<s>
LINC-8	B-Device
was	O
the	O
name	O
of	O
a	O
minicomputer	B-Architecture
manufactured	O
by	O
Digital	O
Equipment	O
Corporation	O
between	O
1966	O
and	O
1969	O
.	O
</s>
<s>
It	O
combined	O
a	O
LINC	B-Device
computer	O
with	O
a	O
PDP-8	B-Device
in	O
one	O
cabinet	O
,	O
thus	O
being	O
able	O
to	O
run	O
programs	O
written	O
for	O
either	O
of	O
the	O
two	O
architectures	O
.	O
</s>
<s>
The	O
LINC-8	B-Device
contained	O
one	O
PDP-8	B-Device
CPU	O
and	O
one	O
LINC	B-Device
CPU	O
,	O
partially	O
emulated	B-Application
by	O
the	O
PDP-8	B-Device
.	O
</s>
<s>
At	O
any	O
one	O
time	O
,	O
the	O
computer	O
was	O
in	O
either	O
'	O
LINC	B-Device
mode	O
 '	O
or	O
'	O
PDP-8	B-Device
mode	O
 '	O
-	O
both	O
processors	O
could	O
not	O
run	O
in	O
parallel	O
.	O
</s>
<s>
In	O
the	O
LINC-8	B-Device
,	O
all	O
interrupts	O
were	O
handled	O
by	O
the	O
PDP-8	B-Device
CPU	O
,	O
and	O
programs	O
that	O
relied	O
on	O
the	O
interrupt	O
architecture	O
of	O
the	O
LINC	B-Device
could	O
not	O
be	O
run	O
.	O
</s>
<s>
The	O
LINC	B-Device
was	O
a	O
12-bit	O
ones	O
 '	O
complement	O
accumulator	B-General_Concept
machine	I-General_Concept
,	O
whereas	O
the	O
PDP-8	B-Device
,	O
while	O
also	O
a	O
12-bit	O
accumulator	B-General_Concept
machine	I-General_Concept
,	O
operated	O
in	O
two	B-General_Concept
's	I-General_Concept
complement	I-General_Concept
arithmetic	I-General_Concept
.	O
</s>
<s>
On	O
the	O
LINC	B-Device
,	O
the	O
full	O
address	O
space	O
was	O
divided	O
into	O
1024-word	O
segments	O
,	O
two	O
of	O
which	O
were	O
selected	O
for	O
use	O
at	O
any	O
one	O
time	O
:	O
the	O
instruction	O
field	O
and	O
the	O
data	O
field	O
.	O
</s>
<s>
As	O
a	O
practical	O
matter	O
,	O
few	O
LINC-8	B-Device
systems	O
ever	O
were	O
expanded	O
to	O
8K	O
total	O
.	O
</s>
<s>
Memory	O
expansion	O
is	O
accomplished	O
first	O
by	O
adding	O
PDP-8	B-Device
memory	O
extension	O
hardware	O
and	O
extended	O
memory	O
instructions	O
and	O
a	O
few	O
minor	O
LINC	B-Device
processor	O
modifications	O
to	O
address	O
the	O
memory	O
beyond	O
the	O
basic	O
4K	O
total	O
.	O
</s>
<s>
As	O
a	O
practical	O
matter	O
,	O
it	O
is	O
always	O
difficult	O
to	O
implement	O
on	O
the	O
"	O
regular	O
"	O
PDP-8	B-Device
,	O
and	O
,	O
in	O
the	O
case	O
of	O
the	O
LINC-8	B-Device
,	O
it	O
became	O
necessary	O
to	O
slow	O
down	O
the	O
CPU	O
slightly	O
just	O
to	O
add	O
on	O
the	O
first	O
additional	O
4K	O
.	O
</s>
<s>
Thus	O
,	O
as	O
a	O
practical	O
matter	O
,	O
LINC-8	B-Device
memory	O
segments	O
are	O
limited	O
to	O
segment	O
0-3	O
,	O
or	O
perhaps	O
0-7	O
on	O
the	O
few	O
8K	O
implementations	O
.	O
</s>
<s>
By	O
convention	O
,	O
the	O
segment	O
0	O
area	O
is	O
not	O
available	O
for	O
normal	O
fully	O
emulated	B-Application
LINC	B-Device
operations	O
.	O
</s>
<s>
This	O
is	O
because	O
the	O
PDP-8	B-Device
program	O
usually	O
known	O
as	O
PROGOFOP	O
is	O
loaded	O
there	O
to	O
handle	O
all	O
interrupts	O
,	O
traps	O
,	O
etc	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
write	O
a	O
program	O
for	O
a	O
"	O
partial	O
"	O
LINC	B-Device
CPU	O
,	O
meaning	O
using	O
