<s>
The	O
Pluribus	B-Device
multiprocessor	B-Operating_System
was	O
an	O
early	O
multi-processor	B-Operating_System
computer	O
designed	O
by	O
BBN	O
for	O
use	O
as	O
a	O
packet	B-Protocol
switch	I-Protocol
in	O
the	O
ARPANET	O
.	O
</s>
<s>
Its	O
design	O
later	O
influenced	O
the	O
BBN	B-Device
Butterfly	I-Device
computer	O
.	O
</s>
<s>
The	O
Pluribus	B-Device
had	O
its	O
beginnings	O
in	O
1972	O
when	O
the	O
need	O
for	O
a	O
second-generation	O
interface	B-Protocol
message	I-Protocol
processor	I-Protocol
(	O
IMP	O
)	O
became	O
apparent	O
.	O
</s>
<s>
These	O
IMPs	O
were	O
Honeywell	B-Device
316	I-Device
and	O
516	O
minicomputers	O
.	O
</s>
<s>
The	O
designers	O
decided	O
on	O
a	O
multiprocessor	B-Operating_System
approach	O
because	O
of	O
its	O
promising	O
potential	O
for	O
modularity	O
,	O
for	O
cost	O
per	O
performance	O
advantages	O
,	O
for	O
reliability	O
,	O
and	O
because	O
the	O
IMP	O
packet	B-Protocol
switch	I-Protocol
algorithms	O
were	O
clearly	O
suitable	O
for	O
parallel	O
processing	O
by	O
independent	O
processors	O
.	O
</s>
<s>
A	O
Pluribus	B-Device
consisted	O
of	O
two	O
or	O
more	O
standard	O
19	O
"	O
electronic	O
equipment	O
racks	O
,	O
each	O
divided	O
into	O
four	O
bays	O
.	O
</s>
<s>
A	O
6-processor	O
Pluribus	B-Device
was	O
used	O
as	O
a	O
network	O
switch	O
to	O
interconnect	O
BBN	O
's	O
five	O
Tenex/	O
"	O
Twenex	O
"	O
timesharing	O
systems	O
along	O
with	O
378	O
terminals	O
on	O
direct	O
serial	O
and	O
dial-in	O
modem	O
lines	O
.	O
</s>
<s>
The	O
Pluribus	B-Device
used	O
the	O
Lockheed	O
SUE	O
as	O
its	O
processor	O
.	O
</s>
<s>
The	O
SUE	O
was	O
similar	O
to	O
DEC	O
's	O
PDP-11	B-Device
.	O
</s>
<s>
The	O
Pluribus	B-Device
software	O
implemented	O
MIMD	B-Operating_System
symmetric	O
multiprocessing	B-Operating_System
.	O
</s>
<s>
Software	O
processes	O
were	O
implemented	O
using	O
non-preemptive	B-Operating_System
multiprogramming	I-Operating_System
.	O
</s>
<s>
An	O
important	O
aspect	O
of	O
the	O
Pluribus	B-Device
software	O
was	O
the	O
"	O
STAGE	O
"	O
system	O
,	O
which	O
detected	O
system	O
errors	O
and	O
took	O
steps	O
to	O
recover	O
from	O
them	O
.	O
</s>
<s>
The	O
recovery	O
process	O
would	O
unlock	O
any	O
locks	B-Operating_System
placed	O
on	O
shared	O
resources	O
,	O
release	O
allocated	O
storage	O
,	O
and	O
restart	O
all	O
processing	O
on	O
all	O
processors	O
.	O
</s>
