<s>
In	O
operating	B-General_Concept
systems	I-General_Concept
,	O
memory	B-General_Concept
management	I-General_Concept
is	O
the	O
function	O
responsible	O
for	O
managing	O
the	O
computer	O
's	O
primary	O
memory	O
.	O
</s>
<s>
The	O
memory	B-General_Concept
management	I-General_Concept
function	O
keeps	O
track	O
of	O
the	O
status	O
of	O
each	O
memory	O
location	O
,	O
either	O
allocated	O
or	O
free	O
.	O
</s>
<s>
This	O
is	O
distinct	O
from	O
application	B-General_Concept
memory	I-General_Concept
management	I-General_Concept
,	O
which	O
is	O
how	O
a	O
process	O
manages	O
the	O
memory	O
assigned	O
to	O
it	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Single	O
allocation	O
is	O
the	O
simplest	O
memory	B-General_Concept
management	I-General_Concept
technique	O
.	O
</s>
<s>
All	O
the	O
computer	O
's	O
memory	O
,	O
usually	O
with	O
the	O
exception	O
of	O
a	O
small	O
portion	O
reserved	O
for	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
is	O
available	O
to	O
a	O
single	O
application	O
.	O
</s>
<s>
MS-DOS	B-Application
is	O
an	O
example	O
of	O
a	O
system	O
that	O
allocates	O
memory	O
in	O
this	O
way	O
.	O
</s>
<s>
An	O
embedded	B-Architecture
system	I-Architecture
running	O
a	O
single	O
application	O
might	O
also	O
use	O
this	O
technique	O
.	O
</s>
<s>
A	O
system	O
using	O
single	O
contiguous	O
allocation	O
may	O
still	O
multitask	B-Operating_System
by	O
swapping	O
the	O
contents	O
of	O
memory	O
to	O
switch	O
among	O
users	O
.	O
</s>
<s>
Early	O
versions	O
of	O
the	O
MUSIC	B-Application
operating	B-General_Concept
system	I-General_Concept
used	O
this	O
technique	O
.	O
</s>
<s>
Each	O
partition	O
might	O
contain	O
all	O
the	O
information	O
for	O
a	O
specific	O
job	B-General_Concept
or	O
task	B-General_Concept
.	O
</s>
<s>
Memory	B-General_Concept
management	I-General_Concept
consists	O
of	O
allocating	O
a	O
partition	O
to	O
a	O
job	B-General_Concept
when	O
it	O
starts	O
and	O
unallocating	O
it	O
when	O
the	O
job	B-General_Concept
ends	O
.	O
</s>
<s>
Partitioned	O
allocation	O
usually	O
requires	O
some	O
hardware	O
support	O
to	O
prevent	O
the	O
jobs	O
from	O
interfering	O
with	O
one	O
another	O
or	O
with	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
IBM	B-Application
System/360	I-Application
used	O
a	O
lock-and-key	O
technique	O
.	O
</s>
<s>
Other	O
systems	O
used	O
base	B-General_Concept
and	I-General_Concept
bounds	I-General_Concept
registers	O
which	O
contained	O
the	O
limits	O
of	O
the	O
partition	O
and	O
flagged	O
invalid	O
accesses	O
.	O
</s>
<s>
The	O
UNIVAC	B-Device
1108	I-Device
Storage	O
Limits	O
Register	O
had	O
separate	O
base/bound	O
sets	O
for	O
instructions	O
and	O
data	O
.	O
</s>
<s>
The	O
system	O
took	O
advantage	O
of	O
memory	B-General_Concept
interleaving	I-General_Concept
to	O
place	O
what	O
were	O
called	O
the	O
i	O
bank	O
and	O
d	O
bank	O
in	O
separate	O
memory	O
modules	O
.	O
</s>
<s>
Partitions	O
may	O
be	O
either	O
static	O
,	O
that	O
is	O
defined	O
at	O
Initial	O
Program	O
Load	O
(	O
IPL	O
)	O
or	O
boot	B-Operating_System
time	I-Operating_System
,	O
or	O
by	O
the	O
computer	O
operator	O
,	O
or	O
dynamic	O
,	O
that	O
is	O
,	O
automatically	O
created	O
for	O
a	O
specific	O
job	B-General_Concept
.	O
</s>
<s>
IBM	B-Application
