<s>
Memory	B-General_Concept
protection	I-General_Concept
is	O
a	O
way	O
to	O
control	O
memory	B-General_Concept
access	O
rights	O
on	O
a	O
computer	O
,	O
and	O
is	O
a	O
part	O
of	O
most	O
modern	O
instruction	B-General_Concept
set	I-General_Concept
architectures	I-General_Concept
and	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
The	O
main	O
purpose	O
of	O
memory	B-General_Concept
protection	I-General_Concept
is	O
to	O
prevent	O
a	O
process	B-Operating_System
from	O
accessing	O
memory	B-General_Concept
that	O
has	O
not	O
been	O
allocated	O
to	O
it	O
.	O
</s>
<s>
This	O
prevents	O
a	O
bug	O
or	O
malware	O
within	O
a	O
process	B-Operating_System
from	O
affecting	O
other	O
processes	O
,	O
or	O
the	O
operating	B-General_Concept
system	I-General_Concept
itself	O
.	O
</s>
<s>
Protection	O
may	O
encompass	O
all	O
accesses	O
to	O
a	O
specified	O
area	O
of	O
memory	B-General_Concept
,	O
write	O
accesses	O
,	O
or	O
attempts	O
to	O
execute	O
the	O
contents	O
of	O
the	O
area	O
.	O
</s>
<s>
An	O
attempt	O
to	O
access	O
unauthorized	O
memory	B-General_Concept
results	O
in	O
a	O
hardware	O
fault	B-Application
,	O
e.g.	O
,	O
a	O
segmentation	B-Error_Name
fault	I-Error_Name
,	O
storage	B-General_Concept
violation	I-General_Concept
exception	O
,	O
generally	O
causing	O
abnormal	B-Application
termination	I-Application
of	O
the	O
offending	O
process	B-Operating_System
.	O
</s>
<s>
Memory	B-General_Concept
protection	I-General_Concept
for	O
computer	O
security	O
includes	O
additional	O
techniques	O
such	O
as	O
address	O
space	O
layout	O
randomization	O
and	O
executable	O
space	O
protection	O
.	O
</s>
<s>
Segmentation	B-General_Concept
refers	O
to	O
dividing	O
a	O
computer	O
's	O
memory	B-General_Concept
into	O
segments	O
.	O
</s>
<s>
A	O
reference	O
to	O
a	O
memory	B-General_Concept
location	O
includes	O
a	O
value	O
that	O
identifies	O
a	O
segment	O
and	O
an	O
offset	O
within	O
that	O
segment	O
.	O
</s>
<s>
A	O
segment	O
descriptor	O
may	O
limit	O
access	O
rights	O
,	O
e.g.	O
,	O
read	O
only	O
,	O
only	O
from	O
certain	O
rings	B-Operating_System
.	O
</s>
<s>
The	O
x86	B-Operating_System
architecture	I-Operating_System
has	O
multiple	O
segmentation	B-General_Concept
features	O
,	O
which	O
are	O
helpful	O
for	O
using	O
protected	B-General_Concept
memory	I-General_Concept
on	O
this	O
architecture	B-General_Concept
.	O
</s>
<s>
On	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
,	O
the	O
Global	B-General_Concept
Descriptor	I-General_Concept
Table	I-General_Concept
and	O
Local	B-General_Concept
Descriptor	I-General_Concept
Tables	I-General_Concept
can	O
be	O
used	O
to	O
reference	O
segments	O
in	O
the	O
computer	O
's	O
memory	B-General_Concept
.	O
</s>
<s>
Pointers	O
to	O
memory	B-General_Concept
segments	I-General_Concept
on	O
x86	B-Operating_System
processors	O
can	O
also	O
be	O
stored	O
in	O
the	O
processor	O
's	O
segment	B-General_Concept
registers	I-General_Concept
.	O
</s>
<s>
Initially	O
x86	B-Operating_System
processors	O
had	O
4	O
segment	B-General_Concept
registers	I-General_Concept
,	O
CS	O
(	O
code	O
segment	O
)	O
,	O
SS	O
(	O
stack	O
segment	O
)	O
,	O
DS	O
(	O
data	O
segment	O
