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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 processes and development of both computer hardware , hardware and soft ...
that can be read and changed in any order, typically used to store working
data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to experience. Standard reference works are often used ...
and
machine code In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, ge ...
. A
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 addressable elements roughly as easily and efficiently as any other ...
memory device allows
data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to experience. Standard reference works are often used ...

data
items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory, in contrast with other direct-access data storage media (such as
hard disk 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
s,
CD-RW CD-RW (Compact Disc-Rewritable) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electron ...
s,
DVD-RW DVD recordable and DVD rewritable are optical disc recording technologies Optical disc authoring requires a number of different optical disc recorder technologies working in tandem, from the optical disc drive. (CD-R), showing character ...
s and the older magnetic tapes and
drum memory Drum memory was a magnetic 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 storage medium. Data Data (; ) are individual facts, statistics, ...
), where the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement. RAM contains
multiplexing In telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, Optical system, optical, or other Electromagnetism, electromagnetic systems. It has its origin in the desire of ...

multiplexing
and
demultiplexing In telecommunications Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines both its essence and the natu ...
circuitry, to connect the data lines to the addressed storage for reading or writing the entry. Usually more than one bit of storage is accessed by the same address, and RAM devices often have multiple data lines and are said to be "8-bit" or "16-bit", etc. devices. In today's technology, random-access memory takes the form of
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 indiv ...

integrated circuit
(IC) chips with
MOS
MOS
(metal-oxide-semiconductor) memory cells. RAM is normally associated with volatile types of memory (such as
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)
modules Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a syst ...
), where stored information is lost if power is removed, although non-volatile RAM has also been developed. Other types of non-volatile memories exist that allow random access for read operations, but either do not allow write operations or have other kinds of limitations on them. These include most types of
ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality The risk of mortality (ROM) provides a medical classification to estimate the likelihood of inhospital death for a patient. The ROM classes are minor, moderate, major, and ex ...
and a type of
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 act ...
called '' NOR-Flash''. The two main types of volatile random-access
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 ...
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). Commercial uses of semiconductor RAM date back to 1965, when IBM introduced the SP95 SRAM chip for their System/360 Model 95 computer, and
Toshiba is a Japanese multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational state, a sovereign st ...
used 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
, both based on
bipolar transistor A bipolar junction transistor (BJT) is a type of transistor upright=1.4, gate A gate or gateway is a point of entry to or from a space enclosed by walls. The word derived from old Norse "gat" meaning road A road is a thoroughfare, r ...

bipolar transistor
s. Commercial MOS memory, based on
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, was developed in the late 1960s, and has since been the basis for all commercial semiconductor memory. 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 ...
, was introduced in October 1970.
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.


History

Early computers used
relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electric Electricity i ...

relay
s,
mechanical counter Mechanical counters are digital counters built using mechanical components. Long before electronics became common, mechanical devices were used to count events. They typically consist of a series of disks mounted on an axle, with the digits zero ...
s or delay lines for main memory functions. Ultrasonic delay lines were serial devices which could only reproduce data in the order it was written.
Drum memory Drum memory was a magnetic 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 storage medium. Data Data (; ) are individual facts, statistics, ...
could be expanded at relatively low cost but efficient retrieval of memory items required knowledge of the physical layout of the drum to optimize speed. Latches built out of
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
triode A triode is an electronic (or ''valve'' in British English) consisting of three s inside an evacuated glass envelope: a heated or , a , and a (). Developed from 's 1906 , a partial vacuum tube that added a grid electrode to the (), the trio ...

triode
s, and later, out of discrete
transistor upright=1.4, gate Candi bentar, a typical Indonesian gate that is often found on the islands of Java">Indonesia.html" ;"title="Candi bentar, a typical Indonesia">Candi bentar, a typical Indonesian gate that is often found on the islands o ...

