Non-volatile random-access memory: Difference between revisions

Revision as of 11:17, 26 April 2020

Revision as of 15:57, 28 May 2020

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{{Memory types}}

{{short description|Type of computer memory}}

'''Non-volatile random-access memory''' ('''NVRAM''') is [[random-access memory]] that retains data without applied power. This is in contrast to [[dynamic random-access memory]] (DRAM) and [[static random-access memory]] (SRAM), which both maintain data only for as long as power is applied, or such forms of memory as magnetic tape, which cannot be randomly accessed but which retains data indefinitely without electric power.

[[Read-only memory]] devices can be used to store system [[firmware]] in [[embedded system]]s such as an automotive ignition system control or home appliance. They are also used to hold the initial processor instructions required to [[Bootstrapping |bootstrap]] a computer system. Read-write memory can be used to store calibration constants, passwords, or setup information, and may be integrated into a [[microcontroller]].

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Custom [[Read-only memory|ROM]] integrated circuits were one solution. The memory contents were stored as a pattern of the last mask used for manufacturing the integrated circuit, and so could not be modified once completed.

[[Programmable read-only memory|PROM]] improved on this design, allowing the chip to be written electrically by the end-user. PROM consists of a series of diodes that are initially all set to a single value, "1" for instance. By applying higher power than normal, a selected diode can be "burned out" (like a [[Fuse (electrical)|fuse]]), thereby permanently setting that bit to "0". PROM facilitated prototyping and small volume manufacturing. Many semiconductor manufacturers provided a PROM version of their mask ROM part, so that development [[firmware]] could be tested before ordering a mask ROM.

Currently, the best-known form of both NV-RAM and [[EEPROM]] memory is [[flash memory]]. Some drawbacks to flash memory include the requirement to write it in larger blocks than many computers can automatically address, and the relatively limited longevity of flash memory due to its finite number of write-erase cycles (as of January 2010 most consumer flash products can withstand only around 100,000 rewrites before memory begins to deteriorate){{Citation needed|date=January 2020}}. Another drawback is the performance limitations preventing flash from matching the response times and, in some cases, the random addressability offered by traditional forms of RAM. Several newer technologies are attempting to replace flash in certain roles, and some even claim to be a truly [[universal memory]], offering the performance of the best SRAM devices with the non-volatility of flash.<ref>"[https://www.academia.edu/6988421/A_Survey_Of_Architectural_Approaches_for_Managing_Embedded_DRAM_and_Non-volatile_On-chip_Caches A Survey Of Architectural Approaches for Managing Embedded DRAM and Non-volatile On-chip Caches]", Mittal et al., IEEE TPDS, 2014.</ref> As of June 2018 these alternatives have not yet become mainstream.

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Another solid-state technology to see more than purely experimental development is [[Phase-change RAM]], or PRAM. PRAM is based on the same storage mechanism as writable [[Compact Disk|CDs]] and [[DVD]]s, but reads them based on their changes in electrical resistance rather than changes in their optical properties. Considered a "dark horse" for some time, in 2006 [[Samsung]] announced the availability of a 512 Mbit part, considerably higher capacity than either MRAM or FeRAM. The areal density of these parts appears to be even higher than modern flash devices, the lower overall storage being due to the lack of multi-bit encoding. This announcement was followed by one from [[Intel]] and [[STMicroelectronics]], who demonstrated their own PRAM devices at the 2006 [[Intel Developer Forum]] in October.

[[Intel]] and [[STMicroelectronics]] are now selling PRAM based devices to consumers, under the names [[3D_XPoint|3D XPoint]] Optane, and QuantX <ref>{{cite web |url=https://pcper.com/2017/06/how-3d-xpoint-phase-change-memory-works/}}</ref>.

===Millipede memory===