<s>
The	O
TOPS-20	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
by	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
is	O
a	O
proprietary	B-Application
OS	O
used	O
on	O
some	O
of	O
DEC	O
's	O
36-bit	O
mainframe	B-Architecture
computers	I-Architecture
.	O
</s>
<s>
The	O
Hardware	O
Reference	O
Manual	O
was	O
described	O
as	O
for	O
"	O
DECsystem-10/DECSYSTEM	O
-20	O
Processor	O
"	O
(	O
meaning	O
the	O
DEC	B-Device
PDP-10	I-Device
and	O
the	O
DECSYSTEM-20	B-Device
)	O
.	O
</s>
<s>
TOPS-20	B-Operating_System
began	O
in	O
1969	O
as	O
the	O
TENEX	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
of	O
Bolt	O
,	O
Beranek	O
and	O
Newman	O
(	O
BBN	O
)	O
and	O
shipped	O
as	O
a	O
product	O
by	O
DEC	O
starting	O
in	O
1976	O
.	O
</s>
<s>
TOPS-20	B-Operating_System
is	O
almost	O
entirely	O
unrelated	O
to	O
the	O
similarly	O
named	O
TOPS-10	B-Operating_System
,	O
but	O
it	O
was	O
shipped	O
with	O
the	O
PA1050	O
TOPS-10	B-Operating_System
Monitor	O
Calls	O
emulation	O
facility	O
which	O
allowed	O
most	O
,	O
but	O
not	O
all	O
,	O
TOPS-10	B-Operating_System
executables	O
to	O
run	O
unchanged	O
.	O
</s>
<s>
As	O
a	O
matter	O
of	O
policy	O
,	O
DEC	O
did	O
not	O
update	O
PA1050	O
to	O
support	O
later	O
TOPS-10	B-Operating_System
additions	O
except	O
where	O
required	O
by	O
DEC	O
software	O
.	O
</s>
<s>
TOPS-20	B-Operating_System
competed	O
with	O
TOPS-10	B-Operating_System
,	O
ITS	B-Application
and	O
WAITS	B-Application
—	O
all	O
of	O
which	O
were	O
notable	O
time-sharing	O
systems	O
for	O
the	O
PDP-10	B-Device
during	O
this	O
timeframe	O
.	O
</s>
<s>
TOPS-20	B-Operating_System
was	O
based	O
upon	O
the	O
TENEX	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
,	O
which	O
had	O
been	O
created	O
by	O
Bolt	O
Beranek	O
and	O
Newman	O
for	O
Digital	O
's	O
PDP-10	B-Device
computer	O
.	O
</s>
<s>
After	O
Digital	O
started	O
development	O
of	O
the	O
KI-10	B-Device
version	O
of	O
the	O
PDP-10	B-Device
,	O
an	O
issue	O
arose	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>
In	O
the	O
end	O
,	O
very	O
little	O
of	O
the	O
original	O
TENEX	B-Operating_System
code	O
remained	O
,	O
and	O
Digital	O
ultimately	O
named	O
the	O
resulting	O
operating	B-General_Concept
system	I-General_Concept
TOPS-20	B-Operating_System
.	O
</s>
<s>
Some	O
of	O
what	O
came	O
with	O
TOPS-20	B-Operating_System
was	O
merely	O
an	O
emulation	O
of	O
the	O
TOPS-10	B-Operating_System
Operating	B-General_Concept
System	I-General_Concept
's	O
calls	O
.	O
</s>
<s>
The	O
package	O
that	O
was	O
mapped	O
into	O
a	O
user	O
's	O
address	O
space	O
was	O
named	O
PA1050	O
:	O
PA	O
as	O
in	O
PAT	O
as	O
in	O
compatibility	O
;	O
10	O
as	O
in	O
DEC	O
or	O
PDP	B-Device
10	I-Device
;	O
50	O
as	O
in	O
a	O
PDP	B-Device
10	I-Device
Model	O
50	O
,	O
10/50	O
,	O
1050	O
.	O
</s>
<s>
Rather	O
advanced	O
for	O
its	B-Application
day	O
were	O
some	O
TOPS-20-specific	O
features	O
:	O
</s>
<s>
One	O
could	O
then	O
type	B-Device
to	O
find	O
out	O
what	O
operands	O
were	O
permitted/required	O
.	O
</s>
<s>
Pressing	O
displays	O
status	B-Device
information	I-Device
.	O
</s>
<s>
The	O
following	O
list	O
of	O
commands	B-Application
are	O
supported	O
by	O
the	O
TOPS-20	B-Operating_System
Command	O
Processor	O
.	O
</s>
<s>
"	O
TOPS-20	B-Operating_System
allows	O
you	O
to	O
use	O
18-bit	O
or	O
30-bit	O
addresses	O
.	O
</s>
<s>
PCL	O
(	O
Programmable	O
Command	O
Language	O
)	O
is	O
a	O
programming	O
language	O
that	O
runs	O
under	O
TOPS-20	B-Operating_System
.	O
</s>
<s>
PCL	O
source	O
programs	O
are	O
,	O
by	O
default	O
,	O
stored	O
with	O
Filetype	O
.PCL	O
,	O
and	O
enable	O
extending	O
the	O
TOPS-20	B-Operating_System
EXEC	O
via	O
a	O
verb	O
named	O
DECLARE	O
.	O
</s>
<s>
Newly	O
compiled	O
commands	B-Application
then	O
become	O
functionally	O
part	O
of	O
the	O
EXEC	O
.	O
</s>
<s>
Paul	O
Allen	O
maintained	O
several	O
publicly	O
accessible	O
historic	O
computer	O
systems	O
before	O
his	O
death	O
,	O
including	O
an	O
XKL	O
TOAD-2	O
running	O
TOPS-20	B-Operating_System
.	O
</s>
