<s>
In	O
computing	O
,	O
virtual	B-Architecture
memory	I-Architecture
,	O
or	O
virtual	B-Architecture
storage	I-Architecture
is	O
a	O
memory	B-General_Concept
management	I-General_Concept
technique	O
that	O
provides	O
an	O
"	O
idealized	O
abstraction	O
of	O
the	O
storage	O
resources	O
that	O
are	O
actually	O
available	O
on	O
a	O
given	O
machine	O
"	O
which	O
"	O
creates	O
the	O
illusion	O
to	O
users	O
of	O
a	O
very	O
large	O
(	O
main	O
)	O
memory	O
"	O
.	O
</s>
<s>
The	O
computer	O
's	O
operating	B-General_Concept
system	I-General_Concept
,	O
using	O
a	O
combination	O
of	O
hardware	B-Architecture
and	O
software	O
,	O
maps	O
memory	O
addresses	O
used	O
by	O
a	O
program	O
,	O
called	O
virtual	B-General_Concept
addresses	I-General_Concept
,	O
into	O
physical	O
addresses	O
in	O
computer	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
Main	O
storage	O
,	O
as	O
seen	O
by	O
a	O
process	B-Operating_System
or	O
task	O
,	O
appears	O
as	O
a	O
contiguous	O
address	B-General_Concept
space	I-General_Concept
or	O
collection	O
of	O
contiguous	O
segments	B-General_Concept
.	O
</s>
<s>
The	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
manages	O
virtual	B-General_Concept
address	I-General_Concept
spaces	I-General_Concept
and	O
the	O
assignment	O
of	O
real	O
memory	O
to	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
Address	O
translation	O
hardware	B-Architecture
in	O
the	O
CPU	B-General_Concept
,	O
often	O
referred	O
to	O
as	O
a	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
(	O
MMU	O
)	O
,	O
automatically	O
translates	O
virtual	B-General_Concept
addresses	I-General_Concept
to	O
physical	O
addresses	O
.	O
</s>
<s>
Software	O
within	O
the	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
may	O
extend	O
these	O
capabilities	O
,	O
utilizing	O
,	O
e.g.	O
,	O
disk	B-Device
storage	I-Device
,	O
to	O
provide	O
a	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
that	O
can	O
exceed	O
the	O
capacity	O
of	O
real	O
memory	O
and	O
thus	O
reference	O
more	O
memory	O
than	O
is	O
physically	O
present	O
in	O
the	O
computer	O
.	O
</s>
<s>
The	O
primary	O
benefits	O
of	O
virtual	B-Architecture
memory	I-Architecture
include	O
freeing	O
applications	O
from	O
having	O
to	O
manage	O
a	O
shared	O
memory	O
space	O
,	O
ability	O
to	O
share	O
memory	O
used	O
by	O
libraries	B-Library
between	O
processes	O
,	O
increased	O
security	O
due	O
to	O
memory	O
isolation	O
,	O
and	O
being	O
able	O
to	O
conceptually	O
use	O
more	O
memory	O
than	O
might	O
be	O
physically	O
available	O
,	O
using	O
the	O
technique	O
of	O
paging	B-Architecture
or	O
segmentation	B-General_Concept
.	O
</s>
<s>
Virtual	B-Architecture
memory	I-Architecture
makes	O
application	O
programming	O
easier	O
by	O
hiding	O
fragmentation	B-Architecture
of	O
physical	O
memory	O
;	O
by	O
delegating	O
to	O
the	O
kernel	O
the	O
burden	O
of	O
managing	O
the	O
memory	O
hierarchy	O
(	O
eliminating	O
the	O
need	O
for	O
the	O
program	O
to	O
handle	O
overlays	B-General_Concept
explicitly	O
)	O
;	O
and	O
,	O
when	O
each	O
process	B-Operating_System
is	O
run	O
in	O
its	O
own	O
dedicated	O
address	B-General_Concept
space	I-General_Concept
,	O
by	O
obviating	O
the	O
need	O
to	B-Library
relocate	I-Library
program	O
code	O
or	O
to	O
access	O
memory	O
with	O
relative	O
addressing	O
.	O
</s>
<s>
Memory	B-General_Concept
virtualization	I-General_Concept
can	O
be	O
considered	O
a	O
generalization	O
of	O
the	O
concept	O
of	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
Virtual	B-Architecture
memory	I-Architecture
is	O
an	O
integral	O
part	O
of	O
a	O
modern	O
computer	B-General_Concept
architecture	I-General_Concept
;	O
implementations	O
usually	O
require	O
hardware	B-Architecture
support	O
,	O
typically	O
in	O
the	O
form	O
of	O
a	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
built	O
into	O
the	O
CPU	B-General_Concept
.	O
</s>
<s>
While	O
not	O
necessary	O
,	O
emulators	B-Application
and	O
virtual	B-Architecture
machines	I-Architecture
can	O
employ	O
hardware	B-Architecture
support	O
to	O
increase	O
performance	O
of	O
their	O
virtual	B-Architecture
memory	I-Architecture
implementations	O
.	O
</s>
<s>
Older	O
operating	B-General_Concept
systems	I-General_Concept
,	O
such	O
as	O
those	O
for	O
the	O
mainframes	B-Architecture
of	O
the	O
1960s	O
,	O
and	O
those	O
for	O
personal	O
computers	O
of	O
the	O
early	O
to	O
mid-1980s	O
(	O
e.g.	O
,	O
DOS	B-Device
)	O
,	O
generally	O
have	O
no	O
virtual	B-Architecture
memory	I-Architecture
functionality	O
,	O
though	O
notable	O
exceptions	O
for	O
mainframes	B-Architecture
of	O
the	O
1960s	O
include	O
:	O
</s>
<s>
During	O
the	O
1960s	O
and	O
early	O
'	O
70s	O
,	O
computer	B-General_Concept
memory	I-General_Concept
