Bit rot

Bit rot, also known as bit decay, data rot, or data decay, is a colloquial computing term used to describe either a gradual decay of storage media or the degradation of a software program over time. The latter use of the term implies that software can 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 discs 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 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 literal rotting. 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 punched 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^[1], a phenomenon known as soft error.

Problems with software

Main article: Software rot

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.

A program may run correctly for years with no problem, then malfunction 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. Some operating systems^[which?] tend to lose stability when left running for long periods, which is why they must be restarted occasionally to remove resident errors that have built up due to software errors.

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. Also, the addition of programs and data on the computer can make operations and searching slower, and sometimes when programs are uninstalled they aren't removed completely. Additionally, fragmentation can slow performance. 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).

See also

• Link rot
• Software rot
• Digital preservation
• Disc rot
• Checksum
• Database integrity

References

1. ^ http://www.research.ibm.com/journal/rd/401/ogorman.pdf "Field testing for cosmic ray soft errors in semiconductor memories"

External links

• "Bit rot" from the Jargon File