<s>
Plessey	B-Device
System	I-Device
250	I-Device
,	O
also	O
known	O
as	O
PP250	O
,	O
was	O
the	O
first	O
operational	O
computer	O
to	O
implement	O
capability-based	B-General_Concept
addressing	I-General_Concept
,	O
to	O
check	O
and	O
balance	O
the	O
computation	O
as	O
a	O
pure	O
Church	O
–	O
Turing	O
machine	O
.	O
</s>
<s>
A	O
Church	O
–	O
Turing	O
machine	O
is	O
a	O
digital	O
computer	O
that	O
encapsulates	O
the	O
symbols	O
in	O
a	O
thread	O
of	O
computation	O
as	O
a	O
chain	O
of	O
protected	O
abstractions	O
by	O
enforcing	O
the	O
dynamic	O
binding	O
laws	O
of	O
Alonzo	O
Church	O
's	O
lambda	B-Language
calculus	I-Language
Other	O
capability	O
based	O
computers	O
,	O
which	O
include	O
and	O
CAP	B-Device
computers	I-Device
,	O
are	O
hybrids	O
.	O
</s>
<s>
It	O
is	O
an	O
unavoidable	O
characteristic	O
of	O
the	O
von	B-Architecture
Neumann	I-Architecture
architecture	I-Architecture
that	O
is	O
founded	O
on	O
shared	O
random	O
access	O
memory	O
and	O
trust	O
in	O
the	O
sharing	O
default	O
access	O
rights	O
.	O
</s>
<s>
For	O
example	O
,	O
every	O
word	O
in	O
every	O
page	O
managed	O
by	O
the	O
virtual	B-Architecture
memory	I-Architecture
manager	O
in	O
an	O
operating	B-General_Concept
system	I-General_Concept
using	O
a	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
(	O
MMU	O
)	O
must	O
be	O
trusted	O
.	O
</s>
<s>
PP250	O
removed	O
not	O
only	O
virtual	B-Architecture
memory	I-Architecture
or	O
any	O
centralized	O
,	O
precompiled	O
operating	B-General_Concept
system	I-General_Concept
,	O
but	O
also	O
the	O
superuser	B-Application
,	O
removing	O
all	O
default	O
machine	O
privileges	O
.	O
</s>
<s>
PP250	O
separated	O
binary	O
data	O
from	O
capability	O
data	O
to	O
protect	O
access	O
rights	O
,	O
simplify	O
the	O
computer	O
and	O
speed	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
<s>
The	O
Church	O
machine	O
encapsulates	O
and	O
context	O
limits	O
the	O
Turing	O
machine	O
by	O
enforcing	O
the	O
laws	O
of	O
the	O
lambda	B-Language
calculus	I-Language
.	O
</s>
<s>
The	O
typed	O
digital	O
media	O
is	O
program	O
controlled	O
by	O
distinctly	O
different	O
machine	B-Language
instructions	I-Language
.	O
</s>
<s>
Mutable	O
binary	O
data	O
is	O
programmed	O
by	O
28	O
RISC	B-Architecture
instruction	I-Architecture
set	I-Architecture
for	O
Imperative	B-Application
programming	I-Application
and	O
procedural	B-Application
programming	I-Application
the	O
binary	O
data	O
using	O
binary	O
data	O
registers	O
confined	O
to	O
a	O
capability	O
limited	O
memory	O
segment	O
.	O
</s>
<s>
Immutable	O
capability	O
keys	O
represent	O
named	O
lambda	B-Language
calculus	I-Language
variables	O
.	O
</s>
<s>
This	O
Church	O
side	O
is	O
a	O
lambda	B-Language
calculus	I-Language
meta-machine	O
.	O
</s>
<s>
The	O
other	O
side	O
is	O
an	O
object-oriented	O
machine	O
of	O
binary	O
objects	O
,	O
programmed	O
functions	O
,	O
capability	O
lists	O
defining	O
function	O
abstractions	O
,	O
storage	O
for	O
threads	O
of	O
computation	O
(	O
lambda	B-Language
calculus	I-Language
applications	O
)	O
or	O
storage	O
for	O
the	O
list	O
of	O
capability	O
keys	O
in	O
a	O
namespace	O
.	O
</s>
<s>
The	O
laws	O
of	O
the	O
lambda	B-Language
calculus	I-Language
are	O
implemented	O
by	O
the	O
Church	O
instructions	O
with	O
micro-programmed	O
access	O
to	O
the	O
reserved	O
