<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
data	O
(	O
treated	O
as	O
singular	O
,	O
plural	O
,	O
or	O
as	O
a	O
mass	O
noun	O
)	O
is	O
any	O
sequence	O
of	O
one	O
or	O
more	O
symbols	O
;	O
datum	O
is	O
a	O
single	O
symbol	O
of	O
data	O
.	O
</s>
<s>
Digital	B-General_Concept
data	I-General_Concept
is	O
data	O
that	O
is	O
represented	O
using	O
the	O
binary	O
number	O
system	O
of	O
ones	O
(	O
1	O
)	O
and	O
zeros	O
(	O
0	O
)	O
,	O
instead	O
of	O
analog	O
representation	O
.	O
</s>
<s>
Data	O
exists	O
in	O
three	O
states	O
:	O
data	B-General_Concept
at	I-General_Concept
rest	I-General_Concept
,	O
data	B-General_Concept
in	I-General_Concept
transit	I-General_Concept
and	O
data	B-General_Concept
in	I-General_Concept
use	I-General_Concept
.	O
</s>
<s>
Data	O
moving	O
to	O
or	O
from	O
a	O
computer	O
,	O
in	O
most	O
cases	O
,	O
moves	B-Protocol
as	I-Protocol
serial	I-Protocol
data	I-Protocol
.	O
</s>
<s>
Data	O
representing	O
quantities	O
,	O
characters	O
,	O
or	O
symbols	O
on	O
which	O
operations	O
are	O
performed	O
by	O
a	O
computer	O
are	O
stored	B-General_Concept
and	O
recorded	O
on	O
magnetic	B-Architecture
,	O
optical	B-General_Concept
,	O
electronic	O
,	O
or	O
mechanical	O
recording	B-General_Concept
media	I-General_Concept
,	O
and	O
transmitted	O
in	O
the	O
form	O
of	O
digital	O
electrical	O
or	O
optical	B-General_Concept
signals	O
.	O
</s>
<s>
Physical	O
computer	B-General_Concept
memory	I-General_Concept
elements	O
consist	O
of	O
an	O
address	O
and	O
a	O
byte/word	O
of	O
data	B-General_Concept
storage	I-General_Concept
.	O
</s>
<s>
Digital	B-General_Concept
data	I-General_Concept
are	O
often	O
stored	B-General_Concept
in	O
relational	O
databases	O
,	O
like	O
tables	B-Application
or	O
SQL	B-Language
databases	I-Language
,	O
and	O
can	O
generally	O
be	O
represented	O
as	O
abstract	O
key/value	O
pairs	O
.	O
</s>
<s>
Data	O
can	O
be	O
organized	O
in	O
many	O
different	O
types	B-General_Concept
of	I-General_Concept
data	I-General_Concept
structures	I-General_Concept
,	O
including	O
arrays	O
,	O
graphs	B-Application
,	O
and	O
objects	O
.	O
</s>
<s>
Data	B-General_Concept
structures	I-General_Concept
can	O
store	O
data	O
of	O
many	O
different	O
types	O
,	O
including	O
numbers	B-Algorithm
,	O
strings	O
and	O
even	O
other	O
data	B-General_Concept
structures	I-General_Concept
.	O
</s>
<s>
A	O
set	O
of	O
instructions	O
to	O
perform	O
a	O
given	O
task	O
(	O
or	O
tasks	O
)	O
is	O
called	O
a	O
program	B-Application
.	O
</s>
<s>
A	O
program	B-Application
is	O
data	O
in	O
the	O
form	O
of	O
coded	O
instructions	O
to	O
control	O
the	O
operation	O
of	O
a	O
computer	O
or	O
other	O
machine	O
.	O
</s>
<s>
In	O
the	O
nominal	O
case	O
,	O
the	O
program	B-Application
,	O
as	O
executed	B-General_Concept
by	O
the	O
computer	O
,	O
will	O
consist	O
of	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
The	O
elements	O
of	O
storage	B-General_Concept
manipulated	O
by	O
the	O
program	B-Application
,	O
but	O
not	O
actually	O
