Flash scalability

Due to its relatively simple structure and high demand for higher capacity, NAND flash memory is the most aggressively scaled technology among electronic devices. The heavy competition among the top few manufacturers only adds to the aggression. Current projections show the technology to reach approximately 20 nm by around 2010. While the expected shrink timeline is a factor of two every three years per original version of Moore's law, this has recently been accelerated in the case of NAND flash to a factor of two every two years.

As the feature size of flash memory cells reach the minimum limit (currently estimated ~20 nm), further Flash density increases will be driven by greater levels of MLC, possibly 3-D stacking of transistors, and process improvements. Even with these advances, it may be impossible to economically scale Flash to smaller and smaller dimensions. Many promising new technologies (such as FeRAM, MRAM, PMC, PCM, and others) are under investigation and development as possible more scalable replacements for Flash.^[35]

Flash memory as a replacement for hard drives

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An obvious extension of flash memory would be as a replacement for hard disks. Flash memory does not have the mechanical limitations and latencies of hard drives, so the idea of a solid-state drive, or SSD, is attractive when considering speed, noise, power consumption, and reliability.

There remain some aspects of flash-based SSDs that make the idea unattractive. Most important, the cost per gigabyte of flash memory remains significantly higher than that of platter-based hard drives. Although this ratio is decreasing rapidly for flash memory, it is not yet clear that flash memory will catch up to the capacities and affordability offered by platter-based storage. Still, research and development is sufficiently vigorous that it is not clear that it will not happen, either.

There is also some concern that the finite number of erase/write cycles of flash memory would render flash memory unable to support an operating system. This seems to be a decreasing issue as warranties on flash-based SSDs are approaching those of current hard drives.^[25][26]

In June, 2006, Samsung Electronics released the first flash-memory based PCs, the Q1-SSD and Q30-SSD, both of which used 32 GB SSDs, and were at least initially available only in South Korea.^[27] Dell Computer introduced a 32GB SSD option on its Latitude D420 and D620 ATG laptops in April 2007—at $549 more than a hard-drive equipped version.^[28]

At the Las Vegas CES 2007 Summit Taiwanese memory company A-DATA showcased SSD hard disk drives based on Flash technology in capacities of 32 GB, 64 GB and 128 GB.^[29] Sandisk announced an OEM 32 GB 1.8" SSD drive at CES 2007.^[30] The XO-1, developed by theOne Laptop Per Child (OLPC) association, uses flash memory rather than a hard drive. As of June 2007, a South Korean company called Mtron claims the fastest SSD with sequential read/write speeds of 100 MB/80 MB per second.^[31]

Rather than entirely replacing the hard drive, hybrid techniques such as hybrid drive and ReadyBoost attempt to combine the advantages of both technologies, using flash as a high-speed cache for files on the disk that are often referenced, but rarely modified, such as application and operating system executable files. Also, Addonics has a PCI adapter for 4 CF cards,^[32] creating a RAID-able array of solid-state storage that is much cheaper than the hardwired-chips PCI card kind.

The ASUS Eee PC uses a flash-based SSD of 2 GB to 20 GB, depending on model. The Apple Inc. Macbook Air has the option to upgrade the standard hard drive to a 128 GB Solid State hard drive. The Lenovo ThinkPad X300 also features a built-in 64 GB Solid State Drive.

Sharkoon has developed a device that uses six SDHC cards in RAID-0 as an SSD alternative; users may use more affordable High-Speed 8GB SDHC cards to get similar or better results than can be obtained from traditional SSDs at a lower cost.

History

Flash memory (both NOR and NAND types) was invented by Dr. Fujio Masuoka while working for Toshiba circa 1980.^[1][2] According to Toshiba, the name "flash" was suggested by Dr. Masuoka's colleague, Mr. Shoji Ariizumi, because the erasure process of the memory contents reminded him of the flash of a camera. Dr. Masuoka presented the invention at the IEEE 1984 International Electron Devices Meeting(IEDM) held in San Francisco, California.

