Non-volatile RAM

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 forms of both NV-RAM and EEP-ROM memory today are 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. To date these alternatives have not yet become mainstream.

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 magnetic-core memory 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.

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