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{{short description|Type of computer memory}}
{{Broader|Non-volatile memory}}
{{See also|Nonvolatile BIOS memory}}
{{Memory types}}
[[File:DS1225.JPG | thumb | right | Non-volatile RAM takes various form factors including this tall IC with pins for a socket or soldering beneath.]]
'''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|sequential-access memory]] such as [[magnetic tape]], which cannot be randomly accessed but which retains data indefinitely without electric power.
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==Early NVRAMs==
Some early computers used [[
Advances in [[semiconductor fabrication]] in the 1970s led to a new generation of [[Solid state (electronics)|solid state]] memories that magnetic-core memory could not match on cost or density. Today dynamic RAM forms the vast majority of a typical computer's [[main memory]]. Many systems require at least some non-volatile memory. Desktop computers require permanent storage of the instructions required to load the operating system. Embedded systems, such as an engine control computer for a car, must retain their instructions when power is removed. Many systems used a combination of RAM and some form of ROM for these roles.
Custom [[
[[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.
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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 10<sup>5</sup> writes to any particular bit.
One approach to overcoming the rewrite count limitation is to have a standard [[
The basis of [[flash memory]] is identical to EEPROM and differs largely in internal layout. Flash allows its memory to be written only in blocks, which greatly simplifies the internal wiring and allows for higher densities. [[Memory storage density]] is the main determinant of cost in most computer memory systems, and due to this flash has evolved into one of the lowest-cost solid-state memory devices available. Starting around 2000, demand for ever-greater quantities of flash have driven manufacturers to use only the latest fabrication systems in order to increase density as much as possible. Although fabrication limits are starting to come into play, new [[Multi-level cell|"multi-bit" techniques]] appear to be able to double or quadruple the density even at existing line widths.
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===Millipede memory===
{{Main|Millipede memory}}
Perhaps one of the more innovative solutions is [[
===FeFET memory===
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