Flash Memory
Flash memory is 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 (pen drive, thumb drives, handy drive, memory stick, flash stick, jump drive) 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 SRAM ("Save RAM", which was not necessarily static RAM) for game save data.
Flash memory is non-volatile, which means that it does not need power to maintain the information stored in the chip. In addition, flash memory offers fast read access times (although not as fast as volatile DRAM memory used for main memory in PCs) and better kinetic shock resistance than hard disks. These characteristics explain the popularity of flash memory for applications such as storage on battery-powered devices. Another feature of flash memory is that when packaged in a "memory card", it is enormously durable, being able to withstand intense pressure, extremes of temperature and immersion in water.
Principles of operation
Flash memory stores information in an array of floating-gate transistors, called "cells". In traditional single-level cell (SLC) devices, each cell stores only one bit of information. Some newer flash memory, known as multi-level cell (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.
History
Flash memory (both NOR and NAND types) was invented by Dr. Fujio Masuoka while working for Toshiba in 1984. According to Toshiba, the name "flash" was suggested by Dr. Masuoka's colleague, Mr. Shoji Ariizumi, because the erasure process of the memory contents reminded him of a flash of a camera. Dr. Masuoka presented the invention at the IEEE 1984 International Electron Devices Meeting (IEDM) held in San Francisco, California. Intel saw the massive potential of the invention and introduced the first commercial NOR type flash chip in 1988.
Limitations
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 must 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.
Another limitation is that flash memory has a finite number of erase-write
cycles (most commercially available flash products are guaranteed to
withstand 100,000 write-erase-cycles for block 0, and no guarantees for
other blocks)[1]. This effect is partially offset by some chip firmware or
file system drivers by counting the writes and dynamically remapping the
blocks in order to spread the write operations between the sectors; this
technique is called wear levelling.
Capacity
Common flash memory parts
(individual internal components or "chips") range widely in capacity from
kilobits to several gigabits each. Multiple chips are often arrayed to
achieve higher capacities for use in devices such as the iPod nano or
SanDisk Sansa e200. The capacity of flash chips generally follows Moore's
Law because they are produced with the same processes used to manufacture
other integrated circuits. However, there have also been jumps beyond
Moore's Law due to innovations in technology.
In 2005, Toshiba and SanDisk developed a NAND flash chip capable of storing
1 GB of data using Multi-level Cell (MLC) technology, capable of storing 2
bits of data per cell. In September 2005, Samsung Electronics announced that
it had developed the world’s first 2 GB chip.[8]
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 of 2006, Samsung announced an 8 GB chip produced using a 40 nm
manufacturing process.[9]
Flash memory products for end-users, such as memory cards and USB drives,
were as of mid 2007 widely available in capacities ranging from 512
megabytes to 16 gigabytes. The highest capacity is found in USB devices with
a current record of 128 GB.[10]
Hitachi (formerly the OEM hard disk unit supplying IBM) has a competing
hard-drive mechanism, the Microdrive, that can fit inside the shell of a
type II CompactFlash card. It has a capacity up to 8 GB.
Contributed by: Mohit Shrivastava, Admin Head - Aptech Singrauli Network