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.Flash memory is an that can be electrically erased and reprogrammed. The two main types of flash memory are named after the. The individual flash, consisting of (floating-gate ), exhibit internal characteristics similar to those of the corresponding gates.Flash memory is a type of memory that was invented at in 1980, based on (electrically erasable programmable ) technology. Toshiba commercially introduced flash memory to the market in 1987. While had to be completely erased before being rewritten, NAND-type flash memory may be erased, written and read in blocks (or pages) which are generally much smaller than the entire device. NOR-type flash allows a single (byte) to be written – to an erased location – or read independently.
Flash memory is an electronic (solid-state) non-volatile computer memory storage medium that. Compared to NOR flash, replacing single transistors with serial-linked groups adds an. Starting in late 2011, as part of Intel's Ultrabook initiative, an increasing number of. Create a book Download as PDF Printable version.
A flash memory device typically consists of one or more flash (each holding many flash memory cells) along with a separate chip.The NAND type is found primarily in, (those produced in 2009 or later), and similar products, for general storage and transfer of data. NAND or NOR flash memory is also often used to store configuration data in numerous digital products, a task previously made possible by EEPROM or battery-powered. One key disadvantage of flash memory is that it can only endure a relatively small number of write cycles in a specific block.Example applications of flash memory include,. In addition to being non-volatile, flash memory offers fast read, although not as fast as static RAM or ROM. Its mechanical shock resistance helps explain its popularity over in portable devices.Although flash memory is technically a type of EEPROM, the term 'EEPROM' is generally used to refer specifically to non-flash EEPROM which is erasable in small blocks, typically bytes. Because erase cycles are slow, the large block sizes used in flash memory erasing give it a significant speed advantage over non-flash EEPROM when writing large amounts of data. As of 2019, flash memory costs much less than byte-programmable EEPROM and had become the dominant memory type wherever a system required a significant amount of non-volatile.
Contents.History Background The origins of flash memory can be traced back to the development of the (FGMOS), also known as the floating-gate transistor. The original (metal-oxide-semiconductor field-effect transistor), also known as the MOS transistor, was invented by Egyptian engineer and Korean engineer at in 1959. Kahng went on to develop a variation, the floating-gate MOSFET, with at Bell Labs in 1967. They proposed that it could be used as floating-gate for storing a form of programmable that is both and.Early types of floating-gate memory included (erasable PROM) and (electrically erasable PROM) in the 1970s.
However, early floating-gate memory required engineers to build a memory cell for each of data, which proved to be cumbersome, slow, and expensive, restricting floating-gate memory to niche applications in the 1970s, such as and the earliest experimental. Invention and commercialization , while working for, proposed a new type of floating-gate memory that allowed entire sections of memory to be erased quickly and easily, by applying a voltage to a single wire connected to a group of cells. This led to Masuoka's invention of flash memory at Toshiba in 1980. According to Toshiba, the name 'flash' was suggested by Masuoka's colleague, Shōji Ariizumi, because the erasure process of the memory contents reminded him of the. Masuoka and colleagues presented the invention of flash in 1984, and then flash at the 1987 International Electron Devices Meeting (IEDM) held in San Francisco.Toshiba commercially launched NAND flash memory in 1987. Introduced the first commercial NOR type flash chip in 1988. NOR-based flash has long erase and write times, but provides full address and data buses, allowing to any memory location.
This makes it a suitable replacement for older (ROM) chips, which are used to store program code that rarely needs to be updated, such as a computer's or the of. Its endurance may be from as little as 100 erase cycles for an on-chip flash memory, to a more typical 10,000 or 100,000 erase cycles, up to 1,000,000 erase cycles. NOR-based flash was the basis of early flash-based removable media; was originally based on it, though later cards moved to less expensive NAND flash.NAND flash has reduced erase and write times, and requires less chip area per cell, thus allowing greater storage density and lower cost per bit than NOR flash; it also has up to 10 times the endurance of NOR flash. However, the I/O interface of NAND flash does not provide a random-access external address bus. Rather, data must be read on a block-wise basis, with typical block sizes of hundreds to thousands of bits. This makes NAND flash unsuitable as a drop-in replacement for program ROM, since most microprocessors and microcontrollers require byte-level random access.
