<s>
The	O
C.mmp	B-Operating_System
was	O
an	O
early	O
multiple	B-Operating_System
instruction	I-Operating_System
,	I-Operating_System
multiple	I-Operating_System
data	I-Operating_System
(	O
MIMD	B-Operating_System
)	O
multiprocessor	B-Operating_System
system	O
developed	O
at	O
Carnegie	O
Mellon	O
University	O
(	O
CMU	O
)	O
by	O
William	O
Wulf	O
(	O
1971	O
)	O
.	O
</s>
<s>
The	O
notation	O
C.mmp	B-Operating_System
came	O
from	O
the	O
PMS	O
notation	O
of	O
Gordon	O
Bell	O
and	O
Allen	O
Newell	O
,	O
where	O
a	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
was	O
designated	O
as	O
C	O
,	O
a	O
variant	O
was	O
noted	O
by	O
the	O
dot	B-Language
notation	I-Language
,	O
and	O
mmp	O
stood	O
for	O
Multi-Mini-Processor	O
.	O
</s>
<s>
Sixteen	O
Digital	O
Equipment	O
Corporation	O
PDP-11	B-Device
minicomputers	O
were	O
used	O
as	O
the	O
processing	O
elements	O
,	O
named	O
Compute	O
Modules	O
(	O
CMs	O
)	O
in	O
the	O
system	O
.	O
</s>
<s>
One	O
of	O
the	O
challenges	O
was	O
that	O
a	O
device	O
was	O
only	O
available	O
through	O
its	O
unique	O
connected	O
processor	O
,	O
so	O
the	O
input/output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
system	O
(	O
designed	O
by	O
Roy	O
Levien	O
)	O
hid	O
the	O
connectivity	O
of	O
the	O
devices	O
and	O
routed	O
the	O
requests	O
to	O
the	O
hosting	O
processor	O
.	O
</s>
<s>
If	O
a	O
processor	O
went	O
down	O
,	O
the	O
devices	O
connected	O
to	O
its	O
Unibus	B-Device
became	O
unavailable	O
,	O
which	O
became	O
a	O
problem	O
in	O
overall	O
system	O
reliability	O
.	O
</s>
<s>
Processor	O
0	O
(	O
the	O
boot	B-Operating_System
processor	O
)	O
had	O
the	O
disk	O
drives	O
attached	O
.	O
</s>
<s>
A	O
16x16	O
crossbar	O
switch	O
–	O
used	O
to	O
connect	O
the	O
16	O
CMs	O
on	O
one	O
side	O
and	O
16	O
banks	O
of	O
shared	B-Operating_System
memory	I-Operating_System
on	O
the	O
other	O
.	O
</s>
<s>
If	O
all	O
16	O
processors	O
were	O
accessing	O
different	O
banks	O
of	O
memory	O
,	O
the	O
memory	O
accesses	O
would	O
all	O
be	O
concurrent	B-Operating_System
.	O
</s>
<s>
Since	O
the	O
PDP-11	B-Device
had	O
a	O
logical	O
address	B-General_Concept
space	I-General_Concept
of	O
16-bits	O
,	O
another	O
address	O
translation	O
unit	O
was	O
added	O
to	O
expand	O
the	O
address	B-General_Concept
space	I-General_Concept
to	O
25	O
bits	O
for	O
the	O
shared	B-Operating_System
memory	I-Operating_System
space	O
.	O
</s>
<s>
The	O
Unibus	B-Device
architecture	O
provided	O
18	O
bits	O
of	O
physical	B-General_Concept
address	I-General_Concept
,	O
and	O
the	O
two	O
high-order	O
bits	O
were	O
used	O
to	O
select	O
one	O
of	O
four	O
relocation	O
registers	O
which	O
selected	O
a	O
bank	O
of	O
memory	O
.	O
</s>
<s>
Properly	O
managing	O
these	O
registers	O
was	O
one	O
of	O
the	O
challenges	O
of	O
programming	O
the	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
original	O
C.mmp	B-Operating_System
design	O
used	O
magnetic-core	O