Intel Corporation saw the massive potential of the invention and introduced the first commercial NOR type flash chip in 1988.^[3] NOR-based flash has long erase and write times, but provides full address and data buses, allowing random access to any memory location. This makes it a suitable replacement for older Read-only memory (ROM) chips, which are used to store program code that rarely needs to be updated, such as a computer's BIOS or the firmware of set-top boxes. Its endurance is 10,000 to 1,000,000 erase cycles.^[4] NOR-based flash was the basis of early flash-based removable media; CompactFlash was originally based on it, though later cards moved to less expensive NAND flash.

Toshiba announced NAND flash at the 1987 International Electron Devices Meeting. It has faster erase and write times, and requires a smaller chip area per cell, thus allowing greater storage densities and lower costs per bit than NOR flash; it also has up to ten 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 made NAND flash unsuitable as a drop-in replacement for program ROM since most microprocessors and microcontrollers required byte-level random access. In this regard NAND flash is similar to other secondary storage devices such as hard disks and optical media, and is thus very suitable for use in mass-storage devices such asmemory cards. The first NAND-based removable media format was SmartMedia in 1995, and many others have followed, includingMultiMediaCard, Secure Digital, Memory Stick and xD-Picture Card. A new generation of memory card formats, including RS-MMC, miniSDand microSD, and Intelligent Stick, feature extremely small form factors. For example, the microSD card has an area of just over 1.5 cm², with a thickness of less than 1 mm; microSD capacities range from 64 MB to 16 GB, as of August 2009.^[5]

Flash memory

Flash memory is a non-volatile computer storage that can be electrically erased and reprogrammed. It is a technology that is primarily used in memory cards and USB flash drives for general storage and transfer of data between computers and other digital products. It is a specific type of EEPROM (Electrically Erasable Programmable Read-Only Memory) that is erased and programmed in large blocks; in early flash the entire chip had to be erased at once. Flash memory costs far less than byte-programmable EEPROM and therefore has become the dominant technology wherever a significant amount of non-volatile, solid state storage is needed. Example applications include PDAs (personal digital assistants), laptop computers, digital audio players, digital cameras and mobile phones. It has also gained popularity in console video game hardware, where it is often used instead of EEPROMs or battery-powered static RAM (SRAM) for game save data.

Since flash memory is non-volatile, no power is needed to maintain the information stored in the chip. In addition, flash memory offers fast read access times (although not as fast as volatile DRAM memory used for main memory in PCs) and better kinetic shock resistance than hard disks. These characteristics explain the popularity of flash memory in portable devices. Another feature of flash memory is that when packaged in a "memory card," it is enormously durable, being able to withstand intense pressure, extremes of temperature, and even immersion in water.^{[citation needed]}

Although 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 old-style EEPROM when writing large amounts of data.^{[citation needed]}

Operation principle

DRAM is usually arranged in a square array of one capacitor and transistor per cell. The illustrations to the right show a simple example with only 4 by 4 cells (modern DRAM can be thousands of cells in length/width).

The long lines connecting each row are known as word lines. Each column is actually composed of two bit lines, each one connected to every other storage cell in the column. (The illustration to the right does not include this important detail.) They are generally known as the + and − bit lines. A sense amplifier is essentially a pair of cross-connected inverters between the bit lines. That is, the first inverter is connected from the + bit line to the − bit line, and the second is connected from the − bit line to the + bit line. This is an example of positive feedback, and the arrangement is only stable with one bit line high and one bit line low.

