Hardware Reference
In-Depth Information
than that.
CD-DA and older CD-ROM drives couldn't read more than one session on a disc, so that is the way
most pressed CDs are recorded. The Orange Book allows multiple sessions on a single disc. To
allow this, the Orange Book defines three main methods or modes of recording:
• Disk At Once (DAO)
• Track At Once (TAO)
• Packet Writing
DAO
DAO is a single-session method of writing CDs in which the lead-in, data tracks, and lead-out are
written in a single operation without the writing laser ever turning off; then the disc is closed. A disc
is considered closed when the last (or only) lead-in is fully written and the next usable address on the
disc is not recorded in that lead-in. In that case, the CD recorder is incapable of writing any further
data on the disc. Note that it is not necessary to close a disc to read it in a normal CD-ROM drive,
although if you were submitting a disc to a CD-duplicating company for replication, most require that
it be closed.
TAO
Multisession discs can be recorded in either TAO or Packet Writing mode. In TAO recording, each
track can be individually written (laser turned on and off) within a session, until the session is closed.
Closing a session is the act of writing the lead-out for that session, which means no more tracks can
be added to that session. If the disc is closed at the same time, no further sessions can be added
either.
The tracks recorded in TAO mode are typically divided by gaps of 2 seconds. Each track written has
150 sectors of overhead for run-in, run-out, pre-gap, and linking. A rewritable drive can read the
tracks even if the session is not closed, but to read them in a non-rewritable CD-DA or CD-ROM
drive, the session must be closed. If you intend to write more sessions to the disc, you can close the
session and not close the disc. At that point, you could start another session of recording to add more
tracks to the disc. The main thing to remember is that each session must be closed (lead-out written)
before another session can be written or before a normal CD-DA or CD-ROM drive can read the
tracks in the session.
Packet Writing
Packet writing is a method whereby multiple writes are allowed within a track, thus reducing the
overhead and wasted space on a disc. Each packet uses four sectors for run-in, two for run-out, and
one for linking. Packets can be of fixed or variable length, but most drives and packet-writing
software use a fixed length because dealing with file systems that way is much easier and more
efficient.
With packet writing, you use the UDF version 1.5 or later file system, which enables the CD to be
treated essentially like a big floppy drive. That is, you can literally drag and drop files to it, use the
copy
command to copy files onto the disc, and so on. The packet-writing software and UDF file
system manage everything. If the disc you are using for packet writing is a CD-R, every time a file is
overwritten or deleted, the file seems to disappear, but you don't get the space back on the disc.
Instead, the file system simply forgets about the file. If the disc is a CD-RW, the space is indeed
reclaimed and the disc won't be full until you literally have more than the limit of active files stored
