Multi-session CDROM access... How?

[Edward Dekkers]

>> If you fixated after the first session, you don't have multiple 
>> sessions but multiple writes.  This is another issue.
> 
> 
> Hmm. Then what exactly is "Multi-session Mode"?
> 
> Mike

OK, let me try to explain.

A CD-R has a table of contents. When you write files to it, it not only 
writes the files, but the table of contents as well. It is this table of 
contents that the reading device sees when you for example get a list of 
files on the CD.

In multi-session mode, what basically happens in the burning software 
(if done properly), is that it reads this table of contents first before 
doing anything else. You can tell the burning app to add, update and 
even delete files. What you must understand is that a CD-R is not 
erasable ever. Basically when you tell the burning app to add a file, it 
writes that file plus a new table of contents including all the files 
previously PLUS the new file. If you tell it to update a file, it will 
write the new file completely, and in this case, when it finishes the 
session and writes a modified table of contents, it points the file name 
to the new position on the disk. The original file is ALSO there, but 
because a disk only has one table of contents, which says the file 
exists at point B now, not point A, the original file is essentially not 
accessible so it looks like it's been updated. Same when you tell the 
burning app to delete a file. It doesn't write ANYTHING file-wise, but 
simply writes out a modified table of contents which does not have a 
pointer for the erased file.

You sort of get what I mean? Multi-session basically keeps a disk 
flexible by moving a table of contents.

Only one table of contents is active (the latest one) at one time.

Multiple Writes is basically the same EXCEPT that the burning app writes 
a completely new table of contents every session WITHOUT ANY DETAILS 
FROM THE PREVIOUS TABLE OF CONTENTS.

So, if you have session 1 with files A, B and C, then write a new 
session with a file D WITHOUT the burning software knowing about the 
previous session's table of contents (this usually happens through user 
error), ONLY file D will be visible.

This is where the session mounting thingy you do becomes necessary. It 
simply uses a different table of contents to the current active one. On 
a properly burned multi-session disk this should really not be necessary.

Regards,
Ed.
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