transistor
s, were used for smaller and faster memories such as registers. Such registers were relatively large and too costly to use for large amounts of data; generally only a few dozen or few hundred bits of such memory could be provided. The first practical form of random-access memory was 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 ...
starting in 1947. It stored data as electrically charged spots on the face of a
cathode-ray tube File:CRT monochrome.png, 250px, Cutaway rendering of a monochrome CRT: 1. Deflection coils2. Electron beam3. Focusing coil4. Phosphor layer on the inner side of the screen; emits light when struck by the electron beam5.&nbs ...
. Since the electron beam of the CRT could read and write the spots on the tube in any order, memory was random access. The capacity of the Williams tube was a few hundred to around a thousand bits, but it was much smaller, faster, and more power-efficient than using individual vacuum tube latches. Developed at the
University of Manchester , mottoeng = Knowledge, Wisdom, Humanity , established = 2004 – University of Manchester Predecessor institutions: 1956 – UMIST , mottoeng = By Knowledge and Work , established = 1824 , closed = 2004 (merge ...
in England, the Williams tube provided the medium on which the first electronically stored program was implemented in the
Manchester Baby The Manchester Baby, also called the Small-Scale Experimental Machine (SSEM), was the first electronic stored-program computer A stored-program computer is a computer A computer is a machine that can be programmed to carry out sequences ...
computer, which first successfully ran a program on 21 June 1948. In fact, rather than the Williams tube memory being designed for the Baby, the Baby was a
testbed A testbed (also spelled test bed) is a platform for conducting rigorous, transparent, and replicable testing of scientific theories, computational tools, and new technologies. The term is used across many disciplines to describe experimental resear ...
to demonstrate the reliability of the memory.
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 ...
was invented in 1947 and developed up until the mid-1970s. It became a widespread form of random-access memory, relying on an array of magnetized rings. By changing the sense of each ring's magnetization, data could be stored with one bit stored per ring. Since every ring had a combination of address wires to select and read or write it, access to any memory location in any sequence was possible. Magnetic core memory was the standard 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 development of both computer hardware , hardware and soft ...
system until displaced by solid-state
MOS
MOS
(
metal–oxide–silicon 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 ...
)
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
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 indiv ...

integrated circuit
s (ICs) during the early 1970s. Prior to the development of integrated
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) circuits, ''permanent'' (or ''read-only'') random-access memory was often constructed using diode matrices driven by
address decoder The four states of a 2-to-4 Decoder In digital electronics Digital electronics is a field of electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in ...
s, or specially wound
core rope memory Core rope memory is a form of read-only memory Read-only memory (ROM) 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 even after power i ...
planes.
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 ...
began in the 1960s with bipolar memory, which used
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 ...
. While it improved performance, it could not compete with the lower price of magnetic core memory.


MOS RAM

The invention of the
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
(metal-oxide-semiconductor field-effect transistor), also known as the MOS transistor, by
Mohamed M. Atalla Mohamed M. Atalla ( ar, محمد عطاالله; August 4, 1924 – December 30, 2009) was an Egyptian-American engineer, physical chemist Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which ...

Mohamed M. Atalla
and
Dawon Kahng Dawon Kahng ( ko, 강대원; May 4, 1931 – May 13, 1992) was a Korean-American electrical engineer and inventor, known for his work in solid-state electronics Solid-state electronics means semiconductor A semiconductor material has an el ...

Dawon Kahng
at
Bell Labs Nokia Bell Labs (formerly named Bell Labs Innovations (1996–2007), AT&T Bell Laboratories (1984–1996) and Bell Telephone Laboratories (1925–1984)) is an American industrial research and scientific development company A company, ab ...
in 1959, led to the development of metal-oxide-semiconductor (MOS) memory 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. 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 electronic 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 application ...
s. MOS memory overtook magnetic core memory as the dominant memory technology in the early 1970s. An integrated bipolar
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) was invented by Robert H. 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 ...
in 1963. It was followed by the development of MOS SRAM by John Schmidt at Fairchild in 1964. SRAM became an alternative to magnetic-core memory, but required six MOS 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 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
introduced the SP95 memory chip for the System/360 Model 95.
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) allowed replacement of a 4 or 6-transistor latch circuit by a single transistor for each memory bit, greatly increasing memory density at the cost of volatility. Data was stored in the tiny capacitance of each transistor, and had to be periodically refreshed every few milliseconds before the charge could leak away.
Toshiba is a Japanese multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational state, a sovereign st ...
's 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
, which was introduced in 1965, used a form of capacitive bipolar DRAM, storing 180-bit data on discrete memory cells, consisting of
germanium Germanium is a chemical element with the Symbol (chemistry), symbol Ge and atomic number 32. It is a lustrous, hard-brittle, grayish-white metalloid in the carbon group, chemically similar to its group neighbors silicon and tin. Pure germanium i ...

