Flash memory

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Flash memory is a form of EEPROM (Electrically Erasable Programmable Read-Only Memory) that allows multiple memory locations to be erased or written in one programming operation. In layman's terms, it is a form of rewritable memory chip that, unlike a Random Access Memory chip, holds its content without the need of a power supply. It is also an example of a Non-Volatile Read Write Memory (NVRWM). The memory is commonly used in memory cards, USB flash drives, MP3 players, digital cameras and mobile phones.

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Overview

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 (though not as fast as volatile DRAM memory used for main memory in PCs) and better shock resistance than hard disks. These characteristics explain the popularity of flash memory for applications such as storage on battery-powered devices.

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Principles of operation

Flash memory stores information in an array of floating gate transistors, called "cells", each of which traditionally stores one bit of information. Newer flash memory devices, sometimes referred to as multi-level cell devices, can store more than 1 bit per cell, by varying the number of electrons placed on the floating gate of a cell.

In NOR flash, each cell looks similar to a standard MOSFET, except that it has two gates instead of just one. One gate is the control gate (CG) like in other MOS transistors, but the second is a floating gate (FG) that is insulated all around by an oxide layer. The FG is between the CG and the substrate. Because the FG is isolated by its insulating oxide layer, any electrons placed on it get trapped there and thus store the information. When electrons are on the FG, they modify (partially cancel out) the electric field coming from the CG, which modifies the threshold voltage (Vt) of the cell. Thus, when the cell is "read" by placing a specific voltage on the CG, electrical current will either flow or not flow, depending on the Vt of the cell, which is controlled by the number of electrons on the FG. This presence or absence of current is sensed and translated into 1's and 0's, reproducing the stored data. In a multi-level cell device, which stores more than 1 bit of information per cell, the amount of current flow will be sensed, rather than simply detecting presence or absence of current, in order to determine the number of electrons stored on the FG.

A NOR flash cell is programmed (set to a specified data value) by starting up electrons flowing from the source to the drain, then a large voltage placed on the CG provides a strong enough electric field to suck them up onto the FG, a process called hot-electron injection. To erase (reset to all 1's, in preparation for reprogramming) a NOR flash cell, a large voltage differential is placed between the CG and source, which pulls the electrons off through quantum tunneling. In single-voltage devices (virtually all chips available today), this high voltage is generated by an on-chip charge pump. Most modern NOR flash memory components are divided into erase segments, usually called either blocks or sectors. All of the memory cells in a block must be erased at the same time. NOR programming, however, can generally be performed one byte or word at a time.

NAND Flash uses tunnel injection for writing and tunnel release for erasing. NAND flash memory forms the core of the removable USB interface storage devices known as keydrives.

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History

Flash memory 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 Integrated Electronics Devices Meeting held in San Jose, California. Intel 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 has a full address/data (memory) interface that allows random access to any location. This makes it suitable for storage of program code that needs to be infrequently updated, such as a computer's BIOS or the firmware of set-top boxes. Its endurance is 10,000 to 1,000,000 erase cycles. NOR-based flash was the basis of early flash-based removable media; Compact Flash was originally based on it, though later cards moved to the cheaper NAND flash.

NAND flash from Samsung and Toshiba followed in 1989. It has faster erase and write times, higher density, and lower cost per bit than NOR flash, and ten times the endurance. However its I/O interface allows only sequential access to data. This makes it suitable for mass-storage devices such as PC cards and various memory cards, and somewhat less useful for computer memory. The first NAND-based removable media format was SmartMedia, and numerous others have followed: MMC, Secure Digital, Memory Stick and xD-Picture Cards. A new generation of these formats is becoming a reality with RS-MMC (Reduced Size MultiMedia Card), the TransFlash and miniSD variants of Secure Digital and the new USB/Memory card hybrid Intelligent Stick. The new formats exhibit a greatly reduced size, usually under 4 cm².

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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. Starting with a freshly erased block, any byte within that block can be programmed. However, once a byte has been programmed, it cannot be changed again until the entire block is erased. In other words, flash memory (specifically NOR flash) offers random-access read and programming operations, but cannot offer random-access rewrite or erase operations. When compared to a hard disk drive, a further limitation is the fact that flash memory has a finite number of erase-write cycles (most commercially available EEPROM products are guaranteed to withstand 106 programming cycles,) so that care has to be taken when moving hard-drive based applications, such as operating systems, to flash-memory based devices such as CompactFlash. This effect is partially offset by some chip firmware or filesystem drivers by counting the writes and dynamically remapping the blocks in order to spread the write operations between the sectors, or by write verification and remapping to spare sectors in case of write failure.