To read a bit from a column, the following operations take place:

1. The sense amplifier is switched off and the bit lines are precharged to exactly matching voltages that are intermediate between high and low logic levels. The bit lines are constructed symmetrically to keep them balanced as precisely as possible.
2. The precharge circuit is switched off. Because the bit lines are very long, theircapacitance will hold the precharge voltage for a brief time. This is an example ofdynamic logic.
3. The selected row's word line is driven high. This connects one storage capacitor to one of the two bit lines. Charge is shared between the selected storage cell and the
   1. appropriate bit line, slightly altering the voltage on the line. Although every effort is made to keep the capacitance of the storage cells high and the capacitance of the bit lines low, capacitance is proportional to physical size, and the length of the bit lines means that the net effect is a very small perturbation of one bit line's voltage.
   2. The sense amplifier is switched on. The positive feedback takes over and amplifies the small voltage difference until one bit line is fully low and the other is fully high. At this point, the row is "open" and a column can be selected.
   3. Read data from the DRAM is taken from the sense amplifiers, selected by the column address. Many reads can be performed while the row is open in this way.
   4. While reads proceed, current is flowing back up the bit lines from the sense amplifiers to the storage cells. This restores (refreshes) the charge in the storage cell. Due to the length of the bit lines, this takes significant time beyond the end of sense amplification, and overlaps with one or more column reads.
   5. When done with the current row, the word line is switched off to disconnect the storage capacitors (the row is "closed"), the sense amplifier is switched off, and the bit lines are precharged again.

   To write to memory, the row is opened and a given column's sense amplifier is temporarily forced to the desired state, so it drives the bit line which charges the capacitor to the desired value. Due to the positive feedback, the amplifier will then hold it stable even after the forcing is removed. During a write to a particular cell, the entire row is read out, one value changed, and then the entire row is written back in, as illustrated in the figure to the right.

   Typically, manufacturers specify that each row should be refreshed every 64 ms or less, according to the JEDEC (Foundation for developing Semiconductor Standards) standard.Refresh logic is commonly used with DRAMs to automate the periodic refresh. This makes the circuit more complicated, but this drawback is usually outweighed by the fact that DRAM is much cheaper and of greater capacity than SRAM. Some systems refresh every row in a tight loop that occurs once every 64 ms. Other systems refresh one row at a time — for example, a system with 2¹³ = 8192 rows would require a refresh rate of one row every 7.8 µs (64 ms / 8192 rows). A few real-time systems refresh a portion of memory at a time based on an external timer that governs the operation of the rest of the system, such as the vertical blanking that occurs every 10 to 20 ms in video equipment. All methods require some sort of counter to keep track of which row is the next to be refreshed. Most DRAM chips include that counter; older kinds require external refresh logic to hold that counter. (Under some conditions, most of the data in DRAM can be recovered even if the DRAM has not been refreshed for several minutes. See dynamic random access memory#Security.)

history Dynamic random access memory

The cryptanalytic machine code-named "Aquarius" used at Bletchley Park during WW II incorporated a hard-wired dynamic memory. Paper tape was read and the characters on it "were remembered in a dynamic store. ... The store used a large bank of capacitors, which were either charged or not, a charged capacitor representing cross (1) and an uncharged capacitor dot (0). Since the charge gradually leaked away, a periodic pulse was applied to top up those which were still charged (hence the term 'dynamic')."^{[1]n 1964, Arnold Farber and Eugene Schlig working for IBM created a memory cell that was hard wired; using a transistor gate and tunnel diode latch, they later replaced the latch with two transistors and two resistors, which became known as the Farber-Schlig cell. In 1965, Benjamin Agusta and his team working for IBM managed to create a 16-bit silicon memory chip based on the Farber-Schlig cell which consisted of 80 transistors, 64 resistors and 4 diodes. In 1966 DRAM was invented by Dr. Robert Dennard at the IBMThomas J. Watson Research Center and he was awarded U.S. patent number 3,387,286 in 1968. Capacitors had been used for earlier memory schemes such as the drum of the Atanasoff–Berry Computer, the Williams tube and the Selectron tube.}

The Toshiba "Toscal" BC-1411 electronic calculator, which went into production in November 1965, uses a form of dynamic RAM built from discrete components.^[2]

In 1969 Honeywell asked Intel to make a DRAM using a 3-transistor cell that they had developed. This became the Intel 1102 (1024x1) in early 1970. However the 1102 had many problems, prompting Intel to begin work on their own improved design (in secrecy to avoid conflict with Honeywell). This became the first commercially-available DRAM memory, the Intel 1103 (1024x1) in October 1970 (despite initial problems with low yield until the 5th revision of the masks).

