Computer memory types

Volatile

DRAM, e.g. DDR SDRAM SRAM Upcoming T-RAM Z-RAM TTRAM Historical Delay line memory Selectron tube Williams tube

Non-volatile

ROM PROM EPROM EEPROM Flash memory Upcoming FeRAM MRAM CBRAM PRAM SONOS RRAM Racetrack memory NRAM Millipede Historical Drum memory Magnetic core memory Plated wire memory Bubble memory Twistor memory

Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, is computer memory that 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 disks, floppy disks, and magnetic tape), optical discs, and early computer storage methods such as paper tape and punch cards.

Non-volatile memory is typically used for the task of secondary storage, or long-term persistent storage. The most widely used form of primary storage today is a volatile form of random access memory (RAM), meaning that when the computer is shut down, anything contained in RAM is lost. Unfortunately, most forms of non-volatile memory have limitations that make them unsuitable for use as primary storage. Typically, non-volatile memory either costs more or performs worse than volatile random access memory.

Several companies are working on developing non-volatile memory systems comparable in speed and capacity to volatile RAM. For instance, IBM is currently developing MRAM (Magnetoresistive RAM). Not only would such technology save energy, but it would allow for computers that could be turned on and off almost instantly, bypassing the slow start-up and shutdown sequence.

Non-volatile data storage can be categorised in electrically addressed systems (read-only memory) and mechanically addressed systems (hard disks, optical disc, magnetic tape, holographic memory and such). Electrically addressed systems are expensive, but fast, whereas mechanically addressed systems have a low price per bit, but are slow. Non-volatile memory may one day eliminate the need for comparatively slow forms of secondary storage systems, which include hard disks.

Contents 1 Electrically addressed 1.1 Mask-programmed ROM 1.2 Programmable ROM 1.3 Erasable PROMs 1.4 UV-erase EPROM 1.5 OTP (one-time programmable) EPROM 1.6 Electrically erasable PROM 1.7 Flash memory 1.8 Magnetoresistive RAM (MRAM) 2 Mechanically addressed systems 2.1 Tape 2.2 Hard disk 2.3 Optical disk 2.4 Nanodrive 2.5 Holographic storage 3 Organic 4 Specifications 5 References

[edit] Electrically addressed

Electrically addressed non-volatile memories based on charge storage can be categorised according to their write mechanism:

[edit] Mask-programmed ROM

One of the earliest forms of non-volatile read-only memory, the mask-programmed ROM was prewired at the design stage to contain specific data; once the mask was used to manufacture the integrated circuits, the data was cast in stone (silicon, actually) and could not be changed.

The mask ROM was therefore useful only for large-volume production, such as for read-only memories containing the startup code in early microcomputers. This program was often referred to as the "bootstrap", as in pulling oneself up by one's own bootstraps.

Due to the very high initial cost and inability to make revisions, the mask ROM is rarely if ever used in new designs.

[edit] Programmable ROM

The next approach was to create a chip which was initially blank; the programmable ROM originally contained silicon or metal fuses, which would be selectively "blown" or destroyed by a device programmer or PROM programmer in order to change 0s to 1s. Once the bits were changed, there was no way to restore them to their original condition. Non-volatile but still somewhat inflexible.

Early PAL programmable array logic chips used a similar programming approach to that used in the fuse-based PROMs.

Newer Antifuse-based PROMs (which are also referred to as one-time-programmable (OTP) memory) are widely used in consumer and automotive electronics, radio-frequency identification devices (RFID), implantable medical devices, and high-definition multimedia interfaces(HDMI) due to their small footprint, reliability, fast read speed, and long data retention rates.

[edit] Erasable PROMs

There are two classes of non-volatile memory chips based on EPROM technology.

[edit] UV-erase EPROM

The original erasable non-volatile memories were EPROM's; these could be readily identified by the distinctive quartz window in the centre of the chip package. These operated by trapping an electrical charge on the gate of a field-effect transistor in order to change a 1 to a 0 in memory. To remove the charge, one would place the chip under an intense short-wavelength fluorescent ultraviolet lamp for 20-30 minutes, returning the entire chip to its original blank (all ones) state.