executed	B-General_Concept
by	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
,	O
are	O
also	O
data	O
.	O
</s>
<s>
At	O
its	O
most	O
essential	O
,	O
a	O
single	O
datum	O
is	O
a	O
value	O
stored	B-General_Concept
at	O
a	O
specific	O
location	O
.	O
</s>
<s>
Therefore	O
,	O
it	O
is	O
possible	O
for	O
computer	B-Application
programs	I-Application
to	O
operate	O
on	O
other	O
computer	B-Application
programs	I-Application
,	O
by	O
manipulating	O
their	O
programmatic	O
data	O
.	O
</s>
<s>
To	O
store	O
data	O
bytes	B-Application
in	O
a	O
file	O
,	O
they	O
have	O
to	O
be	O
serialized	B-Application
in	O
a	O
file	O
format	O
.	O
</s>
<s>
Typically	O
,	O
programs	O
are	O
stored	B-General_Concept
in	O
special	O
file	O
types	O
,	O
different	O
from	O
those	O
used	O
for	O
other	O
data	O
.	O
</s>
<s>
Executable	B-Application
files	I-Application
contain	O
programs	O
;	O
all	O
other	O
files	O
are	O
also	O
data	B-General_Concept
files	I-General_Concept
.	O
</s>
<s>
However	O
,	O
executable	B-Application
files	I-Application
may	O
also	O
contain	O
data	O
used	O
by	O
the	O
program	B-Application
which	O
is	O
built	O
into	O
the	O
program	B-Application
.	O
</s>
<s>
In	O
particular	O
,	O
some	O
executable	B-Application
files	I-Application
have	O
a	O
data	B-General_Concept
segment	I-General_Concept
,	O
which	O
nominally	O
contains	O
constants	O
and	O
initial	O
values	O
for	O
variables	O
,	O
both	O
of	O
which	O
can	O
be	O
considered	O
data	O
.	O
</s>
<s>
The	O
line	O
between	O
program	B-Application
and	O
data	O
can	O
become	O
blurry	O
.	O
</s>
<s>
An	O
interpreter	B-Application
,	O
for	O
example	O
,	O
is	O
a	O
program	B-Application
.	O
</s>
<s>
The	O
input	O
data	O
to	O
an	O
interpreter	B-Application
is	O
itself	O
a	O
program	B-Application
,	O
just	O
not	O
one	O
expressed	O
in	O
native	O
machine	B-Language
language	I-Language
.	O
</s>
<s>
In	O
many	O
cases	O
,	O
the	O
interpreted	O
program	B-Application
will	O
be	O
a	O
human-readable	B-General_Concept
text	B-General_Concept
file	I-General_Concept
,	O
which	O
is	O
manipulated	O
with	O
a	O
text	B-Application
editor	I-Application
program	B-Application
.	O
</s>
<s>
Metaprogramming	B-Application
similarly	O
involves	O
programs	O
manipulating	O
other	O
programs	O
as	O
data	O
.	O
</s>
<s>
Programs	O
like	O
compilers	B-Language
,	O
linkers	B-Application
,	O
debuggers	B-Application
,	O
program	B-Application
updaters	O
,	O
virus	B-Application
scanners	I-Application
and	O
such	O
use	O
other	O
programs	O
as	O
their	O
data	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
user	O
might	O
first	O
instruct	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
load	O
a	O
word	B-General_Concept
processor	I-General_Concept
program	B-Application
from	O
one	O
file	O
,	O
and	O
then	O
use	O
the	O
running	O
program	B-Application
to	O
open	O
and	O
edit	O
a	O
document	O
stored	B-General_Concept
in	O
another	O
file	O
.	O
</s>
<s>
If	O
the	O
word	B-General_Concept
processor	I-General_Concept
