Write Once Read Many
Encyclopedia
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 nearly all storage devices permit unlimited reading of data once written. Deliberately time-limited discs such as Flexplay
, designed for short-term rental of movies, are an exception.)
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. These drives typically used a 12 in (30.5 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 PC's also existed.
These days, the CD-R and DVD-R optical disks for computer
s 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 card
s 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.
Data storage
Data storage can refer to:* Computer data storage; memory, components, devices and media that retain digital computer data used for computing for some interval of time....
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 nearly all storage devices permit unlimited reading of data once written. Deliberately time-limited discs such as Flexplay
Flexplay
Flexplay is a trademark for a DVD-compatible optical video disc format with a time-limited playback time. They are often described as "self-destructing" although the disc merely turns black and does not physically disintegrate. The same technology was used by Disney's Buena Vista Home...
, designed for short-term rental of movies, are an exception.)
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
CD-R
A CD-R is a variation of the Compact Disc invented by Philips and Sony. CD-R is a Write Once Read Many optical medium, though the whole disk does not have to be entirely written in the same session....
and DVD-R
DVD-R
DVD-R is a DVD recordable format. A DVD-R typically has a storage capacity of 4.71 GB. Pioneer has also developed an 8.5 GB dual layer version, DVD-R DL, which appeared on the market in 2005....
. An example was the IBM 3363. These drives typically used a 12 in (30.5 cm) disk in a cartridge, with an ablative
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...
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 PC's also existed.
These days, the CD-R and DVD-R optical disks for computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s 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
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
based on ISO 9660
ISO 9660
ISO 9660, also referred to as CDFS by some hardware and software providers, is a file system standard published by the International Organization for Standardization for optical disc media....
that permits additional files
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
, 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
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
does not allow rewrites of any block of the memory.
Punched card
Punched card
A punched card, punch card, IBM card, or Hollerith card is a piece of stiff paper that contains digital information represented by the presence or absence of holes in predefined positions...
s 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
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
is also a WORM medium. Such memory may contain the instructions to a computer to read the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
from another storage device such as a hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
. The end-user, however, cannot write the ROM even once, but considers it part of the unchangeable computing platform.