<s>
A	O
flash	B-Device
memory	I-Device
controller	I-Device
(	O
or	O
flash	B-Device
controller	I-Device
)	O
manages	O
data	O
stored	O
on	O
flash	B-Device
memory	I-Device
(	O
usually	O
NAND	O
flash	O
)	O
and	O
communicates	O
with	O
a	O
computer	O
or	O
electronic	O
device	O
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
controllers	I-Device
can	O
be	O
designed	O
for	O
operating	O
in	O
low	O
duty-cycle	O
environments	O
like	O
memory	B-Device
cards	I-Device
,	O
or	O
other	O
similar	O
media	B-General_Concept
for	O
use	O
in	O
PDAs	B-Application
,	O
mobile	O
phones	O
,	O
etc	O
.	O
</s>
<s>
USB	B-Protocol
flash	O
drives	O
use	O
flash	B-Device
memory	I-Device
controllers	I-Device
designed	O
to	O
communicate	O
with	O
personal	B-Device
computers	I-Device
through	O
the	O
USB	B-Protocol
port	I-Protocol
at	O
a	O
low	O
duty-cycle	O
.	O
</s>
<s>
Flash	B-Device
controllers	I-Device
can	O
also	O
be	O
designed	O
for	O
higher	O
duty-cycle	O
environments	O
like	O
solid-state	B-Device
drives	I-Device
(	O
SSD	B-Device
)	O
used	O
as	O
data	B-General_Concept
storage	I-General_Concept
for	O
laptop	B-Device
computer	I-Device
systems	O
up	O
to	O
mission-critical	O
enterprise	B-General_Concept
storage	I-General_Concept
arrays	O
.	O
</s>
<s>
After	O
a	O
flash	B-Device
storage	I-Device
device	O
is	O
initially	O
manufactured	O
,	O
the	O
flash	B-Device
controller	I-Device
is	O
first	O
used	O
to	O
format	O
the	O
flash	B-Device
memory	I-Device
.	O
</s>
<s>
This	O
ensures	O
the	O
device	O
is	O
operating	O
properly	O
,	O
it	O
maps	O
out	O
bad	O
flash	B-Device
memory	I-Device
cells	O
,	O
and	O
it	O
allocates	O
spare	O
cells	O
to	O
be	O
substituted	O
for	O
future	O
failed	O
cells	O
.	O
</s>
<s>
Some	O
part	O
of	O
the	O
spare	O
cells	O
is	O
also	O
used	O
to	O
hold	O
the	O
firmware	B-Application
which	O
operates	O
the	O
controller	O
and	O
other	O
special	O
features	O
for	O
a	O
particular	O
storage	O
device	O
.	O
</s>
<s>
A	O
directory	O
structure	O
is	O
created	O
to	O
allow	O
the	O
controller	O
to	O
convert	O
requests	O
for	O
logical	O
sectors	O
into	O
the	O
physical	O
locations	O
on	O
the	O
actual	O
flash	B-Device
memory	I-Device
chips	O
.	O
</s>
<s>
When	O
the	O
system	O
or	O
device	O
needs	O
to	O
read	O
data	O
from	O
or	O
write	O
data	O
to	O
the	O
flash	B-Device
memory	I-Device
,	O
it	O
will	O
communicate	O
with	O
the	O
flash	B-Device
memory	I-Device
controller	I-Device
.	O
</s>
<s>
Simpler	O
devices	O
like	O
SD	O
cards	O
and	O
USB	B-Protocol
flash	O
drives	O
typically	O
have	O
a	O
small	O
number	O
of	O
flash	B-Device
memory	I-Device
die	O
connected	O
simultaneously	O
.	O
</s>
<s>
Operations	O
are	O
limited	O
to	O
the	O
speed	O
of	O
the	O
individual	O
flash	B-Device
memory	I-Device
die	O
.	O
</s>
<s>
In	O
contrast	O
,	O
a	O
high-performance	O
solid-state	B-Device
drive	I-Device
will	O
have	O
more	O
dies	O
organized	O
with	O
parallel	O
communication	O
paths	O
to	O
enable	O
speeds	O
many	O
times	O
greater	O
