The ISO 9660 standard, which has been applicable for CD-ROMs since their inception, has certain limitations which make it inappropriate for DVD, CD-RW and other new disc formats. The UDF (Universal Disc Format) ISO 13346 standard is designed to address these limitations. Specifically, packet writing isn’t entirely compatible with the ISO 9660 logical file system since it needs to know exactly which files will be written during a session to generate the Path Tables and Primary Volume Descriptors, which point to the physical location of files on the disc. UDF allows files to be added to a CD-R or CD-RW disc incrementally, one file at a time, without significant wasted overhead, using a technique called packet writing. Under UDF, even when a file is overwritten, its virtual address remains the same. At the end of each packet-writing session, UDF writes a Virtual Allocation Table (VAT) to the disc that describes the physical locations of each file. Each newly created VAT includes data from the previous VAT, thereby letting UDF locate all the files that have ever written to the disc.

By mid-1998 two versions of UDF had evolved, with future versions planned. UDF 1.02 is the version used on DVD-ROM and DVD-Video discs. UDF 1.5 is a superset that adds support for CD-R and CD-RW. Windows 98 provides support for UDF 1.02. However, in the absence of operating system support for UDF 1.5, special UDF driver software is required to allow packet-writing to the recordable CD formats. Adaptec’s DirectCD V2.0 was the first such software to support both packet-writing and the random erasing of individual files on CD-RW media. The DirectCD V2.0 software allows two kinds of packets can be written: fixed-length and variable-length. Fixed-length packets are more suitable for CD-RW in order to support random erase, because it would be daunting (and slow) to keep track of a large, constantly-changing file system if the packets were not written in fixed locations.

The UDF 1.5 solution is far from ideal however. Quite apart from the difficulties caused by lack of operating system support, there are other issues. The major drawback is that the fixed-length packets (of 32KB as per the UDF standard), take up a great deal of overhead space on the disc. The available capacity of a CD-RW disc formatted for writing in fixed-length packets is reduced to about 550MB. In practice, however, the capacity of a UDF-formatted disc is reduced still further as a consequence of DirectCD’s built-in features to increase the longevity of CD-RW media.

Any particular spot on a CD-RW disc can be erased and rewritten about 1000 times (soon to be improved to 10,000). After that, that particular spot becomes unusable. However, DirectCD is designed to avoid the same physical location being repeatedly written to and erased, using a technique called sparing. This significantly extends the life of a disc, but at the cost of an overhead which reduces effective storage capacity. Even if a particular location on a CD-RW disc does get burned out, DirectCD can mark it unusable and work around it (much the way bad sectors are managed on a hard disk). Consequently, it is highly unlikely that a CD-RW disc will become worn out.

In addition to these issues of reduced capacity, not all CD-R or CD-RW drives support packet writing and it is only MultiRead CD-ROM drives – and only OSTA-endorsed MultiRead drives at that – that can read packet-written discs. To do so requires use of Adaptec’s free UDF Reader software – which enables many MultiRead CD-ROM drives to read discs written in UDF 1.5 format. It is important to note that this software is required in addition to DirectCD – which is itself relevant to CD recorders only.

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