)	O
and	O
ES	O
(	O
extra	O
segment	O
)	O
;	O
later	O
another	O
two	O
segment	B-General_Concept
registers	I-General_Concept
were	O
added	O
FS	O
and	O
GS	O
.	O
</s>
<s>
In	O
paging	B-Architecture
the	O
memory	B-General_Concept
address	O
space	O
or	O
segment	O
is	O
divided	O
into	O
equal-sized	O
blocks	O
called	O
pages	B-General_Concept
.	O
</s>
<s>
Using	O
virtual	B-Architecture
memory	I-Architecture
hardware	O
,	O
each	O
page	O
can	O
reside	O
in	O
any	O
location	O
at	O
a	O
suitable	O
boundary	O
of	O
the	O
computer	O
's	O
physical	O
memory	B-General_Concept
,	O
or	O
be	O
flagged	O
as	O
being	O
protected	O
.	O
</s>
<s>
Virtual	B-Architecture
memory	I-Architecture
makes	O
it	O
possible	O
to	O
have	O
a	O
linear	O
virtual	B-General_Concept
memory	I-General_Concept
address	I-General_Concept
space	I-General_Concept
and	O
to	O
use	O
it	O
to	O
access	O
blocks	O
fragmented	O
over	O
physical	O
memory	B-General_Concept
address	O
space	O
.	O
</s>
<s>
Most	O
computer	B-General_Concept
architectures	I-General_Concept
which	O
support	O
paging	B-Architecture
also	O
use	O
pages	B-General_Concept
as	O
the	O
basis	O
for	O
memory	B-General_Concept
protection	I-General_Concept
.	O
</s>
<s>
A	O
page	B-General_Concept
table	I-General_Concept
maps	O
virtual	B-Architecture
memory	I-Architecture
to	O
physical	O
memory	B-General_Concept
.	O
</s>
<s>
There	O
may	O
be	O
a	O
single	O
page	B-General_Concept
table	I-General_Concept
,	O
a	O
page	B-General_Concept
table	I-General_Concept
for	O
each	O
process	B-Operating_System
,	O
a	O
page	B-General_Concept
table	I-General_Concept
for	O
each	O
segment	O
,	O
or	O
a	O
hierarchy	O
of	O
page	B-General_Concept
tables	I-General_Concept
,	O
depending	O
on	O
the	O
architecture	B-General_Concept
and	O
the	O
OS	O
.	O
</s>
<s>
The	O
page	B-General_Concept
tables	I-General_Concept
are	O
usually	O
invisible	O
to	O
the	O
process	B-Operating_System
.	O
</s>
<s>
Page	B-General_Concept
tables	I-General_Concept
make	O
it	O
easier	O
to	O
allocate	O
additional	O
memory	B-General_Concept
,	O
as	O
each	O
new	O
page	O
can	O
be	O
allocated	O
from	O
anywhere	O
in	O
physical	O
memory	B-General_Concept
.	O
</s>
<s>
On	O
some	O
systems	O
a	O
page	B-General_Concept
table	I-General_Concept
entry	O
can	O
also	O
designate	O
a	O
page	O
as	O
read-only	O
.	O
</s>
<s>
Some	O
operating	B-General_Concept
systems	I-General_Concept
set	O
up	O
a	O
different	O
address	O
space	O
for	O
each	O
process	B-Operating_System
,	O
which	O
provides	O
hard	O
memory	B-General_Concept
protection	I-General_Concept
boundaries	O
.	O
</s>
<s>
It	O
is	O
impossible	O
for	O
an	O
unprivileged	O
application	O
to	O
access	O
a	O
page	O
that	O
has	O
not	O
been	O
explicitly	O
allocated	O
to	O
it	O
,	O
because	O
every	O
memory	B-General_Concept
address	O
either	O
points	O
to	O
a	O
page	O
allocated	O
to	O
that	O
application	O
,	O
or	O
generates	O
an	O
interrupt	B-Application
called	O
a	O
page	B-General_Concept
fault	I-General_Concept
.	O
</s>
<s>
Unallocated	O
pages	B-General_Concept
,	O
and	O
pages	B-General_Concept
allocated	O
to	O
any	O
other	O
application	O
,	O
do	O
not	O
have	O
any	O
addresses	O
from	O
the	O
application	O
point	O