In this regard, NAND flash is similar to other secondary, such as hard disks and, and is thus highly suitable for use in mass-storage devices, such as and (SSD). Flash memory cards and SSDs store data using multiple NAND flash memory chips. The first NAND-based removable memory card format was in 1995, and many others have followed, including:., and.Later developments A new generation of memory card formats, including, and, feature extremely small form factors. For example, the microSD card has an area of just over 1.5 cm 2, with a thickness of less than 1 mm.NAND flash has achieved significant levels of memory as a result of several major technologies that were commercialized during the late 2000s to early 2010s.(MLC) technology stores more than one in each.
Demonstrated (QLC) technology in 1996, with a 64 flash memory chip storing 2-bit data per cell. Also demonstrated quad-level cells in 2000, with a 64 Mb memory chip. In 2009, Toshiba and introduced NAND flash chips with QLC technology storing per cell and holding a capacity of 64 Gb. Introduced (TLC) technology storing 3-bit per cell, and began mass-producing NAND chips with TLC technology in 2010.(CTF) technology was developed during the 1990s to early 2000s. In 1991, researchers including N.
Oyama and Hiroki Shirai described a type of flash memory with a charge trap method. In 1998, Boaz Eitan of (later acquired by ) a flash memory technology named NROM that took advantage of a charge trapping layer to replace the used in conventional flash memory designs. In 2000, an (AMD) research team led by Richard M. Fastow, Egyptian engineer Khaled Z. Ahmed and Jordanian engineer Sameer Haddad (who later joined Spansion) demonstrated a charge-trapping mechanism for NOR flash memory cells.
CTF was later commercialized by AMD and in 2002. 3D (vertical NAND) technology stacks NAND flash memory cells vertically within a chip using 3D charge trap flash (CTP) technology. 3D V-NAND technology was first announced by Toshiba in 2007, and was first commercially released by in 2013.(3D IC) technology stacks (IC) chips vertically into a single 3D IC chip package. Toshiba introduced 3D IC technology to NAND flash memory in April 2007, when they debuted a 16 THGAM embedded NAND flash memory chip, which was manufactured with eight stacked 2 GB NAND flash chips. In September 2007, (now ) introduced 24-layer 3D IC technology, with a 16 GB flash memory chip that was manufactured with 24 stacked NAND flash chips using a wafer bonding process.
Toshiba also used an eight-layer 3D IC for their 32 GB THGBM flash chip in 2008. In 2010, Toshiba used a 16-layer 3D IC for their 128 GB THGBM2 flash chip, which was manufactured with 16 stacked 8 GB chips. In the 2010s, 3D ICs came into widespread commercial use for NAND flash memory in.As of August 2017, microSD cards with a capacity up to 400 (400 billion bytes) are available. The same year, Samsung combined 3D IC chip stacking with its 3D V-NAND and TLC technologies to manufacture its 512 GB KLUFG8R1EM flash memory chip with eight stacked 64-layer V-NAND chips. In 2019, Samsung produced a 1 flash chip, with eight stacked 96-layer V-NAND chips and with QLC technology. Principles of operation. Main article:In flash memory, each memory cell resembles a standard except that the transistor has two gates instead of one.
The cells can be seen as an electrical switch in which current flows between two terminals (source and drain) and is controlled by a floating gate (FG) and a control gate (CG). The CG is similar to the gate in other MOS transistors, but below this, there is the FG insulated all around by an oxide layer. The FG is interposed between the CG and the MOSFET channel. Because the FG is electrically isolated by its insulating layer, electrons placed on it are trapped. When the FG is charged with electrons, this charge the from the CG, thus, increasing the (V T1) of the cell. This means that now a higher voltage (V T2) must be applied to the CG to make the channel conductive. In order to read a value from the transistor, an intermediate voltage between the threshold voltages (V T1 & V T2) is applied to the CG.
