<s>
Density	O
is	O
a	O
measure	O
of	O
the	O
quantity	O
of	O
information	O
bits	O
that	O
can	O
be	O
stored	O
on	O
a	O
given	O
length	O
(	O
linear	O
density	O
)	O
of	O
track	B-Device
,	O
area	O
of	O
surface	O
(	O
areal	B-Device
density	I-Device
)	O
,	O
or	O
in	O
a	O
given	O
volume	O
(	O
volumetric	O
density	O
)	O
of	O
a	O
computer	B-General_Concept
storage	I-General_Concept
medium	I-General_Concept
.	O
</s>
<s>
Density	O
therefore	O
has	O
a	O
direct	O
relationship	O
to	O
storage	B-General_Concept
capacity	I-General_Concept
of	O
a	O
given	O
medium	O
.	O
</s>
<s>
Solid	B-Device
state	I-Device
drives	I-Device
use	O
flash	B-Device
memory	I-Device
to	O
store	O
non-volatile	B-General_Concept
media	I-General_Concept
.	O
</s>
<s>
They	O
are	O
the	O
latest	O
form	O
of	O
mass	O
produced	O
storage	O
and	O
rival	O
magnetic	B-Device
disk	I-Device
media	I-Device
.	O
</s>
<s>
NAND	O
itself	O
is	O
made	O
up	O
of	O
what	O
are	O
called	O
floating	B-Algorithm
gate	I-Algorithm
transistors	I-Algorithm
.	O
</s>
<s>
Hard	B-Device
disk	I-Device
drives	I-Device
store	O
data	O
in	O
the	O
magnetic	O
polarization	O
of	O
small	O
patches	O
of	O
the	O
surface	O
coating	O
on	O
a	O
disk	O
.	O
</s>
<s>
The	O
maximum	O
areal	B-Device
density	I-Device
is	O
defined	O
by	O
the	O
size	O
of	O
the	O
magnetic	O
particles	O
in	O
the	O
surface	O
,	O
as	O
well	O
as	O
the	O
size	O
of	O
the	O
"	O
head	O
"	O
used	O
to	O
read	O
and	O
write	O
the	O
data	O
.	O
</s>
<s>
In	O
1956	O
the	O
first	O
hard	B-Device
drive	I-Device
,	O
the	O
IBM	O
350	O
,	O
had	O
an	O
areal	B-Device
density	I-Device
of	O
2,000	O
bit/in2	O
.	O
</s>
<s>
In	O
2015	O
,	O
Seagate	O
introduced	O
a	O
hard	B-Device
drive	I-Device
with	O
a	O
density	O
of	O
1.34Tbit/in2	O
,	O
more	O
than	O
600	O
million	O
times	O
that	O
of	O
the	O
IBM	O
350	O
.	O
</s>
<s>
New	O
technologies	O
like	O
heat-assisted	O
magnetic	O
recording	O
(	O
HAMR	O
)	O
and	O
microwave-assisted	O
magnetic	O
recording	O
(	O
MAMR	O
)	O
are	O
under	O
development	O
and	O
are	O
expected	O
to	O
allow	O
increases	O
in	O
magnetic	O
areal	B-Device
density	I-Device
to	O
continue	O
.	O
</s>
<s>
Optical	B-Device
discs	I-Device
store	O
data	O
in	O
small	O
pits	O
in	O
a	O
plastic	O
surface	O
that	O
is	O
then	O
covered	O
with	O
a	O
thin	O
layer	O
of	O
reflective	O
metal	O
.	O
</s>
<s>
Single-layer	O
HD	B-Operating_System
DVD	I-Operating_System
and	O
Blu-ray	O
disks	O
offer	O
densities	O
around	O
7.5Gbit/in2	O
and	O
12.5Gbit/in2	O
,	O
respectively	O
.	O
</s>
<s>
When	O
introduced	O
in	O
1982	O
CDs	O
had	O
considerably	O
higher	O
densities	O
than	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
but	O
hard	B-Device
disk	I-Device
drives	I-Device
have	O
since	O
advanced	O
much	O
more	O
quickly	O
and	O
eclipsed	O