also	O
features	O
a	O
spell	B-Application
checker	I-Application
,	O
then	O
the	O
dictionary	O
(	O
word	O
list	O
)	O
for	O
the	O
spell	B-Application
checker	I-Application
would	O
also	O
be	O
considered	O
data	O
.	O
</s>
<s>
The	O
algorithms	O
used	O
by	O
the	O
spell	B-Application
checker	I-Application
to	O
suggest	O
corrections	O
would	O
be	O
either	O
machine	B-Language
code	I-Language
data	O
or	O
text	B-General_Concept
in	O
some	O
interpretable	O
programming	O
language	O
.	O
</s>
<s>
In	O
an	O
alternate	O
usage	O
,	O
binary	O
files	O
(	O
which	O
are	O
not	O
human-readable	B-General_Concept
)	O
are	O
sometimes	O
called	O
data	O
as	O
distinguished	O
from	O
human-readable	B-General_Concept
text	B-General_Concept
.	O
</s>
<s>
The	O
total	O
amount	O
of	O
digital	B-General_Concept
data	I-General_Concept
in	O
2007	O
was	O
estimated	O
to	O
be	O
281	O
billion	O
gigabytes	O
(	O
281	O
exabytes	O
)	O
.	O
</s>
<s>
Random	B-Architecture
access	I-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
holds	O
data	O
that	O
the	O
CPU	O
has	O
direct	O
access	O
to	O
.	O
</s>
<s>
A	O
CPU	O
may	O
only	O
manipulate	O
data	O
within	O
its	O
processor	B-General_Concept
registers	I-General_Concept
or	O
memory	O
.	O
</s>
<s>
This	O
is	O
as	O
opposed	O
to	O
data	B-General_Concept
storage	I-General_Concept
,	O
where	O
the	O
CPU	O
must	O
direct	O
the	O
transfer	O
of	O
data	O
between	O
the	O
storage	B-General_Concept
device	O
(	O
disk	O
,	O
tape	O
...	O
)	O
and	O
memory	O
.	O
</s>
<s>
RAM	B-Architecture
is	O
an	O
array	O
of	O
linear	O
contiguous	B-Architecture
locations	B-General_Concept
that	O
a	O
processor	O
may	O
read	O
or	O
write	O
by	O
providing	O
an	O
address	O
for	O
the	O
read	O
or	O
write	O
operation	O
.	O
</s>
<s>
In	O
RAM	B-Architecture
the	O
smallest	O
element	O
of	O
data	O
is	O
the	O
binary	O
bit	O
.	O
</s>
<s>
The	O
capabilities	O
and	O
limitations	O
of	O
accessing	O
RAM	B-Architecture
are	O
processor	O
specific	O
.	O
</s>
<s>
In	O
general	O
main	B-General_Concept
memory	I-General_Concept
is	O
arranged	O
as	O
an	O
array	O
of	O
locations	B-General_Concept
beginning	O
at	O
address	O
0	O
(	O
hexadecimal	O
0	O
)	O
.	O
</s>
<s>
Each	O
location	O
can	O
store	O
usually	O
8	O
or	O
32	O
bits	O
depending	O
on	O
the	O
computer	B-General_Concept
architecture	I-General_Concept
.	O
</s>
<s>
Indirect	O
,	O
abstract	O
and	O
logical	O
keys	O
codes	O
can	O
be	O
stored	B-General_Concept
in	O
association	O
with	O
values	O
to	O
form	O
a	O
data	B-General_Concept
structure	I-General_Concept
.	O
</s>
<s>
Data	B-General_Concept
structures	I-General_Concept
have	O
predetermined	O
offsets	B-General_Concept
(	O
or	O
links	O
or	O
paths	O
)	O
from	O
the	O
start	O
of	O
the	O
structure	O
,	O
in	O
which	O
data	O
values	O
are	O
stored	B-General_Concept
.	O
</s>
<s>
Therefore	O
,	O
the	O
data	O
key	O
consists	O
of	O
the	O
key	O
to	O
the	O
structure	O
plus	O
the	O
offset	B-General_Concept
(	O
or	O
links	O
or	O
paths	O
)	O
into	O
the	O
structure	O
.	O
</s>
