Flash memory

Flash memory

Overview
Flash memory is a non-volatile
Non-volatile memory
Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, in the most basic sense, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM, most types of magnetic computer...

 computer storage
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....

 chip that can be electrically erased and reprogrammed. It was developed from EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

 (electrically erasable programmable read-only memory) and must be erased in fairly large blocks before these can be rewritten with new data. The high density NAND type must also be programmed and read in (smaller) blocks, or pages, while the NOR type allows a single machine word (byte) to be written and/or read independently.

The NAND type is primarily used in memory card
Memory card
A memory card or flash card is an electronic flash memory data storage device used for storing digital information. They are commonly used in many electronic devices, including digital cameras, mobile phones, laptop computers, MP3 players, and video game consoles...

s, USB flash drive
USB flash drive
A flash drive is a data storage device that consists of flash memory with an integrated Universal Serial Bus interface. flash drives are typically removable and rewritable, and physically much smaller than a floppy disk. Most weigh less than 30 g...

s, solid-state drive
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...

s, and similar products, for general storage and transfer of data.
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Encyclopedia
Flash memory is a non-volatile
Non-volatile memory
Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, in the most basic sense, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM, most types of magnetic computer...

 computer storage
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....

 chip that can be electrically erased and reprogrammed. It was developed from EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

 (electrically erasable programmable read-only memory) and must be erased in fairly large blocks before these can be rewritten with new data. The high density NAND type must also be programmed and read in (smaller) blocks, or pages, while the NOR type allows a single machine word (byte) to be written and/or read independently.

The NAND type is primarily used in memory card
Memory card
A memory card or flash card is an electronic flash memory data storage device used for storing digital information. They are commonly used in many electronic devices, including digital cameras, mobile phones, laptop computers, MP3 players, and video game consoles...

s, USB flash drive
USB flash drive
A flash drive is a data storage device that consists of flash memory with an integrated Universal Serial Bus interface. flash drives are typically removable and rewritable, and physically much smaller than a floppy disk. Most weigh less than 30 g...

s, solid-state drive
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...

s, and similar products, for general storage and transfer of data. The NOR type, which allows true random access and therefore direct code execution, is used as a replacement for the older EPROM
EPROM
An EPROM , or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages...

 and as an alternative to certain kinds of ROM applications. However, NOR flash memory may emulate ROM primarily at the machine code
Machine code
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...

 level; many digital designs need ROM (or PLA
Programmable logic array
A programmable logic array is a kind of programmable logic device used to implement combinational logic circuits. The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output...

) structures for other uses, often at significantly higher speeds than (economical) flash memory may achieve. NAND or NOR flash memory is also often used to store configuration data in numerous digital products, a task previously made possible by EEPROMs or battery-powered static RAM.

Example applications of both types of flash memory include personal computers, PDA
Personal digital assistant
A personal digital assistant , also known as a palmtop computer, or personal data assistant, is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet...

s, digital audio players, digital camera
Digital camera
A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...

s, mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

s, synthesizer
Synthesizer
A synthesizer is an electronic instrument capable of producing sounds by generating electrical signals of different frequencies. These electrical signals are played through a loudspeaker or set of headphones...

s, video games, scientific instrument
Scientific instrument
A scientific instrument can be any type of equipment, machine, apparatus or device as is specifically designed, constructed and often, through trial and error, ingeniously refined to apply utmost efficiency in the utilization of well proven physical principle, relationship or technology to...

ation, industrial robotics, medical electronics, and so on. In addition to being non-volatile, flash memory offers fast read access time
Access time
Access time is the time delay or latency between a request to an electronic system, and the access being completed or the requested data returned....

s, as fast as dynamic RAM, although not as fast as static RAM or ROM. Its mechanic shock resistance explain the popularity over hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

s in portable devices; so does its high durability, being able to withstand high pressure, temperature, immersion in water etc.

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 old-style EEPROM when writing large amounts of data. Flash memory now costs far less than byte-programmable EEPROM and has become the dominant memory type wherever a significant amount of non-volatile, solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 storage is needed.

History


Flash memory (both NOR and NAND types) was invented by Dr. Fujio Masuoka
Fujio Masuoka
Dr. is the inventor of flash memory. He joined Toshiba in 1971. There he also developed SAMOS memory. Dr. Masuoka was excited mostly by the idea of non-volatile memory, memory that would last even when power was turned off. He is now a professor at Tohoku University in Sendai, Japan.Masuoka...

 while working for Toshiba
Toshiba
is a multinational electronics and electrical equipment corporation headquartered in Tokyo, Japan. It is a diversified manufacturer and marketer of electrical products, spanning information & communications equipment and systems, Internet-based solutions and services, electronic components and...

 circa 1980. According to Toshiba, the name "flash" was suggested by Dr. Masuoka's colleague, Mr. Shōji Ariizumi, because the erasure process of the memory contents reminded him of the flash of a camera
Flash (photography)
A flash is a device used in photography producing a flash of artificial light at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a dark scene. Other uses are capturing quickly moving objects or changing the quality of light...

. Dr. Masuoka presented the invention at the IEEE
Institute of Electrical and Electronics Engineers
The Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...