was	O
very	O
expensive	O
.	O
</s>
<s>
The	O
introduction	O
of	O
virtual	B-Architecture
memory	I-Architecture
provided	O
an	O
ability	O
for	O
software	O
systems	O
with	O
large	O
memory	O
demands	O
to	O
run	O
on	O
computers	O
with	O
less	O
real	O
memory	O
.	O
</s>
<s>
The	O
savings	O
from	O
this	O
provided	O
a	O
strong	O
incentive	O
to	O
switch	O
to	O
virtual	B-Architecture
memory	I-Architecture
for	O
all	O
systems	O
.	O
</s>
<s>
The	O
additional	O
capability	O
of	O
providing	O
virtual	B-General_Concept
address	I-General_Concept
spaces	I-General_Concept
added	O
another	O
level	O
of	O
security	O
and	O
reliability	O
,	O
thus	O
making	O
virtual	B-Architecture
memory	I-Architecture
even	O
more	O
attractive	O
to	O
the	O
marketplace	O
.	O
</s>
<s>
Most	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
that	O
support	O
virtual	B-Architecture
memory	I-Architecture
also	O
run	O
each	O
process	B-Operating_System
in	O
its	O
own	O
dedicated	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
Each	O
program	O
thus	O
appears	O
to	O
have	O
sole	O
access	O
to	O
the	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
However	O
,	O
some	O
older	O
operating	B-General_Concept
systems	I-General_Concept
(	O
such	O
as	O
OS/VS1	B-Application
and	O
OS/VS2	B-Application
SVS	I-Application
)	O
and	O
even	O
modern	O
ones	O
(	O
such	O
as	O
IBM	B-Application
i	I-Application
)	O
are	O
single	B-Operating_System
address	I-Operating_System
space	I-Operating_System
operating	I-Operating_System
systems	I-Operating_System
that	O
run	O
all	O
processes	O
in	O
a	O
single	O
address	B-General_Concept
space	I-General_Concept
composed	O
of	O
virtualized	O
memory	O
.	O
</s>
<s>
Embedded	B-Architecture
systems	I-Architecture
and	O
other	O
special-purpose	B-Architecture
computer	I-Architecture
systems	O
that	O
require	O
very	O
fast	O
and/or	O
very	O
consistent	O
response	O
times	O
may	O
opt	O
not	O
to	O
use	O
virtual	B-Architecture
memory	I-Architecture
due	O
to	O
decreased	O
determinism	B-General_Concept
;	O
virtual	B-Operating_System
memory	I-Operating_System
systems	I-Operating_System
trigger	O
unpredictable	O
traps	B-Application
that	O
may	O
produce	O
unwanted	O
and	O
unpredictable	O
delays	O
in	O
response	O
to	O
input	O
,	O
especially	O
if	O
the	O
trap	B-Application
requires	O
that	O
data	O
be	O
read	O
into	O
main	O
memory	O
from	O
secondary	O
memory	O
.	O
</s>
<s>
The	O
hardware	B-Architecture
to	O
translate	O
virtual	B-General_Concept
addresses	I-General_Concept
to	O
physical	O
addresses	O
typically	O
requires	O
a	O
significant	O
chip	O
area	O
to	O
implement	O
,	O
and	O
not	O
all	O
chips	O
used	O
in	O
embedded	B-Architecture
systems	I-Architecture
include	O
that	O
hardware	B-Architecture
,	O
which	O
is	O
another	O
reason	O
some	O
of	O
those	O
systems	O
do	O
n't	O
use	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
In	O
the	O
1950s	O
,	O
all	O
larger	O
programs	O
had	O
to	O
contain	O
logic	O
for	O
managing	O
primary	O
and	O
secondary	O
storage	O
,	O
such	O
as	O
overlaying	B-General_Concept
.	O
</s>
<s>
Virtual	B-Architecture
memory	I-Architecture
was	O
therefore	O
introduced	O
not	O
only	O
to	O
extend	O
primary	O
memory	O
,	O
but	O
to	O
make	O
such	O
an	O
extension	O
as	O
easy	O
as	O
possible	O
for	O
programmers	O
to	O
use	O
.	O
</s>
<s>
To	O
allow	O
for	O
multiprogramming	O
and	O
multitasking	B-Operating_System
,	O
many	O
early	O
systems	O
divided	O
memory	O
between	O
multiple	O
programs	O
without	O
virtual	B-Architecture
memory	I-Architecture
,	O
such	O
as	O
early	O
models	O
of	O
the	O
PDP-10	B-Device
via	O
registers	B-General_Concept
.	O
</s>
<s>
A	O
claim	O
that	O
the	O
concept	O
of	O
virtual	B-Architecture
memory	I-Architecture
was	O
first	O
developed	O
by	O
German	O
physicist	O
Fritz-Rudolf	O
Güntsch	O
at	O
the	O
Technische	O
Universität	O
Berlin	O
in	O
1956	O
in	O
his	O
doctoral	O
thesis	O
,	O
Logical	O
Design	O
of	O
a	O
Digital	O
Computer	O
with	O
Multiple	O
Asynchronous	O
Rotating	O
Drums	B-General_Concept
and	O
Automatic	O
High	O
Speed	O
Memory	O
Operation	O
does	O
not	O
stand	O
up	O
to	O
careful	O
scrutiny	O
.	O
</s>
<s>
The	O
computer	O
proposed	O
by	O
Güntsch	O
(	O
but	O
never	O
built	O
)	O
had	O
an	O
address	B-General_Concept
space	I-General_Concept
of	O
105	O
words	O
which	O
mapped	O
exactly	O
onto	O
the	O
105	O
words	O
of	O
the	O
drums	B-General_Concept
,	O
i.e.	O
</s>
<s>
the	O
addresses	O
were	O
real	O
addresses	O
and	O
there	O
was	O
no	O
form	O
of	O
indirect	O
mapping	O
,	O
a	O
key	O
feature	O
of	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
What	O
Güntsch	O
did	O
invent	O
was	O
a	O
form	O
of	O
cache	B-General_Concept
memory	I-General_Concept
,	O
since	O
his	O
