Things seemed complicated enough when we were mastering a disc. Now, with UDF and packet writing, they're getting more confused. Formatting, finalizing - what's going on?
The cases are very different for write-once and erasable media - variable- and fixed-length packets. I pleaded with Adaptec to use different terms when UDF was being introduced, but I was overruled. Something about a standards committee. <G>
When you 'format' a write-once blank (CD-R), you don't really format the disc. You simply establish the directory for an unusual sort of session. Just as you can add audio tracks to an open session one at a time, you can add data files to an open UDF session one at a time. Since the session is open, it has no runout track and can be read only in a writer. Since it is not in a format known to the OS, it can be read only in a writer capable of UDF with a UDF program such as DCD operating. (In fact, a program such as Infinadyne's Diagnostic can also interpret the format, but that is because it does so the way that DCD does.)
When you add a file to a disc in variable-length packets, it goes to the UDF 'driver' part of DCD - a part you don't see but which has been installed when you boot up. That driver translates the file into one or more packets and writes them to the disc as a sort of 'track'. (Because the laser beam must be positioned for this task in a way not needed for mastering, not all drives will permit packet writing; that's why some drives do not support UDF.) After a while, you may want to make your disc readable in a CD-ROM drive, not merely in your writer. Then you close the session with a visible part of DCD. When you do that, you create a TOC similar to the one made when you master a disc - but not identical in form or content. Because closing a variable-length packet session changes the type of the directory, it is sometimes called "finalizing".
When you master a disc, a file is written in contiguous blocks - the file is read sequentially. When you write variable-length packets, the packets may *not* be in sequence. If you are copying two large files during the same period, they may be interleaved. So the format used when you close the session is ISO 9660 Level 3, not the standard Level 1 (I haven't found Level 2 yet in my reading - it may show up yet). Because it is not Level 1, it cannot be read in DOS, Win 3.x, early Mac OS's and so on. When you close a session, you have the option to close the disc. If you do, it is no longer writable. If you do not, then you can add another session by "formatting" again. That will open a fresh directory space and link the session to its predecessor. Since the earlier session is in Level 3, you are not able to master to the disc to add a Level 1 session - the TOC would become hopelessly confused.
NOTE: As you will see in the page of this primer on Psychic Software, the program does not know whether you will be closing the session or the disc and does not make allowance for it. Therefore, you can write so much to the disc that there will not be room to close the session.
Now a word on behavior with fixed-length packets. When you format an erasable blank (CD-RW), you actually do format the whole disc. You write markers to delineate the segments in which packets will be written and finish it up with a runout track at the end of the disc. As a result, the disc can be read in a CD-ROM drive - but only if that drive is MultiRead (can read an erasable) and has a translator from UDF to a format the OS will recognize. Of course, that translator takes the form of another of those 'hidden' parts of DCD. It actually reads the information about the disc's contents so that it presents the disc to the OS as though it were a sort of oversize floppy. The translation is done in RAM - which introduces a whole set of other issues. The bottom line is that a disc in fixed-length packets is always single-session and the session is technically closed although you can still add and delete data.
Isn't it nice that this stuff is so simple?
E-mail me at cdrecording@mrichter.com
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