germanium
bipolar transistors and capacitors. While it offered improved performance over magnetic-core memory, 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, Dr.
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 capable of building
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 under IBM for a single-transistor DRAM memory cell, based on MOS technology. The first commercial DRAM IC chip was 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 ...
, which was
manufactured Manufacturing is the creation or production Production may be: Economics and business * Production (economics) * Production, the act of manufacturing goods * Production, in the outline of industrial organization, the act of making products (g ...
on an 8µm MOS process with a capacity of 1 kbit, and was released in 1970.
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) was developed by
Samsung Electronics Samsung Electronics Co., Ltd. (; lit. "tristar electronics", sometimes shortened to SEC and stylized as SΛMSUNG) is a South Korean multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in ...
. The first commercial SDRAM chip was the Samsung KM48SL2000, which had a capacity of 16
Mbit The megabit is a multiple of the unit 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 a ...
. It was introduced by
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 ...

Samsung
in 1992, and mass-produced in 1993. The first commercial
DDR SDRAM Double Data Rate Synchronous Dynamic Random-Access Memory (DDR SDRAM) is a double data rate In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experiment ...
(
double data rate 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 common ...
SDRAM) memory chip was Samsung's 64Mbit DDR SDRAM chip, released in June 1998.
GDDR Graphics DDR SDRAM (GDDR SDRAM) is a type of 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 c ...
(graphics DDR) is a form of DDR
SGRAM Synchronous dynamic random-access memory (synchronous dynamic RAM or SDRAM) is any DRAM photograph of the Micron Technology MT4C1024 DRAM integrated circuit (1994). It has a capacity of 1 megabit equivalent of 2^bits or Dynamic rand ...
(synchronous graphics RAM), which was first released by Samsung as a 16Mbit memory chip in 1998.


Types

The two widely used forms of modern RAM are
static RAM Static random-access memory (static RAM or SRAM) is a type of 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 c ...
(SRAM) and
dynamic RAM photograph of the Micron Technology Micron Technology, Inc. is an American producer of computer memory and computer data storage including dynamic random-access memory, flash memory Flash memory is an electronic non-volatile ...
(DRAM). In SRAM, a bit of data is stored using the state of a six-
transistor upright=1.4, gate Candi bentar, a typical Indonesian gate that is often found on the islands of Java">Indonesia.html" ;"title="Candi bentar, a typical Indonesia">Candi bentar, a typical Indonesian gate that is often found on the islands o ...

transistor
memory cell, typically using six
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
s (metal-oxide-semiconductor field-effect transistors). This form of RAM is more expensive to produce, but is generally faster and requires less dynamic power than DRAM. In modern computers, SRAM is often used as cache memory for the CPU. DRAM stores a bit of data using a transistor and
capacitor A capacitor is a device that stores electric charge in an electric field. It is a passivity (engineering), passive electronic component with two terminal (electronics), terminals. The effect of a capacitor is known as capacitance. While som ...

