<s>
The	O
PCI	B-General_Concept
hole	I-General_Concept
or	O
PCI	B-General_Concept
memory	I-General_Concept
hole	I-General_Concept
is	O
a	O
limitation	O
of	O
32-bit	O
hardware	O
and	O
32-bit	O
operating	O
systems	O
that	O
causes	O
a	O
computer	O
to	O
appear	O
to	O
have	O
less	O
memory	O
available	O
than	O
is	O
physically	O
installed	O
.	O
</s>
<s>
This	O
memory	O
addressing	O
limitation	O
and	O
the	O
later	O
workarounds	O
necessary	O
to	O
overcome	O
it	O
are	O
functionally	O
similar	O
to	O
the	O
memory	O
limits	O
of	O
the	O
early	O
8088	O
IBM	B-Device
PC	I-Device
memory	O
architecture	O
(	O
see	O
Conventional	B-Device
memory	I-Device
)	O
.	O
</s>
<s>
The	O
original	O
IBM	B-Device
PC	I-Device
was	O
typically	O
supplied	O
with	O
64KB	O
of	O
memory	O
or	O
less	O
;	O
it	O
was	O
designed	O
to	O
take	O
a	O
maximum	O
of	O
640KB	O
,	O
far	O
more	O
than	O
it	O
was	O
thought	O
would	O
ever	O
be	O
needed	O
.	O
</s>
<s>
This	O
rapidly	O
became	O
a	O
restriction	O
that	O
had	O
to	O
be	O
handled	O
by	O
complex	O
DOS	B-General_Concept
memory	I-General_Concept
management	I-General_Concept
.	O
</s>
<s>
Similar	O
successive	O
restrictions	O
in	O
size	O
have	O
been	O
imposed	O
and	O
overcome	O
on	O
hard	B-Device
drives	I-Device
.	O
</s>
<s>
The	O
loss	O
of	O
usable	O
memory	O
caused	O
by	O
the	O
PCI	B-General_Concept
hole	I-General_Concept
,	O
when	O
using	O
memory-mapped	B-Architecture
I/O	I-Architecture
,	O
is	O
caused	O
by	O
using	O
the	O
same	O
address	O
space	O
for	O
both	O
physical	O
memory	O
and	O
to	O
communicate	O
with	O
hardware	O
devices	O
.	O
</s>
<s>
The	O
amount	O
of	O
system	O
memory	O
that	O
is	O
hidden	O
and	O
unavailable	O
varies	O
widely	O
with	O
the	O
actual	O
mainboard	O
and	O
chipset	B-Device
,	O
the	O
BIOS	O
,	O
the	O
amount	O
of	O
physical	O
memory	O
,	O
the	O
amount	O
of	O
video	O
RAM	O
installed	O
on	O
graphics	O
cards	O
,	O
and	O
the	O
number	O
and	O
type	O
of	O
PCI	B-Protocol
cards	I-Protocol
installed	O
in	O
the	O
system	O
.	O
</s>
<s>
More	O
than	O
a	O
gigabyte	O
of	O
32-bit	O
system	O
memory	O
can	O
be	O
unavailable	O
when	O
four	O
gigabytes	O
of	O
physical	O
memory	O
and	O
multiple	O
3D	O
cards	O
with	O
large	O
amounts	O
of	O
video	O
memory	O
are	O
installed	O
;	O
on	O
some	O
mainboards	O
,	O
the	O
hole	O
is	O
always	O
at	O
least	O
one	O
gigabyte	O
in	O
size	O
regardless	O
of	O
the	O
installed	O
expansion	B-Device
cards	I-Device
.	O
</s>
<s>
A	O
workaround	O
first	O
developed	O
in	O
the	O
Pentium	O
Pro	O
,	O
known	O
as	O
Physical	B-General_Concept
Address	I-General_Concept
Extension	I-General_Concept
(	O
PAE	O
)	O
,	O
allows	O
certain	O
32-bit	O
operating	O
systems	O
to	O
access	O
up	O
to	O
36-bit	O
memory	O
addresses	O
,	O
even	O
though	O
individual	O
programs	O
are	O
still	O
limited	O
to	O
operating	O
within	O
32	O
bits	O
of	O
address	O
space	O
.	O
</s>
<s>
But	O
PAE	O
alone	O
is	O
not	O
enough	O
to	O
address	O
the	O
PCI	B-General_Concept
hole	I-General_Concept
issue	O
,	O
as	O
memory	O
addresses	O
and	O
I/O	O
PCI	B-Protocol
addresses	O
are	O
still	O
overlapping	O
somewhere	O
between	O
the	O
3rd	O
and	O
4th	O
gigabyte	O
.	O
</s>
<s>
A	O
PAE	O
compatible	O
operating	O
system	O
together	O
with	O
a	O
PAE	O
compatible	O
CPU	B-Device
cannot	O
do	O
better	O
than	O
accessing	O
memory	O
from	O
the	O
1st	O
to	O
the	O
3rd	O
gigabyte	O
,	O
then	O
from	O
the	O
