<s>
In	O
computing	O
,	O
memory	O
is	O
a	O
device	O
or	O
system	O
that	O
is	O
used	O
to	O
store	O
information	O
for	O
immediate	O
use	O
in	O
a	O
computer	O
or	O
related	O
computer	B-Architecture
hardware	I-Architecture
and	O
digital	B-General_Concept
electronic	O
devices	O
.	O
</s>
<s>
The	O
term	O
memory	O
is	O
often	O
synonymous	O
with	O
the	O
term	O
primary	O
storage	B-General_Concept
or	O
main	O
memory	O
.	O
</s>
<s>
Computer	B-General_Concept
memory	I-General_Concept
operates	O
at	O
a	O
high	O
speed	O
compared	O
to	O
storage	B-General_Concept
that	O
is	O
slower	O
but	O
less	O
expensive	O
and	O
higher	O
in	O
capacity	O
.	O
</s>
<s>
Besides	O
storing	O
opened	O
programs	O
,	O
computer	B-General_Concept
memory	I-General_Concept
serves	O
as	O
disk	B-General_Concept
cache	I-General_Concept
and	O
write	B-General_Concept
buffer	I-General_Concept
to	O
improve	O
both	O
reading	O
and	O
writing	O
performance	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
borrow	O
RAM	B-Architecture
capacity	O
for	O
caching	O
so	O
long	O
as	O
not	O
needed	O
by	O
running	O
software	O
.	O
</s>
<s>
If	O
needed	O
,	O
contents	O
of	O
the	O
computer	B-General_Concept
memory	I-General_Concept
can	O
be	O
transferred	O
to	O
storage	B-General_Concept
;	O
a	O
common	O
way	O
of	O
doing	O
this	O
is	O
through	O
a	O
memory	O
management	O
technique	O
called	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
Modern	O
memory	O
is	O
implemented	O
as	O
semiconductor	B-Architecture
memory	I-Architecture
,	O
where	O
data	O
is	O
stored	O
within	O
memory	B-Algorithm
cells	I-Algorithm
built	O
from	O
MOS	B-Architecture
transistors	I-Architecture
and	O
other	O
components	O
on	O
an	O
integrated	O
circuit	O
.	O
</s>
<s>
There	O
are	O
two	O
main	O
kinds	O
of	O
semiconductor	B-Architecture
memory	I-Architecture
,	O
volatile	B-General_Concept
and	O
non-volatile	B-General_Concept
.	O
</s>
<s>
Examples	O
of	O
non-volatile	B-General_Concept
memory	I-General_Concept
are	O
flash	B-Device
memory	I-Device
and	O
ROM	B-Device
,	O
PROM	B-General_Concept
,	O
EPROM	B-General_Concept
and	O
EEPROM	B-General_Concept
memory	O
.	O
</s>
<s>
Examples	O
of	O
volatile	B-General_Concept
memory	I-General_Concept
are	O
dynamic	O
random-access	B-Architecture
memory	I-Architecture
(	O
DRAM	O
)	O
used	O
for	O
primary	O
storage	B-General_Concept
,	O
and	O
static	B-Architecture
random-access	I-Architecture
memory	I-Architecture
(	O
SRAM	O
)	O
used	O
for	O
CPU	B-General_Concept
cache	I-General_Concept
.	O
</s>
<s>
Most	O
semiconductor	B-Architecture
memory	I-Architecture
is	O
organized	O
into	O
memory	B-Algorithm
cells	I-Algorithm
each	O
storing	O
one	O
bit	O
(	O
0	O
or	O
1	O
)	O
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
organization	O
includes	O
both	O
one	O
bit	O
per	O
memory	B-Algorithm
cell	I-Algorithm
and	O
multi-level	B-Device
cell	I-Device
capable	O
of	O
storing	O
multiple	O
bits	O
per	O
cell	O
.	O
</s>
<s>
The	O
memory	B-Algorithm
cells	I-Algorithm