of	O
view	O
.	O
</s>
<s>
A	O
page	B-General_Concept
fault	I-General_Concept
may	O
not	O
necessarily	O
indicate	O
an	O
error	O
.	O
</s>
<s>
Page	B-General_Concept
faults	I-General_Concept
are	O
not	O
only	O
used	O
for	O
memory	B-General_Concept
protection	I-General_Concept
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
may	O
manage	O
the	O
page	B-General_Concept
table	I-General_Concept
in	O
such	O
a	O
way	O
that	O
a	O
reference	O
to	O
a	O
page	O
that	O
has	O
been	O
previously	O
paged	B-Architecture
out	I-Architecture
to	O
secondary	O
storage	O
causes	O
a	O
page	B-General_Concept
fault	I-General_Concept
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
intercepts	O
the	O
page	B-General_Concept
fault	I-General_Concept
,	O
loads	O
the	O
required	O
memory	B-General_Concept
page	I-General_Concept
,	O
and	O
the	O
application	O
continues	O
as	O
if	O
no	O
fault	B-Application
had	O
occurred	O
.	O
</s>
<s>
This	O
scheme	O
,	O
a	O
type	O
of	O
virtual	B-Architecture
memory	I-Architecture
,	O
allows	O
in-memory	O
data	O
not	O
currently	O
in	O
use	O
to	O
be	O
moved	O
to	O
secondary	O
storage	O
and	O
back	O
in	O
a	O
way	O
which	O
is	O
transparent	O
to	O
applications	O
,	O
to	O
increase	O
overall	O
memory	B-General_Concept
capacity	O
.	O
</s>
<s>
On	O
some	O
systems	O
,	O
a	O
request	O
for	O
virtual	B-Architecture
storage	I-Architecture
may	O
allocate	O
a	O
block	O
of	O
virtual	O
addresses	O
for	O
which	O
no	O
page	B-General_Concept
frames	I-General_Concept
have	O
been	O
assigned	O
,	O
and	O
the	O
system	O
will	O
only	O
assign	O
and	O
initialize	O
page	B-General_Concept
frames	I-General_Concept
when	O
page	B-General_Concept
faults	I-General_Concept
occur	O
.	O
</s>
<s>
On	O
some	O
systems	O
a	O
guard	B-Error_Name
page	I-Error_Name
may	O
be	O
used	O
,	O
either	O
for	O
error	O
detection	O
or	O
to	O
automatically	O
grow	O
data	O
structures	O
.	O
</s>
<s>
On	O
some	O
systems	O
,	O
the	O
page	B-General_Concept
fault	I-General_Concept
mechanism	O
is	O
also	O
used	O
for	O
executable	O
space	O
protection	O
such	O
as	O
W^X	O
.	O
</s>
<s>
A	O
memory	B-General_Concept
protection	I-General_Concept
key	O
(	O
MPK	O
)	O
mechanism	O
divides	O
physical	O
memory	B-General_Concept
into	O
blocks	O
of	O
a	O
particular	O
size	O
(	O
e.g.	O
,	O
4KiB	O
)	O
,	O
each	O
of	O
which	O
has	O
an	O
associated	O
numerical	O
value	O
called	O
a	O
protection	O
key	O
.	O
</s>
<s>
Each	O
process	B-Operating_System
also	O
has	O
a	O
protection	O
key	O
value	O
associated	O
with	O
it	O
.	O
</s>
<s>
On	O
a	O
memory	B-General_Concept
access	O
the	O
hardware	O
checks	O
that	O
the	O
current	O
process	B-Operating_System
's	O
protection	O
key	O
matches	O
the	O
value	O
associated	O
with	O
the	O
memory	B-General_Concept
block	O
being	O
accessed	O
;	O
if	O
not	O
,	O
an	O
exception	O
occurs	O
.	O
</s>
<s>
This	O
mechanism	O
was	O
introduced	O
in	O
the	O
System/360	B-Application
architecture	B-General_Concept
.	O
</s>
<s>
It	O
is	O
available	O
on	O
today	O
's	O
System	B-Device
z	I-Device
mainframes	B-Architecture
and	O
heavily	O
used	O
by	O
System	B-Device
z	I-Device
operating	B-General_Concept
systems	I-General_Concept
and	O
their	O
subsystems	O
