<s>
The	O
PDP-8	B-Device
is	O
a	O
12-bit	B-Device
minicomputer	B-Architecture
that	O
was	O
produced	O
by	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
.	O
</s>
<s>
It	O
was	O
the	O
first	O
commercially	O
successful	O
minicomputer	B-Architecture
,	O
with	O
over	O
50,000	O
units	O
being	O
sold	O
over	O
the	O
model	O
's	O
lifetime	O
.	O
</s>
<s>
Its	O
basic	O
design	O
follows	O
the	O
pioneering	O
LINC	B-Device
but	O
has	O
a	O
smaller	O
instruction	B-General_Concept
set	I-General_Concept
,	O
which	O
is	O
an	O
expanded	O
version	O
of	O
the	O
PDP-5	B-Device
instruction	B-General_Concept
set	I-General_Concept
.	O
</s>
<s>
Similar	O
machines	O
from	O
DEC	O
are	O
the	O
PDP-12	B-Device
which	O
is	O
a	O
modernized	O
version	O
of	O
the	O
PDP-8	B-Device
and	O
LINC	B-Device
concepts	O
,	O
and	O
the	O
PDP-14	B-Device
industrial	O
controller	O
system	O
.	O
</s>
<s>
The	O
earliest	O
PDP-8	B-Device
model	O
,	O
informally	O
known	O
as	O
a	O
"	O
Straight-8	O
"	O
,	O
was	O
introduced	O
on	O
22	O
March	O
1965	O
priced	O
at	O
$	O
18,500	O
(	O
)	O
.	O
</s>
<s>
It	O
used	O
diode	B-General_Concept
–	I-General_Concept
transistor	I-General_Concept
logic	I-General_Concept
packaged	O
on	O
flip	B-Device
chip	I-Device
cards	O
in	O
a	O
machine	O
about	O
the	O
size	O
of	O
a	O
small	O
household	O
refrigerator	O
.	O
</s>
<s>
The	O
Straight-8	O
was	O
supplanted	O
in	O
1966	O
by	O
the	O
PDP-8/S	B-Device
,	O
which	O
was	O
available	O
in	O
desktop	O
and	O
rack-mount	O
models	O
.	O
</s>
<s>
Using	O
a	O
one-bit	B-General_Concept
serial	I-General_Concept
arithmetic	B-General_Concept
logic	I-General_Concept
unit	I-General_Concept
(	O
ALU	O
)	O
allowed	O
the	O
PDP-8/S	B-Device
to	O
be	O
smaller	O
and	O
less	O
expensive	O
,	O
although	O
slower	O
than	O
the	O
original	O
PDP-8	B-Device
.	O
</s>
<s>
Later	O
systems	O
(	O
the	O
PDP-8/I	O
and	O
/L	O
,	O
the	O
PDP-8/E	B-Device
,	O
/F	O
,	O
and	O
/M	O
,	O
and	O
the	O
PDP-8/A	O
)	O
returned	O
to	O
a	O
faster	O
,	O
fully	O
parallel	O
implementation	O
but	O
use	O
much	O
less	O
costly	O
transistor	B-General_Concept
–	I-General_Concept
transistor	I-General_Concept
logic	I-General_Concept
(	O
TTL	B-General_Concept
)	O
MSI	O
logic	O
.	O
</s>
<s>
Most	O
surviving	O
PDP-8s	B-Device
are	O
from	O
this	O
era	O
.	O
</s>
<s>
The	O
PDP-8/E	B-Device
is	O
common	O
,	O
and	O
well-regarded	O
because	O
many	O
types	O
of	O
I/O	B-General_Concept
devices	I-General_Concept
were	O
available	O
for	O
it	O
.	O
</s>
<s>
The	O
last	O
commercial	O
PDP-8	B-Device
models	O
introduced	O
in	O
1979	O
are	O
called	O
"	O
CMOS-8s	O
"	O
,	O
based	O
on	O
CMOS	B-Device
microprocessors	B-Architecture
.	O
</s>
<s>
Intersil	O
sold	O
the	O
integrated	O
circuits	O
commercially	O
through	O
1982	O
as	O
the	O
Intersil	B-General_Concept
6100	I-General_Concept
family	I-General_Concept
.	O
</s>
<s>
By	O
virtue	O
of	O
their	O
CMOS	B-Device
technology	O
they	O
had	O
low	O
power	O
requirements	O
and	O
were	O
used	O
in	O
some	O
embedded	O
military	O
systems	O
.	O
</s>
<s>
The	O
chief	O
engineer	O
who	O
designed	O
the	O
initial	O
version	O
of	O
the	O
PDP-8	B-Device
was	O
Edson	O
de	O
Castro	O
,	O
who	O
later	O
founded	O
Data	O
General	O
.	O
</s>
<s>
The	O
PDP-8	B-Device
combines	O
low	O
cost	O
,	O
simplicity	O
,	O
expandability	O
,	O
and	O
careful	O
engineering	O
for	O
value	O
.	O
</s>
<s>
The	O
greatest	O
historical	O
significance	O
was	O
that	O
the	O
PDP-8	B-Device
'	O
s	O
low	O
cost	O
and	O
high	O
volume	O
made	O
a	O
computer	O
available	O
to	O
many	O
new	O
customers	O
for	O
many	O
new	O
uses	O
.	O
</s>
<s>
It	O
made	O
programming	O
cumbersome	O
,	O
as	O
is	O
seen	O
in	O
the	O
examples	O
in	O
this	O
article	O
and	O
from	O
the	O
discussion	O
of	O
"	O
pages	B-General_Concept
"	O
and	O
"	O
fields	O
"	O
.	O
</s>
<s>
For	O
example	O
,	O
subtracting	O
a	O
number	O
involves	O
computing	O
its	O
two	B-General_Concept
's	I-General_Concept
complement	I-General_Concept
then	O
adding	O
it	O
;	O
writing	O
a	O
conditional	O
jump	O
involves	O
writing	O
a	O
conditional	O
skip	O
around	O
the	O
jump	O
,	O
the	O
skip	O
coding	O
the	O
condition	O
negative	O
to	O
the	O
one	O
desired	O
.	O
</s>
<s>
Subsequent	O
computer	O
designs	O
emphasized	O
ease	O
of	O
programming	O
,	O
typically	O
using	O
larger	O
and	O
more	O
intuitive	O
instruction	B-General_Concept
sets	I-General_Concept
.	O
</s>
<s>
Eventually	O
,	O
most	O
machine	O
code	O
was	O
generated	O
by	O
compilers	B-Language
and	O
report	O
generators	O
.	O
</s>
<s>
The	O
reduced	B-Architecture
instruction	I-Architecture
set	I-Architecture
computer	I-Architecture