are	O
grouped	O
into	O
words	O
of	O
fixed	O
word	O
length	O
,	O
for	O
example	O
,	O
1	O
,	O
2	O
,	O
4	O
,	O
8	O
,	O
16	O
,	O
32	O
,	O
64	O
or	O
128	O
bits	O
.	O
</s>
<s>
In	O
the	O
early	O
1940s	O
,	O
memory	O
technology	O
often	O
permitted	O
a	O
capacity	O
of	O
a	O
few	O
bytes	B-Application
.	O
</s>
<s>
The	O
first	O
electronic	O
programmable	O
digital	B-General_Concept
computer	O
,	O
the	O
ENIAC	B-Device
,	O
using	O
thousands	O
of	O
vacuum	O
tubes	O
,	O
could	O
perform	O
simple	O
calculations	O
involving	O
20	O
numbers	O
of	O
ten	O
decimal	O
digits	O
stored	O
in	O
the	O
vacuum	O
tubes	O
.	O
</s>
<s>
The	O
next	O
significant	O
advance	O
in	O
computer	B-General_Concept
memory	I-General_Concept
came	O
with	O
acoustic	O
delay-line	O
memory	O
,	O
developed	O
by	O
J	O
.	O
Presper	O
Eckert	O
in	O
the	O
early	O
1940s	O
.	O
</s>
<s>
Through	O
the	O
construction	O
of	O
a	O
glass	O
tube	O
filled	O
with	O
mercury	O
and	O
plugged	O
at	O
each	O
end	O
with	O
a	O
quartz	O
crystal	O
,	O
delay	O
lines	O
could	O
store	O
bits	O
of	O
information	O
in	O
the	O
form	O
of	O
sound	O
waves	O
propagating	O
through	O
the	O
mercury	O
,	O
with	O
the	O
quartz	O
crystals	O
acting	O
as	O
transducers	B-Algorithm
to	O
read	O
and	O
write	O
bits	O
.	O
</s>
<s>
Two	O
alternatives	O
to	O
the	O
delay	O
line	O
,	O
the	O
Williams	B-General_Concept
tube	I-General_Concept
and	O
Selectron	O
tube	O
,	O
originated	O
in	O
1946	O
,	O
both	O
using	O
electron	O
beams	O
in	O
glass	O
tubes	O
as	O
means	O
of	O
storage	B-General_Concept
.	O
</s>
<s>
Using	O
cathode	B-Device
ray	I-Device
tubes	I-Device
,	O
Fred	O
Williams	O
invented	O
the	O
Williams	B-General_Concept
tube	I-General_Concept
,	O
which	O
was	O
the	O
first	O
random-access	B-Architecture
computer	I-Architecture
memory	I-Architecture
.	O
</s>
<s>
The	O
Williams	B-General_Concept
tube	I-General_Concept
was	O
able	O
to	O
store	O
more	O
information	O
than	O
the	O
Selectron	O
tube	O
(	O
the	O
Selectron	O
was	O
limited	O
to	O
256	O
bits	O
,	O
while	O
the	O
Williams	B-General_Concept
tube	I-General_Concept
could	O
store	O
thousands	O
)	O
and	O
less	O
expensive	O
.	O
</s>
<s>
The	O
Williams	B-General_Concept
tube	I-General_Concept
was	O
nevertheless	O
frustratingly	O
sensitive	O
to	O
environmental	O
disturbances	O
.	O
</s>
<s>
Efforts	O
began	O
in	O
the	O
late	O
1940s	O
to	O
find	O
non-volatile	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
It	O
was	O
developed	O
by	O
Frederick	O
W	O
.	O
Viehe	O
and	O
An	O
Wang	O
in	O
the	O
late	O
1940s	O
,	O
and	O
improved	O
by	O
Jay	O
Forrester	O
and	O
Jan	O
A	O
.	O
Rajchman	O
in	O
the	O
early	O
1950s	O
,	O
before	O
being	O
commercialised	O
with	O
the	O
Whirlwind	B-Device
computer	I-Device
in	O
1953	O
.	O
</s>
<s>
Magnetic-core	O
memory	O
was	O
the	O
dominant	O
form	O
of	O
memory	O
until	O
the	O
development	O
of	O
MOS	B-Architecture
semiconductor	B-Architecture
memory	I-Architecture
in	O
the	O
1960s	O
.	O
</s>
<s>
The	O
first	O