.	O
</s>
<s>
The	O
System/360	B-Application
protection	O
keys	O
described	O
above	O
are	O
associated	O
with	O
physical	O
addresses	O
.	O
</s>
<s>
This	O
is	O
different	O
from	O
the	O
protection	O
key	O
mechanism	O
used	O
by	O
architectures	O
such	O
as	O
the	O
Hewlett-Packard/Intel	O
IA-64	B-General_Concept
and	O
Hewlett-Packard	O
PA-RISC	B-Device
,	O
which	O
are	O
associated	O
with	O
virtual	O
addresses	O
,	O
and	O
which	O
allow	O
multiple	O
keys	O
per	O
process	B-Operating_System
.	O
</s>
<s>
In	O
the	O
Itanium	O
and	O
PA-RISC	B-Device
architectures	O
,	O
translations	O
(	O
TLB	B-Architecture
entries	O
)	O
have	O
keys	O
(	O
Itanium	O
)	O
or	O
access	O
ids	O
(	O
PA-RISC	B-Device
)	O
associated	O
with	O
them	O
.	O
</s>
<s>
A	O
running	O
process	B-Operating_System
has	O
several	O
protection	O
key	O
registers	O
(	O
16	O
for	O
Itanium	O
,	O
4	O
for	O
PA-RISC	B-Device
)	O
.	O
</s>
<s>
A	O
translation	O
selected	O
by	O
the	O
virtual	B-General_Concept
address	I-General_Concept
has	O
its	O
key	O
compared	O
to	O
each	O
of	O
the	O
protection	O
key	O
registers	O
.	O
</s>
<s>
If	O
none	O
match	O
,	O
a	O
fault	B-Application
or	O
exception	O
is	O
generated	O
.	O
</s>
<s>
The	O
software	O
fault	B-Application
handler	O
can	O
,	O
if	O
desired	O
,	O
check	O
the	O
missing	O
key	O
against	O
a	O
larger	O
list	O
of	O
keys	O
maintained	O
by	O
software	O
;	O
thus	O
,	O
the	O
protection	O
key	O
registers	O
inside	O
the	O
processor	O
may	O
be	O
treated	O
as	O
a	O
software-managed	O
cache	O
of	O
a	O
larger	O
list	O
of	O
keys	O
associated	O
with	O
a	O
process	B-Operating_System
.	O
</s>
<s>
PA-RISC	B-Device
has	O
15	O
–	O
18	O
bits	O
of	O
key	O
;	O
Itanium	O
mandates	O
at	O
least	O
18	O
.	O
</s>
<s>
In	O
the	O
x86	B-Operating_System
,	O
the	O
protection	O
keys	O
architecture	B-General_Concept
allows	O
tagging	O
virtual	O
addresses	O
for	O
user	O
pages	B-General_Concept
with	O
any	O
of	O
16	O
protection	O
keys	O
.	O
</s>
<s>
All	O
the	O
pages	B-General_Concept
tagged	O
with	O
the	O
same	O
protection	O
key	O
constitute	O
a	O
protection	O
domain	O
.	O
</s>
<s>
Load	O
and	O
store	O
operations	O
are	O
checked	O
against	O
both	O
the	O
page	B-General_Concept
table	I-General_Concept
permissions	O
and	O
the	O
protection	O
key	O
permissions	O
associated	O
with	O
the	O
protection	O
domain	O
of	O
the	O
virtual	B-General_Concept
address	I-General_Concept
,	O
and	O
only	O
allowed	O
if	O
both	O
permissions	O
allow	O
the	O
access	O
.	O
</s>
<s>
Since	O
the	O
protection	O
keys	O
are	O
associated	O
with	O
a	O
virtual	B-General_Concept
address	I-General_Concept
,	O
the	O
protection	O
domains	O
are	O
per	O
address	O
space	O
,	O
so	O
processes	O
running	O
in	O
different	O
address	O
spaces	O
can	O
each	O
use	O
all	O
16	O
domains	O
.	O
</s>
<s>
In	O
Multics	B-Application
and	O
systems	O
derived	O
from	O
it	O
,	O
each	O
segment	O
has	O
a	O
protection	B-Operating_System
ring	I-Operating_System
for	O
reading	O
,	O
writing	O
and	O
execution	O
;	O
an	O
attempt	O
by	O
a	O
process	B-Operating_System
with	O
a	O
higher	O
ring	O
number	O
than	O
the	O
ring	O
number	O
for	O
the	O
segment	O
causes	O
a	O
fault	B-Application
.	O
</s>
<s>
