<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
capability-based	B-General_Concept
addressing	I-General_Concept
is	O
a	O
scheme	O
used	O
by	O
some	O
computers	O
to	O
control	O
access	O
to	O
memory	O
as	O
an	O
efficient	O
implementation	O
of	O
capability-based	O
security	O
.	O
</s>
<s>
Under	O
a	O
capability-based	B-General_Concept
addressing	I-General_Concept
scheme	O
,	O
pointers	O
are	O
replaced	O
by	O
protected	O
objects	O
(	O
called	O
capabilities	O
)	O
that	O
can	O
be	O
created	O
only	O
through	O
the	O
use	O
of	O
privileged	O
instructions	O
which	O
may	O
be	O
executed	O
only	O
by	O
either	O
the	O
kernel	B-Operating_System
or	O
some	O
other	O
privileged	O
process	O
authorised	O
to	O
do	O
so	O
.	O
</s>
<s>
Thus	O
,	O
a	O
kernel	B-Operating_System
can	O
limit	O
application	O
code	O
and	O
other	O
subsystems	O
access	O
to	O
the	O
minimum	O
necessary	O
portions	O
of	O
memory	O
(	O
and	O
disable	O
write	O
access	O
where	O
appropriate	O
)	O
,	O
without	O
the	O
need	O
to	O
use	O
separate	O
address	B-General_Concept
spaces	I-General_Concept
and	O
therefore	O
require	O
a	O
context	B-Operating_System
switch	I-Operating_System
when	O
an	O
access	O
occurs	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Plessey	B-Device
System	I-Device
250	I-Device
required	O
that	O
all	O
capabilities	O
be	O
stored	O
in	O
capability-list	O
segments	O
.	O
</s>
<s>
This	O
is	O
a	O
generalization	O
of	O
the	O
use	O
of	O
tag	B-Architecture
bits	I-Architecture
to	O
protect	O
segment	O
descriptors	O
in	O
the	O
Burroughs	B-Device
large	I-Device
systems	I-Device
,	O
and	O
it	O
was	O
used	O
to	O
protect	O
capabilities	O
in	O
the	O
IBM	B-Device
System/38	I-Device
.	O
</s>
<s>
The	O
designers	O
of	O
the	O
System/38	B-Device
'	O
s	O
descendent	O
systems	O
,	O
including	O
AS/400	B-Device
and	O
IBM	B-Application
i	I-Application
,	O
removed	O
capability-based	B-General_Concept
addressing	I-General_Concept
.	O
</s>
<s>
The	O
reason	O
given	O
for	O
this	O
decision	O
is	O
that	O
they	O
could	O
find	O
no	O
way	O
to	O
revoke	O
capabilities	O
(	O
although	O
patterns	O
for	O
implementing	O
revocation	O
in	O
capability	O
systems	O
had	O
been	O
published	O
as	O
early	O
as	O
1974	O
,	O
even	O
before	O
the	O
introduction	O
of	O
System/38	B-Device
)	O
.	O
</s>