5th	O
to	O
the	O
64th	O
gigabyte	O
.	O
</s>
<s>
The	O
PCI	B-General_Concept
hole	I-General_Concept
is	O
still	O
there	O
.	O
</s>
<s>
On	O
a	O
4GB	O
host	O
,	O
and	O
in	O
the	O
absence	O
of	O
one	O
or	O
another	O
additional	O
workaround	O
,	O
PAE	O
does	O
nothing	O
for	O
accessing	O
the	O
~	O
1GB	O
memory	O
overlapped	O
by	O
the	O
PCI	B-Protocol
I/O	O
.	O
</s>
<s>
PAE	O
was	O
fully	O
supported	O
in	O
Windows	O
XP	O
up	O
to	O
the	O
Service	O
Pack	O
1	O
(	O
SP1	O
)	O
release	O
,	O
but	O
then	O
withdrawn	O
for	O
SP2	O
;	O
the	O
only	O
32-bit	O
versions	O
of	O
Microsoft	B-Application
Windows	I-Application
to	O
fully	O
support	O
this	O
are	O
certain	O
high-end	O
server	O
versions	O
of	O
Windows	B-Application
Server	I-Application
2003	I-Application
and	O
earlier	O
;	O
as	O
of	O
2014	O
,	O
it	O
is	O
mainly	O
in	O
use	O
by	O
32-bit	O
Linux	B-Application
distributions	O
;	O
Ubuntu	B-Operating_System
has	O
made	O
it	O
mandatory	O
for	O
its	O
32-bit	O
version	O
since	O
2013	O
.	O
</s>
<s>
Many	O
versions	O
of	O
MS	B-Application
Windows	I-Application
can	O
activate	O
what	O
is	O
still	O
called	O
PAE	O
for	O
the	O
purpose	O
of	O
using	O
the	O
NX	B-General_Concept
bit	I-General_Concept
,	O
but	O
this	O
no	O
longer	O
extends	O
the	O
address	O
space	O
.	O
</s>
<s>
The	O
limitations	O
of	O
the	O
32-bit	O
PCI	B-General_Concept
hole	I-General_Concept
can	O
affect	O
purely	O
64-bit	O
operating	O
systems	O
as	O
the	O
system	O
BIOS	O
must	O
cater	O
for	O
all	O
operating	O
systems	O
which	O
are	O
supported	O
by	O
the	O
hardware	O
(	O
16-	O
,	O
32-	O
,	O
and	O
64-bit	O
operating	O
systems	O
all	O
run	O
on	O
the	O
same	O
hardware	O
)	O
.	O
</s>
<s>
Many	O
BIOSes	O
can	O
be	O
configured	O
by	O
the	O
user	O
to	O
fill	O
the	O
memory	O
hole	O
by	O
mapping	O
devices	O
high	O
up	O
in	O
the	O
64-bit	O
address	O
space	O
,	O
so	O
long	O
as	O
the	O
devices	O
,	O
their	O
drivers	O
,	O
and	O
the	O
chipset	B-Device
all	O
support	O
this	O
.	O
</s>
<s>
Another	O
way	O
to	O
remove	O
the	O
PCI	B-General_Concept
hole	I-General_Concept
,	O
which	O
is	O
only	O
useful	O
for	O
64-bit	O
operating	O
systems	O
and	O
those	O
32-bit	O
systems	O
that	O
support	O
the	O
Physical	B-General_Concept
Address	I-General_Concept
Extension	I-General_Concept
method	O
described	O
above	O
,	O
is	O
to	O
"	O
remap	O
"	O
some	O
or	O
all	O
of	O
the	O
memory	O
between	O
the	O
two	O
-	O
and	O
four-gigabyte	O
limits	O
to	O
addresses	O
above	O
four	O
gigabytes	O
.	O
</s>
<s>
This	O
needs	O
to	O
be	O
supported	O
by	O
the	O
chipset	B-Device
of	O
the	O
computer	O
and	O
can	O
usually	O
be	O
activated	O
in	O
the	O
BIOS	O
Setup	O
.	O
</s>
<s>
This	O
remapping	O
works	O
on	O
the	O
level	O
of	O
physical	O
addresses	O
,	O
unlike	O
the	O
higher-level	O
remapping	O
of	O
virtual	B-General_Concept
to	O
physical	O
addresses	O
that	O
happens	O
inside	O
the	O
CPU	B-Device
core	O
.	O
</s>
<s>
Activating	O
this	O
for	O
traditional	O
32-bit	O
operating	O
systems	O
does	O
more	O
harm	O
than	O
good	O
,	O
as	O
the	O
remapped	O
memory	O
(	O
often	O
larger	O
than	O
the	O
PCI	B-General_Concept
hole	I-General_Concept
itself	O
)	O
is	O
unusable	O
to	O
such	O
operating	O
systems	O
,	O
even	O
though	O
e.g.	O
</s>
<s>
Windows	B-Application
Vista	I-Application
will	O
show	O
such	O
memory	O
to	O
physically	O
exist	O
on	O
the	O
"	O
System	O
Properties	O
"	O
page	O
.	O
</s>
