<s>
TENEX	B-Operating_System
is	O
an	O
operating	B-General_Concept
system	I-General_Concept
developed	O
in	O
1969	O
by	O
BBN	O
for	O
the	O
PDP-10	B-Device
,	O
which	O
later	O
formed	O
the	O
basis	O
for	O
Digital	O
Equipment	O
Corporation	O
's	O
TOPS-20	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
In	O
the	O
1960s	O
,	O
BBN	O
was	O
involved	O
in	O
a	O
number	O
of	O
LISP-based	O
artificial	B-Application
intelligence	I-Application
projects	O
for	O
DARPA	O
,	O
many	O
of	O
which	O
had	O
very	O
large	O
(	O
for	O
the	O
era	O
)	O
memory	O
requirements	O
.	O
</s>
<s>
One	O
solution	O
to	O
this	O
problem	O
was	O
to	O
add	O
paging	B-Architecture
software	O
to	O
the	O
LISP	B-Language
language	I-Language
,	O
allowing	O
it	O
to	O
write	O
out	O
unused	O
portions	O
of	O
memory	O
to	O
disk	O
for	O
later	O
recall	O
if	O
needed	O
.	O
</s>
<s>
One	O
such	O
system	O
had	O
been	O
developed	O
for	O
the	O
PDP-1	B-Device
at	O
MIT	O
by	O
Daniel	O
Murphy	O
before	O
he	O
joined	O
BBN	O
.	O
</s>
<s>
Early	O
DEC	O
machines	O
were	O
based	O
on	O
an	O
18-bit	B-General_Concept
word	O
,	O
allowing	O
addresses	O
to	O
encode	O
for	O
a	O
256	O
kiloword	O
memory	O
.	O
</s>
<s>
The	O
pager	O
used	O
the	O
most	O
significant	O
bits	O
of	O
the	O
address	O
to	O
index	O
a	O
table	O
of	O
blocks	O
on	O
a	O
magnetic	B-General_Concept
drum	I-General_Concept
that	O
acted	O
as	O
the	O
pager	O
's	O
backing	O
store	O
.	O
</s>
<s>
The	O
software	O
would	O
fetch	O
the	O
pages	O
if	O
needed	O
,	O
and	O
then	O
resolve	O
the	O
address	O
to	O
the	O
proper	O
area	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
In	O
1964	O
DEC	O
announced	O
the	O
PDP-6	B-Device
.	O
</s>
<s>
DEC	O
was	O
still	O
heavily	O
involved	O
with	O
MIT	O
's	O
AI	B-Application
Lab	O
,	O
and	O
many	O
feature	O
requests	O
from	O
the	O
LISP	B-Language
hackers	O
were	O
moved	O
into	O
this	O
machine	O
.	O
</s>
<s>
36-bit	O
computing	O
was	O
especially	O
useful	O
for	O
LISP	B-Language
programming	O
because	O
with	O
an	O
18-bit	B-General_Concept
address	O
space	O
,	O
a	O
word	O
of	O
storage	O
on	O
these	O
systems	O
contained	O
two	O
addresses	O
,	O
a	O
perfect	O
match	O
for	O
the	O
common	O
LISP	B-Language
CAR	B-Language
and	I-Language
CDR	I-Language
operations	O
.	O
</s>
<s>
BBN	O
became	O
interested	O
in	O
buying	O
one	O
for	O
their	O
AI	B-Application
work	O
when	O
they	O
became	O
available	O
,	O
but	O
wanted	O
DEC	O
to	O
add	O
a	O
hardware	O
version	O
of	O
Murphy	O
's	O
pager	O
directly	O
into	O
the	O
system	O
.	O
</s>
<s>
With	O
such	O
an	O
addition	O
,	O
every	O
program	O
on	O
the	O
system	O
would	O
have	O
paging	B-Architecture
support	O
invisibly	O
,	O
making	O
it	O
much	O
easier	O
to	O
do	O