[edit] OTP (one-time programmable) EPROM

An OTP is electrically an EPROM, but with the quartz window physically missing. Like the fuse PROM it can be written once, but cannot be erased.

[edit] Electrically erasable PROM

Electrically erasable PROMs have the advantage of being able to selectively erase any part of the chip without the need to erase the entire chip and without the need to remove the chip from the circuit. While an erase and rewrite of a location appears nearly instantaneous to the user, the write process is slightly slower than the read process; the chip can be read at full system speeds.

The limited number of times a single location can be rewritten is usually in the 10000-100000 range; the capacity of an EEPROM also tends to be smaller than that of other non-volatile memories. Nonetheless, EEPROMs are useful for storing settings or configuration for devices ranging from dial-up modems to satellite receivers.

[edit] Flash memory

The flash memory chip is a close relative to the EEPROM; it differs in that it can only be erased one block or "page" at a time. Capacity is substantially larger than that of an EEPROM, making these chips a popular choice for digital cameras and desktop PC BIOS chips.

[edit] Magnetoresistive RAM (MRAM)

Magnetoresistive RAM is one of the newest approaches to non-volatile memory and stores data in magnetic storage elements called magnetic tunnel junctions (MTJ's). MRAM has an especially promising future as it seeks to encompass all the desirable features of the other popular types of memory (non-volatility, infinite endurance, high-speed reading/writing, low cost)^[1].

The 1st generation of MRAM, such as Everspin Technologies' 4 Mbit, utilized field induced writing^[2]. The 2nd generation is being developed mainly through two approaches: Thermal Assisted Switching (TAS)^[1] which is being developed by Crocus Technology, and Spin Torque Transfer (STT) which Crocus, Hynix, IBM, and several other companies are developing^[3].

[edit] Mechanically addressed systems

Mechanically addressed systems utilize a contact structure ('head') to read and write on a designated storage medium. Since circuitry layout is not a key factor for data density, the amount of storage is typically much larger than for electrically addressed systems.

[edit] Tape

[edit] Hard disk

[edit] Optical disk

[edit] Nanodrive

[edit] Holographic storage

[edit] Organic

There are polymer printed ferroelectric memory (i.e.Thin Film Electronics).

This section requires expansion.

[edit] Specifications

Specification March 2007 [4]	2.5" HDD	1" Microdrive	Flash Memory	Optical Disk	Tape	MRAM
Device Model:	Hitachi Travelstar 5k160	Hitachi Microdrive 3k8	Hynix HY27UH08AG5M	Blu Ray	HP Ultrium 960	Everspin[5] MR2A16A
Density (GBit/cm2)	20.3	18.4	6.7	3.8	0.047	0.0021
Capacity (GByte)	160	8	2	50	400	0.004
Price per bit (Eur/GByte)	1.5	9.0	6.0	1.25	0.075	35000
Price per unit (Eur)	110	87	14	635	2340	17.4
Price per medium (Eur) [6]	nd	nd	nd	40	30	nd
Data rate (Mbit/s)	540	80	23	144	640	436
Access time (ms) [7]	11	12	0.025	180	72000	0.035
Power consumption (W) [8]	1.8	0.6	0.1	25	20	0.08
Form factor h x w x d (cm)	0.95x7x10	0.5x3x4	0.1x1.2x2	4x15x19	2x10x10	0.1x1x1.8

[edit] References

1. ^ ^{a b} The Emergence of Practical MRAM http://www.crocus-technology.com/pdf/BH%20GSA%20Article.pdf
2. ^ http://www.everspin.com/technology.html
3. ^ http://www.eetimes.com/news/latest/showArticle.jhtml?articleID=218000269
4. ^ A justification for this table can be found here:
5. ^ formerly of Freescale Semiconductor
6. ^ For removables
7. ^ Average/typical)
8. ^ Average