semiconductor	B-Architecture
memory	I-Architecture
was	O
implemented	O
as	O
a	O
flip-flop	B-General_Concept
circuit	O
in	O
the	O
early	O
1960s	O
using	O
bipolar	O
transistors	O
.	O
</s>
<s>
Semiconductor	B-Architecture
memory	I-Architecture
made	O
from	O
discrete	O
devices	O
was	O
first	O
shipped	O
by	O
Texas	O
Instruments	O
to	O
the	O
United	O
States	O
Air	O
Force	O
in	O
1961	O
.	O
</s>
<s>
The	O
first	O
bipolar	O
semiconductor	B-Architecture
memory	I-Architecture
IC	O
chip	O
was	O
the	O
SP95	O
introduced	O
by	O
IBM	O
in	O
1965	O
.	O
</s>
<s>
While	O
semiconductor	B-Architecture
memory	I-Architecture
offered	O
improved	O
performance	O
over	O
magnetic-core	O
memory	O
,	O
it	O
remain	O
larger	O
and	O
more	O
expensive	O
and	O
did	O
not	O
displace	O
magnetic-core	O
memory	O
until	O
the	O
late	O
1960s	O
.	O
</s>
<s>
The	O
invention	O
of	O
the	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
field-effect	I-Architecture
transistor	I-Architecture
(	O
MOSFET	B-Architecture
)	O
enabled	O
the	O
practical	O
use	O
of	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
(	O
MOS	B-Architecture
)	O
transistors	O
as	O
memory	B-Algorithm
cell	I-Algorithm
storage	B-General_Concept
elements	O
.	O
</s>
<s>
MOS	B-Architecture
memory	I-Architecture
was	O
developed	O
by	O
John	O
Schmidt	O
at	O
Fairchild	O
Semiconductor	O
in	O
1964	O
.	O
</s>
<s>
In	O
addition	O
to	O
higher	O
performance	O
,	O
MOS	B-Architecture
semiconductor	B-Architecture
memory	I-Architecture
was	O
cheaper	O
and	O
consumed	O
less	O
power	O
than	O
magnetic	O
core	O
memory	O
.	O
</s>
<s>
Wood	O
and	O
R	O
.	O
Ball	O
of	O
the	O
Royal	O
Radar	O
Establishment	O
proposed	O
digital	B-General_Concept
storage	B-General_Concept
systems	I-General_Concept
that	O
use	O
CMOS	B-Device
(	O
complementary	O
MOS	B-Architecture
)	O
memory	B-Algorithm
cells	I-Algorithm
,	O
in	O
addition	O
to	O
MOSFET	B-Architecture
power	O
devices	O
for	O
the	O
power	O
supply	O
,	O
switched	O
cross-coupling	O
,	O
switches	B-Device
and	O
delay-line	O
storage	B-General_Concept
.	O
</s>
<s>
The	O
development	O
of	O
silicon-gate	O
MOS	B-Architecture
integrated	O
circuit	O
(	O
MOS	B-Architecture
IC	O
)	O
technology	O
by	O
Federico	O
Faggin	O
at	O
Fairchild	O
in	O
1968	O
enabled	O
the	O
production	O
of	O
MOS	B-Architecture
memory	I-Architecture
chips	O
.	O
</s>
<s>
NMOS	B-Algorithm
memory	O
was	O
commercialized	O
by	O
IBM	O
in	O
the	O
early	O
1970s	O
.	O
</s>
<s>
MOS	B-Architecture
memory	I-Architecture
overtook	O
magnetic	O
core	O
memory	O
as	O
the	O
dominant	O
memory	O
technology	O
in	O
the	O
early	O
1970s	O
.	O
</s>
<s>
The	O
two	O
main	O
types	O
of	O
volatile	B-General_Concept
random-access	B-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
are	O
static	B-Architecture
random-access	I-Architecture
memory	I-Architecture
(	O
SRAM	O
)	O
and	O
dynamic	O
random-access	B-Architecture
memory	I-Architecture
(	O
DRAM	O
)	O
.	O
</s>
<s>
Bipolar	O
SRAM	O
was	O
invented	O
by	O
Robert	O
Norman	O
