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In
computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and software. It has sci ...

computing
, memory is a device or system that is used to store information for immediate use in a
computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These programs enable compu ...

computer
or related
computer hardware Computer hardware includes the physical parts of a computer, such as the computer case, case, central processing unit (CPU), Computer monitor, monitor, Computer mouse, mouse, Computer keyboard, keyboard, computer data storage, graphics card, so ...

computer hardware
and
digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electronics comprises the physics, engineering, technology a ...
electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow b ...
devices. The term ''memory'' is often synonymous with the term ''
primary storage A spindle of DVD-RW's. Computer data storage is a technology consisting of computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perfor ...
'' or ''
main memory Computer data storage is a technology consisting of computer components and Data storage device, recording media that are used to retain digital data (computing), data. It is a core function and fundamental component of computers. The cent ...
''. An archaic synonym for memory is store. Computer memory operates at a high speed compared to storage that is slower but less expensive and higher in capacity. Besides storing opened programs, computer memory serves as disk cache to improve both reading and writing performance. Operating systems borrow RAM capacity for caching so long as not needed by running software. If needed, contents of the computer memory can be transferred to storage; a common way of doing this is through a memory management technique called ''
virtual memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...

virtual memory
''. Modern memory is implemented as
semiconductor memory Semiconductor memory is a used for , such as . It typically refers to MOS memory, where data is stored within (MOS) on a memory chip. There are numerous different types using different semiconductor technologies. The two main types of (RA ...
, where data is stored within memory cells built from
MOS transistor The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), is a type of insulated-gate field-effect transistor The field-effect trans ...
s and other components on an
integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuit 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of indivi ...

integrated circuit
. There are two main kinds of semiconductor memory, volatile and
non-volatile Non-volatile memory (NVM) or non-volatile storage is a type of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of ...
. Examples of
non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data. Examples of non-volatile ...
are
flash memory Flash memory is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses acti ...
and
ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality, a medical classification to estimate the likelihood of death for a patient * Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac * ...
,
PROM A promenade dance, commonly called a prom, is a dance party A dance party (also referred to as a dance) is a social gathering where dancing is the primary activity. Some dance parties are held in a casual setting and open to the public, such a ...
,
EPROM An EPROM (rarely EROM), or erasable programmable read-only memory, is a type of programmable read-only memory A programmable read-only memory (PROM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. (eF ...

EPROM
and
EEPROM EEPROM (also E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information ...
memory. Examples of
volatile memory Volatile memory, in contrast to non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant pow ...
are
dynamic random-access memory Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access memory, random-access semiconductor memory that stores each bit of data in a memory cell (computing), memory cell, usually consisting of a tiny capacitor and a tr ...
(DRAM) used for primary storage, and
static random-access memory clone (2K × 8 bits) Static random-access memory (static RAM or SRAM) is a type of random-access memory File:Random Access Memory HyperX.jpg, 8GB DDR3 RAM stick with a white Heat sink, heatsink Random-access memory (RAM; ) is a f ...
(SRAM) used for
CPU cache A CPU cache is a hardware cache In computing, a cache ( , or in Australian English) is a hardware or software component that stores data so that future requests for that data can be served faster; the data stored in a cache might be the resu ...
. Most semiconductor memory is organized into memory cells each storing one
bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algo ...
(0 or 1).
Flash memory Flash memory is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses acti ...
organization includes both one bit per memory cell and
multi-level cell In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplificati ...
capable of storing multiple bits per cell. The memory cells are grouped into words of fixed
word length In computing, a word is the natural unit of data used by a particular Central processing unit, processor design. A word is a fixed-sized Data (computing), piece of data handled as a unit by the instruction set or the hardware of the processor. Th ...
, for example, 1, 2, 4, 8, 16, 32, 64 or 128 bits. Each word can be accessed by a binary address of ''N'' bits, making it possible to store 2''N'' words in the memory.


History

In the early 1940s, memory technology often permitted a capacity of a few bytes. The first electronic programmable
digital computer A computer is a machine A machine is a man-made device that uses power to apply forces and control movement to perform an action. Machines can be driven by animals and people A people is a plurality of person A person (plural ...
, the
ENIAC ENIAC (; Electronic Numerical Integrator and Computer) was the first programmable, electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission ...

