<s>
The	O
DECSYSTEM-20	B-Device
was	O
a	O
36-bit	O
Digital	O
Equipment	O
Corporation	O
PDP-10	B-Device
mainframe	B-Architecture
computer	I-Architecture
running	O
the	O
TOPS-20	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
(	O
products	O
introduced	O
in	O
1977	O
)	O
.	O
</s>
<s>
PDP-10	B-Device
computers	O
running	O
the	O
TOPS-10	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
were	O
labeled	O
DECsystem-10	B-Device
as	O
a	O
way	O
of	O
differentiating	O
them	O
from	O
the	O
PDP-11	B-Device
.	O
</s>
<s>
Later	O
on	O
,	O
those	O
systems	O
running	O
TOPS-20	B-Operating_System
(	O
on	O
the	O
KL10	B-Device
PDP-10	B-Device
processors	O
)	O
were	O
labeled	O
DECSYSTEM-20	B-Device
(	O
the	O
block	O
capitals	O
being	O
the	O
result	O
of	O
a	O
lawsuit	O
brought	O
against	O
DEC	O
by	O
Singer	O
,	O
which	O
once	O
made	O
a	O
computer	O
called	O
"	O
The	B-Device
System	I-Device
Ten	I-Device
"	O
)	O
.	O
</s>
<s>
The	O
DECSYSTEM-20	B-Device
was	O
sometimes	O
called	O
PDP-20	B-Device
,	O
although	O
this	O
designation	O
was	O
never	O
used	O
by	O
DEC	O
.	O
</s>
<s>
The	O
only	O
significant	O
difference	O
the	O
user	O
could	O
see	O
between	O
a	O
DECsystem-10	B-Device
and	O
a	O
DECSYSTEM-20	B-Device
was	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
the	O
color	O
of	O
the	O
paint	O
.	O
</s>
<s>
Most	O
(	O
but	O
not	O
all	O
)	O
machines	O
sold	O
to	O
run	O
TOPS-10	B-Operating_System
were	O
painted	O
"	O
Blasi	O
Blue	O
"	O
,	O
whereas	O
most	O
TOPS-20	B-Operating_System
machines	O
were	O
painted	O
"	O
Terracotta	O
"	O
(	O
often	O
mistakenly	O
called	O
"	O
Chinese	O
Red	O
"	O
or	O
orange	O
;	O
the	O
actual	O
name	O
of	O
the	O
color	O
on	O
the	O
paint	O
cans	O
was	O
Terra	O
Cotta	O
)	O
.	O
</s>
<s>
There	O
were	O
some	O
significant	O
internal	O
differences	O
between	O
the	O
earlier	O
KL10	B-Device
Model	O
A	O
processors	O
,	O
used	O
in	O
the	O
earlier	O
DECsystem-10s	B-Device
running	O
on	O
KL10	B-Device
processors	O
,	O
and	O
the	O
later	O
KL10	B-Device
Model	O
Bs	O
,	O
used	O
for	O
the	O
DECSYSTEM-20s	B-Device
.	O
</s>
<s>
Model	O
As	O
used	O
the	O
original	O
PDP-10	B-Device
memory	O
bus	O
,	O
with	O
external	O
memory	O
modules	O
.	O
</s>
<s>
The	O
later	O
Model	O
B	O
processors	O
used	O
in	O
the	O
DECSYSTEM-20	B-Device
used	O
internal	O
memory	O
,	O
mounted	O
in	O
the	O
same	O
cabinet	O
as	O
the	O
CPU	B-General_Concept
.	O
</s>
<s>
The	O
Model	O
As	O
also	O
had	O
different	O
packaging	O
;	O
they	O
came	O
in	O
the	O
original	O
tall	O
PDP-10	B-Device
cabinets	O
,	O
rather	O
than	O
the	O
short	O
ones	O
used	O
later	O
on	O
for	O
the	O
DECSYSTEM-20	B-Device
.	O
</s>
<s>
The	O
last	O
released	O
implementation	O
of	O
DEC	O
's	O
36-bit	O
architecture	O
was	O
the	O
single	O
cabinet	O
DECSYSTEM-2020	O
,	O
using	O
a	O
KS10	B-Device
processor	O
.	O
</s>
<s>
The	O
DECSYSTEM-20	B-Device
was	O
primarily	O
designed	O
and	O
used	O
as	O
a	O
small	O
mainframe	B-Architecture
for	O
timesharing	B-General_Concept
.	O
</s>
<s>
Separate	O
disk	O
allocations	O
were	O
maintained	O
for	O
all	O
users	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
and	O
various	O
levels	O
of	O
protection	O
could	O
be	O
maintained	O
by	O
for	O
System	O
,	O
Owner	O
,	O
Group	O
,	O
and	O
World	O
users	O
.	O
</s>
<s>
The	O
Living	O
Computer	O
Museum	O
of	O
Seattle	O
,	O
Washington	O
maintains	O
a	O
2065	O
running	O
TOPS-10	B-Operating_System
,	O
which	O
is	O
available	O
to	O
interested	O
parties	O
via	O
SSH	O
upon	O
registration	O
(	O
at	O
no	O
cost	O
)	O
at	O
their	O
website	O
.	O
</s>