at	O
Fairchild	O
Semiconductor	O
in	O
1963	O
,	O
followed	O
by	O
the	O
development	O
of	O
MOS	B-Architecture
SRAM	O
by	O
John	O
Schmidt	O
at	O
Fairchild	O
in	O
1964	O
.	O
</s>
<s>
Commercial	O
use	O
of	O
SRAM	O
began	O
in	O
1965	O
,	O
when	O
IBM	O
introduced	O
their	O
SP95	O
SRAM	O
chip	O
for	O
the	O
System/360	B-Application
Model	I-Application
95	I-Application
.	O
</s>
<s>
Toshiba	O
introduced	O
bipolar	O
DRAM	O
memory	B-Algorithm
cells	I-Algorithm
for	O
its	O
Toscal	O
BC-1411	O
electronic	O
calculator	O
in	O
1965	O
.	O
</s>
<s>
MOS	B-Architecture
technology	I-Architecture
is	O
the	O
basis	O
for	O
modern	O
DRAM	O
.	O
</s>
<s>
In	O
1966	O
,	O
Robert	O
H	O
.	O
Dennard	O
at	O
the	O
IBM	O
Thomas	O
J	O
.	O
Watson	O
Research	O
Center	O
was	O
working	O
on	O
MOS	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
While	O
examining	O
the	O
characteristics	O
of	O
MOS	B-Architecture
technology	I-Architecture
,	O
he	O
found	O
it	O
was	O
possible	O
to	O
build	O
capacitors	O
,	O
and	O
that	O
storing	O
a	O
charge	O
or	O
no	O
charge	O
on	O
the	O
MOS	B-Architecture
capacitor	O
could	O
represent	O
the	O
1	O
and	O
0	O
of	O
a	O
bit	O
,	O
while	O
the	O
MOS	B-Architecture
transistor	I-Architecture
could	O
control	O
writing	O
the	O
charge	O
to	O
the	O
capacitor	O
.	O
</s>
<s>
This	O
led	O
to	O
his	O
development	O
of	O
a	O
single-transistor	O
DRAM	O
memory	B-Algorithm
cell	I-Algorithm
.	O
</s>
<s>
In	O
1967	O
,	O
Dennard	O
filed	O
a	O
patent	O
for	O
a	O
single-transistor	O
DRAM	O
memory	B-Algorithm
cell	I-Algorithm
based	O
on	O
MOS	B-Architecture
technology	I-Architecture
.	O
</s>
<s>
This	O
led	O
to	O
the	O
first	O
commercial	O
DRAM	O
IC	O
chip	O
,	O
the	O
Intel	B-General_Concept
1103	I-General_Concept
in	O
October	O
1970	O
.	O
</s>
<s>
Synchronous	O
dynamic	O
random-access	B-Architecture
memory	I-Architecture
(	O
SDRAM	O
)	O
later	O
debuted	O
with	O
the	O
Samsung	B-Application
KM48SL2000	O
chip	O
in	O
1992	O
.	O
</s>
<s>
The	O
term	O
memory	O
is	O
also	O
often	O
used	O
to	O
refer	O
to	O
non-volatile	B-General_Concept
memory	I-General_Concept
including	O
read-only	B-Device
memory	I-Device
(	O
ROM	B-Device
)	O
through	O
modern	O
flash	B-Device
memory	I-Device
.	O
</s>
<s>
Programmable	B-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
(	O
PROM	B-General_Concept
)	O
was	O
invented	O
by	O
Wen	O
Tsing	O
Chow	O
in	O
1956	O
,	O
while	O
working	O
for	O
the	O
Arma	O
Division	O
of	O
the	O
American	O
Bosch	O
Arma	O
Corporation	O
.	O
</s>
<s>
In	O
1967	O
,	O
Dawon	O
Kahng	O
and	O
Simon	O
Sze	O
of	O
Bell	O
Labs	O
proposed	O
that	O
the	O
floating	B-Algorithm
gate	I-Algorithm
of	O
a	O
MOS	B-Architecture
semiconductor	O
device	O
could	O
be	O
used	O
for	O
the	O
cell	O
of	O
a	O
reprogrammable	O
ROM	B-Device
,	O
which	O
led	O
to	O
Dov	O
Frohman	O
of	O
Intel	O
inventing	O
EPROM	B-General_Concept
(	O
erasable	O
PROM	B-General_Concept
)	O
in	O
1971	O
.	O
</s>
<s>
EEPROM	B-General_Concept
(	O
electrically	O
erasable	O