Simulation	O
is	O
the	O
use	O
of	O
a	O
monitoring	B-Application
program	B-Application
to	O
interpret	O
the	O
machine	O
code	O
instructions	O
of	O
some	O
computer	B-General_Concept
architectures	I-General_Concept
.	O
</s>
<s>
Such	O
an	O
instruction	B-Application
set	I-Application
simulator	I-Application
can	O
provide	O
memory	B-General_Concept
protection	I-General_Concept
by	O
using	O
a	O
segmentation-like	O
scheme	O
and	O
validating	O
the	O
target	O
address	O
and	O
length	O
of	O
each	O
instruction	O
in	O
real	O
time	O
before	O
actually	O
executing	O
them	O
.	O
</s>
<s>
The	O
simulator	O
must	O
calculate	O
the	O
target	O
address	O
and	O
length	O
and	O
compare	O
this	O
against	O
a	O
list	O
of	O
valid	O
address	O
ranges	O
that	O
it	O
holds	O
concerning	O
the	O
thread	B-Operating_System
's	I-Operating_System
environment	O
,	O
such	O
as	O
any	O
dynamic	O
memory	B-General_Concept
blocks	O
acquired	O
since	O
the	O
thread	B-Operating_System
's	I-Operating_System
inception	O
,	O
plus	O
any	O
valid	O
shared	O
static	O
memory	B-General_Concept
slots	O
.	O
</s>
<s>
The	O
meaning	O
of	O
"	O
valid	O
"	O
may	O
change	O
throughout	O
the	O
thread	B-Operating_System
's	I-Operating_System
life	O
depending	O
upon	O
context	O
.	O
</s>
<s>
It	O
is	O
generally	O
not	O
advisable	O
to	O
use	O
this	O
method	O
of	O
memory	B-General_Concept
protection	I-General_Concept
where	O
adequate	O
facilities	O
exist	O
on	O
a	O
CPU	O
,	O
as	O
this	O
takes	O
valuable	O
processing	O
power	O
from	O
the	O
computer	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
generally	O
used	O
for	O
debugging	O
and	O
testing	O
purposes	O
to	O
provide	O
an	O
extra	O
fine	O
level	O
of	O
granularity	O
to	O
otherwise	O
generic	O
storage	B-General_Concept
violations	I-General_Concept
and	O
can	O
indicate	O
precisely	O
which	O
instruction	O
is	O
attempting	O
to	O
overwrite	O
the	O
particular	O
section	O
of	O
storage	O
which	O
may	O
have	O
the	O
same	O
storage	O
key	O
as	O
unprotected	O
storage	O
.	O
</s>
<s>
Capability-based	B-General_Concept
addressing	I-General_Concept
is	O
a	O
method	O
of	O
memory	B-General_Concept
protection	I-General_Concept
that	O
is	O
unused	O
in	O
modern	O
commercial	O
computers	O
.	O
</s>
<s>
In	O
this	O
method	O
,	O
pointers	O
are	O
replaced	O
by	O
protected	O
objects	O
(	O
called	O
capabilities	O
)	O
that	O
can	O
only	O
be	O
created	O
using	O
privileged	O
instructions	O
which	O
may	O
only	O
be	O
executed	O
by	O
the	O
kernel	O
,	O
or	O
some	O
other	O
process	B-Operating_System
authorized	O
to	O
do	O
so	O
.	O
</s>
<s>
This	O
effectively	O
lets	O
the	O
kernel	O
control	O
which	O
processes	O
may	O
access	O
which	O
objects	O
in	O
memory	B-General_Concept
,	O
with	O
no	O
need	O
to	O
use	O
separate	O
address	O
spaces	O
or	O
context	B-Operating_System
switches	I-Operating_System
.	O
</s>
<s>
Only	O
a	O
few	O
commercial	O
products	O
used	O
capability	O
based	O
security	O
:	O
Plessey	B-Device
System	I-Device
250	I-Device
,	O
IBM	B-Device
System/38	I-Device
,	O
Intel	B-Device
iAPX	I-Device
432	I-Device
architecture	B-General_Concept
and	O
KeyKOS	B-Operating_System
.	O
</s>
<s>
Capability	O
approaches	O
are	O
widely	O
