drives, each with its own file allocation table, root directory and
so on. But what if 'the rules' stated that a partition table, which defines
the start and finish addresses and total capacity of the drive, could only be
written once, but your first partitioning attempt only utilised 20% of your
hard disk's capacity. You suddenly find yourself with 80% of your hard disk
(or a writable CD-ROM disk or photo-CD) capacity unuseable, just because 
a partition table has been unalterably burned onto your disk.

The solution that has been arrived at, at least for Orange Book CDs, is to
permit extra partion tables (and the partition data itself) to be added
later, while existing data on the disk remains unaltered.

Because this is a 'new' standard we are left with a lot of drives produced
without the ability to understand the presence of or read any of these
'partition tables' after the first. It would seem that this problem could
be overcome by software or firmware changes. Unfortuantely this is not
always the case because multisession discs use the altered physical data
structures of CD-ROM/XA, which most older drives are physicall incapable of
interpretting.

I welcome comments from those with more technical CD-ROM knowledge on my
interpretation of these issues.

Peter Swiatkowski
University of Melbourne
Victoria, AUSTRALIA
Newsgroups: alt.cd-rom