Non-volatile random-access memory
Computer memory and Computer data storage types |
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Volatile |
Non-volatile |
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 forms of sequential-access memory such 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 systems such as an automotive ignition system control or home appliance. They are also used to hold the initial processor instructions required to bootstrap a computer system. Read-write memory such as NVRAM can be used to store calibration constants, passwords, or setup information, and may be integrated into a microcontroller.
If the main memory of a computer system were non-volatile, it would greatly reduce the time required to start a system after a power interruption. Current existing types of semiconductor non-volatile memory have limitations in memory size, power consumption, or operating life that make them impractical for main memory. Development is going on for the use of non-volatile memory chips as a system's main memory, as persistent memory. A standard for persistent memory known as NVDIMM-P has been published in 2021.[1][2][3]
Early NVRAMs
Some early computers used magnetic drum which was non-volatile as a byproduct of its construction. The industry moved to magnetic-core memory in the later 1950s, which stored data in the polarity of small magnets. Since the magnets held their state even with the power removed, core memory was also non-volatile. Other memory types required constant power to retain data, such as vacuum tube or solid-state flip-flops, Williams tube, and semiconductor memory (static or dynamic RAM).
Advances in
Custom 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.
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]. 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. As of June 2018 these alternatives have not yet become mainstream.
Those who required real RAM-like performance and non-volatility typically have had to use conventional RAM devices and a battery backup. For example, IBM PC's and successors beginning with the
Floating-gate MOSFET
A huge advance in NVRAM technology was the introduction of the floating-gate MOSFET transistor, which led to the introduction of erasable programmable read-only memory, or EPROM. EPROM consists of a grid of transistors whose gate terminal (the switch) is protected by a high-quality insulator. By pushing electrons onto the base with the application of higher-than-normal voltage, the electrons become trapped on the far side of the insulator, thereby permanently switching the transistor on (1). EPROM can be reset to the base state (all 1s or 0s, depending on the design) by applying ultraviolet light (UV). The UV photons have enough energy to push the electrons through the insulator and return the base to a ground state. At that point the EPROM can be re-written from scratch.
An improvement on EPROM, EEPROM, soon followed. The extra E stands for electrically, referring to the ability to reset EEPROM using electricity instead of UV, making the devices much easier to use in practice. The bits are re-set with the application of even higher power through the other terminals of the transistor (source and drain). This high-power pulse, in effect, sucks the electrons through the insulator, returning it to the ground state. This process has the disadvantage of mechanically degrading the chip, however, so memory systems based on floating-gate transistors in general have short write-lifetimes, on the order of 105 writes to any particular bit.
One approach to overcoming the rewrite count limitation is to have a standard
The basis of
Commercialized Alternatives
Flash and EEPROM's limited write-cycles are a serious problem for any real RAM-like role. In addition, the high power needed to write the cells is a problem in low-power roles, where NVRAM is often used. The power also needs time to be built up in a device known as a charge pump, which makes writing dramatically slower than reading, often as much as 1,000 times. A number of new memory devices have been proposed to address these shortcomings.
Ferroelectric RAM
To date, the only such system to enter widespread production is
Magnetoresistive RAM
Another approach to see major development effort is
STT-MRAM appears to allow for much higher densities than those of the first generation, but is lagging behind flash for the same reasons as FeRAM – enormous competitive pressures in the flash market.Phase-change RAM
Another solid-state technology to see more than purely experimental development is
Intel and Micron Technology had a joint venture to sell PRAM devices under the names 3D XPoint, Optane and QuantX, which was discontinued in July 2022.[9][10]
STMicroelectronics manufactures phase-change memory devices for automotive applications.
Researched Alternatives
Millipede memory
Perhaps one of the more innovative solutions is
FeFET memory
An alternative application of (hafnium oxide based)
See also
References
- ^ "JEDEC DDR5 & NVDIMM-P Standards Under Development" (Press release). JEDEC. 2017-03-30.
- ^ "JEDEC to Hold Workshops for DDR5, LPDDR5 & NVDIMM-P Standards" (Press release). JEDEC. 2019-09-05.
- ^ "JEDEC Publishes DDR4 NVDIMM-P Bus Protocol Standard" (Press release). JEDEC. 2021-02-17.
- ^ Chan, Peter (2005-04-21). "X4C105 NOVRAM Features and Applications" (PDF). Intersil. Archived from the original (PDF) on 2007-06-14.
- ^ "F-RAM Memory Technology". Ramtron. Archived from the original on 2012-04-18. Retrieved 2012-06-08.
- ^ "Technology". Everspin. Archived from the original on June 10, 2009.
- ^ Hoberman, Barry. "The Emergence of Practical MRAM" (PDF). Crocus Technology. Archived from the original (PDF) on 2011-04-27. Retrieved 2009-07-20.
- ^ LaPedus, Mark (2009-06-18). "Tower invests in Crocus, tips MRAM foundry deal". EE Times. Retrieved 2020-01-09.
- ^ Mann, Tobias (2022-07-29). "Why Intel killed its Optane memory business". The Register. Situation Publishing. Retrieved 2022-11-18.
- ^ Allyn Malventano (June 2, 2017). "HOW 3D XPOINT PHASE-CHANGE MEMORY WORKS". PC Perspective.
- Hitachi Global Storage Technologies. 2011-08-03. Archived from the originalon 2011-10-26. Retrieved 2011-12-17.
- ^ Johnston, Casey (2011-05-07). "New hard drive write method packs in one terabit per inch". Ars Technica. Retrieved 2011-12-17.
External links
- Supporting filesystems in persistent memory, LWN.net, September 2, 2014, by Jonathan Corbet