used	O
in	O
research	O
systems	O
such	O
as	O
EROS	B-Operating_System
and	O
Combex	O
DARPA	O
browser	O
.	O
</s>
<s>
They	O
are	O
used	O
conceptually	O
as	O
the	O
basis	O
for	O
some	O
virtual	B-Architecture
machines	I-Architecture
,	O
most	O
notably	O
Smalltalk	B-Application
and	O
Java	B-Device
.	O
</s>
<s>
Dynamic	O
tainting	O
is	O
a	O
technique	O
for	O
protecting	O
programs	O
from	O
illegal	O
memory	B-General_Concept
accesses	O
.	O
</s>
<s>
When	O
memory	B-General_Concept
is	O
allocated	O
,	O
at	O
runtime	O
,	O
this	O
technique	O
taints	O
both	O
the	O
memory	B-General_Concept
and	O
the	O
corresponding	O
pointer	O
using	O
the	O
same	O
taint	O
mark	O
.	O
</s>
<s>
Taint	O
marks	O
are	O
then	O
suitably	O
propagated	O
while	O
the	O
program	B-Application
executes	O
and	O
are	O
checked	O
every	O
time	O
a	O
memory	B-General_Concept
address	O
m	O
is	O
accessed	O
through	O
a	O
pointer	O
p	O
;	O
if	O
the	O
taint	O
marks	O
associated	O
with	O
m	O
and	O
p	O
differ	O
,	O
the	O
execution	O
is	O
stopped	O
and	O
the	O
illegal	O
access	O
is	O
reported	O
.	O
</s>
<s>
SPARC	B-Device
M7	I-Device
processors	O
(	O
and	O
higher	O
)	O
implement	O
dynamic	O
tainting	O
in	O
hardware	O
.	O
</s>
<s>
Oracle	O
markets	O
this	O
feature	O
as	O
Silicon	O
Secured	O
Memory	B-General_Concept
(	O
SSM	O
)	O
(	O
previously	O
branded	O
as	O
Application	O
Data	O
Integrity	O
(	O
ADI	O
)	O
)	O
.	O
</s>
<s>
The	O
lowRISC	O
CPU	O
design	O
includes	O
dynamic	O
tainting	O
under	O
the	O
name	O
Tagged	O
Memory	B-General_Concept
.	O
</s>
<s>
Different	O
operating	B-General_Concept
systems	I-General_Concept
use	O
different	O
forms	O
of	O
memory	B-General_Concept
protection	I-General_Concept
or	O
separation	O
.	O
</s>
<s>
Although	O
memory	B-General_Concept
protection	I-General_Concept
was	O
common	O
on	O
most	O
mainframes	B-Architecture
and	O
many	O
minicomputer	B-Architecture
systems	O
from	O
the	O
1960s	O
,	O
true	O
memory	B-General_Concept
separation	O
was	O
not	O
used	O
in	O
home	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
until	O
OS/2	B-Application
(	O
and	O
in	O
RISC	B-Operating_System
OS	I-Operating_System
)	O
was	O
released	O
in	O
1987	O
.	O
</s>
<s>
On	O
prior	O
systems	O
,	O
such	O
lack	O
of	O
protection	O
was	O
even	O
used	O
as	O
a	O
form	O
of	O
interprocess	B-Operating_System
communication	I-Operating_System
,	O
by	O
sending	O
a	O
pointer	O
between	O
processes	O
.	O
</s>
<s>
It	O
is	O
possible	O
for	O
processes	O
to	O
access	O
System	O
Memory	B-General_Concept
in	O
the	O
Windows	O
9x	O
family	O
of	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Some	O
operating	B-General_Concept
systems	I-General_Concept
that	O
do	O
implement	O
memory	B-General_Concept
protection	I-General_Concept
include	O
:	O
</s>
<s>
RISC	B-Operating_System
OS	I-Operating_System
(	O
1987	O
)	O
(	O
The	O
OS	O
memory	B-General_Concept
protection	I-General_Concept
is	O
not	O
comprehensive	O
.	O
)	O
</s>
<s>
On	O
Unix-like	B-Operating_System
systems	I-Operating_System
,	O
the	O
mprotect	B-General_Concept
system	B-Operating_System
call	I-Operating_System
is	O
used	O
to	O
control	O
memory	B-General_Concept
protection	I-General_Concept
.	O
</s>