memory	O
,	O
but	O
during	O
its	O
lifetime	O
,	O
higher	O
performance	O
dynamic	O
random-access	B-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
became	O
available	O
and	O
the	O
system	O
was	O
upgraded	O
.	O
</s>
<s>
The	O
original	O
processors	O
were	O
PDP-11/20	O
processors	O
,	O
but	O
in	O
the	O
final	O
system	O
,	O
only	O
five	O
of	O
these	O
were	O
used	O
;	O
the	O
remaining	O
11	O
were	O
PDP-11/40	B-Device
processors	O
,	O
which	O
were	O
modified	O
by	O
having	O
extra	O
writeable	O
microcode	B-Device
space	O
.	O
</s>
<s>
Most	O
of	O
the	O
11/20	O
modifications	O
were	O
custom	O
changes	O
to	O
the	O
wire-wrapped	O
backplane	B-Architecture
,	O
but	O
because	O
the	O
PDP-11/40	B-Device
was	O
implemented	O
in	O
microcode	B-Device
,	O
a	O
separate	O
proc-mod	O
board	O
was	O
designed	O
that	O
intercepted	O
certain	O
instructions	O
and	O
implemented	O
the	O
protected	O
operating	B-General_Concept
system	I-General_Concept
requirements	O
.	O
</s>
<s>
For	O
example	O
,	O
it	O
was	O
necessary	O
,	O
for	O
operating	B-General_Concept
system	I-General_Concept
integrity	O
,	O
that	O
the	O
stack	O
pointer	O
register	O
never	O
be	O
odd	O
.	O
</s>
<s>
On	O
the	O
11/40	O
,	O
any	O
access	O
to	O
the	O
stack	O
was	O
intercepted	O
by	O
the	O
proc-mod	O
board	O
and	O
generated	O
an	O
illegal	O
data	O
access	O
trap	B-Application
if	O
the	O
low-order	O
bit	O
was	O
1	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
was	O
named	O
Hydra	B-Application
.	O
</s>
<s>
It	O
was	O
capability-based	O
,	O
object-oriented	B-Language
,	O
multi-user	B-Operating_System
,	O
and	O
a	O
microkernel	B-Operating_System
.	O
</s>
<s>
The	O
OS	O
and	O
most	O
application	B-Application
software	I-Application
was	O
written	O
in	O
the	O
programming	O
language	O
BLISS-11	O
,	O
which	O
required	O
cross-compiling	B-Application
on	O
a	O
PDP-10	B-Device
.	O
</s>
<s>
The	O
OS	O
used	O
very	O
little	O
assembly	B-Language
language	I-Language
.	O
</s>
<s>
Among	O
the	O
programming	O
languages	O
available	O
on	O
the	O
system	O
was	O
an	O
ALGOL	B-Language
68	I-Language
variant	O
which	O
included	O
extensions	O
supporting	O
parallel	B-Operating_System
computing	I-Operating_System
,	O
to	O
make	O
good	O
use	O
of	O
the	O
C.mmp	B-Operating_System
.	O
</s>
<s>
The	O
ALGOL	O
compiler	B-Language
ran	O
native	O
on	O
Hydra	B-Application
OS	O
.	O
</s>
<s>
When	O
it	O
had	O
passed	O
a	O
first	O
set	O
of	O
diagnostics	O
,	O
it	O
was	O
partitioned	O
back	O
in	O
as	O
an	O
I/O	B-General_Concept
processor	O
and	O
would	O
not	O
run	O
application	O
code	O
(	O
but	O
its	O
peripheral	O
devices	O
were	O
now	O
available	O
)	O
;	O
it	O
continued	O
to	O
run	O
diagnostics	O
.	O
</s>
<s>
Thus	O
,	O
the	O
OS	O
became	O
an	O
early	O
example	O
of	O
a	O
fault-tolerant	B-General_Concept
system	I-General_Concept
,	O
able	O
to	O
deal	O
with	O
hardware	O
problems	O
which	O
arose	O
,	O
inevitably	O
.	O
</s>