The first DRAM with multiplexed row and column address lines was the Mostek MK4096 (4096x1) designed by Robert Proebsting and introduced in 1973. This addressing scheme, a radical advance, allowed it to fit into packages with fewer pins, a cost advantage that would grow with every jump in memory size. The MK4096 also proved to be very robust design in customer applications. At the 16K density the cost advantage increased, and the Mostek MK4116 16K DRAM achieved greater than 75% worldwide DRAM market share. However, as density increased to 64K Mostek was overtaken by Japanese DRAM manufacturers selling higher quality DRAMs using the same multiplexing scheme at below-cost prices.

Dynamic random access memory

Dynamic random access memory (DRAM) is a type of random access memory that stores each bit of data in a separate capacitor within an integrated circuit. Since real capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically. Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and other static memory.

The advantage of DRAM is its structural simplicity: only one transistor and a capacitor are required per bit, compared to six transistors in SRAM. This allows DRAM to reach very high density. Unlike flash memory, it is volatile memory (cf. non-volatile memory), since it loses its data when the power supply is removed

computer memory

In computing, an aperture is a portion of the address space which is persistently associated with a particular peripheral device or a memoryunit. Apertures may reach external devices such as ROM or RAM chips, or internal memory on the CPU itself.

Typically a memory device attached to a computer accepts addresses starting at zero, and so a system with more than one such device would have ambiguous addressing. To resolve this, the memory logic will contain several aperture selectors, each containing a range selector and an interface to one of the memory devices. The set of selector address ranges of the apertures are disjoint. When the CPU presents a physical address within the range recognized by an aperture, the aperture unit routes the request (with the address remapped to a zero base) to the attached device. Thus apertures form a layer of address translation below the level of the usual virtual-to-physical mapping.

Robotic storage

Large quantities of individual magnetic tapes, and optical or magneto-optical discs may be stored in robotic tertiary storage devices. In tape storage field they are known as tape libraries, and in optical storage field optical jukeboxes, or optical disk libraries per analogy. Smallest forms of either technology containing just one drive device are referred to as autoloaders or autochangers.

Robotic-access storage devices may have a number of slots, each holding individual media, and usually one or more picking robots that traverse the slots and load media to built-in drives. The arrangement of the slots and picking devices affects performance. Important characteristics of such storage are possible expansion options: adding slots, modules, drives, robots. Tape libraries may have from 10 to more than 100,000 slots, and provide terabytes or petabytes of near-line information. Optical jukeboxes are somewhat smaller solutions, up to 1,000 slots.

Robotic storage is used for backups, and for high-capacity archives in imaging, medical, and video industries. Hierarchical storage management is a most known archiving strategy of automatically migrating long-unused files from fast hard disk storage to libraries or jukeboxes. If the files are needed, they are retrieved back to disk.

Related technologies

Network connectivity

A secondary or tertiary storage may connect to a computer utilizing computer networks. This concept does not pertain to the primary storage, which is shared between multiple processors in a much lesser degree.

• Direct-attached storage (DAS) is a traditional mass storage, that does not use any network. This is still a most popular approach. This term was coined lately, together with NAS and SAN.
• Network-attached storage (NAS) is mass storage attached to a computer which another computer can access at file level over a local area network, a private wide area network, or in the case of online file storage, over the Internet. NAS is commonly associated with the NFSand CIFS/SMB protocols.
• Storage area network (SAN) is a specialized network, that provides other computers with storage capacity. The crucial difference between NAS and SAN is the former presents and manages file systems to client computers, whilst a latter provides access at block-addressing (raw) level, leaving it to attaching systems to manage data or file systems within the provided capacity. SAN is commonly associated with Fibre Channel networks

Paper data storage media

Paper data storage, typically in the form of paper tape or punched cards, has long been used to store information for automatic processing, particularly before general-purpose computers existed. Information was recorded by punching holes into the paper or cardboard medium and was read mechanically (or later optically) to determine whether a particular location on the medium was solid or contained a hole. A few technologies allow people to make marks on paper that are easily read by machine—these are widely used for tabulating votes and grading standardized tests. Barcodes made it possible for any object that was to be sold or transported to have some computer readable information securely attached to it.

