<s>
CPU	B-General_Concept
modes	I-General_Concept
(	O
also	O
called	O
processor	B-General_Concept
modes	I-General_Concept
,	O
CPU	O
states	O
,	O
CPU	O
privilege	O
levels	O
and	O
other	O
names	O
)	O
are	O
operating	O
modes	O
for	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
of	O
some	O
computer	B-General_Concept
architectures	I-General_Concept
that	O
place	O
restrictions	O
on	O
the	O
type	O
and	O
scope	O
of	O
operations	O
that	O
can	O
be	O
performed	O
by	O
certain	O
processes	O
being	O
run	O
by	O
the	O
CPU	O
.	O
</s>
<s>
This	O
design	O
allows	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
run	O
with	O
more	O
privileges	O
than	O
application	B-Application
software	I-Application
.	O
</s>
<s>
Ideally	O
,	O
only	O
highly	O
trusted	O
kernel	B-Operating_System
code	O
is	O
allowed	O
to	O
execute	O
in	O
the	O
unrestricted	O
mode	O
;	O
everything	O
else	O
(	O
including	O
non-supervisory	O
portions	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
)	O
runs	O
in	O
a	O
restricted	O
mode	O
and	O
must	O
use	O
a	O
system	B-Operating_System
call	I-Operating_System
(	O
via	O
interrupt	B-Application
)	O
to	O
request	O
the	O
kernel	B-Operating_System
perform	O
on	O
its	O
behalf	O
any	O
operation	O
that	O
could	O
damage	O
or	O
compromise	O
the	O
system	O
,	O
making	O
it	O
impossible	O
for	O
untrusted	O
programs	O
to	O
alter	O
or	O
damage	O
other	O
programs	O
(	O
or	O
the	O
computing	O
system	O
itself	O
)	O
.	O
</s>
<s>
In	O
practice	O
,	O
however	O
,	O
system	B-Operating_System
calls	I-Operating_System
take	O
time	O
and	O
can	O
hurt	O
the	O
performance	O
of	O
a	O
computing	O
system	O
,	O
so	O
it	O
is	O
not	O
uncommon	O
for	O
system	O
designers	O
to	O
allow	O
some	O
time-critical	B-General_Concept
software	O
(	O
especially	O
device	B-Application
drivers	I-Application
)	O
to	O
run	O
with	O
full	O
kernel	B-General_Concept
privileges	I-General_Concept
.	O
</s>
<s>
Multiple	O
modes	O
can	O
be	O
implemented	O
—	O
allowing	O
a	O
hypervisor	B-Operating_System
to	O
run	O
multiple	O
operating	B-General_Concept
system	I-General_Concept
supervisors	O
beneath	O
it	O
,	O
which	O
is	O
the	O
basic	O
design	O
of	O
many	O
virtual	B-Architecture
machine	I-Architecture
systems	O
available	O
today	O
.	O
</s>
<s>
The	O
unrestricted	O
mode	O
is	O
often	O
called	O
kernel	B-Operating_System
mode	O
,	O
but	O
many	O
other	O
designations	O
exist	O
(	O
master	O
mode	O
,	O
supervisor	O
mode	O
,	O
privileged	O
mode	O
,	O
etc	O
.	O
)	O
.	O
</s>
<s>
In	O
kernel	B-Operating_System
mode	O
,	O
the	O
CPU	O
may	O
perform	O
any	O
operation	O
allowed	O
by	O
its	O
architecture	O
;	O
any	O
instruction	O
may	O
be	O
executed	O
,	O
any	O
I/O	O
operation	O
initiated	O
,	O
any	O
area	O
of	O
memory	O
accessed	O
,	O
and	O
so	O
on	O
.	O
</s>
<s>
In	O
the	O
other	O
CPU	B-General_Concept
modes	I-General_Concept
,	O
certain	O
restrictions	O
on	O
CPU	O
operations	O
are	O
enforced	O
by	O
the	O
hardware	O
.	O
</s>
<s>
User-mode	O
capabilities	O
of	O
the	O
CPU	O
are	O
typically	O
a	O
subset	O
of	O
those	O
available	O
in	O
kernel	B-Operating_System
mode	O
,	O
but	O
in	O
some	O
cases	O
,	O
such	O
as	O
hardware	O
emulation	O
of	O
non-native	O
architectures	O
,	O
they	O
may	O
be	O
significantly	O
different	O
from	O
those	O
available	O
in	O
standard	O
kernel	B-Operating_System
mode	O
.	O
</s>
<s>
These	O
architectures	O
are	O
often	O
said	O
to	O
have	O
ring-based	O
security	O
,	O
wherein	O
the	O
hierarchy	O
of	O
privileges	O
resembles	O
a	O
set	O
of	O
concentric	O
rings	O
,	O
with	O
the	O
kernel	B-Operating_System
mode	O
in	O
the	O
center	O
.	O
</s>
<s>
Multics	B-Application
hardware	O
was	O
the	O
first	O
significant	O
implementation	O
of	O
ring	O
security	O
,	O
but	O
many	O
other	O
hardware	O
platforms	O
have	O
been	O
designed	O
along	O
similar	O
lines	O
,	O
including	O
the	O
Intel	B-General_Concept
80286	I-General_Concept
protected	B-Application
mode	I-Application
,	O
and	O
the	O
IA-64	B-General_Concept
as	O
well	O
,	O
though	O
it	O
is	O
referred	O
to	O
by	O
a	O
different	O
name	O
in	O
these	O
cases	O
.	O
</s>
<s>
Hardware	O
registers	O
track	O
the	O
current	O
operating	O
mode	O
of	O
the	O
CPU	O
,	O
but	O
additional	O
virtual-memory	B-Architecture
registers	O
,	O
page-table	B-General_Concept
entries	O
,	O
and	O
other	O
data	O
may	O
track	O
mode	O
identifiers	O
for	O
other	O
resources	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
CPU	O
may	O
be	O
operating	O
in	O
Ring	O
0	O
as	O
indicated	O
by	O
a	O
status	O
word	O
in	O
the	O
CPU	O
itself	O
,	O
but	O
every	O
access	O
to	O
memory	O
may	O
additionally	O
be	O
validated	O
against	O
a	O
separate	O
ring	O
number	O
for	O
the	O
virtual-memory	B-Architecture
segment	O
targeted	O
by	O
the	O
access	O
,	O
and/or	O
against	O
a	O
ring	O
number	O
for	O
the	O
physical	O
page	O
(	O
if	O
any	O
)	O
being	O
targeted	O
.	O
</s>
<s>
This	O
has	O
been	O
demonstrated	O
with	O
the	O
PSP	B-Operating_System
handheld	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
Hardware	O
that	O
meets	O
the	O
Popek	B-Architecture
and	I-Architecture
Goldberg	I-Architecture
virtualization	I-Architecture
requirements	I-Architecture
makes	O
writing	O
software	O
to	O
efficiently	O
support	O
a	O
virtual	B-Architecture
machine	I-Architecture
much	O
simpler	O
.	O
</s>