PROM	B-General_Concept
)	O
was	O
developed	O
by	O
Yasuo	O
Tarui	O
,	O
Yutaka	O
Hayashi	O
and	O
Kiyoko	O
Naga	O
at	O
the	O
Electrotechnical	O
Laboratory	O
in	O
1972	O
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
was	O
invented	O
by	O
Fujio	O
Masuoka	O
at	O
Toshiba	O
in	O
the	O
early	O
1980s	O
.	O
</s>
<s>
Toshiba	O
commercialized	O
NAND	O
flash	B-Device
memory	I-Device
in	O
1987	O
.	O
</s>
<s>
Volatile	B-General_Concept
memory	I-General_Concept
is	O
computer	B-General_Concept
memory	I-General_Concept
that	O
requires	O
power	O
to	O
maintain	O
the	O
stored	O
information	O
.	O
</s>
<s>
Most	O
modern	O
semiconductor	O
volatile	B-General_Concept
memory	I-General_Concept
is	O
either	O
static	B-Architecture
RAM	I-Architecture
(	O
SRAM	O
)	O
or	O
dynamic	O
RAM	B-Architecture
(	O
DRAM	O
)	O
.	O
</s>
<s>
DRAM	O
dominates	O
for	O
desktop	B-General_Concept
system	I-General_Concept
memory	O
.	O
</s>
<s>
SRAM	O
is	O
used	O
for	O
CPU	B-General_Concept
cache	I-General_Concept
.	O
</s>
<s>
SRAM	O
is	O
also	O
found	O
in	O
small	O
embedded	B-Architecture
systems	I-Architecture
requiring	O
little	O
memory	O
.	O
</s>
<s>
Dynamic	O
RAM	B-Architecture
is	O
more	O
complicated	O
for	O
interfacing	O
and	O
control	O
,	O
needing	O
regular	O
refresh	O
cycles	O
to	O
prevent	O
losing	O
its	O
contents	O
,	O
but	O
uses	O
only	O
one	O
transistor	O
and	O
one	O
capacitor	O
per	O
bit	O
,	O
allowing	O
it	O
to	O
reach	O
much	O
higher	O
densities	O
and	O
much	O
cheaper	O
per-bit	O
costs	O
.	O
</s>
<s>
Non-volatile	B-General_Concept
memory	I-General_Concept
can	O
retain	O
the	O
stored	O
information	O
even	O
when	O
not	O
powered	O
.	O
</s>
<s>
Examples	O
of	O
non-volatile	B-General_Concept
memory	I-General_Concept
include	O
read-only	B-Device
memory	I-Device
,	O
flash	B-Device
memory	I-Device
,	O
most	O
types	O
of	O
magnetic	O
computer	B-General_Concept
storage	I-General_Concept
devices	I-General_Concept
(	O
e.g.	O
</s>
<s>
hard	B-Device
disk	I-Device
drives	I-Device
,	O
floppy	B-Device
disks	I-Device
and	O
magnetic	O
tape	O
)	O
,	O
optical	B-Device
discs	I-Device
,	O
and	O
early	O
computer	B-General_Concept
storage	I-General_Concept
methods	O
such	O
as	O
paper	O
tape	O
and	O
punched	B-Architecture
cards	I-Architecture
.	O
</s>
<s>
Non-volatile	B-General_Concept
memory	I-General_Concept
technologies	O
under	O
development	O
include	O
ferroelectric	O
RAM	B-Architecture
,	O
programmable	B-General_Concept
metallization	I-General_Concept
cell	I-General_Concept
,	O
Spin-transfer	O
torque	O
magnetic	O
RAM	B-Architecture
,	O
SONOS	B-Algorithm
,	O
resistive	B-General_Concept
random-access	I-General_Concept
memory	I-General_Concept
,	O
racetrack	B-General_Concept
memory	I-General_Concept
,	O
Nano-RAM	O
,	O
3D	B-Device
XPoint	I-Device
,	O
and	O
millipede	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
A	O
third	O
category	O
of	O
memory	O
is	O
semi-volatile	O
.	O
</s>
<s>
The	O
term	O
is	O
used	O
to	O
describe	O
a	O
memory	O
that	O
