<s>
The	O
kernel	B-Operating_System
is	O
a	O
computer	B-Application
program	I-Application
at	O
the	O
core	O
of	O
a	O
computer	O
's	O
operating	B-General_Concept
system	I-General_Concept
and	O
generally	O
has	O
complete	O
control	O
over	O
everything	O
in	O
the	O
system	O
.	O
</s>
<s>
It	O
is	O
the	O
portion	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
code	O
that	O
is	O
always	O
resident	O
in	O
memory	O
and	O
facilitates	O
interactions	O
between	O
hardware	O
and	O
software	O
components	O
.	O
</s>
<s>
A	O
full	O
kernel	B-Operating_System
controls	O
all	O
hardware	O
resources	O
(	O
e.g.	O
</s>
<s>
I/O	B-General_Concept
,	O
memory	O
,	O
cryptography	O
)	O
via	O
device	B-Application
drivers	I-Application
,	O
arbitrates	O
conflicts	O
between	O
processes	O
concerning	O
such	O
resources	O
,	O
and	O
optimizes	O
the	O
utilization	O
of	O
common	O
resources	O
e.g.	O
</s>
<s>
CPU	B-General_Concept
&	O
cache	O
usage	O
,	O
file	B-Application
systems	I-Application
,	O
and	O
network	O
sockets	O
.	O
</s>
<s>
On	O
most	O
systems	O
,	O
the	O
kernel	B-Operating_System
is	O
one	O
of	O
the	O
first	O
programs	O
loaded	O
on	O
startup	B-Operating_System
(	O
after	O
the	O
bootloader	B-Application
)	O
.	O
</s>
<s>
It	O
handles	O
the	O
rest	O
of	O
startup	B-Operating_System
as	O
well	O
as	O
memory	O
,	O
peripherals	O
,	O
and	O
input/output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
requests	O
from	O
software	O
,	O
translating	O
them	O
into	O
data-processing	B-General_Concept
instructions	O
for	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
.	O
</s>
<s>
The	O
critical	O
code	O
of	O
the	O
kernel	B-Operating_System
is	O
usually	O
loaded	O
into	O
a	O
separate	O
area	O
of	O
memory	O
,	O
which	O
is	O
protected	O
from	O
access	O
by	O
application	B-Application
software	I-Application
or	O
other	O
less	O
critical	O
parts	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
kernel	B-Operating_System
performs	O
its	O
tasks	O
,	O
such	O
as	O
running	O
processes	O
,	O
managing	O
hardware	O
devices	O
such	O
as	O
the	O
hard	B-Device
disk	I-Device
,	O
and	O
handling	O
interrupts	B-Application
,	O
in	O
this	O
protected	O
kernel	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
In	O
contrast	O
,	O
application	B-Application
programs	I-Application
such	O
as	O
browsers	O
,	O
word	O
processors	O
,	O
or	O
audio	O
or	O
video	O
players	O
use	O
a	O
separate	O
area	O
of	O
memory	O
,	O
user	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
This	O
separation	O
prevents	O
user	O
data	O
and	O
kernel	B-Operating_System
data	O
from	O
interfering	O
with	O
each	O
other	O
and	O
causing	O
instability	O
and	O
slowness	O
,	O
as	O
well	O
as	O
preventing	O
malfunctioning	O
applications	O
from	O
affecting	O
other	O
applications	O
or	O
crashing	O
the	O
entire	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Even	O
in	O
systems	O
where	O
the	O
kernel	B-Operating_System
is	O
included	O
in	O
application	O
address	B-General_Concept
spaces	I-General_Concept
,	O
memory	B-General_Concept
protection	I-General_Concept
is	O
used	O
to	O
prevent	O
unauthorized	O
applications	O
from	O
modifying	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
kernel	B-Operating_System
's	O
interface	B-General_Concept
is	O
a	O
low-level	O
abstraction	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
When	O
a	O
process	B-Operating_System
requests	O
a	O
service	O
from	O
the	O
kernel	B-Operating_System
,	O
it	O
must	O
invoke	O
a	O
system	B-Operating_System
call	I-Operating_System
,	O
usually	O
through	O
a	O
wrapper	O
function	O
.	O
</s>
<s>
There	O
are	O
different	O
kernel	B-Operating_System
architecture	O
designs	O
.	O
</s>
<s>
Monolithic	B-Operating_System
kernels	I-Operating_System
run	O
entirely	O
in	O
a	O
single	O
address	B-General_Concept
space	I-General_Concept
with	O
the	O
CPU	B-General_Concept
executing	O
in	O
supervisor	B-Operating_System
mode	I-Operating_System
,	O
mainly	O
for	O
speed	O
.	O
</s>
<s>
Microkernels	B-Operating_System
run	O
most	O
but	O
not	O
all	O
of	O
their	O
services	O
in	O
user	B-Operating_System
space	I-Operating_System
,	O
like	O
user	O
processes	O
do	O
,	O
mainly	O
for	O
resilience	O
and	O
modularity	B-Architecture
.	O
</s>
<s>
MINIX	B-Application
3	I-Application
is	O
a	O
notable	O
example	O
of	O
microkernel	B-Operating_System
design	O
.	O
</s>
<s>
Instead	O
,	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
is	O
monolithic	O
,	O
although	O
it	O
is	O
also	O
modular	B-Architecture
,	O
for	O
it	O
can	O
insert	O
and	O
remove	O
loadable	B-Application
kernel	I-Application
modules	I-Application
at	B-Library
runtime	I-Library
.	O
</s>
<s>
The	O
kernel	B-Operating_System
takes	O
responsibility	O
for	O
deciding	O
at	O
any	O
time	O
which	O
of	O
the	O
many	O
running	O
programs	O
should	O
be	O
allocated	O
to	O
the	O
processor	O
or	O
processors	O
.	O
</s>
<s>
Random-access	B-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
is	O
used	O
to	O
store	O
both	O
program	O
instructions	O
and	O
data	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
is	O
responsible	O
for	O
deciding	O
which	O
memory	O
each	O
process	B-Operating_System
can	O
use	O
,	O
and	O
determining	O
what	O
to	O
do	O
when	O
not	B-General_Concept
enough	I-General_Concept
memory	I-General_Concept
is	O
available	O
.	O
</s>
<s>
I/O	B-General_Concept
devices	I-General_Concept
include	O
such	O
peripherals	O
as	O
keyboards	O
,	O
mice	O
,	O
disk	O
drives	O
,	O
printers	O
,	O
USB	B-Protocol
devices	O
,	O
network	O
adapters	O
,	O
and	O
display	B-Device
devices	I-Device
.	O
</s>
<s>
The	O
kernel	B-Operating_System
allocates	O
requests	O
from	O
applications	O
to	O
perform	O
I/O	B-General_Concept
to	O
an	O
appropriate	O
device	O
and	O
provides	O
convenient	O
methods	O
for	O
using	O
the	O
device	O
(	O
typically	O
abstracted	O
to	O
the	O
point	O
where	O
the	O
application	O
does	O
not	O
need	O
to	O
know	O
implementation	O
details	O
of	O
the	O
device	O
)	O
.	O
</s>
<s>
Key	O
aspects	O
necessary	O
in	O
resource	O
management	O
are	O
defining	O
the	O
execution	O
domain	O
(	O
address	B-General_Concept
space	I-General_Concept
)	O
and	O
the	O
protection	O
mechanism	O
used	O
to	O
mediate	O
access	O
to	O
the	O
resources	O
within	O
a	O
domain	O
.	O
</s>
<s>
Kernels	B-Operating_System
also	O
provide	O
methods	O
for	O
synchronization	O
and	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
.	O
</s>
<s>
These	O
implementations	O
may	O
be	O
located	O
within	O
the	O
kernel	B-Operating_System
itself	O
or	O
the	O
kernel	B-Operating_System
can	O
also	O
rely	O
on	O
other	O
processes	O
it	O
is	O
running	O
.	O
</s>
<s>
Although	O
the	O
kernel	B-Operating_System
must	O
provide	O
IPC	O
in	O
order	O
to	O
provide	O
access	O
to	O
the	O
facilities	O
provided	O
by	O
each	O
other	O
,	O
kernels	B-Operating_System
must	O
also	O
provide	O
running	O
programs	O
with	O
a	O
method	O
to	O
make	O
requests	O
to	O
access	O
these	O
facilities	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
is	O
also	O
responsible	O
for	O
context	B-Operating_System
switching	I-Operating_System
between	O
processes	O
or	O
threads	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
has	O
full	O
access	O
to	O
the	O
system	O
's	O
memory	O
and	O
must	O
allow	O
processes	O
to	O
safely	O
access	O
this	O
memory	O
as	O
they	O
require	O
it	O
.	O
</s>
<s>
Often	O
the	O
first	O
step	O
in	O
doing	O
this	O
is	O
virtual	B-General_Concept
addressing	I-General_Concept
,	O
usually	O
achieved	O
by	O
paging	B-Architecture
and/or	O