(	O
hidden	O
)	O
capability	O
registers	O
.	O
</s>
<s>
As	O
a	O
real-time	B-General_Concept
controller	O
,	O
the	O
PP250	O
provided	O
fail-safe	O
software	O
applications	O
for	O
computerized	O
telephone	O
and	O
military	O
communication	O
systems	O
with	O
decades	O
of	O
software	O
and	O
hardware	O
reliability	O
.	O
</s>
<s>
Furthermore	O
,	O
no	O
unfair	O
,	O
default	O
privileges	O
exist	O
for	O
an	O
operating	B-General_Concept
system	I-General_Concept
or	O
a	O
superuser	B-Application
,	O
thereby	O
blocking	O
all	O
hacking	O
and	O
malware	O
.	O
</s>
<s>
The	O
multiprocessing	B-Operating_System
hardware	O
architecture	O
and	O
the	O
dynamically	O
bound	O
,	O
type	O
limited	O
memory	O
,	O
exclusively	O
accessed	O
through	O
capability	O
limited	O
addressing	O
,	O
replace	O
the	O
statically	O
bound	O
,	O
page	O
based	O
linear	O
compilations	O
with	O
dynamically	O
bound	O
instructions	O
,	O
crosschecked	O
and	O
authorized	O
at	O
run	O
time	O
.	O
</s>
<s>
The	O
imperative	B-Application
Turing	O
commands	O
must	O
bind	O
to	O
binary	O
data	O
objects	O
as	O
defined	O
by	O
the	O
selected	O
capability	O
register	O
.	O
</s>
<s>
The	O
access	O
rights	O
of	O
the	O
selected	O
capability	O
register	O
must	O
approve	O
data	O
access	O
rights	O
(	O
Read	O
Binary	O
Data	O
,	O
Write	O
Binary	O
Data	O
or	O
Execute	O
Machine	B-Language
Code	I-Language
)	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
object-oriented	O
machine	B-Language
code	I-Language
is	O
encapsulated	O
as	O
a	O
function	O
abstraction	O
in	O
private	O
execution	O
space	O
.	O
</s>
<s>
It	O
is	O
a	O
register-oriented	B-General_Concept
architecture	I-General_Concept
,	O
with	O
8	O
program	O
accessible	O
data	O
registers	O
and	O
8	O
program	O
accessible	O
capability	O
registers	O
.	O
</s>
<s>
Instructions	O
that	O
access	O
memory	O
have	O
an	O
opcode	B-Language
,	O
a	O
field	O
specifying	O
a	O
data	O
register	O
operand	O
,	O
a	O
field	O
specifying	O
a	O
data	O
register	O
used	O
as	O
an	O
index	B-General_Concept
register	I-General_Concept
containing	O
an	O
offset	O
into	O
a	O
segment	O
,	O
a	O
field	O
specifying	O
a	O
capability	O
register	O
referring	O
to	O
the	O
segment	O
containing	O
the	O
memory	O
location	O
,	O
and	O
a	O
field	O
containing	O
a	O
base	O
offset	O
into	O
the	O
segment	O
.	O
</s>
<s>
The	O
offset	O
into	O
the	O
segment	O
is	O
the	O
sum	O
of	O
the	O
base	O
offset	O
and	O
the	O
contents	O
of	O
the	O
index	B-General_Concept
register	I-General_Concept
.	O
</s>
<s>
The	O
software	O
was	O
modular	O
based	O
on	O
the	O
universal	O
model	O
of	O
computation	O
and	O
the	O
lambda	B-Language
calculus	I-Language
.	O
</s>
<s>
Six	O
Church	O
instructions	O
hide	O
the	O
details	O
of	O
a	O
named	O
function	B-Algorithm
application	I-Algorithm
using	O
capability	O
keys	O
for	O
the	O
typed	O
concepts	O
of	O
variables	O
,	O
functions	O
,	O
abstractions	O
,	O
applications	O
and	O
a	O
namespace	O
.	O
</s>
<s>
Manufactured	O
by	O
Plessey	O
company	O
plc	O
in	O
the	O
United	O
Kingdom	O
in	O
1970	O
,	O
it	O
was	O
successfully	O
deployed	O
by	O
the	O
Ministry	O
of	O
Defence	O
for	O
the	O
British	O
Army	O
Ptarmigan	O
project	O
and	O
served	O
in	O
the	O
first	O
Gulf	B-Application
War	I-Application
as	O
a	O
tactical	O
mobile	O
communication	O
network	B-Protocol
switch	I-Protocol
.	O
</s>