<s>
When	O
such	O
a	O
structure	O
is	O
repeated	O
,	O
storing	O
variations	O
of	O
the	O
data	O
values	O
and	O
the	O
data	O
keys	O
within	O
the	O
same	O
repeating	O
structure	O
,	O
the	O
result	O
can	O
be	O
considered	O
to	O
resemble	O
a	O
table	B-Application
,	O
in	O
which	O
each	O
element	O
of	O
the	O
repeating	O
structure	O
is	O
considered	O
to	O
be	O
a	O
column	O
and	O
each	O
repetition	O
of	O
the	O
structure	O
is	O
considered	O
as	O
a	O
row	O
of	O
the	O
table	B-Application
.	O
</s>
<s>
The	O
tabular	B-Application
view	O
of	O
repeating	O
data	B-General_Concept
structures	I-General_Concept
is	O
only	O
one	O
of	O
many	O
possibilities	O
.	O
</s>
<s>
Repeating	O
data	B-General_Concept
structures	I-General_Concept
can	O
be	O
organised	O
hierarchically	O
,	O
such	O
that	O
nodes	O
are	O
linked	O
to	O
each	O
other	O
in	O
a	O
cascade	O
of	O
parent-child	O
relationships	O
.	O
</s>
<s>
Thus	O
the	O
nodal	O
hierarchy	O
provides	O
the	O
key	O
for	O
addressing	O
the	O
data	B-General_Concept
structures	I-General_Concept
associated	O
with	O
the	O
nodes	O
.	O
</s>
<s>
This	O
representation	O
can	O
be	O
thought	O
of	O
as	O
an	O
inverted	B-Application
tree	I-Application
.	O
</s>
<s>
modern	O
computer	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
file	B-Application
systems	I-Application
are	O
a	O
common	O
example	O
;	O
and	O
XML	B-Protocol
is	O
another	O
.	O
</s>
<s>
When	O
passing	O
sequentially	O
through	O
groups	O
of	O
the	O
data	O
with	O
the	O
same	O
key	O
,	O
or	O
a	O
subset	O
of	O
the	O
key	O
changes	O
,	O
this	O
is	O
referred	O
to	O
in	O
data	O
processing	O
circles	O
as	O
a	O
break	O
,	O
or	O
a	O
control	B-General_Concept
break	I-General_Concept
.	O
</s>
<s>
Until	O
the	O
advent	O
of	O
bulk	O
non-volatile	B-General_Concept
memory	I-General_Concept
like	O
flash	B-Device
,	O
persistent	O
data	B-General_Concept
storage	I-General_Concept
was	O
traditionally	O
achieved	O
by	O
writing	O
the	O
data	O
to	O
external	O
block	O
devices	O
like	O
magnetic	B-Architecture
tape	O
and	O
disk	B-General_Concept
drives	I-General_Concept
.	O
</s>
<s>
These	O
devices	O
typically	O
seek	O
to	O
a	O
location	O
on	O
the	O
magnetic	B-Architecture
media	O
and	O
then	O
read	O
or	O
write	O
blocks	B-General_Concept
of	I-General_Concept
data	I-General_Concept
of	O
a	O
predetermined	O
size	O
.	O
</s>
<s>
Early	O
used	O
raw	O
disk	O
data	O
file-systems	O
or	O
disc	O
operating	B-General_Concept
systems	I-General_Concept
reserved	O
contiguous	B-Architecture
blocks	O
on	O
the	O
disc	O
drive	O
for	O
data	B-General_Concept
files	I-General_Concept
.	O
</s>
<s>
Later	O
file-systems	B-Application
introduced	O
partitions	B-Application
.	O
</s>
<s>
They	O
reserved	O
blocks	O
of	O
disc	O
data	O
space	O
for	O
partitions	B-Application
and	O
used	O
the	O
allocated	O
blocks	O
more	O
economically	O
,	O
by	O
dynamically	O
assigning	O
blocks	O
of	O
a	O
partition	O
to	O
a	O
file	O
as	O
needed	O
.	O
</s>
<s>
To	O
achieve	O
this	O
,	O
the	O
file	B-Application