returned	O
full-circle	O
to	O
the	O
PDP-8	B-Device
'	O
s	O
emphasis	O
on	O
a	O
simple	O
instruction	B-General_Concept
set	I-General_Concept
and	O
achieving	O
multiple	O
actions	O
in	O
a	O
single	O
instruction	O
cycle	O
,	O
in	O
order	O
to	O
maximize	O
execution	O
speed	O
,	O
although	O
the	O
newer	O
computers	O
have	O
much	O
longer	O
instruction	O
words	O
.	O
</s>
<s>
The	O
PDP-8	B-Device
used	O
ideas	O
from	O
several	O
12-bit	B-Device
predecessors	O
such	O
as	O
the	O
LINC	B-Device
designed	O
by	O
W.A.	O
</s>
<s>
Molnar	O
,	O
who	O
were	O
inspired	O
by	O
Seymour	O
Cray	O
's	O
CDC	B-Device
160	I-Device
minicomputer	B-Architecture
.	O
</s>
<s>
The	O
PDP-8	B-Device
uses	O
12	B-Device
bits	I-Device
for	O
its	O
word	O
size	O
and	O
arithmetic	O
(	O
on	O
unsigned	O
integers	O
from	O
0	O
to	O
4095	O
or	O
signed	O
integers	O
from	O
−2048	O
to	O
+2047	O
)	O
.	O
</s>
<s>
However	O
,	O
software	O
can	O
do	O
multiple-precision	B-Algorithm
arithmetic	I-Algorithm
.	O
</s>
<s>
An	O
interpreter	O
was	O
available	O
for	O
floating	B-Algorithm
point	I-Algorithm
operations	O
,	O
for	O
example	O
,	O
that	O
uses	O
a	O
36-bit	O
floating	B-Algorithm
point	I-Algorithm
representation	I-Algorithm
with	O
a	O
two-word	O
(	O
24-bit	O
)	O
significand	B-Algorithm
(	O
mantissa	B-Algorithm
)	O
and	O
one-word	O
exponent	O
.	O
</s>
<s>
Subject	O
to	O
speed	O
and	O
memory	O
limitations	O
,	O
the	O
PDP-8	B-Device
can	O
perform	O
calculations	O
similar	O
to	O
more	O
expensive	O
contemporary	O
electronic	O
computers	O
,	O
such	O
as	O
the	O
IBM	B-Device
1130	I-Device
and	O
various	O
models	O
of	O
the	O
IBM	B-Application
System/360	I-Application
,	O
while	O
being	O
easier	O
to	O
interface	O
with	O
external	O
devices	O
.	O
</s>
<s>
The	O
memory	O
address	O
space	O
is	O
also	O
12	B-Device
bits	I-Device
,	O
so	O
the	O
PDP-8	B-Device
'	O
s	O
basic	O
configuration	O
has	O
a	O
main	O
memory	O
of	O
4,096	O
(	O
212	O
)	O
twelve-bit	O
words	O
,	O
or	O
6kB	O
in	O
modern	O
terms	O
.	O
</s>
<s>
An	O
optional	O
memory-expansion	O
unit	O
can	O
switch	B-General_Concept
banks	I-General_Concept
of	I-General_Concept
memories	I-General_Concept
using	O
an	O
IOT	O
instruction	O
.	O
</s>
<s>
The	O
1974	O
Pocket	O
Reference	O
Card	O
for	O
the	O
PDP-8/E	B-Device
gives	O
a	O
basic	O
instruction	O
time	O
of	O
1.2	O
microseconds	O
,	O
or	O
2.6	O
microseconds	O
for	O
instructions	O
that	O
reference	O
memory	O
.	O
</s>
<s>
The	O
PDP-8	B-Device
was	O
designed	O
in	O
part	O
to	O
handle	O
contemporary	O
telecommunications	O
and	O
text	O
.	O
</s>
<s>
Six-bit	O
character	O
codes	O
were	O
in	O
widespread	O
use	O
at	O
the	O
time	O
,	O
and	O
the	O
PDP-8	B-Device
'	O
s	O
twelve-bit	O
words	O
can	O
efficiently	O
store	O
two	O
such	O
characters	O
.	O
</s>
<s>
In	O
addition	O
,	O
a	O
six-bit	O
teleprinter	O
code	O
called	O
the	O
teletypesetting	O
or	O
TTS	O
code	O
was	O
in	O
widespread	O
use	O
by	O
the	O
news	O
wire	O
services	O
,	O
and	O
an	O
early	O
application	O
for	O
the	O
PDP-8	B-Device
was	O
typesetting	O
using	O
this	O
code	O
.	O
</s>
<s>
PDP-8	B-Device
instructions	O
have	O
a	O
3-bit	O
opcode	O
,	O
so	O
there	O
are	O
only	O
eight	O
instructions	O
.	O
</s>
<s>
The	O
assembler	B-Language
provides	O
more	O
instruction	O
mnemonics	O
to	O
a	O
programmer	O
by	O
translating	O
I/O	B-General_Concept
and	O
operate-mode	O
instructions	O
to	O
combinations	O
of	O
the	O
op-codes	O
and	O
instruction	O
fields	O
.	O
</s>
<s>
It	O
also	O
has	O
only	O
three	O
programmer-visible	O
registers	O
:	O
A	O
12-bit	B-Device
accumulator	B-General_Concept
(	O
AC	O
)	O
,	O
a	O
program	B-General_Concept
counter	I-General_Concept
(	O
PC	O
)	O
,	O
and	O
a	O
carry	B-Algorithm
flag	I-Algorithm
called	O
the	O
"	O
link	O
register	B-General_Concept
"	O
(	O
L	B-Algorithm
)	O
.	O
</s>
<s>
For	O
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
,	O
the	O
PDP-8	B-Device
has	O
a	O
single	O
interrupt	B-Application
shared	O
by	O
all	O
devices	O
,	O
an	O
I/O	B-General_Concept
bus	O
accessed	O
by	O
I/O	B-General_Concept
instructions	O
and	O
a	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
(	O
DMA	B-General_Concept
)	O
channel	O
.	O
</s>
<s>
The	O
programmed	O
I/O	B-General_Concept
bus	O
typically	O
runs	O
low	O
to	O
medium-speed	O
peripherals	O
,	O
such	O
as	O
printers	O
,	O
teletypes	O
,	O
paper	O
tape	O
punches	O
and	O
readers	O
,	O
while	O
DMA	B-General_Concept
is	O
used	O
for	O
cathode-ray	B-Device
tube	I-Device
screens	O
with	O
a	O
light	B-Device
pen	I-Device
,	O
analog-to-digital	O
converters	O
,	O
digital-to-analog	O
converters	O