ENIAC
, using thousands of
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστή ...
s, could perform simple calculations involving 20 numbers of ten decimal digits stored in the vacuum tubes. The next significant advance in computer memory came with acoustic
delay-line memory Delay-line memory is a form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and developmen ...
, developed by
J. Presper Eckert John Adam Presper Eckert Jr. (April 9, 1919 – June 3, 1995) was an American electrical engineer and computer pioneer. With John Mauchly John William Mauchly (August 30, 1907 – January 8, 1980) was an American physicist A physicist ...
in the early 1940s. Through the construction of a glass tube filled with
mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

mercury
and plugged at each end with a quartz crystal, delay lines could store bits of information in the form of sound waves propagating through the mercury, with the quartz crystals acting as
transducer A transducer is a device that energy from one form to another. Usually a transducer converts a in one form of energy to a signal in another. Transducers are often employed at the boundaries of , , and s, where electrical signals are converted t ...

transducer
s to read and write bits. Delay-line memory was limited to a capacity of up to a few thousand bits. Two alternatives to the delay line, the
Williams tube The Williams tube, or the Williams–Kilburn tube after inventors Freddie Williams and Tom Kilburn, is an early form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing ...
and
Selectron tube The Selectron was an early form of digital computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes a ...
, originated in 1946, both using electron beams in glass tubes as means of storage. Using
cathode ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, the beams of which are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms (oscilloscope), pictures (televisi ...

cathode ray tube
s, Fred Williams invented the Williams tube, which was the first random-access computer memory. The Williams tube was able to store more information than the Selectron tube (the Selectron was limited to 256 bits, while the Williams tube could store thousands) and less expensive. The Williams tube was nevertheless frustratingly sensitive to environmental disturbances. Efforts began in the late 1940s to find
non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data. Examples of non-volatile ...
.
Magnetic-core memory Magnetic-core memory was the predominant form of random-access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of ...
allowed for recall of memory after power loss. It was developed by Frederick W. Viehe and
An Wang An Wang (; February 7, 1920 – March 24, 1990) was a Chinese–American computer engineer and inventor, and co-founder of computer company Wang Laboratories, which was known primarily for its dedicated word processing machines. An Wang was a ...
in the late 1940s, and improved by
Jay Forrester Jay Wright Forrester (July 14, 1918 – November 16, 2016) was a pioneering American computer engineer Computer engineering (CoE or CpE) is a branch of engineering Engineering is the use of scientific method, scientific principles to ...
and Jan A. Rajchman in the early 1950s, before being commercialised with the
Whirlwind A whirlwind is a weather phenomenon in which a vortex , revealed by colored smoke In fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. ...
computer in 1953. Magnetic-core memory was the dominant form of memory until the development of
MOS
MOS
semiconductor memory Semiconductor memory is a used for , such as . It typically refers to MOS memory, where data is stored within (MOS) on a memory chip. There are numerous different types using different semiconductor technologies. The two main types of (RA ...
in the 1960s. The first
semiconductor memory Semiconductor memory is a used for , such as . It typically refers to MOS memory, where data is stored within (MOS) on a memory chip. There are numerous different types using different semiconductor technologies. The two main types of (RA ...
was implemented as a flip-flop circuit in the early 1960s using
bipolar transistors A bipolar junction transistor (BJT) is a type of transistor file:MOSFET Structure.png, upright=1.4, Metal-oxide-semiconductor field-effect transistor (MOSFET), showing Metal gate, gate (G), body (B), source (S) and drain (D) terminals. The ga ...
. Semiconductor memory made from
discrete device An electronic component is any basic discrete device or physical entity in an electronic system Electronic may refer to: *Electronics, the science of how to control electric energy in semiconductor *Electronics (magazine), ''Electronics'' (maga ...
s was first shipped by
Texas Instruments Texas Instruments Incorporated (TI) is an America The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America North America is a ...
to the
United States Air Force The United States Air Force (USAF) is the air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosph ...