any	O
sort	O
of	O
programming	O
on	O
the	O
machine	O
.	O
</s>
<s>
DEC	O
was	O
initially	O
interested	O
,	O
but	O
soon	O
(	O
1966	O
)	O
announced	O
they	O
were	O
in	O
fact	O
dropping	O
the	O
PDP-6	B-Device
and	O
concentrating	O
solely	O
on	O
their	O
smaller	O
18-bit	B-General_Concept
and	O
new	O
16-bit	O
lines	O
.	O
</s>
<s>
The	O
PDP-6	B-Device
was	O
expensive	O
and	O
complex	O
,	O
and	O
had	O
not	O
sold	O
well	O
for	O
these	O
reasons	O
.	O
</s>
<s>
It	O
was	O
not	O
long	O
until	O
it	O
became	O
clear	O
that	O
DEC	O
was	O
once	O
again	O
entering	O
the	O
36-bit	O
business	O
with	O
what	O
would	O
become	O
the	O
PDP-10	B-Device
.	O
</s>
<s>
BBN	O
started	O
talks	O
with	O
DEC	O
to	O
get	O
a	O
paging	B-Architecture
subsystem	O
in	O
the	O
new	O
machine	O
,	O
then	O
known	O
by	O
its	O
CPU	O
name	O
,	O
the	O
KA-10	B-Device
.	O
</s>
<s>
However	O
,	O
one	O
development	O
of	O
these	O
talks	O
was	O
support	O
for	O
a	O
second	O
virtual	B-Architecture
memory	I-Architecture
segment	O
,	O
allowing	O
half	O
of	O
the	O
user	O
address	O
space	O
to	O
be	O
mapped	O
to	O
a	O
separate	O
(	O
potentially	O
read-only	O
)	O
region	O
of	O
physical	O
memory	O
.	O
</s>
<s>
Additionally	O
,	O
DEC	O
was	O
firm	O
on	O
keeping	O
the	O
cost	O
of	O
the	O
machine	O
as	O
low	O
as	O
possible	O
,	O
such	O
as	O
supporting	O
bare-bones	O
systems	O
with	O
a	O
minimum	O
of	O
16K	O
words	O
of	O
core	O
,	O
and	O
omitting	O
the	O
fast	O
semiconductor	O
register	B-General_Concept
option	O
(	O
substituting	O
core	O
)	O
,	O
at	O
the	O
cost	O
of	O
a	O
considerable	O
performance	O
decrease	O
.	O
</s>
<s>
BBN	O
nevertheless	O
went	O
ahead	O
with	O
its	O
purchase	O
of	O
several	O
PDP-10s	B-Device
,	O
and	O
decided	O
to	O
build	O
their	O
own	O
hardware	O
pager	O
.	O
</s>
<s>
During	O
this	O
period	O
a	O
debate	O
began	O
on	O
what	O
operating	B-General_Concept
system	I-General_Concept
to	O
run	O
on	O
the	O
new	O
machines	O
.	O
</s>
<s>
Strong	O
arguments	O
were	O
made	O
for	O
the	O
continued	O
use	O
of	O
TOPS-10	B-Operating_System
,	O
in	O
order	O
to	O
keep	O
their	O
existing	O
software	O
running	O
with	O
minimum	O
effort	O
.	O
</s>
<s>
This	O
would	O
require	O
a	O
re-write	O
of	O
TOPS-10	B-Operating_System
to	O
support	O
the	O
paging	B-Architecture
system	O
,	O
and	O
this	O
seemed	O
like	O
a	O
major	O
problem	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
TOPS-10	B-Operating_System
did	O
not	O
support	O
a	O
number	O
of	O
features	O
the	O
developers	O
wanted	O
.	O
</s>
<s>
In	O
the	O
end	O
they	O
decided	O
to	O
make	O
a	O
new	O
system	O
,	O
but	O
include	O
an	O
emulation	O
library	O
that	O
would	O
allow	O
it	O
to	O
run	O
existing	O
TOPS-10	B-Operating_System
software	O