high-speed	O
memory	O
was	O
intended	O
to	O
contain	O
a	O
copy	O
of	O
some	O
blocks	O
of	O
code	O
or	O
data	O
taken	O
from	O
the	O
drums	B-General_Concept
.	O
</s>
<s>
This	O
is	O
exactly	O
the	O
situation	O
in	O
computers	O
with	O
cache	B-General_Concept
memory	I-General_Concept
,	O
one	O
of	O
the	O
earliest	O
commercial	O
examples	O
of	O
which	O
was	O
the	O
IBM	O
System/360	O
Model	O
85	O
.	O
</s>
<s>
A	O
semiconductor	O
cache	B-General_Concept
store	O
,	O
invisible	O
to	O
the	O
user	O
,	O
held	O
the	O
contents	O
of	O
parts	O
of	O
the	O
main	O
store	O
in	O
use	O
by	O
the	O
currently	O
executing	O
program	O
.	O
</s>
<s>
This	O
is	O
exactly	O
analogous	O
to	O
Güntsch	O
's	O
system	O
,	O
designed	O
as	O
a	O
means	O
to	O
improve	O
performance	O
,	O
rather	O
than	O
to	O
solve	O
the	O
problems	O
involved	O
in	O
multi-programming	B-Operating_System
.	O
</s>
<s>
The	O
first	O
true	O
virtual	B-Operating_System
memory	I-Operating_System
system	I-Operating_System
was	O
that	O
implemented	O
at	O
the	O
University	O
of	O
Manchester	O
to	O
create	O
a	O
one-level	O
storage	O
system	O
as	O
part	O
of	O
the	O
Atlas	B-Device
Computer	I-Device
.	O
</s>
<s>
It	O
used	O
a	O
paging	B-Architecture
mechanism	O
to	O
map	O
the	O
virtual	B-General_Concept
addresses	I-General_Concept
available	O
to	O
the	O
programmer	O
onto	O
the	O
real	O
memory	O
that	O
consisted	O
of	O
16,384	O
words	O
of	O
primary	O
core	O
memory	O
with	O
an	O
additional	O
98,304	O
words	O
of	O
secondary	O
drum	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
The	O
addition	O
of	O
virtual	B-Architecture
memory	I-Architecture
into	O
the	O
Atlas	B-Device
also	O
eliminated	O
a	O
looming	O
programming	O
problem	O
:	O
planning	O
and	O
scheduling	O
data	O
transfers	O
between	O
main	O
and	O
secondary	O
memory	O
and	O
recompiling	O
programs	O
for	O
each	O
change	O
of	O
size	O
of	O
main	O
memory	O
.	O
</s>
<s>
The	O
first	O
Atlas	B-Device
was	O
commissioned	O
in	O
1962	O
but	O
working	O
prototypes	O
of	O
paging	B-Architecture
had	O
been	O
developed	O
by	O
1959	O
.	O
</s>
<s>
In	O
1961	O
,	O
the	O
Burroughs	O
Corporation	O
independently	O
released	O
the	O
first	O
commercial	O
computer	O
with	O
virtual	B-Architecture
memory	I-Architecture
,	O
the	O
B5000	O
,	O
with	O
segmentation	B-General_Concept
rather	O
than	O
paging	B-Architecture
.	O
</s>
<s>
IBM	O
developed	O
the	O
concept	O
of	O
hypervisors	B-Operating_System
in	O
their	O
CP-40	B-Application
and	O
CP-67	B-Application
,	O
and	O
in	O
1972	O
provided	O
it	O
for	O
the	O
S/370	B-Device
as	O
Virtual	B-Architecture
Machine	I-Architecture
Facility/370	O
.	O
</s>
<s>
IBM	O
introduced	O
the	O
Start	O
Interpretive	O
Execution	O
(	O
SIE	O
)	O
instruction	O
as	O
part	O
of	O
370-XA	O
on	O
the	O
3081	O
,	O
and	O
VM/XA	O
versions	O
of	O
VM	B-Application
to	O
exploit	O
it	O
.	O
</s>
<s>
Before	O
virtual	B-Architecture
memory	I-Architecture
could	O
be	O
implemented	O
in	O
mainstream	O
operating	B-General_Concept
systems	I-General_Concept
,	O
many	O
problems	O
had	O
to	O
be	O
addressed	O
.	O
</s>
<s>
Dynamic	O
address	O
translation	O
required	O
expensive	O
and	O
difficult-to-build	O
specialized	O
hardware	B-Architecture
;	O
initial	O
implementations	O
slowed	O
down	O
access	O
to	O
memory	O
slightly	O
.	O
</s>
<s>
By	O
1969	O
,	O
the	O
debate	O
over	O
virtual	B-Architecture
memory	I-Architecture
for	O
commercial	O
computers	O
was	O
over	O
;	O
an	O
IBM	O
research	O
team	O
led	O
by	O
David	O
Sayre	O
showed	O
that	O
their	O
virtual	B-Architecture
memory	I-Architecture
overlay	B-General_Concept
system	I-General_Concept
consistently	O
worked	O
better	O
than	O
the	O
best	O
manually	O
controlled	O
systems	O
.	O
</s>
<s>
Throughout	O
the	O
1970s	O
,	O
the	O
IBM	B-Device
370	I-Device
series	O
running	O
their	O
virtual-storage	O
based	O
operating	B-General_Concept
systems	I-General_Concept
provided	O
a	O
means	O
for	O
business	O
users	O
to	O
migrate	O
multiple	O
older	O
systems	O
into	O
fewer	O
,	O
more	O
powerful	O
,	O
mainframes	B-Architecture
that	O
had	O
improved	O
price/performance	O
.	O
</s>
<s>
The	O
first	O
minicomputer	B-Architecture
to	O
introduce	O
virtual	B-Architecture
memory	I-Architecture
was	O
the	O
Norwegian	O
NORD-1	B-Device
;	O
during	O
the	O
1970s	O
,	O
other	O
minicomputers	B-Architecture
implemented	O
virtual	B-Architecture
memory	I-Architecture
,	O
notably	O
VAX	B-Device
models	O
running	O
VMS	B-Operating_System
.	O
</s>
<s>
Virtual	B-Architecture
memory	I-Architecture
was	O
introduced	O
to	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
with	O
the	O
protected	B-Application
mode	I-Application
of	O
the	O
Intel	B-General_Concept
80286	I-General_Concept
processor	O
,	O
but	O
its	O
segment	O
