Editing Computer memory (section)

==Volatility categories==

=== Volatile memory ===
[[File:Kinds-of-RAM.JPG|thumb|Various memory modules containing different types of DRAM (from top to bottom): DDR SDRAM, SDRAM, EDO DRAM, and FPM DRAM]]
{{Main|Volatile memory}}

Volatile memory is computer memory that requires power to maintain the stored information. Most modern [[semiconductor]] volatile memory is either [[static RAM]] (SRAM) or [[dynamic RAM]] (DRAM).{{efn|Other volatile memory technologies that have attempted to compete or replace SRAM and DRAM include [[Z-RAM]] and [[A-RAM]].}} DRAM dominates for desktop system memory. SRAM is used for [[CPU cache]]. SRAM is also found in small [[embedded system]]s requiring little memory.

SRAM retains its contents as long as the power is connected and may use a simpler interface, but [[Static random-access memory#Design|commonly uses six transistors per bit]]. Dynamic RAM is more complicated for interfacing and control, needing regular refresh cycles to prevent losing its contents, but uses only one transistor and one capacitor per bit, allowing it to reach much higher densities and much cheaper per-bit costs.<ref name=":1" /><ref name="HC" /><ref name=":2" />

=== Non-volatile memory ===
{{Main|Non-volatile memory}}

Non-volatile memory can retain the stored information even when not powered. Examples of non-volatile memory include [[read-only memory]], [[flash memory]], most types of magnetic computer storage devices (e.g. [[hard disk drive]]s, [[floppy disk]]s and [[magnetic tape]]), [[optical disc]]s, and early computer storage methods such as [[magnetic drum]], [[paper tape]] and [[punched card]]s.<ref name=":2" />

Non-volatile memory technologies under development include [[ferroelectric RAM]], [[programmable metallization cell]], [[Spin-transfer torque magnetic RAM]], [[SONOS]], [[resistive random-access memory]], [[racetrack memory]], [[Nano-RAM]], [[3D XPoint]], and [[millipede memory]].

=== Semi-volatile memory ===
A third category of memory is ''semi-volatile''. The term is used to describe a memory that has some limited non-volatile duration after power is removed, but then data is ultimately lost. A typical goal when using a semi-volatile memory is to provide the high performance and durability associated with volatile memories while providing some benefits of non-volatile memory.

For example, some non-volatile memory types experience wear when written. A ''worn'' cell has increased volatility but otherwise continues to work. Data locations which are written frequently can thus be directed to use worn circuits.  As long as the location is updated within some known retention time, the data stays valid.  After a period of time without update, the value is copied to a less-worn circuit with longer retention.  Writing first to the worn area allows a high write rate while avoiding wear on the not-worn circuits.<ref>{{cite web|last1=Montierth, Briggs, Keithley|title=Semi-volatile NAND flash memory|url=https://patents.google.com/patent/US7710777B1/|access-date=20 May 2018}}</ref>

As a second example, an [[STT-RAM]] can be made non-volatile by building large cells, but doing so raises the cost per bit and power requirements and reduces the write speed. Using small cells improves cost, power, and speed, but leads to semi-volatile behavior. In some applications, the increased volatility can be managed to provide many benefits of a non-volatile memory, for example by removing power but forcing a wake-up before data is lost; or by caching read-only data and discarding the cached data if the power-off time exceeds the non-volatile threshold.<ref>{{cite web|last1=Keppel, Naeimi, Nasrullah|title=Method and apparatus for managing a spin-transfer torque memory|url=https://patents.google.com/patent/US9342403B2/|website=Google Patents|access-date=20 May 2018}}</ref>

The term semi-volatile is also used to describe semi-volatile behavior constructed from other memory types, such as [[nvSRAM]], which combines [[Static random-access memory|SRAM]] and a non-volatile memory on the same [[Microchip|chip]], where an external signal copies data from the volatile memory to the non-volatile memory, but if power is removed before the copy occurs, the data is lost. Another example is [[Battery-backed memory|battery-backed RAM]], which uses an external [[Electric battery|battery]] to power the memory device in case of external power loss. If power is off for an extended period of time, the battery may run out, resulting in data loss.<ref name=":2" />