<s>
The	O
PDP-6	B-Device
,	O
short	O
for	O
Programmed	B-Device
Data	I-Device
Processor	I-Device
model	O
6	O
,	O
is	O
a	O
computer	O
developed	O
by	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
during	O
1963	O
and	O
first	O
delivered	O
in	O
the	O
summer	O
of	O
1964	O
.	O
</s>
<s>
It	O
was	O
an	O
expansion	O
of	O
DEC	O
's	O
existing	O
18-bit	B-General_Concept
systems	I-General_Concept
to	O
use	O
a	O
36-bit	O
data	O
word	O
,	O
which	O
was	O
at	O
that	O
time	O
a	O
common	O
word	O
size	O
for	O
large	O
machines	O
like	O
IBM	B-Device
mainframes	I-Device
.	O
</s>
<s>
The	O
system	O
was	O
constructed	O
using	O
the	O
same	O
germanium	O
transistor-based	O
System	B-Device
Module	I-Device
layout	O
as	O
DEC	O
's	O
earlier	O
machines	O
,	O
like	O
the	O
PDP-1	B-Device
and	O
PDP-4	B-Device
.	O
</s>
<s>
The	O
system	O
was	O
designed	O
with	O
real-time	B-General_Concept
computing	I-General_Concept
use	O
in	O
mind	O
,	O
not	O
just	O
batch	B-General_Concept
processing	I-General_Concept
as	O
was	O
typical	O
for	O
most	O
mainframes	B-Architecture
.	O
</s>
<s>
This	O
made	O
it	O
popular	O
in	O
university	O
settings	O
and	O
its	B-Application
support	O
for	O
the	O
Lisp	B-Language
language	I-Language
made	O
it	O
particularly	O
useful	O
in	O
artificial	B-Application
intelligence	I-Application
labs	O
like	O
Project	O
MAC	O
at	O
MIT	O
.	O
</s>
<s>
It	O
was	O
also	O
complex	O
,	O
expensive	O
,	O
and	O
unreliable	O
as	O
a	O
result	O
of	O
its	B-Application
use	O
of	O
so	O
many	O
early-model	O
transistors	O
.	O
</s>
<s>
The	O
lasting	O
influence	O
of	O
the	O
PDP-6	B-Device
was	O
its	B-Application
re-implementation	O
using	O
modern	O
silicon	O
transistors	O
and	O
the	O
newer	O
Flip-Chip	B-Device
module	I-Device
packaging	O
to	O
produce	O
the	O
PDP-10	B-Device
.	O
</s>
<s>
The	O
instruction	B-General_Concept
sets	I-General_Concept
of	O
the	O
two	O
machines	O
are	O
almost	O
identical	O
.	O
</s>
<s>
The	O
PDP-10	B-Device
was	O
less	O
expensive	O
and	O
more	O
reliable	O
,	O
and	O
about	O
1500	O
were	O
sold	O
during	O
its	B-Application
lifetime	O
.	O
</s>
<s>
DEC	O
soon	O
introduced	O
the	O
PDP-1	B-Device
which	O
was	O
built	O
out	O
of	O
large	O
numbers	O
of	O
these	O
modules	O
,	O
now	O
known	O
as	O
System	B-Device
Building	I-Device
Blocks	I-Device
or	O
System	B-Device
Modules	I-Device
.	O
</s>
<s>
The	O
PDP-1	B-Device
used	O
an	O
18-bit	B-General_Concept
word	O
.	O
</s>
<s>
Word	O
lengths	O
in	O
the	O
early	O
1960s	O
were	O
generally	O
some	O
multiple	O
of	O
six	O
bits	O
,	O
as	O
the	O
character	B-General_Concept
codes	I-General_Concept
of	O
the	O
era	O
were	O
6	O
bits	O
long	O
and	O
it	O
was	O
also	O
a	O