If the channel conducts at this intermediate voltage, the FG must be uncharged (if it was charged, we would not get conduction because the intermediate voltage is less than V T2), and hence, a logical '1' is stored in the gate. If the channel does not conduct at the intermediate voltage, it indicates that the FG is charged, and hence, a logical '0' is stored in the gate.
The presence of a logical '0' or '1' is sensed by determining whether there is current flowing through the transistor when the intermediate voltage is asserted on the CG. In a multi-level cell device, which stores more than one per cell, the amount of current flow is sensed (rather than simply its presence or absence), in order to determine more precisely the level of charge on the FG.Fowler–Nordheim tunneling The process of moving electrons from the control gate and into the floating gate is called, and it fundamentally changes the characteristics of the cell by increasing the MOSFET's threshold voltage. This, in turn, changes the drain-source current that flows through the transistor for a given gate voltage, which is ultimately used to encode a binary value. The Fowler-Nordheim tunneling effect is reversible, so electrons can be added to or removed from the floating gate, processes traditionally known as writing and erasing. Internal charge pumps Despite the need for relatively high programming and erasing voltages, virtually all flash chips today require only a single supply voltage and produce the high voltages that are required using on-chip.Over half the energy used by a 1.8 V NAND flash chip is lost in the charge pump itself. Since are inherently more efficient than charge pumps, researchers developing SSDs have proposed returning to the dual Vcc/Vpp supply voltages used on all early flash chips, driving the high Vpp voltage for all flash chips in an SSD with a single shared external boost converter.In spacecraft and other high-radiation environments, the on-chip charge pump is the first part of the flash chip to fail, although flash memories will continue to work – in read-only mode – at much higher radiation levels. NOR flash.
NOR flash memory wiring and structure on siliconIn NOR flash, each cell has one end connected directly to ground, and the other end connected directly to a bit line. This arrangement is called 'NOR flash' because it acts like a when one of the word lines (connected to the cell's CG) is brought high, the corresponding storage transistor acts to pull the output bit line low. NOR flash continues to be the technology of choice for embedded applications requiring a discrete non-volatile memory device. The low read latencies characteristic of NOR devices allow for both direct code execution and data storage in a single memory product.
Programming. Erasing a NOR memory cell (setting it to logical 1), via quantum tunnelingA single-level NOR flash cell in its default state is logically equivalent to a binary '1' value, because current will flow through the channel under application of an appropriate voltage to the control gate, so that the bitline voltage is pulled down. NAND flash memory wiring and structure on silicon NAND flash NAND flash also uses, but they are connected in a way that resembles a: several transistors are connected in series, and the bit line is pulled low only if all the word lines are pulled high (above the transistors' V T). These groups are then connected via some additional transistors to a NOR-style bit line array in the same way that single transistors are linked in NOR flash.Compared to NOR flash, replacing single transistors with serial-linked groups adds an extra level of addressing. Whereas NOR flash might address memory by page then word, NAND flash might address it by page, word and bit.
Bit-level addressing suits bit-serial applications (such as hard disk emulation), which access only one bit at a time. Execute-in-place applications, on the other hand, require every bit in a word to be accessed simultaneously.
