<s>
In	O
computing	O
,	O
a	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
(	O
VAS	O
)	O
or	O
address	B-General_Concept
space	I-General_Concept
is	O
the	O
set	O
of	O
ranges	O
of	O
virtual	O
addresses	O
that	O
an	O
operating	B-General_Concept
system	I-General_Concept
makes	O
available	O
to	O
a	O
process	O
.	O
</s>
<s>
The	O
range	O
of	O
virtual	O
addresses	O
usually	O
starts	O
at	O
a	O
low	O
address	O
and	O
can	O
extend	O
to	O
the	O
highest	O
address	O
allowed	O
by	O
the	O
computer	O
's	O
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
and	O
supported	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
pointer	O
size	O
implementation	O
,	O
which	O
can	O
be	O
4	O
bytes	B-Application
for	O
32-bit	O
or	O
8	O
bytes	B-Application
for	O
64-bit	B-Device
OS	I-Device
versions	O
.	O
</s>
<s>
This	O
provides	O
several	O
benefits	O
,	O
one	O
of	O
which	O
is	O
security	O
through	O
process	O
isolation	O
assuming	O
each	O
process	O
is	O
given	O
a	O
separate	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
In	O
the	O
following	O
description	O
,	O
the	O
terminology	O
used	O
will	O
be	O
particular	O
to	O
the	O
Windows	B-Device
NT	I-Device
operating	B-General_Concept
system	I-General_Concept
,	O
but	O
the	O
concepts	O
are	O
applicable	O
to	O
other	O
virtual	O
memory	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
When	O
a	O
new	O
application	O
on	O
a	O
32-bit	O
OS	O
is	O
executed	O
,	O
the	O
process	O
has	O
a	O
VAS	O
:	O
each	O
one	O
of	O
the	O
memory	O
addresses	O
(	O
from	O
0	O
to	O
232	O
−	O
1	O
)	O
in	O
that	O
space	O
can	O
have	O
a	O
single	O
byte	B-Application
as	O
a	O
value	O
.	O
</s>
<s>
Using	O
or	O
setting	O
values	O
in	O
such	O
a	O
VAS	O
would	O
cause	O
a	O
memory	B-General_Concept
exception	I-General_Concept
.	O
</s>
<s>
Addresses	O
in	O
the	O
process	O
VAS	O
are	O
mapped	O
to	O
bytes	B-Application
in	O
the	O
exe	O
file	O
.	O
</s>
<s>
The	O
v	O
's	O
are	O
values	O
from	O
bytes	B-Application
in	O
the	O
mapped	B-General_Concept
file	I-General_Concept
.	O
</s>
<s>
The	O
process	O
then	O
starts	O
executing	O
bytes	B-Application
in	O
the	O
exe	O
file	O
.	O
</s>
<s>
However	O
,	O
the	O
only	O
way	O
the	O
process	O
can	O
use	O
or	O
set	O
'	O
-	O
'	O
values	O
in	O
its	O
VAS	O
is	O
to	O
ask	O
the	O
OS	O
to	O
map	O
them	O
to	O
bytes	B-Application
from	O
a	O
file	O
.	O
</s>
<s>
A	O
common	O
way	O
to	O
use	O
VAS	O
memory	O
in	O
this	O
way	O
is	O
to	O
map	O
it	O
to	O
the	O
page	B-Architecture
file	I-Architecture
.	O
</s>
<s>
The	O
page	B-Architecture
file	I-Architecture
is	O
a	O
single	O
file	O
,	O
but	O
multiple	O
distinct	O
sets	O
of	O
contiguous	O
bytes	B-Application
can	O
be	O
mapped	O
into	O
a	O
VAS	O
:	O
</s>
<s>
And	O
different	O
parts	O
of	O
the	O
page	B-Architecture
file	I-Architecture
can	O
map	O
into	O
the	O
VAS	O
of	O
different	O
processes	O
:	O
</s>
<s>
On	O
Microsoft	B-Application
Windows	I-Application
32-bit	O
,	O
by	O
default	O
,	O
only	O
are	O
made	O
available	O
to	O
processes	O
for	O
their	O
own	O
use	O
.	O
</s>
<s>
The	O
other	O
are	O
used	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
On	O
later	O
32-bit	O
editions	O
of	O
Microsoft	B-Application
Windows	I-Application
it	O
is	O
possible	O
to	O
extend	O
the	O
user-mode	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
to	O
while	O
only	O
is	O
left	O
for	O
kernel-mode	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
by	O
marking	O
the	O
programs	O
as	O
IMAGE_FILE_LARGE_ADDRESS_AWARE	O
and	O
enabling	O
the	O
/3GB	O
switch	O
in	O
the	O
boot.ini	O
file	O
.	O
</s>
<s>
On	O
Microsoft	B-Application
Windows	I-Application
64-bit	B-Device
,	O
in	O
a	O
process	O
running	O
an	O
executable	O
that	O
was	O
linked	O
with	O
/LARGEADDRESSAWARE:NO	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
artificially	O
limits	O
the	O
user	O
mode	O
portion	O
of	O
the	O
process	O
's	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
to	O
2	O
GiB	O
.	O
</s>
<s>
This	O
applies	O
to	O
both	O
32	O
-	O
and	O
64-bit	B-Device
executables	O
.	O
</s>
<s>
Processes	O
running	O
executables	O
that	O
were	O
linked	O
with	O
the	O
/LARGEADDRESSAWARE:YES	O
option	O
,	O
which	O
is	O
the	O
default	O
for	O
64-bit	B-Device
Visual	O
Studio	O
2010	O
and	O
later	O
,	O
have	O
access	O
to	O
more	O
than	O
of	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
:	O
up	O
to	O
for	O
32-bit	O
executables	O
,	O
up	O
to	O
for	O
64-bit	B-Device
executables	O
in	O
Windows	O
through	O
Windows	O
8	O
,	O
and	O
up	O
to	O
for	O
64-bit	B-Device
executables	O
in	O
Windows	O
8.1	O
and	O
later	O
.	O
</s>
<s>
page	B-Architecture
file	I-Architecture
as	O
the	O
backing	O
store	O
for	O
any	O
new	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
.	O
</s>
<s>
However	O
,	O
a	O
process	O
can	O
also	O
explicitly	B-General_Concept
map	I-General_Concept
file	O
bytes	B-Application
.	O
</s>
<s>
For	O
x86	B-Operating_System
CPUs	O
,	O
Linux	B-Application
32-bit	O
allows	O
splitting	O
the	O
user	O
and	O
kernel	O
address	B-General_Concept
ranges	I-General_Concept
in	O
different	O
ways	O
:	O
3G/1G	O
user/kernel	O
(	O
default	O
)	O
,	O
1G/3G	O
user/kernel	O
or	O
2G/2G	O
user/kernel	O
.	O
</s>