 1984 International Electron Devices Meeting
International Electron Devices Meeting
The IEEE International Electron Devices Meeting is an annual electronics conference held alternately in San Francisco, California and Washington D.C. each December...

(IEDM) held in San Francisco, California
San Francisco, California
San Francisco , officially the City and County of San Francisco, is the financial, cultural, and transportation center of the San Francisco Bay Area, a region of 7.15 million people which includes San Jose and Oakland...

.

Intel Corporation
Intel Corporation
Intel Corporation is an American multinational semiconductor chip maker corporation headquartered in Santa Clara, California, United States and the world's largest semiconductor chip maker, based on revenue. It is the inventor of the x86 series of microprocessors, the processors found in most...

 saw the massive potential of the invention and 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 random access
Random access
In computer science, random access is the ability to access an element at an arbitrary position in a sequence in equal time, independent of sequence size. The position is arbitrary in the sense that it is unpredictable, thus the use of the term "random" in "random access"...

 to any memory location. This makes it a suitable replacement for older read-only memory
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...

 (ROM) chips, which are used to store program code that rarely needs to be updated, such as a computer's BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

 or the firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...

 of set-top box
Set-top box
A set-top box or set-top unit is an information appliance device that generally contains a tuner and connects to a television set and an external source of signal, turning the signal into content which is then displayed on the television screen or other display device.-History:Before the...

es. Its endurance is 10,000 to 1,000,000 erase cycles. NOR-based flash was the basis of early flash-based removable media; CompactFlash
CompactFlash
CompactFlash is a mass storage device format used in portable electronic devices. Most CompactFlash devices contain flash memory in a standardized enclosure. The format was first specified and produced by SanDisk in 1994...

 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 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 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 data storage device
Data storage device
thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....

s such as hard disks and optical media, and is thus very suitable for use in mass-storage devices such as memory card
Memory card
A memory card or flash card is an electronic flash memory data storage device used for storing digital information. They are commonly used in many electronic devices, including digital cameras, mobile phones, laptop computers, MP3 players, and video game consoles...

s. The first NAND-based removable media format was SmartMedia
SmartMedia
SmartMedia is a flash memory card standard owned by Toshiba, with capacities ranging from 2 MB to 128 MB. SmartMedia memory cards are no longer manufactured.- History :...

 in 1995, and many others have followed, including MultiMediaCard
MultiMediaCard
The MultiMediaCard is a flash memory memory card standard. Unveiled in 1997 by Siemens AG and SanDisk, it is based on Toshiba's NAND-based flash memory, and is therefore much smaller than earlier systems based on Intel NOR-based memory such as CompactFlash. MMC is about the size of a postage...

, Secure Digital
Secure Digital
Secure Digital is a non-volatile memory card format developed by the SD Card Association for use in portable devices. The SD technology is used by more than 400 brands across dozens of product categories and more than 8,000 models, and is considered the de-facto industry standard.Secure Digital...

, Memory Stick
Memory Stick
Memory Stick is a removable flash memory card format, launched by Sony in October 1998, and is also used in general to describe the whole family of Memory Sticks...

 and xD-Picture Card
XD-Picture Card
xD-Picture Card is a flash memory card format, used mainly in older digital cameras. xD stands for Extreme Digital.xD cards are available in capacities of 16 MiB up to 2 GiB.- History :...

. A new generation of memory card formats, including RS-MMC, miniSD and microSD, and Intelligent Stick
Intelligent Stick
"Intelligent Stick" is a brand name for a USB flash drive memory card which was developed by the Power Quotient International company in 2002. Also called "I-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 64 GB, as of May 2011.

Principles of operation







Flash memory stores information in an array of memory cells made from floating-gate transistors. In traditional single-level cell (SLC) devices, each cell stores only one bit of information. Some newer flash memory, known as multi-level cell
Multi-level cell
In electronics, a multi-level cell is a memory element capable of storing more than a single bit of information.MLC NAND flash is a flash memory technology using multiple levels per cell to allow more bits to be stored using the same number of transistors...

 (MLC) devices, can store more than one bit per cell by choosing between multiple levels of electrical charge to apply to the floating gates of its cells.

The floating gate may be conductive (typically polysilicon
Polycrystalline silicon
Polycrystalline silicon, also called polysilicon, is a material consisting of small silicon crystals. It differs from single-crystal silicon, used for electronics and solar cells, and from amorphous silicon, used for thin film devices and solar cells....

 in most kinds of flash memory) or non-conductive (as in SONOS
SONOS
SONOS, short for "Silicon-Oxide-Nitride-Oxide-Silicon", is a type of non-volatile computer memory closely related to Flash RAM. It is distinguished from mainstream flash by the use of silicon nitride instead of polysilicon for the charge storage material. A further variant is "SHINOS"...

 flash memory).

Floating-gate transistor


In flash memory, each memory cell resembles a standard MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...