segmentation	B-General_Concept
.	O
</s>
<s>
Virtual	B-General_Concept
addressing	I-General_Concept
allows	O
the	O
kernel	B-Operating_System
to	O
make	O
a	O
given	O
physical	O
address	O
appear	O
to	O
be	O
another	O
address	O
,	O
the	O
virtual	B-General_Concept
address	I-General_Concept
.	O
</s>
<s>
Virtual	B-General_Concept
address	I-General_Concept
spaces	I-General_Concept
may	O
be	O
different	O
for	O
different	O
processes	O
;	O
the	O
memory	O
that	O
one	O
process	B-Operating_System
accesses	O
at	O
a	O
particular	O
(	O
virtual	O
)	O
address	O
may	O
be	O
different	O
memory	O
from	O
what	O
another	O
process	B-Operating_System
accesses	O
at	O
the	O
same	O
address	O
.	O
</s>
<s>
This	O
allows	O
every	O
program	O
to	O
behave	O
as	O
if	O
it	O
is	O
the	O
only	O
one	O
(	O
apart	O
from	O
the	O
kernel	B-Operating_System
)	O
running	O
and	O
thus	O
prevents	O
applications	O
from	O
crashing	O
each	O
other	O
.	O
</s>
<s>
On	O
many	O
systems	O
,	O
a	O
program	O
's	O
virtual	B-General_Concept
address	I-General_Concept
may	O
refer	O
to	O
data	O
which	O
is	O
not	O
currently	O
in	O
memory	O
.	O
</s>
<s>
The	O
layer	O
of	O
indirection	O
provided	O
by	O
virtual	B-General_Concept
addressing	I-General_Concept
allows	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
use	O
other	O
data	O
stores	O
,	O
like	O
a	O
hard	B-Device
drive	I-Device
,	O
to	O
store	O
what	O
would	O
otherwise	O
have	O
to	O
remain	O
in	O
main	O
memory	O
(	O
RAM	B-Architecture
)	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
operating	B-General_Concept
systems	I-General_Concept
can	O
allow	O
programs	O
to	O
use	O
more	O
memory	O
than	O
the	O
system	O
has	O
physically	O
available	O
.	O
</s>
<s>
When	O
a	O
program	O
needs	O
data	O
which	O
is	O
not	O
currently	O
in	O
RAM	B-Architecture
,	O
the	O
CPU	B-General_Concept
signals	O
to	O
the	O
kernel	B-Operating_System
that	O
this	O
has	O
happened	O
,	O
and	O
the	O
kernel	B-Operating_System
responds	O
by	O
writing	O
the	O
contents	O
of	O
an	O
inactive	O
memory	O
block	O
to	O
disk	O
(	O
if	O
necessary	O
)	O
and	O
replacing	O
it	O
with	O
the	O
data	O
requested	O
by	O
the	O
program	O
.	O
</s>
<s>
This	O
scheme	O
is	O
generally	O
known	O
as	O
demand	B-General_Concept
paging	I-General_Concept
.	O
</s>
<s>
Virtual	B-General_Concept
addressing	I-General_Concept
also	O
allows	O
creation	O
of	O
virtual	O
partitions	O
of	O
memory	O
in	O
two	O
disjointed	O
areas	O
,	O
one	O
being	O
reserved	O
for	O
the	O
kernel	B-Operating_System
(	O
kernel	B-Operating_System
space	I-Operating_System
)	O
and	O
the	O
other	O
for	O
the	O
applications	O
(	O
user	B-Operating_System
space	I-Operating_System
)	O
.	O
</s>
<s>
The	O
applications	O
are	O
not	O
permitted	O
by	O
the	O
processor	O
to	O
address	O
kernel	B-Operating_System
memory	O
,	O
thus	O
preventing	O
an	O
application	O
from	O
damaging	O
the	O
running	O
kernel	B-Operating_System
.	O
</s>
<s>
This	O
fundamental	O
partition	O
of	O
memory	O
space	O
has	O
contributed	O
much	O
to	O
the	O
current	O
designs	O
of	O
actual	O
general-purpose	O
kernels	B-Operating_System
and	O
is	O
almost	O
universal	O
in	O
such	O
systems	O
,	O
although	O
some	O
research	O
kernels	B-Operating_System
(	O
e.g.	O
,	O
Singularity	B-Operating_System
)	O
take	O
other	O
approaches	O
.	O
</s>
<s>
To	O
perform	O
useful	O
functions	O
,	O
processes	O
need	O
access	O
to	O
the	O
peripherals	O
connected	O
to	O
the	O
computer	O
,	O
which	O
are	O
controlled	O
by	O
the	O
kernel	B-Operating_System
through	O
device	B-Application
drivers	I-Application
.	O
</s>
<s>
A	O
device	B-Application
driver	I-Application
is	O
a	O
computer	B-Application
program	I-Application
encapsulating	O
,	O
monitoring	O
and	O
controlling	O
a	O
hardware	O
device	O
(	O
via	O
its	O
)	O
on	O
behalf	O
of	O
the	O
OS	B-General_Concept
.	O
</s>
<s>
It	O
provides	O
the	O
operating	B-General_Concept
system	I-General_Concept
with	O
an	O
API	B-General_Concept
,	O
procedures	O
and	O
information	O
about	O
how	O
to	O
control	O
and	O
communicate	O
with	O
a	O
certain	O
piece	O
of	O
hardware	O
.	O
</s>
<s>
Device	B-Application
drivers	I-Application
are	O
an	O
important	O
and	O
vital	O
dependency	O
for	O
all	O
OS	B-General_Concept
and	O
their	O
applications	O
.	O
</s>
<s>
The	O
design	O
goal	O
of	O
a	O
driver	O
is	O
abstraction	O
;	O
the	O
function	O
of	O
the	O
driver	O
is	O
to	O
translate	O
the	O
OS-mandated	O
abstract	O
function	O
calls	O
(	O
programming	O
calls	O
)	O
into	O
device-specific	O
calls	O
.	O
</s>
<s>
Device	B-Application
drivers	I-Application
are	O
used	O
for	O
e.g.	O
</s>
<s>
At	O
the	O
hardware	O
level	O
,	O
common	O
abstractions	O
of	O
device	B-Application
drivers	I-Application
include	O
:	O
</s>
<s>
And	O
at	O
the	O
software	O
level	O
,	O
device	B-Application
driver	I-Application
abstractions	O
include	O
:	O
</s>
<s>
For	O
example	O
,	O
to	O
show	O
the	O
user	O
something	O
on	O
the	O
screen	O
,	O
an	O
application	O
would	O
make	O
a	O
request	O
to	O
the	O
kernel	B-Operating_System
,	O
which	O
would	O
forward	O
the	O
request	O
to	O
its	O
display	O
driver	O
,	O
which	O
is	O
then	O
responsible	O
for	O
actually	O
plotting	O
the	O
character/pixel	O
.	O
</s>
<s>
A	O
kernel	B-Operating_System
must	O
maintain	O
a	O
list	O
of	O
available	O
devices	O
.	O
</s>
<s>
This	O
list	O
may	O
be	O
known	O
in	O
advance	O
(	O
e.g.	O
,	O
on	O
an	O
embedded	B-Architecture
system	I-Architecture
where	O
the	O
kernel	B-Operating_System
will	O
be	O
rewritten	O
if	O
the	O
available	O
hardware	O
changes	O
)	O
,	O
configured	O
by	O
the	O
user	O
(	O
typical	O
on	O
older	O
PCs	B-Device
and	O
on	O
systems	O
that	O
are	O
not	O
designed	O
for	O
personal	O
use	O
)	O
or	O
detected	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
at	O
run	B-Library
time	I-Library
(	O
normally	O
called	O
plug	B-Device
and	I-Device
play	I-Device
)	O
.	O
</s>
<s>
In	O
plug-and-play	B-Device
systems	O
,	O
a	O
device	O
manager	O
first	O
performs	O
a	O
scan	O
on	O
different	O
peripheral	O
buses	O
,	O
such	O
as	O
Peripheral	B-Protocol
Component	I-Protocol
Interconnect	I-Protocol
(	O
PCI	B-Protocol
)	O
or	O
Universal	B-Protocol
Serial	I-Protocol
Bus	I-Protocol
(	O
USB	B-Protocol
)	O
,	O
to	O
detect	O
installed	O
devices	O
,	O
then	O
searches	O
for	O
the	O
appropriate	O
drivers	O
.	O
</s>
<s>
As	O
device	O
management	O
is	O
a	O
very	O
OS-specific	O
topic	O
,	O
these	O
drivers	O
are	O
handled	O
differently	O
by	O
each	O
kind	O
of	O
kernel	B-Operating_System
design	I-Operating_System
,	O
but	O
in	O
every	O
case	O
,	O
the	O
kernel	B-Operating_System
has	O
to	O
provide	O
the	O
I/O	B-General_Concept
to	O
allow	O
drivers	O
to	O
physically	O
access	O
their	O
devices	O
through	O
some	O
port	B-Protocol
or	O
memory	O
location	O
.	O
</s>
<s>
Important	O
decisions	O
have	O
to	O
be	O
made	O
when	O
designing	O
the	O
device	O
management	O
system	O
,	O
as	O
in	O
some	O
designs	O
accesses	O
may	O
involve	O
context	B-Operating_System
switches	I-Operating_System
,	O
making	O
the	O
operation	O
very	O
CPU-intensive	O
and	O
easily	O
causing	O
a	O
significant	O
performance	O
overhead	O
.	O
</s>
<s>
In	O
computing	O
,	O
a	O
system	B-Operating_System
call	I-Operating_System
is	O
how	O
a	O
process	B-Operating_System
requests	O
a	O
service	O
from	O
an	O
operating	B-General_Concept
system	I-General_Concept
's	O
kernel	B-Operating_System
that	O
it	O
does	O
not	O
normally	O
have	O
permission	O
to	O
run	O
.	O
</s>
<s>
System	B-Operating_System
calls	I-Operating_System
provide	O
the	O
interface	B-General_Concept
