<s>
The	O
Scalable	B-General_Concept
Coherent	I-General_Concept
Interface	I-General_Concept
or	O
Scalable	B-General_Concept
Coherent	I-General_Concept
Interconnect	I-General_Concept
(	O
SCI	O
)	O
,	O
is	O
a	O
high-speed	O
interconnect	B-General_Concept
standard	O
for	O
shared	O
memory	O
multiprocessing	O
and	O
message	O
passing	O
.	O
</s>
<s>
The	O
goal	O
was	O
to	O
scale	O
well	O
,	O
provide	O
system-wide	O
memory	B-Operating_System
coherence	I-Operating_System
and	O
a	O
simple	O
interface	O
;	O
i.e.	O
</s>
<s>
The	O
IEEE	O
Std	O
1596-1992	O
,	O
IEEE	O
Standard	O
for	O
Scalable	B-General_Concept
Coherent	I-General_Concept
Interface	I-General_Concept
(	O
SCI	O
)	O
was	O
approved	O
by	O
the	O
IEEE	O
standards	O
board	O
on	O
March	O
19	O
,	O
1992	O
.	O
</s>
<s>
Soon	O
after	O
the	O
Fastbus	B-Architecture
(	O
IEEE	O
960	O
)	O
follow-on	O
Futurebus	B-Architecture
(	O
IEEE	B-Architecture
896	I-Architecture
)	O
project	O
in	O
1987	O
,	O
some	O
engineers	O
predicted	O
it	O
would	O
already	O
be	O
too	O
slow	O
for	O
the	O
high	B-Architecture
performance	I-Architecture
computing	I-Architecture
marketplace	O
by	O
the	O
time	O
it	O
would	O
be	O
released	O
in	O
the	O
early	O
1990s	O
.	O
</s>
<s>
It	O
was	O
essentially	O
a	O
subset	O
of	O
Futurebus	B-Architecture
features	O
that	O
could	O
be	O
easily	O
implemented	O
at	O
high	O
speed	O
,	O
along	O
with	O
minor	O
additions	O
to	O
make	O
it	O
easier	O
to	O
connect	O
to	O
other	O
systems	O
,	O
such	O
as	O
VMEbus	B-Architecture
.	O
</s>
<s>
Most	O
of	O
the	O
developers	O
had	O
their	O
background	O
from	O
high-speed	O
computer	B-General_Concept
buses	I-General_Concept
.	O
</s>
<s>
Stein	O
Gjessing	O
’s	O
group	O
at	O
the	O
University	O
of	O
Oslo	O
used	O
formal	O
methods	O
to	O
verify	O
the	O
coherence	B-General_Concept
protocol	I-General_Concept
and	O
Dolphin	O
Server	O
Technology	O
implemented	O
a	O
node	O
controller	O
chip	O
including	O
the	O
cache	B-General_Concept
coherence	I-General_Concept
logic	O
.	O
</s>
<s>
Different	O
versions	O
and	O
derivatives	O
of	O
SCI	O
were	O
implemented	O
by	O
companies	O
like	O
Dolphin	O
Interconnect	B-General_Concept
Solutions	O
,	O
Convex	O
,	O
Data	B-Device
General	I-Device
AViiON	I-Device
(	O
using	O
cache	O
controller	O
and	O
link	O
controller	O
chips	O
from	O
Dolphin	O
)	O
,	O
Sequent	O
and	O
Cray	O
Research	O
.	O
</s>
<s>
Dolphin	O
Interconnect	B-General_Concept
Solutions	O
implemented	O
a	O
PCI	O
and	O
PCI-Express	O
connected	O
derivative	O
of	O
SCI	O
that	O
provides	O
non-coherent	O
shared	O
memory	O
access	O
.	O
</s>
<s>
SCI	O
was	O
often	O
used	O
to	O
implement	O
non-uniform	B-Operating_System
memory	I-Operating_System
access	I-Operating_System
architectures	O
.	O
</s>
<s>
It	O
was	O
also	O
used	O
by	O
Sequent	O
Computer	O
Systems	O
as	O
the	O
processor	O
memory	O
bus	O
in	O
their	O
NUMA-Q	B-Operating_System
systems	O
.	O
</s>
<s>
The	O
logical	O
level	O
that	O
describes	O
the	O
address	O
space	O
,	O
data	O
transfer	O
protocols	O
,	O
cache	B-General_Concept
coherence	I-General_Concept
mechanisms	O
,	O
synchronization	O
primitives	O
,	O
control	O
and	O
status	O
registers	O
,	O
and	O
initialization	O
and	O
error	O
recovery	O
facilities	O
.	O
</s>