, except the transistor has two gates instead of one. On top is the control gate (CG), as in other MOS transistors, but below this there is a floating gate (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, any electrons placed on it are trapped there and, under normal conditions, will not discharge for many years. When the FG holds a charge, it screens
Electric field screening
Screening is the damping of electric fields caused by the presence of mobile charge carriers. It is an important part of the behavior of charge-carrying fluids, such as ionized gases and conduction electrons in semiconductors and metals....

 (partially cancels) the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 from the CG, which modifies the threshold voltage
Threshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...

 (VT) of the cell. During read-out, a voltage intermediate between the possible threshold voltages is applied to the CG, and the MOSFET channel will become conducting or remain insulating, depending on the VT of the cell, which is in turn controlled by charge on the FG. The current flow through the MOSFET channel is sensed and forms a binary code
Binary code
A binary code is a way of representing text or computer processor instructions by the use of the binary number system's two-binary digits 0 and 1. This is accomplished by assigning a bit string to each particular symbol or instruction...

, reproducing the stored data. In a multi-level cell device, which stores more than one bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

 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.

NOR flash


In NOR gate
NOR gate
The NOR gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output results if both the inputs to the gate are LOW . If one or both input is HIGH , a LOW output results. NOR is the result of the negation of the OR operator...

 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 NOR gate: when one of the word lines 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


A 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. A NOR flash cell can be programmed, or set to a binary "0" value, by the following procedure:
  • an elevated on-voltage (typically >5 V) is applied to the CG
  • the channel is now turned on, so electrons can flow from the source to the drain (assuming an NMOS transistor)
  • the source-drain current is sufficiently high to cause some high energy electrons to jump through the insulating layer onto the FG, via a process called hot-electron injection
    Hot carrier injection
    Hot carrier injection is a phenomenon in solid-state electronic devices where an electron or a “hole” gains sufficient kinetic energy to overcome a potential barrier necessary to break an interface state. The term "hot" refers to the effective temperature used to model carrier density, not to the...


Erasing


To erase a NOR flash cell (resetting it to the "1" state), a large voltage of the opposite polarity is applied between the CG and source, pulling the electrons off the FG through quantum tunneling. Modern NOR flash memory chips are divided into erase segments (often called blocks or sectors). The erase operation can only be performed on a block-wise basis; all the cells in an erase segment must be erased together. Programming of NOR cells, however, can generally be performed one byte or word at a time.

Internal charge pumps


Despite the need for high programming and erasing voltages, virtually all flash chips today require only a single supply voltage, and produce the high voltages via on-chip charge pump
Charge pump
A charge pump is a kind of DC to DC converter that uses capacitors as energy storage elements to create either a higher or lower voltage power source. Charge pump circuits are capable of high efficiencies, sometimes as high as 90–95% while being electrically simple circuits.Charge pumps use some...

s.

NAND flash


NAND flash also uses floating-gate transistor
Floating Gate MOSFET
The Floating Gate MOSFET is a field effect transistor, whose structure is similar to a conventional MOSFET. The gate of the FGMOS is electrically isolated, creating a floating node in DC, and a number of secondary gates or inputs are deposited above the floating gate and are electrically isolated...

s, but they are connected in a way that resembles a NAND gate
NAND gate
The Negated AND, NO AND or NAND gate is the opposite of the digital AND gate, and behaves in a manner that corresponds to the opposite of AND gate, as shown in the truth table on the right. A LOW output results only if both the inputs to the gate are HIGH...

: several transistors are connected in series, and only if all word lines are pulled high (above the transistors' VT) is the bit line pulled low. These groups are then connected via some additional transistors to a NOR-style bit line array.

To read, most of the word lines are pulled up above the VT of a programmed bit, while one of them is pulled up to just over the VT 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. In addition, NAND flash is typically permitted to contain a certain number of faults (NOR flash, as is used for a BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

 ROM, is expected to be fault-free). Manufacturers try to maximize the amount of usable storage by shrinking the size of the transistor below the size where they can be made reliably, to the size where further reductions would increase the number of faults faster than it would increase the total storage available.

NAND flash uses tunnel injection
Tunnel injection
Tunnel injection is the quantum tunneling effect, also called Fowler-Nordheim tunnel injection, when charge carriers are injected to an electric conductor through a thin layer of an electric insulator....

 for writing and tunnel release for erasing. NAND flash memory forms the core of the removable USB
Universal Serial Bus
USB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....

 storage devices known as USB flash drive
USB flash drive
A flash drive is a data storage device that consists of flash memory with an integrated Universal Serial Bus interface. flash drives are typically removable and rewritable, and physically much smaller than a floppy disk. Most weigh less than 30 g...

s, as well as most memory card
Memory card
A memory card or flash card is an electronic flash memory data storage device used for storing digital information. They are commonly used in many electronic devices, including digital cameras, mobile phones, laptop computers, MP3 players, and video game consoles...

 formats and solid-state drive
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...

s available today.

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 only be erased 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 cannot 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 value's. For example, a nibble
Nibble
In computing, a nibble is a four-bit aggregation, or half an octet...

 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, and programming can only clear bits. Filesystems designed for flash devices can make use of this capability to represent sector metadata.