has	O
some	O
limited	O
non-volatile	B-General_Concept
duration	O
after	O
power	O
is	O
removed	O
,	O
but	O
then	O
data	O
is	O
ultimately	O
lost	O
.	O
</s>
<s>
A	O
typical	O
goal	O
when	O
using	O
a	O
semi-volatile	O
memory	O
is	O
to	O
provide	O
the	O
high	O
performance	O
and	O
durability	O
associated	O
with	O
volatile	B-General_Concept
memories	I-General_Concept
while	O
providing	O
some	O
benefits	O
of	O
non-volatile	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
some	O
non-volatile	B-General_Concept
memory	I-General_Concept
types	O
experience	O
wear	O
when	O
written	O
.	O
</s>
<s>
As	O
a	O
second	O
example	O
,	O
an	O
STT-RAM	O
can	O
be	O
made	O
non-volatile	B-General_Concept
by	O
building	O
large	O
cells	O
,	O
but	O
doing	O
so	O
raises	O
the	O
cost	O
per	O
bit	O
and	O
power	O
requirements	O
and	O
reduces	O
the	O
write	O
speed	O
.	O
</s>
<s>
Using	O
small	O
cells	O
improves	O
cost	O
,	O
power	O
,	O
and	O
speed	O
,	O
but	O
leads	O
to	O
semi-volatile	O
behavior	O
.	O
</s>
<s>
In	O
some	O
applications	O
,	O
the	O
increased	O
volatility	O
can	O
be	O
managed	O
to	O
provide	O
many	O
benefits	O
of	O
a	O
non-volatile	B-General_Concept
memory	I-General_Concept
,	O
for	O
example	O
by	O
removing	O
power	O
but	O
forcing	O
a	O
wake-up	O
before	O
data	O
is	O
lost	O
;	O
or	O
by	O
caching	O
read-only	O
data	O
and	O
discarding	O
the	O
cached	O
data	O
if	O
the	O
power-off	O
time	O
exceeds	O
the	O
non-volatile	B-General_Concept
threshold	O
.	O
</s>
<s>
The	O
term	O
semi-volatile	O
is	O
also	O
used	O
to	O
describe	O
semi-volatile	O
behavior	O
constructed	O
from	O
other	O
memory	O
types	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
volatile	B-General_Concept
and	O
a	O
non-volatile	B-General_Concept
memory	I-General_Concept
may	O
be	O
combined	O
,	O
where	O
an	O
external	O
signal	O
copies	O
data	O
from	O
the	O
volatile	B-General_Concept
memory	I-General_Concept
to	O
the	O
non-volatile	B-General_Concept
memory	I-General_Concept
,	O
but	O
if	O
power	O
is	O
removed	O
before	O
the	O
copy	O
occurs	O
,	O
the	O
data	O
is	O
lost	O
.	O
</s>
<s>
Or	O
,	O
a	O
battery-backed	O
volatile	B-General_Concept
memory	I-General_Concept
,	O
and	O
if	O
external	O
power	O
is	O
lost	O
there	O
is	O
some	O
known	O
period	O
where	O
the	O
battery	O
can	O
continue	O
to	O
power	O
the	O
volatile	B-General_Concept
memory	I-General_Concept
,	O
but	O
if	O
power	O
is	O
off	O
for	O
an	O
extended	O
time	O
,	O
the	O
battery	O
runs	O
down	O
and	O
data	O
is	O
lost	O
.	O
</s>
<s>
Modern	O
operating	B-General_Concept
systems	I-General_Concept
have	O
complex	O
systems	O
to	O
properly	O
manage	O
memory	O
.	O
</s>
<s>
A	O
memory	B-Error_Name
leak	I-Error_Name
occurs	O
when	O
a	O
program	O
requests	O
memory	O
from	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
never	O
returns	O
the	O
memory	O
when	O
it	O
's	O
done	O
with	O
it	O
.	O
</s>
<s>
A	O
program	O
with	O
this	O
bug	O
will	O
gradually	O
require	O
more	O
and	O
more	O
memory	O