United States Air Force
in 1961. The same year, the concept of solid-state memory on an
integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuit 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of indivi ...

integrated circuit
(IC) chip was proposed by applications engineer Bob Norman at
Fairchild Semiconductor Fairchild Semiconductor International, Inc. was an American semiconductor A semiconductor material has an electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental pr ...
. The first bipolar semiconductor memory IC chip was the SP95 introduced by
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
in 1965. While semiconductor memory offered improved performance over magnetic-core memory, it remain larger and more expensive and did not displace magnetic-core memory until the late 1960s.


MOS memory

The invention of the metal–oxide–semiconductor field-effect transistor (
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), is a type of insulated-gate field-effect transistor that is fabricated by th ...

MOSFET
) enabled the practical use of
metal–oxide–semiconductor The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), is a type of insulated-gate field-effect transistor The field-effect tran ...
(MOS) transistors as memory cell storage elements. MOS memory was developed by John Schmidt at
Fairchild Semiconductor Fairchild Semiconductor International, Inc. was an American semiconductor A semiconductor material has an electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental pr ...
in 1964. In addition to higher performance, MOS
semiconductor memory Semiconductor memory is a used for , such as . It typically refers to MOS memory, where data is stored within (MOS) on a memory chip. There are numerous different types using different semiconductor technologies. The two main types of (RA ...
was cheaper and consumed less power than magnetic core memory. In 1965, J. Wood and R. Ball of the
Royal Radar Establishment The Royal Radar Establishment was a research centre in Malvern, Worcestershire in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardi ...
proposed digital storage systems that use
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "see-moss"), also known as complementary-symmetry metal–oxide–semiconductor (COS-MOS), is a type of metal–oxide–semiconductor field-effect transistor The metal–oxide–sem ...
(complementary MOS) memory cells, in addition to MOSFET power devices for the
power supply #REDIRECT Power supply#REDIRECT Power supply A power supply is an electrical device that supplies electric power to an electrical load. The primary function of a power supply is to convert electric current from a source to the correct voltage, el ...

power supply
, switched cross-coupling,
switches In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an ident ...

switches
and delay-line storage. The development of silicon-gate
MOS integrated circuit file:MOSFET Structure.png, upright=1.6, MOSFET, showing metal gate, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an gate oxide, insulating layer (pink). The metal–oxide–semiconductor field-eff ...
(MOS IC) technology by
Federico Faggin Federico Faggin (, ; born 1 December 1941) is an Italian-American physicist, engineer, inventor and entrepreneur. He is best known for designing the first commercial microprocessor, the Intel 4004. He led the Intel 4004, 4004 (MCS-4) project and ...

Federico Faggin
at Fairchild in 1968 enabled the production of MOS
memory chip Semiconductor memory is a digital electronics, digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to MOS memory, where data is stored within metal–oxide–semiconductor (MOS) memo ...
s. NMOS memory was commercialized by
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
in the early 1970s. MOS memory overtook magnetic core memory as the dominant memory technology in the early 1970s. The two main types of volatile
random-access memory Random-access memory (RAM; ) is a form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic proces ...
(RAM) are
static random-access memory clone (2K × 8 bits) Static random-access memory (static RAM or SRAM) is a type of random-access memory File:Random Access Memory HyperX.jpg, 8GB DDR3 RAM stick with a white Heat sink, heatsink Random-access memory (RAM; ) is a f ...
(SRAM) and
dynamic random-access memory Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access memory, random-access semiconductor memory that stores each bit of data in a memory cell (computing), memory cell, usually consisting of a tiny capacitor and a tr ...
(DRAM). Bipolar SRAM was invented by Robert Norman at Fairchild Semiconductor in 1963, followed by the development of MOS SRAM by John Schmidt at Fairchild in 1964. SRAM became an alternative to magnetic-core memory, but requires six transistors for each
bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algo ...
of data. Commercial use of SRAM began in 1965, when IBM introduced their SP95 SRAM chip for the System/360 Model 95.
Toshiba is a Japanese headquartered in , . Its diversified products and services include power, industrial and social infrastructure systems, elevators and escalators, electronic components, s, s (HDD), printers, batteries, lighting, as well as sol ...
introduced bipolar DRAM memory cells for its Toscal BC-1411
electronic calculator An electronic calculator is typically a portable Electronics, electronic device used to perform calculations, ranging from basic arithmetic to complex mathematics. The first Solid-state electronics, solid-state electronic calculator was created ...