between	O
a	O
process	B-Operating_System
and	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Most	O
operations	O
interacting	O
with	O
the	O
system	O
require	O
permissions	O
not	O
available	O
to	O
a	O
user-level	B-Operating_System
process	B-Operating_System
,	O
e.g.	O
,	O
I/O	B-General_Concept
performed	O
with	O
a	O
device	O
present	O
on	O
the	O
system	O
,	O
or	O
any	O
form	O
of	O
communication	O
with	O
other	O
processes	O
requires	O
the	O
use	O
of	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
A	O
system	B-Operating_System
call	I-Operating_System
is	O
a	O
mechanism	O
that	O
is	O
used	O
by	O
the	O
application	B-Application
program	I-Application
to	O
request	O
a	O
service	O
from	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
They	O
use	O
a	O
machine-code	B-Language
instruction	O
that	O
causes	O
the	O
processor	O
to	O
change	O
mode	O
.	O
</s>
<s>
An	O
example	O
would	O
be	O
from	O
supervisor	B-Operating_System
mode	I-Operating_System
to	O
protected	O
mode	O
.	O
</s>
<s>
This	O
is	O
where	O
the	O
operating	B-General_Concept
system	I-General_Concept
performs	O
actions	O
like	O
accessing	O
hardware	O
devices	O
or	O
the	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
.	O
</s>
<s>
Generally	O
the	O
operating	B-General_Concept
system	I-General_Concept
provides	O
a	O
library	O
that	O
sits	O
between	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
normal	O
user	O
programs	O
.	O
</s>
<s>
Usually	O
it	O
is	O
a	O
C	B-Language
library	I-Language
such	O
as	O
Glibc	B-Language
or	O
Windows	O
API	B-General_Concept
.	O
</s>
<s>
The	O
library	O
handles	O
the	O
low-level	O
details	O
of	O
passing	O
information	O
to	O
the	O
kernel	B-Operating_System
and	O
switching	O
to	O
supervisor	B-Operating_System
mode	I-Operating_System
.	O
</s>
<s>
System	B-Operating_System
calls	I-Operating_System
include	O
close	O
,	O
open	O
,	O
read	O
,	O
wait	O
and	O
write	O
.	O
</s>
<s>
To	O
actually	O
perform	O
useful	O
work	O
,	O
a	O
process	B-Operating_System
must	O
be	O
able	O
to	O
access	O
the	O
services	O
provided	O
by	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
This	O
is	O
implemented	O
differently	O
by	O
each	O
kernel	B-Operating_System
,	O
but	O
most	O
provide	O
a	O
C	B-Language
library	I-Language
or	O
an	O
API	B-General_Concept
,	O
which	O
in	O
turn	O
invokes	O
the	O
related	O
kernel	B-Operating_System
functions	O
.	O
</s>
<s>
The	O
method	O
of	O
invoking	O
the	O
kernel	B-Operating_System
function	O
varies	O
from	O
kernel	B-Operating_System
to	O
kernel	B-Operating_System
.	O
</s>
<s>
If	O
memory	B-General_Concept
isolation	I-General_Concept
is	O
in	O
use	O
,	O
it	O
is	O
impossible	O
for	O
a	O
user	O
process	B-Operating_System
to	O
call	O
the	O
kernel	B-Operating_System
directly	O
,	O
because	O
that	O
would	O
be	O
a	O
violation	O
of	O
the	O
processor	O
's	O
access	O
control	O
rules	O
.	O
</s>
<s>
Using	O
a	O
software-simulated	O
interrupt	B-Application
.	O
</s>
<s>
Using	O
a	O
call	B-Device
gate	I-Device
.	O
</s>
<s>
A	O
call	B-Device
gate	I-Device
is	O
a	O
special	O
address	O
stored	O
by	O
the	O
kernel	B-Operating_System
in	O
a	O
list	O
in	O
kernel	B-Operating_System
memory	O
at	O
a	O
location	O
known	O
to	O
the	O
processor	O
.	O
</s>
<s>
Using	O
a	O
special	O
system	B-Operating_System
call	I-Operating_System
instruction	O
.	O
</s>
<s>
This	O
technique	O
requires	O
special	O
hardware	O
support	O
,	O
which	O
common	O
architectures	O
(	O
notably	O
,	O
x86	B-Operating_System
)	O
may	O
lack	O
.	O
</s>
<s>
System	B-Operating_System
call	I-Operating_System
instructions	O
have	O
been	O
added	O
to	O
recent	O
models	O
of	O
x86	B-Operating_System
processors	O
,	O
however	O
,	O
and	O
some	O
operating	B-General_Concept
systems	I-General_Concept
for	O
PCs	B-Device
make	O
use	O
of	O
them	O
when	O
available	O
.	O
</s>
<s>
An	O
application	O
that	O
makes	O
large	O
numbers	O
of	O
requests	O
but	O
does	O
not	O
need	O
to	O
wait	O
for	O
the	O
result	O
of	O
each	O
may	O
add	O
details	O
of	O
requests	O
to	O
an	O
area	O
of	O
memory	O
that	O
the	O
kernel	B-Operating_System
periodically	O
scans	O
to	O
find	O
requests	O
.	O
</s>
<s>
An	O
important	O
consideration	O
in	O
the	O
design	O
of	O
a	O
kernel	B-Operating_System
is	O
the	O
support	O
it	O
provides	O
for	O
protection	O
from	O
faults	O
(	O
fault	B-General_Concept
tolerance	I-General_Concept
)	O
and	O
from	O
malicious	O
behaviours	O
(	O
security	O
)	O
.	O
</s>
<s>
These	O
two	O
aspects	O
are	O
usually	O
not	O
clearly	O
distinguished	O
,	O
and	O
the	O
adoption	O
of	O
this	O
distinction	O
in	O
the	O
kernel	B-Operating_System
design	I-Operating_System
leads	O
to	O
the	O
rejection	O
of	O
a	O
hierarchical	B-Operating_System
structure	I-Operating_System
for	I-Operating_System
protection	I-Operating_System
.	O
</s>
<s>
The	O
mechanisms	O
or	O
policies	O
provided	O
by	O
the	O
kernel	B-Operating_System
can	O
be	O
classified	O
according	O
to	O
several	O
criteria	O
,	O
including	O
:	O
static	O
(	O
enforced	O
at	O
compile	B-Application
time	I-Application
)	O
or	O
dynamic	O
(	O
enforced	O
at	O
run	B-Library
time	I-Library
)	O
;	O
pre-emptive	O
or	O
post-detection	O
;	O
according	O
to	O
the	O
protection	O
principles	O
they	O
satisfy	O
(	O
e.g.	O
,	O
Denning	O
)	O
;	O
whether	O
they	O
are	O
hardware	O
supported	O
or	O
language	O
based	O
;	O
whether	O
they	O
are	O
more	O
an	O
open	O
mechanism	O
or	O
a	O
binding	O
policy	O
;	O
and	O
many	O
more	O
.	O
</s>
<s>
Support	O
for	O
hierarchical	B-Operating_System
protection	I-Operating_System
domains	I-Operating_System
is	O
typically	O
implemented	O
using	O
CPU	B-General_Concept
modes	I-General_Concept
.	O
</s>
<s>
Many	O
kernels	B-Operating_System
provide	O
implementation	O
of	O
"	O
capabilities	O
"	O
,	O
i.e.	O
,	O
objects	O
that	O
are	O
provided	O
to	O
user	O
code	O
which	O
allow	O
limited	O
access	O
to	O
an	O
underlying	O
object	O
managed	O
by	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
kernel	B-Operating_System
may	O
be	O
able	O
to	O
perform	O
many	O
different	O
operations	O
,	O
including	O
read	O
,	O
write	O
,	O
delete	O
or	O
execute	O
,	O
but	O
a	O
user-level	B-Operating_System
application	O
may	O
only	O
be	O
permitted	O
to	O
perform	O
some	O
of	O
these	O
operations	O
(	O
e.g.	O
,	O
it	O
may	O
only	O
be	O
allowed	O
to	O
read	O
the	O
file	O
)	O
.	O
</s>
<s>
A	O
common	O
implementation	O
of	O
this	O
is	O
for	O
the	O
kernel	B-Operating_System
to	O
provide	O
an	O
object	O
to	O
the	O
application	O
(	O
typically	O
so	O
called	O
a	O
"	O
file	O
handle	O
"	O
)	O
which	O
the	O
application	O
may	O
then	O
invoke	O
operations	O
on	O
,	O
the	O
validity	O
of	O
which	O
the	O
kernel	B-Operating_System
checks	O
at	O
the	O
time	O
the	O
operation	O
is	O
requested	O
.	O
</s>
<s>
Such	O
a	O
system	O
may	O
be	O
extended	O
to	O
cover	O
all	O
objects	O
that	O
the	O
kernel	B-Operating_System
manages	O
,	O
and	O
indeed	O
to	O
objects	O
provided	O
by	O
other	O
user	O
applications	O
.	O
</s>
<s>
An	O
efficient	O
and	O
simple	O
way	O
to	O
provide	O
hardware	O
support	O
of	O
capabilities	O
is	O
to	O
delegate	O
to	O
the	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
(	O
MMU	O
)	O
the	O
responsibility	O
of	O
checking	O
access-rights	O
for	O
every	O
memory	O
access	O
,	O
a	O
mechanism	O
called	O
capability-based	B-General_Concept
addressing	I-General_Concept
.	O
</s>
<s>
Most	O
commercial	O
computer	B-General_Concept
architectures	I-General_Concept
lack	O
such	O
MMU	O
support	O
for	O
capabilities	O
.	O
</s>
<s>
In	O
this	O
