Computer memory can be considered as a location where information is stored, that can be utilized by the operating system, software programs, hardware devices and by the user. There are two types of memory as volatile memory and non-volatile memory on the basis of their information retention ability. Volatile memory is memory that loses its contents when the computer or hardware device loses power. Computer RAM (Random Access Memory) is a good example of a volatile memory. Non-volatile memory, (sometimes abbreviated as NVRAM) is memory that keeps its contents even if the power is lost. CMOS (Complementary metal-oxide semiconductor) is a good example of a non-volatile memory.
Memory is capable of retaining digital information under certain conditions. This retained material might be operational code or data files or a combination of the two. An ideal memory subsystem optimises density, preserves critical material in a non-volatile condition, is easy to program and re-program, can be read fast, and is cost-effective for the application. Some memory technologies meet one or more of these requirements very well, but there are limitations that can prevent the product from becoming a genuine solution, especially in newer applications.
Non-volatile memory
Non-volatile memory is the type of computer memory that can retain the stored information even when not powered. Read-only memory, flash memory, most types of magnetic computer storage devices (e.g. hard disks, floppy disk drives, and magnetic tape), optical disc drives, and early computer storage methods such as paper tape and punch cards are some examples of non-volatile memory.
Non-volatile memory is typically used for the task of secondary storage or long-term persistent storage. The most widely used form of primary storage today is a volatile form of random access memory (RAM). When the computer is shut down, anything contained in RAM gets lost. Unfortunately, most forms of non-volatile memory have limitations that make them unsuitable for use as primary storage. Typically, non-volatile memory either costs more or performs worse than volatile random access memory. But, in terms of secondary storage, non-volatile memory plays a major role.
Several companies are working on developing non-volatile memory systems comparable in speed and capacity to volatile RAM. For instance, IBM is currently developing MRAM (Magnetic RAM). Not only would such technology save energy, but it would allow for computers that could be turned on and off almost instantly, bypassing the slow start-up and shutdown sequence.
Non-volatile data storage can be categorized in electrically addressed systems random access memory and mechanically addressed systems hard disks, optical disc, magnetic tape, holographic memory and such. Electrically addressed systems are expensive, but fast, whereas mechanically addressed systems have a low price per bit, but are slow. Non-volatile memory may one day eliminate the need for comparatively slow forms of secondary storage systems, which include hard disks.
Electrically addressed non-volatile memories based on charge storage can be categorised according to their write mechanism as follows.
Mask-programmed ROM
The mask-programmed ROM was one of the earliest forms of non-volatile read-only memory. It was pre-wired at the design stage to contain specific data. Once the mask was used to manufacture the integrated circuits, the data was cast in stone and could not be changed. Whatever '1's and '0's were in memory when it left the factory were there for life. The mask ROM was therefore useful only for large-volume production, such as for read-only memories containing the start-up code in early microcomputers. Due to the very high initial cost and inability to make revisions, the mask ROM is rarely if ever used in new designs.
Programmable ROM (PROM)
The next approach was to create a chip which was initially blank. The resultant was the programmable ROM. Originally it contained silicon or metal fuses, which would be selectively "blown" or destroyed by a device programmer or PROM programmer in order to change '0's to '1's. Once the bits were changed, there was no way to restore them to their original condition. Therefore, even though the Programmable ROM was non-volatile, still it was somewhat inflexible.
Erasable PROMs (EPROM)
The original erasable non-volatile memories were EPROM's. These could be readily identified by the distinctive quartz window in the centre of the chip package. There are two classes of non-volatile memory chips based on EPROM technology, the UV-erase EPROMs and one-time programmable ROMs.
UV-erase EPROM
These operated by trapping an electrical charge on the gate of a field-effect transistor in order to change a '1' to a '0' in memory. To remove the charge, the chip should be placed under an intense short-wavelength fluorescent ultraviolet lamp for 20-30 minutes. This action will return the entire chip to its original blank (all '1's) state.