System/360	I-Application
Operating	I-Application
System	I-Application
Multiprogramming	O
with	O
a	O
Fixed	O
Number	O
of	O
Tasks	O
(	O
MFT	O
)	O
is	O
an	O
example	O
of	O
static	O
partitioning	O
,	O
and	O
Multiprogramming	O
with	O
a	O
Variable	O
Number	O
of	O
Tasks	O
(	O
MVT	O
)	O
is	O
an	O
example	O
of	O
dynamic	O
.	O
</s>
<s>
Partitions	O
may	O
be	O
relocatable	O
using	O
hardware	O
typed	O
memory	O
,	O
like	O
the	O
Burroughs	B-Device
Corporation	I-Device
B5500	I-Device
,	O
or	O
base	B-General_Concept
and	I-General_Concept
bounds	I-General_Concept
registers	O
like	O
the	O
PDP-10	B-Device
or	O
GE-635	B-Device
.	O
</s>
<s>
Relocatable	O
partitions	O
are	O
able	O
to	O
be	O
compacted	B-General_Concept
to	O
provide	O
larger	O
chunks	O
of	O
contiguous	O
physical	O
memory	O
.	O
</s>
<s>
Early	O
versions	O
of	O
IBM	O
's	O
Time	B-Application
Sharing	I-Application
Option	I-Application
(	O
TSO	O
)	O
swapped	O
users	O
in	O
and	O
out	O
of	O
time-sharing	B-General_Concept
partitions	O
.	O
</s>
<s>
Paged	O
allocation	O
divides	O
the	O
computer	O
's	O
primary	O
memory	O
into	O
fixed-size	O
units	O
called	O
page	B-General_Concept
frames	I-General_Concept
,	O
and	O
the	O
program	O
's	O
virtual	O
address	B-General_Concept
space	I-General_Concept
into	O
pages	B-General_Concept
of	O
the	O
same	O
size	O
.	O
</s>
<s>
The	O
hardware	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
maps	O
pages	B-General_Concept
to	O
frames	O
.	O
</s>
<s>
The	O
physical	O
memory	O
can	O
be	O
allocated	O
on	O
a	O
page	O
basis	O
while	O
the	O
address	B-General_Concept
space	I-General_Concept
appears	O
contiguous	O
.	O
</s>
<s>
Usually	O
,	O
with	O
paged	B-General_Concept
memory	I-General_Concept
management	I-General_Concept
,	O
each	O
job	B-General_Concept
runs	O
in	O
its	O
own	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
However	O
,	O
there	O
are	O
some	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
within	O
a	O
single	O
address	B-General_Concept
space	I-General_Concept
,	O
such	O
as	O
IBM	B-Application
i	I-Application
,	O
which	O
runs	O
all	O
processes	O
within	O
a	O
large	O
address	B-General_Concept
space	I-General_Concept
,	O
and	O
IBM	O
OS/VS2	B-Application
(	O
SVS	O
)	O
,	O
which	O
ran	O
all	O
jobs	O
in	O
a	O
single	O
16MiB	O
virtual	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
Paged	O
memory	O
can	O
be	O
demand-paged	B-General_Concept
when	O
the	O
system	O
can	O
move	O
pages	B-General_Concept
as	O
required	O
between	O
primary	O
and	O
secondary	O
memory	O
.	O
</s>
<s>
Segmented	O
memory	O
is	O
the	O
only	O
memory	B-General_Concept
management	I-General_Concept
technique	O
that	O
does	O
not	O
provide	O
the	O
user	O
's	O
program	O
with	O
a	O
"	O
linear	O
and	O
contiguous	O
address	O
space.	O
"	O
</s>
<s>
With	O
paging	O
support	O
the	O
pages	B-General_Concept
are	O
usually	O
the	O
unit	O
of	O
swapping	O
and	O
segmentation	O
only	O
adds	O
an	O
additional	O
level	O
of	O
security	O
.	O
</s>
<s>
The	O
Intel	B-Device
IA-32	I-Device
(	O
x86	O
)	O
architecture	O
allows	O
a	O
process	O
to	O
have	O
up	O
to	O
16,383	O
segments	O
of	O
up	O
to	O
4GiB	O
each	O
.	O
</s>
<s>
IA-32	B-Device
segments	O
are	O
subdivisions	O
of	O
the	O
computer	O
's	O
linear	O
address	B-General_Concept
