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Computerworld June 7, 2010
Flash memory
is inside your smartphone, GPS, MP3 player, digital camera, PC and the USB drive on your key chain.
Solid-state drives (SSD) using
flash memory
are replacing hard drives in
netbooks
and
PCs
and even some server
installations. Needing no
batteries
or other power to retain data, flash is convenient and relatively
foolproof
.
Flash memory
is a solid-state chip that maintains stored data without any external
power source
. It is commonly
used in portable electronics and
removable storage
devices, and to replace com-puter hard drives.
As with other solid-state technologies,
flash memory
's history includes rapidly increasing capacity, ever-smaller
physical sizes and continually falling prices.
Flash memory
is a type of electronically erasable programmable read-only memory (
EEPROM
),
memory chips
that
retain information without requiring power. (This is different from flash RAM, which does need power to retain data.)
Regular
EEPROM
erases content byte by byte; most
flash memory
erases data in whole blocks, making it
suitable
for
use with applications where large amounts of data require frequent updates. Inside the flash chip, data is stored in
cells protected by floating gates. Tunneling electrons change the gate's electronic
charge
in "a flash" (hence the
name), clearing the cell of its contents so it can be rewritten.
Flash
memory devices
use two different logical technologies -- NOR and
NAND
-- to map data. NOR flash
provides
high-speed
random access, reading and writing data in specific memory lo-cations; it can retrieve as little as
a single byte. NOR is used to store cell phones' operating systems; it's also used in computers for the BIOS program
that runs at start-up.
NAND
flash reads and writes sequentially at high speed, handling data in small
blocks called
pages. This flash is used
in solid-state and
USB flash drives
,
digital cameras
, audio and
video players
, and TV
set-top boxes
.
NAND
flash reads
faster than it writes, quickly transferring whole pages of data. Less expensive than NOR flash,
NAND
technology
offers higher capacity for the same-size silicon.
As a
NAND
chip
wears
out, erase/program operations slow down considerably, causing more retries and bad
block
remapping
. Moving many small files could further
degrade
transfer rates
. Catastrophic failure happens only
with extended use (after thousands of writes and
accesses); periodic
backup and replacement forestall this problem.
USB drives
: Introduced in 2002,
USB drives
encapsulate flash with a
memory controller
in a small package offering
high capacity, fast
transfer rates
,
flexibility
and
convenience
; some feature built-in hardware encryption and
password
protection
. Compared with floppy or
optical drives
,
USB flash drives
store more data and provide easy file transfer
between most devices with a USB interface.
In December 2004, Computerworld described a 2GB flash drive selling for more than $400; nowadays, 2GB devices
can commonly be found for under $10. This February, Kingston Tech-nology Corp. announced U.S. availability of a
256GB flash drive -- the biggest yet -- for $1,100.
Memory cards
: These have evolved from the matchbook-size
CompactFlash
cards introduced in 1994 through 2001's
postage-stamp-size
Secure Digital
cards to the latest
miniSD
and microSD cards, with higher capacities and faster
transfer speeds at every step.
Solid-state drives: The newest
flash memory
application,
SSDs
can replace a computer's hard drive. They have no
moving parts, so mechanical failure is near zero. Solid-state drives are quieter and smaller than hard drives, and they
provide faster response, access and boot-up times but consume much less power and run
cooler
. Traditional hard
drives currently offer greater ca-pacity and a lower price, but this will likely change. Early concerns that
flash
memory
's
finite
number of erase/
write cycles
would be a problem are abating as
warranties
for flash-
based
SSDs
approach
those of hard drives.
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