<s>
In	O
computing	O
,	O
MOESI	B-General_Concept
is	O
a	O
full	O
cache	B-General_Concept
coherency	I-General_Concept
protocol	O
that	O
encompasses	O
all	O
of	O
the	O
possible	O
states	O
commonly	O
used	O
in	O
other	O
protocols	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
four	O
common	O
MESI	B-General_Concept
protocol	I-General_Concept
states	O
,	O
there	O
is	O
a	O
fifth	O
"	O
Owned	O
"	O
state	O
representing	O
data	O
that	O
is	O
both	O
modified	O
and	O
shared	O
.	O
</s>
<s>
As	O
discussed	O
in	O
AMD64	B-Device
Architecture	O
Programmer	O
's	O
Manual	O
Vol	O
.	O
</s>
<s>
2	O
'	O
System	O
Programming	O
 '	O
,	O
each	O
cache	B-General_Concept
line	I-General_Concept
is	O
in	O
one	O
of	O
five	O
states	O
:	O
</s>
<s>
Modified	O
This	O
cache	O
has	O
the	O
only	O
valid	O
copy	O
of	O
the	O
cache	B-General_Concept
line	I-General_Concept
,	O
and	O
has	O
made	O
changes	O
to	O
that	O
copy	O
.	O
</s>
<s>
OwnedThis	O
cache	O
is	O
one	O
of	O
several	O
with	O
a	O
valid	O
copy	O
of	O
the	O
cache	B-General_Concept
line	I-General_Concept
,	O
but	O
has	O
the	O
exclusive	O
right	O
to	O
make	O
changes	O
to	O
it	O
—	O
other	O
caches	O
may	O
read	O
but	O
not	O
write	O
the	O
cache	B-General_Concept
line	I-General_Concept
.	O
</s>
<s>
When	O
this	O
cache	O
changes	O
data	O
on	O
the	O
cache	B-General_Concept
line	I-General_Concept
,	O
it	O
must	O
broadcast	O
those	O
changes	O
to	O
all	O
other	O
caches	O
sharing	O
the	O
line	O
.	O
</s>
<s>
The	O
cache	B-General_Concept
line	I-General_Concept
may	O
be	O
changed	O
to	O
the	O
Modified	O
state	O
after	O
invalidating	O
all	O
shared	O
copies	O
,	O
or	O
changed	O
to	O
the	O
Shared	O
state	O
by	O
writing	O
the	O
modifications	O
back	O
to	O
main	O
memory	O
.	O
</s>
<s>
Owned	O
cache	B-General_Concept
lines	I-General_Concept
must	O
respond	O
to	O
a	O
snoop	B-General_Concept
request	O
with	O
data	O
.	O
</s>
<s>
Unlike	O
the	O
MESI	B-General_Concept
protocol	I-General_Concept
,	O
a	O
shared	O
cache	B-General_Concept
line	I-General_Concept
may	O
be	O
dirty	O
with	O
respect	O
to	O
memory	O
;	O
if	O
it	O
is	O
,	O
some	O
cache	O
has	O
a	O
copy	O
in	O
the	O
Owned	O
state	O
,	O
and	O
that	O
cache	O
is	O
responsible	O
for	O
eventually	O
updating	O
main	O
memory	O
.	O
</s>
<s>
The	O
cache	B-General_Concept
line	I-General_Concept
may	O
not	O
be	O
written	O
,	O
but	O
may	O
be	O
changed	O
to	O
the	O
Exclusive	O
or	O
Modified	O
state	O
after	O
invalidating	O
all	O
shared	O
copies	O
.	O
</s>
<s>
(	O
If	O
the	O
cache	B-General_Concept
line	I-General_Concept
was	O
Owned	O
before	O
,	O
the	O
invalidate	O
response	O
will	O
indicate	O
this	O
,	O
and	O
the	O
state	O
will	O
become	O
Modified	O
,	O
so	O
the	O
obligation	O
to	O
eventually	O
write	O
the	O
data	O
back	O
to	O
memory	O
is	O
not	O
forgotten	O
.	O
)	O
</s>
<s>
Shared	O
cache	B-General_Concept
lines	I-General_Concept
may	O
not	O
respond	O
to	O
a	O
snoop	B-General_Concept
request	O
with	O
data	O
.	O
</s>
<s>
For	O
any	O
given	O
pair	O
of	O
caches	O
,	O
the	O
permitted	O
states	O
of	O
a	O
given	O
cache	B-General_Concept
line	I-General_Concept
are	O
as	O
follows	O
:	O
</s>
<s>
(	O
The	O
order	O
in	O
which	O
the	O
states	O
are	O
normally	O
listed	O
serves	O
only	O
to	O
make	O
the	O
acronym	O
"	O
MOESI	B-General_Concept
"	O
pronounceable	O
.	O
)	O
</s>
<s>
This	O
protocol	O
,	O
a	O
more	O
elaborate	O
version	O
of	O
the	O
simpler	O
MESI	B-General_Concept
protocol	I-General_Concept
(	O
but	O
not	O
in	O
extended	O
MESI	B-General_Concept
-	O
see	O
Cache	B-General_Concept
coherency	I-General_Concept
)	O
,	O
avoids	O
the	O
need	O
to	O
write	O
a	O
dirty	O
cache	B-General_Concept
line	I-General_Concept
back	O
to	O
main	O
memory	O
when	O
another	O
processor	O
tries	O
to	O
read	O
it	O
.	O
</s>
<s>
While	O
MOESI	B-General_Concept
can	O
quickly	O
share	O
dirty	O
cache	B-General_Concept
lines	I-General_Concept
from	O
cache	O
,	O
it	O
cannot	O
quickly	O
share	O
clean	O
lines	O
from	O
cache	O
.	O
</s>
<s>
If	O
a	O
cache	B-General_Concept
line	I-General_Concept
is	O
clean	O
with	O
respect	O
to	O
memory	O
and	O
in	O
the	O
shared	O
state	O
,	O
then	O
any	O
snoop	B-General_Concept
request	O
to	O
that	O
cache	B-General_Concept
line	I-General_Concept
will	O
be	O
filled	O
from	O
memory	O
,	O
rather	O
than	O
a	O
cache	O
.	O
</s>
<s>
If	O
a	O
processor	O
wishes	O
to	O
write	O
to	O
an	O
Owned	O
cache	B-General_Concept
line	I-General_Concept
,	O
it	O
must	O
notify	O
the	O
other	O
processors	O
that	O
are	O
sharing	O
that	O
cache	B-General_Concept
line	I-General_Concept
.	O
</s>
