<s>
LOCUS	B-Operating_System
is	O
a	O
discontinued	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
developed	O
at	O
UCLA	O
during	O
the	O
1980s	O
.	O
</s>
<s>
It	O
was	O
notable	O
for	O
providing	O
an	O
early	O
implementation	O
of	O
the	O
single-system	B-Operating_System
image	I-Operating_System
idea	O
,	O
where	O
a	O
cluster	B-Architecture
of	O
machines	O
appeared	O
to	O
be	O
one	O
larger	O
machine	O
.	O
</s>
<s>
A	O
desire	O
to	O
commercialize	O
the	O
technologies	O
developed	O
for	O
LOCUS	B-Operating_System
inspired	O
the	O
creation	O
of	O
the	O
Locus	B-Operating_System
Computing	O
Corporation	O
which	O
went	O
on	O
to	O
include	O
ideas	O
from	O
LOCUS	B-Operating_System
in	O
various	O
products	O
,	O
including	O
OSF/1	O
AD	O
and	O
,	O
finally	O
,	O
the	O
SCO	O
–	O
Tandem	O
UnixWare	B-Operating_System
NonStop	I-Operating_System
Clusters	I-Operating_System
product	O
.	O
</s>
<s>
The	O
LOCUS	B-Operating_System
system	O
was	O
created	O
at	O
UCLA	O
between	O
1980	O
and	O
1983	O
,	O
initial	O
implementation	O
was	O
on	O
a	O
cluster	B-Architecture
of	O
PDP-11/45s	B-Device
using	O
1	O
and	O
10	O
megabit	O
ring	B-Architecture
networks	I-Architecture
,	O
by	O
1983	O
the	O
system	O
was	O
running	O
on	O
17	O
VAX-11/750s	O
using	O
a	O
10	O
megabit	O
Ethernet	O
.	O
</s>
<s>
The	O
system	O
was	O
Unix	B-Application
compatible	O
and	O
provided	O
both	O
a	O
single	O
root	O
view	O
of	O
the	O
file	O
system	O
and	O
a	O
unified	O
process	O
space	O
across	O
all	O
nodes	O
.	O
</s>
<s>
The	O
development	O
of	O
LOCUS	B-Operating_System
was	O
supported	O
by	O
an	O
ARPA	O
research	O
contract	O
,	O
DSS-MDA-903-82-C-0189	O
.	O
</s>
<s>
In	O
order	O
to	O
allow	O
reliable	O
and	O
rapid	O
access	O
to	O
the	O
cluster	B-Architecture
wide	O
filesystem	O
LOCUS	B-Operating_System
used	O
replication	B-Architecture
,	O
the	O
data	O
of	O
files	O
could	O
be	O
stored	O
on	O
more	O
than	O
one	O
node	O
and	O
LOCUS	B-Operating_System
would	O
keep	O
the	O
various	O
copies	O
up	O
to	O
date	O
.	O
</s>
<s>
In	O
order	O
to	O
ensure	O
that	O
all	O
access	O
was	O
made	O
to	O
the	O
most	O
recent	O
version	O
of	O
any	O
file	O
LOCUS	B-Operating_System
would	O
nominate	O
one	O
node	O
as	O
the	O
"	O
current	O
synchronization	O
site	O
"	O
(	O
CSS	O
)	O
for	O
a	O
particular	O
file	O
system	O
.	O
</s>
<s>
As	O
with	O
other	O
SSI	B-Operating_System
systems	O
LOCUS	B-Operating_System
sometimes	O
found	O
it	O
necessary	O
to	O
break	O
the	O
illusion	O
of	O
a	O
single	O
system	O
,	O
notably	O
to	O
allow	O
some	O
files	O
to	O
be	O
different	O
on	O
a	O
per-node	O
basis	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
user	O
was	O
running	O
on	O
one	O
of	O
the	O
PDP-11/45	B-Device
'	O
s	O
and	O
typed	O
the	O
command	O
/bin/who	O
then	O
the	O
system	O
would	O
find	O
that	O
/bin/who	O
was	O
actually	O
a	O
hidden	O
directory	O
and	O
run	O
the	O
command	O
/bin/who/45	O
.	O
</s>
<s>
LOCUS	B-Operating_System
provided	O
remote	O
access	O
to	O
I/O	O
devices	O
.	O
</s>
<s>
LOCUS	B-Operating_System
provided	O
a	O
single	O
process	O
space	O
.	O
</s>
<s>
Both	O
the	O
Unix	B-Application
fork	B-Language
and	O
exec	B-Operating_System
calls	O
would	O
examine	O
an	O
advice	O
list	O
which	O
determined	O
on	O
which	O
node	O
the	O
process	O
would	O
be	O
run	O
.	O
</s>
<s>
LOCUS	B-Operating_System
was	O
designed	O
to	O
work	O
with	O
heterogeneous	O
nodes	O
,	O
(	O
e.g.	O
,	O
a	O
mix	O
of	O
VAX	O
750s	O
and	O
PDP	B-Device
11/45s	I-Device
)	O
and	O
could	O
decide	O
to	O
execute	O
a	O
process	O
on	O
a	O
different	O
node	O
if	O
it	O
needed	O
a	O
particular	O
instruction	O
set	O
.	O
</s>
<s>
As	O
an	O
optimization	O
a	O
run	O
call	O
was	O
added	O
which	O
was	O
equivalent	O
to	O
a	O
combined	O
fork	B-Language
and	O
exec	B-Operating_System
,	O
thus	O
avoiding	O
the	O
overhead	O
of	O
copying	O
the	O
process	O
memory	O
image	O
to	O
another	O
node	O
before	O
overwriting	O
it	O
by	O
the	O
new	O
image	O
.	O
</s>
<s>
Processes	O
could	O
use	O
pipes	B-Operating_System
for	O
inter	O
node	O
communication	O
,	O
including	O
named	B-Operating_System
pipes	I-Operating_System
,	O
</s>
<s>
The	O
LOCUS	B-Operating_System
system	O
was	O
designed	O
to	O
be	O
able	O
to	O
cope	O
with	O
network	B-Algorithm
partitioning	I-Algorithm
-	O
one	O
or	O
more	O
nodes	O
becoming	O
disconnected	O
from	O
the	O
rest	O
of	O
the	O
system	O
.	O
</s>
<s>
As	O
the	O
file	O
system	O
was	O
replicated	B-Architecture
the	O
disconnected	O
nodes	O
could	O
continue	O
to	O
access	O
files	O
.	O
</s>