swapping	O
technique	O
scaled	O
poorly	O
to	O
larger	O
segment	O
sizes	O
.	O
</s>
<s>
The	O
Intel	B-General_Concept
80386	I-General_Concept
introduced	O
paging	B-Architecture
support	O
underneath	O
the	O
existing	O
segmentation	B-General_Concept
layer	O
,	O
enabling	O
the	O
page	B-General_Concept
fault	I-General_Concept
exception	B-Application
to	O
chain	O
with	O
other	O
exceptions	O
without	O
double	B-General_Concept
fault	I-General_Concept
.	O
</s>
<s>
However	O
,	O
loading	O
segment	O
descriptors	O
was	O
an	O
expensive	O
operation	O
,	O
causing	O
operating	B-General_Concept
system	I-General_Concept
designers	O
to	O
rely	O
strictly	O
on	O
paging	B-Architecture
rather	O
than	O
a	O
combination	O
of	O
paging	B-Architecture
and	O
segmentation	B-General_Concept
.	O
</s>
<s>
Nearly	O
all	O
current	O
implementations	O
of	O
virtual	B-Architecture
memory	I-Architecture
divide	O
a	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
into	O
pages	O
,	O
blocks	O
of	O
contiguous	O
virtual	B-Architecture
memory	I-Architecture
addresses	O
.	O
</s>
<s>
Pages	O
on	O
contemporary	O
systems	O
are	O
usually	O
at	O
least	O
4	O
kilobytes	O
in	O
size	O
;	O
systems	O
with	O
large	O
virtual	B-General_Concept
address	I-General_Concept
ranges	O
or	O
amounts	O
of	O
real	O
memory	O
generally	O
use	O
larger	O
page	B-General_Concept
sizes	I-General_Concept
.	O
</s>
<s>
Page	B-General_Concept
tables	I-General_Concept
are	O
used	O
to	O
translate	O
the	O
virtual	B-General_Concept
addresses	I-General_Concept
seen	O
by	O
the	O
application	O
into	O
physical	O
addresses	O
used	O
by	O
the	O
hardware	B-Architecture
to	O
process	B-Operating_System
instructions	O
;	O
such	O
hardware	B-Architecture
that	O
handles	O
this	O
specific	O
translation	O
is	O
often	O
known	O
as	O
the	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
.	O
</s>
<s>
Each	O
entry	O
in	O
the	O
page	B-General_Concept
table	I-General_Concept
holds	O
a	O
flag	O
indicating	O
whether	O
the	O
corresponding	O
page	B-General_Concept
is	O
in	O
real	O
memory	O
or	O
not	O
.	O
</s>
<s>
If	O
it	O
is	O
in	O
real	O
memory	O
,	O
the	O
page	B-General_Concept
table	I-General_Concept
entry	O
will	O
contain	O
the	O
real	O
memory	B-General_Concept
address	I-General_Concept
at	O
which	O
the	O
page	B-General_Concept
is	O
stored	O
.	O
</s>
<s>
When	O
a	O
reference	O
is	O
made	O
to	O
a	O
page	B-General_Concept
by	O
the	O
hardware	B-Architecture
,	O
if	O
the	O
page	B-General_Concept
table	I-General_Concept
entry	O
for	O
the	O
page	B-General_Concept
indicates	O
that	O
it	O
is	O
not	O
currently	O
in	O
real	O
memory	O
,	O
the	O
hardware	B-Architecture
raises	O
a	O
page	B-General_Concept
fault	I-General_Concept
exception	B-Application
,	O
invoking	O
the	O
paging	B-Architecture
supervisor	O
component	O
of	O
the	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
Systems	O
can	O
have	O
,	O
e.g.	O
,	O
one	O
page	B-General_Concept
table	I-General_Concept
for	O
the	O
whole	O
system	O
,	O
separate	O
page	B-General_Concept
tables	I-General_Concept
for	O
each	O
address	B-General_Concept
space	I-General_Concept
or	O
process	B-Operating_System
,	O
separate	O
page	B-General_Concept
tables	I-General_Concept
for	O
each	O
segment	O
;	O
similarly	O
,	O
systems	O
can	O
have	O
,	O
e.g.	O
,	O
no	O
segment	O
table	O
,	O
one	O
segment	O
table	O
for	O
the	O
whole	O
system	O
,	O
separate	O
segment	O
tables	O
for	O
each	O
address	B-General_Concept
space	I-General_Concept
or	O
process	B-Operating_System
,	O
separate	O
segment	O
tables	O
for	O
each	O
region	O
in	O
a	O
tree	O
of	O
region	O
tables	O
for	O
each	O
address	B-General_Concept
space	I-General_Concept
or	O
process	B-Operating_System
.	O
</s>
<s>
If	O
there	O
is	O
only	O
one	O
page	B-General_Concept
table	I-General_Concept
,	O
different	O
applications	O
running	O
at	O
the	O
same	O
time	O
use	O
different	O
parts	O
of	O
a	O
single	O
range	O
of	O
virtual	B-General_Concept
addresses	I-General_Concept
.	O
</s>
<s>
If	O
there	O
are	O
multiple	O
page	B-General_Concept
or	O
segment	O
tables	O
,	O
there	O
are	O
multiple	O
virtual	B-General_Concept
address	I-General_Concept
spaces	I-General_Concept
and	O
concurrent	O
applications	O
with	O
separate	O
page	B-General_Concept
tables	I-General_Concept
redirect	O
to	O
different	O
real	O
addresses	O
.	O
</s>
<s>
Some	O
earlier	O
systems	O
with	O
smaller	O
real	O
memory	O
sizes	O
,	O
such	O
as	O
the	O
SDS	B-Device
940	I-Device
,	O
used	O
page	B-General_Concept
registers	I-General_Concept
instead	O
of	O
page	B-General_Concept
tables	I-General_Concept
in	O
memory	O
for	O
address	O
translation	O
.	O
</s>
<s>
This	O
part	O
of	O
the	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
creates	O
and	O
manages	O