approach	O
,	O
each	O
protected	O
object	O
must	O
reside	O
in	O
an	O
address	B-General_Concept
space	I-General_Concept
that	O
the	O
application	O
does	O
not	O
have	O
access	O
to	O
;	O
the	O
kernel	B-Operating_System
also	O
maintains	O
a	O
list	O
of	O
capabilities	O
in	O
such	O
memory	O
.	O
</s>
<s>
When	O
an	O
application	O
needs	O
to	O
access	O
an	O
object	O
protected	O
by	O
a	O
capability	O
,	O
it	O
performs	O
a	O
system	B-Operating_System
call	I-Operating_System
and	O
the	O
kernel	B-Operating_System
then	O
checks	O
whether	O
the	O
application	O
's	O
capability	O
grants	O
it	O
permission	O
to	O
perform	O
the	O
requested	O
action	O
,	O
and	O
if	O
it	O
is	O
permitted	O
performs	O
the	O
access	O
for	O
it	O
(	O
either	O
directly	O
,	O
or	O
by	O
delegating	O
the	O
request	O
to	O
another	O
user-level	B-Operating_System
process	B-Operating_System
)	O
.	O
</s>
<s>
The	O
performance	O
cost	O
of	O
address	B-General_Concept
space	I-General_Concept
switching	O
limits	O
the	O
practicality	O
of	O
this	O
approach	O
in	O
systems	O
with	O
complex	O
interactions	O
between	O
objects	O
,	O
but	O
it	O
is	O
used	O
in	O
current	O
operating	B-General_Concept
systems	I-General_Concept
for	O
objects	O
that	O
are	O
not	O
accessed	O
frequently	O
or	O
which	O
are	O
not	O
expected	O
to	O
perform	O
quickly	O
.	O
</s>
<s>
If	O
the	O
firmware	O
does	O
not	O
support	O
protection	O
mechanisms	O
,	O
it	O
is	O
possible	O
to	O
simulate	O
protection	O
at	O
a	O
higher	O
level	O
,	O
for	O
example	O
by	O
simulating	O
capabilities	O
by	O
manipulating	O
page	B-General_Concept
tables	I-General_Concept
,	O
but	O
there	O
are	O
performance	O
implications	O
.	O
</s>
<s>
An	O
important	O
kernel	B-Operating_System
design	I-Operating_System
decision	O
is	O
the	O
choice	O
of	O
the	O
abstraction	B-Architecture
levels	I-Architecture
where	O
the	O
security	O
mechanisms	O
and	O
policies	O
should	O
be	O
implemented	O
.	O
</s>
<s>
Kernel	B-Operating_System
security	O
mechanisms	O
play	O
a	O
critical	O
role	O
in	O
supporting	O
security	O
at	O
higher	O
levels	O
.	O
</s>
<s>
One	O
approach	O
is	O
to	O
use	O
firmware	O
and	O
kernel	B-Operating_System
support	O
for	O
fault	B-General_Concept
tolerance	I-General_Concept
(	O
see	O
above	O
)	O
,	O
and	O
build	O
the	O
security	O
policy	O
for	O
malicious	O
behavior	O
on	O
top	O
of	O
that	O
(	O
adding	O
features	O
such	O
as	O
cryptography	O
mechanisms	O
where	O
necessary	O
)	O
,	O
delegating	O
some	O
responsibility	O
to	O
the	O
compiler	B-Language
.	O
</s>
<s>
Approaches	O
that	O
delegate	O
enforcement	O
of	O
security	O
policy	O
to	O
the	O
compiler	B-Language
and/or	O
the	O
application	O
level	O
are	O
often	O
called	O
language-based	O
security	O
.	O
</s>
<s>
The	O
lack	O
of	O
many	O
critical	O
security	O
mechanisms	O
in	O
current	O
mainstream	O
operating	B-General_Concept
systems	I-General_Concept
impedes	O
the	O
implementation	O
of	O
adequate	O
security	O
policies	O
at	O
the	O
application	O
abstraction	B-Architecture
level	I-Architecture
.	O
</s>
<s>
In	O
fact	O
,	O
a	O
common	O
misconception	O
in	O
computer	O
security	O
is	O
that	O
any	O
security	O
policy	O
can	O
be	O
implemented	O
in	O
an	O
application	O
regardless	O
of	O
kernel	B-Operating_System
support	O
.	O
</s>
<s>
According	O
to	O
Mars	O
Research	O
Group	O
developers	O
,	O
a	O
lack	O
of	O
isolation	O
is	O
one	O
of	O
the	O
main	O
factors	O
undermining	O
kernel	B-Operating_System
security	O
.	O
</s>
<s>
They	O
propose	O
their	O
driver	O
isolation	O
framework	O
for	O
protection	O
,	O
primarily	O
in	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
The	O
processor	O
monitors	O
the	O
execution	O
and	O
stops	O
a	O
program	O
that	O
violates	O
a	O
rule	O
,	O
such	O
as	O
a	O
user	O
process	B-Operating_System
that	O
tries	O
to	O
write	O
to	O
kernel	B-Operating_System
memory	O
.	O
</s>
<s>
In	O
systems	O
that	O
lack	O
support	O
for	O
capabilities	O
,	O
processes	O
are	O
isolated	O
from	O
each	O
other	O
by	O
using	O
separate	O
address	B-General_Concept
spaces	I-General_Concept
.	O
</s>
<s>
Calls	O
from	O
user	O
processes	O
into	O
the	O
kernel	B-Operating_System
are	O
regulated	O
by	O
requiring	O
them	O
to	O
use	O
one	O
of	O
the	O
above-described	O
system	B-Operating_System
call	I-Operating_System
methods	O
.	O
</s>
<s>
In	O
a	O
language-based	B-Operating_System
protection	I-Operating_System
system	I-Operating_System
,	O
the	O
kernel	B-Operating_System
will	O
only	O
allow	O
code	O
to	O
execute	O
that	O
has	O
been	O
produced	O
by	O
a	O
trusted	O
language	B-Language
compiler	I-Language
.	O
</s>
<s>
No	O
need	O
for	O
separate	O
address	B-General_Concept
spaces	I-General_Concept
.	O
</s>
<s>
Switching	O
between	O
address	B-General_Concept
spaces	I-General_Concept
is	O
a	O
slow	O
operation	O
that	O
causes	O
a	O
great	O
deal	O
of	O
overhead	O
,	O
and	O
a	O
lot	O
of	O
optimization	O
work	O
is	O
currently	O
performed	O
in	O
order	O
to	O
prevent	O
unnecessary	O
switches	O
in	O
current	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Switching	O
is	O
completely	O
unnecessary	O
in	O
a	O
language-based	B-Operating_System
protection	I-Operating_System
system	I-Operating_System
,	O
as	O
all	O
code	O
can	O
safely	O
operate	O
in	O
the	O
same	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
Longer	O
application	O
startup	B-Operating_System
time	O
.	O
</s>
<s>
Applications	O
must	O
be	O
verified	O
when	O
they	O
are	O
started	O
to	O
ensure	O
they	O
have	O
been	O
compiled	B-Language
by	O
the	O
correct	O
compiler	B-Language
,	O
or	O
may	O
need	O
recompiling	O
either	O
from	O
source	O
code	O
or	O
from	O
bytecode	O
.	O
</s>
<s>
On	O
traditional	O
systems	O
,	O
applications	O
frequently	O
perform	O
operations	O
that	O
are	O
not	O
type	B-Language
safe	I-Language
.	O
</s>
<s>
Such	O
operations	O
cannot	O
be	O
permitted	O
in	O
a	O
language-based	B-Operating_System
protection	I-Operating_System
system	I-Operating_System
,	O
which	O
means	O
that	O
applications	O
may	O
need	O
to	O
be	O
rewritten	O
and	O
may	O
,	O
in	O
some	O
cases	O
,	O
lose	O
performance	O
.	O
</s>
<s>
Examples	O
of	O
systems	O
with	O
language-based	O
protection	O
include	O
JX	B-Language
and	O
Microsoft	O
's	O
Singularity	B-Operating_System
.	O
</s>
<s>
Edsger	O
Dijkstra	O
proved	O
that	O
from	O
a	O
logical	O
point	O
of	O
view	O
,	O
atomic	B-General_Concept
lock	B-Operating_System
and	O
unlock	O
operations	O
operating	O
on	O
binary	B-Operating_System
semaphores	I-Operating_System
are	O
sufficient	O
primitives	O
to	O
express	O
any	O
functionality	O
of	O
process	B-Operating_System
cooperation	O
.	O
</s>
<s>
However	O
this	O
approach	O
is	O
generally	O
held	O
to	O
be	O
lacking	O
in	O
terms	O
of	O
safety	O
and	O
efficiency	O
,	O
whereas	O
a	O
message	B-Architecture
passing	I-Architecture
approach	O
is	O
more	O
flexible	O
.	O
</s>
<s>
A	O
number	O
of	O
other	O
approaches	O
(	O
either	O
lower	O
-	O
or	O
higher-level	O
)	O
are	O
available	O
as	O
well	O
,	O
with	O
many	O
modern	O
kernels	B-Operating_System
providing	O
support	O
for	O
systems	O
such	O
as	O
shared	B-Operating_System
memory	I-Operating_System
and	O
remote	B-Operating_System
procedure	I-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
The	O
idea	O
of	O
a	O
kernel	B-Operating_System
where	O
I/O	B-General_Concept
devices	I-General_Concept
are	O
handled	O
uniformly	O
with	O
other	O
processes	O
,	O
as	O
parallel	O
co-operating	O
processes	O
,	O
was	O
first	O
proposed	O
and	O
implemented	O
by	O
Brinch	O
Hansen	O
(	O
although	O
similar	O
ideas	O