OTP (one-time programmable) ROM
An OTP is electrically an EPROM, but with the quartz window physically missing. Like the fuse PROM it can be written once, but cannot be erased. It has largely replaced PROM chips in electronic production as an EPROM with no window is inexpensive to manufacture and can be programmed using identical equipment to that used to write to the UV-window EPROM.
Electrically Erasable PROM (EEPROM)
Electrically erasable PROM's have the advantage of being able to selectively erase any part of the chip without the need to erase the entire chip and without the need to remove the chip from the circuit. While an erase and rewrite of a location appears nearly instantaneous to the user, the write process is slightly slower than the read process. The chip can be read at full system speeds.
The limited number of times a single location can be rewritten is usually in the range of 10,000-100,000. The capacity of an EEPROM also tends to be smaller than that of other non-volatile memories. However, EEPROMs are useful for storing settings or configuration for devices ranging from dial-up modems to satellite receivers.
Flash Memory
The flash memory chip is a close relative to the EEPROM. It differs in that it can only be erased one "block" or "page" at a time. Capacity is substantially larger than that of an EEPROM, making these chips a popular choice for digital cameras and desktop PC BIOS chips.
Battery-Backed Static RAM
This is a volatile memory chip to which a battery has been added in order to preserve the contents in the absence of external power. These used to be typically manufactured with CMOS technology to minimise power consumption. A lithium cell can easily power a small memory for a few years. It is now common to use SDRAM with a Lithium ion battery. It is possible to preserve a gigabyte of such memory for days. The settings from the BIOS menus which appear on start-up on most desktop PCs are stored in battery-backed CMOS static RAM as a battery must already be present on the mainboard to keep the real-time clock running when the computer is not in use.
Mechanically addressed systems
Tape, Hard disk, Optical disk, Nanodrive, Holographic storage are some of the mechanically addressed systems.
What is Flash Memory?
Flash memory is a type of non-volatile computer memory that can be electrically erased and reprogrammed. It is a technology that is primarily used in memory cards and USB flash drives (thumb drives, handy drive, memory stick, flash stick, jump drive), which are used for general storage and transfer of data between computers and other digital products. It is a specific type of EEPROM 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. Examples of applications include PDAs and laptop computers, digital audio players, digital cameras and mobile phones. It has also gained some popularity in the game console market, where it is often used instead of EEPROMs or battery-powered static RAM (SRAM) for game save data.
Flash memory stores information in an array of floating gate transistors called 'cells' each of which traditionally stores one bit of information. The two major types of flash memory are known as NOR architecture and NAND architecture.
NOR Flash Memory
NOR, named after the specific data mapping technology (Not OR), is a high-speed Flash technology. NOR Flash memory provides high-speed random-access capabilities, being able to read and write data in specific locations in the memory without having to access the memory in sequential mode. NOR Flash allows the retrieval of data as small as a single byte. NOR Flash excels in applications where data is randomly retrieved or written. NOR is most often found built into cellular phones (to store the phone’s operating system) and PDAs and is also used in computers to store the BIOS program that runs to provide the start-up functionality.
NAND Flash Memory
NAND Flash was invented after NOR Flash, and is named after the specific mapping technology used for data (Not AND). NAND Flash memory reads and writes in high speed sequential mode, handling data in small block sizes (“pages”). NAND Flash can retrieve or write data as single pages, but cannot retrieve individual bytes like NOR Flash. NAND Flash memory is commonly found in solid-state hard drives, audio and video Flash media devices, television set-top boxes, digital cameras, cell phones (for data storage) and other devices where data is generally written or read sequentially. For example, most digital cameras use NAND-Flash based digital film, as pictures are usually taken and stored sequentially.
NAND-Flash is also more efficient when pictures are read back, as it transfers whole pages of data very quickly. As a sequential storage medium, NAND Flash is ideal for data storage. NAND Flash memory is less expensive than NOR Flash memory, and can accommodate more storage capacity in the same die size.