optical	B-Device
media	I-Device
in	O
both	O
areal	B-Device
density	I-Device
and	O
capacity	O
per	O
device	O
.	O
</s>
<s>
The	O
first	O
magnetic	O
tape	O
drive	O
,	O
the	O
Univac	O
Uniservo	B-Device
,	O
recorded	O
at	O
the	O
density	O
of	O
128bit/in	O
on	O
a	O
half-inch	O
magnetic	O
tape	O
,	O
resulting	O
in	O
the	O
areal	B-Device
density	I-Device
of	O
256bit/in2	O
.	O
</s>
<s>
In	O
2015	O
,	O
IBM	O
and	O
Fujifilm	O
claimed	O
a	O
new	O
record	O
for	O
the	O
magnetic	O
tape	O
areal	B-Device
density	I-Device
of	O
123Gbit/in2	O
,	O
while	O
LTO-6	B-Operating_System
,	O
the	O
highest-density	O
production	O
tape	O
shipping	O
in	O
2015	O
,	O
provides	O
an	O
areal	B-Device
density	I-Device
of	O
0.84Gbit/in2	O
.	O
</s>
<s>
IBM	O
aimed	O
to	O
commercialize	O
their	O
Millipede	B-General_Concept
memory	I-General_Concept
system	O
at	O
1Tbit/in2	O
in	O
2007	O
but	O
development	O
appears	O
to	O
be	O
moribund	O
.	O
</s>
<s>
A	O
newer	O
IBM	O
technology	O
,	O
racetrack	B-General_Concept
memory	I-General_Concept
,	O
uses	O
an	O
array	O
of	O
many	O
small	O
nanoscopic	O
wires	O
arranged	O
in	O
3D	O
,	O
each	O
holding	O
numerous	O
bits	O
to	O
improve	O
density	O
.	O
</s>
<s>
Holographic	B-Algorithm
storage	I-Algorithm
technologies	O
are	O
also	O
attempting	O
to	O
leapfrog	O
existing	O
systems	O
,	O
but	O
they	O
too	O
have	O
been	O
losing	O
the	O
race	O
,	O
and	O
are	O
estimated	O
to	O
offer	O
1Tbit/in2	O
as	O
well	O
,	O
with	O
about	O
250GB/in2	O
being	O
the	O
best	O
demonstrated	O
to	O
date	O
for	O
non-quantum	O
holography	O
systems	O
.	O
</s>
<s>
By	O
far	O
the	O
densest	O
type	O
of	O
memory	O
storage	O
experimentally	O
to	O
date	O
is	O
electronic	B-Algorithm
quantum	I-Algorithm
holography	I-Algorithm
.	O
</s>
<s>
By	O
superimposing	O
images	O
of	O
different	O
wavelengths	O
into	O
the	O
same	O
hologram	O
,	O
in	O
2009	O
a	O
Stanford	O
research	O
team	O
achieved	O
a	O
bit	B-Device
density	I-Device
of	O
35	O
bit/electron	O
(	O
approximately	O
3	O
exabytes/in2	O
)	O
using	O
electron	O
microscopes	O
and	O
a	O
copper	O
medium	O
.	O
</s>
<s>
With	O
the	O
notable	O
exception	O
of	O
NAND	O
Flash	B-Device
memory	I-Device
,	O
increasing	O
storage	B-Device
density	I-Device
of	O
a	O
medium	O
typically	O
improves	O
the	O
transfer	O
speed	O
at	O
which	O
that	O
medium	O
can	O
operate	O
.	O
</s>
<s>
A	O
standard	O
3½-inch	O
floppy	O
disk	O
spins	O
at	O
300rpm	O
,	O
and	O
the	O
innermost	O
track	B-Device
is	O
about	O
66mm	O
long	O
(	O
10.5mm	O
radius	O
)	O
.	O
</s>
<s>
Along	O
that	O
track	B-Device
the	O
bits	O
are	O
stored	O
at	O
a	O
density	O
of	O
686bit/mm	O
,	O
which	O
means	O
that	O
the	O
head	O
sees	O
686bit/mm	O
×	O
330mm/s	O
=	O
226,380	O
bit/s	O
(	O
or	O
28.3KB/s	O
)	O
.	O
</s>
<s>
Now	O
consider	O
an	O
improvement	O
