<s>
In	O
computing	O
,	O
the	O
MSI	B-General_Concept
protocol	I-General_Concept
-	O
a	O
basic	O
cache-coherence	B-General_Concept
protocol	I-General_Concept
-	O
operates	O
in	O
multiprocessor	O
systems	O
.	O
</s>
<s>
As	O
with	O
other	O
cache	B-General_Concept
coherency	I-General_Concept
protocols	O
,	O
the	O
letters	O
of	O
the	O
protocol	O
name	O
identify	O
the	O
possible	O
states	O
in	O
which	O
a	O
cache	B-General_Concept
line	O
can	O
be	O
.	O
</s>
<s>
In	O
MSI	O
,	O
each	O
block	O
contained	O
inside	O
a	O
cache	B-General_Concept
can	O
have	O
one	O
of	O
three	O
possible	O
states	O
:	O
</s>
<s>
Modified	O
:	O
The	O
block	O
has	O
been	O
modified	O
in	O
the	O
cache	B-General_Concept
.	O
</s>
<s>
The	O
data	O
in	O
the	O
cache	B-General_Concept
is	O
then	O
inconsistent	O
with	O
the	O
backing	O
store	O
(	O
e.g.	O
</s>
<s>
A	O
cache	B-General_Concept
with	O
a	O
block	O
in	O
the	O
"	O
M	O
"	O
state	O
has	O
the	O
responsibility	O
to	O
write	O
the	O
block	O
to	O
the	O
backing	O
store	O
when	O
it	O
is	O
evicted	O
.	O
</s>
<s>
Shared	O
:	O
This	O
block	O
is	O
unmodified	O
and	O
exists	O
in	O
read-only	O
state	O
in	O
at	O
least	O
one	O
cache	B-General_Concept
.	O
</s>
<s>
The	O
cache	B-General_Concept
can	O
evict	O
the	O
data	O
without	O
writing	O
it	O
to	O
the	O
backing	O
store	O
.	O
</s>
<s>
Invalid	O
:	O
This	O
block	O
is	O
either	O
not	O
present	O
in	O
the	O
current	O
cache	B-General_Concept
or	O
has	O
been	O
invalidated	O
by	O
a	O
bus	O
request	O
,	O
and	O
must	O
be	O
fetched	O
from	O
memory	O
or	O
another	O
cache	B-General_Concept
if	O
the	O
block	O
is	O
to	O
be	O
stored	O
in	O
this	O
cache	B-General_Concept
.	O
</s>
<s>
When	O
a	O
read	O
request	O
arrives	O
at	O
a	O
cache	B-General_Concept
for	O
a	O
block	O
in	O
the	O
"	O
M	O
"	O
or	O
"	O
S	O
"	O
states	O
,	O
the	O
cache	B-General_Concept
supplies	O
the	O
data	O
.	O
</s>
<s>
If	O
the	O
block	O
is	O
not	O
in	O
the	O
cache	B-General_Concept
(	O
in	O
the	O
"	O
I	O
"	O
state	O
)	O
,	O
it	O
must	O
verify	O
that	O
the	O
block	O
is	O
not	O
in	O
the	O
"	O
M	O
"	O
state	O
in	O
any	O
other	O
cache	B-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
bus	O
architectures	O
often	O
perform	O
snooping	B-General_Concept
,	O
where	O
the	O
read	O
request	O
is	O
broadcast	O
to	O
all	O
of	O
the	O
caches	O
.	O
</s>
<s>
Other	O
architectures	O
include	O
cache	B-Architecture
directories	I-Architecture
which	O
have	O
agents	O
(	O
directories	O
)	O
that	O
know	O
which	O
caches	O
last	O
had	O
copies	O
of	O
a	O
particular	O
cache	B-General_Concept
block	I-General_Concept