This requires word-level addressing. In any case, both bit and word addressing modes are possible with either NOR or NAND flash.To read data, first the desired group is selected (in the same way that a single transistor is selected from a NOR array). Next, most of the word lines are pulled up above the V T of a programmed bit, while one of them is pulled up to just over the V T of an erased bit. The series group will conduct (and pull the bit line low) if the selected bit has not been programmed.Despite the additional transistors, the reduction in ground wires and bit lines allows a denser layout and greater storage capacity per chip. (The ground wires and bit lines are actually much wider than the lines in the diagrams.) In addition, NAND flash is typically permitted to contain a certain number of faults (NOR flash, as is used for a ROM, is expected to be fault-free). Manufacturers try to maximize the amount of usable storage by shrinking the size of the transistors.Writing and erasing NAND flash uses for writing and for erasing. NAND flash memory forms the core of the removable storage devices known as, as well as most formats and available today.The architecture of NAND Flash means that data can be read and programmed in pages, typically between 4 KiB and 16 KiB in size, but can only be erased at the level of entire blocks consisting of multiple pages and MB in size.
When a block is erased all the cells are logically set to 1. Data can only be programmed in one pass to a page in a block that was erased. Any cells that have been set to 0 by programming can only be reset to 1 by erasing the entire block.
This means that before new data can be programmed into a page that already contains data, the current contents of the page plus the new data must be copied to a new, erased page. If a suitable page is available, the data can be written to it immediately.
If no erased page is available, a block must be erased before copying the data to a page in that block. The old page is then marked as invalid and is available for erasing and reuse. Vertical NAND Vertical NAND (V-NAND) or 3D NAND memory stacks memory cells vertically and uses a architecture. The vertical layers allow larger areal bit densities without requiring smaller individual cells. It is also known as 3D NAND or BiCS Flash. 3D NAND was first announced by in 2007.
V-NAND was first commercially manufactured by in 2013. Structure V-NAND uses a geometry (which was commercially introduced in 2002 by and ) that stores charge on an embedded film. Such a film is more robust against point defects and can be made thicker to hold larger numbers of electrons. V-NAND wraps a planar charge trap cell into a cylindrical form.The hierarchical structure of NAND Flash starts at a cell level which establishes strings, then pages, blocks, planes and ultimately a die.
A string is a series of connected NAND cells in which the source of one cell is connected to the drain of the next one. Depending on the NAND technology, a string typically consists of 32 to 128 NAND cells. Strings are organised into pages which are then organised into blocks in which each string is connected to a separate line called a bitline (BL) All cells with the same position in the string are connected through the control gates by a wordline (WL) A plane contains a certain number of blocks that are connected through the same BL. A Flash die consists of one or more planes, and the peripheral circuitry that is needed to perform all the read/ write/ erase operations.An individual memory cell is made up of one planar polysilicon layer containing a hole filled by multiple concentric vertical cylinders. The hole's polysilicon surface acts as the gate electrode.
The outermost silicon dioxide cylinder acts as the gate dielectric, enclosing a silicon nitride cylinder that stores charge, in turn enclosing a silicon dioxide cylinder as the tunnel dielectric that surrounds a central rod of conducting polysilicon which acts as the conducting channel.Memory cells in different vertical layers do not interfere with each other, as the charges cannot move vertically through the silicon nitride storage medium, and the electric fields associated with the gates are closely confined within each layer. The vertical collection is electrically identical to the serial-linked groups in which conventional NAND flash memory is configured. Construction Growth of a group of V-NAND cells begins with an alternating stack of conducting (doped) polysilicon layers and insulating silicon dioxide layers.The next step is to form a cylindrical hole through these layers. In practice, a 128 V-NAND chip with 24 layers of memory cells requires about 2.9 billion such holes. Next, the hole's inner surface receives multiple coatings, first silicon dioxide, then silicon nitride, then a second layer of silicon dioxide. Finally, the hole is filled with conducting (doped) polysilicon.
Performance As of 2013, V-NAND flash architecture allows read and write operations twice as fast as conventional NAND and can last up to 10 times as long, while consuming 50 percent less power. They offer comparable physical bit density using 10-nm lithography but may be able to increase bit density by up to two orders of magnitude. Limitations Block erasure One limitation of flash memory is that, although it can be read or programmed a byte or a word at a time in a random access fashion, it can be erased only a block at a time. This generally sets all bits in the block to 1. Starting with a freshly erased block, any location within that block can be programmed.