,	O
tape	O
drives	O
,	O
and	O
disk	B-Device
drives	I-Device
.	O
</s>
<s>
To	O
save	O
money	O
,	O
the	O
design	O
uses	O
inexpensive	O
main	O
memory	O
for	O
many	O
purposes	O
that	O
are	O
served	O
by	O
more	O
expensive	O
flip-flop	B-General_Concept
registers	O
in	O
other	O
computers	O
,	O
such	O
as	O
auxiliary	O
counters	O
and	O
subroutine	O
linkage	O
.	O
</s>
<s>
For	O
faster	O
math	O
,	O
the	O
Extended	O
Arithmetic	O
Element	O
(	O
EAE	O
)	O
provides	O
multiply	O
and	O
divide	O
instructions	O
with	O
an	O
additional	O
register	B-General_Concept
,	O
the	O
Multiplier/Quotient	O
(	O
MQ	O
)	O
register	B-General_Concept
.	O
</s>
<s>
The	O
EAE	O
was	O
an	O
option	O
on	O
the	O
original	O
PDP-8	B-Device
,	O
the	O
8/I	O
,	O
and	O
the	O
8/E	O
,	O
but	O
it	O
is	O
an	O
integral	O
part	O
of	O
the	O
Intersil	B-General_Concept
6100	I-General_Concept
microprocessor	B-Architecture
.	O
</s>
<s>
The	O
PDP-8	B-Device
is	O
optimized	O
for	O
simplicity	B-Architecture
of	I-Architecture
design	I-Architecture
.	O
</s>
<s>
The	O
electronics	O
of	O
a	O
basic	O
PDP-8	B-Device
CPU	O
has	O
only	O
four	O
12-bit	B-Device
registers	O
:	O
the	O
accumulator	B-General_Concept
,	O
program	B-General_Concept
counter	I-General_Concept
,	O
memory-buffer	B-General_Concept
register	I-General_Concept
,	O
and	O
memory-address	B-General_Concept
register	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
the	O
memory	B-General_Concept
buffer	I-General_Concept
register	I-General_Concept
provides	O
arithmetic	O
operands	O
,	O
is	O
part	O
of	O
the	O
instruction	O
register	B-General_Concept
,	O
and	O
stores	O
data	O
to	O
rewrite	O
the	O
core	O
memory	O
.	O
</s>
<s>
Because	O
of	O
their	O
simplicity	O
,	O
early	O
PDP-8	B-Device
models	O
were	O
less	O
expensive	O
than	O
most	O
other	O
commercially	O
available	O
computers	O
.	O
</s>
<s>
They	O
used	O
thousands	O
of	O
very	O
small	O
,	O
standardized	O
logic-modules	O
,	O
with	O
gold	O
connectors	O
,	O
integrated	O
by	O
a	O
costly	O
,	O
complex	O
wire-wrapped	O
backplane	B-Architecture
in	O
a	O
large	O
cabinet	O
.	O
</s>
<s>
In	O
the	O
later	O
8/S	O
model	O
,	O
introduced	O
in	O
August	O
1966	O
,	O
two	O
different	O
logic	O
voltages	O
increased	O
the	O
fan-out	B-General_Concept
of	O
the	O
inexpensive	O
diode	B-General_Concept
–	I-General_Concept
transistor	I-General_Concept
logic	I-General_Concept
.	O
</s>
<s>
The	O
CPU	O
of	O
the	O
PDP-8/S	B-Device
has	O
only	O
about	O
519	O
logic	O
gates	O
.	O
</s>
<s>
In	O
comparison	O
,	O
small	O
microcontrollers	B-Architecture
(	O
as	O
of	O
2008	O
)	O
usually	O
have	O
15,000	O
or	O
more	O
.	O
</s>
<s>
The	O
even	O
later	O
PDP-8/E	B-Device
is	O
a	O
larger	O
,	O
more	O
capable	O
computer	O
,	O
but	O
further	O
reengineered	O
for	O
better	O
value	O
.	O
</s>
<s>
It	O
employs	O
faster	O
transistor	B-General_Concept
–	I-General_Concept
transistor	I-General_Concept
logic	I-General_Concept
,	O
in	O
integrated	O
circuits	O
.	O
</s>
<s>
It	O
allows	O
expansion	O
with	O
less	O
expense	O
because	O
it	O
uses	O
the	O
OMNIBUS	O
in	O
place	O
of	O
the	O
wire-wrapped	O
backplane	B-Architecture
on	O
earlier	O
models	O
.	O
</s>
<s>
(	O
A	O
personal	O
account	O
of	O
the	O
development	O
of	O
the	O
PDP-8/E	B-Device
can	O
be	O
read	O
on	O
the	O
Engineering	O
and	O
Technology	O
History	O
Wiki	O
.	O
)	O
</s>
<s>
The	O
total	O
sales	O
figure	O
for	O
the	O
PDP-8	B-Device
family	O
has	O
been	O
estimated	O
at	O
over	O
300,000	O
machines	O
.	O
</s>
<s>
+	O
PDP-8	B-Device
Models	O
Summary	O
Model	O
Description	O
Year	O
Price	O
Quantity	O
produced	O
Weight	O
PDP-8	B-Device
Semi-discrete	O
components	O
.	O
</s>
<s>
DTL	B-General_Concept
.	O
</s>
<s>
The	O
PDP-8	B-Device
is	O
readily	O
emulated	B-Application
,	O
as	O
its	O
instruction	B-General_Concept
set	I-General_Concept
is	O
much	O
simpler	O
than	O
modern	O
architectures	O
.	O
</s>
<s>
Enthusiasts	O
have	O
created	O
entire	O
PDP-8s	B-Device
using	O
single	O
FPGA	B-Architecture
devices	O
.	O
</s>
<s>
Several	O
software	O
simulations	O
of	O
a	O
PDP-8	B-Device
are	O
available	O
on	O
the	O
Internet	O
,	O
as	O
well	O
as	O
open-source	O
hardware	O
re-implementations	O
.	O
</s>
<s>
The	O
software	O
simulations	O
often	O
simulate	O
late-model	O
PDP-8s	B-Device
with	O
all	O
possible	O
peripherals	O
.	O
</s>
<s>
One	O
of	O
the	O
first	O
commercial	O
versions	O
of	O
a	O
PDP-8/S	B-Device
virtual	O
machine	O
ran	O
on	O
a	O
Kaypro	O
386	O
(	O
an	O
80386-based	O
computer	O
)	O
and	O
was	O
written	O
in	O
the	O
C	O
computer	O
language	O
(	O
before	O
the	O
ANSI-C	O
standard	O
was	O
finalized	O
)	O
and	O
