random-access memory
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Random-access memory (RAM; ) is a form of
computer memory In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and Digital data, digital Electronics, electronic devices. The term ''memory'' is often synonymous with th ...
that can be read and changed in any order, typically used to store working
data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interp ...
and
machine code In computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programm ...
. A random-access memory device allows
data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interp ...
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 that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk platter, platters ...
s,
CD-RW CD-RW (Compact Disc-Rewritable) is a digital media, digital optical disc data storage device, storage format introduced in 1997. A CD-RW compact disc (CD-RWs) can be written, read, erased, and re-written. CD-RWs, as opposed to CDs, require ...
s,
DVD-RW DVD recordable and DVD rewritable are optical disc recording technologies. Both terms describe DVD optical discs that can be written to by a DVD recorder, whereas only 'rewritable' discs are able to erase and rewrite data. Data is written ('burne ...
s and the older magnetic tapes and
drum memory Drum memory was a magnetic data storage device invented by Gustav Tauschek in 1932 in Austria. Drums were widely used in the 1950s and into the 1960s as computer memory. For many early computers, drum memory formed the main working memory of ...
), 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 and Computer network, computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a sc ...
and demultiplexing 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 circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of ti ...
(IC) chips with MOS (metal-oxide-semiconductor) memory cells. RAM is normally associated with volatile types of memory where stored information is lost if power is removed. The two main types of volatile random-access
semiconductor memory Semiconductor memory is a digital electronics, digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memo ...
are
static random-access memory Static random-access memory (static RAM or SRAM) is a type of random-access memory (RAM) that uses Flip-flop (electronics), latching circuitry (flip-flop) to store each bit. SRAM is volatile memory; data is lost when power is removed. The ...
(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). Non-volatile RAM has also been developed and other types of non-volatile memories 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 e ...
and a type of
flash memory 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 t ...
called '' NOR-Flash''. Use of semiconductor RAM dated back to 1965, when IBM introduced the monolithic (single-chip) 16-bit SP95 SRAM chip for their System/360 Model 95 computer, and
Toshiba , commonly known as Toshiba and stylized as TOSHIBA, is a Japanese multinational corporation, multinational Conglomerate (company), conglomerate corporation headquartered in Minato, Tokyo, Minato, Tokyo, Japan. Its diversified products and serv ...
used discrete DRAM memory cells for its 180-bit 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 crea ...
, both based on
bipolar transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar t ...
s. While it offered improved performance over
magnetic-core memory Magnetic-core memory was the predominant form of random access, random-access computer memory for 20 years between about 1955 and 1975. Such memory is often just called core memory, or, informally, core. Core memory uses toroids (rings) of a ...
, bipolar DRAM could not compete with the lower price of the then-dominant magnetic-core memory. MOS memory, based on MOS transistors, was developed in the late 1960s, and was the basis for all early commercial semiconductor memory. The first commercial DRAM IC chip, the 1K Intel 1103, 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 affi ...
KM48SL2000 chip in 1992.


History

Early computers used
relay file:Delta Electronics DPS-350FB A - board 1 - OEG SDT-SS-112M - case removed-3045.jpg, A relay file:Kontakt.svg, Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts file:R ...
s, mechanical counters 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 invented by Gustav Tauschek in 1932 in Austria. Drums were widely used in the 1950s and into the 1960s as computer memory. For many early computers, drum memory formed the main working memory of ...
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 amplifier, amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated Electrical filament, filament or cathode, a control grid, grid, and a Plat ...
s, and later, out of discrete
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 gate is separated from the body by an insulating layer (pink). A ...
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 named after inventors Frederic Calland Williams, Freddie Williams and Tom Kilburn, is an early form of computer memory. It was the first Random-access memory, random-access digital storage devic ...
starting in 1947. It stored data as electrically charged spots on the face of a
cathode-ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a Phosphorescence, phosphorescent screen. The images may represent electrical waveforms (osci ...