system	I-Application
had	O
to	O
keep	O
track	O
of	O
which	O
blocks	O
were	O
used	O
or	O
unused	O
by	O
data	B-General_Concept
files	I-General_Concept
in	O
a	O
catalog	O
or	O
file	O
allocation	O
table	B-Application
.	O
</s>
<s>
Though	O
this	O
made	O
better	O
use	O
of	O
the	O
disc	O
data	O
space	O
,	O
it	O
resulted	O
in	O
fragmentation	B-Architecture
of	O
files	O
across	O
the	O
disc	O
,	O
and	O
a	O
concomitant	O
performance	O
overhead	O
due	O
additional	O
seek	O
time	O
to	O
read	O
the	O
data	O
.	O
</s>
<s>
Modern	O
file	B-Application
systems	I-Application
reorganize	O
fragmented	O
files	O
dynamically	O
to	O
optimize	O
file	B-Application
access	I-Application
times	O
.	O
</s>
<s>
Further	O
developments	O
in	O
file	B-Application
systems	I-Application
resulted	O
in	O
virtualization	B-General_Concept
of	O
disc	O
drives	O
i.e.	O
</s>
<s>
where	O
a	O
logical	O
drive	O
can	O
be	O
defined	O
as	O
partitions	B-Application
from	O
a	O
number	O
of	O
physical	O
drives	O
.	O
</s>
<s>
Indexes	O
are	O
a	O
way	O
to	O
copy	O
out	O
keys	O
and	O
location	O
addresses	O
from	O
data	B-General_Concept
structures	I-General_Concept
in	O
files	O
,	O
tables	B-Application
and	O
data	B-General_Concept
sets	I-General_Concept
,	O
then	O
organize	O
them	O
using	O
inverted	B-Application
tree	I-Application
structures	I-Application
to	O
reduce	O
the	O
time	O
taken	O
to	O
retrieve	O
a	O
subset	O
of	O
the	O
original	O
data	O
.	O
</s>
<s>
The	O
most	O
popular	O
indexes	O
are	O
the	O
B-tree	B-Architecture
and	O
the	O
dynamic	O
hash	B-Error_Name
key	O
indexing	O
methods	O
.	O
</s>
<s>
Object-oriented	B-Language
programming	I-Language
uses	O
two	O
basic	O
concepts	O
for	O
understanding	O
data	O
and	O
software	O
:	O
</s>
<s>
The	O
memory	B-General_Concept
locations	I-General_Concept
where	O
the	O
object	O
's	O
data	O
was	O
stored	B-General_Concept
are	O
garbage	B-General_Concept
and	O
are	O
reclassified	O
as	O
unused	O
memory	O
available	O
for	O
reuse	O
.	O
</s>
<s>
The	O
advent	O
of	O
databases	O
introduced	O
a	O
further	O
layer	B-Architecture
of	I-Architecture
abstraction	I-Architecture
for	O
persistent	O
data	B-General_Concept
storage	I-General_Concept
.	O
</s>
<s>
Databases	O
use	O
metadata	O
,	O
and	O
a	O
structured	B-Language
query	I-Language
language	I-Language
protocol	O
between	O
client	B-Operating_System
and	I-Operating_System
server	I-Operating_System
systems	O
,	O
communicating	O
over	O
a	O
computer	B-Architecture
network	I-Architecture
,	O
using	O
a	O
two	B-General_Concept
phase	I-General_Concept
commit	I-General_Concept
logging	O
system	O
to	O
ensure	O
transactional	B-General_Concept
completeness	O
,	O
when	O
saving	O
data	O
.	O
</s>
<s>
Modern	O
scalable	O
and	O
high-performance	O
data	O
persistence	O
technologies	O
,	O
such	O
as	O
Apache	B-Application
Hadoop	I-Application
,	O
rely	O
on	O
massively	O
parallel	O
distributed	O
data	O
processing	O
across	O
many	O
commodity	O
computers	O
on	O
a	O
high	O
bandwidth	O
network	O
.	O
</s>