<s>
Scalability	O
for	O
large	O
systems	O
is	O
achieved	O
through	O
a	O
distributed	O
directory-based	B-Architecture
cache	I-Architecture
coherence	I-Architecture
model	O
.	O
</s>
<s>
(	O
The	O
other	O
popular	O
models	O
for	O
cache	B-General_Concept
coherency	I-General_Concept
are	O
based	O
on	O
system-wide	O
eavesdropping	O
(	O
snooping	B-General_Concept
)	O
of	O
memory	O
transactions	O
–	O
a	O
scheme	O
which	O
is	O
not	O
very	O
scalable	O
.	O
)	O
</s>
<s>
In	O
SCI	O
each	O
node	O
contains	O
a	O
directory	O
with	O
a	O
pointer	O
to	O
the	O
next	O
node	O
in	O
a	O
linked	B-Data_Structure
list	I-Data_Structure
that	O
shares	O
a	O
particular	O
cache	O
line	O
.	O
</s>
<s>
The	O
SCI	O
standard	O
defines	O
a	O
packet	B-Protocol
switched	I-Protocol
network	I-Protocol
.	O
</s>
<s>
Cache	B-General_Concept
coherence	I-General_Concept
ensures	O
data	O
consistency	O
in	O
multiprocessor	O
systems	O
.	O
</s>
<s>
The	O
simplest	O
form	O
applied	O
in	O
earlier	O
systems	O
was	O
based	O
on	O
clearing	O
the	O
cache	O
contents	O
between	O
context	B-Operating_System
switches	I-Operating_System
and	O
disabling	O
the	O
cache	O
for	O
data	O
that	O
were	O
shared	O
between	O
two	O
or	O
more	O
processors	O
.	O
</s>
<s>
Bus	O
based	O
systems	O
use	O
eavesdropping	O
(	O
snooping	B-General_Concept
)	O
methods	O
since	O
buses	O
are	O
inherently	O
broadcast	O
.	O
</s>
<s>
Modern	O
systems	O
with	O
point-to	O
point	O
links	O
use	O
broadcast	O
methods	O
with	O
snoop	B-General_Concept
filter	I-General_Concept
options	O
to	O
improve	O
performance	O
.	O
</s>
<s>
Instead	O
,	O
SCI	O
uses	O
a	O
distributed	O
directory-based	B-Architecture
cache	I-Architecture
coherence	I-Architecture
protocol	O
with	O
a	O
linked	B-Data_Structure
list	I-Data_Structure
of	O
nodes	O
containing	O
processors	O
that	O
share	O
a	O
particular	O
cache	O
line	O
.	O
</s>
<s>
The	O
memory	O
tag	O
holds	O
a	O
pointer	O
to	O
the	O
head	O
of	O
the	O
linked	B-Data_Structure
list	I-Data_Structure
and	O
a	O
state	O
code	O
for	O
the	O
line	O
(	O
three	O
states	O
–	O
home	O
,	O
fresh	O
,	O
gone	O
)	O
.	O
</s>
<s>
Associated	O
with	O
each	O
node	O
is	O
also	O
a	O
cache	O
for	O
holding	O
remote	O
data	O
with	O
a	O
directory	O
containing	O
forward	O
and	O
backward	O
pointers	O
to	O
nodes	O
in	O
the	O
linked	B-Data_Structure
list	I-Data_Structure
sharing	O
the	O
cache	O
line	O
.	O
</s>
<s>
The	O
overhead	O
for	O
the	O
directory	O
based	O
cache	B-General_Concept
coherence	I-General_Concept
is	O
a	O
constant	O
percentage	O
of	O
the	O
node	O
’s	O
memory	O
and	O
cache	O
.	O
</s>
<s>
Gustavson	O
led	O
a	O
group	O
called	O
the	O
Scalable	B-General_Concept
Coherent	I-General_Concept
Interface	I-General_Concept
and	O
Serial	O
Express	O
Users	O
,	O
Developers	O
,	O
and	O
Manufacturers	O
Association	O
and	O
maintained	O
a	O
web	O
site	O
for	O
the	O
technology	O
starting	O
in	O
1996	O
.	O
</s>
<s>
After	O
the	O
first	O
1992	O
edition	O
,	O
follow-on	O
projects	O
defined	O
shared	O
data	O
formats	O
in	O
1993	O
,	O
a	O
version	O
using	O
low-voltage	B-Architecture
differential	I-Architecture
signaling	I-Architecture
in	O
1996	O
,	O
and	O
a	O
memory	O
interface	O
known	O
as	O
Ramlink	O
later	O
in	O
1996	O
.	O
</s>