Flash memory which stores a single bit per cell (e.g., a value of “0” or “1” per cell) is known as Single-Level Cell (SLC) Flash. Newer flash memory devices, sometimes referred to as Multi-Level Cell devices, can store more than one bit per cell, by using more than two levels of electrical charges, placed on the floating gate of a cell.
General Flash characteristics
Although flash shares many characteristics with EPROM and EEPROM, current-generation flash differs in that ERASE operations are done in blocks. Flash, EPROM and EEPROM all must be erased before being written. When erasing EPROM, the entire chip is erased with a UV light source. EEPROM is automatically erased before a WRITE on a byte basis. Flash is either erased in blocks (boot block or sectored erase block flash) or the entire chip at once (bulk erase flash).
Boot block devices have erase blocks that vary in size from 4KB to 128KB. Sectored erase block flash has blocks of equal size, some with no additional hardware protection. This configuration is suited for mass storage or firmware applications. Although flash is erased on a block basis, WRITE and READ operations are done on a random byte or word basis.
Flash Cell Structure
Most flash devices share basically the same cell structure as the EPROM cell. Both the flash and EPROM cells are dual polysilicon (poly), floating-gate CMOS field effect transistors. The first poly layer is isolated from the control gate by an interpoly dielectric layer and from the substrate by a thin oxide layer.
This isolation allows the first poly layer (floating gate) to store charge. The second poly layer is connected to the wordline and functions as the control gate. However, there are two main differences between a flash cell and an EPROM cell that allow for electrical erase of the flash cell. Flash has a thinner oxide layer of approximately 100 angstroms to enable Fowler-Nordheim tunneling of electrons from the floating gate during an ERASE. In addition, flash has a deeper source diffusion to further enhance ERASE performance.
How flash Cell Works?
Automated write and erase
One feature that many current-generation flash devices have is an on-chip state machine that automates WRITE and ERASE. First-generation flash and EPROM typically require the host system or programmer to execute complex algorithms to write and erase. These algorithms are required to write any flash cell, but on current-generation flash the algorithms are executed internally by a state machine. This frees the host system to do other tasks while the state machine writes or erases the flash memory and simplifies designing of flash by reducing software overhead necessary to write or erase the device.
During a WRITE, the state machine controls the WRITE pulse timing to the cell, tracks the number of pulses issued, controls the voltages applied to the cell and verifies that the data was written correctly. When executing an ERASE, the state machine first writes all locations within the block to “0” so that each cell contains uniform charge. The state machine then issues the ERASE pulses to the cells within the block and monitors the ERASE for completion. At any time during a WRITE or ERASE, the status register may be read to monitor the WRITE or ERASE in progress or to check for the completion of the WRITE or ERASE cycle.
WRITE
Flash and EPROM implement hot electron injection to place charge on the floating gate during a WRITE. During a WRITE, a high programming voltage (VPP = 12V) is placed on the control gate. This forces an inversion region to form in the p-type substrate. The drain voltage is increased to approximately half the control gate voltage (6 volts) while the source is grounded (0 volts), increasing the voltage drop between the drain and source. (Figure 2.) With the inversion region formed, the current between drain and source increases. The resulting high electron flow from source to drain increases the kinetic energy of the electrons. This causes the electrons to gain enough energy to overcome the oxide barrier and collect on the floating gate.
After the WRITE is completed, the negative charge on the floating gate raises the cell’s threshold voltage (VT) above the wordline logic 1 voltage. When a written cell’s wordline is brought to a logic 1 during a READ, the cell will not turn on. The sense amps detect and amplify the cell current and output a “0” for a written cell.
ERASE
Flash employs “Fowler-Nordheim tunnelling” (a process whereby electrons tunnel through a barrier in the presence of a high electric field) to remove charge from the floating gate to bring it to the erased state. Using high-voltage source erase, the source is brought to a high voltage (VPP = 12V), the control gate grounded (0 volts) and the drain left unconnected. (Figure 3.) The large positive voltage on the source, as compared to the floating gate, attracts the negatively charged electrons from the floating gate to the source through the thin oxide. Because the drain is not connected, the ERASE function is a much lower current-per-cell operation than a WRITE that uses hot electron injection.
