Write once read many
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A write once read many or WORM drive is a data storage device where information, once written, cannot be modified. On ordinary data storage devices, the number of times data can be modified is not limited, except by the rated lifespan of the device, as modification involves physical changes that may cause wear to the device. The "read many" aspect is unremarkable, as modern storage devices permit unlimited reading of data once written.[Note 1]
WORM devices are useful in archiving information when users want the security of knowing it has not been modified since the initial write, which might imply tampering.
WORM drives preceded the invention of the CD-R and DVD-R. An example was the IBM 3363.[1] These drives typically used a 12 in (30 cm) disk in a cartridge, with an ablative optical layer that could be written to only once, and were often used in places like libraries that needed to store large amounts of data. Interfaces to connect these to PCs also existed.
These days, the CD-R and DVD-R optical disks for computers are the most common WORM device. On these disks, no region of the disk can be recorded a second time. However, these disks often use a file system based on ISO 9660 that permits additional files, and even revised versions of a file by the same name, to be recorded in a different region of the disk. To the user of the disk, the disk appears to allow additions and revisions until all the disk space is used.
A version of the SecureDigital flash memory card exists in which the internal microprocessor does not allow rewrites of any block of the memory.
Punched cards and paper tape are obsolete WORM media. Although any unpunched area of the medium could be punched after the first write of the medium, doing so was virtually never useful. Read-only memory is also a WORM medium. Such memory may contain the instructions to a computer to read the operating system from another storage device such as a hard disk. The end-user, however, cannot write the ROM even once, but considers it part of the unchangeable computing platform.
In recent years there has been a renewed interest in WORM based on organic components, such as PEDOT:PSS [2] [3] [4] or other polymers such as PVK [5] or PCz [6]. Organic WORM devices, considered organic memory, could be used as memory elements for low-power RFID tags [7].
[edit] Notes
- ^ Historical exceptions include time-limited discs such as Flexplay, designed for short-term rental of movies; and early non-volatile memory technologies such as magnetic-core memory and Bubble memory, from which reading data also erased it.
[edit] References
- ^ IBM 3363 Optical WORM drive
- ^ Organic-inorganic heterojunction WORM memory
- ^ Möller et al. "A polymer/semiconductor write-once read-many-times memory"
- ^ Smith and Forrest "A low switching voltage organic-on-inorganic heterojunction memory element utilizing a conductive polymer fuse on a doped silicon substrate"
- ^ Lin and Ma "Realization of write-once-read-many-times memory devices based on poly(N-vinylcarbazole) by thermally annealing"
- ^ Teo et al. "An Organic-Based Diode–Memory Device With Rectifying Property for Crossbar Memory Array Applications"
- ^ "Holst Centre reports major step towards organic RFID"