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 Multi-level Cell
Multi-level cell
In electronics, a multi-level cell is a memory element capable of storing more than a single bit of information.MLC NAND flash is a flash memory technology using multiple levels per cell to allow more bits to be stored using the same number of transistors...

 devices, where one memory cell holds more than one bit.

Common flash devices such as USB keys and memory cards provide only a block-level interface, or flash translation layer (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 abusive and/or poorly designed hardware/software. For example, nearly all consumer devices ship formatted with MS-FAT file system, which pre-dates flash memory, having been designed for DOS, and disk media.

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. Micron Technology
Micron Technology
Micron Technology, Inc. is an American multinational corporation based in Boise, Idaho, USA, best known for producing many forms of semiconductor devices. This includes DRAM, SDRAM, flash memory, SSD and CMOS image sensing chips. Consumers may be more familiar with its consumer brand Crucial...

 and Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...

 announced an SLC NAND flash memory chip rated for 1,000,000 P/E cycles on December 17, 2008.

The guaranteed cycle count may apply only to block zero (as is the case with TSOP
Thin small-outline package
Thin small-outline packages, or TSOPs are a type of surface mount IC package. They are notably very low-profile and have tight lead spacing ....

 NAND devices), or to all blocks (as in NOR). This effect is partially offset 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 wear leveling
Wear leveling
Wear leveling is a technique for prolonging the service life of some kinds of erasable computer storage media, such as Flash memory used in solid-state drives and USB Flash drives...

. Another approach is to perform write verification and remapping to spare sectors in case of write failure, a technique called Bad Block
Bad Sector
Bad Sector is an ambient/noise project formed in 1992 in Tuscany, Italy by Massimo Magrini. While working at the Computer Art Lab of ISTI in Pisa , he developed original gesture interfaces that he uses in live performances: 'Aerial Painting Hand' , 'UV-Stick' Bad Sector is an ambient/noise...

 Management (BBM). For portable consumer devices, these wearout 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 thin clients and routers, which are only programmed once or at most a few times during their lifetimes.

Read disturb


The method used to read NAND flash memory can cause other cells near the cell being read to change over time if the surrounding cells of the block are not rewritten. This is generally in the hundreds of thousands of reads without a rewrite of those cells. The error does not appear when reading the original cell, but rather shows up when finally reading one of the surrounding cells. If the flash controller does not track the total number of reads across the whole storage device and rewrite the surrounding data periodically as a precaution, a read disturb error will likely occur with data loss as a result.

Low-level access


The low-level interface to flash memory chips differs from those of other memory types such as DRAM
Dynamic random access memory
Dynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...

, ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...

, and EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

, which support bit-alterability (both zero to one and one to zero) and random-access via externally accessible address bus
Address bus
An address bus is a computer bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus...

es.

While NOR memory provides an external address bus for read and program operations (and thus supports random-access); unlocking and erasing NOR memory must proceed on a block-by-block basis. With NAND flash memory, read and programming operations must be performed page-at-a-time while unlocking and erasing must happen in block-wise fashion.

NOR memories


Reading 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 execute in place
Execute in place
In computer science, execute in place is a method of executing programs directly from long term storage rather than copying it into RAM. It is an extension of using shared memory to reduce the total amount of memory required....

 (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 KB.

Bad block management is a relatively new feature in NOR chips. In older NOR devices not supporting bad block management, the software or device driver
Device driver
In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....

 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, a special set of Common Flash Memory Interface
Common Flash Memory Interface
The Common Flash memory Interface is an open standard jointly developed by AMD, Intel, Sharp and Fujitsu. AMD has sold its flash memory products division to Spansion. An overview about the specification is available at...

 (CFI) commands allow the device to identify itself and its critical operating parameters.

Apart from being used as random-access ROM, NOR memories can also be used as storage devices 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.

NAND memories


NAND flash architecture was introduced by Toshiba in 1989. These memories are accessed much like block devices such as hard disks or memory cards. Each block consists of a number of pages. The pages are typically 512 or 2,048 or 4,096 bytes in size. Associated with each page are a few bytes (typically 1/32 of the data size) that can be used for storage of an error correcting code (ECC) checksum
Checksum
A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...

.

Typical block sizes include:
  • 32 pages of 512+16 bytes each for a block size of 16 KB
  • 64 pages of 2,048+64 bytes each for a block size of 128 KB
  • 64 pages of 4,096+128 bytes each for a block size of 256 KB
  • 128 pages of 4,096+128 bytes each for a block size of 512 KB.


While reading and programming is performed on a page basis, erasure can only be performed on a block basis. Number of Operations (NOPs) is the number of times the pages can be programmed. So far this number for MLC flash is always one whereas for SLC flash it is four.

NAND devices also require bad block management by the device driver software, or by a separate controller
Flash memory controller
A flash memory controller manages the data stored on flash memory and communicates with a computer or electronic device. Flash memory controllers can be designed for operating in low duty-cycle environments like SD cards, CompactFlash cards, or other similar media for use in digital cameras, PDAs,...

 chip. SD cards, for example, include controller circuitry to perform bad block management and wear leveling
Wear leveling
Wear leveling is a technique for prolonging the service life of some kinds of erasable computer storage media, such as Flash memory used in solid-state drives and USB Flash drives...

