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Flash Memory: Theory and Applications
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 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 has prevented flash from becoming a solid state replacement for the
hard disk drive on normal desktop and laptop computers.
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