After the ERASE is completed, the lack of charge on the floating gate lowers the cell’s VT below the wordline logic 1 voltage. When an erased cell’s wordline is brought to a logic 1 during a READ, the transistor will turn on and conduct more current than a written cell. Some flash devices use Fowler-Nordheim tunneling for WRITEs as well as ERASEs.
Array architecture
Flash products are available in symmetrical as well as asymmetrical blocking architecture. The flexible blocking architecture enables system integration of code and data within a single flash device.
During a READ of a byte or word of data, the addressed row (wordline) is brought to a logic 1 level (> VT of an erased cell). This condition turns on erased cells which allow current to flow from drain to source, while written cells remain in the off state with little current flow from drain to source. The cell current is detected by the sense amps and amplified to the appropriate logic level to the outputs. All other wordlines within the array remain low. Because only one wordline needs to be controlled at a time during a READ, the decode overhead is minimized. As a result, high random-access READ performance is achieved with the NOR architecture.
Flash memory devices
There are many products can be identified that use the flash technology. USB flash drive is a device commonly used by most of the people in there day to day life in many activities. Therefore it is important that studying the functioning and features with the USB flash drive.
USB flash drive
They are NAND-type flash memory data storage devices integrated with a USB (universal serial bus) interface. They are typically small, lightweight, removable and rewritable. Capacity is limited only by current flash memory densities, although cost per megabyte may increase rapidly at higher capacities due to the expensive components.
USB flash drives offer potential advantages over other portable storage devices, particularly the floppy disk. They are more compact, generally faster, hold more data, and are more reliable (due to both their lack of moving parts, and their more durable design) than floppy disks. These types of drives use the USB mass storage standard, supported natively by modern operating systems such as Linux, Mac OS X, Unix, and Windows.
A flash drive consists of a small printed circuit board encased in a plastic or metal casing, making the drive sturdy enough to be carried about in a pocket, as a key fob, or on a lanyard. Only the USB connector protrudes from this protection, and is usually covered by a removable cap. Most flash drives use a standard type-A USB connection allowing them to be connected directly to a port on a personal computer.
To access the data stored in a flash drive, the drive must be connected to a computer, either by plugging it into a USB host controller built into the computer, or into a USB hub. Flash drives are active only when plugged into a USB connection and draw all necessary power from the supply provided by that connection. However, some flash drives, especially high-speed drives utilizing the USB 2.0 standard, may require more power than the limited amount provided by a bus-powered USB hub, such as those built into some computer keyboards or monitors. These drives will not work unless plugged directly into a host controller or a self-powered hub.
Components of a USB flash drive
One end of the device is fitted with a single male type-A USB connector. Inside the plastic casing is a small printed circuit board. Mounted on this board is some simple power circuitry and a small number of surface-mounted integrated circuits (ICs). Typically, one of these ICs provides an interface to the USB port, another drives the onboard memory, and the other is the flash memory.
Essential components
There are typically four parts to a flash drive:
- Male type-A USB connector - provides an interface to the host computer.
- USB mass storage controller - implements the USB host controller and provides a linear interface to block-oriented serial flash devices while hiding the complexities of block-orientation, block erasure, and wear levelling, or wear balancing. The controller contains a small RISC microprocessor and a small amount of on-chip ROM and RAM.
- NAND flash memory chip - stores data. NAND flash is typically also used in digital cameras.
- Crystal oscillator - produces the device's main 12 MHz clock signal and controls the device's data output through a phase-locked loop.
The typical device may also include:
- Jumpers and test pins - for testing during the flash drive's manufacturing or loading code into the microprocessor.
- LEDs - indicate data transfers or data reads and writes.
- Write-protect switches - indicate whether the device should be in "write-protection" mode.