. When a logical block is accessed by high-level software, it is mapped to a physical block by the device driver or controller. A number of blocks on the flash chip may be set aside for storing mapping tables to deal with bad blocks, or the system may simply check each block at power-up to create a bad block map in RAM. The overall memory capacity gradually shrinks as more blocks are marked as bad.

NAND relies on ECC to compensate for bits that may spontaneously fail during normal device operation. A typical ECC will correct a one bit error in each 2048 bits (256 bytes) using 22 bits of ECC code, or a one bit error in each 4096 bits (512 bytes) using 24 bits of ECC code. If ECC cannot correct the error during read, it may still detect the error. When doing erase or program operations, the device can detect blocks that fail to program or erase and mark them bad. The data is then written to a different, good block, and the bad block map is updated.

Most NAND devices are shipped from the factory with some bad blocks which are typically identified and marked according to a specified bad block marking strategy. By allowing some bad blocks, the manufacturers achieve far higher yields than would be possible if all blocks had to be verified good. This significantly reduces NAND flash costs and only slightly decreases the storage capacity of the parts.

When executing software from NAND memories, virtual memory
Virtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...

 strategies are often used: memory contents must first be paged
Paging
In computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called...

 or copied into memory-mapped RAM and executed there (leading to the common combination of NAND + RAM). A memory management unit
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...

 (MMU) in the system is helpful, but this can also be accomplished with overlays
Overlay (programming)
In a general computing sense, overlaying means "replacement of a block of stored instructions or data with another" Overlaying is a programming method that allows programs to be larger than the computer's main memory...

. For this reason, some systems will use a combination of NOR and NAND memories, where a smaller NOR memory is used as software ROM and a larger NAND memory is partitioned with a file system for use as a non-volatile data storage area.

NAND is best suited to systems requiring high capacity data storage. This type of flash architecture offers higher densities and larger capacities at lower cost with faster erase, sequential write, and sequential read speeds, sacrificing the random-access and execute in place advantage of the NOR architecture.

Standardization


A group called the Open NAND Flash Interface Working Group
Open NAND Flash Interface Working Group
The Open NAND Flash Interface Working Group , is a consortium of technology companies working to develop open standards for NAND flash memory chips and devices that communicate with them...

 (ONFI) has developed a standardized low-level interface for NAND flash chips. This allows interoperability between conforming NAND devices from different vendors. The ONFI specification version 1.0 was released on December 28, 2006. It specifies:
  • a standard physical interface (pinout
    Pinout
    In electronics, a pinout is a cross-reference between the contacts, or pins, of an electrical connector or electronic component, and their functions.- Purpose :...

    ) for NAND flash in TSOP
    Thin small-outline package
    Thin small-outline packages, or TSOPs are a type of surface mount IC package. They are notably very low-profile and have tight lead spacing ....

    -48, WSOP-48, LGA
    Land grid array
    The land grid array is a type of surface-mount packaging for integrated circuits that is notable for having the pins on the socket rather than the integrated circuit...

    -52, and BGA
    Ball grid array
    A ball grid array is a type of surface-mount packaging used for integrated circuits.- Description :The BGA is descended from the pin grid array , which is a package with one face covered with pins in a grid pattern. These pins conduct electrical signals from the integrated circuit to the printed...

    -63 packages
  • a standard command set for reading, writing, and erasing NAND flash chips
  • a mechanism for self-identification (comparable to the Serial Presence Detect
    Serial Presence Detect
    Serial presence detect refers to a standardized way to automatically access information about a computer memory module. Earlier 72-pin SIMMs included 5 pins which provided 5 bits of parallel presence detect data, but the 168-pin DIMM standard changed to a serial presence detect to encode much...

    ion feature of SDRAM memory modules)


The ONFI group is supported by major NAND flash manufacturers, including Hynix
Hynix
Hynix Semiconductor Inc. chips and flash memory chips. Founded in 1983, Hynix is the world's second-largest memory chipmaker, the largest being Samsung Electronics. Formerly known as Hyundai Electronics, the company has manufacturing sites in Korea, the U.S., China and Taiwan...

, Intel, Micron Technology
Micron Technology
Micron Technology, Inc. is an American multinational corporation based in Boise, Idaho, USA, best known for producing many forms of semiconductor devices. This includes DRAM, SDRAM, flash memory, SSD and CMOS image sensing chips. Consumers may be more familiar with its consumer brand Crucial...

, and Numonyx
Numonyx
Numonyx is a semiconductor company making flash memories, which was founded on March 31, 2008 by Intel Corporation, STMicroelectronics and Francisco Partners. It was acquired by Micron Technology on February 9, 2010....

, as well as by major manufacturers of devices incorporating NAND flash chips.

A group of vendors, including Intel, Dell
Dell
Dell, Inc. is an American multinational information technology corporation based in 1 Dell Way, Round Rock, Texas, United States, that develops, sells and supports computers and related products and services. Bearing the name of its founder, Michael Dell, the company is one of the largest...