page	B-General_Concept
tables	I-General_Concept
and	O
lists	O
of	O
free	O
page	B-General_Concept
frames	I-General_Concept
.	O
</s>
<s>
In	O
order	O
to	O
ensure	O
that	O
there	O
will	O
be	O
enough	O
free	O
page	B-General_Concept
frames	I-General_Concept
to	O
quickly	O
resolve	O
page	B-General_Concept
faults	I-General_Concept
,	O
the	O
system	O
may	O
periodically	O
steal	O
allocated	O
page	B-General_Concept
frames	I-General_Concept
,	O
using	O
a	O
page	B-General_Concept
replacement	I-General_Concept
algorithm	I-General_Concept
,	O
e.g.	O
,	O
a	O
Least	B-General_Concept
recently	I-General_Concept
used	I-General_Concept
(	O
LRU	O
)	O
algorithm	O
.	O
</s>
<s>
Stolen	O
page	B-General_Concept
frames	I-General_Concept
that	O
have	O
been	O
modified	O
are	O
written	O
back	O
to	O
auxiliary	O
storage	O
before	O
they	O
are	O
added	O
to	O
the	O
free	O
queue	O
.	O
</s>
<s>
On	O
some	O
systems	O
the	O
paging	B-Architecture
supervisor	O
is	O
also	O
responsible	O
for	O
managing	O
translation	O
registers	B-General_Concept
that	O
are	O
not	O
automatically	O
loaded	O
from	O
page	B-General_Concept
tables	I-General_Concept
.	O
</s>
<s>
Typically	O
,	O
a	O
page	B-General_Concept
fault	I-General_Concept
that	O
cannot	O
be	O
resolved	O
results	O
in	O
an	O
abnormal	O
termination	O
of	O
the	O
application	O
.	O
</s>
<s>
However	O
,	O
some	O
systems	O
allow	O
the	O
application	O
to	O
have	O
exception	B-Application
handlers	O
for	O
such	O
errors	O
.	O
</s>
<s>
The	O
paging	B-Architecture
supervisor	O
may	O
handle	O
a	O
page	B-General_Concept
fault	I-General_Concept
exception	B-Application
in	O
several	O
different	O
ways	O
,	O
depending	O
on	O
the	O
details	O
:	O
</s>
<s>
If	O
the	O
virtual	B-General_Concept
address	I-General_Concept
is	O
invalid	O
,	O
the	O
paging	B-Architecture
supervisor	O
treats	O
it	O
as	O
an	O
error	O
.	O
</s>
<s>
If	O
the	O
page	B-General_Concept
is	O
valid	O
and	O
the	O
page	B-General_Concept
information	O
is	O
not	O
loaded	O
into	O
the	O
MMU	O
,	O
the	O
page	B-General_Concept
information	O
will	O
be	O
stored	O
into	O
one	O
of	O
the	O
page	B-General_Concept
registers	I-General_Concept
.	O
</s>
<s>
If	O
the	O
page	B-General_Concept
is	O
uninitialized	O
,	O
a	O
new	O
page	B-General_Concept
frame	I-General_Concept
may	O
be	O
assigned	O
and	O
cleared	O
.	O
</s>
<s>
If	O
there	O
is	O
a	O
stolen	O
page	B-General_Concept
frame	I-General_Concept
containing	O
the	O
desired	O
page	B-General_Concept
,	O
that	O
page	B-General_Concept
frame	I-General_Concept
will	O
be	O
reused	O
.	O
</s>
<s>
For	O
a	O
fault	O
due	O
to	O
a	O
write	O
attempt	O
into	O
a	O
read-protected	O
page	B-General_Concept
,	O
if	O
it	O
is	O
a	O
copy-on-write	O
page	B-General_Concept
then	O
a	O
free	O
page	B-General_Concept
frame	I-General_Concept
will	O
be	O
assigned	O
and	O
the	O
contents	O
of	O
the	O
old	O
page	B-General_Concept
copied	O
;	O
otherwise	O
it	O
is	O
treated	O
as	O
an	O
error	O
.	O
</s>
<s>
If	O
the	O
virtual	B-General_Concept
address	I-General_Concept
is	O
a	O
valid	O
page	B-Architecture
in	I-Architecture
a	O
memory-mapped	O
file	O
or	O
a	O
paging	B-Architecture
file	I-Architecture
,	O
a	O
free	O
page	B-General_Concept
frame	I-General_Concept
will	O
be	O
assigned	O
and	O
the	O
page	B-General_Concept
read	O
in	O
.	O
</s>
<s>
In	O
most	O
cases	O
,	O
there	O
will	O
be	O
an	O
update	O
to	O
the	O
page	B-General_Concept
table	I-General_Concept
,	O
possibly	O
followed	O
by	O
purging	O
the	O
Translation	O
Lookaside	O
Buffer	B-General_Concept
(	O
TLB	O
)	O
,	O
and	O
the	O
system	O
restarts	O
the	O
instruction	O
that	O
causes	O
the	O
exception	B-Application
.	O
</s>
<s>
If	O
the	O
free	O
page	B-General_Concept
frame	I-General_Concept
queue	O
is	O
empty	O
then	O
the	O
paging	B-Architecture
supervisor	O
must	O
free	O
a	O
page	B-General_Concept
frame	I-General_Concept
using	O
the	O
same	O
page	B-General_Concept
replacement	I-General_Concept
algorithm	I-General_Concept
for	O
page	B-General_Concept
stealing	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
have	O
memory	O
areas	O
that	O
are	O
pinned	O
(	O
never	O
swapped	O
to	O
secondary	O
storage	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
interrupt	B-Application
mechanisms	O
rely	O
on	O
an	O
array	O
of	O
pointers	O
to	O
their	O
handlers	O
,	O
such	O
as	O
I/O	B-General_Concept
completion	O
and	O
page	B-General_Concept
fault	I-General_Concept
.	O
</s>
<s>
If	O
the	O
pages	O
containing	O
these	O
pointers	O
or	O
the	O
code	O
that	O
they	O
invoke	O
were	O
pageable	O
,	O
interrupt-handling	O
would	O
become	O
far	O
more	O
complex	O
and	O
time-consuming	O
,	O
particularly	O
in	O
the	O
case	O
of	O
page	B-General_Concept
fault	I-General_Concept
interruptions	O
.	O
</s>