. 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 The University of Manchester is a public university, public research university in Manchester, England. The main campus is south of Manchester city centre, Manchester City Centre on Wilmslow Road, Oxford Road. The university owns and operates majo ...
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. It was built at the University of Manchester by Frederic Calland Williams, Frederic C. Williams, Tom Kilburn, and Ge ...
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 rese ...
to demonstrate the reliability of the memory.
Magnetic-core memory Magnetic-core memory was the predominant form of random access, random-access computer memory for 20 years between about 1955 and 1975. Such memory is often just called core memory, or, informally, core. Core memory uses toroids (rings) of a ...
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, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and Digital data, digital Electronics, electronic devices. The term ''memory'' is often synonymous with th ...
system until displaced by solid-state MOS ( metal–oxide–silicon)
semiconductor memory Semiconductor memory is a digital electronics, digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memo ...
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 circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of ti ...
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 soft ...
(ROM) circuits, ''permanent'' (or ''read-only'') random-access memory was often constructed using diode matrices driven by
address decoder In digital electronics, an address decoder is a binary decoder that has two or more inputs for address bus, address bits and one or more outputs for device selection signals. When the address for a particular device appears on the address inputs, t ...
s, or specially wound
core rope memory Core rope memory is a form of read-only memory (ROM) for computers, first used in the 1960s by Mariner program, early NASA Mars space probes and then in the Apollo Guidance Computer (AGC) and programmed by the Massachusetts Institute of Techno ...
planes.
Semiconductor memory Semiconductor memory is a digital electronics, digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memo ...
began in the 1960s with bipolar memory, which used
bipolar transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar t ...
s. 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) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which ...
(metal-oxide-semiconductor field-effect transistor), also known as the MOS transistor, by 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. He is best known for inventing the MOSFET (metal–oxide–semiconductor field-effe ...
at
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company A com ...
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 company based in San Jose, California. Founded in 1957 as a division of Fairchild Camera and Instrument, it became a pioneer in the manufacturing of transistors and of ...
in 1964. In addition to higher performance, MOS
semiconductor memory Semiconductor memory is a digital electronics, digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memo ...
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- ...
(MOS IC) technology by
Federico Faggin Federico Faggin (, ; born 1 December 1941) is an Italian 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 the desi ...
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 devices in which data is stored within metal–oxide–semiconductor (MOS) memo ...
s. MOS memory overtook magnetic core memory as the dominant memory technology in the early 1970s. An integrated bipolar
static random-access memory Static random-access memory (static RAM or SRAM) is a type of random-access memory (RAM) that uses Flip-flop (electronics), latching circuitry (flip-flop) to store each bit. SRAM is volatile memory; data is lost when power is removed. The ...
(SRAM) was invented by Robert H. Norman at
Fairchild Semiconductor Fairchild Semiconductor International, Inc. was an American semiconductor company based in San Jose, California. Founded in 1957 as a division of Fairchild Camera and Instrument, it became a pioneer in the manufacturing of transistors and of ...
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 of data. Commercial use of SRAM began in 1965, when 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 , commonly known as Toshiba and stylized as TOSHIBA, is a Japanese multinational corporation, multinational Conglomerate (company), conglomerate corporation headquartered in Minato, Tokyo, Minato, Tokyo, Japan. Its diversified products and serv ...
'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 crea ...
, 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 lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to it ...
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 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
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passivity (engineering), passive electronic component with two termi ...
s, 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, which was
manufactured Manufacturing is the creation or production of goods In economics, goods are items that satisfy human wants and provide utility, for example, to a consumer making a purchase of a satisfying Product (business), product. A common distincti ...
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. (, sometimes shortened to SEC and stylized as SΛMSUNG) is a South Korean multinational corporation, multinational electronics corporation headquartered in Yeongtong-gu, Suwon, South Korea. It is the pinnacle of ...
