<s>
In	O
cache	B-General_Concept
coherency	I-General_Concept
protocol	O
literature	O
,	O
Write-Once	B-General_Concept
was	O
the	O
first	O
MESI	B-General_Concept
protocol	I-General_Concept
defined	O
.	O
</s>
<s>
It	O
has	O
the	O
optimization	O
of	O
executing	O
write-through	O
on	O
the	O
first	O
write	O
and	O
a	O
write-back	O
on	O
all	O
subsequent	O
writes	O
,	O
reducing	O
the	O
overall	O
bus	B-General_Concept
traffic	O
in	O
consecutive	O
writes	O
to	O
the	O
computer	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
Cache	B-General_Concept
coherence	I-General_Concept
protocols	O
are	O
an	O
important	O
issue	O
in	O
Symmetric	B-Operating_System
multiprocessing	I-Operating_System
systems	O
,	O
where	O
each	O
CPU	B-General_Concept
maintains	O
a	O
cache	B-General_Concept
of	O
the	O
memory	O
.	O
</s>
<s>
In	O
this	O
protocol	O
,	O
each	O
block	O
in	O
the	O
local	O
cache	B-General_Concept
is	O
in	O
one	O
of	O
these	O
four	O
states	O
:	O
</s>
<s>
This	O
is	O
the	O
only	O
state	O
that	O
generates	O
a	O
write-back	O
when	O
the	O
block	O
is	O
replaced	O
in	O
the	O
cache	B-General_Concept
.	O
</s>
<s>
These	O
states	O
have	O
exactly	O
the	O
same	O
meanings	O
as	O
the	O
four	O
states	O
of	O
the	O
MESI	B-General_Concept
protocol	I-General_Concept
(	O
they	O
are	O
simply	O
listed	O
in	O
reverse	O
order	O
)	O
,	O
but	O
this	O
is	O
a	O
simplified	O
form	O
of	O
it	O
that	O
avoids	O
the	O
Read	O
for	O
Ownership	O
operation	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
abbreviated	O
in	O
the	O
order	O
above	O
)	O
:	O
</s>
<s>
Read	O
hit	O
:	O
The	O
information	O
is	O
supplied	O
by	O
the	O
current	O
cache	B-General_Concept
.	O
</s>
<s>
The	O
read	O
is	O
snooped	B-General_Concept
by	O
other	O
caches	O
;	O
if	O
any	O
of	O
them	O
have	O
the	O
line	O
in	O
the	O
Dirty	O
state	O
,	O
the	O
read	O
is	O
interrupted	O
long	O
enough	O
to	O
write	O
the	O
data	O
back	O
to	O
memory	O
before	O
it	O
is	O
allowed	O
to	O
continue	O
.	O
</s>
<s>
Write	O
hit	O
:	O
If	O
the	O
information	O
in	O
the	O
cache	B-General_Concept
is	O
in	O
Dirty	O
or	O
Reserved	O
state	O
,	O
the	O
cache	B-General_Concept
line	O
is	O
updated	O
in	O
place	O
and	O
its	O
state	O
is	O
set	O
to	O
Dirty	O
without	O
updating	O
memory	O
.	O
</s>
<s>
Write	O
miss	O
:	O
A	O
partial	O
cache	B-General_Concept
line	O
write	O
is	O
handled	O
as	O
a	O
read	O
miss	O
(	O
if	O
necessary	O
to	O
fetch	O
the	O
unwritten	O
portion	O
of	O
the	O
cache	B-General_Concept
line	O
)	O
followed	O
by	O
a	O
write	O
hit	O
.	O
</s>
<s>
This	O
leaves	O
all	O
other	O
caches	O
in	O
the	O
Invalid	O
state	O
,	O
and	O
the	O
current	O
cache	B-General_Concept
in	O
the	O
Reserved	O
state	O
.	O
</s>
<s>
This	O
is	O
a	O
variant	O
of	O
the	O
MESI	B-General_Concept
protocol	I-General_Concept
,	O
but	O
there	O
is	O
no	O
explicit	O
read-for-ownership	O
or	O
broadcast	O
invalidate	O
operation	O
to	O
bring	O
a	O
line	O
into	O
cache	B-General_Concept
in	O
the	O
Exclusive	O
state	O
without	O
performing	O
a	O
main	O
memory	O
write	O
.	O
</s>
<s>
Shared	O
)	O
cache	B-General_Concept
line	O
performs	O
a	O
write	O
through	O
to	O
memory	O
,	O
which	O
implicitly	O
invalidates	O
other	O
caches	O
.	O
</s>
<s>
After	O
that	O
,	O
the	O
line	O
is	O
in	O
the	O
Reserved	O
(	O
Exclusive	O
)	O
state	O
,	O
and	O
further	O
writes	O
can	O
be	O
done	O
without	O
reference	O
to	O
main	O
memory	O
,	O
leaving	O
the	O
cache	B-General_Concept
line	O
in	O
the	O
Dirty	O
(	O
Modified	O
)	O
state	O
.	O
</s>