.	O
</s>
<s>
If	O
another	O
cache	B-General_Concept
has	O
the	O
block	O
in	O
the	O
"	O
M	O
"	O
state	O
,	O
it	O
must	O
write	O
back	O
the	O
data	O
to	O
the	O
backing	O
store	O
and	O
go	O
to	O
the	O
"	O
S	O
"	O
or	O
"	O
I	O
"	O
states	O
.	O
</s>
<s>
Once	O
any	O
"	O
M	O
"	O
line	O
is	O
written	O
back	O
,	O
the	O
cache	B-General_Concept
obtains	O
the	O
block	O
from	O
either	O
the	O
backing	O
store	O
,	O
or	O
another	O
cache	B-General_Concept
with	O
the	O
data	O
in	O
the	O
"	O
S	O
"	O
state	O
.	O
</s>
<s>
The	O
cache	B-General_Concept
can	O
then	O
supply	O
the	O
data	O
to	O
the	O
requester	O
.	O
</s>
<s>
After	O
supplying	O
the	O
data	O
,	O
the	O
cache	B-General_Concept
block	I-General_Concept
is	O
in	O
the	O
"	O
S	O
"	O
state	O
.	O
</s>
<s>
When	O
a	O
write	O
request	O
arrives	O
at	O
a	O
cache	B-General_Concept
for	O
a	O
block	O
in	O
the	O
"	O
M	O
"	O
state	O
,	O
the	O
cache	B-General_Concept
modifies	O
the	O
data	O
locally	O
.	O
</s>
<s>
If	O
the	O
block	O
is	O
in	O
the	O
"	O
S	O
"	O
state	O
,	O
the	O
cache	B-General_Concept
must	O
notify	O
any	O
other	O
caches	O
that	O
might	O
contain	O
the	O
block	O
in	O
the	O
"	O
S	O
"	O
state	O
that	O
they	O
must	O
evict	O
the	O
block	O
.	O
</s>
<s>
This	O
notification	O
may	O
be	O
via	O
bus	B-General_Concept
snooping	I-General_Concept
or	O
a	O
directory	O
,	O
as	O
described	O
above	O
.	O
</s>
<s>
If	O
the	O
block	O
is	O
in	O
the	O
"	O
I	O
"	O
state	O
,	O
the	O
cache	B-General_Concept
must	O
notify	O
any	O
other	O
caches	O
that	O
might	O
contain	O
the	O
block	O
in	O
the	O
"	O
S	O
"	O
or	O
"	O
M	O
"	O
states	O
that	O
they	O
must	O
evict	O
the	O
block	O
.	O
</s>
<s>
If	O
the	O
block	O
is	O
in	O
another	O
cache	B-General_Concept
in	O
the	O
"	O
M	O
"	O
state	O
,	O
that	O
cache	B-General_Concept
must	O
either	O
write	O
the	O
data	O
to	O
the	O
backing	O
store	O
or	O
supply	O
it	O
to	O
the	O
requesting	O
cache	B-General_Concept
.	O
</s>
<s>
If	O
at	O
this	O
point	O
the	O
cache	B-General_Concept
does	O
not	O
yet	O
have	O
the	O
block	O
locally	O
,	O
the	O
block	O
is	O
read	O
from	O
the	O
backing	O
store	O
before	O
being	O
modified	O
in	O
the	O
cache	B-General_Concept
.	O
</s>
<s>
After	O
the	O
data	O
is	O
modified	O
,	O
the	O
cache	B-General_Concept
block	I-General_Concept
is	O
in	O
the	O
"	O
M	O
"	O
state	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	O
are	O
as	O
follows	O
:	O
</s>
<s>
Processor	O
requests	O
to	O
the	O
cache	B-General_Concept
include	O
:	O
</s>
<s>
PrRd	O
:	O
Processor	O
request	O
to	O
read	O
a	O
cache	B-General_Concept
block	I-General_Concept
