Non-volatile random-access memory: Difference between revisions

Revision as of 15:24, 14 March 2020

Revision as of 15:43, 14 March 2020

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'''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 [[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]].

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.

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]]. Known as [[NVDIMM#Types|NVDIMM-P]], it is expected to be released in 2020.<ref>{{cite press release|url=https://www.jedec.org/news/pressreleases/jedec-ddr5-nvdimm-p-standards-under-development|title=JEDEC DDR5 & NVDIMM-P Standards Under Development|date=2017-03-30|publisher=[[JEDEC]]}}</ref><ref>{{cite press release|url=https://www.jedec.org/news/pressreleases/jedec-hold-workshops-ddr5-lpddr5-nvdimm-p-standards|title=JEDEC to Hold Workshops for DDR5, LPDDR5 & NVDIMM-P Standards|date=2019-09-05|publisher=JEDEC}}</ref>

==Early NVRAMs==

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[[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.

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 [[IBM PC AT]] used [[nonvolatile BIOS memory]], often called ''CMOS RAM'' or ''parameter RAM'', and this was a common solution in other early microcomputer systems like the original [[Apple Macintosh]], which used a small amount of memory powered by a battery for storing basic setup information like the selected boot volume. (The original IBM PC and PC XT instead used DIP switches to represent up to 24 bits of system configuration data; DIP or similar switches are another, primitive type of programmable ROM device that was widely used in the 1970s and 1980s for very small amounts of data&mdash;typically no more than 8 bytes.) Before industry standardization on the IBM PC architecture, some other microcomputer models used battery-backed RAM more extensively: for example, in the [[TRS-80 Model 100]]/Tandy 102, all of the main memory (8 KB minimum, 32 KB maximum) is battery-backed SRAM. Also, in the 1990s many video game software cartridges (e.g. for consoles such as the [[Sega Genesis]]) included battery-backed RAM to retain saved games, high scores, and similar data. Also, some arcade video game cabinets contain CPU modules that include battery-backed RAM containing keys for on-the-fly game software decryption. Much larger battery backed memories are still used today as [[cache (computing)|caches]] for high-speed [[database]]s that require a performance level newer NVRAM devices have not yet managed to meet.