with	O
minor	O
effort	O
.	O
</s>
<s>
The	O
developer	O
team	O
—	O
amongst	O
them	O
Daniel	O
Murphy	O
and	O
Daniel	O
G	O
.	O
Bobrow	O
—	O
chose	O
the	O
name	O
TENEX	B-Operating_System
(	O
TEN-EXtended	O
)	O
for	O
the	O
new	O
system	O
.	O
</s>
<s>
It	O
included	O
a	O
full	O
virtual	B-Operating_System
memory	I-Operating_System
system	I-Operating_System
—	O
that	O
is	O
,	O
not	O
only	O
could	O
programs	O
access	O
a	O
full	O
18	B-General_Concept
bit	I-General_Concept
address	O
space	O
of	O
262144	O
words	O
of	O
virtual	B-Architecture
memory	I-Architecture
,	O
every	O
program	O
could	O
do	O
so	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
The	O
only	O
change	O
needed	O
was	O
for	O
the	O
pager	O
to	O
be	O
able	O
to	O
hold	O
several	O
sets	O
of	O
mappings	O
between	O
RAM	B-Architecture
and	O
store	O
,	O
one	O
for	O
each	O
program	O
using	O
the	O
system	O
.	O
</s>
<s>
One	O
notable	O
feature	O
of	O
TENEX	B-Operating_System
was	O
its	O
user-oriented	O
command	B-Application
line	I-Application
interpreter	O
.	O
</s>
<s>
Unlike	O
typical	O
systems	O
of	O
the	O
era	O
,	O
TENEX	B-Operating_System
deliberately	O
used	O
long	O
command	O
names	O
and	O
even	O
included	O
non-significant	O
noise	O
words	O
to	O
further	O
expand	O
the	O
commands	O
for	O
clarity	O
.	O
</s>
<s>
For	O
instance	O
,	O
Unix	B-Application
uses	O
ls	O
to	O
print	O
a	O
list	O
of	O
files	O
in	O
a	O
directory	O
,	O
whereas	O
TENEX	B-Operating_System
used	O
DIRECTORY	O
(	O
OF	O
FILES	O
)	O
.	O
</s>
<s>
To	O
relieve	O
users	O
of	O
the	O
need	O
to	O
type	O
these	O
long	O
commands	O
,	O
TENEX	B-Operating_System
used	O
a	O
command	O
completion	O
system	O
that	O
understood	O
unambiguously	O
abbreviated	O
command	O
words	O
,	O
and	O
expanded	O
partial	O
command	O
words	O
into	O
complete	O
words	O
or	O
phrases	O
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
user	O
could	O
type	O
DIR	O
and	O
the	O
escape	O
key	O
,	O
at	O
which	O
point	O
TENEX	B-Operating_System
would	O
replace	O
DIR	O
with	O
the	O
full	O
command	O
.	O
</s>
<s>
TENEX	B-Operating_System
also	O
included	O
a	O
command	O
recognition	O
help	O
system	O
:	O
typing	O
a	O
question	O
mark	O
(	O
?	O
</s>
<s>
)	O
,	O
printed	O
out	O
a	O
list	O
of	O
possible	O
matching	O
commands	O
and	O
then	O
return	O
the	O
user	O
to	O
the	O
command	B-Application
line	I-Application
with	O
the	O
question	O
mark	O
removed	O
.	O
</s>
<s>
The	O
command	B-Application
line	I-Application
completion	O
and	O
help	O
live	O
on	O
in	O
current	O
CLIs	O
like	O
tcsh	B-Language
.	O
</s>
<s>
TENEX	B-Operating_System
became	O
fairly	O
popular	O
in	O
the	O
small	O
PDP-10	B-Device
market	O
,	O
and	O
the	O
external	O
pager	O
hardware	O