electronic calculator
in 1965. While it offered improved performance, bipolar DRAM could not compete with the lower price of the then dominant magnetic-core memory. MOS technology is the basis for modern DRAM. In 1966,
Robert H. Dennard Robert Dennard (born September 5, 1932) is an American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of America (USA), commonly k ...
at the IBM Thomas J. Watson Research Center was working on MOS memory. While examining the characteristics of MOS technology, he found it was possible to build
capacitors A capacitor is a device that stores electric charge in an electric field An electric field (sometimes E-field) is the physical field that surrounds each electric charge Electric charge is the physical property of matter that causes it t ...
, and that storing a charge or no charge on the MOS capacitor could represent the 1 and 0 of a bit, while the MOS transistor could control writing the charge to the capacitor. This led to his development of a single-transistor DRAM memory cell. In 1967, Dennard filed a patent for a single-transistor DRAM memory cell based on MOS technology. This led to the first commercial DRAM IC chip, the
Intel 1103 The 1103 is a dynamic random-access memory (DRAM) integrated circuit (IC) developed and Semiconductor fabrication, fabricated by Intel. Introduced in October 1970, the 1103 was the first commercially available DRAM IC; and due to its small physica ...
in October 1970.''The DRAM memory of Robert Dennard''
history-computer.com
Synchronous dynamic random-access memory Synchronous dynamic random-access memory (synchronous dynamic RAM or SDRAM) is any Dynamic RAM, DRAM where the operation of its external pin interface is coordinated by an externally supplied clock signal. DRAM integrated circuits (ICs) produce ...
(SDRAM) later debuted with the
Samsung The Samsung Group (or simply Samsung) ( ko, 삼성) is a South Korean Multinational corporation, multinational manufacturing Conglomerate (company), conglomerate headquartered in Samsung Town, Seoul, South Korea. It comprises numerous affil ...
KM48SL2000 chip in 1992. The term ''memory'' is also often used to refer to
non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data. Examples of non-volatile ...
, specifically
flash memory Flash memory is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses acti ...
. It has origins in
read-only memory Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
(ROM).
Programmable read-only memory A programmable read-only memory (PROM) is a form of digital memory where the contents can be changed once after manufacture of the device. The data is then permanent and cannot be changed. It is one type of read-only memory Read-only memory (RO ...
(PROM) was invented by
Wen Tsing Chow Wen Tsing Chow (; 1918–2001), was a Chinese-born American missile guidance 300px, A guided bomb strikes a practice target Missile guidance refers to a variety of methods of guiding a missile In military terminology, a missile, also known a ...
in 1956, while working for the Arma Division of the American Bosch Arma Corporation. In 1967, Dawon Kahng and Simon Sze of Bell Labs proposed that the
floating gateThe floating-gate MOSFET (FGMOS), also known as a floating-gate MOS transistor or floating-gate transistor, is a type of metal–oxide–semiconductor field-effect transistor The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS- ...
of a MOS
semiconductor device A device is an that relies on the properties of a material (primarily , , and , as well as s) for its function. Semiconductor devices have replaced s in most applications. They in the , rather than as free electrons across a (typically l ...
could be used for the cell of a reprogrammable
read-only memory Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
(ROM), which led to
Dov Frohman Dov Frohman (Hebrew Hebrew (, , or ) is a Northwest Semitic languages, Northwest Semitic language of the Afroasiatic languages, Afroasiatic language family. Historically, it is regarded as the language of the Israelites, Judeans and their ...

Dov Frohman
of
Intel Intel Corporation is an American multinational corporation A multinational company (MNC) is a corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., is a Legal personalit ...

Intel
inventing
EPROM An EPROM (rarely EROM), or erasable programmable read-only memory, is a type of programmable read-only memory A programmable read-only memory (PROM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. (eF ...