<s>
Hence	O
,	O
some	O
part	O
of	O
the	O
page	B-General_Concept
table	I-General_Concept
structures	O
is	O
not	O
pageable	O
.	O
</s>
<s>
The	O
paging	B-Architecture
supervisor	O
code	O
and	O
drivers	O
for	O
secondary	O
storage	O
devices	O
on	O
which	O
pages	O
reside	O
must	O
be	O
permanently	O
pinned	O
,	O
as	O
otherwise	O
paging	B-Architecture
would	O
n't	O
even	O
work	O
because	O
the	O
necessary	O
code	O
would	O
n't	O
be	O
available	O
.	O
</s>
<s>
Timing-dependent	O
components	O
may	O
be	O
pinned	O
to	O
avoid	O
variable	O
paging	B-Architecture
delays	O
.	O
</s>
<s>
Data	B-General_Concept
buffers	I-General_Concept
that	O
are	O
accessed	O
directly	O
by	O
peripheral	O
devices	O
that	O
use	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
or	O
I/O	B-Device
channels	I-Device
must	O
reside	O
in	O
pinned	O
pages	O
while	O
the	O
I/O	B-General_Concept
operation	I-General_Concept
is	O
in	O
progress	O
because	O
such	O
devices	O
and	O
the	O
buses	B-General_Concept
to	O
which	O
they	O
are	O
attached	O
expect	O
to	O
find	O
data	B-General_Concept
buffers	I-General_Concept
located	O
at	O
physical	O
memory	O
addresses	O
;	O
regardless	O
of	O
whether	O
the	O
bus	O
has	O
a	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
for	I-General_Concept
I/O	I-General_Concept
,	O
transfers	O
cannot	O
be	O
stopped	O
if	O
a	O
page	B-General_Concept
fault	I-General_Concept
occurs	O
and	O
then	O
restarted	O
when	O
the	O
page	B-General_Concept
fault	I-General_Concept
has	O
been	O
processed	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
data	O
could	O
come	O
from	O
a	O
measurement	O
sensor	O
unit	O
and	O
lost	O
real	O
time	O
data	O
that	O
got	O
lost	O
because	O
of	O
a	O
page	B-General_Concept
fault	I-General_Concept
ca	O
n't	O
be	O
recovered	O
.	O
</s>
<s>
In	O
IBM	O
's	O
operating	B-General_Concept
systems	I-General_Concept
for	O
System/370	B-Device
and	O
successor	O
systems	O
,	O
the	O
term	O
is	O
"	O
fixed	O
"	O
,	O
and	O
such	O
pages	O
may	O
be	O
long-term	O
fixed	O
,	O
or	O
may	O
be	O
short-term	O
fixed	O
,	O
or	O
may	O
be	O
unfixed	O
(	O
i.e.	O
,	O
pageable	O
)	O
.	O
</s>
<s>
System	O
control	O
structures	O
are	O
often	O
long-term	O
fixed	O
(	O
measured	O
in	O
wall-clock	O
time	O
,	O
i.e.	O
,	O
time	O
measured	O
in	O
seconds	O
,	O
rather	O
than	O
time	O
measured	O
in	O
fractions	O
of	O
one	O
second	O
)	O
whereas	O
I/O	B-General_Concept
buffers	B-General_Concept
are	O
usually	O
short-term	O
fixed	O
(	O
usually	O
measured	O
in	O
significantly	O
less	O
than	O
wall-clock	O
time	O
,	O
possibly	O
for	O
tens	O
of	O
milliseconds	O
)	O
.	O
</s>
<s>
Indeed	O
,	O
the	B-Operating_System
OS	I-Operating_System
has	O
a	O
special	O
facility	O
for	O
"	O
fast	O
fixing	O
"	O
these	O
short-term	O
fixed	O
data	B-General_Concept
buffers	I-General_Concept
(	O
fixing	O
which	O
is	O
performed	O
without	O
resorting	O
to	O
a	O
time-consuming	O
Supervisor	B-Operating_System
Call	I-Operating_System
instruction	I-Operating_System
)	O
.	O
</s>
<s>
Multics	B-Application
used	O
the	O
term	O
"	O
wired	O
"	O
.	O
</s>
<s>
OpenVMS	B-Operating_System
and	O
Windows	B-Application
refer	O
to	O
pages	O
temporarily	O
made	O
nonpageable	O
(	O
as	O
for	O
I/O	B-General_Concept
buffers	B-General_Concept
)	O
as	O
"	O
locked	O
"	O
,	O
and	O
simply	O
"	O
nonpageable	O
"	O
for	O
those	O
that	O
are	O
never	O
pageable	O
.	O
</s>
<s>
The	O
Single	O
UNIX	O
Specification	O
also	O
uses	O
the	O
term	O
"	O
locked	O
"	O
in	O
the	O
specification	O
for	O
,	O
as	O
do	O
the	O
man	B-Protocol
pages	I-Protocol
on	O
many	O
Unix-like	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
In	O
OS/VS1	B-Application
and	O
similar	O
OSes	O
,	O
some	O
parts	O
of	O
systems	O
memory	O
are	O
managed	O
in	O
"	O
virtual-real	O
"	O
mode	O
,	O
called	O
"	O
V=	O
R	O
"	O
.	O
</s>
<s>
In	O
this	O
mode	O
every	O
virtual	B-General_Concept
address	I-General_Concept
corresponds	O
to	O
the	O
same	O
real	B-General_Concept
address	I-General_Concept
.	O
</s>
<s>
This	O
mode	O
is	O
used	O
for	O
interrupt	B-Application
mechanisms	O
,	O
for	O
the	O
paging	B-Architecture
supervisor	O
and	O
page	B-General_Concept
tables	I-General_Concept
in	O
older	O
systems	O
,	O
and	O
for	O
application	O
programs	O
using	O
non-standard	O
I/O	B-General_Concept
management	O
.	O
</s>
<s>
For	O
example	O
,	O
IBM	O
's	O
z/OS	B-Application
has	O
3	O
modes	O
(	O
virtual-virtual	O
,	O
virtual-real	O
and	O
virtual-fixed	O
)	O
.	O
</s>
<s>
When	O
paging	B-Architecture
and	O
page	B-General_Concept
stealing	O
are	O
used	O
,	O
a	O
problem	O
called	O
"	O
thrashing	B-General_Concept
"	O
can	O
occur	O
,	O
in	O
which	O
the	O
computer	O
spends	O
an	O
unsuitably	O
large	O
amount	O
of	O
time	O
transferring	O
pages	O
to	O