. The first commercial SDRAM chip was the Samsung KM48SL2000, which had a capacity of 16 Mbit. 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 affi ...
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 (DDR) synchronous dynamic random-access memory (SDRAM) class of memory integrated circuits used in computers. DDR SDRAM, also retroactively called DDR1 ...
(
double data rate In computing, a computer bus operating with double data rate (DDR) transfers data on both the rising and falling edges of the clock signal. This is also known as double pumped, dual-pumped, and double transition. The term toggle mode is used in ...
SDRAM) memory chip was Samsung's 64Mbit DDR SDRAM chip, released in June 1998. GDDR (graphics DDR) is a form of DDR SGRAM (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 that can be read and changed in any order, typically used to store working Data (computing) ...
(SRAM) and
dynamic RAM Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory Semiconductor memory is a digital electronics, digital electronic semiconductor device used for digital data storage, such as computer m ...
(DRAM). In SRAM, a bit of data is stored using the state of a six-
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 gate is separated from the body by an insulating layer (pink). A ...
memory cell, typically using six MOSFETs. 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 electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passivity (engineering), passive electronic component with two termi ...
pair (typically a MOSFET and
MOS capacitor The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the thermal oxidation, controlled oxidation of silicon. It has an insulated gate, the ...
, 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 soft ...
(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 called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or as a ...
and NOR flash) share properties of both ROM and RAM, enabling data to persist without power and to be updated without requiring special equipment.
ECC memory Error correction code memory (ECC memory) is a type of computer data storage that uses an error correction code (ECC) to detect and correct n-bit data corruption which occurs in memory. ECC memory is used in most computers where data corruption c ...
(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 added to a string of binary code. Parity bits are a simple form of Error detection and correction, error detecting code. Parity bits are generally applied to the smallest units of a communication protocol, ...
s or error correction codes. 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-based disc specification presented in 1996 by the DVD Forum, which specifies rewritable DVD-RAM media and the appropriate DVD writers. DVD-RAM media have been used in computers as well as camcorders an ...
is somewhat of a misnomer since, unlike
CD-RW CD-RW (Compact Disc-Rewritable) is a digital media, digital optical disc data storage device, storage format introduced in 1997. A CD-RW compact disc (CD-RWs) can be written, read, erased, and re-written. CD-RWs, as opposed to CDs, require ...
or
DVD-RW DVD recordable and DVD rewritable are optical disc recording technologies. Both terms describe DVD optical discs that can be written to by a DVD recorder, whereas only 'rewritable' discs are able to erase and rewrite data. Data is written ('burne ...
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, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and Digital data, digital Electronics, electronic devices. The term ''memory'' is often synonymous with th ...
. The memory cell is an
electronic circuit An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or Conductive trace, traces through which electric current can flow. It is a t ...
that stores one bit 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 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 gat ...
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 A processor register is a quickly accessible location available to a computer's Processor (computing), processor. Registers usually consist of a small amount of fast Computer storage, storage, although some registers have specific hardware functi ...
s, on- die SRAM caches, external caches,
DRAM 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 ...
,
paging In computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as Co ...
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 "cr ...
or swap space 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 between a request to an electronic system, and the access being completed or the requested data returned * In a computer A computer is a machine that can be programmed to carry out sequences of ...
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, Memory rank, rank, channel, or interleaved memory, interleave organization of the components make the access time variable, although not to the extent that access time to rotating storage media 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, memory modules 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 and other Integrated circuit, ICs on the motherboard, as well as in hard-drives, CD-ROMs, 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 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 "paging, swap" 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 (computer science), 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 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. As a common example, the 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 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 NVRAM, ''non-volatile'' RAM, which preserve data while powered down, are under development. The technologies used include carbon nanotubes and approaches utilizing Tunnel magnetoresistance. Amongst the 1st generation Magnetoresistive random-access memory, MRAM, 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, 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, *