assembler	B-Language
by	O
David	O
Beecher	O
of	O
Denver	O
,	O
Colorado	O
.	O
</s>
<s>
It	O
replaced	O
a	O
failing	O
PDP-8/S	B-Device
computer	O
that	O
operated	O
the	O
fuel	O
handling	O
machine	O
at	O
Reactor	O
#85	O
,	O
the	O
Platteville	O
,	O
Colorado	O
Nuclear	O
Fuel	O
powered	O
Electric	O
Generating	O
Station	O
,	O
Ft	B-Application
.	O
St.	O
Vrain	O
.	O
</s>
<s>
It	O
included	O
a	O
simulated	O
paper	O
tape	O
loader	O
and	O
front	B-Device
panel	I-Device
.	O
</s>
<s>
The	O
I/O	B-General_Concept
systems	O
underwent	O
huge	O
changes	O
during	O
the	O
PDP-8	B-Device
era	O
.	O
</s>
<s>
Early	O
PDP-8	B-Device
models	O
use	O
a	O
front	B-Device
panel	I-Device
interface	O
,	O
a	O
paper-tape	O
reader	O
and	O
a	O
teletype	O
printer	O
with	O
an	O
optional	O
paper-tape	O
punch	O
.	O
</s>
<s>
Over	O
time	O
,	O
I/O	B-General_Concept
systems	O
such	O
as	O
magnetic	O
tape	O
,	O
RS-232	O
and	O
current	O
loop	O
dumb	B-General_Concept
terminals	I-General_Concept
,	O
punched	B-Architecture
card	I-Architecture
readers	O
,	O
and	O
fixed-head	B-General_Concept
disks	I-General_Concept
were	O
added	O
.	O
</s>
<s>
Toward	O
the	O
end	O
of	O
the	O
PDP-8	B-Device
era	O
,	O
floppy	B-Device
disks	I-Device
and	O
moving-head	O
cartridge	B-Operating_System
disk	B-Device
drives	I-Device
were	O
popular	O
I/O	B-General_Concept
devices	I-General_Concept
.	O
</s>
<s>
Modern	O
enthusiasts	O
have	O
created	O
standard	O
PC	O
style	O
IDE	B-Protocol
hard	B-Device
disk	I-Device
adapters	O
for	O
real	O
and	O
simulated	O
PDP-8	B-Device
computers	O
.	O
</s>
<s>
Several	O
types	O
of	O
I/O	B-General_Concept
are	O
supported	O
:	O
</s>
<s>
The	O
Omnibus	O
(	O
a	O
backplane	B-Architecture
of	O
undedicated	O
system	B-Architecture
bus	I-Architecture
slots	O
)	O
introduced	O
in	O
the	O
PDP-8/E	B-Device
.	O
</s>
<s>
A	O
simplified	O
,	O
inexpensive	O
form	O
of	O
DMA	B-General_Concept
called	O
"	O
three-cycle	O
data	O
break	O
"	O
is	O
supported	O
;	O
this	O
requires	O
the	O
assistance	O
of	O
the	O
processor	O
.	O
</s>
<s>
The	O
"	O
data	O
break	O
"	O
method	O
moves	O
some	O
of	O
common	O
logic	O
needed	O
to	O
implement	O
DMA	B-General_Concept
I/O	B-General_Concept
from	O
each	O
I/O	B-General_Concept
device	I-General_Concept
into	O
one	O
common	O
copy	O
of	O
the	O
logic	O
within	O
the	O
processor	O
.	O
</s>
<s>
"	O
Data	O
break	O
"	O
places	O
the	O
processor	O
in	O
charge	O
of	O
maintaining	O
the	O
DMA	B-General_Concept
address	O
and	O
word	O
count	O
registers	O
.	O
</s>
<s>
In	O
three	O
successive	O
memory	O
cycles	O
,	O
the	O
processor	O
updates	O
the	O
word	O
count	O
,	O
updates	O
the	O
transfer	O
address	O
,	O
and	O
stores	O
or	O
retrieves	O
the	O
actual	O
I/O	B-General_Concept
data	O
word	O
.	O
</s>
<s>
One-cycle	O
data	O
break	O
effectively	O
triples	O
the	O
DMA	B-General_Concept
transfer	O
rate	O
because	O
only	O
the	O
target	O
data	O
needed	O
to	O
be	O
transferred	O
to	O
and	O
from	O
the	O
core	O
memory	O
.	O
</s>
<s>
However	O
,	O
the	O
I/O	B-General_Concept
devices	I-General_Concept
need	O
more	O
electronic	O
logic	O
to	O
manage	O
their	O
own	O
word	O
count	O
and	O
transfer	O
address	B-General_Concept
registers	I-General_Concept
.	O
</s>
<s>
By	O
the	O
time	O
the	O
PDP-8/E	B-Device
was	O
introduced	O
,	O
electronic	O
logic	O
had	O
become	O
less	O
expensive	O
and	O
"	O
one-cycle	O
data	O
break	O
"	O
became	O
more	O
popular	O
.	O
</s>
<s>
Early	O
PDP-8	B-Device
systems	O
were	O
shipped	O
with	O
no	O
pre-installed	O
software	O
;	O
each	O
time	O
the	O
PDP-8	B-Device
was	O
powered	O
up	O
,	O
the	O
user	O
hand-entered	O
instructions	O
using	O
a	O
bank	O
of	O
12	O
toggle	O
switches	O
.	O
</s>
<s>
Typically	O
,	O
these	O
instructions	O
were	O
a	O
bootstrap	B-Operating_System
loader	I-Operating_System
to	O
read	O
a	O
program	O
from	O
the	O
paper	O
tape	O
reader	O
.	O
</s>
<s>
Program	O
development	O
could	O
then	O
proceed	O
,	O
using	O
paper	O
tape	O
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
.	O
</s>
<s>
Paper-tape	O
versions	O
of	O
a	O
number	O
of	O
programming	O
languages	O
became	O
available	O
,	O
including	O
DEC	O
's	O
FOCAL	B-Language
interpreter	O
and	O
a	O
4K	O
FORTRAN	B-Application
compiler	B-Language
and	O
runtime	O
.	O
</s>
<s>
Toward	O
the	O
end	O
of	O
the	O
PDP-8	B-Device
era	O
,	O
operating	O
systems	O
such	O
as	O
OS/8	B-Operating_System
and	O
COS-310	O
allowed	O
a	O
traditional	O
line	B-Application
mode	I-Application
editor	B-Application
and	O
command-line	B-Application
compiler	B-Language
development	O
system	O
using	O
languages	O
such	O
as	O
PAL-III	B-Device
