<s>
In	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
,	O
demand	B-General_Concept
paging	I-General_Concept
(	O
as	O
opposed	O
to	O
anticipatory	O
paging	B-Architecture
)	O
is	O
a	O
method	O
of	O
virtual	B-Architecture
memory	I-Architecture
management	O
.	O
</s>
<s>
In	O
a	O
system	O
that	O
uses	O
demand	B-General_Concept
paging	I-General_Concept
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
copies	O
a	O
disk	B-General_Concept
page	I-General_Concept
into	O
physical	O
memory	O
only	O
if	O
an	O
attempt	O
is	O
made	O
to	O
access	O
it	O
and	O
that	O
page	B-Architecture
is	O
not	O
already	O
in	O
memory	O
(	O
i.e.	O
,	O
if	O
a	O
page	B-General_Concept
fault	I-General_Concept
occurs	O
)	O
.	O
</s>
<s>
It	O
follows	O
that	O
a	O
process	B-Operating_System
begins	O
execution	O
with	O
none	O
of	O
its	O
pages	O
in	O
physical	O
memory	O
,	O
and	O
many	O
page	B-General_Concept
faults	I-General_Concept
will	O
occur	O
until	O
most	O
of	O
a	O
process	B-Operating_System
's	O
working	B-General_Concept
set	I-General_Concept
of	O
pages	O
are	O
located	O
in	O
physical	O
memory	O
.	O
</s>
<s>
This	O
is	O
an	O
example	O
of	O
a	O
lazy	B-General_Concept
loading	I-General_Concept
technique	O
.	O
</s>
<s>
Demand	B-General_Concept
paging	I-General_Concept
follows	O
that	O
pages	O
should	O
only	O
be	O
brought	O
into	O
memory	O
if	O
the	O
executing	O
process	B-Operating_System
demands	O
them	O
.	O
</s>
<s>
This	O
is	O
often	O
referred	O
to	O
as	O
lazy	O
evaluation	O
as	O
only	O
those	O
pages	O
demanded	O
by	O
the	O
process	B-Operating_System
are	O
swapped	O
from	O
secondary	O
storage	O
to	O
main	O
memory	O
.	O
</s>
<s>
Contrast	O
this	O
to	O
pure	O
swapping	O
,	O
where	O
all	O
memory	O
for	O
a	O
process	B-Operating_System
is	O
swapped	O
from	O
secondary	O
storage	O
to	O
main	O
memory	O
during	O
the	O
process	B-Operating_System
startup	O
.	O
</s>
<s>
Commonly	O
,	O
to	O
achieve	O
this	O
process	B-Operating_System
a	O
page	B-General_Concept
table	I-General_Concept
implementation	O
is	O
used	O
.	O
</s>
<s>
The	O
page	B-General_Concept
table	I-General_Concept
maps	O
logical	B-General_Concept
memory	I-General_Concept
to	O
physical	O
memory	O
.	O
</s>
<s>
The	O
page	B-General_Concept
table	I-General_Concept
uses	O
a	O
bitwise	O
operator	O
to	O
mark	O
if	O
a	O
page	B-Architecture
is	O
valid	O
or	O
invalid	O
.	O
</s>
<s>
A	O
valid	O
page	B-Architecture
is	O
one	O
that	O
currently	O
resides	O
in	O
main	O
memory	O
.	O
</s>
<s>
An	O
invalid	O
page	B-Architecture
is	O
one	O
that	O
currently	O
resides	O
in	O
secondary	O
memory	O
.	O
</s>
<s>
When	O
a	O
process	B-Operating_System
tries	O
to	O
access	O
a	O
page	B-Architecture
,	O
the	O
following	O
steps	O
are	O
generally	O
followed	O
:	O
</s>
<s>
Attempt	O
to	O
access	O
page	B-Architecture
.	O
</s>
<s>
If	O
page	B-Architecture
is	O
valid	O
(	O
in	O
memory	O
)	O
then	O
continue	O
processing	O
instruction	O
as	O
normal	O
.	O
</s>
<s>
If	O
page	B-Architecture
is	O
invalid	O
then	O
a	O
page-fault	O
trap	O
occurs	O
.	O
</s>
<s>
If	O
not	O
,	O
the	O
process	B-Operating_System
is	O
terminated	O
(	O
illegal	O
memory	O
access	O
)	O
.	O
</s>
<s>
Otherwise	O
,	O
we	O
have	O
to	O
page	B-Architecture
in	I-Architecture
the	O
required	O
page	B-Architecture
.	O
</s>
<s>
Schedule	O
disk	O
operation	O
to	O
read	O
the	O
desired	O
page	B-Architecture
into	O
main	O
memory	O
.	O
</s>
<s>
Restart	O
the	O
instruction	O
that	O
was	O
interrupted	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
trap	O
.	O
</s>
<s>
Demand	B-General_Concept
paging	I-General_Concept
,	O
as	O
opposed	O
to	O
loading	O
all	O
pages	O
immediately	O
:	O
</s>
<s>
Only	O
loads	O
pages	O
that	O
are	O
demanded	O
by	O
the	O
executing	O
process	B-Operating_System
.	O
</s>
<s>
As	O
there	O
is	O
more	O
space	O
in	O
main	O
memory	O
,	O
more	O
processes	O
can	O
be	O
loaded	O
,	O
reducing	O
the	O
context	B-Operating_System
switching	I-Operating_System
time	O
,	O
which	O
utilizes	O
large	O
amounts	O
of	O
resources	O
.	O
</s>
<s>
Individual	O
programs	O
face	O
extra	O
latency	O
when	O
they	O
access	O
a	O
page	B-Architecture
for	O
the	O
first	O
time	O
.	O
</s>
<s>
Low-cost	O
,	O
low-power	O
embedded	B-Architecture
systems	I-Architecture
may	O
not	O
have	O
a	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
that	O
supports	O
page	B-General_Concept
replacement	I-General_Concept
.	O
</s>
<s>
Memory	B-General_Concept
management	I-General_Concept
with	O
page	B-General_Concept
replacement	I-General_Concept
algorithms	I-General_Concept
becomes	O
slightly	O
more	O
complex	O
.	O
</s>
<s>
Possible	O
security	O
risks	O
,	O
including	O
vulnerability	O
to	O
timing	O
attacks	O
;	O
see	O
(	O
specifically	O
the	O
virtual	B-Architecture
memory	I-Architecture
attack	O
in	O
section	O
2	O
)	O
.	O
</s>
<s>
Thrashing	B-General_Concept
which	O
may	O
occur	O
due	O
to	O
repeated	O
page	B-General_Concept
faults	I-General_Concept
.	O
</s>
