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Non-volatile random-access memory (NVRAM) is random-access memory that retains its information when power is turned off (non-volatile). 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. The best-known form of NVRAM memory today 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 (most consumer flash products at the time of writing can withstand only around 100,000 rewrites before memory begins to deteriorate). 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.〔"(A Survey Of Architectural Approaches for Managing Embedded DRAM and Non-volatile On-chip Caches )", Mittal et al., IEEE TPDS, 2014.〕 To date these alternatives have not yet become mainstream. ==Early NVRAMs== Early computers used a variety of memory systems, some of which happened to be non-volatile, although not typically by design but simply as a side-effect of their construction. The most common form of memory through the 1960s was magnetic-core memory, 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. Such memory contrasted sharply with memory based on active electronic devices, originally tube (or thermionic valve) based flip-flop devices, and later semiconductor based flip-flop (SRAM), or even charge storage systems (DRAM). Rapid advances in semiconductor fabrication in the 1970s led to a new generation of solid state memories that core simply could not compete with. Relentless market forces have dramatically improved these devices over the years, and today the low-cost and high-performance DRAM forms the vast majority of a typical computer's main memory. However there are many roles where non-volatility is important, either in cases where the power will be removed for periods of time or where the constant power needs of DRAM conflicts with low-power devices. For many years, there was no practical RAM-like device to fill this niche, and many systems used a combination of RAM and some form of ROM for these roles. Custom ROM was the earliest solution, but had the disadvantage of being able to be written to only once, when the chip was initially designed. ROMs consist of a series of diodes permanently wired to return the required data, the diodes being built in this configuration when they are being fabricated. 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), thereby permanently setting that bit to "0". PROM was a boon to companies who wished to update the contents with new revisions, or alternately produce a number of different products using the same chip. For instance, PROM was widely used for game console cartridges in the 1980s. Those who required real RAM-like performance and non-volatility typically have had to use conventional RAM devices and a battery backup. This nonvolatile BIOS memory, often called ''CMOS RAM'' or ''parameter RAM'', was a common solution in earlier computer 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. Much larger battery backed memories are still used today as caches for high-speed databases, requiring a performance level newer NVRAM devices have not yet managed to meet. ==The floating-gate transistor== A huge advance in NVRAM technology was the introduction of the floating-gate 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 re-set to the "base state" (all "1"s or "0"s, 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 SRAM where each bit is backed up by an EEPROM bit. In normal operation the chip functions as a fast SRAM and in case of power failure the content is quickly transferred to the EEPROM part, from where it gets loaded back at the next power up. Such chips were called NOVRAMs〔(X4C105 NOVRAM Features and Applications, Intersil Application Note )〕 by their manufacturers. 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-bit" techniques appear to be able to double or quadruple the density even at existing linewidths. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Non-volatile random-access memory」の詳細全文を読む スポンサード リンク
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