Mass Storage Basics - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 14

Technology
Both flash memory and EEPROM provide non-volatile, electrically erasable
storage. Compared to EEPROM, flash-memory cells are physically smaller,
can withstand more erase/write cycles, and are cheaper to manufacture. The
main disadvantage of flash memory is that unlike EEPROM, flash memory
is erasable only in blocks, not by individual byte. Even so, for most storage
devices, flash memory is the more practical choice, while EEPROM is useful
for storing infrequently changed configuration settings.
Two flash-memory technologies in popular use are NOR and NAND.
NOR flash is suited for storing program code, where the CPU wants fast
read access but rarely writes to the memory. NOR flash has fast read times
but slow erase and write times. NOR flash has low density, so large amounts
of storage may require multiple chips. To access NOR flash, a CPU uses the
same data and address lines used to access other parallel memory chips.
Storage devices use NAND flash, which has fast erase and write times.
NAND flash also has lower power consumption and is much cheaper than
NOR flash. A CPU accesses NAND flash chips via data lines and command
and address registers. NAND flash has high density, so large amounts of
memory can fit in a small package. The advantages of NAND flash are so
attractive that some devices use NAND flash for program memory along
with a RAM cache to improve performance.
Three varieties of NAND flash are Old Single-level Cell (SLC), New SLC,
and Multi-level Cell (MLC). MLC memory stores multiple bits in each cell
and is popular because it's cheaper to manufacture. However, compared to
SLC memory, MLC memory supports fewer erase cycles, has slower write
times, and consumes more power. These are the typical number of erase
cycles supported by each memory type: Old SLC: 1,000,000, New SLC:
100,000, MLC: 10,000.
Wear Leveling
Wear-leveling techniques can extend the useful life of flash memory by writ-
ing to different physical locations in each erase/write cycle. A typical write
operation accesses only a portion of the memory in a flash-memory card.
Writing to different locations with each write operation helps to spread the
erase/write operations evenly to all areas of the memory and extends the life