[edit]Uncommon

Vacuum tube memory

A Williams tube used a cathode ray tube, and a Selectron tube used a large vacuum tube to store information. These primary storage devices were short-lived in the market, since Williams tube was unreliable and Selectron tube was expensive.

Electro-acoustic memory

Delay line memory used sound waves in a substance such as mercury to store information. Delay line memory was dynamic volatile, cycle sequential read/write storage, and was used for primary storage.

Optical tape

is a medium for optical storage generally consisting of a long and narrow strip of plastic onto which patterns can be written and from which the patterns can be read back. It shares some technologies with cinema film stock and optical discs, but is compatible with neither. The motivation behind developing this technology was the possibility of far greater storage capacities than either magnetic tape or optical discs.

Phase-change memory

uses different mechanical phases of Phase Change Material to store information in an X-Y addressable matrix, and reads the information by observing the varying electrical resistance of the material. Phase-change memory would be non-volatile, random access read/write storage, and might be used for primary, secondary and off-line storage. Most rewritable and many write once optical disks already use phase change material to store information.

Holographic data storage

stores information optically inside crystals or photopolymers. Holographic storage can utilize the whole volume of the storage medium, unlike optical disc storage which is limited to a small number of surface layers. Holographic storage would be non-volatile, sequential access, and either write once or read/write storage. It might be used for secondary and off-line storage. See Holographic Versatile Disc(HVD).

Molecular memory

stores information in polymers that can store electric charge. Molecular memory might be especially suited for primary storage. The theoretical storage capacity of molecular memory is 10 terabits per square inch. ^[13]

[edit]Related technologies

Optical storage media

Optical storage, the typical Optical disc, stores information in deformities on the surface of a circular disc and reads this information by illuminating the surface with a laser diode and observing the reflection. Optical disc storage is non-volatile. The deformities may be permanent (read only media ), formed once (write once media) or reversible (recordable or read/write media). The following forms are currently in common use:^[12]

• CD, CD-ROM, DVD, BD-ROM: Read only storage, used for mass distribution of digital information (music, video, computer programs)
• CD-R, DVD-R, DVD+R, BD-R: Write once storage, used for tertiary and off-line storage
• CD-RW, DVD-RW, DVD+RW, DVD-RAM, BD-RE: Slow write, fast read storage, used for tertiary and off-line storage
• Ultra Density Optical or UDO is similar in capacity to BD-R or BD-RE and is slow write, fast read storage used for tertiary and off-line storage.

Magneto-optical disc storage is optical disc storage where the magnetic state on a ferromagnetic surface stores information. The information is read optically and written by combining magnetic and optical methods. Magneto-optical disc storage is non-volatile, sequential access, slow write, fast read storage used for tertiary and off-line storage.

3D optical data storage has also been proposed.

Magnetic

Magnetic storage uses different patterns of magnetization on a magnetically coated surface to store information. Magnetic storage is non-volatile. The information is accessed using one or more read/write heads which may contain one or more recording transducers. A read/write head only covers a part of the surface so that the head or medium or both must be moved relative to another in order to access data. In modern computers, magnetic storage will take these forms:

• Magnetic disk
  • Floppy disk, used for off-line storage
  • Hard disk drive, used for secondary storage
• Magnetic tape data storage, used for tertiary and off-line storage

In early computers, magnetic storage was also used for primary storage in a form of magnetic drum, or core memory, core rope memory, thin-film memory, twistor memory or bubble memory. Also unlike today, magnetic tape was often used for secondary storage

Characteristics of storage

Storage technologies at all levels of the storage hierarchy can be differentiated by evaluating certain core characteristics as well as measuring characteristics specific to a particular implementation. These core characteristics are volatility, mutability, accessibility, and addressibility. For any particular implementation of any storage technology, the characteristics worth measuring are capacity and performance.