However, once a bit has been set to 0, only by erasing the entire block can it be changed back to 1. In other words, flash memory (specifically NOR flash) offers random-access read and programming operations but does not offer arbitrary random-access rewrite or erase operations. A location can, however, be rewritten as long as the new value's 0 bits are a superset of the over-written values. For example, a value may be erased to 1111, then written as 1110. Successive writes to that nibble can change it to 1010, then 0010, and finally 0000. Essentially, erasure sets all bits to 1, and programming can only clear bits to 0.Some file systems designed for flash devices make use of this rewrite capability, for example, to represent sector metadata.Other flash file systems, such as, never make use of this 'rewrite' capability—they do a lot of extra work to meet a 'write once rule'.Although data structures in flash memory cannot be updated in completely general ways, this allows members to be 'removed' by marking them as invalid. This technique may need to be modified for devices, where one memory cell holds more than one bit.Common flash devices such as and memory cards provide only a block-level interface, or (FTL), which writes to a different cell each time to wear-level the device.
This prevents incremental writing within a block; however, it does help the device from being prematurely worn out by intensive write patterns.Memory wear Another limitation is that flash memory has a finite number of program – erase cycles (typically written as P/E cycles). Most commercially available flash products are guaranteed to withstand around 100,000 P/E cycles before the wear begins to deteriorate the integrity of the storage. And announced an SLC NAND flash memory chip rated for 1,000,000 P/E cycles on 17 December 2008.The guaranteed cycle count may apply only to block zero (as is the case with NAND devices), or to all blocks (as in NOR). This effect is mitigated in some chip firmware or file system drivers by counting the writes and dynamically remapping blocks in order to spread write operations between sectors; this technique is called. Another approach is to perform write verification and remapping to spare sectors in case of write failure, a technique called management (BBM).
For portable consumer devices, these wear out management techniques typically extend the life of the flash memory beyond the life of the device itself, and some data loss may be acceptable in these applications. For high-reliability data storage, however, it is not advisable to use flash memory that would have to go through a large number of programming cycles. This limitation is meaningless for 'read-only' applications such as and, which are programmed only once or at most a few times during their lifetimes.In December 2012, Taiwanese engineers from Macronix revealed their intention to announce at the 2012 IEEE International Electron Devices Meeting that they had figured out how to improve NAND flash storage read/write cycles from 10,000 to 100 million cycles using a 'self-healing' process that used a flash chip with 'onboard heaters that could anneal small groups of memory cells.' The built-in thermal annealing was to replace the usual erase cycle with a local high temperature process that not only erased the stored charge, but also repaired the electron-induced stress in the chip, giving write cycles of at least 100 million. The result was to be a chip that could be erased and rewritten over and over, even when it should theoretically break down. As promising as Macronix's breakthrough might have been for the mobile industry, however, there were no plans for a commercial product to be released any time in the near future. Read disturb The method used to read NAND flash memory can cause nearby cells in the same memory block to change over time (become programmed).
This is known as read disturb. The threshold number of reads is generally in the hundreds of thousands of reads between intervening erase operations. If reading continually from one cell, that cell will not fail but rather one of the surrounding cells on a subsequent read. To avoid the read disturb problem the flash controller will typically count the total number of reads to a block since the last erase. When the count exceeds a target limit, the affected block is copied over to a new block, erased, then released to the block pool.
The original block is as good as new after the erase. If the flash controller does not intervene in time, however, a read disturb error will occur with possible data loss if the errors are too numerous to correct with an. X-ray effects Most flash ICs come in (BGA) packages, and even the ones that do not are often mounted on a PCB next to other BGA packages.
