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A flash memory controller (or flash controller) manages data stored on
flash memory Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both us ...
(usually
NAND flash Flash memory is an Integrated circuit, electronic Non-volatile memory, non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for t ...
) and communicates with a
computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as C ...
or
electronic device The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
. Flash memory controllers can be designed for operating in low
duty-cycle A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a formu ...
environments like
memory card A memory card is an electronic data storage device used for storing digital information, typically using flash memory. These are commonly used in digital portable electronic devices. They allow adding memory to such devices using a card in a so ...
s, or other similar
media Media may refer to: Communication * Media (communication), tools used to deliver information or data ** Advertising media, various media, content, buying and placement for advertising ** Broadcast media, communications delivered over mass e ...
for use in PDAs,
mobile phone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whil ...
s, etc.
USB flash drive A USB flash drive (also called a thumb drive) is a data storage device that includes flash memory with an integrated USB interface. It is typically removable, rewritable and much smaller than an optical disc. Most weigh less than . Since firs ...
s use flash memory controllers designed to communicate with
personal computer A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tec ...
s through the
USB port Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...
at a low duty-cycle. Flash controllers can also be designed for higher duty-cycle environments like
solid-state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is a ...
s (SSD) used as data storage for laptop computer systems up to
mission-critical A mission critical factor of a system is any factor (component, equipment, personnel, process, procedure, software, etc.) that is essential to business operation or to an organization. Failure or disruption of mission critical factors will resu ...
enterprise storage arrays.


Initial setup

After a flash storage device is initially manufactured, the flash controller is first used to format the flash memory. This ensures the device is operating properly, it maps out bad flash memory cells, and it allocates spare cells to be substituted for future failed cells. Some part of the spare cells is also used to hold the
firmware In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide h ...
which operates the controller and other special features for a particular storage device. A directory structure is created to allow the controller to convert requests for logical sectors into the physical locations on the actual flash memory chips.


Reading, writing, and erasing

When the system or device needs to read data from or write data to the flash memory, it will communicate with the flash memory controller. Simpler devices like SD cards and USB flash drives typically have a small number of flash memory die connected simultaneously. Operations are limited to the speed of the individual flash memory die. In contrast, a high-performance solid-state drive will have as many as 100 or more dies organized in a matrix with parallel communication paths to enable speeds many times greater than that of a single flash die.


Wear-leveling and block picking

Flash memory can withstand a limited number of program-erase cycles. If a particular flash memory block were programmed and erased repeatedly without writing to any other blocks, the one block would wear out before all the other blocks thereby prematurely ending the life of the storage device. For this reason flash controllers use a technique called
wear leveling Wear leveling (also written as wear levelling) is a technique Wear leveling techniques for flash memory systems. for prolonging the service life of some kinds of erasable computer storage media, such as flash memory, which is used in solid-state d ...
to distribute writes as evenly as possible across all the flash blocks in the SSD. In a perfect scenario this would enable every block to be written to its maximum life so they all fail at the same time.


Flash translation layer (FTL) and mapping

Usually, flash memory controllers also include the "flash translation layer" (FTL), a layer below the file system that maps host side or file system logical block addresses (LBAs) to the physical address of the flash memory (logical-to-physical mapping). The LBAs refer to sector numbers and to a mapping unit of 512 bytes. All LBAs that represent the logical size visible to and managed by the file system are mapped to a physical location (block ID, page ID and sector ID) of the Flash. As part of the
wear leveling Wear leveling (also written as wear levelling) is a technique Wear leveling techniques for flash memory systems. for prolonging the service life of some kinds of erasable computer storage media, such as flash memory, which is used in solid-state d ...
and other flash management algorithms (bad block management, read disturb management, safe flash handling etc.), the physical location of an LBA might dynamically change frequently. The mapping units of an FTL can differ so that LBAs are mapped block-, page- or even sub-page-based. Depending on the usage pattern, a finer mapping granularity can significantly reduce the flash wear out and maximize the endurance of a flash based storage media. The deduplication function to eliminate redundant data and duplicate writes is also added in FTL. As the FTL metadata takes up its own flash space, it too needs protection in case of power loss. In addition, it is possible for the mapping table to wear out before other parts of the flash memory has, prematurely ending the life of a storage device. This is usually avoided in enterprise devices by allocating an oversized space for spares, although more durable forms of storage like
MRAM Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory which stores data in magnetic domains. Developed in the mid-1980s, proponents have argued that magnetoresistive RAM will eventually surpass competing tec ...
has been proposed for FTL too.


Garbage collection

Once every block of a solid-state storage device has been written one time, the flash controller will need to return to some of the initial blocks which no longer have current data (also called stale blocks). The data in these blocks were replaced with newly written blocks and now they are waiting to be erased so that new data can be written into them. This is a process called ''
garbage collection Waste collection is a part of the process of waste management. It is the transfer of solid waste from the point of use and disposal to the point of treatment or landfill. Waste collection also includes the curbside collection of recyclabl ...
'' (GC). All SSDs, CF Cards, and other flash storage devices will include some level of garbage collection. The speed at which a flash controller will do this can vary.


References

{{DEFAULTSORT:Flash Memory Controller Computer memory Solid-state computer storage