were	O
suggested	O
in	O
1967	O
)	O
.	O
</s>
<s>
In	O
Hansen	O
's	O
description	O
of	O
this	O
,	O
the	O
"	O
common	O
"	O
processes	O
are	O
called	O
internal	O
processes	O
,	O
while	O
the	O
I/O	B-General_Concept
devices	I-General_Concept
are	O
called	O
external	O
processes	O
.	O
</s>
<s>
Because	O
of	O
this	O
,	O
providing	O
a	O
more	O
abstract	B-Architecture
interface	I-Architecture
to	O
manage	O
the	O
device	O
is	O
important	O
.	O
</s>
<s>
This	O
interface	B-General_Concept
is	O
normally	O
done	O
by	O
a	O
device	B-Application
driver	I-Application
or	O
hardware	O
abstraction	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
The	O
kernel	B-Operating_System
must	O
maintain	O
the	O
list	O
of	O
these	O
devices	O
by	O
querying	O
the	O
system	O
for	O
them	O
in	O
some	O
way	O
.	O
</s>
<s>
This	O
can	O
be	O
done	O
through	O
the	O
BIOS	B-Operating_System
,	O
or	O
through	O
one	O
of	O
the	O
various	O
system	O
buses	O
(	O
such	O
as	O
PCI/PCIE	O
,	O
or	O
USB	B-Protocol
)	O
.	O
</s>
<s>
Using	O
an	O
example	O
of	O
a	O
video	O
driver	O
,	O
when	O
an	O
application	O
requests	O
an	O
operation	O
on	O
a	O
device	O
,	O
such	O
as	O
displaying	O
a	O
character	O
,	O
the	O
kernel	B-Operating_System
needs	O
to	O
send	O
this	O
request	O
to	O
the	O
current	O
active	O
video	O
driver	O
.	O
</s>
<s>
This	O
is	O
an	O
example	O
of	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
.	O
</s>
<s>
The	O
principle	O
of	O
separation	O
of	O
mechanism	O
and	O
policy	O
is	O
the	O
substantial	O
difference	O
between	O
the	O
philosophy	O
of	O
micro	O
and	O
monolithic	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
In	O
minimal	O
microkernel	B-Operating_System
just	O
some	O
very	O
basic	O
policies	O
are	O
included	O
,	O
and	O
its	O
mechanisms	O
allows	O
what	O
is	O
running	O
on	O
top	O
of	O
the	O
kernel	B-Operating_System
(	O
the	O
remaining	O
part	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
the	O
other	O
applications	O
)	O
to	O
decide	O
which	O
policies	O
to	O
adopt	O
(	O
as	O
memory	B-General_Concept
management	I-General_Concept
,	O
high	O
level	O
process	B-Operating_System
scheduling	O
,	O
file	B-Application
system	I-Application
management	O
,	O
etc	O
.	O
)	O
.	O
</s>
<s>
A	O
monolithic	B-Operating_System
kernel	I-Operating_System
instead	O
tends	O
to	O
include	O
many	O
policies	O
,	O
therefore	O
restricting	O
the	O
rest	O
of	O
the	O
system	O
to	O
rely	O
on	O
them	O
.	O
</s>
<s>
The	O
failure	O
to	O
properly	O
fulfill	O
this	O
separation	O
is	O
one	O
of	O
the	O
major	O
causes	O
of	O
the	O
lack	O
of	O
substantial	O
innovation	O
in	O
existing	O
operating	B-General_Concept
systems	I-General_Concept
,	O
a	O
problem	O
common	O
in	O
computer	B-General_Concept
architecture	I-General_Concept
.	O
</s>
<s>
The	O
monolithic	O
design	O
is	O
induced	O
by	O
the	O
"	O
kernel	B-Operating_System
mode	O
"	O
/	O
"	O
user	B-Operating_System
mode	I-Operating_System
"	O
architectural	O
approach	O
to	O
protection	O
(	O
technically	O
called	O
hierarchical	B-Operating_System
protection	I-Operating_System
domains	I-Operating_System
)	O
,	O
which	O
is	O
common	O
in	O
conventional	O
commercial	O
systems	O
;	O
in	O
fact	O
,	O
every	O
module	B-Application
needing	O
protection	O
is	O
therefore	O
preferably	O
included	O
into	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
This	O
link	O
between	O
monolithic	O
design	O
and	O
"	O
privileged	B-Operating_System
mode	I-Operating_System
"	O
can	O
be	O
reconducted	O
to	O
the	O
key	O
issue	O
of	O
mechanism-policy	O
separation	O
;	O
in	O
fact	O
the	O
"	O
privileged	B-Operating_System
mode	I-Operating_System
"	O
architectural	O
approach	O
melds	O
together	O
the	O
protection	O
mechanism	O
with	O
the	O
security	O
policies	O
,	O
while	O
the	O
major	O
alternative	O
architectural	O
approach	O
,	O
capability-based	B-General_Concept
addressing	I-General_Concept
,	O
clearly	O
distinguishes	O
between	O
the	O
two	O
,	O
leading	O
naturally	O
to	O
a	O
microkernel	B-Operating_System
design	O
(	O
see	O
Separation	O
of	O
protection	O
and	O
security	O
)	O
.	O
</s>
<s>
While	O
monolithic	B-Operating_System
kernels	I-Operating_System
execute	O
all	O
of	O
their	O
code	O
in	O
the	O
same	O
address	B-General_Concept
space	I-General_Concept
(	O
kernel	B-Operating_System
space	I-Operating_System
)	O
,	O
microkernels	B-Operating_System
try	O
to	O
run	O
most	O
of	O
their	O
services	O
in	O
user	B-Operating_System
space	I-Operating_System
,	O
aiming	O
to	O
improve	O
maintainability	O
and	O
modularity	B-Architecture
of	O
the	O
codebase	O
.	O
</s>
<s>
Most	O
kernels	B-Operating_System
do	O
not	O
fit	O
exactly	O
into	O
one	O
of	O
these	O
categories	O
,	O
but	O
are	O
rather	O
found	O
in	O
between	O
these	O
two	O
designs	O
.	O
</s>
<s>
These	O
are	O
called	O
hybrid	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
More	O
exotic	O
designs	O
such	O
as	O
nanokernels	O
and	O
exokernels	B-Operating_System
are	O
available	O
,	O
but	O
are	O
seldom	O
used	O
for	O
production	O
systems	O
.	O
</s>
<s>
The	O
Xen	B-Operating_System
hypervisor	I-Operating_System
,	O
for	O
example	O
,	O
is	O
an	O
exokernel	B-Operating_System
.	O
</s>
<s>
In	O
a	O
monolithic	B-Operating_System
kernel	I-Operating_System
,	O
all	O
OS	B-General_Concept
services	O
run	O
along	O
with	O
the	O
main	O
kernel	B-Operating_System
thread	O
,	O
thus	O
also	O
residing	O
in	O
the	O
same	O
memory	O
area	O
.	O
</s>
<s>
monolithic	B-Operating_System
kernel	I-Operating_System
"	O
.	O
</s>
<s>
The	O
main	O
disadvantages	O
of	O
monolithic	B-Operating_System
kernels	I-Operating_System
are	O
the	O
dependencies	O
between	O
system	O
components	O
a	O
bug	O
in	O
a	O
device	B-Application
driver	I-Application
might	O
crash	O
the	O
entire	O
system	O
and	O
the	O
fact	O
that	O
large	O
kernels	B-Operating_System
can	O
become	O
very	O
difficult	O
to	O
maintain	O
;	O
Thompson	O
also	O
stated	O
that	O
"	O
It	O
is	O
also	O
easier	O
for	O
[	O
a	O
monolithic	O
kernel ]	O
to	O
turn	O
into	O
a	O
mess	O
in	O
a	O
hurry	O
as	O
it	O
is	O
modified.	O
"	O
</s>
<s>
Monolithic	B-Operating_System
kernels	I-Operating_System
,	O
which	O
have	O
traditionally	O
been	O
used	O
by	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
contain	O
all	O
the	O
operating	B-General_Concept
system	I-General_Concept
core	O
functions	O
and	O
the	O
device	B-Application
drivers	I-Application
.	O
</s>
<s>
A	O
monolithic	B-Operating_System
kernel	I-Operating_System
is	O
one	O
single	O
program	O
that	O
contains	O
all	O
of	O
the	O
code	O
necessary	O
to	O
perform	O
every	O
kernel-related	O
task	O
.	O
</s>
<s>
Every	O
part	O
which	O
is	O
to	O
be	O
accessed	O
by	O
most	O
programs	O
which	O
cannot	O
be	O
put	O
in	O
a	O
library	O
is	O
in	O
the	O
kernel	B-Operating_System
space	I-Operating_System
:	O
Device	B-Application
drivers	I-Application
,	O
scheduler	O
,	O
memory	O
handling	O
,	O
file	B-Application
systems	I-Application
,	O
and	O
network	B-Protocol
stacks	I-Protocol
.	O
</s>
<s>
Many	O
system	B-Operating_System
calls	I-Operating_System
are	O
provided	O
to	O
applications	O
,	O
to	O
allow	O
them	O
to	O
access	O
all	O
those	O
services	O
.	O
</s>
<s>
A	O
monolithic	B-Operating_System
kernel	I-Operating_System
,	O
while	O
initially	O
loaded	O
with	O
subsystems	O
that	O
may	O
not	O
be	O
needed	O
,	O
can	O
be	O
tuned	O
to	O
a	O
point	O
where	O
it	O
is	O
as	O
fast	O
as	O
or	O
faster	O
than	O
the	O
one	O
that	O
was	O
specifically	O
designed	O
for	O
the	O
hardware	O
,	O
although	O
more	O
relevant	O
in	O
a	O
general	O
sense	O