After, boards with BGA packages are often X-rayed to see if the balls are making proper connections to the proper pad, or if the BGA needs. These X-rays can erase programmed bits in a flash chip (convert programmed '0' bits into erased '1' bits). Erased bits ('1' bits) are not affected by X-rays.Some manufacturers are now making X-ray proof SD and USB memory devices.Low-level access The low-level interface to flash memory chips differs from those of other memory types such as, and, which support bit-alterability (both zero to one and one to zero) and via externally accessible.NOR memory has an external address bus for reading and programming. For NOR memory, reading and programming are random-access, and unlocking and erasing are block-wise. For NAND memory, reading and programming are page-wise, and unlocking and erasing are block-wise.NOR memories.
NOR flash by IntelReading from NOR flash is similar to reading from random-access memory, provided the address and data bus are mapped correctly. Because of this, most microprocessors can use NOR flash memory as (XIP) memory, meaning that programs stored in NOR flash can be executed directly from the NOR flash without needing to be copied into RAM first. NOR flash may be programmed in a random-access manner similar to reading. Programming changes bits from a logical one to a zero. Bits that are already zero are left unchanged.
Erasure must happen a block at a time, and resets all the bits in the erased block back to one. Typical block sizes are 64, 128, or 256.Bad block management is a relatively new feature in NOR chips. In older NOR devices not supporting bad block management, the software or controlling the memory chip must correct for blocks that wear out, or the device will cease to work reliably.The specific commands used to lock, unlock, program, or erase NOR memories differ for each manufacturer. To avoid needing unique driver software for every device made, special (CFI) commands allow the device to identify itself and its critical operating parameters.Besides its use as random-access ROM, NOR flash can also be used as a storage device, by taking advantage of random-access programming. Some devices offer read-while-write functionality so that code continues to execute even while a program or erase operation is occurring in the background. For sequential data writes, NOR flash chips typically have slow write speeds, compared with NAND flash.Typical NOR flash does not need an.
NAND memories NAND flash architecture was introduced by Toshiba in 1989. These memories are accessed much like, such as hard disks. Each block consists of a number of pages. The pages are typically 512, 2,048 or 4,096 bytes in size. Main article:Because of the particular characteristics of flash memory, it is best used with either a controller to perform wear leveling and error correction or specifically designed flash file systems, which spread writes over the media and deal with the long erase times of NOR flash blocks. The basic concept behind flash file systems is the following: when the flash store is to be updated, the file system will write a new copy of the changed data to a fresh block, remap the file pointers, then erase the old block later when it has time.In practice, flash file systems are used only for (MTDs), which are embedded flash memories that do not have a controller.
Removable flash and have built-in controllers to perform wear leveling and error correction so use of a specific flash file system does not add any benefit.Capacity Multiple chips are often arrayed to achieve higher capacities for use in consumer electronic devices such as multimedia players. The capacity of flash chips generally follows because they are manufactured with many of the same techniques and equipment.Consumer flash storage devices typically are advertised with usable sizes expressed as a small integer power of two (2, 4, 8, etc.) and a designation of megabytes (MB) or gigabytes (GB); e.g., 512 MB, 8 GB. This includes marketed as hard drive replacements, in accordance with traditional, which use. Thus, an SSD marked as '64 ' is at least 64 × 1000 3 bytes (64 GB).
Serial Flash: Silicon Storage Tech SST25VF080BSerial flash is a small, low-power flash memory that provides only serial access to the data - rather than addressing individual bytes, the user reads or writes large contiguous groups of bytes in the address space serially. (SPI) is a typical protocol for accessing the device.When incorporated into an, serial flash requires fewer wires on the than parallel flash memories, since it transmits and receives data one bit at a time. This may permit a reduction in board space, power consumption, and total system cost.There are several reasons why a serial device, with fewer external pins than a parallel device, can significantly reduce overall cost:. Many are pad-limited, meaning that the size of the is constrained by the number of pads, rather than the complexity and number of gates used for the device logic. Eliminating bond pads thus permits a more compact integrated circuit, on a smaller die; this increases the number of dies that may be fabricated on a, and thus reduces the cost per die. Reducing the number of external pins also reduces assembly and costs.
