en.wikipedia.org/wiki/Flash_memory
edit 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.
transistor, 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. Because the FG is isolated by its insulating oxide layer, any electrons placed on it get trapped there and thus store the information.
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.
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.
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 cm^2.
edit 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.
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.
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.
edit 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.
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.
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 uses 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 GB flashdrives are still very expensive for a normal consumer.
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