assembly	B-Language
language	I-Language
,	O
FORTRAN	B-Application
,	O
BASIC	O
,	O
and	O
DIBOL	B-Language
.	O
</s>
<s>
Fairly	O
modern	O
and	O
advanced	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
(	O
RTOS	B-Operating_System
)	O
and	O
preemptive	O
multitasking	O
multi-user	O
systems	O
were	O
available	O
:	O
a	O
real-time	O
system	O
(	O
RTS-8	O
)	O
was	O
available	O
as	O
were	O
multiuser	O
commercial	O
systems	O
(	O
COS-300	O
and	O
COS-310	O
)	O
and	O
a	O
dedicated	O
single-user	O
word-processing	O
system	O
(	O
WPS-8	O
)	O
.	O
</s>
<s>
A	O
time-sharing	B-General_Concept
system	I-General_Concept
,	O
TSS-8	B-Operating_System
,	O
was	O
also	O
available	O
.	O
</s>
<s>
TSS-8	B-Operating_System
allows	O
multiple	O
users	O
to	O
log	O
into	O
the	O
system	O
via	O
110-baud	O
terminals	O
,	O
and	O
edit	O
,	O
compile	B-Language
and	O
debug	O
programs	O
.	O
</s>
<s>
Languages	O
include	O
a	O
special	O
version	O
of	O
BASIC	O
,	O
a	O
FORTRAN	B-Application
subset	O
similar	O
to	O
FORTRAN-1	O
(	O
no	O
user-written	O
subroutines	O
or	O
functions	O
)	O
,	O
an	O
ALGOL	B-Language
subset	O
,	O
FOCAL	B-Language
,	O
and	O
an	O
assembler	B-Language
called	O
PAL-D	O
.	O
</s>
<s>
A	O
fair	O
amount	O
of	O
user-donated	O
software	O
for	O
the	O
PDP-8	B-Device
was	O
available	O
from	O
DECUS	O
,	O
the	O
Digital	O
Equipment	O
Corporation	O
User	O
Society	O
,	O
and	O
often	O
came	O
with	O
full	O
source	O
listings	O
and	O
documentation	O
.	O
</s>
<s>
The	O
three	O
high-order	O
bits	O
of	O
the	O
12-bit	B-Device
instruction	O
word	O
(	O
labelled	O
bits	O
0	O
through	O
2	O
)	O
are	O
the	O
operation	O
code	O
.	O
</s>
<s>
Bit	O
4	O
,	O
if	O
set	O
,	O
says	O
to	O
complete	O
the	O
address	O
using	O
the	O
5	O
high-order	O
bits	O
of	O
the	O
program	B-General_Concept
counter	I-General_Concept
(	O
PC	O
)	O
register	B-General_Concept
,	O
meaning	O
that	O
the	O
addressed	O
location	O
was	O
within	O
the	O
same	O
128	O
words	O
as	O
the	O
instruction	O
.	O
</s>
<s>
Bit	O
3	O
specifies	O
indirection	O
;	O
if	O
set	O
,	O
the	O
address	O
obtained	O
as	O
described	O
so	O
far	O
points	O
to	O
a	O
12-bit	B-Device
value	O
in	O
memory	O
that	O
gives	O
the	O
actual	O
effective	O
address	O
for	O
the	O
instruction	O
;	O
this	O
allows	O
operands	O
to	O
be	O
anywhere	O
in	O
memory	O
at	O
the	O
expense	O
of	O
an	O
additional	O
word	O
.	O
</s>
<s>
This	O
use	O
of	O
the	O
instruction	O
word	O
divides	O
the	O
4,096	O
-word	O
memory	O
into	O
128-word	O
pages	B-General_Concept
;	O
bit	O
4	O
of	O
the	O
instruction	O
selects	O
either	O
the	O
current	O
page	O
or	O
page	B-General_Concept
0	I-General_Concept
(	O
addresses	O
0000	O
–	O
0177	O
in	O
octal	O
)	O
.	O
</s>
<s>
Memory	O
in	O
page	B-General_Concept
0	I-General_Concept
is	O
at	O
a	O
premium	O
,	O
since	O
variables	O
placed	O
here	O
can	O
be	O
addressed	O
directly	O
from	O
any	O
page	O
.	O
</s>
<s>
(	O
Moreover	O
,	O
address	O
0000	O
is	O
where	O
any	O
interrupt	B-Application
service	O
routine	O
must	O
start	O
,	O
and	O
addresses	O
0010	O
–	O
0017	O
have	O
the	O
special	O
property	O
of	O
auto-incrementing	O
preceding	O
any	O
indirect	O
reference	O
through	O
them	O
.	O
)	O
</s>
<s>
The	O
standard	O
assembler	B-Language
places	O
constant	O
values	O
for	O
arithmetic	O
in	O
the	O
current	O
page	O
.	O
</s>
<s>
It	O
was	O
important	O
to	O
write	O
routines	O
to	O
fit	O
within	O
128-word	O
pages	B-General_Concept
,	O
or	O
to	O
arrange	O
routines	O
to	O
minimize	O
page	O
transitions	O
,	O
as	O
references	O
and	O
jumps	O
outside	O
the	O
current	O
page	O
require	O
an	O
extra	O
word	O
.	O
</s>
<s>
001	O
–	O
TAD	O
–	O
Two	B-General_Concept
's	I-General_Concept
complement	I-General_Concept
ADd	O
the	O
memory	O
operand	O
to	O
<L,AC>	O
(	O
a	O
12	B-Device
bit	I-Device
signed	O
value	O
(	O
AC	O
)	O
w	O
.	O
carry	O
in	O
L	B-Algorithm
)	O
.	O
</s>
<s>
010	O
–	O
ISZ	O
–	O
Increment	O
the	O
memory	O
operand	O
and	O
Skip	O
next	B-General_Concept
instruction	I-General_Concept
if	O
result	O
is	O
Zero	O
.	O
</s>
<s>
100	O
–	O
JMS	O
–	O
JuMp	O
to	O
Subroutine	O
(	O
storing	O
return	B-Language
address	I-Language
in	O
first	O
word	O
of	O
subroutine	O
!	O
</s>
<s>
110	O
–	O
IOT	O
–	O
Input/Output	B-General_Concept
Transfer	O
(	O
see	O
below	O
)	O
.	O
</s>
<s>
The	O
PDP-8	B-Device
processor	O
defined	O
few	O
of	O
the	O
IOT	O
instructions	O
,	O
but	O
simply	O
provided	O
a	O
framework	O
.	O
</s>
<s>
Most	O
IOT	O
instructions	O
were	O
defined	O
by	O
the	O
individual	O
I/O	B-General_Concept
devices	I-General_Concept
.	O
</s>
<s>
Bits	O
3	O
through	O
8	O
of	O
an	O
IOT	O
instruction	O