useful	O
size	O
for	O
storing	O
binary	O
coded	O
decimal	O
digits	O
with	O
an	O
optional	O
sign	O
,	O
as	O
commonly	O
used	O
on	O
IBM	O
machines	O
of	O
the	O
era	O
.	O
</s>
<s>
The	O
PDP-1	B-Device
'	O
s	O
18-bit	B-General_Concept
length	O
allowed	O
the	O
machine	O
to	O
be	O
simpler	O
and	O
less	O
expensive	O
than	O
these	O
,	O
at	O
.	O
</s>
<s>
Proposals	O
for	O
a	O
PDP-2	B-Device
and	O
PDP-3	B-Device
were	O
turned	O
down	O
,	O
and	O
DEC	O
's	O
next	O
machine	O
,	O
the	O
PDP-4	B-Device
,	O
was	O
essentially	O
a	O
smaller	O
and	O
less-expensive	O
PDP-1	B-Device
that	O
sold	O
for	O
roughly	O
half	O
the	O
cost	O
.	O
</s>
<s>
The	O
PDP-5	B-Device
was	O
built	O
from	O
the	O
same	O
components	O
,	O
but	O
used	O
a	O
12-bit	O
word	O
for	O
even	O
greater	O
savings	O
and	O
sold	O
for	O
about	O
.	O
</s>
<s>
The	O
PDP-6	B-Device
was	O
DEC	O
's	O
first	O
"	O
big	O
"	O
machine	O
.	O
</s>
<s>
It	O
used	O
36-bit	O
words	O
,	O
in	O
common	O
with	O
other	O
large	B-Architecture
computers	I-Architecture
at	O
the	O
time	O
from	O
companies	O
like	O
IBM	O
,	O
Honeywell	O
and	O
General	O
Electric	O
.	O
</s>
<s>
Unlike	O
those	O
machines	O
,	O
the	O
PDP-6	B-Device
was	O
supplied	O
with	O
a	O
timesharing	B-General_Concept
system	O
"	O
out	O
of	O
the	O
box	O
"	O
.	O
</s>
<s>
Timesharing	B-General_Concept
had	O
been	O
available	O
for	O
other	O
machines	O
,	O
most	O
famously	O
the	O
PDP-1	B-Device
at	O
Project	O
MAC	O
,	O
but	O
the	O
PDP-6	B-Device
was	O
the	O
first	O
such	O
system	O
to	O
be	O
supported	O
by	O
the	O
manufacturer	O
.	O
</s>
<s>
Worldwide	O
,	O
only	O
23	O
PDP-6	B-Device
'	O
s	O
were	O
sold	O
,	O
the	O
smallest	O
number	O
of	O
any	O
DEC	O
machine	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
sales	O
force	O
found	O
the	O
PDP-6	B-Device
to	O
be	O
a	O
"	O
hard	O
sell	O
"	O
.	O
</s>
<s>
Nevertheless	O
,	O
the	O
company	O
later	O
considered	O
the	O
PDP-6	B-Device
to	O
be	O
a	O
success	O
:	O
</s>
<s>
Fortunately	O
,	O
this	O
decision	O
was	O
reversed	O
and	O
a	O
new	O
36-bit	O
system	O
was	O
designed	O
using	O
the	O
new	O
Flip-Chip	B-Device
modules	I-Device
and	O
much	O
smaller	O
system	O
boards	O
to	O
improve	O
density	O
and	O
reliability	O
.	O
</s>
<s>
The	O
resulting	O
design	O
was	O
released	O
in	O
1966	O
as	O
the	O
PDP-10	B-Device
.	O
</s>
<s>
DEC	O
describes	O
this	O
machine	O
as	O
the	O
successor	O
to	O
the	O
PDP-6	B-Device
.	O
</s>
<s>
Although	O
it	O
was	O
program-compatible	O
with	O
the	O
PDP-6	B-Device
,	O
it	O
ran	O
roughly	O
twice	O
as	O
fast	O
,	O
had	O
both	O
timesharing	B-General_Concept
and	O
batch	B-General_Concept
processing	I-General_Concept
features	O
,	O
and	O