Off-line storage

Off-line storage, also known as disconnected storage, is a computer data storage on a medium or a device that is not under the control of a processing unit.^[4] The medium is recorded, usually in a secondary or tertiary storage device, and then physically removed or disconnected. It must be inserted or connected by a human operator before a computer can access it again. Unlike tertiary storage, it cannot be accessed without human interaction.

Off-line storage is used to transfer information, since the detached medium can be easily physically transported. Additionally, in case a disaster, for example a fire, destroys the original data, a medium in a remote location will be probably unaffected, enabling disaster recovery. Off-line storage increases general information security, since it is physically inaccessible from a computer, and data confidentiality or integrity cannot be affected by computer-based attack techniques. Also, if the information stored for archival purposes is accessed seldom or never, off-line storage is less expensive than tertiary storage.

In modern personal computers, most secondary and tertiary storage media are also used for off-line storage. Optical discs and flash memory devices are most popular, and to much lesser extent removable hard disk drives. In enterprise uses, magnetic tape is predominant. Older examples are floppy disks, Zip disks, or punched cards.

Tertiary storage

Tertiary storage or tertiary memory,^[3] provides a third level of storage. Typically it involves a robotic mechanism which will mount (insert) and dismount removable mass storage media into a storage device according to the system's demands; this data is often copied to secondary storage before use. It is primarily used for archival of rarely accessed information since it is much slower than secondary storage (e.g. 5–60 seconds vs. 1-10 milliseconds). This is primarily useful for extraordinarily large data stores, accessed without human operators. Typical examples include tape libraries and optical jukeboxes.

When a computer needs to read information from the tertiary storage, it will first consult a catalog databaseto determine which tape or disc contains the information. Next, the computer will instruct a robotic arm to fetch the medium and place it in a drive. When the computer has finished reading the information, the robotic arm will return the medium to its place in the library

Secondary storage

Secondary storage (or external memory) differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfers the desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down—it is non-volatile. Per unit, it is typically also an order of magnitude less expensive than primary storage. Consequently, modern computer systems typically have an order of magnitude more secondary storage than primary storage and data is kept for a longer time there.

In modern computers, hard disk drives are usually used as secondary storage. The time taken to access a given byte of information stored on a hard disk is typically a few thousandths of a second, or milliseconds. By contrast, the time taken to access a given byte of information stored in random access memory is measured in billionths of a second, or nanoseconds. This illustrates the very significant access-time difference which distinguishes solid-state memory from rotating magnetic storage devices: hard disks are typically about a million times slower than memory. Rotating optical storage devices, such as CD and DVD drives, have even longer access times. With disk drives, once the disk read/write head reaches the proper placement and the data of interest rotates under it, subsequent data on the track are very fast to access. As a result, in order to hide the initial seek time and rotational latency, data are transferred to and from disks in large contiguous blocks.

When data reside on disk, block access to hide latency offers a ray of hope in designing efficient external memory algorithms. Sequential or block access on disks is orders of magnitude faster than random access, and many sophisticated paradigms have been developed to design efficient algorithms based upon sequential and block access . Another way to reduce the I/O bottleneck is to use multiple disks in parallel in order to increase the bandwidth between primary and secondary memory.^{[2]Most computer operating systems use the concept of virtual memory, allowing utilization of more primary storage capacity than is physically available in the system. As the primary memory fills up, the system moves the least-used chunks (pages) to secondary storage devices (to aswap file or page file), retrieving them later when they are needed. As more of these retrievals from slower secondary storage are necessary, the more the overall system performance is degraded.}

Some other examples of secondary storage technologies are: flash memory (e.g. USB flash drives or keys), floppy disks, magnetic tape, paper tape, punched cards, standalone RAM disks, and Iomega Zip drives.

The secondary storage is often formatted according to a file system format, which provides the abstraction necessary to organize data into filesand directories, providing also additional information (called metadata) describing the owner of a certain file, the access time, the access permissions, and other information.