than	O
that	O
of	O
a	O
single	O
flash	O
die	O
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
can	O
withstand	O
a	O
limited	O
number	O
of	O
program-erase	O
cycles	O
.	O
</s>
<s>
If	O
a	O
particular	O
flash	B-Device
memory	I-Device
block	O
were	O
programmed	O
and	O
erased	O
repeatedly	O
without	O
writing	O
to	O
any	O
other	O
blocks	O
,	O
the	O
one	O
block	O
would	O
wear	O
out	O
before	O
all	O
the	O
other	O
blocks	O
thereby	O
prematurely	O
ending	O
the	O
life	O
of	O
the	O
storage	O
device	O
.	O
</s>
<s>
For	O
this	O
reason	O
flash	B-Device
controllers	I-Device
use	O
a	O
technique	O
called	O
wear	B-Application
leveling	I-Application
to	O
distribute	O
writes	O
as	O
evenly	O
as	O
possible	O
across	O
all	O
the	O
flash	O
blocks	O
in	O
the	O
SSD	B-Device
.	O
</s>
<s>
Usually	O
,	O
flash	B-Device
memory	I-Device
controllers	I-Device
also	O
include	O
the	O
"	O
flash	O
translation	O
layer	O
"	O
(	O
FTL	O
)	O
,	O
a	O
layer	O
below	O
the	O
file	O
system	O
that	O
maps	O
host	O
side	O
or	O
file	O
system	O
logical	O
block	O
addresses	O
(	O
LBAs	O
)	O
to	O
the	O
physical	O
address	O
of	O
the	O
flash	B-Device
memory	I-Device
(	O
logical-to-physical	O
mapping	O
)	O
.	O
</s>
<s>
As	O
part	O
of	O
the	O
wear	B-Application
leveling	I-Application
and	O
other	O
flash	O
management	O
algorithms	O
(	O
bad	O
block	O
management	O
,	O
read	O
disturb	O
management	O
,	O
safe	O
flash	O
handling	O
etc	O
.	O
</s>
<s>
Depending	O
on	O
the	O
usage	O
pattern	O
,	O
a	O
finer	O
mapping	O
granularity	O
can	O
significantly	O
reduce	O
the	O
flash	O
wear	O
out	O
and	O
maximize	O
the	O
endurance	O
of	O
a	O
flash	O
based	O
storage	B-General_Concept
media	I-General_Concept
.	O
</s>
<s>
In	O
addition	O
,	O
it	O
is	O
possible	O
for	O
the	O
mapping	O
table	O
to	O
wear	O
out	O
before	O
other	O
parts	O
of	O
the	O
flash	B-Device
memory	I-Device
has	O
,	O
prematurely	O
ending	O
the	O
life	O
of	O
a	O
storage	O
device	O
.	O
</s>
<s>
This	O
is	O
usually	O
avoided	O
in	O
enterprise	O
devices	O
by	O
allocating	O
an	O
oversized	O
space	O
for	O
spares	O
,	O
although	O
more	O
durable	O
forms	O
of	O
storage	O
like	O
MRAM	B-General_Concept
has	O
been	O
proposed	O
for	O
FTL	O
too	O
.	O
</s>
<s>
Once	O
every	O
block	O
of	O
a	O
solid-state	B-Device
storage	I-Device
device	I-Device
has	O
been	O
written	O
one	O
time	O
,	O
the	O
flash	B-Device
controller	I-Device
will	O
need	O
to	O
return	O
to	O
some	O
of	O
the	O
initial	O
blocks	O
which	O
no	O
longer	O
have	O
current	O
data	O
(	O
also	O
called	O
stale	O
blocks	O
)	O
.	O
</s>
<s>
This	O
is	O
a	O
process	O
called	O
garbage	B-General_Concept
collection	I-General_Concept
(	O
GC	O
)	O
.	O
</s>
<s>
All	O
SSDs	B-Device
,	O
CF	O
Cards	O
,	O
and	O
other	O
flash	B-Device
storage	I-Device
devices	O
will	O
include	O
some	O
level	O
of	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
<s>
The	O
speed	O
at	O
which	O
a	O
flash	B-Device
controller	I-Device
will	O
do	O
this	O
can	O
vary	O
.	O
</s>