offered	O
a	O
wide	O
variety	O
of	O
expansions	O
and	O
input/output	B-General_Concept
options	O
.	O
</s>
<s>
Addressing	O
remained	O
18-bit	B-General_Concept
,	O
as	O
in	O
earlier	O
DEC	O
machines	O
,	O
allowing	O
for	O
a	O
256kword	O
main	O
memory	O
,	O
about	O
1MB	O
in	O
modern	O
terms	O
.	O
</s>
<s>
The	O
use	O
of	O
a	O
36-bit	O
word	O
allowed	O
two	O
18-bit	B-General_Concept
addresses	O
to	O
be	O
stored	O
in	O
a	O
single	O
memory	O
location	O
;	O
this	O
made	O
it	O
suited	O
to	O
the	O
storage	O
of	O
a	O
cons	B-Protocol
,	O
a	O
widely	O
used	O
structure	O
in	O
the	O
Lisp	B-Language
language	I-Language
,	O
meaning	O
the	O
PDP-6	B-Device
could	O
store	O
a	O
cons	B-Protocol
in	O
a	O
single	O
word	O
and	O
read	O
and	O
write	O
one	O
in	O
a	O
single	O
operation	O
.	O
</s>
<s>
The	O
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
could	O
be	O
categorized	O
as	O
"	O
one-and-a-half	O
address	O
"	O
.	O
</s>
<s>
The	O
opcode	B-Language
was	O
stored	O
in	O
the	O
most	O
significant	O
bits	O
of	O
the	O
36-bit	O
word	O
,	O
using	O
9	O
bits	O
.	O
</s>
<s>
The	O
last	O
18	B-General_Concept
bits	I-General_Concept
indicated	O
an	O
address	O
.	O
</s>
<s>
This	O
left	O
another	O
five	O
bits	O
in	O
the	O
instruction	B-Language
word	I-Language
,	O
bits	O
13	O
through	O
17	O
.	O
</s>
<s>
The	O
remaining	O
four	O
bits	O
offered	O
a	O
similar	O
functionality	O
by	O
selecting	O
a	O
second	O
register	O
as	O
an	O
index	B-General_Concept
register	I-General_Concept
,	O
allowing	O
memory	O
to	O
be	O
stepped	O
through	O
by	O
changing	O
the	O
value	O
in	O
the	O
register	O
rather	O
than	O
in	O
main	O
memory	O
,	O
which	O
is	O
generally	O
faster	O
.	O
</s>
<s>
The	O
registers	O
in	O
the	O
PDP-6	B-Device
were	O
simply	O
the	O
first	O
16	O
memory	O
locations	O
of	O
main	O
memory	O
.	O
</s>
<s>
Most	O
,	O
if	O
not	O
all	O
,	O
PDP-6	B-Device
systems	O
were	O
equipped	O
with	O
the	O
optional	O
Type	O
162	O
"	O
Fast	O
Memory	O
"	O
,	O
which	O
constructed	O
these	O
16	O
memory	O
locations	O
from	O
discrete-transistor	O
flip-flops	O
.	O
</s>
<s>
These	O
operated	O
four	O
times	O
as	O
fast	O
as	O
the	O
core	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
The	O
PDP-6	B-Device
weighed	O
about	O
,	O
with	O
"	O
Fast	O
Memory	O
"	O
.	O
</s>
<s>
The	O
PDP-6	B-Device
was	O
infamous	O
because	O
of	O
the	O
6205	O
board	O
,	O
a	O
large	O
(	O
11	O
×	O
9inches	O
)	O
board	O
which	O
contained	O
1	O
bit	O
of	O
arithmetic	O
register	O
(	O
AR	O
)	O
,	O
memory	O
buffer	O
(	O
MB	O
)	O
,	O
and	O
multiplier-quotient	O
register	O
(	O
MQ	O
)	O
.	O
</s>
<s>
There	O
was	O
also	O
a	O
great	O
fear	O
of	O
powering	O
off	O
a	O
PDP-6	B-Device
