Universal Disk Format
Now there is a packet writing - which has two different flavors. The idea of writing packages is that if you do not worry about the TOC, you can write data to CD-R in the form of blocks or packets. The information that goes into each packet needs to be accessible by the writer, but not necessarily by an ordinary reader. When writing an appropriate set of functions in the heart of OS - at about extensions for DOS - the supplier can provide the set of opportunities for writers with the appropriate hardware and firmware. However, there is no TOC at Level 1 or Level 3 sense that the information on the disk and in other ways. This requires additional space on disk and indicates that the data should be interpreted when UDF disc inserted in the drive.
When a write-once disc is formatted for variable-length packets, space is committed for the TOC, but no TOC is written. Nothing else of importance is done, so the process is quick. When a file is written to the disc, it becomes a set of packets headed by the information corresponding to what would be in its FAT entry and including its length so that the location of the next packet can be determined. At the same time, DCD holds in memory a virtual FAT (VFAT) which gives access to that file just as though it were on a conventional disc. Write another file and the VFAT is updated appropriately. When the disc is ejected, the VFAT is removed from memory and nothing is written to the disc.
If the session is closed when the disc is ejected, the information for the TOC is written to the disc from the VFAT but the packets themselves are not altered. The TOC is then accessed through the usual extension. Until the disc is closed, there is no usable TOC information on it - which is why it cannot be read conventionally. A disc with a closed session has a TOC in Level 3 format; that TOC may point to the place another TOC will be written or may say: this disc is closed. During the time that a packet is being prepared and written, a catastrophic system failure would cause the write to be incomplete and might make the disc useless; at all other times, even if the system is powered down with the disc in the drive, the information is safe. The laser is off, everything needed is already written and nothing can happen. Apply power, return to your OS, and the packets are read to form the VFAT.
In the first version(s) of UDF, only conventional, non-erasable media were considered. For them, a very efficient system could be devised. At the head of each packet is the information on the file it represents including its length. Each packet contains an integral number of blocks; the only space wasted is for that header and for the leftover bytes of the last 2K block. A variable-length packet can be converted to ISO 9660 Level 3 format because all the bytes of the file are in complete blocks. Those blocks may not be consecutive, however; if one writes more than one file at a time to a packet disc, the packets may be interleaved. When the session is closed to Level 3, the TOC is written in a more or less conventional fashion, but data interpretation requires that the OS ignore the old header information. In other words, the TOC is similar to that of Level 1, but both the TOC and the data are different enough to require changes that make the old extension useless. (Note that the Level 3 TOC may be written either to close a session or to close the disc. If the session is closed, another UDF session can be started and finalized/closed; when a disc is closed, the TOC indicates it and will not permit reopening.)