and	O
from	O
a	O
backing	B-General_Concept
store	I-General_Concept
,	O
hence	O
slowing	O
down	O
useful	O
work	O
.	O
</s>
<s>
A	O
task	O
's	O
working	B-General_Concept
set	I-General_Concept
is	O
the	O
minimum	O
set	O
of	O
pages	O
that	O
should	O
be	O
in	O
memory	O
in	O
order	O
for	O
it	O
to	O
make	O
useful	O
progress	O
.	O
</s>
<s>
Thrashing	B-General_Concept
occurs	O
when	O
there	O
is	O
insufficient	B-General_Concept
memory	I-General_Concept
available	O
to	O
store	O
the	O
working	B-General_Concept
sets	I-General_Concept
of	O
all	O
active	O
programs	O
.	O
</s>
<s>
Adding	O
real	O
memory	O
is	O
the	O
simplest	O
response	O
,	O
but	O
improving	O
application	O
design	O
,	O
scheduling	O
,	O
and	O
memory	B-General_Concept
usage	I-General_Concept
can	O
help	O
.	O
</s>
<s>
This	O
reduces	O
demand	O
on	O
real	O
memory	O
by	O
swapping	O
out	O
the	O
entire	O
working	B-General_Concept
set	I-General_Concept
of	O
one	O
or	O
more	O
processes	O
.	O
</s>
<s>
A	O
system	O
thrashing	B-General_Concept
is	O
often	O
a	O
result	O
of	O
a	O
sudden	O
spike	O
in	O
page	B-General_Concept
demand	O
from	O
a	O
small	O
number	O
of	O
running	O
programs	O
.	O
</s>
<s>
Swap-token	O
is	O
a	O
lightweight	O
and	O
dynamic	O
thrashing	B-General_Concept
protection	O
mechanism	O
.	O
</s>
<s>
The	O
basic	O
idea	O
is	O
to	O
set	O
a	O
token	O
in	O
the	O
system	O
,	O
which	O
is	O
randomly	O
given	O
to	O
a	O
process	B-Operating_System
that	O
has	O
page	B-General_Concept
faults	I-General_Concept
when	O
thrashing	B-General_Concept
happens	O
.	O
</s>
<s>
The	O
process	B-Operating_System
that	O
has	O
the	O
token	O
is	O
given	O
a	O
privilege	O
to	O
allocate	O
more	O
physical	O
memory	B-General_Concept
pages	I-General_Concept
to	O
build	O
its	O
working	B-General_Concept
set	I-General_Concept
,	O
which	O
is	O
expected	O
to	O
quickly	O
finish	O
its	O
execution	O
and	O
to	O
release	O
the	O
memory	B-General_Concept
pages	I-General_Concept
to	O
other	O
processes	O
.	O
</s>
<s>
The	O
first	O
version	O
of	O
swap-token	O
is	O
implemented	O
in	O
Linux	O
The	O
second	O
version	O
is	O
called	O
preempt	O
swap-token	O
In	O
this	O
updated	O
swap-token	O
implementation	O
,	O
a	O
priority	O
counter	O
is	O
set	O
for	O
each	O
process	B-Operating_System
to	O
track	O
the	O
number	O
of	O
swap-out	O
pages	O
.	O
</s>
<s>
The	O
token	O
is	O
always	O
given	O
to	O
the	O
process	B-Operating_System
with	O
a	O
high	O
priority	O
,	O
which	O
has	O
a	O
high	O
number	O
of	O
swap-out	O
pages	O
.	O
</s>
<s>
The	O
length	O
of	O
the	O
time	O
stamp	O
is	O
not	O
a	O
constant	O
but	O
is	O
determined	O
by	O
the	O
priority	O
:	O
the	O
higher	O
the	O
number	O
of	O
swap-out	O
pages	O
of	O
a	O
process	B-Operating_System
,	O
the	O
longer	O
the	O
time	O
stamp	O
for	O
it	O
will	O
be	O
.	O
</s>
<s>
Some	O
systems	O
,	O
such	O
as	O
the	O
Burroughs	O
B5500	O
,	O
use	O
segmentation	B-General_Concept
instead	O
of	O
paging	B-Architecture
,	O
dividing	O
virtual	B-General_Concept
address	I-General_Concept
spaces	I-General_Concept
into	O
variable-length	O
segments	B-General_Concept
.	O
</s>
<s>
A	O
virtual	B-General_Concept
address	I-General_Concept
here	O
consists	O
of	O
a	O
segment	O
number	O
and	O
an	O
offset	O
within	O
the	O
segment	O
.	O
</s>
<s>
The	O
Intel	B-General_Concept
80286	I-General_Concept
supports	O
a	O
similar	O
segmentation	B-General_Concept
scheme	O
as	O
an	O
option	O
,	O
but	O
it	O
is	O
rarely	O
used	O
.	O
</s>
<s>
Segmentation	B-General_Concept
and	O
paging	B-Architecture
can	O
be	O
used	O
together	O
by	O
dividing	O
each	O
segment	O
into	O
pages	O
;	O
systems	O
with	O
this	O
memory	O
structure	O
,	O
such	O
as	O
Multics	B-Application
and	O
IBM	B-Device
System/38	I-Device
,	O
are	O
usually	O
paging-predominant	O
,	O
segmentation	B-General_Concept
providing	O
memory	O
protection	O
.	O
</s>
<s>
In	O
the	O
Intel	B-General_Concept
80386	I-General_Concept
and	O
later	O
IA-32	B-Device
processors	O
,	O
the	O
segments	B-General_Concept
reside	O
in	O
a	O
32-bit	O
linear	O
,	O
paged	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
Segments	B-General_Concept
can	O
be	O
moved	O
in	O
and	O
out	O
of	O
that	O
space	O
;	O
pages	O
there	O
can	O
"	O
page	B-General_Concept
"	O
in	O
and	O
out	O
of	O
main	O
memory	O
,	O
providing	O
two	O
levels	O
of	O
virtual	B-Architecture
memory	I-Architecture
;	O
few	O
if	O
any	O
operating	B-General_Concept
systems	I-General_Concept
do	O
so	O
,	O
instead	O
using	O
only	O
paging	B-Architecture
.	O
</s>
<s>
Early	O
non-hardware-assisted	O
x86	B-General_Concept
virtualization	I-General_Concept
solutions	O
combined	O
paging	B-Architecture
and	O
segmentation	B-General_Concept
because	O
x86	B-Operating_System
paging	B-Architecture
offers	O
only	O