only	O
the	O
hardware	O
that	O
actually	O
exists	O
.	O
</s>
<s>
Whenever	O
an	O
operation	O
is	O
performed	O
that	O
it	O
cannot	O
handle	O
,	O
the	O
PDP-8	B-Device
operation	O
resumes	O
.	O
</s>
<s>
However	O
,	O
the	O
LINC	B-Device
operation	O
could	O
have	O
been	O
terminated	O
for	O
a	O
variety	O
of	O
reasons	O
.	O
</s>
<s>
As	O
such	O
,	O
it	O
is	O
always	O
recommended	O
that	O
PROGOFOP	O
be	O
loaded	O
when	O
attempting	O
to	O
use	O
"	O
complete	O
"	O
LINC	B-Device
programs	O
on	O
this	O
system	O
.	O
</s>
<s>
Many	O
operating	O
systems	O
were	O
written	O
for	O
this	O
machine	O
;	O
some	O
were	O
essentially	O
slightly	O
modified	O
versions	O
designed	O
for	O
the	O
original	O
LINC	B-Device
CPU	O
it	O
is	O
partially	O
based	O
on	O
.	O
</s>
<s>
Bootup	O
conventions	O
allowed	O
an	O
image	O
of	O
a	O
custom	O
version	O
of	O
PROGOFOP	O
to	O
first	O
be	O
loaded	O
,	O
followed	O
by	O
executing	O
tape	O
instructions	O
to	O
load	O
the	O
LINC-based	O
operating	O
system	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
the	O
bootup	O
procedure	O
was	O
accomplished	O
manually	O
right	O
on	O
the	O
LINC	B-Device
console	O
switches	O
;	O
later	O
systems	O
self-started	O
the	O
system	O
after	O
loading	O
PROGOFOP	O
.	O
</s>
<s>
Other	O
operating	O
systems	O
are	O
actually	O
more	O
generic	O
and	O
are	O
designed	O
to	O
mostly	O
ignore	O
the	O
LINC	B-Device
side	O
of	O
things	O
.	O
</s>
<s>
These	O
are	O
PDP-8-only	O
systems	O
,	O
although	O
perhaps	O
custom	O
configured	O
for	O
the	O
vagaries	O
of	O
the	O
specifics	O
of	O
a	O
LINC-8	B-Device
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
this	O
means	O
that	O
they	O
cannot	O
be	O
run	O
on	O
any	O
other	O
machine	O
;	O
in	O
other	O
cases	O
,	O
the	O
LINC-8	B-Device
merely	O
represented	O
a	O
normal	O
variation	O
of	O
drivers	O
off	O
of	O
an	O
otherwise	O
non-descript	O
PDP-8	B-Device
system	O
.	O
</s>
<s>
An	O
advantage	O
of	O
a	O
PDP-8-based	O
system	O
is	O
that	O
PROGOFOP	O
is	O
superfluous	O
here	O
.	O
</s>
<s>
If	O
needed	O
,	O
the	O
PDP-8	B-Device
system	O
could	O
load	O
PROGOFOP	O
as	O
well	O
as	O
a	O
user	O
program	O
primarily	O
LINC-oriented	O
to	O
get	O
at	O
the	O
laboratory	O
peripherals	O
.	O
</s>
<s>
The	O
LINC	B-Device
convention	O
of	O
the	O
entire	O
first	O
1K	O
being	O
unavailable	O
reserved	O
for	O
PROGOFOP	O
is	O
exchanged	O
for	O
the	O
far	O
smaller	O
PDP-8	B-Device
convention	O
of	O
reserving	O
only	O
07600-07777	O
or	O
the	O
last	O
128-word	O
page	O
of	O
the	O
first	O
4K	O
of	O
the	O
machine	O
.	O
</s>
<s>
This	O
corresponds	O
to	O
a	O
small	O
reserved	O
area	O
at	O
the	O
end	O
of	O
LINC	B-Device
segment	O
3	O
in	O
exchange	O
for	O
much	O
greater	O
overall	O
flexibility	O
.	O
</s>
<s>
The	O
PDP-8	B-Device
divided	O
its	O
memory	O
into	O
128-word	O
pages	O
.	O
</s>
<s>
Again	O
,	O
hardware	O
limitations	O
of	O
the	O
LINC-8	B-Device
make	O
it	O
hard	O
to	O
achieve	O
a	O
total	O
size	O
of	O
more	O
than	O
8K	O
total	O
.	O
</s>
<s>
The	O
computer	O
included	O
a	O
number	O
of	O
LINC	B-Device
peripherals	O
,	O
which	O
were	O