The cost per byte of flash memory remains significantly higher than the corresponding cost of a hard disk drive, and that (on top of finite number of erase-write cycles previously mentioned) has prevented flash from becoming a solid state replacement for the hard disk drive on normal desktop and laptop computers.

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Low-level access

Low-level access to a physical flash memory by device driver software is different from accessing common memories. Whereas a common RAM will simply respond to read and write operations by returning the contents or altering them immediately, flash memories need special considerations, especially when used as program memory akin to a read-only memory (ROM).

While reading data can be performed on individual addresses on NOR memories (not on NAND memories) unlocking (making available for erase or write), erasing and writing operations are performed block-wise on all flash memories. A typical block size will be 64, 128, or 256 KiB.

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NOR memories

The read-only mode of NOR memories is similar to reading from a common memory, provided address and data bus is mapped correctly, so NOR flash memory is much like any address-mapped memory. NOR flash memories can be used as execute-in-place memory, meaning it behaves as a ROM memory mapped to a certain address.

When unlocking, erasing or writing NOR memories, special commands are written to the first page of the mapped memory. These commands are defined as the common flash interface (defined by Intel) and the flash circuit will provide a list of all available commands to the physical driver.

Apart from being used as a ROM, the NOR memories can of course also be partitioned with a file system and used as any storage device.

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NAND memories

NAND flash memories cannot provide execute-in-place due to their different construction principles. These memories are accessed much like block devices such as hard disks or memory cards. When executing software from NAND memories, virtual memory strategies are used: memory contents must first be paged into memory-mapped RAM and executed there, making the presence of a memory management unit (MMU) on the system absolutely necessary.

For this reason some systems will use a combination of NOR and NAND memories, where the NOR memory is used as software ROM and the NAND memory is partitioned with a file system and used as a random access storage area.

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Flash file systems

Because of the particular characteristics of flash memory, it is best used with specifically designed 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 over to a fresh block, remap the file pointers, then erase the old block later when it has time.

One of the earliest flash file systems was Microsoft's FFS2 (presumably preceded by FFS1), for use with MS-DOS in the early 1990s. Around 1994, the PCMCIA industry group approved the FTL (Flash Translation Layer) specification, which allowed a flash device to look like a FAT disk, but still have effective wear levelling. Other commercial systems such as FlashFX by Datalight were created to avoid patent concerns with FTL.

JFFS was the first flash file system for Linux, but it was quickly superseded by JFFS2, originally developed for NOR flash. Then YAFFS was released in 2003, dealing specifically with NAND flash, and JFFS2 was updated to support NAND flash too. However, in practice most flash media is used with the old FAT filesystem for compatibility purposes.

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Capacity

Common flash memory parts (individual internal components or "chips") range widely in capacity from kilobits to hundreds of megabits each.

Toshiba and SanDisk have developed a NAND flash chip capable of storing 8 gigabits (1 gigabyte) of data using MLC (multi-level cell) technology, capable of storing 2 bits of data per cell.

In September 2005, Samsung Electronics, by far the world's largest manufacturer of NAND flash with ~40% of bit market share, announced that it had developed the world’s first 16 gigabit and later 32 gigabit NAND flash memory chip. In March 2006, the same company announced memories with the capacity of 32 gigabyte, essentially in the same order of magnitude as smaller laptop harddrives.

With the introduction of Samsung's 16 gigabit chips came the iPod nano, a flash device available in 1 GB, 2 GB and 4 GB capacities, which use one or two 1 gigabyte Toshiba chips and one 4 gigabyte Samsung chips respectively, according to the autopsies done by Ars Technica, Systm and Impress Direct.

Up to date NAND spot pricing is available at dramexchange.

In efforts to focus on increasing capacities, 64 MB and smaller capacity flash memory has been largely discontinued, 128 MB capacity flash memory is being given a large discount to attract entry level buyers (people who do not generally require one, or use it only once in a long time), 256 MB capacity flash memory is the normal storage space for any people who do not extensively use flash memory, while more and more people are adopting 512 MB or 1 GB flash drives. 2 and 4 GB flashdrives are still rather expensive for a normal consumer.

IBM has substituted flash memory with a hard drive mechanism that can fit inside the shell of a CompactFlash card so it can last longer, it has a capacity of 1 GB and more.

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Related links

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External links

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