until	O
the	O
program	O
fails	O
as	O
the	O
operating	B-General_Concept
system	I-General_Concept
runs	O
out	O
.	O
</s>
<s>
A	O
segmentation	B-Error_Name
fault	I-Error_Name
results	O
when	O
a	O
program	O
tries	O
to	O
access	O
memory	O
that	O
it	O
does	O
not	O
have	O
permission	O
to	O
access	O
.	O
</s>
<s>
Generally	O
,	O
a	O
program	O
doing	O
so	O
will	O
be	O
terminated	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
A	O
buffer	B-General_Concept
overflow	I-General_Concept
occurs	O
when	O
a	O
program	O
writes	O
data	O
to	O
the	O
end	O
of	O
its	O
allocated	O
space	O
and	O
then	O
continues	O
to	O
write	O
data	O
beyond	O
this	O
to	O
memory	O
that	O
has	O
been	O
allocated	O
for	O
other	O
purposes	O
.	O
</s>
<s>
This	O
location	O
was	O
a	O
physical	O
location	O
on	O
the	O
actual	O
memory	O
hardware	B-Architecture
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
the	O
error	O
might	O
overwrite	O
memory	O
used	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Hackers	B-Application
can	O
take	O
advantage	O
of	O
this	O
lack	O
of	O
protection	O
to	O
create	O
viruses	O
and	O
malware	O
.	O
</s>
<s>
Virtual	B-Architecture
memory	I-Architecture
is	O
a	O
system	O
where	O
physical	O
memory	O
is	O
managed	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
typically	O
with	O
assistance	O
from	O
a	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
some	O
data	O
can	O
be	O
stored	O
in	O
RAM	B-Architecture
while	O
other	O
data	O
is	O
stored	O
on	O
a	O
hard	B-Device
drive	I-Device
(	O
e.g.	O
</s>
<s>
in	O
a	O
swapfile	B-Architecture
)	O
,	O
functioning	O
as	O
an	O
extension	O
of	O
the	O
cache	B-General_Concept
hierarchy	I-General_Concept
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
will	O
place	O
actively	O
used	O
data	O
in	O
RAM	B-Architecture
,	O
which	O
is	O
much	O
faster	O
than	O
hard	B-Device
disks	I-Device
.	O
</s>
<s>
When	O
the	O
amount	O
of	O
RAM	B-Architecture
is	O
not	O
sufficient	O
to	O
run	O
all	O
the	O
current	O
programs	O
,	O
it	O
can	O
result	O
in	O
a	O
situation	O
where	O
the	O
computer	O
spends	O
more	O
time	O
moving	O
data	O
from	O
RAM	B-Architecture
to	O
disk	O
and	O
back	O
than	O
it	O
does	O
accomplishing	O
tasks	O
;	O
this	O
is	O
known	O
as	O
thrashing	B-General_Concept
.	O
</s>
<s>
If	O
the	O
operating	B-General_Concept
system	I-General_Concept
detects	O
that	O
a	O
program	O
has	O
tried	O
to	O
alter	O
memory	O
that	O
does	O
not	O
belong	O
to	O
it	O
,	O
the	O
program	O
is	O
terminated	O
(	O
or	O
otherwise	O
restricted	O
or	O
redirected	O
)	O
.	O
</s>
<s>
If	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
memory	O
is	O
corrupted	O
,	O
the	O
entire	O
computer	O
system	O
may	O
crash	O
and	O
need	O
to	O
be	O
rebooted	B-Device
.	O
</s>
<s>
It	O
may	O
also	O
be	O
used	O
benignly	O
by	O
desirable	O
programs	O
which	O
are	O
intended	O
to	O
modify	O
other	O
programs	O
,	O
debuggers	B-Application
,	O
for	O
example	O
,	O
to	O
insert	O
breakpoints	O
or	O
hooks	O
.	O
</s>
