<s>
The	O
Firefly	B-Device
cache	B-General_Concept
coherence	I-General_Concept
protocol	O
is	O
the	O
schema	O
used	O
in	O
the	O
DEC	B-Device
Firefly	I-Device
multiprocessor	O
workstation	O
,	O
developed	O
by	O
DEC	O
Systems	O
Research	O
Center	O
.	O
</s>
<s>
This	O
protocol	O
is	O
a	O
3	O
State	O
Write	O
Update	O
Cache	B-General_Concept
Coherence	I-General_Concept
Protocol	O
.	O
</s>
<s>
Unlike	O
the	O
Dragon	B-General_Concept
protocol	I-General_Concept
,	O
the	O
Firefly	B-Device
protocol	O
updates	O
the	O
Main	O
Memory	O
as	O
well	O
as	O
the	O
Local	O
caches	O
on	O
Write	O
Update	O
Bus	O
Transition	O
.	O
</s>
<s>
Thus	O
the	O
Shared	O
Clean	O
and	O
Shared	O
Modified	O
States	O
present	O
in	O
case	O
of	O
Dragon	B-General_Concept
Protocol	I-General_Concept
,	O
are	O
not	O
distinguished	O
between	O
in	O
case	O
of	O
Firefly	B-Device
Protocol	O
.	O
</s>
<s>
These	O
states	O
correspond	O
to	O
the	O
Exclusive	O
,	O
Shared	O
,	O
and	O
Modified	O
states	O
of	O
the	O
MESI	B-General_Concept
protocol	I-General_Concept
.	O
</s>
<s>
Bus-side	O
requests	O
are	O
the	O
requests	O
generated	O
in	O
response	O
to	O
the	O
processor	O
-	O
side	O
requests	O
to	O
maintain	O
cache	B-General_Concept
coherence	I-General_Concept
.	O
</s>
<s>
These	O
are	O
classified	O
into	O
two	O
types	O
in	O
the	O
Firefly	B-Device
protocol	O
,	O
namely	O
:	O
</s>
<s>
All	O
other	O
caches	O
snoop	B-General_Concept
all	O
memory	O
operations	O
and	O
raise	O
the	O
CopiesExist(C )	O
if	O
they	O
detect	O
a	O
"	O
snoop	B-General_Concept
hit	O
"	O
,	O
i.e.	O
</s>
<s>
If	O
there	O
's	O
a	O
CPU	O
read	O
hit	O
,	O
the	O
block	O
stays	O
in	O
whatever	O
state	O
it	O
is	O
already	O
in	O
—	O
just	O
like	O
in	O
the	O
Dragon	B-General_Concept
protocol	I-General_Concept
.	O
</s>
<s>
Unlike	O
MESI	B-General_Concept
,	O
in	O
the	O
Firefly	B-Device
update	O
protocol	O
,	O
write	O
propagation	O
is	O
ensured	O
by	O
directly	O
updating	O
all	O
other	O
copies	O
on	O
a	O
write	O
request	O
by	O
the	O
processors	O
(	O
PrWr	O
)	O
.	O
</s>
<s>
In	O
update	O
protocols	O
,	O
by	O
contrast	O
,	O
the	O
new	O
data	O
value	O
has	O
to	O
be	O
sent	O
along	O
with	O
the	O
BusUpdate	O
signal	O
to	O
allow	O
memory	O
and	O
other	O
caches	O
to	O
snoop	B-General_Concept
and	O
update	O
their	O
data	O
.	O
</s>