.	O
</s>
<s>
Modern	O
monolithic	B-Operating_System
kernels	I-Operating_System
,	O
such	O
as	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
the	O
FreeBSD	B-Operating_System
kernel	B-Operating_System
,	O
the	O
AIX	B-Application
kernel	B-Operating_System
,	O
the	O
HP-UX	B-Application
kernel	B-Operating_System
,	O
and	O
the	O
Solaris	B-Application
kernel	I-Application
,	O
all	O
of	O
which	O
fall	O
into	O
the	O
category	O
of	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
support	O
loadable	B-Application
kernel	I-Application
modules	I-Application
,	O
allowing	O
modules	B-Application
to	O
be	O
loaded	O
into	O
the	O
kernel	B-Operating_System
at	B-Library
runtime	I-Library
,	O
permitting	O
easy	O
extension	O
of	O
the	O
kernel	B-Operating_System
's	O
capabilities	O
as	O
required	O
,	O
while	O
helping	O
to	O
minimize	O
the	O
amount	O
of	O
code	O
running	O
in	O
kernel	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
Most	O
work	O
in	O
the	O
monolithic	B-Operating_System
kernel	I-Operating_System
is	O
done	O
via	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
These	O
are	O
interfaces	O
,	O
usually	O
kept	O
in	O
a	O
tabular	O
structure	O
,	O
that	O
access	O
some	O
subsystem	O
within	O
the	O
kernel	B-Operating_System
such	O
as	O
disk	O
operations	O
.	O
</s>
<s>
Essentially	O
calls	O
are	O
made	O
within	O
programs	O
and	O
a	O
checked	O
copy	O
of	O
the	O
request	O
is	O
passed	O
through	O
the	O
system	B-Operating_System
call	I-Operating_System
.	O
</s>
<s>
The	O
monolithic	O
Linux	B-Operating_System
kernel	I-Operating_System
can	O
be	O
made	O
extremely	O
small	O
not	O
only	O
because	O
of	O
its	O
ability	O
to	O
dynamically	O
load	O
modules	B-Application
but	O
also	O
because	O
of	O
its	O
ease	O
of	O
customization	O
.	O
</s>
<s>
single	O
floppy	O
disk	O
and	O
still	O
provide	O
a	O
fully	O
functional	O
operating	B-General_Concept
system	I-General_Concept
(	O
one	O
of	O
the	O
most	O
popular	O
of	O
which	O
is	O
muLinux	O
)	O
.	O
</s>
<s>
This	O
ability	O
to	O
miniaturize	O
its	O
kernel	B-Operating_System
has	O
also	O
led	O
to	O
a	O
rapid	O
growth	O
in	O
the	O
use	O
of	O
Linux	B-Application
in	O
embedded	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
These	O
types	O
of	O
kernels	B-Operating_System
consist	O
of	O
the	O
core	O
functions	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
the	O
device	B-Application
drivers	I-Application
with	O
the	O
ability	O
to	O
load	O
modules	B-Application
at	B-Library
runtime	I-Library
.	O
</s>
<s>
They	O
provide	O
a	O
small	O
set	O
of	O
simple	O
hardware	O
abstractions	O
and	O
use	O
applications	O
called	O
servers	B-Application
to	O
provide	O
more	O
functionality	O
.	O
</s>
<s>
This	O
particular	O
approach	O
defines	O
a	O
high-level	O
virtual	O
interface	B-General_Concept
over	O
the	O
hardware	O
,	O
with	O
a	O
set	O
of	O
system	B-Operating_System
calls	I-Operating_System
to	O
implement	O
operating	B-Operating_System
system	I-Operating_System
services	I-Operating_System
such	O
as	O
process	B-Operating_System
management	O
,	O
concurrency	O
and	O
memory	B-General_Concept
management	I-General_Concept
in	O
several	O
modules	B-Application
that	O
run	O
in	O
supervisor	B-Operating_System
mode	I-Operating_System
.	O
</s>
<s>
Coding	O
in	O
kernel	B-Operating_System
can	O
be	O
challenging	O
,	O
in	O
part	O
because	O
one	O
cannot	O
use	O
common	O
libraries	O
(	O
like	O
a	O
full-featured	O
libc	B-Language
)	O
,	O
and	O
because	O
one	O
needs	O
to	O
use	O
a	O
source-level	B-Application
debugger	I-Application
like	O
gdb	B-Language
.	O
</s>
<s>
Bugs	O
in	O
one	O
part	O
of	O
the	O
kernel	B-Operating_System
have	O
strong	O
side	O
effects	O
;	O
since	O
every	O
function	O
in	O
the	O
kernel	B-Operating_System
has	O
all	O
the	O
privileges	O
,	O
a	O
bug	O
in	O
one	O
function	O
can	O
corrupt	O
data	O
structure	O
of	O
another	O
,	O
totally	O
unrelated	O
part	O
of	O
the	O
kernel	B-Operating_System
,	O
or	O
of	O
any	O
running	O
program	O
.	O
</s>
<s>
Kernels	B-Operating_System
often	O
become	O
very	O
large	O
and	O
difficult	O
to	O
maintain	O
.	O
</s>
<s>
Even	O
if	O
the	O
modules	B-Application
servicing	O
these	O
operations	O
are	O
separate	O
from	O
the	O
whole	O
,	O
the	O
code	O
integration	O
is	O
tight	O
and	O
difficult	O
to	O
do	O
correctly	O
.	O
</s>
<s>
Since	O
the	O
modules	B-Application
run	O
in	O
the	O
same	O
address	B-General_Concept
space	I-General_Concept
,	O
a	O
bug	O
can	O
bring	O
down	O
the	O
entire	O
system	O
.	O
</s>
<s>
Microkernel	B-Operating_System
(	O
also	O
abbreviated	O
μK	O
or	O
uK	O
)	O
is	O
the	O
term	O
describing	O
an	O
approach	O
to	O
operating	B-General_Concept
system	I-General_Concept
design	O
by	O
which	O
the	O
functionality	O
of	O
the	O
system	O
is	O
moved	O
out	O
of	O
the	O
traditional	O
"	O
kernel	B-Operating_System
"	O
,	O
into	O
a	O
set	O
of	O
"	O
servers	B-Application
"	O
that	O
communicate	O
through	O
a	O
"	O
minimal	O
"	O
kernel	B-Operating_System
,	O
leaving	O
as	O
little	O
as	O
possible	O
in	O
"	O
system	O
space	O
"	O
and	O
as	O
much	O
as	O
possible	O
in	O
"	O
user	B-Operating_System
space	I-Operating_System
"	O
.	O
</s>
<s>
A	O
microkernel	B-Operating_System
that	O
is	O
designed	O
for	O
a	O
specific	O
platform	O
or	O
device	O
is	O
only	O
ever	O
going	O
to	O
have	O
what	O
it	O
needs	O
to	O
operate	O
.	O
</s>
<s>
The	O
microkernel	B-Operating_System
approach	O
consists	O
of	O
defining	O
a	O
simple	O
abstraction	O
over	O
the	O
hardware	O
,	O
with	O
a	O
set	O
of	O
primitives	O
or	O
system	B-Operating_System
calls	I-Operating_System
to	O
implement	O
minimal	O
OS	B-General_Concept
services	O
such	O
as	O
memory	B-General_Concept
management	I-General_Concept
,	O
multitasking	B-Operating_System
,	O
and	O
inter-process	B-Operating_System
communication	I-Operating_System
.	O
</s>
<s>
Other	O
services	O
,	O
including	O
those	O
normally	O
provided	O
by	O
the	O
kernel	B-Operating_System
,	O
such	O
as	O
networking	B-Architecture
,	O
are	O
implemented	O
in	O
user-space	B-Operating_System
programs	O
,	O
referred	O
to	O
as	O
servers	B-Application
.	O
</s>
<s>
Microkernels	B-Operating_System
are	O
easier	O
to	O
maintain	O
than	O
monolithic	B-Operating_System
kernels	I-Operating_System
,	O
but	O
the	O
large	O
number	O
of	O
system	B-Operating_System
calls	I-Operating_System
and	O
context	B-Operating_System
switches	I-Operating_System
might	O
slow	O
down	O
the	O
system	O
because	O
they	O
typically	O
generate	O
more	O
overhead	O
than	O
plain	O
function	O
calls	O
.	O
</s>
<s>
Only	O
parts	O
which	O
really	O
require	O
being	O
in	O
a	O
privileged	B-Operating_System
mode	I-Operating_System
are	O
in	O
kernel	B-Operating_System
space	I-Operating_System
:	O
IPC	O
(	O
Inter-Process	B-Operating_System
Communication	I-Operating_System
)	O
,	O
basic	O
scheduler	O
,	O
or	O
scheduling	O
primitives	O
,	O
basic	O
memory	O
handling	O
,	O
basic	O
I/O	B-General_Concept
primitives	O
.	O
</s>
<s>
Many	O
critical	O
parts	O
are	O
now	O
running	O
in	O
user	B-Operating_System
space	I-Operating_System
:	O
The	O
complete	O
scheduler	O
,	O
memory	O
handling	O
,	O
file	B-Application
systems	I-Application
,	O
and	O
network	B-Protocol
stacks	I-Protocol
.	O
</s>
<s>
Micro	B-Operating_System
kernels	I-Operating_System
were	O
invented	O
as	O
a	O
reaction	O
to	O
traditional	O
"	O
monolithic	O
"	O
kernel	B-Operating_System
design	I-Operating_System
,	O
whereby	O
all	O
system	O
functionality	O
was	O
put	O
in	O
a	O
one	O
static	O
program	O
running	O
in	O
a	O
special	O
"	O
system	O