two	O
protection	O
domains	O
whereas	O
a	O
VMM	O
,	O
guest	O
OS	O
or	O
guest	O
application	O
stack	O
needs	O
three	O
.	O
</s>
<s>
The	O
difference	O
between	O
paging	B-Architecture
and	O
segmentation	B-General_Concept
systems	O
is	O
not	O
only	O
about	O
memory	O
division	O
;	O
segmentation	B-General_Concept
is	O
visible	O
to	O
user	O
processes	O
,	O
as	O
part	O
of	O
memory	O
model	O
semantics	O
.	O
</s>
<s>
This	O
difference	O
has	O
important	O
consequences	O
;	O
a	O
segment	O
is	O
not	O
a	O
page	B-General_Concept
with	O
variable	O
length	O
or	O
a	O
simple	O
way	O
to	O
lengthen	O
the	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
Segmentation	B-General_Concept
that	O
can	O
provide	O
a	O
single-level	O
memory	O
model	O
in	O
which	O
there	O
is	O
no	O
differentiation	O
between	O
process	B-Operating_System
memory	O
and	O
file	O
system	O
consists	O
of	O
only	O
a	O
list	O
of	O
segments	B-General_Concept
(	O
files	O
)	O
mapped	O
into	O
the	O
process	B-Operating_System
's	O
potential	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
This	O
is	O
not	O
the	O
same	O
as	O
the	O
mechanisms	O
provided	O
by	O
calls	O
such	O
as	O
mmap	B-Operating_System
and	O
Win32	B-Library
's	O
MapViewOfFile	O
,	O
because	O
inter-file	O
pointers	O
do	O
not	O
work	O
when	O
mapping	O
files	O
into	O
semi-arbitrary	O
places	O
.	O
</s>
<s>
In	O
Multics	B-Application
,	O
a	O
file	O
(	O
or	O
a	O
segment	O
from	O
a	O
multi-segment	O
file	O
)	O
is	O
mapped	O
into	O
a	O
segment	O
in	O
the	O
address	B-General_Concept
space	I-General_Concept
,	O
so	O
files	O
are	O
always	O
mapped	O
at	O
a	O
segment	O
boundary	O
.	O
</s>
<s>
A	O
file	O
's	O
linkage	O
section	O
can	O
contain	O
pointers	O
for	O
which	O
an	O
attempt	O
to	O
load	O
the	O
pointer	O
into	O
a	O
register	O
or	O
make	O
an	O
indirect	O
reference	O
through	O
it	O
causes	O
a	O
trap	B-Application
.	O
</s>
<s>
The	O
unresolved	O
pointer	O
contains	O
an	O
indication	O
of	O
the	O
name	O
of	O
the	O
segment	O
to	O
which	O
the	O
pointer	O
refers	O
and	O
an	O
offset	O
within	O
the	O
segment	O
;	O
the	O
handler	O
for	O
the	O
trap	B-Application
maps	O
the	O
segment	O
into	O
the	O
address	B-General_Concept
space	I-General_Concept
,	O
puts	O
the	O
segment	O
number	O
into	O
the	O
pointer	O
,	O
changes	O
the	O
tag	O
field	O
in	O
the	O
pointer	O
so	O
that	O
it	O
no	O
longer	O
causes	O
a	O
trap	B-Application
,	O
and	O
returns	O
to	O
the	O
code	O
where	O
the	O
trap	B-Application
occurred	O
,	O
re-executing	O
the	O
instruction	O
that	O
caused	O
the	O
trap	B-Application
.	O
</s>
<s>
This	O
eliminates	O
the	O
need	O
for	O
a	O
linker	B-Application
completely	O
and	O
works	O
when	O
different	O
processes	O
map	O
the	O
same	O
file	O
into	O
different	O
places	O
in	O
their	O
private	O
address	B-General_Concept
spaces	I-General_Concept
.	O
</s>
<s>
Some	O
operating	B-General_Concept
systems	I-General_Concept
provide	O
for	O
swapping	O
entire	O
address	B-General_Concept
spaces	I-General_Concept
,	O
in	O
addition	O
to	O
whatever	O
facilities	O
they	O
have	O
for	O
paging	B-Architecture
and	O
segmentation	B-General_Concept
.	O
</s>
<s>
When	O
this	O
occurs	O
,	O
the	B-Operating_System
OS	I-Operating_System
writes	O
those	O
pages	O
and	O
segments	B-General_Concept
currently	O
in	O
real	O
memory	O
to	O
swap	O
files	O
.	O
</s>
<s>
In	O
a	O
swap-in	O
,	O
the	B-Operating_System
OS	I-Operating_System
reads	O
back	O
the	O
data	O
from	O
the	O
swap	O
files	O
but	O
does	O
not	O
automatically	O
read	O
back	O
pages	O
that	O
had	O
been	O
paged	O
out	O
at	O
the	O
time	O
of	O
the	O
swap	O
out	O
operation	O
.	O
</s>
<s>
IBM	O
's	O
MVS	B-Application
,	O
from	O
OS/VS2	B-Application
Release	O
2	O
through	O
z/OS	B-Application
,	O
provides	O
for	O
marking	O
an	O
address	B-General_Concept
space	I-General_Concept
as	O
unswappable	O
;	O
doing	O
so	O
does	O
not	O
pin	O
any	O
pages	O
in	O
the	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
In	O
addition	O
,	O
privileged	O
code	O
can	O
temporarily	O
make	O
an	O
address	B-General_Concept
space	I-General_Concept
unswappable	O
using	O
a	O
SYSEVENT	O
Supervisor	B-Operating_System
Call	I-Operating_System
instruction	I-Operating_System
(	O
SVC	O
)	O
;	O
certain	O
changes	O
in	O
the	O
address	B-General_Concept
space	I-General_Concept
properties	O
require	O
that	O
the	B-Operating_System
OS	I-Operating_System
swap	O
it	O
out	O
and	O
then	O
swap	O
it	O
back	O
in	O
,	O
using	O
SYSEVENT	O
TRANSWAP	O
.	O
</s>
<s>
Swapping	O
does	O
not	O
necessarily	O
require	O
memory	B-General_Concept
management	I-General_Concept
hardware	I-General_Concept
,	O
if	O
,	O
for	O
example	O
,	O
multiple	O
jobs	O
are	O
swapped	O
in	O
and	O
out	O
of	O
the	O
same	O
area	O
of	O
storage	O
.	O
</s>