, and Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 formed a Non-Volatile Memory Host Controller Interface (NVMHCI) Working Group. The goal of the group is to provide standard software and hardware programming interfaces for nonvolatile memory subsystems, including the "flash cache" device connected to the PCI Express
PCI Express
PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...

 bus.

Distinction between NOR and NAND flash


NOR and NAND flash differ in two important ways:
  • the connections of the individual memory cells are different
  • the interface provided for reading and writing the memory is different (NOR allows random-access for reading, NAND allows only page access)

These two are linked by the design choices made in the development of NAND flash. A goal of NAND flash development was to reduce the chip area required to implement a given capacity of flash memory, and thereby to reduce cost per bit and increase maximum chip capacity so that flash memory could compete with magnetic storage devices like hard disks.

NOR and NAND flash get their names from the structure of the interconnections between memory cells. In NOR flash, cells are connected in parallel to the bit lines, allowing cells to be read and programmed individually. The parallel connection of cells resembles the parallel connection of transistors in a CMOS NOR gate. In NAND flash, cells are connected in series, resembling a NAND gate. The series connections consume less space than parallel ones, reducing the cost of NAND flash. It does not, by itself, prevent NAND cells from being read and programmed individually.

When NOR flash was developed, it was envisioned as a more economical and conveniently rewritable ROM than contemporary EPROM
EPROM
An EPROM , or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages...

 and EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

 memories. Thus random-access reading circuitry was necessary. However, it was expected that NOR flash ROM would be read much more often than written, so the write circuitry included was fairly slow and could only erase in a block-wise fashion. On the other hand, applications that use flash as a replacement for disk drives do not require word-level write address, which would only add to the complexity and cost unnecessarily.

Because of the series connection and removal of wordline contacts, a large grid of NAND flash memory cells will occupy perhaps only 60% of the area of equivalent NOR cells (assuming the same CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

 process resolution, e.g. 130nm, 90 nm, 65 nm). NAND flash's designers realized that the area of a NAND chip, and thus the cost, could be further reduced by removing the external address and data bus circuitry. Instead, external devices could communicate with NAND flash via sequential-accessed command and data registers, which would internally retrieve and output the necessary data. This design choice made random-access of NAND flash memory impossible, but the goal of NAND flash was to replace hard disks, not to replace ROMs.

Write endurance


The write endurance of SLC floating-gate NOR flash is typically equal to or greater than that of NAND flash, while MLC NOR and NAND flash have similar endurance capabilities. Example Endurance cycle ratings listed in datasheets for NAND and NOR flash are provided.
  • SLC NAND flash is typically rated at about 100k cycles (Samsung OneNAND KFW4G16Q2M)
  • MLC NAND flash used to be rated at about 5–10k cycles (Samsung K9G8G08U0M) but is now typically 1k - 3k cycles
  • TLC NAND flash is typically rated at about 100-500 cycles
  • SLC floating-gate NOR flash has typical endurance rating of 100k to 1M cycles (Numonyx M58BW 100k; Spansion
    Spansion
    Spansion Inc. is a Flash memory chip maker that designs, develops and manufactures NOR flash memory products. The company has over 3,400 employees and is headquartered in Sunnyvale, California. Spansion is a former joint-venture between AMD and Fujitsu....

     S29CD016J 1,000k)
  • MLC floating-gate NOR flash has typical endurance rating of 100k cycles (Numonyx J3 flash)


However, by applying certain algorithms and design paradigms such as wear leveling and memory over-provisioning, the endurance of a storage system can be tuned to serve specific requirements.

Flash file systems


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: 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 only used for memory technology device
Memory Technology Device
Memory technology devices are a new type of device file in Linux for interacting with flash memory. The MTD subsystem was created to provide an abstraction layer between the hardware-specific device drivers and higher-level applications...

s (MTDs), which are embedded flash memories that do not have a controller. Removable flash memory card
Memory card
A memory card or flash card is an electronic flash memory data storage device used for storing digital information. They are commonly used in many electronic devices, including digital cameras, mobile phones, laptop computers, MP3 players, and video game consoles...

s and USB flash drive
USB flash drive
A flash drive is a data storage device that consists of flash memory with an integrated Universal Serial Bus interface. flash drives are typically removable and rewritable, and physically much smaller than a floppy disk. Most weigh less than 30 g...

s 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 or GPS. The capacity of flash chips generally follows Moore's Law
Moore's Law
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....

 because they are manufactured with many of the same integrated circuits techniques and equipment.

Consumer flash drives typically have sizes measured in powers of two (e.g., 512 MB, 8 GB). This includes SSD
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...

s as hard drive replacements, even though traditional hard drives tend to use decimal units
SI prefix
The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol...

. Thus, an SSD marked as "64 GB
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...

" is actually 64 × 1,0243 bytes (64 GiB
Gib
Gib may refer to:* A castrated male cat or ferret* Gibibit , measurement unit of digitally stored computer information* Gibraltar, British overseas territory* Drywall, construction material...

). In reality, most users will have slightly less capacity than this available, due to the space taken by file system metadata.