"	O
mode	O
of	O
the	O
processor	O
.	O
</s>
<s>
In	O
the	O
microkernel	B-Operating_System
,	O
only	O
the	O
most	O
fundamental	O
of	O
tasks	O
are	O
performed	O
such	O
as	O
being	O
able	O
to	O
access	O
some	O
(	O
not	O
necessarily	O
all	O
)	O
of	O
the	O
hardware	O
,	O
manage	O
memory	O
and	O
coordinate	O
message	B-Architecture
passing	I-Architecture
between	O
the	O
processes	O
.	O
</s>
<s>
Some	O
systems	O
that	O
use	O
micro	B-Operating_System
kernels	I-Operating_System
are	O
QNX	B-Operating_System
and	O
the	B-Application
HURD	I-Application
.	O
</s>
<s>
In	O
the	O
case	O
of	O
QNX	B-Operating_System
and	O
Hurd	B-Application
user	O
sessions	O
can	O
be	O
entire	O
snapshots	O
of	O
the	O
system	O
itself	O
or	O
views	O
as	O
it	O
is	O
referred	O
to	O
.	O
</s>
<s>
The	O
very	O
essence	O
of	O
the	O
microkernel	B-Operating_System
architecture	O
illustrates	O
some	O
of	O
its	O
advantages	O
:	O
</s>
<s>
Rapid	O
development	O
time	O
and	O
new	O
software	O
can	O
be	O
tested	O
without	O
having	O
to	O
reboot	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
Most	O
microkernels	B-Operating_System
use	O
a	O
message	B-Architecture
passing	I-Architecture
system	O
to	O
handle	O
requests	O
from	O
one	O
server	O
to	O
another	O
.	O
</s>
<s>
The	O
message	B-Architecture
passing	I-Architecture
system	O
generally	O
operates	O
on	O
a	O
port	B-Protocol
basis	O
with	O
the	O
microkernel	B-Operating_System
.	O
</s>
<s>
As	O
an	O
example	O
,	O
if	O
a	O
request	O
for	O
more	O
memory	O
is	O
sent	O
,	O
a	O
port	B-Protocol
is	O
opened	O
with	O
the	O
microkernel	B-Operating_System
and	O
the	O
request	O
sent	O
through	O
.	O
</s>
<s>
Once	O
within	O
the	O
microkernel	B-Operating_System
,	O
the	O
steps	O
are	O
similar	O
to	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
The	O
rationale	O
was	O
that	O
it	O
would	O
bring	O
modularity	B-Architecture
in	O
the	O
system	O
architecture	O
,	O
which	O
would	O
entail	O
a	O
cleaner	O
system	O
,	O
easier	O
to	O
debug	O
or	O
dynamically	O
modify	O
,	O
customizable	O
to	O
users	O
 '	O
needs	O
,	O
and	O
more	O
performing	O
.	O
</s>
<s>
They	O
are	O
part	O
of	O
the	O
operating	B-General_Concept
systems	I-General_Concept
like	O
GNU	B-Application
Hurd	I-Application
,	O
MINIX	B-Operating_System
,	O
MkLinux	B-Application
,	O
QNX	B-Operating_System
and	O
Redox	B-Operating_System
OS	I-Operating_System
.	O
</s>
<s>
Although	O
microkernels	B-Operating_System
are	O
very	O
small	O
by	O
themselves	O
,	O
in	O
combination	O
with	O
all	O
their	O
required	O
auxiliary	O
code	O
they	O
are	O
,	O
in	O
fact	O
,	O
often	O
larger	O
than	O
monolithic	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
Advocates	O
of	O
monolithic	B-Operating_System
kernels	I-Operating_System
also	O
point	O
out	O
that	O
the	O
two-tiered	O
structure	O
of	O
microkernel	B-Operating_System
systems	O
,	O
in	O
which	O
most	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
does	O
not	O
interact	O
directly	O
with	O
the	O
hardware	O
,	O
creates	O
a	O
not-insignificant	O
cost	O
in	O
terms	O
of	O
system	O
efficiency	O
.	O
</s>
<s>
These	O
types	O
of	O
kernels	B-Operating_System
normally	O
provide	O
only	O
the	O
minimal	O
services	O
such	O
as	O
defining	O
memory	O
address	B-General_Concept
spaces	I-General_Concept
,	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
and	O
the	O
process	B-Operating_System
management	O
.	O
</s>
<s>
The	O
other	O
functions	O
such	O
as	O
running	O
the	O
hardware	O
processes	O
are	O
not	O
handled	O
directly	O
by	O
microkernels	B-Operating_System
.	O
</s>
<s>
Proponents	O
of	O
micro	B-Operating_System
kernels	I-Operating_System
point	O
out	O
those	O
monolithic	B-Operating_System
kernels	I-Operating_System
have	O
the	O
disadvantage	O
that	O
an	O
error	O
in	O
the	O
kernel	B-Operating_System
can	O
cause	O
the	O
entire	O
system	O
to	O
crash	O
.	O
</s>
<s>
However	O
,	O
with	O
a	O
microkernel	B-Operating_System
,	O
if	O
a	O
kernel	B-Operating_System
process	B-Operating_System
crashes	O
,	O
it	O
is	O
still	O
possible	O
to	O
prevent	O
a	O
crash	O
of	O
the	O
system	O
as	O
a	O
whole	O
by	O
merely	O
restarting	O
the	O
service	O
that	O
caused	O
the	O
error	O
.	O
</s>
<s>
Other	O
services	O
provided	O
by	O
the	O
kernel	B-Operating_System
such	O
as	O
networking	B-Architecture
are	O
implemented	O
in	O
user-space	B-Operating_System
programs	O
referred	O
to	O
as	O
servers	B-Application
.	O
</s>
<s>
Servers	B-Application
allow	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
be	O
modified	O
by	O
simply	O
starting	O
and	O
stopping	O
programs	O
.	O
</s>
<s>
For	O
a	O
machine	O
without	O
networking	B-Architecture
support	O
,	O
for	O
instance	O
,	O
the	O
networking	B-Architecture
server	O
is	O
not	O
started	O
.	O
</s>
<s>
The	O
task	O
of	O
moving	O
in	O
and	O
out	O
of	O
the	O
kernel	B-Operating_System
to	O
move	O
data	O
between	O
the	O
various	O
applications	O
and	O
servers	B-Application
creates	O
overhead	O
which	O
is	O
detrimental	O
to	O
the	O
efficiency	O
of	O
micro	B-Operating_System
kernels	I-Operating_System
in	O
comparison	O
with	O
monolithic	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
Disadvantages	O
in	O
the	O
microkernel	B-Operating_System
exist	O
however	O
.	O
</s>
<s>
Messaging	O
bugs	O
can	O
be	O
harder	O
to	O
fix	O
due	O
to	O
the	O
longer	O
trip	O
they	O
have	O
to	O
take	O
versus	O
the	O
one	O
off	O
copy	O
in	O
a	O
monolithic	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
Process	B-Operating_System
management	O
in	O
general	O
can	O
be	O
very	O
complicated	O
.	O
</s>
<s>
The	O
disadvantages	O
for	O
microkernels	B-Operating_System
are	O
extremely	O
context-based	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
they	O
work	O
well	O
for	O
small	O
single-purpose	O
(	O
and	O
critical	O
)	O
systems	O
because	O
if	O
not	O
many	O
processes	O
need	O
to	O
run	O
,	O
then	O
the	O
complications	O
of	O
process	B-Operating_System
management	O
are	O
effectively	O
mitigated	O
.	O
</s>
<s>
A	O
microkernel	B-Operating_System
allows	O
the	O
implementation	O
of	O
the	O
remaining	O
part	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
as	O
a	O
normal	O
application	B-Application
program	I-Application
written	O
in	O
a	O
high-level	B-Language
language	I-Language
,	O
and	O
the	O
use	O
of	O
different	O
operating	B-General_Concept
systems	I-General_Concept
on	O
top	O
of	O
the	O
same	O
unchanged	O
kernel	B-Operating_System
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
dynamically	O
switch	O
among	O
operating	B-General_Concept
systems	I-General_Concept
and	O
to	O
have	O
more	O
than	O
one	O
active	O
simultaneously	O
.	O
</s>
<s>
As	O
the	O
computer	O
kernel	B-Operating_System
grows	O
,	O
so	O
grows	O
the	O
size	O
and	O
vulnerability	O
of	O
its	O
trusted	O
computing	O
base	O
;	O
and	O
,	O
besides	O
reducing	O
security	O
,	O
there	O
is	O
the	O
problem	O
of	O
enlarging	O
the	O
memory	O
footprint	O
.	O
</s>
<s>
This	O
is	O
mitigated	O
to	O
some	O
degree	O
by	O
perfecting	O
the	O
virtual	B-Architecture
memory	I-Architecture
system	O
,	O
but	O
not	O
all	O
computer	B-General_Concept
architectures	I-General_Concept
have	O
virtual	B-Architecture
memory	I-Architecture
support	O
.	O
</s>
<s>
To	O
reduce	O
the	O
kernel	B-Operating_System
's	O
footprint	O
,	O
extensive	O
editing	O
has	O
to	O
be	O
performed	O
to	O
carefully	O
remove	O
unneeded	O
code	O
,	O
which	O
can	O
be	O
very	O
difficult	O
with	O
non-obvious	O
interdependencies	O
between	O
parts	O
of	O
a	O
kernel	B-Operating_System
with	O
millions	O