to	O
the	O
design	O
that	O
doubles	O
the	O
density	O
of	O
the	O
bits	O
by	O
reducing	O
sample	O
length	O
and	O
keeping	O
the	O
same	O
track	B-Device
spacing	O
.	O
</s>
<s>
These	O
,	O
too	O
,	O
were	O
completely	O
overwhelmed	O
by	O
newer	O
devices	O
like	O
the	O
LS-120	B-Device
,	O
which	O
were	O
forced	O
to	O
use	O
higher-speed	O
interfaces	O
such	O
as	O
IDE	B-Protocol
.	O
</s>
<s>
Although	O
the	O
effect	O
on	O
performance	O
is	O
most	O
obvious	O
on	O
rotating	O
media	O
,	O
similar	O
effects	O
come	O
into	O
play	O
even	O
for	O
solid-state	O
media	O
like	O
Flash	B-Device
RAM	I-Device
or	O
DRAM	O
.	O
</s>
<s>
In	O
this	O
case	O
the	O
performance	O
is	O
generally	O
defined	O
by	O
the	O
time	O
it	O
takes	O
for	O
the	O
electrical	O
signals	O
to	O
travel	O
through	O
the	O
computer	B-General_Concept
bus	I-General_Concept
to	O
the	O
chips	O
,	O
and	O
then	O
through	O
the	O
chips	O
to	O
the	O
individual	O
"	O
cells	O
"	O
used	O
to	O
store	O
data	O
(	O
each	O
cell	O
holds	O
one	O
bit	O
)	O
.	O
</s>
<s>
As	O
the	O
cell	O
size	O
decreases	O
,	O
through	O
the	O
improvements	O
in	O
semiconductor	B-Architecture
fabrication	I-Architecture
that	O
led	O
to	O
Moore	O
's	O
Law	O
,	O
the	O
resistance	O
is	O
reduced	O
and	O
less	O
power	O
is	O
needed	O
to	O
operate	O
the	O
cells	O
.	O
</s>
<s>
As	O
fabrication	B-Architecture
has	O
improved	O
,	O
solid-state	O
memory	O
has	O
improved	O
dramatically	O
in	O
terms	O
of	O
performance	O
.	O
</s>
<s>
A	O
less	O
obvious	O
effect	O
is	O
that	O
as	O
density	O
improves	O
,	O
the	O
number	O
of	O
DIMMs	B-General_Concept
needed	O
to	O
supply	O
any	O
particular	O
amount	O
of	O
memory	O
decreases	O
,	O
which	O
in	O
turn	O
means	O
less	O
DIMMs	B-General_Concept
overall	O
in	O
any	O
particular	O
computer	O
.	O
</s>
<s>
Storage	B-Device
density	I-Device
also	O
has	O
a	O
strong	O
effect	O
on	O
the	O
price	O
of	O
memory	O
,	O
although	O
in	O
this	O
case	O
,	O
the	O
reasons	O
are	O
not	O
so	O
obvious	O
.	O
</s>
<s>
The	O
highest	O
capacity	O
drives	O
use	O
more	O
platters	O
,	O
essentially	O
individual	O
hard	B-Device
drives	I-Device
within	O
the	O
case	O
.	O
</s>
<s>
Hard	B-Device
drives	I-Device
are	O
often	O
measured	O
in	O
terms	O
of	O
cost	O
per	O
bit	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
first	O
commercial	O
hard	B-Device
drive	I-Device
,	O
IBM	B-Device
's	I-Device
RAMAC	I-Device
in	O
1957	O
,	O
supplied	O
3.75MB	O
for	O
$34	O
,	O
500	O
,	O
or	O
$	O
9,200	O
per	O
megabyte	O
.	O
</s>
<s>
In	O
1989	O
,	O
a	O
40MB	O
hard	B-Device
drive	I-Device
cost	O
$1200	O
,	O
or	O
$	O
30/MB	O
.	O
</s>
<s>
Fabrication	B-Architecture
has	O
improved	O
yields	O
over	O
time	O
by	O
using	O
larger	O
wafers	O
,	O
and	O
producing	O
wafers	O
with	O
fewer	O
failures	O
.	O
</s>