controlled	O
by	O
special	O
LINC	B-Device
mode	O
instructions	O
.	O
</s>
<s>
These	O
devices	O
included	O
analog	O
inputs	O
in	O
the	O
forms	O
of	O
knobs	O
and	O
jacks	O
,	O
relays	O
for	O
control	O
of	O
external	O
equipment	O
,	O
LINCtape	O
drives	O
(	O
the	O
predecessor	O
of	O
the	O
DECtape	O
)	O
,	O
an	O
oscilloscope-like	O
cathode	B-Device
ray	I-Device
tube	I-Device
under	O
program	O
control	O
,	O
as	O
well	O
as	O
a	O
Teletype	B-Application
Model	I-Application
33	I-Application
ASR	O
.	O
</s>
<s>
Some	O
of	O
these	O
peripherals	O
are	O
simulated	O
and	O
are	O
actually	O
peripherals	O
of	O
the	O
PDP-8	B-Device
.	O
</s>
<s>
Any	O
unimplemented	O
operation	O
stops	O
the	O
LINC	B-Device
CPU	O
and	O
interrupts	O
the	O
PDP-8	B-Device
processor	O
to	O
handle	O
the	O
specifics	O
.	O
</s>
<s>
Most	O
notably	O
,	O
the	O
LINCtape	O
is	O
actually	O
a	O
PDP-8	B-Device
peripheral	O
;	O
the	O
tape	O
class	O
of	O
LINC	B-Device
instructions	O
are	O
trapped	O
and	O
interrupt	O
the	O
PDP-8	B-Device
which	O
then	O
emulates	B-Application
how	O
a	O
real	O
LINC	B-Device
or	O
PDP-12	B-Device
would	O
carry	O
out	O
the	O
specifics	O
of	O
the	O
latest	O
tape	O
instruction	O
.	O
</s>
<s>
Pressing	O
a	O
variety	O
of	O
keys	O
on	O
the	O
seemingly	O
present	O
LINC	B-Device
console	O
all	O
cause	O
PDP-8	B-Device
interrupts	O
;	O
PROGOFOP	O
is	O
designed	O
to	O
emulate	B-Application
the	O
functions	O
as	O
they	O
would	O
appear	O
on	O
the	O
original	O
LINC	B-Device
.	O
</s>
<s>
An	O
interesting	O
feature	O
is	O
the	O
FETCH/EXEC	O
stop	O
,	O
which	O
is	O
implemented	O
in	O
all	O
hardware	O
in	O
the	O
LINC	B-Device
and	O
PDP-12	B-Device
.	O
</s>
<s>
This	O
will	O
cause	O
a	O
PDP-8	B-Device
interrupt	O
stalling	O
the	O
LINC	B-Device
program	O
.	O
</s>
<s>
In	O
essence	O
,	O
the	O
LINC-8	B-Device
implements	O
all	O
of	O
the	O
functions	O
of	O
the	O
console	O
panel	O
of	O
the	O
"	O
real	O
"	O
LINC	B-Device
,	O
then	O
uses	O
the	O
PDP-8	B-Device
to	O
simulate	O
most	O
of	O
them	O
.	O
</s>
<s>
The	O
LINC-8	B-Device
was	O
built	O
as	O
a	O
laboratory	O
computer	O
.	O
</s>
<s>
The	O
LINCtape	O
magnetic	O
tape	O
drive	O
,	O
designed	O
by	O
Wesley	O
A	O
.	O
Clark	O
for	O
the	O
LINC	B-Device
,	O
was	O
suitable	O
for	O
handling	O
in	O
a	O
laboratory	O
environment	O
,	O
and	O
the	O
tapes	O
could	O
be	O
carelessly	O
pocketed	O
,	O
dropped	O
,	O
or	O
even	O
pierced	O
and	O
cut	O
without	O
losing	O
the	O
data	O
stored	O
on	O
them	O
.	O
</s>
<s>
In	O
1969	O
,	O
DEC	O
improved	O
upon	O
the	O
LINC-8	B-Device
with	O
the	O
PDP-12	B-Device
,	O
a	O
similar	O
combination	O
computer	O
for	O
lab	O
use	O
,	O
and	O
the	O
LINC-8	B-Device
was	O
cancelled	O
.	O
</s>
<s>
Few	O
LINC-8	B-Device
computers	O
were	O
ever	O
built	O
,	O
numbering	O
only	O
in	O
the	O
low	O
hundreds	O
,	O
and	O
so	O
the	O
model	O
is	O
a	O
rare	O
sight	O
today	O
.	O
</s>
<s>
As	O
of	O
2008	O
,	O
a	O
project	O
to	O
emulate	B-Application
the	O
LINC-8	B-Device
on	O
modern	O
hardware	O
is	O
underway	O
within	O
the	O
Update	O
computer	O
society	O
at	O
Uppsala	O
University	O
.	O
</s>