developed	O
into	O
a	O
small	O
business	O
of	O
its	O
own	O
.	O
</s>
<s>
In	O
early	O
1970	O
DEC	O
started	O
work	O
on	O
an	O
upgrade	O
to	O
the	O
PDP-10	B-Device
processor	O
,	O
the	O
KI-10	B-Device
.	O
</s>
<s>
This	O
compromise	O
impacted	O
system	O
sales	O
;	O
by	O
this	O
point	O
TENEX	B-Operating_System
was	O
the	O
most	O
popular	O
customer-written	O
PDP-10	B-Device
operating	B-General_Concept
systems	I-General_Concept
,	O
but	O
it	O
would	O
not	O
run	O
on	O
the	O
new	O
,	O
faster	O
KI-10s	B-Device
.	O
</s>
<s>
To	O
correct	O
this	O
problem	O
,	O
the	O
DEC	B-Device
PDP-10	I-Device
sales	O
manager	O
purchased	O
the	O
rights	O
to	O
TENEX	B-Operating_System
from	O
BBN	O
and	O
set	O
up	O
a	O
project	O
to	O
port	O
it	O
to	O
the	O
new	O
machine	O
.	O
</s>
<s>
Most	O
of	O
the	O
work	O
centered	O
on	O
emulating	O
the	O
BBN	O
pager	O
hardware	O
in	O
a	O
combination	O
of	O
software	O
and	O
the	O
KI-10	B-Device
'	O
s	O
simpler	O
hardware	O
.	O
</s>
<s>
The	O
speed	O
of	O
the	O
KI-10	B-Device
compared	O
to	O
the	O
PDP-6	B-Device
made	O
this	O
possible	O
.	O
</s>
<s>
Additionally	O
the	O
porting	O
effort	O
required	O
a	O
number	O
of	O
new	O
device	B-Application
drivers	I-Application
to	O
support	O
the	O
newer	O
backing	O
store	O
devices	O
being	O
used	O
.	O
</s>
<s>
Just	O
as	O
the	O
new	O
TENEX	B-Operating_System
was	O
shipping	O
,	O
DEC	O
started	O
work	O
on	O
the	O
KL-10	B-Device
,	O
intended	O
to	O
be	O
a	O
low-cost	O
version	O
of	O
the	O
KI-10	B-Device
.	O
</s>
<s>
While	O
this	O
was	O
going	O
on	O
,	O
Stanford	O
University	O
AI	B-Application
programmers	O
,	O
many	O
of	O
them	O
MIT	O
alumni	O
,	O
were	O
working	O
on	O
their	O
own	O
project	O
to	O
build	O
a	O
PDP-10	B-Device
that	O
was	O
ten	O
times	O
faster	O
than	O
the	O
original	O
KA-10	B-Device
.	O
</s>
<s>
DEC	O
visited	O
them	O
and	O
many	O
of	O
their	O
ideas	O
were	O
then	O
folded	O
into	O
the	O
KL-10	B-Device
project	O
.	O
</s>
<s>
The	O
same	O
year	O
IBM	O
also	O
announced	O
their	O
own	O
machine	O
with	O
virtual	B-Architecture
memory	I-Architecture
,	O
making	O
it	O
a	O
standard	O
requirement	O
for	O
any	O
computer	O
.	O
</s>
<s>
From	O
the	O
start	O
,	O
the	O
new	O
DECSYSTEM-20	B-Device
would	O
run	O
a	O
version	O
of	O
TENEX	B-Operating_System
as	O
its	O
default	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Functional	O
upgrades	O
for	O
the	O
KL-10	B-Device
processor	O
architecture	O
were	O
limited	O
.	O
</s>
<s>
Some	O
effective	O
address	O
calculations	O
by	O
instructions	O
located	O
beyond	O
the	O
original	O
18-bit	B-General_Concept
address	O
space	O
were	O
performed	O
to	O
30	O
significant	O
bits	O
,	O
although	O
only	O
a	O
23-bit	O
virtual	O
address	O
