Flash memory. Flash memory differs from main memory in that stored data
are retained even if power is turned off (or fails). There are two types of flash
memory, called NAND and NOR flash. Of these, NAND flash has a much
higher storage capacity for a given cost, and is widely used for data storage
in devices such as cameras, music players, and cell phones, and increasingly,
in laptop computers as well. Flash memory has a lower cost per byte than
main memory, in addition to being nonvolatile; that is, it retains stored data
even if power is switched off.
Flash memory is alsowidely used for storing data in “USB keys,” which can
be plugged into the Universal Serial Bus (USB) slots of computing devices.
Such USB keys have become a popular means of transporting data between
computer systems (“floppy disks” played the same role in earlier days, but
their limited capacity has made them obsolete now).
Flash memory is also increasingly used as a replacement for magnetic
disks for storing moderate amounts of data. Such disk-drive replacements
are called solid-state drives. As of 2009, a 64 GB solid-state hard drive costs
less than $200, and capacities range up to 160 GB. Further, flash memory
is increasingly being used in server systems to improve performance by
caching frequently used data, since it provides faster access than disk, with
larger storage capacity than main memory (for a given cost).