In 2005, Toshiba and SanDisk
SanDisk
SanDisk Corporation is an American multinational corporation that designs, develops and manufactures data storage solutions in a range of form factors using the flash memory, controller and firmware technologies. It was founded in 1988 by Dr. Eli Harari and Sanjay Mehrotra, non-volatile memory...

 developed a NAND flash chip capable of storing 1 GB of data using multi-level cell
Multi-level cell
In electronics, a multi-level cell is a memory element capable of storing more than a single bit of information.MLC NAND flash is a flash memory technology using multiple levels per cell to allow more bits to be stored using the same number of transistors...

 (MLC) technology, capable of storing two bits of data per cell. In September 2005, Samsung Electronics
Samsung Electronics
Samsung Electronics is a South Korean multinational electronics and information technology company headquartered in Samsung Town, Seoul...

 announced that it had developed the world’s first 2 GB chip.

In March 2006, Samsung announced flash hard drives with a capacity of 4 GB, essentially the same order of magnitude as smaller laptop hard drives, and in September 2006, Samsung announced an 8 GB chip produced using a 40-nm manufacturing process.
In January 2008, Sandisk announced availability of their 16 GB MicroSDHC and 32 GB SDHC Plus cards.

In 2009, Kingston announced a 256 GB flash drive available only in the UK and other parts of Europe. As of 2010, however, it is available in the USA.

An PCI-e SSD with a maximum capacity of 5 TB was announced by Fusion-Io on November 17, 2009, as being capable of sustaining 6 GB per second transfer rate.

There are still flash-chips manufactured with capacities under or around 1 MB, e.g., for BIOS-ROMs and embedded applications.

Transfer rates


NAND flash memory cards are much faster at reading than writing so it is the maximum read speed that is commonly advertised.

As a chip wears out, its erase/program operations slow down considerably, requiring more retries and bad block remapping. Transferring multiple small files, each smaller than the chip-specific block size, could lead to a much lower rate. Access latency also influences performance, but less so than with their hard drive counterpart.

The speed is sometimes quoted in MB/s (megabytes per second), or as a multiple of that of a legacy single speed CD-ROM, such as 60×, 100× or 150×. Here 1× is equivalent to 150 kB/s. For example, a 100× memory card gives 150 kB/s × 100 = 15,000 kB/s = 14.65 MB/s.

Performance also depends on the quality of memory controllers. Even when the only change to manufacturing is die-shrink, the absence of an appropriate controller can result in degraded speeds.

Serial flash


Serial flash is a small, low-power flash memory that uses a serial interface, typically SPI
Serial Peripheral Interface Bus
The Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link standard named by Motorola that operates in full duplex mode. Devices communicate in master/slave mode where the master device initiates the data frame. Multiple slave devices are allowed with individual slave select ...

, for sequential data access. When incorporated into an embedded system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...

, serial flash requires fewer wires on the PCB
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

 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 ASICs
    Application-specific integrated circuit
    An application-specific integrated circuit is an integrated circuit customized for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an ASIC...

     are pad-limited, meaning that the size of the die
    Die (integrated circuit)
    A die in the context of integrated circuits is a small block of semiconducting material, on which a given functional circuit is fabricated.Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon or other semiconductor through processes such as...

     is constrained by the number of wire bond 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 wafer
    Wafer (electronics)
    A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...

    , and thus reduces the cost per die.
  • Reducing the number of external pins also reduces assembly and packaging 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 routing
    Routing (EDA)
    In electronic design, wire routing, commonly called simply routing, is a step in the design of printed circuit boards and integrated circuits . It builds on a preceding step, called placement, which determines the location of each active element of an IC or component on a 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 AT45 DataFlash™ or the Micron Technology
Micron Technology
Micron Technology, Inc. is an American multinational corporation based in Boise, Idaho, USA, best known for producing many forms of semiconductor devices. This includes DRAM, SDRAM, flash memory, SSD and CMOS image sensing chips. Consumers may be more familiar with its consumer brand Crucial...

 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 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.

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 SRAM offers access times below 10 ns
Nanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....

, while DDR2
DDR2 SDRAM
DDR2 SDRAM is a double data rate synchronous dynamic random-access memory interface. It supersedes the original DDR SDRAM specification and has itself been superseded by DDR3 SDRAM...

 SDRAM
SDRAM
Synchronous dynamic random access memory is dynamic random access memory that is synchronized with the system bus. Classic DRAM has an asynchronous interface, which means that it responds as quickly as possible to changes in control inputs...

 offers access times below 20 ns. Because of this, it is often desirable to shadow 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 firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...

 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 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 hard drives, Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

 controllers, DSL modems, wireless network devices, etc.

Flash memory as a replacement for hard drives


One more recent application for flash memory is as a replacement for hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

s. Flash memory does not have the mechanical limitations and latencies of hard drives, so a solid-state drive
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...

 (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 RAID
RAID
RAID is a storage technology that combines multiple disk drive components into a logical unit...

 and SAN
Storage area network
A storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...

 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.

In June 2006, Samsung Electronics
Samsung Electronics
Samsung Electronics is a South Korean multinational electronics and information technology company headquartered in Samsung Town, Seoul...