space	O
was	O
supported	O
.	O
</s>
<s>
Program	O
code	O
located	O
in	O
the	O
original	O
18-bit	B-General_Concept
address	O
space	O
had	O
unchanged	O
semantics	O
,	O
for	O
backward	B-General_Concept
compatibility	I-General_Concept
.	O
</s>
<s>
The	O
first	O
in-house	O
code	O
name	O
for	O
the	O
operating	B-General_Concept
system	I-General_Concept
was	O
VIROS	O
(	O
VIRtual	B-Architecture
memory	I-Architecture
Operating	B-General_Concept
System	I-General_Concept
)	O
;	O
when	O
customers	O
started	O
asking	O
questions	O
,	O
the	O
name	O
was	O
changed	O
to	O
SNARK	O
so	O
that	O
DEC	O
could	O
truthfully	O
deny	O
that	O
there	O
was	O
any	O
project	O
called	O
VIROS	O
.	O
</s>
<s>
Ultimately	O
DEC	O
picked	O
TOPS-20	B-Operating_System
as	O
the	O
name	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
and	O
it	O
was	O
as	O
TOPS-20	B-Operating_System
that	O
it	O
was	O
marketed	O
.	O
</s>
<s>
The	O
hacker	O
community	O
,	O
mindful	O
of	O
its	O
origins	O
,	O
quickly	O
dubbed	O
it	O
TWENEX	B-Operating_System
(	O
a	O
portmanteau	O
of	O
"	O
twenty	O
TENEX	B-Operating_System
"	O
)	O
,	O
even	O
though	O
by	O
this	O
point	O
very	O
little	O
of	O
the	O
original	O
TENEX	B-Operating_System
code	O
remained	O
(	O
analogously	O
to	O
the	O
differences	O
between	O
AT&T	O
V7	O
Unix	B-Application
and	O
BSD	B-Operating_System
)	O
.	O
</s>
<s>
DEC	O
people	O
cringed	O
when	O
they	O
heard	O
"	O
TWENEX	B-Operating_System
"	O
,	O
but	O
the	O
term	O
caught	O
on	O
nevertheless	O
(	O
the	O
written	O
abbreviation	O
"	O
20x	O
"	O
was	O
also	O
used	O
)	O
.	O
</s>
<s>
TWENEX	B-Operating_System
was	O
successful	O
and	O
very	O
popular	O
;	O
in	O
fact	O
,	O
there	O
was	O
a	O
period	O
in	O
the	O
early	O
1980s	O
when	O
it	O
commanded	O
as	O
fervent	O
a	O
culture	O
of	O
partisans	O
as	O
Unix	B-Application
or	O
ITS	O
—	O
but	O
DEC	O
's	O
decision	O
to	O
scrap	O
all	O
the	O
internal	O
rivals	O
to	O
the	O
VAX	B-Device
architecture	O
and	O
its	O
VMS	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
killed	O
the	O
DEC-20	B-Device
and	O
put	O
an	O
end	O
to	O
TWENEX	B-Operating_System
's	O
brief	O
period	O
of	O
popularity	O
.	O
</s>
<s>
DEC	O
attempted	O
to	O
convince	O
TOPS-20	B-Operating_System
users	O
to	O
convert	O
to	O
VMS	B-Operating_System
,	O
but	O
instead	O
,	O
by	O
the	O
late	O
1980s	O
,	O
most	O
of	O
the	O
TOPS-20	B-Operating_System
users	O
had	O
migrated	O
to	O
Unix	B-Application
.	O
</s>
<s>
A	O
loyal	O
group	O
of	O
TOPS-20	B-Operating_System
enthusiasts	O
kept	O
working	O
on	O
various	O
projects	O
to	O
preserve	O
and	O
extend	O
TOPS-20	B-Operating_System
,	O
notably	O
Mark	O
Crispin	O
and	O
the	O
Panda	O
TOPS-20	B-Operating_System
distribution	O
.	O
</s>
