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Bit rot

Bit rot, or bit decay, is a colloquial computing term used either to describe gradual decay of storage media or to facetiously describe the spontaneous degradation of a software program over time. The latter use of the term implies that software can literally wear out or rust like a physical tool. More commonly, bit rot refers to the decay of physical storage media.

Decay of storage media

Bit rot is often defined as the event in which the small electric charge of a bit in memory disperses, possibly altering program code.

Bit rot can also be used to describe the phenomenon of data stored in EPROMs and flash memory gradually decaying over the duration of many years, or in the decay of data stored on CD or DVD disks or other types of consumer storage.

The cause of bit rot varies depending on the medium. EPROMs and flash memory store data using electrical charges, which can slowly leak away due to imperfect insulation. The chip itself is not affected by this, so that re-programming it once per decade or so will prevent the bit rot.

Floppy disk and magnetic tape storage may experience bit rot as bits lose magnetic orientation, and in warm, humid conditions these media are prone to literally rot. In optical discs such as CDs and DVDs the breakdown of the material onto which the data is stored may cause bit rot. This can be mitigated by storing disks in a dark, cool location with low humidity. Archival quality disks are also available. Old punch cards and punched tape may also experience literal rotting.

Bit rot is also used to describe the idea that semiconductor RAM may occasionally be altered by cosmic rays.

Problems with software

The term "bit rot" is often used to refer to dormant code rot, i.e. the fact that dormant (unused or little-used) code gradually decays in correctness as a result of interface changes in active code that is called from the dormant code.

When a program that has been running correctly for an extended time suddenly malfunctions for no apparent reason, programmers often jokingly attribute the failure to bit rot. Such an effect may be due to a memory leak or other non-obvious software bug. Often, although there is no obvious change in the program's operating environment, a subtle difference has occurred that is triggering a latent software error. The error in the software may also originate by human operation which allows the construction or derivation of false-positive behavior to occur within the code.

The term is also used to describe the slowing of performance of a PC over time from continued use. One cause of this is installing software or software components that run when the user logs in, causing a noticeable delay in boot time. Normally, unused data (such as a text file containing some notes) does not impede performance of a PC (with the exception of software that, for example, indexes files on a disk to make file searching faster).

Other uses of the term

Bit rot may refer to the tendency of a patch file generated based on a given version of a program's source code to fail to have the intended effect when applied to the current version, increasingly as that current version continues to evolve. When this occurs, the patch file is often said to have bit rotted.

Rarely, bit rot is referred to as the process by which data becomes inaccessible due to the lack of working mechanisms to read old data storage formats. This could mean that programs originally used to create or access the data are no longer available, operating systems used to run such programs are no longer available, or hardware used to access the physical data store is no longer available. For example, a game stored on a floppy disk may be referred to as having succumbed to bit rot if the user no longer possesses a floppy disk drive to read the disk, or such a game may not be playable due to lack of system and/or software support even if the physical media is readable.

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