Editing Flash memory (section)

===Block erasure===
One limitation of flash memory is that it can be erased only 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&nbsp;flash) offers random-access read and programming operations but does not offer arbitrary random-access rewrite or erase operations. A location can, however, be rewritten as long as the new value's 0 bits are a superset of the over-written values. For example, a [[nibble]] value may be erased to 1111, then written as 1110. Successive writes to that nibble can change it to 1010, then 0010, and finally 0000. Essentially, erasure sets all bits to 1, and programming can only clear bits to 0.<ref>
[http://ww1.microchip.com/downloads/en/AppNotes/doc2546.pdf "AVR105: Power Efficient High Endurance Parameter Storage in Flash Memory"].
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Some file systems designed for flash devices make use of this rewrite capability, for example [[YAFFS|YAFFS1]], to represent sector metadata. Other flash file systems, such as [[YAFFS|YAFFS2]], never make use of this "rewrite" capability – they do a lot of extra work to meet a "write once rule".

Although data structures in flash memory cannot be updated in completely general ways, this allows members to be "removed" by marking them as invalid. This technique may need to be modified for [[multi-level cell]] devices, where one memory cell holds more than one bit.

Common flash devices such as [[USB flash drive]]s and memory cards provide only a block-level interface, or [[flash translation layer]] (FTL), which writes to a different cell each time to wear-level the device. This prevents incremental writing within a block; however, it does help the device from being prematurely worn out by intensive write patterns.