<s>
Memory	B-Operating_System
coherence	I-Operating_System
is	O
an	O
issue	O
that	O
affects	O
the	O
design	O
of	O
computer	O
systems	O
in	O
which	O
two	O
or	O
more	O
processors	O
or	O
cores	B-Architecture
share	O
a	O
common	O
area	O
of	O
memory	B-General_Concept
.	O
</s>
<s>
In	O
a	O
uniprocessor	B-Operating_System
system	I-Operating_System
(	O
whereby	O
,	O
in	O
today	O
's	O
terms	O
,	O
there	O
exists	O
only	O
one	O
core	O
)	O
,	O
there	O
is	O
only	O
one	O
processing	O
element	O
doing	O
all	O
the	O
work	O
and	O
therefore	O
only	O
one	O
processing	O
element	O
that	O
can	O
read	O
or	O
write	O
from/to	O
a	O
given	O
memory	B-General_Concept
location	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
when	O
a	O
value	O
is	O
changed	O
,	O
all	O
subsequent	O
read	O
operations	O
of	O
the	O
corresponding	O
memory	B-General_Concept
location	O
will	O
see	O
the	O
updated	O
value	O
,	O
even	O
if	O
it	O
is	O
cached	O
.	O
</s>
<s>
Conversely	O
,	O
in	O
multiprocessor	B-Operating_System
(	O
or	O
multicore	B-Architecture
)	O
systems	O
,	O
there	O
are	O
two	O
or	O
more	O
processing	O
elements	O
working	O
at	O
the	O
same	O
time	O
,	O
and	O
so	O
it	O
is	O
possible	O
that	O
they	O
simultaneously	O
access	O
the	O
same	O
memory	B-General_Concept
location	O
.	O
</s>
<s>
Provided	O
none	O
of	O
them	O
changes	O
the	O
data	O
in	O
this	O
location	O
,	O
they	O
can	O
share	O
it	O
indefinitely	O
and	O
cache	B-General_Concept
it	O
as	O
they	O
please	O
.	O
</s>
<s>
But	O
as	O
soon	O
as	O
one	O
updates	O
the	O
location	O
,	O
the	O
others	O
might	O
work	O
on	O
an	O
out-of-date	O
copy	O
that	O
,	O
e.g.	O
,	O
resides	O
in	O
their	O
local	O
cache	B-General_Concept
.	O
</s>
<s>
Consequently	O
,	O
some	O
scheme	O
is	O
required	O
to	O
notify	O
all	O
the	O
processing	O
elements	O
of	O
changes	O
to	O
shared	O
values	O
;	O
such	O
a	O
scheme	O
is	O
known	O
as	O
a	O
memory	B-Operating_System
coherence	I-Operating_System
protocol	I-Operating_System
,	O
and	O
if	O
such	O
a	O
protocol	O
is	O
employed	O
the	O
system	O
is	O
said	O
to	O
have	O
a	O
coherent	B-Operating_System
memory	I-Operating_System
.	O
</s>
<s>
The	O
exact	O
nature	O
and	O
meaning	O
of	O
the	O
memory	B-Operating_System
coherency	I-Operating_System
is	O
determined	O
by	O
the	O
consistency	B-General_Concept
model	I-General_Concept
that	O
the	O
coherence	B-General_Concept
protocol	I-General_Concept
implements	O
.	O
</s>
<s>
In	O
order	O
to	O
write	O
correct	O
concurrent	O
programs	O
,	O
programmers	O
must	O
be	O
aware	O
of	O
the	O
exact	O
consistency	B-General_Concept
model	I-General_Concept
that	O
is	O
employed	O
by	O
their	O
systems	O
.	O
</s>
<s>
When	O
implemented	O
in	O
hardware	O
,	O
the	O
coherency	O
protocol	O
can	O
,	O
for	O
example	O
,	O
be	O
directory-based	B-Architecture
or	O
snooping-based	O
(	O
also	O
called	O
sniffing	O
)	O
.	O
</s>
<s>
Specific	O
protocols	O
include	O
the	O
MSI	B-General_Concept
protocol	I-General_Concept
and	O
its	O
derivatives	O
MESI	B-General_Concept
,	O
MOSI	B-General_Concept
and	O
MOESI	B-General_Concept
.	O
</s>
