Example of writable but volatile random access memory: Synchronous Dynamic RAM modules, primarily used as main memory in personal computers, workstations, and servers. Read-write memory is a type of Computer memory that may be relatively easily written to as well as read from Volatile memory, also known as volatile storage or primary storage device, is Computer memory that requires power to maintain the stored information A DIMM, or dual in-line memory module, comprises a series of Dynamic random access memory Integrated circuits These modules are mounted on a Printed A personal computer ( PC) is any Computer whose original sales price size and capabilities make it useful for individuals and which is intended to be operated A workstation, such as a Unix workstation, RISC workstation or Engineering workstation, is a high-end Microcomputer

Random access memory (usually known by its acronym, RAM) is a type of computer data storage. DDR SDRAM ( double data rate synchronous dynamic random access memory) is a class of memory Integrated circuit used in Computers It achieves nearly twice Static random access memory (SRAM is a type of Semiconductor memory where the word static indicates that unlike ''dynamic'' RAM (DRAM, it does not Z-RAM, short for " zero capacitor RAM " is a new type of Computer memory in development by Innovative Silicon Inc Twin Transistor RAM ( TTRAM) is a new type of Computer memory in development by Renesas The Williams tube or the Williams-Kilburn tube (after inventors Freddie Williams and Tom Kilburn) developed about 1946 or 1947 Genesis in radar The basic concept of the delay line originated with World War II Radar research as a system to reduce clutter from reflections from the ground Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, is Computer memory that can retain the stored information A programmable read-only memory ( PROM) or field programmable read-only memory ( FPROM) is a form of digital memory where the setting of each bit is EEPROM (also written E2PROM and pronounced e-e-prom or simply e-squared which stands for E lectrically E rasable P rogrammable An EPROM, or E rasable P rogrammable '''''R'''ead-'''O'''nly '''M'''emory'', is a type of memory chip that retains its EEPROM (also written E2PROM and pronounced e-e-prom or simply e-squared which stands for E lectrically E rasable P rogrammable Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed Ferroelectric RAM ( FeRAM or FRAM) is a Random access memory similar in construction to DRAM but uses a Ferroelectric layer instead Magnetoresistive Random Access Memory ( MRAM) is a non-volatile computer memory ( NVRAM) technology which has been under development since The programmable metallization cell, or PMC, is a new form of non-volatile Computer memory being developed at Arizona State University and Phase-change memory (also known as PCM, PRAM, PCRAM, Ovonic Unified Memory, Chalcogenide RAM and C-RAM) is a type This article is about the music device manufacturer For the computer memory system see SONOS. Resistive random-access memory ( RRAM) is a new Non-volatile memory type being developed by Fujitsu, Sharp, Samsung, Micron IBM Racetrack Memory is an experimental Non-volatile memory device under development at IBM 's Almaden Research Center by a team led by Stuart Nano-RAM is a proprietary Computer memory technology from the company Nantero. Drum memory is a magnetic Data storage device and was an early form of Computer memory widely used in the 1950s and into the 1960s invented by Gustav Tauschek Magnetic core memory, or ferrite-core memory, is an early form of Random access Computer memory. Prehistory twistor memory Bubble memory is largely the brainchild of a single person Andrew Bobeck. Twistor is a form of Computer memory, similar to Core memory, formed by wrapping or closing Magnetic tape around a current-carrying wire Acronyms, initialisms, and alphabetisms are Abbreviations that are formed using the initial components in a phrase or name Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital Today it takes the form of integrated circuits that allow the stored data to be accessed in any order, i. Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside Debt AIDS Trade in Africa (or DATA) is a Multinational non-government organization founded in January 2002 in London by U2 's e. at random. In Computer science, random access (sometimes called direct access) is the ability to access an arbitrary element of a sequence in equal time The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data. In Computational complexity theory, constant time, or O (1 time refers to the computation time of a problem when the time needed to solve that problem does not depend ^[1]

This contrasts with storage mechanisms such as tapes, magnetic discs and optical discs, which rely on the physical movement of the recording medium or a reading head. Magnetic tape is a medium for Magnetic recording generally consisting of a thin magnetizable coating on a long and narrow strip of Plastic. In these devices, the movement takes longer than the data transfer, and the retrieval time varies depending on the physical location of the next item.