EPROM
(erasable PROM) in 1971.
EEPROM EEPROM (also E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information ...
(electrically erasable PROM) was developed by Yasuo Tarui, Yutaka Hayashi and Kiyoko Naga at the
Electrotechnical Laboratory The , or AIST, is a Japanese research facility headquartered in Tokyo Tokyo ( , ; Japanese language, Japanese: 東京, ''Tōkyō'' ), officially the Tokyo Metropolis (Japanese language, Japanese: 東京都, ''Tōkyō-to''), is the capital ...
in 1972. Flash memory was invented by
Fujio Masuoka is a Japanese engineer, who has worked for Toshiba is a Japanese headquartered in , . Its diversified products and services include power, industrial and social infrastructure systems, elevators and escalators, electronic components, s, s ...
at
Toshiba is a Japanese headquartered in , . Its diversified products and services include power, industrial and social infrastructure systems, elevators and escalators, electronic components, s, s (HDD), printers, batteries, lighting, as well as sol ...
in the early 1980s. Masuoka and colleagues presented the invention of
NOR flash Flash memory is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses act ...
in 1984, and then
NAND flash Flash memory is an Integrated circuit, electronic Non-volatile memory, non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for ...
in 1987. Toshiba commercialized NAND flash memory in 1987. Developments in technology and economies of scale have made possible so-called Very Large Memory (VLM) computers.


Volatile memory

Volatile memory is computer memory that requires power to maintain the stored information. Most modern
semiconductor A semiconductor material has an value falling between that of a , such as metallic copper, and an , such as glass. Its falls as its temperature rises; metals behave in the opposite way. Its conducting properties may be altered in useful ways ...
volatile memory is either static RAM ( SRAM) or dynamic RAM (
DRAM Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum fr ...

DRAM
). SRAM retains its contents as long as the power is connected and is simpler for interfacing, but 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. SRAM is not worthwhile for desktop system memory, where DRAM dominates, but is used for their cache memories. SRAM is commonplace in small embedded systems, which might only need tens of kilobytes or less. Volatile memory technologies that have attempted to compete or replace SRAM and DRAM include Z-RAM and A-RAM.


Non-volatile memory

Non-volatile memory is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory (see
ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality, a medical classification to estimate the likelihood of death for a patient * Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac * ...
),
flash memory Flash memory is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses acti ...
, most types of magnetic computer storage devices (e.g.
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data stora ...

hard disk drive
s,
floppy disk A floppy disk or floppy diskette (sometimes casually referred to as a floppy or diskette) is a type of disk storage Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by vario ...

floppy disk
s and
magnetic tape Magnetic tape is a medium for , made of a thin, magnetizable coating on a long, narrow strip of . It was developed in Germany in 1928, based on . Devices that record and playback audio and video using magnetic tape are s and s respectively. A ...

magnetic tape
),
optical disc In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest ...
s, and early computer storage methods such as
paper tape Five- and eight-hole punched paper tape file:Harwell-dekatron-witch-10.jpg, Paper tape reader on the Harwell computer with a small piece of five-hole tape connected in a circle – creating a physical program loop Punched tape or perforated pape ...

paper tape
and
punched card A punched card (also punch cardSteven Pinker, in ''The Stuff of Thought'', Viking, 2007, p.362, notes the loss of ''-ed'' in pronunciation ''as it did in ice cream, mincemeat, and box set, formerly iced cream, minced meat, and boxed set.'' or p ...

punched card
s. Forthcoming non-volatile memory technologies include FERAM,
CBRAM The programmable metallization cell, or PMC, is a non-volatile Non-volatile memory (NVM) or non-volatile storage is a type of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating ...
, PRAM, STT-RAM, SONOS, RRAM, racetrack memory, Nano-RAM, NRAM, 3D XPoint, and millipede memory.


Semi-volatile memory

A third category of memory is "semi-volatile". The term is used to describe a memory which 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 high performance/durability/etc. associated with volatile memories, while providing some benefits of a true non-volatile memory. For example, some non-volatile memory types can wear out, where 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. If the retention time "expires" without an update, then 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. As a second example, an Spin-transfer_torque#Spin-transfer_torque_memory, STT-RAM can be made non-volatile by building large cells, but the cost per bit and write power go up, while the write speed goes down. 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. The term semi-volatile is also used to describe semi-volatile behavior constructed from other memory types. For example, a volatile and a non-volatile memory may be combined, where an external signal copies data from the volatile memory to the non-volatile memory, but if power is removed without copying, the data is lost. Or, a battery-backed volatile memory, and if external power is lost there is some known period where the battery can continue to power the volatile memory, but if power is off for an extended time, the battery runs down and data is lost.