select	O
an	O
I/O	B-General_Concept
device	I-General_Concept
.	O
</s>
<s>
00	O
is	O
handled	O
by	O
the	O
processor	O
and	O
not	O
sent	O
to	O
any	O
I/O	B-General_Concept
device	I-General_Concept
(	O
see	O
below	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
ION	O
(	O
6001	O
)	O
enables	O
interrupt	B-Application
processing	O
,	O
and	O
IOFF	O
(	O
6002	O
)	O
disables	O
it	O
.	O
</s>
<s>
Bit	O
11	O
causes	O
the	O
processor	O
to	O
skip	O
the	O
next	B-General_Concept
instruction	I-General_Concept
if	O
the	O
I/O	B-General_Concept
device	I-General_Concept
is	O
ready	O
.	O
</s>
<s>
Bit	O
9	O
moves	O
a	O
word	O
between	O
AC	O
and	O
the	O
device	O
,	O
initiates	O
another	O
I/O	B-General_Concept
transfer	O
,	O
and	O
clears	O
the	O
device	O
's	O
"	O
ready	O
"	O
flag	O
.	O
</s>
<s>
More	O
complicated	O
devices	O
,	O
such	O
as	O
disk	B-Device
drives	I-Device
,	O
use	O
these	O
3	O
bits	O
in	O
device-specific	O
fashions	O
.	O
</s>
<s>
In	O
use	O
,	O
a	O
programmer	O
can	O
write	O
several	O
instruction	O
mnemonics	O
alongside	O
one	O
another	O
,	O
and	O
the	O
assembler	B-Language
combines	O
them	O
with	O
OR	O
to	O
devise	O
the	O
actual	O
instruction	O
word	O
.	O
</s>
<s>
Many	O
I/O	B-General_Concept
devices	I-General_Concept
support	O
"	O
microcoded	O
"	O
IOT	O
instructions	O
.	O
</s>
<s>
For	O
example	O
,	O
combining	O
CLA	O
(	O
CLear	O
Accumulator	B-General_Concept
)	O
,	O
CLL	O
(	O
CLear	O
Link	O
)	O
,	O
and	O
IAC	O
(	O
Increment	O
ACcumulator	B-General_Concept
)	O
first	O
clears	O
the	O
AC	O
and	O
Link	O
,	O
then	O
increments	O
the	O
accumulator	B-General_Concept
,	O
leaving	O
it	O
set	O
to	O
1	O
.	O
</s>
<s>
Adding	O
RAL	O
to	O
the	O
mix	O
(	O
so	O
CLA	O
CLL	O
IAC	O
RAL	O
)	O
causes	O
the	O
accumulator	B-General_Concept
to	O
be	O
cleared	O
,	O
incremented	O
,	O
then	O
rotated	O
left	O
,	O
leaving	O
it	O
set	O
to	O
2	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
small	O
integer	O
constants	O
were	O
placed	O
in	O
the	O
accumulator	B-General_Concept
with	O
a	O
single	O
instruction	O
.	O
</s>
<s>
The	O
combination	O
CMA	O
IAC	O
,	O
which	O
the	O
assembler	B-Language
lets	O
you	O
abbreviate	O
as	O
CIA	O
,	O
produces	O
the	O
arithmetic	O
inverse	O
of	O
AC	O
:	O
the	O
twos-complement	B-General_Concept
negation	O
.	O
</s>
<s>
Since	O
there	O
is	O
no	O
subtraction	O
instruction	O
,	O
only	O
the	O
twos-complement	B-General_Concept
add	O
(	O
TAD	O
)	O
,	O
computing	O
the	O
difference	O
of	O
two	O
operands	O
,	O
requires	O
first	O
negating	O
the	O
subtrahend	O
.	O
</s>
<s>
The	O
programmer	O
can	O
write	O
NOP	B-Language
(	O
No	B-Language
Operation	I-Language
)	O
to	O
assemble	O
such	O
an	O
instruction	O
.	O
</s>
<s>
A	O
Group	O
2	O
OPR	O
instruction	O
that	O
has	O
none	O
of	O
the	O
microprogrammed	O
bits	O
set	O
is	O
another	O
No-Op	B-Language
instruction	O
.	O
</s>
<s>
Unused	O
bit	O
combinations	O
of	O
OPR	O
are	O
defined	O
as	O
a	O
third	O
Group	O
of	O
microprogrammed	O
actions	O
mostly	O
affecting	O
the	O
MQ	O
(	O
Multiplier/Quotient	O
)	O
register	B-General_Concept
.	O
</s>
<s>
The	O
MQ	O
register	B-General_Concept
and	O
the	O
extended	O
arithmetic	O
element	O
(	O
EAE	O
)	O
instructions	O
are	O
optional	O
and	O
only	O
exist	O
when	O
EAE	O
option	O
was	O
purchased	O
.	O
</s>
<s>
A	O
12-bit	B-Device
word	O
can	O
have	O
4,096	O
different	O
values	O
,	O
and	O
this	O
is	O
the	O
maximum	O
number	O
of	O
words	O
the	O
original	O
PDP-8	B-Device
can	O
address	O
indirectly	O
through	O
a	O
word	O
pointer	O
.	O
</s>
<s>
4,096	O
12-bit	B-Device
words	O
represent	O
6,144	O
bytes	O
in	O
modern	O
terminology	O
,	O
or	O
6kB	O
.	O
</s>
<s>
The	O
IF	O
register	B-General_Concept
specifies	O
the	O
field	O
for	O
instruction	O
fetches	O
and	O
direct	O
memory	O
references	O
;	O
the	O
DF	O
register	B-General_Concept
specifies	O
the	O
field	O
for	O
indirect	O
data	O
accesses	O
.	O
</s>
<s>
A	O
set	O
of	O
I/O	B-General_Concept
instructions	O
in	O
the	O
range	O
6200	O
through	O
6277	O
is	O
handled	O
by	O
the	O
Memory	O
Extension	O
Controller	O
and	O
give	O
access	O
to	O
the	O
DF	O
and	O
IF	O
registers	O
.	O
</s>
<s>
It	O
was	O
more	O
complicated	O
for	O
multiple-field	O
programs	O
to	O
deal	O
with	O
field	O
boundaries	O
and	O
the	O
DF	O
and	O
IF	O
registers	O
than	O
it	O
would	O
have	O
been	O
if	O
they	O
could	O
simply	O
generate	O
15-bit	O
addresses	O
,	O
but	O
the	O
design	O
provided	O
backward	O
compatibility	O
and	O
is	O
consistent	O
with	O
the	O
12-bit	B-Device
architecture	O
used	O
throughout	O
the	O
PDP-8	B-Device
.	O
</s>
<s>
Compare	O
the	O
later	O