- Unpopulated space - provides space to include a second memory chip. Having this second space allows the manufacturer to develop only one printed circuit board that can be used for more than one storage size device, to meet the needs of the market.
- USB connector cover or cap - reduces the risk of damage due to static electricity, and improves overall device appearance. Some flash drives do not feature a cap, but instead have retractable USB connectors. Other flash drives have a "swivel" cap that is permanently connected to the drive itself and eliminates the chance of losing the cap.
- Transport aid - In some cases, the cap or the main body contains a hole suitable for connection to a key chain or lanyard or to otherwise aid transport and storage of the USB flash device.
USB flash drives have been integrated into other things such as a watch or a pen.
Common uses of a USB flash drive
Personal data transport
The most common use of flash drives is to transport and store personal files such as documents, pictures and video. Individuals also store medical alert information for use in emergencies and for disaster preparation.
Computer repair
Flash drives are useful in the PC repair field as a means to transfer recovery and antivirus software to infected PCs, while allowing a portion of the host machine's data to be archived in case of emergency.
System administration
Flash drives are particularly popular among system and network administrators, who load them with configuration information and software used for system maintenance, troubleshooting, and recovery.
Application carriers
Flash drives are used to carry applications that run on the host computer without requiring installation. U3, backed by flash drive vendors, offers an API to flash drive-specific functions. “airWRX” is an application framework that runs from a flash drive and turns its PC host and other nearby PCs into a multi-screen, web-like work environment.
Audio players
Many companies make solid state digital audio players in a small form factor, essentially producing flash drives with sound output and a simple user interface. Probably the best-known of these has been Apple Computer's “iPod shuffle”, and the Creative Labs “MuVo”.
To boot operating systems
In a way similar to that used in LiveCD, one can launch any operating system from a bootable flash drive, known as a “LiveUSB”.
Windows
In Windows Vista, the “ReadyBoost” feature allows use of a flash drive to augment system memory.
Strengths and weaknesses of Flash Memory Devices
Many Flash Memory devices are nearly impervious to the scratches and dust that were problematic for previous forms of portable storage, such as compact discs and floppy disks. Their durable solid-state design means they often survive casual abuse. This makes them ideal for transporting personal data or work files from one location to another, such as from home to office or for carrying around personal data that the user typically wants to access in a variety of places. The near ubiquity of USB support on modern computers means that such a drive will work in most places. A drawback to the small size is that they are easy to misplace, leave behind, or otherwise lose.
Flash memory devices are relatively dense form of storage, where even the cheapest will store dozens of floppy disks worth of data. Some can hold more data than a CD (700 MB). Top of the line flash drives can store more data than a double-sided dual-layer DVD - even 64 GB and more.
USB Flash drives implement the USB mass storage device class, meaning that most modern operating systems can read and write to flash drives without any additional device drivers. The flash drives present a simple block-structured logical unit to the host operating system, hiding the individual complex implementation details of the various underlying flash memory devices. The operating system can use whatever type of file system or block addressing scheme it wants. Some computers have the ability to boot up from flash drives.
All flash memory devices can sustain only a limited number of write and erase cycles before failure. Mid-range flash devices under normal conditions will support several hundred thousand cycles, although write operations will gradually slow as the device ages. This should be a consideration when using a flash drive to run application software or an operating system. To address this, as well as space limitations, some developers have produced special versions of operating systems (such as Linux) or commonplace applications (such as Mozilla Firefox) designed to run from flash drives. These are typically optimized for size and configured to place temporary or intermediate files in the computer's main RAM memory rather than store them temporarily on the flash drive.
Most USB flash drives do not include a write-protect mechanism. Such a switch on the housing of the drive itself would keep the host computer from writing or modifying data on the drive. Write-protection would make a device suitable for repairing virus-contaminated host computers without infecting the USB flash drive itself.
Flash devices are much more tolerant of abuse than mechanical drives, but can still be damaged or have data corrupted by severe physical impacts. Improperly wired USB ports can also destroy the circuitry of a flash drive, a danger in home-built desktop PCs.