The word RAM is mostly associated with volatile types of memory (such as DRAM memory modules), where the information is lost after the power is switched off. Volatile memory, also known as volatile storage or primary storage device, is Computer memory that requires power to maintain the stored information A DIMM, or dual in-line memory module, comprises a series of Dynamic random access memory Integrated circuits These modules are mounted on a Printed However, many other types of memory are RAM as well (i. e. Random Access Memory), including most types of ROM and a kind of flash memory called NOR-Flash. Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed

History

The first type of random access memory was the magnetic core memory, developed in 1951, and used in all computers up until the development of the static and dynamic RAM integrated circuits in the late 1960s and early 1970s. Magnetic core memory, or ferrite-core memory, is an early form of Random access Computer memory. Prior to the development of the magnetic core memory, computers used relays or vacuum tubes to perform memory functions.

Overview

Types of RAM

Modern types of writable RAM generally store a bit of data in either the state of a flip-flop, as in SRAM (static RAM), or as a charge in a capacitor (or transistor gate), as in DRAM (dynamic RAM), EPROM, EEPROM and Flash. In Digital circuits a flip-flop is a term referring to an Electronic circuit (a Bistable Multivibrator) that has two stable states and thereby Static random access memory (SRAM is a type of Semiconductor memory where the word static indicates that unlike ''dynamic'' RAM (DRAM, it does not Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. A capacitor is a passive electrical component that can store Energy in the Electric field between a pair of conductors In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals An EPROM, or E rasable P rogrammable '''''R'''ead-'''O'''nly '''M'''emory'', is a type of memory chip that retains its EEPROM (also written E2PROM and pronounced e-e-prom or simply e-squared which stands for E lectrically E rasable P rogrammable Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed Some types have circuitry to detect and/or correct random faults called memory errors in the stored data, using parity bits or error correction codes. RAM parity determines whether a Random access memory unit stores a Parity bit for Error detection purposes In Mathematics, Computer science, Telecommunication, and Information theory, error detection and correction has great practical importance in RAM of the read-only type, ROM, instead uses a metal mask to permanently enable/disable selected transistors, instead of storing a charge in them.

As both SRAM and DRAM are volatile, other forms of computer storage, such as disks and magnetic tapes, have been used as "permanent" storage in traditional computers. Disk storage is a general category of a Computer storage mechanisms in which data is recorded on planar round and rotating surfaces ( disks, discs, or Magnetic tape has been used for Data storage for over 50 years Many newer products instead rely on flash memory to maintain data between sessions of use: examples include PDAs, small music players, mobile phones, synthesizers, advanced calculators, industrial instrumentaion and robotics, and many other types of products; even certain categories of personal computers, such as the OLPC XO-1, Asus Eee PC, and others, have begun replacing magnetic disk with so called flash drives (similar to fast memory cards equipped with an IDE or SATA interface). Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed A personal computer ( PC) is any Computer whose original sales price size and capabilities make it useful for individuals and which is intended to be operated The XO-1, previously known as the $100 Laptop or Children's Machine, is an inexpensive Laptop computer intended to be distributed to children in developing The ASUS Eee PC (pronounced as the letter e, IPA /iː/ is a Subnotebook / Netbook computer designed by ASUS. A memory card or flash memory card is a solid-state electronic Flash memory Data storage device used with Digital AT Attachment with Packet Interface ( ATA/ATAPI) is a standard interface used to connect storage devices such as Hard disks Solid-state

There are two basic types of flash memory: the NOR type, which is capable of true random access, and the NAND type, which is not; the former is therefore often used in place of ROM, while the latter is used in most memory cards and solid-state drives, due to a lower price. Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed A memory card or flash memory card is a solid-state electronic Flash memory Data storage device used with Digital A solid-state drive ( SSD) is a Data storage device that uses solid-state memory to store persistent Data.