capacitor
pair (typically a MOSFET and
MOS capacitor 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 ...
, respectively), which together comprise a DRAM cell. The capacitor holds a high or low charge (1 or 0, respectively), and the transistor acts as a switch that lets the control circuitry on the chip read the capacitor's state of charge or change it. As this form of memory is less expensive to produce than static RAM, it is the predominant form of computer memory used in modern computers. Both static and dynamic RAM are considered ''volatile'', as their state is lost or reset when power is removed from the system. By contrast,
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) stores data by permanently enabling or disabling selected transistors, such that the memory cannot be altered. Writeable variants of ROM (such as
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 ...
and
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 act ...
) share properties of both ROM and RAM, enabling data to persist without power and to be updated without requiring special equipment. These persistent forms of semiconductor ROM include
USB Universal Serial Bus (USB) is an industry standard ''Industry Standard'' is a 1982 album by Dixie Dregs, The Dregs. It is their only album featuring vocals (by Alex Ligertwood of Santana (band), Santana and Patrick Simmons of The Doobie Brot ...
flash drives, memory cards for cameras and portable devices, and
solid-state drives A solid-state drive (SSD) is a solid-state storageSolid-state storage (sometimes abbreviated as SSS) is a type of non-volatile computer storage that stores and retrieves digital information using only electronic circuit File:PExdcr01CJC.jpg ...
.
ECC memory Error correction code memory (ECC memory) is a type of computer data storage File:Maxell DVD-RW 4.7GB crop 20051120.jpg, A spindle of DVD-RWs Computer data storage is a technology consisting of computer components and Data storage device, ...
(which can be either SRAM or DRAM) includes special circuitry to detect and/or correct random faults (memory errors) in the stored data, using
parity bit A parity bit, or check bit, is a 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 al ...
s or
error correction codes In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for error control, controlling errors in data over unreliable or noisy communication channels. The ce ...
. In general, the term ''RAM'' refers solely to solid-state memory devices (either DRAM or SRAM), and more specifically the main memory in most computers. In optical storage, the term
DVD-RAM DVD-RAM (DVD Random Access Memory) is a DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data storage format invented and developed in 1995 and released in late 1996. The medium ...
is somewhat of a misnomer since, unlike
CD-RW CD-RW (Compact Disc-Rewritable) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electron ...
or
DVD-RW DVD recordable and DVD rewritable are optical disc recording technologies Optical disc authoring requires a number of different optical disc recorder technologies working in tandem, from the optical disc drive. (CD-R), showing character ...
it does not need to be erased before reuse. Nevertheless, a DVD-RAM behaves much like a hard disc drive if somewhat slower.


Memory cell

The memory cell is the fundamental building block 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 development of both computer hardware , hardware and soft ...
. The memory cell is an
electronic circuit An electronic circuit is composed of individual electronic component 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 electr ...

electronic circuit
that stores 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 ...
of binary information and it must be set to store a logic 1 (high voltage level) and reset to store a logic 0 (low voltage level). Its value is maintained/stored until it is changed by the set/reset process. The value in the memory cell can be accessed by reading it. In SRAM, the memory cell is a type of flip-flop circuit, usually implemented using
FET The field-effect transistor (FET) is a type of transistor upright=1.4, gate Candi bentar, a typical Indonesian gate that is often found on the islands of Java">Indonesia.html" ;"title="Candi bentar, a typical Indonesia">Candi bentar, ...
s. This means that SRAM requires very low power when not being accessed, but it is expensive and has low storage density. A second type, DRAM, is based around a capacitor. Charging and discharging this capacitor can store a "1" or a "0" in the cell. However, the charge in this capacitor slowly leaks away, and must be refreshed periodically. Because of this refresh process, DRAM uses more power, but it can achieve greater storage densities and lower unit costs compared to SRAM.


Addressing

To be useful, memory cells must be readable and writeable. Within the RAM device, multiplexing and demultiplexing circuitry is used to select memory cells. Typically, a RAM device has a set of address lines A0... An, and for each combination of bits that may be applied to these lines, a set of memory cells are activated. Due to this addressing, RAM devices virtually always have a memory capacity that is a power of two. Usually several memory cells share the same address. For example, a 4 bit 'wide' RAM chip has 4 memory cells for each address. Often the width of the memory and that of the microprocessor are different, for a 32 bit microprocessor, eight 4 bit RAM chips would be needed. Often more addresses are needed than can be provided by a device. In that case, external multiplexors to the device are used to activate the correct device that is being accessed.


Memory hierarchy

One can read and over-write data in RAM. Many computer systems have a memory hierarchy consisting of
processor register Processor may refer to: Computing Hardware * Processor (computing) In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic p ...
s, on-die SRAM caches, external caches,
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
,
paging In 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 prog ...

paging
systems and
virtual memory In computing, virtual memory, or virtual storage is a Memory management (operating systems), memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "cre ...