A serial device may be packaged in a smaller and simpler package than a parallel device. Smaller and lower pin-count packages occupy less PCB area. Lower pin-count devices simplify PCB.There are two major SPI flash types.
The first type is characterized by small pages and one or more internal SRAM page buffers allowing a complete page to be read to the buffer, partially modified, and then written back (for example, the Atmel DataFlash or the Page Erase NOR Flash). The second type has larger sectors. The smallest sectors typically found in an SPI flash are 4 kB, but they can be as large as 64 kB.
Since the SPI flash lacks an internal SRAM buffer, the complete page must be read out and modified before being written back, making it slow to manage. SPI flash is cheaper than DataFlash and is therefore a good choice when the application is code shadowing.The two types are not easily exchangeable, since they do not have the same pinout, and the command sets are incompatible.Most are based on SRAM configuration cells and require an external configuration device, often a serial flash chip, to reload the configuration every power cycle. Firmware storage With the increasing speed of modern CPUs, parallel flash devices are often much slower than the memory bus of the computer they are connected to. Conversely, modern offers access times below 10, while offers access times below 20 ns. Because of this, it is often desirable to code stored in flash into RAM; that is, the code is copied from flash into RAM before execution, so that the CPU may access it at full speed.
Device may be stored in a serial flash device, and then copied into SDRAM or SRAM when the device is powered-up. Using an external serial flash device rather than on-chip flash removes the need for significant process compromise (a manufacturing process that is good for high-speed logic is generally not good for flash and vice versa). Once it is decided to read the firmware in as one big block it is common to add compression to allow a smaller flash chip to be used. Typical applications for serial flash include storing firmware for, controllers, etc.Flash memory as a replacement for hard drives. Main article:One more recent application for flash memory is as a replacement for.
Flash memory does not have the mechanical limitations and latencies of hard drives, so a (SSD) is attractive when considering speed, noise, power consumption, and reliability. Flash drives are gaining traction as mobile device secondary storage devices; they are also used as substitutes for hard drives in high-performance desktop computers and some servers with and architectures.There remain some aspects of flash-based SSDs that make them unattractive. The cost per gigabyte of flash memory remains significantly higher than that of hard disks.
Also flash memory has a finite number of P/E cycles, but this seems to be currently under control since warranties on flash-based SSDs are approaching those of current hard drives. See also: and Flash memory shipments ( est.
Example of random-access memory: Synchronous, primarily used as main memory in, and.Random-access memory ( RAM ) is a form of that can be read and changed in any order, typically used to store working. A memory device allows items to be 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, and the older and, 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 and 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 (IC) chips with (metal-oxide-semiconductor).
RAM is normally associated with types of memory (such as ), where stored information is lost if power is removed, although non-volatile RAM has also been developed. Other types of 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 and a type of called.The two main types of volatile random-access are (SRAM) and (DRAM). Commercial uses of semiconductor RAM date back to 1965, when IBM introduced the SP95 SRAM chip for their computer, and used DRAM memory cells for its Toscal BC-1411, both based on. Commercial MOS memory, based on, was developed in the late 1960s, and has since been the basis for all commercial semiconductor memory. The first commercial DRAM IC chip, the, was introduced in October 1970.
(SDRAM) later debuted with the KM48SL2000 chip in 1992. 1 (MB) chip, one of the last models developed by in 1989.Early computers used, or for main memory functions. Ultrasonic delay lines could only reproduce data in the order it was written.
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, and later, out of discrete, 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 starting in 1947. It stored data as electrically charged spots on the face of a. 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 in England, the Williams tube provided the medium on which the first electronically stored program was implemented in the 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 to demonstrate the reliability of the memory.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.
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Magnetic core memory was the standard form of system until displaced by in (ICs) during the early 1970s.Prior to the development of integrated (ROM) circuits, permanent (or read-only) random-access memory was often constructed using driven by, or specially wound planes. began in the 1960s with bipolar memory, which used. While it improved performance, it could not compete with the lower price of magnetic core memory.
