<s>
High	B-Device
memory	I-Device
is	O
the	O
part	O
of	O
physical	B-General_Concept
memory	I-General_Concept
in	O
a	O
computer	O
which	O
is	O
not	O
directly	O
mapped	O
by	O
the	O
page	B-General_Concept
tables	I-General_Concept
of	O
its	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
The	O
phrase	O
is	O
also	O
sometimes	O
used	O
as	O
shorthand	O
for	O
the	O
High	B-Device
Memory	I-Device
Area	I-Device
,	O
which	O
is	O
a	O
different	O
concept	O
entirely	O
.	O
</s>
<s>
Some	O
operating	B-Operating_System
system	I-Operating_System
kernels	I-Operating_System
,	O
such	O
as	O
Linux	B-Application
,	O
divide	O
their	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
into	O
two	O
regions	O
,	O
devoting	O
the	O
larger	O
to	O
user	B-Operating_System
space	I-Operating_System
and	O
the	O
smaller	O
to	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
In	O
current	O
32-bit	O
x86	B-Operating_System
computers	O
,	O
this	O
commonly	O
(	O
although	O
does	O
not	O
have	O
to	O
,	O
as	O
this	O
is	O
a	O
configurable	O
option	O
)	O
takes	O
the	O
form	O
of	O
a	O
3GB/1GB	O
split	O
of	O
the	O
4GB	O
address	O
space	O
,	O
so	O
kernel	B-Operating_System
virtual	O
addresses	O
start	O
at	O
0xC0000000	O
and	O
go	O
to	O
0xFFFFFFFF	O
.	O
</s>
<s>
The	O
lower	O
896MB	O
,	O
from	O
0xC0000000	O
to	O
0xF7FFFFFF	O
,	O
is	O
directly	O
mapped	O
to	O
the	O
kernel	B-Operating_System
physical	O
address	O
space	O
,	O
and	O
the	O
remaining	O
128MB	O
,	O
from	O
0xF8000000	O
to	O
0xFFFFFFFF	O
,	O
is	O
used	O
on	O
demand	O
by	O
the	O
kernel	B-Operating_System
to	O
be	O
mapped	O
to	O
high	B-Device
memory	I-Device
.	O
</s>
<s>
When	O
in	O
user	B-Operating_System
mode	I-Operating_System
,	O
translations	O
are	O
only	O
effective	O
for	O
the	O
first	O
region	O
,	O
thus	O
protecting	O
the	O
kernel	B-Operating_System
from	O
user	O
programs	B-Application
,	O
but	O
when	O
in	O
kernel	B-Operating_System
mode	O
,	O
translations	O
are	O
effective	O
for	O
both	O
regions	O
,	O
thus	O
giving	O
the	O
kernel	B-Operating_System
an	O
easy	O
way	O
to	O
refer	O
to	O
the	O
buffers	B-General_Concept
of	O
processes	O
—	O
it	O
just	O
uses	O
the	O
process	O
 '	O
own	O
mappings	O
.	O
</s>
<s>
However	O
,	O
if	O
the	O
kernel	B-Operating_System
needs	O
to	O
refer	O
to	O
physical	B-General_Concept
memory	I-General_Concept
for	O
which	O
a	O
userspace	B-Operating_System
translation	O
has	O
not	O
already	O
been	O
provided	O
,	O
it	O
has	O
only	O
1GB	O
(	O
for	O
example	O
)	O
of	O
virtual	O
memory	O
to	O
use	O
.	O
</s>
<s>
On	O
computers	O
with	O
a	O
lot	O
of	O
physical	B-General_Concept
memory	I-General_Concept
,	O
this	O
can	O
mean	O
that	O
there	O
exists	O
memory	O
that	O
the	O
kernel	B-Operating_System
cannot	O
refer	O
to	O
directly	O
—	O
this	O
is	O
called	O
high	B-Device
memory	I-Device
.	O
</s>
<s>
When	O
the	O
kernel	B-Operating_System
wishes	O
to	O
address	O
high	B-Device
memory	I-Device
,	O
it	O
creates	O
a	O
mapping	O
on	O
the	O
fly	O
and	O
destroys	O
the	O
mapping	O
when	O
done	O
,	O
which	O
incurs	O
a	O
performance	O
penalty	O
.	O
</s>