Intel	B-General_Concept
8086	I-General_Concept
,	O
whose	O
16-bit	B-Device
memory	O
addresses	O
are	O
expanded	O
to	O
20	O
bits	O
by	O
combining	O
them	O
with	O
the	O
contents	O
of	O
a	O
specified	O
or	O
implied	O
segment	B-General_Concept
register	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
4K	O
FOCAL	B-Language
normally	O
had	O
about	O
3K	O
of	O
code	O
with	O
only	O
1K	O
left	O
over	O
for	O
user	O
program	O
and	O
data	O
.	O
</s>
<s>
With	O
a	O
few	O
patches	O
,	O
FOCAL	B-Language
could	O
use	O
a	O
second	O
4K	O
field	O
for	O
user	O
program	O
and	O
data	O
.	O
</s>
<s>
Moreover	O
,	O
additional	O
4K	O
fields	O
could	O
be	O
allocated	O
to	O
separate	O
users	O
,	O
turning	O
4K	O
FOCAL	B-Language
into	O
a	O
multi-user	O
timesharing	B-General_Concept
system	O
.	O
</s>
<s>
On	O
the	O
PDP-8/E	B-Device
and	O
later	O
models	O
,	O
the	O
Memory	O
Extension	O
Controller	O
was	O
enhanced	O
to	O
enable	O
machine	O
virtualization	O
.	O
</s>
<s>
A	O
program	O
written	O
to	O
use	O
a	O
PDP-8	B-Device
'	O
s	O
entire	O
resources	O
can	O
coexist	O
with	O
other	O
such	O
programs	O
on	O
the	O
same	O
PDP-8	B-Device
under	O
the	O
control	O
of	O
a	O
virtual	O
machine	O
manager	O
.	O
</s>
<s>
The	O
manager	O
can	O
make	O
all	O
I/O	B-General_Concept
instructions	O
(	O
including	O
those	O
that	O
operated	O
on	O
the	O
Memory	O
Extension	O
Controller	O
)	O
cause	O
a	O
trap	B-Application
(	O
an	O
interrupt	B-Application
handled	O
by	O
the	O
manager	O
)	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
the	O
manager	O
can	O
map	O
memory	O
references	O
,	O
map	O
data	O
or	O
instruction	O
fields	O
,	O
and	O
redirect	O
I/O	B-General_Concept
to	O
different	O
devices	O
.	O
</s>
<s>
New	O
I/O	B-General_Concept
instructions	O
to	O
the	O
Memory	O
Extension	O
Controller	O
retrieve	O
the	O
current	O
value	O
of	O
the	O
data	O
and	O
instruction	O
fields	O
,	O
letting	O
software	O
save	O
and	O
restore	O
most	O
of	O
the	O
machine	O
state	O
across	O
a	O
trap	B-Application
.	O
</s>
<s>
The	O
manager	O
has	O
to	O
include	O
a	O
complete	O
PDP-8	B-Device
emulator	B-Application
(	O
not	O
difficult	O
for	O
an	O
8-instruction	O
machine	O
)	O
.	O
</s>
<s>
Whenever	O
a	O
CIF	O
instruction	O
traps	B-Application
to	O
the	O
manager	O
,	O
it	O
has	O
to	O
emulate	O
the	O
instructions	O
up	O
to	O
the	O
next	O
jump	O
.	O
</s>
<s>
Fortunately	O
,	O
as	O
a	O
jump	O
usually	O
is	O
the	O
next	B-General_Concept
instruction	I-General_Concept
after	O
CIF	O
,	O
this	O
emulation	B-Application
does	O
not	O
slow	O
programs	O
down	O
much	O
,	O
but	O
it	O
is	O
a	O
large	O
workaround	O
to	O
a	O
seemingly	O
small	O
design	O
deficiency	O
.	O
</s>
<s>
By	O
the	O
time	O
of	O
the	O
PDP-8/A	O
,	O
memory	O
prices	O
had	O
fallen	O
enough	O
that	O
memory	O
exceeding	O
32K	O
was	O
desirable	O
.	O
</s>
<s>
However	O
,	O
by	O
this	O
time	O
,	O
the	O
PDP-8	B-Device
was	O
in	O
decline	O
,	O
so	O
very	O
little	O
standard	O
software	O
was	O
modified	O
to	O
use	O
these	O
new	O
features	O
.	O
</s>
<s>
The	O
following	O
examples	O
show	O
code	O
in	O
PDP-8	B-Device
assembly	B-Language
language	I-Language
as	O
one	O
might	O
write	O
for	O
the	O
PAL-III	B-Device
assembler	B-Language
.	O
</s>
<s>
As	O
shown	O
,	O
much	O
of	O
the	O
text	O
of	O
a	O
typical	O
PDP-8	B-Device
program	O
focuses	O
not	O
on	O
the	O
author	O
's	O
intended	O
algorithm	O
but	O
on	O
low-level	O
mechanics	O
.	O
</s>
<s>
This	O
complete	O
PDP-8	B-Device
assembly	B-Language
language	I-Language
program	O
outputs	O
"	O
Hello	O
,	O
world	O
!	O
"	O
</s>
<s>
The	O
PDP-8	B-Device
processor	O
does	O
not	O
implement	O
a	O
stack	B-Application
upon	O
which	O
to	O
store	O
registers	O
or	O
other	O
context	B-Operating_System
when	O
a	O
subroutine	O
is	O
called	O
or	O
an	O
interrupt	B-Application
occurs	O
.	O
</s>
<s>
(	O
A	O
stack	B-Application
can	O
be	O
implemented	O
in	O
software	O
,	O
as	O
demonstrated	O
in	O
the	O
next	O
section	O
.	O
)	O
</s>
<s>
Instead	O
,	O
the	O
JMS	O
instruction	O
simply	O
stores	O
the	O
updated	O
PC	O
(	O
pointing	O
past	O
JMS	O
,	O
to	O
the	O
return	B-Language
address	I-Language
)	O
at	O
the	O
effective	O
address	O
and	O
jumps	O
to	O
the	O
effective	O
address	O
plus	O
one	O
.	O
</s>
<s>
The	O
fact	O
that	O
the	O
JMS	O
instruction	O
uses	O
the	O
word	O
just	O
before	O
the	O
code	O
of	O
the	O
subroutine	O
to	O
deposit	O
the	O
return	B-Language
address	I-Language
prevents	O
reentrancy	B-Operating_System
and	O
recursion	O
without	O
additional	O
work	O
by	O
the	O
programmer	O
.	O
</s>
<s>
It	O
also	O
makes	O
it	O
difficult	O
to	O
use	O
ROM	B-Device
