Editing Dynamic random-access memory (section)

==Error detection and correction==
{{Main|RAM parity|ECC memory}}

Electrical or magnetic interference inside a computer system can cause a single bit of DRAM to [[RAM parity|spontaneously flip]] to the opposite state. The majority of one-off ("[[soft error|soft]]") errors in DRAM chips occur as a result of [[background radiation]], chiefly [[neutron]]s from [[cosmic ray]] secondaries, which may change the contents of one or more memory cells or interfere with the circuitry used to read/write them.

The problem can be mitigated by using [[Redundancy (engineering)|redundant]] memory bits and additional circuitry that use these bits to detect and correct soft errors. In most cases, the detection and correction are performed by the [[memory controller]]; sometimes, the required logic is transparently implemented within DRAM chips or modules, enabling the ECC memory functionality for otherwise ECC-incapable systems.<ref>{{cite web
 |url         = http://www.intelligentmemory.com/ECC-DRAM/
 |title       = ECC DRAM – Intelligent Memory
 |access-date  = 2015-01-16
 |website     = intelligentmemory.com
 |url-status  = dead
 |archive-url  = https://web.archive.org/web/20141223152744/http://www.intelligentmemory.com/ECC-DRAM/
 |archive-date = 2014-12-23
}}</ref> The extra memory bits are used to record [[RAM parity|parity]] and to enable missing data to be reconstructed by [[error-correcting code]] (ECC). Parity allows the detection of all single-bit errors (actually, any odd number of wrong bits). The most common error-correcting code, a [[Hamming code#Hamming codes with additional parity (SECDED)|SECDED Hamming code]], allows a single-bit error to be corrected and, in the usual configuration, with an extra parity bit, double-bit errors to be detected.<ref>{{cite web|author1=Mastipuram, Ritesh|author2=Wee, Edwin C|title=Soft errors' impact on system reliability|url=http://www.edn.com/article/CA454636.html|website=EDN|publisher=Cypress Semiconductor|archive-url=https://web.archive.org/web/20070416115228/http://www.edn.com/article/CA454636.html|archive-date=16 April 2007|date=30 September 2004}}</ref>

Recent studies give widely varying error rates with over seven orders of magnitude difference, ranging from {{nowrap|10<sup>&minus;10</sup>−10<sup>−17</sup> error/bit·h}}, roughly one bit error, per hour, per gigabyte of memory to one bit error, per century, per gigabyte of memory.<ref name="Borucki1">Borucki, "Comparison of Accelerated DRAM Soft Error Rates Measured at Component and System Level", 46th Annual International Reliability Physics Symposium, Phoenix, 2008, pp. 482–487</ref><ref name="Schroeder1">[[Bianca Schroeder|Schroeder, Bianca]] et al. (2009). [http://www.cs.toronto.edu/~bianca/papers/sigmetrics09.pdf "DRAM errors in the wild: a large-scale field study"] {{webarchive|url=https://web.archive.org/web/20150310193355/http://www.cs.toronto.edu/~bianca/papers/sigmetrics09.pdf |date=2015-03-10 }}. ''Proceedings of the Eleventh International Joint Conference on Measurement and Modeling of Computer Systems'', pp.&nbsp;193–204.</ref><ref name="Xin1">{{cite web|url=http://www.ece.rochester.edu/~xinli/usenix07/|title=A Memory Soft Error Measurement on Production Systems|website=www.ece.rochester.edu|access-date=8 May 2018|url-status=dead|archive-url=https://web.archive.org/web/20170214005146/http://www.ece.rochester.edu/~xinli/usenix07/|archive-date=14 February 2017}}</ref> The Schroeder et al. 2009 study reported a 32% chance that a given computer in their study would suffer from at least one correctable error per year, and provided evidence that most such errors are intermittent hard rather than soft errors and that trace amounts of radioactive material that had gotten into the chip packaging were emitting alpha particles and corrupting the data.<ref>{{cite web |url=https://spectrum.ieee.org/drams-damning-defects-and-how-they-cripple-computers |title=DRAM's Damning Defects—and How They Cripple Computers - IEEE Spectrum |access-date=2015-11-24 |url-status=live |archive-url=https://web.archive.org/web/20151124182515/https://spectrum.ieee.org/computing/hardware/drams-damning-defects-and-how-they-cripple-computers |archive-date=2015-11-24 }}</ref> A 2010 study at the University of Rochester also gave evidence that a substantial fraction of memory errors are intermittent hard errors.<ref>{{cite web|url=http://www.cs.rochester.edu/~kshen/papers/usenix2010-li.pdf|title="A Realistic Evaluation of Memory Hardware Errors and Software System Susceptibility". Usenix Annual Tech Conference 2010|author1=Li, Huang|author2=Shen, Chu|year=2010|url-status=live|archive-url=https://web.archive.org/web/20150515214728/http://www.cs.rochester.edu/%7Ekshen/papers/usenix2010-li.pdf|archive-date=2015-05-15}}</ref> Large scale studies on non-ECC main memory in PCs and laptops suggest that undetected memory errors account for a substantial number of system failures: the 2011 study reported a 1-in-1700 chance per 1.5% of memory tested (extrapolating to an approximately 26% chance for total memory) that a computer would have a memory error every eight months.<ref>{{cite web|url=http://research.microsoft.com/pubs/144888/eurosys84-nightingale.pdf|title=Cycles, cells and platters: an empirical analysis of hardware failures on a million consumer PCs. Proceedings of the sixth conference on Computer systems (EuroSys '11). pp 343-356|year=2011|url-status=live|archive-url=https://web.archive.org/web/20121114111006/http://research.microsoft.com/pubs/144888/eurosys84-nightingale.pdf|archive-date=2012-11-14}}</ref>