 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
South Korea
The Republic of Korea , , is a sovereign state in East Asia, located on the southern portion of the Korean Peninsula. It is neighbored by the People's Republic of China to the west, Japan to the east, North Korea to the north, and the East China Sea and Republic of China to the south...

. 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.

At the Las Vegas
Las Vegas metropolitan area
The Las Vegas Valley is the heart of the Las Vegas-Paradise, NV MSA also known as the Las Vegas–Paradise–Henderson MSA which includes all of Clark County, Nevada, and is a metropolitan area in the southern part of the U.S. state of Nevada. The Valley is defined by the Las Vegas Valley landform, a ...

 CES 2007
Consumer Electronics Show
The International Consumer Electronics Show is a major technology-related trade show held each January in the Las Vegas Convention Center, Las Vegas, Nevada, United States. Not open to the public, the Consumer Electronics Association-sponsored show typically hosts previews of products and new...

 Summit Taiwan
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...

ese memory company A-DATA showcased flash memory based SSD
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...

 units in capacities of 32 GB, 64 GB and 128 GB. The XO-1, developed by the One Laptop Per Child (OLPC) association, uses flash memory rather than a hard drive. As of March 2009, a Salt Lake City company called Fusion-io claims the fastest SSD with sequential read/write speeds of 1500 MB/1400 MB's per second.

There are also hybrid techniques such as hybrid drive
Hybrid drive
A Hybrid Drive, Hybrid Hard Drive , or Hybrid Hard Disk Drive is a type of large-buffer computer hard disk drive. It is different from standard hard drives in that it integrates a cache using non-volatile memory or even a small solid-state drive...

 and ReadyBoost
ReadyBoost
ReadyBoost is a disk cache component of Microsoft Windows, first introduced with Microsoft's Windows Vista in 2006 and bundled with Windows 7 in 2009...

 that attempt to combine the advantages of both technologies, using flash as a high-speed non-volatile cache
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...

 for files on the disk that are often referenced, but rarely modified, such as application and operating system executable
Executable
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...

 files.

Early versions of the ASUS Eee PC
ASUS Eee PC
The Asus Eee PC is a subnotebook/netbook computer line from ASUSTeK Computer Incorporated, and a part of the Asus Eee product family. At the time of its introduction in late 2007, it was noted for its combination of a light weight, Linux operating system, solid-state drive , and relatively low cost...

 used a flash-based SSD of 2 GB to 20 GB, depending on model, although later versions of the machine use hard disks. The Apple Inc. Macbook Air
MacBook Air
The MacBook Air family is a line of Apple ultraportable Macintosh notebook computers.The first-generation MacBook Air was a 13.3"-only model, previously promoted as the World's Thinnest Notebook, introduced at the Macworld Conference & Expo on January 15, 2008. It featured a custom Intel Merom CPU...

 notebooks have the option of a 256 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
RAID
RAID is a storage technology that combines multiple disk drive components into a logical unit...

-0 configuration as an SSD. They claim users may use more affordable High-Speed 8 GB SDHC cards to get similar or better results than can be obtained from traditionally built SSDs at a lower cost.

Industry


One source states that, in 2008, the flash memory industry includes about US$9.1 billion in production and sales. Other sources put the flash memory market at a size of more than US$20 billion in 2006, accounting for more than eight percent of the overall semiconductor market and more than 34 percent of the total semiconductor memory market.

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 aggressiveness in shrinking the design rule or process technology node.Current projections show the technology to reach approximately 20 nm by around late 2011. While the expected shrink timeline is a factor of two every three years per original version of Moore's law
Moore's Law
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....

, 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 (19 nm as of April 2011 ), further flash density increases will be driven by greater levels of MLC, possibly 3-D stacking of transistors, and improvements to the manufacturing process. The decrease in endurance and increase in uncorrectable bit error rates that accompany feature size shrinking can be compensated by improved error correction mechanisms. Even with these advances, it may be impossible to economically scale flash to smaller and smaller dimensions. Many promising new technologies (such as FeRAM
Ferroelectric RAM
Ferroelectric RAM is a random-access memory similar in construction to DRAM but uses a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FeRAM is one of a growing number of alternative non-volatile memory technologies that offer the same functionality as Flash memory...

, MRAM, PMC
Programmable metallization cell
The programmable metallization cell, or PMC, is a new form of non-volatile computer memory being developed at Arizona State University and its spinoff, Axon Technologies....

, PCM
Phase-change memory
Phase-change memory is a type of non-volatile computer memory. PRAMs exploit the unique behavior of chalcogenide glass. Heat produced by the passage of an electric current switches this material between two states, crystalline and amorphous...

, and others) are under investigation and development as possible more scalable replacements for flash.

See also

  • List of flash file systems
  • Secure USB drive
  • Open NAND Flash Interface Working Group
    Open NAND Flash Interface Working Group
    The Open NAND Flash Interface Working Group , is a consortium of technology companies working to develop open standards for NAND flash memory chips and devices that communicate with them...

  • Write amplification
    Write amplification
    Write amplification is an undesirable phenomenon associated with Flash memory and solid-state drives . Because Flash memory must be erased before it can be rewritten, the process to perform these operations results in moving user data and metadata more than once...


External links