.	O
</s>
<s>
PrWr	O
:	O
Processor	O
request	O
to	O
write	O
a	O
cache	B-General_Concept
block	I-General_Concept
.	O
</s>
<s>
BusRd	O
:	O
When	O
a	O
read	O
miss	O
occurs	O
in	O
a	O
processor	O
's	O
cache	B-General_Concept
,	O
it	O
sends	O
a	O
BusRd	O
request	O
on	O
the	O
bus	O
and	O
expects	O
to	O
receive	O
the	O
cache	B-General_Concept
block	I-General_Concept
in	O
return	O
.	O
</s>
<s>
BusRdX	O
:	O
When	O
a	O
write	O
miss	O
occurs	O
in	O
a	O
processor	O
's	O
cache	B-General_Concept
,	O
it	O
sends	O
a	O
BusRdX	O
request	O
on	O
the	O
bus	O
which	O
returns	O
the	O
cache	B-General_Concept
block	I-General_Concept
and	O
invalidates	O
the	O
block	O
in	O
the	O
caches	O
of	O
other	O
processors	O
.	O
</s>
<s>
BusUpgr	O
:	O
When	O
there	O
's	O
a	O
write	O
hit	O
in	O
a	O
processor	O
's	O
cache	B-General_Concept
,	O
it	O
sends	O
a	O
BusUpgr	O
request	O
on	O
the	O
bus	O
to	O
invalidate	O
the	O
block	O
in	O
the	O
caches	O
of	O
other	O
processors	O
.	O
</s>
<s>
Flush	O
:	O
Request	O
that	O
indicates	O
that	O
a	O
whole	O
cache	B-General_Concept
block	I-General_Concept
is	O
being	O
written	O
back	O
to	O
the	O
memory	O
.	O
</s>
<s>
On	O
a	O
BusRd	O
,	O
the	O
cache	B-General_Concept
block	I-General_Concept
is	O
flushed	O
onto	O
the	O
bus	O
and	O
state	O
changes	O
to	O
Shared	O
.	O
</s>
<s>
On	O
a	O
BusRdX	O
,	O
the	O
cache	B-General_Concept
block	I-General_Concept
is	O
flushed	O
onto	O
the	O
bus	O
and	O
state	O
changes	O
to	O
Invalid	O
.	O
</s>
<s>
Note	O
that	O
by	O
being	O
in	O
the	O
Modified	O
state	O
in	O
one	O
particular	O
processor	O
,	O
a	O
cache	B-General_Concept
block	I-General_Concept
has	O
to	O
be	O
in	O
the	O
Invalid	O
state	O
in	O
all	O
other	O
processor(s )	O
,	O
as	O
the	O
Modified	O
state	O
is	O
allowed	O
in	O
either	O
none	O
or	O
only	O
one	O
processor	O
.	O
</s>
<s>
Modern	O
systems	O
use	O
variants	O
of	O
the	O
MSI	B-General_Concept
protocol	I-General_Concept
to	O
reduce	O
the	O
amount	O
of	O
traffic	O
in	O
the	O
coherency	O
interconnect	O
.	O
</s>
<s>
The	O
MESI	B-General_Concept
protocol	I-General_Concept
adds	O
an	O
"	O
Exclusive	O
"	O
state	O
to	O
reduce	O
the	O
traffic	O
caused	O
by	O
writes	O
of	O
blocks	O
that	O
only	O
exist	O
in	O
one	O
cache	B-General_Concept
.	O
</s>
<s>
The	O
MOSI	B-General_Concept
protocol	I-General_Concept
adds	O
an	O
"	O
Owned	O
"	O
state	O
to	O
reduce	O
the	O
traffic	O
caused	O
by	O
write-backs	O
of	O
blocks	O
that	O
are	O
read	O
by	O
other	O
caches	O
.	O
</s>
<s>
The	O
MOESI	B-General_Concept
protocol	I-General_Concept
does	O
both	O
of	O
these	O
things	O
.	O
</s>