Memory hierarchy

One module of 128MB NEC SD-RAM.

Many computer systems have a memory hierarchy consisting of CPU registers, on-die SRAM caches, external caches, DRAM, paging systems, and virtual memory or swap space on a hard drive. In Computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage Static random access memory (SRAM is a type of Semiconductor memory where the word static indicates that unlike ''dynamic'' RAM (DRAM, it does not In Computer science, a cache (kæʃ like "cash") is a collection of data duplicating original In Computer Operating systems that have their Main memory divided into pages, paging (sometimes called swapping) is a transfer Virtual memory is a Computer system technique which gives an application program the impression that it has contiguous working memory while in fact it may be physically In Computer Operating systems that have their Main memory divided into pages, paging (sometimes called swapping) is a transfer This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different access times, violating the original concept behind the random access term in RAM. 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 Even within a hierarchy level such as DRAM, the specific row, column, bank, rank, channel, or interleave organization of the components make the access time variable, although not to the extent that rotating storage media or a tape is variable. Interleaving in Computer science is a way to arrange Data in a non- Contiguous way in order to increase performance A data storage device is a device for recording (storing information (data (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 most modern personal computers, the RAM comes in easily upgraded form of modules called memory modules or DRAM modules about the size of a few sticks of chewing gum. A DIMM, or dual in-line memory module, comprises a series of Dynamic random access memory Integrated circuits These modules are mounted on a Printed A DIMM, or dual in-line memory module, comprises a series of Dynamic random access memory Integrated circuits These modules are mounted on a Printed These can quickly be replaced should they become damaged or too small for current purposes. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the CPU and other ICs on the motherboard, as well as in hard-drives, CD-ROMs, and several other parts of the computer system. A motherboard is the central or primary Printed circuit board (PCB making up a complex electronic system such as a modern Computer or Laptop CD-ROM (an initialism of "Compact Disc Read-Only Memory " is a pre-pressed Compact Disc that contains data accessible to but not writable

Swapping

If a computer becomes low on RAM during intensive application cycles, the computer can resort to swapping. In this case, the computer temporarily uses hard drive space as additional memory. A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device Constantly relying on this type of backup memory is called thrashing, which is generally undesirable because it lowers overall system performance. In Computer science, thrash (verb is the term used to describe a degenerate situation on a computer where increasing resources are used to do a decreasing amount of work In order to reduce the dependency on swapping, more RAM can be installed.

Other uses of the term

Other physical devices with read/write capability can have "RAM" in their names: for example, DVD-RAM. DVD-RAM ( DVD – Random Access Memory) is a disc specification presented in 1996 by the DVD Forum, which specifies rewritable DVD-RAM media and the appropriate "Random access" is also the name of an indexing method: hence, disk storage is often called "random access" because the reading head can move relatively quickly from one piece of data to another, and does not have to read all the data in between. However the final "M" is crucial: "RAM" (provided there is no additional term as in "DVD-RAM") always refers to a solid-state device.

"RAM disks"

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 is a software layer that enables applications to transparently use RAM, often a segment of Main memory, as if it was a Hard disk Unless the memory used is non-volatile, a RAM disk loses the stored data when the computer is shut down. However, volatile memory can retain its data when the computer is shut down if it has a separate power source, usually a battery. In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy

Recent developments

Several new types of non-volatile RAM, which will preserve data while powered down, are under development. Non-volatile Random access memory ( NVRAM) is the general name used to describe any type of random access memory which does not lose its information The technologies used include carbon nanotubes and the magnetic tunnel effect. See also Graphene, Buckypaper Carbon nanotubes (CNTs are Allotropes of carbon with a nanostructure that can have a length-to-diameter In Physics, the tunnel magnetoresistance effect (TMR, occurs when two Ferromagnets are separated by a thin (about 1 nm) insulator. In summer 2003, a 128 KB magnetic RAM chip manufactured with 0. A kilobyte (derived from the SI prefix Kilo -, meaning 1000 is a unit of Information or Computer storage equal to either 1024 Magnetoresistive Random Access Memory ( MRAM) is a non-volatile computer memory ( NVRAM) technology which has been under development since 18 µm technology was introduced. The core technology of MRAM is based on the magnetic tunnel effect. In June 2004, Infineon Technologies unveiled a 16 MB prototype again based on 0. Infineon Technologies AG () was founded in April 1999 when the Semiconductor operations of parent company Siemens AG, were spun off to form a separate A megabyte is a unit of Information or Computer storage equal to either 106 (1000000 Bytes or 220 (1048576 bytes depending on 18 µm technology. Nantero built a functioning carbon nanotube memory prototype 10 GB array in 2004. Nantero is a Technology Company based in Woburn Massachusetts dedicated to developing a new form of non-volatile RAM, which they call Nano-RAM A gigabyte (derived from the SI prefix Giga-) is a unit of Information or Computer Whether some of these technologies will be able to eventually take a significant market share from either DRAM, SRAM, or flash-memory technology, remains to be seen however.

In 2006, "Solid-state drives" (based on flash memory) with capacities exceeding 150 gigabytes and speeds far exceeding traditional disks have become available. A solid-state drive ( SSD) is a Data storage device that uses solid-state memory to store persistent Data. This development has started to blur the definition between traditional random access memory and "disks", dramatically reducing the difference in performance.

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. From 1986 to 2000, 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 in computer performance. In Engineering, bottleneck is a phenomenon by which the performance or capacity of an entire system is severely limited by a single component ^[2]

Currently, CPU speed improvements have slowed significantly partly due to major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Intel summarized these causes in their Platform 2015 documentation (PDF):

“First of all, as chip geometries shrink and clock frequencies rise, the transistor leakage current increases, leading to excess power consumption and heat (more on power consumption below). 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 bottleneck), further undercutting any gains that frequency increases might otherwise buy. The von Neumann architecture is a design model for a stored-program Digital computer that uses a processing unit and a single separate storage structure In addition, partly due to limitations in the means of producing inductance within solid state devices, resistance-capacitance (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address. A resistor–capacitor circuit (RC circuit, or RC filter or RC network, is an Electric circuit composed of resistors and capacitors driven by ”

The RC delays in signal transmission were also noted in Clock Rate versus IPC: The End of the Road for Conventional Microarchitectures which projects a maximum of 12. 5% average annual CPU performance improvement between 2000 and 2014. The data on Intel Processors clearly shows a slowdown in performance improvements in recent processors. However, Intel's new processors, Core 2 Duo (codenamed Conroe) show a significant improvement over previous Pentium 4 processors; due to a more efficient architecture, performance increased while clock rate actually decreased. The Core 2 brand refers to a range of Intel 's consumer 64-bit dual-core and 2x2 MCM quad-core CPUs with the X86-64 instruction set The Pentium 4 brand refers to Intel 's line of single- core mainstream desktop and Laptop Central processing units (CPUs introduced

Security concerns

Contrary to simple models (and perhaps common belief), the contents of modern SDRAM modules aren't lost immediately when the computer is shutdown; instead, the contents fade away - a process that takes only seconds at room temperatures, but which can be extended to minutes at low temperatures. As an example, it is therefore possible to get hold of an encryption key if it was stored in ordinary working memory (i. e. the SDRAM modules). ^[3]

See also

Terminology

DRAM

Dynamic random access memory

SRAM

Static random access memory

Notes and references

External links

• How RAM Works – Article by Jeff Tyson and Dave Coustan
• What kind of RAM – Pictures and descriptions of RAM from Darrell's computer help and information site.