Management

Proper management of memory is vital for a computer system to operate properly. Modern operating systems have complex systems to properly manage memory. Failure to do so can lead to bugs, slow performance, and at worst case, takeover by virus (computing), viruses and malicious software.


Bugs

Improper management of memory is a common cause of bugs, including the following types: * In an arithmetic overflow, a calculation results in a number larger than the allocated memory permits. For example, a signed 8-bit integer allows the numbers −128 to +127. If its value is 127 and it is instructed to add one, the computer can not store the number 128 in that space. Such a case will result in an undesired operation, such as changing the number's value to −128 instead of +128. * A memory leak occurs when a program requests memory from the operating system and never returns the memory when it's done with it. A program with this bug will gradually require more and more memory until the program fails as it runs out. * A segmentation fault results when a program tries to access memory that it does not have permission to access. Generally, a program doing so will be terminated by the operating system. * A buffer overflow means that a program writes data to the end of its allocated space and then continues to write data to memory that has been allocated for other purposes. This may result in erratic program behavior, including memory access errors, incorrect results, a crash, or a breach of system security. They are thus the basis of many software vulnerabilities and can be maliciously exploited.


Early computer systems

In early computer systems, programs typically specified the location to write memory and what data to put there. This location was a physical location on the actual memory hardware. The slow processing of such computers did not allow for the complex memory management systems used today. Also, as most such systems were single-task, sophisticated systems were not required as much. This approach has its pitfalls. If the location specified is incorrect, this will cause the computer to write the data to some other part of the program. The results of an error like this are unpredictable. In some cases, the incorrect data might overwrite memory used by the operating system. Computer crackers can take advantage of this to create Malware, viruses and malware.


Virtual memory

Virtual memory is a system where all physical memory is controlled by the operating system. When a program needs memory, it requests it from the operating system. The operating system then decides in what physical location to place the program's code and data. This offers several advantages. Computer programmers no longer need to worry about where their data is physically stored or whether the user's computer will have enough memory. It also allows multiple types of memory to be used. For example, some data can be stored in physical RAM chips while other data is stored on a hard drive (e.g. in a swapfile), functioning as an extension of the cache hierarchy. This drastically increases the amount of memory available to programs. The operating system will place actively used data in physical RAM, which is much faster than hard disks. When the amount of RAM is not sufficient to run all the current programs, it can result in a situation where the computer spends more time moving data from RAM to disk and back than it does accomplishing tasks; this is known as Thrashing (computer science), thrashing.


Protected memory

Protected memory is a system where each program is given an area of memory to use and is not permitted to go outside that range. Use of protected memory greatly enhances both the reliability and security of a computer system. Without protected memory, it is possible that a bug in one program will alter the memory used by another program. This will cause that other program to run off of corrupted memory with unpredictable results. If the operating system's memory is corrupted, the entire computer system may crash and need to be Reboot (computing), rebooted. At times programs intentionally alter the memory used by other programs. This is done by viruses and malware to take over computers. It may also be used benignly by desirable programs which are intended to modify other programs; in the modern age, this is generally considered bad programming practice for application programs, but it may be used by system development tools such as debuggers, for example to insert breakpoints or hooks. Protected memory assigns programs their own areas of memory. If the operating system detects that a program has tried to alter memory that does not belong to it, the program is terminated (or otherwise restricted or redirected). This way, only the offending program crashes, and other programs are not affected by the misbehavior (whether accidental or intentional). Protected memory systems almost always include virtual memory as well.


See also

* Memory geometry * Memory hierarchy * Memory organization * Processor registers store data but normally are not considered as memory, since they only store one word and do not include an addressing mechanism. * Semiconductor memory * Units of information


Notes


References


Further reading

* * {{Benchmark Computer memory, MOSFETs Digital electronics