Comparison to other portable memory forms:
Flash storage devices are often compared to other common, portable, swappable data storage devices, such as floppy disks, Zip disks, miniCD / miniDVD, CD-R/CD-RW and DVD-RW discs.
Floppy disks were the first popular method of file transport, but have been almost completely phased out due to their low capacity, low speed, and low durability. Virtually all new computers no longer include floppy drives, and do include USB ports, the Apple iMac being the first to ship this way. However, floppy disks are still in use because of their low cost; they are often the easiest or only way to share files with older systems; floppy drives can be added to new systems either internally or externally.
Attempts to extend the floppy standard (such as the Imation SuperDisk) were not successful because of a reputation for unreliability and the lack of a single standard for PC vendors to adopt. The Iomega Zip drive enjoyed some popularity, but never reached the point of ubiquity in computers. Also, the larger sizes of Zip (now up to 750 MB) cannot be read on older drives. Unless one were to carry an external drive, their usefulness as a means of moving data was rather limited. The cost per megabyte was fairly high, with individual disks. Because moving parts are involved and the material used for creating the storage medium in Zip disks is similar to that used in floppy disks, Zip disks have a high risk of failure and data loss compared to flash drives. Larger removable storage media, like Iomega's Jaz drive, had even higher costs for both drives and media, and as such were not pervasively adopted as a floppy alternative.
CD-R and CD-RW are swappable storage media alternatives. Unlike Zip and floppy drives, DVD and CD recorders are now common in personal computer systems. CD-Rs can be written to only once. But CD-RWs are rated at up to 1,000 erase/write cycles, and modern NAND-based flash drives often last for 500,000 or more erase/write cycles. Optical storage devices also usually are slower than their flash-based counterparts. And, compact discs with a 12 cm diameter can be inconveniently large and, unlike flash drives, cannot fit into a pocket or hang from a key chain. There are smaller CD-R media such as business card CD-Rs, which have the same dimensions as a credit card, and (slightly less convenient but have more storage) 8 cm CD-Rs. But these are harder to obtain and generally more expensive than the standard 12 cm version. There also is no standard file system for rewritable optical media. Packet-writing utilities like DirectCD and InCD exist but produce discs that are not universally readable, despite their claiming to be based on the UDF standard. The upcoming Mount Rainier standard addresses this shortcoming in CD-RW media, but it still is not supported by most DVD and CD recorders or major operating systems. As a result, CDs/DVDs are a good way to record a great deal of information cheaply but not good for making ongoing small changes to a large collection of information; flash drives' ability to do this is their major advantage.
USB Flash drives and Security
Some flash drives feature encryption of the data stored on them, generally using full disk encryption below the file system. This prevents an unauthorized person from accessing the data stored on it. The disadvantage is that the drive is accessible only in the minority of computers which have compatible encryption software, for which no portable standard is widely deployed.
Some encryption applications allow running without installation. The executable files can be stored on the USB drive, together with the encrypted file image. The encrypted partition can be accessed on any computer running Microsoft Windows. Other flash drives allow the user to configure secure and public partitions of different sizes. Executable files for Windows, Macintosh, and Linux may be on the drive, depending on manufacturer support. Some security software may require administrative rights on the host PC to access data.
Newer flash drives support biometric fingerprinting to confirm the user's identity. As of mid-2005, this was a relatively costly alternative to standard password protection offered on many new USB flash storage d
evices. Most fingerprint scanning drives rely upon the host operating system to validate the fingerprint via a software driver, restricting the drive to Microsoft Windows computers.
Some manufacturers deploy physical authentication tokens in the form of a flash drive. These are used to control access to a sensitive system by containing encryption keys or, more commonly, communicating with security software on the target machine. The system is designed so the target machine will not operate except when the flash drive device is plugged into it. Some of these "PC lock" devices also function as normal flash drives when plugged into other machines.
All such forms of data protection security involve an increased risk of loss of access to the data by the legitimate owner/user.
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