<s>
The	O
Heterogeneous	B-Operating_System
Element	I-Operating_System
Processor	I-Operating_System
(	O
HEP	O
)	O
was	O
introduced	O
by	O
Denelcor	O
,	O
Inc	O
.	O
in	O
1982	O
.	O
</s>
<s>
A	O
HEP	O
system	O
,	O
as	O
the	O
name	O
implies	O
,	O
was	O
pieced	O
together	O
from	O
many	O
heterogeneous	O
components	O
--	O
processors	B-General_Concept
,	O
data	O
memory	O
modules	O
,	O
and	O
I/O	O
modules	O
.	O
</s>
<s>
The	O
components	O
were	O
connected	O
via	O
a	O
switched	B-Protocol
network	I-Protocol
.	O
</s>
<s>
A	O
single	O
processor	O
,	O
called	O
a	O
PEM	O
,	O
in	O
a	O
HEP	O
system	O
(	O
up	O
to	O
sixteen	O
PEMs	O
could	O
be	O
connected	O
)	O
was	O
rather	O
unconventional	O
;	O
via	O
a	O
"	O
program	O
status	O
word	O
(	O
PSW	O
)	O
queue	O
,	O
"	O
up	O
to	O
fifty	O
processes	B-Operating_System
could	O
be	O
maintained	O
in	O
hardware	O
at	O
once	O
.	O
</s>
<s>
The	O
eight-stage	O
instruction	B-General_Concept
pipeline	I-General_Concept
allowed	O
instructions	O
from	O
eight	O
different	O
processes	B-Operating_System
to	O
proceed	O
at	O
once	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
full	O
processor	O
throughput	O
of	O
10	O
MIPS	O
could	O
only	O
be	O
achieved	O
when	O
eight	O
or	O
more	O
processes	B-Operating_System
were	O
active	O
;	O
no	O
single	O
process	O
could	O
achieve	O
throughput	O
greater	O
than	O
1.25	O
MIPS	O
.	O
</s>
<s>
This	O
type	O
of	O
multithreading	B-General_Concept
processing	O
classifies	O
the	O
HEP	O
as	O
a	O
barrel	B-Operating_System
processor	I-Operating_System
.	O
</s>
<s>
The	O
hardware	O
implementation	O
of	O
the	O
HEP	O
PEM	O
was	O
emitter-coupled	B-General_Concept
logic	I-General_Concept
.	O
</s>
<s>
Processes	B-Operating_System
were	O
classified	O
as	O
either	O
user-level	O
or	O
supervisor-level	O
.	O
</s>
<s>
User-level	O
processes	B-Operating_System
could	O
create	O
supervisor-level	O
processes	B-Operating_System
,	O
which	O
were	O
used	O
to	O
manage	O
user-level	O
processes	B-Operating_System
and	O
perform	O
I/O	O
.	O
</s>
<s>
Processes	B-Operating_System
of	O
the	O
same	O
class	O
were	O
required	O
to	O
be	O
grouped	O
into	O
one	O
of	O
seven	O
user	O
tasks	O
and	O
seven	O
supervisor	O
tasks	O
.	O
</s>
<s>
Each	O
processor	O
,	O
in	O
addition	O
to	O
the	O
PSW	O
queue	O
and	O
instruction	B-General_Concept
pipeline	I-General_Concept
,	O
contained	O
instruction	O
memory	O
,	O
2,048	O
64-bit	O
general	O
purpose	O
registers	B-General_Concept
and	O
4,096	O
constant	O
registers	B-General_Concept
.	O
</s>
<s>
Constant	O
registers	B-General_Concept
were	O
differentiated	O
by	O
the	O
fact	O
that	O
only	O
supervisor	O
processes	B-Operating_System
could	O
modify	O
their	O
contents	O
.	O
</s>
<s>
The	O
processors	B-General_Concept
themselves	O
contained	O
no	O
data	O
memory	O
;	O
instead	O
,	O
data	O
memory	O
modules	O
could	O
be	O
separately	O
attached	O
to	O
the	O
switched	B-Protocol
network	I-Protocol
.	O
</s>
<s>
The	O
HEP	O
implemented	O
a	O
type	O
of	O
mutual	B-Operating_System
exclusion	I-Operating_System
in	O
which	O
all	O
registers	B-General_Concept
and	O
locations	O
in	O
data	O
memory	O
had	O
associated	O
"	O
empty	O
"	O
and	O
"	O
full	O
"	O
states	O
.	O
</s>
<s>
A	O
programmer	O
could	O
allow	O
processes	B-Operating_System
to	O
halt	O
after	O
trying	O
to	O
read	O
from	O
an	O
empty	O
location	O
or	O
write	O
to	O
a	O
full	O
location	O
,	O
enforcing	O
critical	O
sections	O
.	O
</s>
<s>
The	O
switched	B-Protocol
network	I-Protocol
between	O
elements	O
resembled	O
,	O
in	O
many	O
ways	O
,	O
a	O
modern	O
computer	O
network	O
.	O
</s>
<s>
Another	O
component	O
of	O
the	O
switched	B-Protocol
network	I-Protocol
was	O
the	O
sO	O
System	O
,	O
with	O
its	O
own	O
memory	O
and	O
many	O
individual	O
DEC	O
UNIBUS	O
buses	O
attached	O
for	O
disks	O
and	O
other	O
peripherals	O
.	O
</s>
<s>
The	O
HEP	O
's	O
primary	O
application	O
programming	O
language	O
was	O
a	O
unique	O
Fortran	B-Application
variant	O
.	O
</s>
<s>
In	O
time	O
C	B-Language
,	O
Pascal	B-Application
,	O
and	O
SISAL	B-Language
were	O
added	O
.	O
</s>
<s>
Mike	O
Muuss	O
was	O
involved	O
in	O
a	O
Unix	B-Application
port	O
for	O
the	O
Ballistic	O
Research	O
Laboratory	O
.	O
</s>
<s>
HEPOS	O
was	O
not	O
a	O
Unix-like	O
operating	O
system	O
.	O
</s>
<s>
The	O
HEP	O
had	O
the	O
performance	O
of	O
a	O
CDC	O
7600-class	O
computer	O
in	O
the	O
Cray-1	B-Device
era	O
.	O
</s>
<s>
The	O
BRL	O
system	O
was	O
used	O
to	O
prepare	O
a	O
movie	O
using	O
the	O
BRL-CAD	B-Language
software	O
as	O
its	O
only	O
real	O
application	O
.	O
</s>