with	O
the	O
PDP-8	B-Device
because	O
read-write	O
return-address	O
storage	O
is	O
commingled	O
with	O
read-only	O
code	O
storage	O
in	O
the	O
address	O
space	O
.	O
</s>
<s>
They	O
are	O
placed	O
into	O
special	O
ROM	B-Device
cards	O
that	O
provide	O
a	O
few	O
words	O
of	O
read/write	O
memory	O
,	O
accessed	O
indirectly	O
through	O
the	O
use	O
of	O
a	O
thirteenth	O
flag	O
bit	O
in	O
each	O
ROM	B-Device
word	O
.	O
</s>
<s>
They	O
avoid	O
the	O
use	O
of	O
subroutines	O
;	O
or	O
use	O
code	O
such	O
as	O
the	O
following	O
,	O
instead	O
of	O
the	O
JMS	O
instruction	O
,	O
to	O
put	O
the	O
return	B-Language
address	I-Language
in	O
read-write	O
memory	O
:	O
</s>
<s>
If	O
a	O
programmer	O
makes	O
the	O
mistake	O
of	O
having	O
a	O
subroutine	O
call	O
itself	O
,	O
directly	O
or	O
by	O
an	O
intermediate	O
subroutine	O
,	O
then	O
the	O
return	B-Language
address	I-Language
for	O
the	O
outer	O
call	O
is	O
destroyed	O
by	O
the	O
return	B-Language
address	I-Language
of	O
the	O
subsequent	O
call	O
,	O
leading	O
to	O
an	O
infinite	O
loop	O
.	O
</s>
<s>
If	O
one	O
module	O
is	O
coded	O
with	O
an	O
incorrect	O
or	O
obsolete	O
address	O
for	O
a	O
subroutine	O
,	O
it	O
would	O
not	O
just	O
fail	O
to	O
execute	O
the	O
entire	O
code	O
sequence	O
of	O
the	O
subroutine	O
,	O
it	O
might	O
modify	O
a	O
word	O
of	O
the	O
subroutine	O
's	O
code	O
,	O
depositing	O
a	O
return	B-Language
address	I-Language
that	O
the	O
processor	O
might	O
interpret	O
as	O
an	O
instruction	O
during	O
a	O
subsequent	O
correct	O
call	O
to	O
the	O
subroutine	O
.	O
</s>
<s>
Though	O
the	O
PDP-8	B-Device
does	O
not	O
have	O
a	O
hardware	O
stack	B-Application
,	O
stacks	B-Application
can	O
be	O
implemented	O
in	O
software	O
.	O
</s>
<s>
Here	O
are	O
example	O
PUSH	B-Application
and	I-Application
POP	I-Application
subroutines	O
,	O
simplified	O
to	O
omit	O
issues	O
such	O
as	O
testing	O
for	O
stack	B-Application
overflow	O
and	O
underflow	O
:	O
</s>
<s>
And	O
here	O
is	O
"	O
Hello	O
World	O
"	O
with	O
this	O
"	O
stack	B-Application
"	O
implemented	O
,	O
and	O
"	O
OUT	O
"	O
subroutine	O
:	O
</s>
<s>
Another	O
possible	O
subroutine	O
for	O
the	O
PDP-8	B-Device
is	O
a	O
linked	O
list	O
.	O
</s>
<s>
There	O
is	O
a	O
single	O
interrupt	B-Application
line	I-Application
on	O
the	O
PDP-8	B-Device
I/O	B-General_Concept
bus	O
.	O
</s>
<s>
The	O
processor	O
handles	O
any	O
interrupt	B-Application
by	O
disabling	O
further	O
interrupts	B-Application
and	O
executing	O
a	O
JMS	O
to	O
location	O
0000	O
.	O
</s>
<s>
As	O
it	O
is	O
difficult	O
to	O
write	O
reentrant	B-Operating_System
subroutines	O
,	O
it	O
is	O
difficult	O
to	O
nest	O
interrupts	B-Application
and	O
this	O
is	O
usually	O
not	O
done	O
;	O
each	O
interrupt	B-Application
runs	O
to	O
completion	O
and	O
re-enables	O
interrupts	B-Application
just	O
before	O
executing	O
the	O
JMP	O
I	O
0	O
instruction	O
that	O
returns	O
from	O
the	O
interrupt	B-Application
.	O
</s>
<s>
Because	O
there	O
is	O
only	O
a	O
single	O
interrupt	B-Application
line	I-Application
on	O
the	O
I/O	B-General_Concept
bus	O
,	O
the	O
occurrence	O
of	O
an	O
interrupt	B-Application
does	O
not	O
inform	O
the	O
processor	O
of	O
the	O
source	O
of	O
the	O
interrupt	B-Application
.	O
</s>
<s>
Instead	O
,	O
the	O
interrupt	B-Application
service	O
routine	O
has	O
to	O
serially	O
poll	O
each	O
active	O
I/O	B-General_Concept
device	I-General_Concept
to	O
see	O
if	O
it	O
is	O
the	O
source	O
.	O
</s>
<s>
The	O
code	O
that	O
does	O
this	O
is	O
called	O
a	O
skip	O
chain	O
because	O
it	O
consists	O
of	O
a	O
series	O
of	O
PDP-8	B-Device
"	O
test	O
and	O
skip	O
if	O
flag	O
set	O
"	O
I/O	B-General_Concept
instructions	O
.	O
</s>
<s>
The	O
relative	O
interrupt	B-Application
priority	O
of	O
the	O
I/O	B-General_Concept
devices	I-General_Concept
is	O
determined	O
by	O
their	O
position	O
in	O
the	O
skip	O
chain	O
:	O
If	O
several	O
devices	O
interrupt	B-Application
,	O
the	O
device	O
tested	O
earlier	O
in	O
the	O
skip	O
chain	O
is	O
serviced	O
first	O
.	O
</s>
<s>
An	O
engineering	O
textbook	O
popular	O
in	O
the	O
1980s	O
,	O
The	O
Art	O
of	O
Digital	O
Design	O
by	O
David	O
Winkel	O
and	O
Franklin	O
Prosser	O
,	O
contains	O
an	O
example	O
problem	O
spanning	O
several	O
chapters	O
in	O
which	O
the	O
authors	O
demonstrate	O
the	O
process	O
of	O
designing	O
a	O
computer	O
that	O
is	O
compatible	O
with	O
the	O
PDP-8/I	O
.	O
</s>
<s>
The	O
USSR	O
produced	O
the	O
minicomputers	B-Architecture
Saratov-1	O
and	O
Saratov-2	O
,	O
which	O
cloned	O
the	O
PDP-8	B-Device
and	O
PDP-8/E	B-Device
,	O
respectively	O
.	O
</s>