virtual memory
or
swap space In 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 prog ...
on a hard drive. This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different
access time Access time is the time delay or latency Latency or latent may refer to: Science and technology * Latent heat, energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process * Latent variable, a variable ...
s, violating the original concept behind the ''random access'' term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank,
rank Rank is the relative position, value, worth, complexity, power, importance, authority, level, etc. of a person or object within a ranking A ranking is a relationship between a set of items such that, for any two items, the first is either "rank ...
, channel, or interleave organization of the components make the access time variable, although not to the extent that access time to rotating
storage media On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data storage medium. Data Data are units of information Information can be thought of as the resolution of uncertaint ...
or a tape is variable. The overall goal of using a memory hierarchy is to obtain the highest possible average access performance while minimizing the total cost of the entire memory system (generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom). In many modern personal computers, the RAM comes in an easily upgraded form of modules called
memory module In computing, a memory module or RAM (random-access memory) stick is a printed circuit board on which Computer memory, memory integrated circuits are mounted. Memory modules permit easy installation and replacement in electronic systems, especially ...
s or DRAM modules about the size of a few sticks of chewing gum. These can quickly be replaced should they become damaged or when changing needs demand more storage capacity. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of ...

CPU
and other
ICs ICS may refer to: Computing * Image Cytometry Standard, a digital multidimensional image file format used in life sciences microscopy * Industrial control system, computer systems and networks used to control industrial plants and infrastructure ...

ICs
on the
motherboard A motherboard (also called mainboard, main circuit board, or mobo) is the main printed circuit board A printed circuit board (PCB) is a laminated sandwich structure of conductive and insulating layers. PCBs have two complementary functions. ...

motherboard
, as well as in hard-drives,
CD-ROM A CD-ROM (, compact disc read-only memory) is a pre-pressed optical compact disc The compact disc (CD) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital ...

CD-ROM
s, and several other parts of the computer system.


Other uses of RAM

In addition to serving as temporary storage and working space for the operating system and applications, RAM is used in numerous other ways.


Virtual memory

Most modern operating systems employ a method of extending RAM capacity, known as "virtual memory". A portion of the computer's
hard 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 ...
is set aside for a ''paging file'' or a ''scratch partition'', and the combination of physical RAM and the paging file form the system's total memory. (For example, if a computer has 2 GB (10243 B) of RAM and a 1 GB page file, the operating system has 3 GB total memory available to it.) When the system runs low on physical memory, it can "" portions of RAM to the paging file to make room for new data, as well as to read previously swapped information back into RAM. Excessive use of this mechanism results in thrashing and generally hampers overall system performance, mainly because hard drives are far slower than RAM.


RAM disk

Software can "partition" a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a
RAM disk A RAM drive (also called a RAM disk) is a block of random-access memory (primary storage or volatile memory) that a computer's software is treating as if the memory were a disk drive (secondary storage). It is sometimes referred to as a virtual RA ...
. A RAM disk loses the stored data when the computer is shut down, unless memory is arranged to have a standby battery source, or changes to the RAM disk are written out to a nonvolatile disk. The RAM disk is reloaded from the physical disk upon RAM disk initialization.


Shadow RAM

Sometimes, the contents of a relatively slow ROM chip are copied to read/write memory to allow for shorter access times. The ROM chip is then disabled while the initialized memory locations are switched in on the same block of addresses (often write-protected). This process, sometimes called ''shadowing'', is fairly common in both computers and
embedded systems An embedded system is a computer system A computer is a machine that can be programmed to carry out Sequence, sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Com ...
. As a common example, the
BIOS In computing, BIOS (, ); an acronym for Basic Input/Output System and also known as the System BIOS, ROM BIOS or PC BIOS) is firmware used to perform Computer hardware, hardware initialization during the booting process (power-on startup), and t ...

BIOS
in typical personal computers often has an option called “use shadow BIOS” or similar. When enabled, functions that rely on data from the BIOS's ROM instead use DRAM locations (most can also toggle shadowing of video card ROM or other ROM sections). Depending on the system, this may not result in increased performance, and may cause incompatibilities. For example, some hardware may be inaccessible to the
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
if shadow RAM is used. On some systems the benefit may be hypothetical because the BIOS is not used after booting in favor of direct hardware access. Free memory is reduced by the size of the shadowed ROMs.