MOS RAMThe invention of the (metal-oxide-semiconductor field-effect transistor), also known as the MOS transistor, by and at in 1959, led to the development of (MOS) memory by John Schmidt at in 1964. In addition to higher performance, MOS was cheaper and consumed less power than magnetic core memory. The development of (MOS IC) technology by at Fairchild in 1968 enabled the production of MOS. MOS memory overtook magnetic core memory as the dominant memory technology in the early 1970s.An integrated bipolar (SRAM) was invented by Robert H. Norman at 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 of data. Commercial use of SRAM began in 1965, when introduced the SP95 memory chip for the.(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. 's Toscal BC-1411, which was introduced in 1965, used a form of capacitive bipolar DRAM, storing 180-bit data on discrete, consisting of 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.
At the was working on MOS memory. While examining the characteristics of MOS technology, he found it was capable of building, 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, which was on an MOS process with a capacity of 1, and was released in 1970.(SDRAM) was developed. The first commercial SDRAM chip was the Samsung KM48SL2000, which had a capacity of 16. It was introduced by in 1992, and mass-produced in 1993. The first commercial ( SDRAM) memory chip was Samsung's 64 DDR SDRAM chip, released in June 1998. (graphics DDR) is a form of DDR (synchronous graphics RAM), which was first released by Samsung as a 16 Mb memory chip in 1998. TypesThe two widely used forms of modern RAM are (SRAM) and (DRAM). In SRAM, a is stored using the state of a six-, typically using six (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. DRAM stores a bit of data using a transistor and pair (typically a MOSFET and, 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, (ROM) stores data by permanently enabling or disabling selected transistors, such that the memory cannot be altered. Writeable variants of ROM (such as and ) share properties of both ROM and RAM, enabling data to without power and to be updated without requiring special equipment.
These persistent forms of semiconductor ROM include flash drives, memory cards for cameras and portable devices, and.(which can be either SRAM or DRAM) includes special circuitry to detect and/or correct random faults (memory errors) in the stored data, using or.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 is somewhat of a misnomer since, unlike or 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. Main article:The memory cell is the fundamental building block of. The memory cell is an that stores one 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 circuit, usually implemented using. 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.
DRAM Cell (1 Transistor and one capacitor)AddressingTo 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. Main article:One can read and over-write data in RAM.
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Many computer systems have a memory hierarchy consisting of, on-die caches, external, systems and or 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, violating the original concept behind the random access term in RAM.
Even within a hierarchy level such as DRAM, the specific row, column, bank, channel, or organization of the components make the access time variable, although not to the extent that access time to rotating 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 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 and other on the, as well as in hard-drives, and several other parts of the computer system.Other uses of RAM. Main article:Most modern operating systems employ a method of extending RAM capacity, known as 'virtual memory'. A portion of the computer's 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 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 and generally hampers overall system performance, mainly because hard drives are far slower than RAM.RAM disk. Main article:Software can 'partition' a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a. A RAM disk loses the stored data when the computer is shut down, unless memory is arranged to have a standby battery source.Shadow RAMSometimes, 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.As a common example, the 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 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 developmentsSeveral new types of, which preserve data while powered down, are under development. The technologies used include and approaches utilizing.
Amongst the 1st generation, a 128 ( 128 × 2 10 bytes) chip was manufactured with 0.18 µm technology in the summer of 2003. In June 2004, unveiled a 16 (16 × 2 20 bytes) prototype again based on 0.18 µm technology. There are two 2nd generation techniques currently in development: (TAS) which is being developed by, and (STT) on which, and several other companies are working. Built a functioning carbon nanotube memory prototype 10 (10 × 2 30 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, ' (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, where is more important than speed. Memory wallThe '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, 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 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. Summarized these causes in a 2005 document.First of all, as chip geometries shrink and clock frequencies rise, the transistor increases, leading to excess power consumption and heat.
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