,	O
since	O
it	O
would	O
generally	O
result	O
in	O
at	O
least	O
one	O
6205	O
board	O
failing	O
.	O
</s>
<s>
The	O
experience	O
with	O
the	O
6205	O
led	O
the	O
designers	O
of	O
the	O
first	O
models	O
of	O
PDP-10	B-Device
,	O
the	O
KA10	O
and	O
KI10	B-Device
,	O
to	O
use	O
only	O
small	O
boards	O
.	O
</s>
<s>
It	O
was	O
not	O
until	O
the	O
KL10	B-Device
that	O
large	O
boards	O
were	O
used	O
again	O
.	O
</s>
<s>
The	O
PDP-6	B-Device
supported	O
time-sharing	B-General_Concept
through	O
the	O
use	O
of	O
a	O
status	O
bit	O
selecting	O
between	O
two	O
operating	O
modes	O
(	O
"	O
Executive	O
"	O
and	O
"	O
User	O
"	O
,	O
with	O
access	O
to	O
input/output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
,	O
etc.	O
,	O
being	O
restricted	O
in	O
the	O
latter	O
)	O
,	O
and	O
a	O
single	O
relocation/protection	O
register	O
which	O
allowed	O
a	O
user	O
's	O
address	B-General_Concept
space	I-General_Concept
to	O
be	O
limited	O
to	O
a	O
set	O
section	O
of	O
main	O
memory	O
(	O
a	O
second	O
relocation/protection	O
register	O
for	O
shareable	O
"	O
high	O
segments	O
"	O
was	O
added	O
on	O
the	O
PDP-10	B-Device
)	O
.	O
</s>
<s>
The	O
main	O
operating	B-General_Concept
system	I-General_Concept
used	O
on	O
the	O
machine	O
was	O
an	O
early	O
version	O
of	O
what	O
later	O
became	O
TOPS-10	B-Operating_System
,	O
and	O
several	O
sites	O
made	O
custom	O
versions	O
of	O
the	O
system	O
,	O
which	O
was	O
available	O
in	O
source	O
code	O
form	O
.	O
</s>
<s>
MIT	O
's	O
Incompatible	B-Application
Timesharing	I-Application
System	I-Application
(	O
ITS	B-Application
)	O
operating	B-General_Concept
system	I-General_Concept
also	O
began	O
on	O
the	O
PDP-6	B-Device
.	O
</s>
<s>
Although	O
it	O
was	O
possible	O
to	O
time-share	O
a	O
PDP-6	B-Device
without	O
a	O
disk	O
drive	O
,	O
configuring	O
it	O
with	O
four	O
dual	O
DECtape	O
drives	O
"	O
could	O
effectively	O
support	O
about	O
4-6	O
simultaneous	O
users.	O
"	O
</s>
<s>
The	O
same	O
system	O
,	O
with	O
a	O
single	O
disk	O
drive	O
,	O
resulted	O
in	O
"	O
real	O
time-sharing	B-General_Concept
(	O
and	O
)	O
could	O
easily	O
handle	O
20-30	O
users.	O
"	O
</s>
<s>
Stanford	O
's	O
PDP-6	B-Device
was	O
shown	O
at	O
DECUS	O
in	O
1984	O
.	O
</s>
<s>
The	O
Fast	O
Memory	O
cabinet	O
from	O
the	O
Stanford	O
PDP-6	B-Device
was	O
part	O
of	O
that	O
donation	O
.	O
</s>
<s>
There	O
is	O
no	O
evidence	O
that	O
the	O
modules	O
sold	O
at	O
the	O
Boston	O
computer	O
museum	O
gift	O
shop	O
were	O
from	O
the	O
Stanford	O
PDP-6	B-Device
,	O
nor	O
is	O
there	O
any	O
evidence	O
that	O
the	O
museum	O
had	O
ever	O
had	O
this	O
machine	O
in	O
its	B-Application
possession	O
.	O
</s>