Recent developments

Several new types of ''non-volatile'' RAM, which preserve data while powered down, are under development. The technologies used include
carbon nanotube image of a single-walled carbon nanotube. Carbon nanotubes (CNTs) are tubes made of carbon with diameters typically measured in nanometers The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI ...

carbon nanotube
s and approaches utilizing
Tunnel magnetoresistance Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ), which is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough (typically a fe ...
. Amongst the 1st generation
MRAM Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory Non-volatile random-access memory (NVRAM) is random-access memory that retains data without applied power. This is in contrast to dynamic random-acces ...
, a 128 kilobit, kbit ( bytes) chip was manufactured with 0.18 µm technology in the summer of 2003. In June 2004, Infineon Technologies unveiled a 16 Megabyte, MB (16 × 220 bytes) prototype again based on 0.18 µm technology. There are two 2nd generation techniques currently in development: thermal-assisted switching (TAS) which is being developed by Crocus Technology, and spin-transfer torque (STT) on which Crocus Technology, Crocus, Hynix,
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
, and several other companies are working. Nantero built a functioning carbon nanotube memory prototype 10 Gigabyte, GB (10 × 230 bytes) array in 2004. Whether some of these technologies can eventually take significant market share from either DRAM, SRAM, or flash-memory technology, however, remains to be seen. Since 2006, "solid-state drives" (based on flash memory) with capacities exceeding 256 gigabytes and performance far exceeding traditional disks have become available. This development has started to blur the definition between traditional random-access memory and "disks", dramatically reducing the difference in performance. Some kinds of random-access memory, such as "EcoRAM", are specifically designed for server farms, where low-power electronics, low power consumption is more important than speed.


Memory wall

The "memory wall" is the growing disparity of speed between CPU and memory outside the CPU chip. An important reason for this disparity is the limited communication bandwidth beyond chip boundaries, which is also referred to as ''bandwidth wall''. From 1986 to 2000, Central processing unit, CPU speed improved at an annual rate of 55% while memory speed only improved at 10%. Given these trends, it was expected that memory latency would become an overwhelming bottleneck (engineering), bottleneck in computer performance. CPU speed improvements slowed significantly partly due to major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Intel Corporation, Intel summarized these causes in a 2005 document.
First of all, as chip geometries shrink and clock frequencies rise, the transistor Leakage (electronics), leakage current increases, leading to excess power consumption and heat... Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies. Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called Von Neumann architecture#Von Neumann bottleneck, Von Neumann bottleneck), further undercutting any gains that frequency increases might otherwise buy. In addition, partly due to limitations in the means of producing inductance within solid state devices, RC time constant#Delay, resistance-capacitance (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address.
The RC delays in signal transmission were also noted in "Clock Rate versus IPC: The End of the Road for Conventional Microarchitectures" which projected a maximum of 12.5% average annual CPU performance improvement between 2000 and 2014. A different concept is the processor-memory performance gap, which can be addressed by Three-dimensional integrated circuit, 3D integrated circuits that reduce the distance between the logic and memory aspects that are further apart in a 2D chip. Memory subsystem design requires a focus on the gap, which is widening over time. The main method of bridging the gap is the use of Cache (computing), caches; small amounts of high-speed memory that houses recent operations and instructions nearby the processor, speeding up the execution of those operations or instructions in cases where they are called upon frequently. Multiple levels of caching have been developed to deal with the widening gap, and the performance of high-speed modern computers relies on evolving caching techniques. There can be up to a 53% difference between the growth in speed of processor and the lagging speed of main memory access. Solid-state drive, Solid-state hard drives have continued to increase in speed, from ~400 Mbit/s via Serial ATA, SATA3 in 2012 up to ~3 GB/s via NVM Express, NVMe/PCI Express, PCIe in 2018, closing the gap between RAM and hard disk speeds, although RAM continues to be an order of magnitude faster, with single-lane DDR4 SDRAM, DDR4 3200 capable of 25 GB/s, and modern GDDR SDRAM, GDDR even faster. Fast, cheap, Non-volatile memory, non-volatile solid state drives have replaced some functions formerly performed by RAM, such as holding certain data for immediate availability in server farms - 1 terabyte of SSD storage can be had for $200, while 1 TB of RAM would cost thousands of dollars.


Timeline


SRAM


DRAM


SDRAM


See also


References


External links

* {{DEFAULTSORT:Random-Access Memory American inventions Computer architecture Computer memory Types of RAM, *