space	I-General_Concept
,	O
the	O
virtual	O
address	B-General_Concept
space	I-General_Concept
provided	O
by	O
the	O
paging	O
hardware	O
.	O
</s>
<s>
The	O
Multics	B-Application
operating	I-Application
system	I-Application
is	O
probably	O
the	O
best	O
known	O
system	O
implementing	O
segmented	O
memory	O
.	O
</s>
<s>
Multics	B-Application
segments	O
are	O
subdivisions	O
of	O
the	O
computer	O
's	O
physical	O
memory	O
of	O
up	O
to	O
256	O
pages	B-General_Concept
,	O
each	O
page	O
being	O
1K	O
36-bit	O
words	O
in	O
size	O
,	O
resulting	O
in	O
a	O
maximum	O
segment	O
size	O
of	O
1MiB	O
(	O
with	O
9-bit	O
bytes	O
,	O
as	O
used	O
in	O
Multics	B-Application
)	O
.	O
</s>
<s>
Rollout/rollin	O
(	O
RO/RI	O
)	O
is	O
a	O
computer	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
memory	B-General_Concept
management	I-General_Concept
technique	O
where	O
the	O
entire	O
non-shared	O
code	O
and	O
data	O
of	O
a	O
running	O
program	O
is	O
swapped	O
out	O
to	O
auxiliary	O
memory	O
(	O
disk	O
or	O
drum	O
)	O
to	O
free	O
main	O
storage	O
for	O
another	O
task	B-General_Concept
.	O
</s>
<s>
Rollout/rollin	O
was	O
commonly	O
used	O
in	O
time-sharing	B-General_Concept
systems	I-General_Concept
,	O
where	O
the	O
user	O
's	O
"	O
think	O
time	O
"	O
was	O
relatively	O
long	O
compared	O
to	O
the	O
time	O
to	O
do	O
the	O
swap	O
.	O
</s>
<s>
Unlike	O
virtual	B-Architecture
storage	I-Architecture
—	O
paging	O
or	O
segmentation	O
,	O
rollout/rollin	O
does	O
not	O
require	O
any	O
special	O
memory	B-General_Concept
management	I-General_Concept
hardware	I-General_Concept
;	O
however	O
,	O
unless	O
the	O
system	O
has	O
relocation	O
hardware	O
such	O
as	O
a	O
memory	B-General_Concept
map	I-General_Concept
or	O
base	B-General_Concept
and	I-General_Concept
bounds	I-General_Concept
registers	O
,	O
the	O
program	O
must	O
be	O
rolled	O
back	O
in	O
to	O
its	O
original	O
memory	O
locations	O
.	O
</s>
<s>
Rollout/rollin	O
has	O
been	O
largely	O
superseded	O
by	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
Rollout/rollin	O
allows	O
the	O
temporary	O
,	O
dynamic	O
expansion	O
of	O
a	O
particular	O
job	B-General_Concept
beyond	O
its	O
originally	O
specified	O
region	O
.	O
</s>
<s>
When	O
a	O
job	B-General_Concept
needs	O
more	O
space	O
,	O
rollout/rollin	O
attempts	O
to	O
obtain	O
unassigned	O
storage	O
for	O
the	O
job	B-General_Concept
's	O
use	O
.	O
</s>
<s>
If	O
there	O
is	O
no	O
such	O
unassigned	O
storage	O
,	O
another	O
job	B-General_Concept
is	O
rolled	O
out	O
—	O
i.e.	O
,	O
is	O
transferred	O
to	O
auxiliary	O
storage	O
—	O
so	O
that	O
its	O
region	O
may	O
be	O
used	O
by	O
the	O
first	O
job	B-General_Concept
.	O
</s>
<s>
When	O
released	O
by	O
the	O
first	O
job	B-General_Concept
,	O
this	O
additional	O
storage	O
is	O
again	O
available	O
,	O
either	O
(	O
1	O
)	O
as	O
unassigned	O
storage	O
,	O
if	O
that	O
was	O
its	O
source	O
,	O
or	O
(	O
2	O
)	O
to	O
receive	O
the	O
job	B-General_Concept
to	O
be	O
transferred	O
back	O
into	O
main	O
storage	O
(	O
rolled	O
in	O
)	O
.	O
</s>
<s>
In	O
OS/360	B-Application
,	O
rollout/rollin	O
was	O
used	O
only	O
for	O
batch	B-General_Concept
jobs	I-General_Concept
,	O
and	O
rollin	O
does	O
not	O
occur	O
until	O
the	O
jobstep	O
borrowing	O
the	O
region	O
terminates	O
.	O
</s>