of	O
lines	O
of	O
code	O
.	O
</s>
<s>
By	O
the	O
early	O
1990s	O
,	O
due	O
to	O
the	O
various	O
shortcomings	O
of	O
monolithic	B-Operating_System
kernels	I-Operating_System
versus	O
microkernels	B-Operating_System
,	O
monolithic	B-Operating_System
kernels	I-Operating_System
were	O
considered	O
obsolete	O
by	O
virtually	O
all	O
operating	B-General_Concept
system	I-General_Concept
researchers	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
design	O
of	O
Linux	B-Application
as	O
a	O
monolithic	B-Operating_System
kernel	I-Operating_System
rather	O
than	O
a	O
microkernel	B-Operating_System
was	O
the	O
topic	O
of	O
a	O
famous	O
debate	O
between	O
Linus	O
Torvalds	O
and	O
Andrew	O
Tanenbaum	O
.	O
</s>
<s>
There	O
is	O
merit	O
on	O
both	O
sides	O
of	O
the	O
argument	O
presented	O
in	O
the	O
Tanenbaum	B-Language
–	I-Language
Torvalds	I-Language
debate	I-Language
.	O
</s>
<s>
Monolithic	B-Operating_System
kernels	I-Operating_System
are	O
designed	O
to	O
have	O
all	O
of	O
their	O
code	O
in	O
the	O
same	O
address	B-General_Concept
space	I-General_Concept
(	O
kernel	B-Operating_System
space	I-Operating_System
)	O
,	O
which	O
some	O
developers	O
argue	O
is	O
necessary	O
to	O
increase	O
the	O
performance	O
of	O
the	O
system	O
.	O
</s>
<s>
The	O
monolithic	O
model	O
tends	O
to	O
be	O
more	O
efficient	O
through	O
the	O
use	O
of	O
shared	O
kernel	B-Operating_System
memory	O
,	O
rather	O
than	O
the	O
slower	O
IPC	O
system	O
of	O
microkernel	B-Operating_System
designs	O
,	O
which	O
is	O
typically	O
based	O
on	O
message	B-Architecture
passing	I-Architecture
.	O
</s>
<s>
The	O
performance	O
of	O
microkernels	B-Operating_System
was	O
poor	O
in	O
both	O
the	O
1980s	O
and	O
early	O
1990s	O
.	O
</s>
<s>
However	O
,	O
studies	O
that	O
empirically	O
measured	O
the	O
performance	O
of	O
these	O
microkernels	B-Operating_System
did	O
not	O
analyze	O
the	O
reasons	O
of	O
such	O
inefficiency	O
.	O
</s>
<s>
The	O
explanations	O
of	O
this	O
data	O
were	O
left	O
to	O
"	O
folklore	O
"	O
,	O
with	O
the	O
assumption	O
that	O
they	O
were	O
due	O
to	O
the	O
increased	O
frequency	O
of	O
switches	O
from	O
"	O
kernel-mode	B-Operating_System
"	O
to	O
"	O
user-mode	B-Operating_System
"	O
,	O
to	O
the	O
increased	O
frequency	O
of	O
inter-process	B-Operating_System
communication	I-Operating_System
and	O
to	O
the	O
increased	O
frequency	O
of	O
context	B-Operating_System
switches	I-Operating_System
.	O
</s>
<s>
In	O
fact	O
,	O
as	O
guessed	O
in	O
1995	O
,	O
the	O
reasons	O
for	O
the	O
poor	O
performance	O
of	O
microkernels	B-Operating_System
might	O
as	O
well	O
have	O
been	O
:	O
(	O
1	O
)	O
an	O
actual	O
inefficiency	O
of	O
the	O
whole	O
microkernel	B-Operating_System
approach	O
,	O
(	O
2	O
)	O
the	O
particular	O
concepts	O
implemented	O
in	O
those	O
microkernels	B-Operating_System
,	O
and	O
(	O
3	O
)	O
the	O
particular	O
implementation	O
of	O
those	O
concepts	O
.	O
</s>
<s>
Therefore	O
it	O
remained	O
to	O
be	O
studied	O
if	O
the	O
solution	O
to	O
build	O
an	O
efficient	O
microkernel	B-Operating_System
was	O
,	O
unlike	O
previous	O
attempts	O
,	O
to	O
apply	O
the	O
correct	O
construction	O
techniques	O
.	O
</s>
<s>
On	O
the	O
other	O
end	O
,	O
the	O
hierarchical	B-Operating_System
protection	I-Operating_System
domains	I-Operating_System
architecture	O
that	O
leads	O
to	O
the	O
design	O
of	O
a	O
monolithic	B-Operating_System
kernel	I-Operating_System
has	O
a	O
significant	O
performance	O
drawback	O
each	O
time	O
there	O
's	O
an	O
interaction	O
between	O
different	O
levels	O
of	O
protection	O
(	O
i.e.	O
,	O
when	O
a	O
process	B-Operating_System
has	O
to	O
manipulate	O
a	O
data	O
structure	O
both	O
in	O
"	O
user	B-Operating_System
mode	I-Operating_System
"	O
and	O
"	O
supervisor	B-Operating_System
mode	I-Operating_System
"	O
)	O
,	O
since	O
this	O
requires	O
message	O
copying	O
by	O
value	O
.	O
</s>
<s>
Hybrid	B-Operating_System
kernels	I-Operating_System
are	O
used	O
in	O
most	O
commercial	O
operating	B-General_Concept
systems	I-General_Concept
such	O
as	O
Microsoft	B-Device
Windows	I-Device
NT	I-Device
3.1	I-Device
,	O
NT	O
3.5	O
,	O
NT	O
3.51	O
,	O
NT	O
4.0	O
,	O
2000	O
,	O
XP	B-Application
,	O
Vista	O
,	O
7	O
,	O
8	O
,	O
8.1	O
and	O
10	O
.	O
</s>
<s>
Apple	O
's	O
own	O
macOS	B-Application
uses	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
called	O
XNU	B-Operating_System
which	O
is	O
based	O
upon	O
code	O
from	O
OSF/1	B-Operating_System
'	O
s	O
Mach	B-Operating_System
kernel	I-Operating_System
(	O
OSFMK	O
7.3	O
)	O
and	O
FreeBSD	B-Operating_System
's	O
monolithic	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
They	O
are	O
similar	O
to	O
micro	B-Operating_System
kernels	I-Operating_System
,	O
except	O
they	O
include	O
some	O
additional	O
code	O
in	O
kernel-space	B-Operating_System
to	O
increase	O
performance	O
.	O
</s>
<s>
These	O
kernels	B-Operating_System
represent	O
a	O
compromise	O
that	O
was	O
implemented	O
by	O
some	O
developers	O
to	O
accommodate	O
the	O
major	O
advantages	O
of	O
both	O
monolithic	O
and	O
micro	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
These	O
types	O
of	O
kernels	B-Operating_System
are	O
extensions	O
of	O
micro	B-Operating_System
kernels	I-Operating_System
with	O
some	O
properties	O
of	O
monolithic	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
Unlike	O
monolithic	B-Operating_System
kernels	I-Operating_System
,	O
these	O
types	O
of	O
kernels	B-Operating_System
are	O
unable	O
to	O
load	O
modules	B-Application
at	B-Library
runtime	I-Library
on	O
their	O
own	O
.	O
</s>
<s>
Hybrid	B-Operating_System
kernels	I-Operating_System
are	O
micro	B-Operating_System
kernels	I-Operating_System
that	O
have	O
some	O
"	O
non-essential	O
"	O
code	O
in	O
kernel-space	B-Operating_System
in	O
order	O
for	O
the	O
code	O
to	O
run	O
more	O
quickly	O
than	O
it	O
would	O
were	O
it	O
to	O
be	O
in	O
user-space	B-Operating_System
.	O
</s>
<s>
Hybrid	B-Operating_System
kernels	I-Operating_System
are	O
a	O
compromise	O
between	O
the	O
monolithic	O
and	O
microkernel	B-Operating_System
designs	O
.	O
</s>
<s>
This	O
implies	O
running	O
some	O
services	O
(	O
such	O
as	O
the	O
network	B-Protocol
stack	I-Protocol
or	O
the	O
filesystem	B-Application
)	O
in	O
kernel	B-Operating_System
space	I-Operating_System
to	O
reduce	O
the	O
performance	O
overhead	O
of	O
a	O
traditional	O
microkernel	B-Operating_System
,	O
but	O
still	O
running	O
kernel	B-Operating_System
code	O
(	O
such	O
as	O
device	B-Application
drivers	I-Application
)	O
as	O
servers	B-Application
in	O
user	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
Many	O
traditionally	O
monolithic	B-Operating_System
kernels	I-Operating_System
are	O
now	O
at	O
least	O
adding	O
(	O
or	O
else	O
using	O
)	O
the	O
module	B-Application
capability	O
.	O
</s>
<s>
The	O
most	O
well	O
known	O
of	O
these	O
kernels	B-Operating_System
is	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
The	O
modular	B-Application
kernel	I-Application
essentially	O
can	O
have	O
parts	O
of	O
it	O
that	O
are	O
built	O
into	O
the	O
core	O
kernel	B-Operating_System
binary	O
or	O
binaries	O
that	O
load	O
into	O
memory	O
on	O
demand	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
note	O
that	O
a	O
code	O
tainted	O
module	B-Application
has	O
the	O
potential	O
to	O
destabilize	O
a	O
running	O
kernel	B-Operating_System
.	O
</s>
<s>
Many	O
people	O
become	O
confused	O
on	O
this	O
point	O
when	O
discussing	O
micro	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
write	O
a	O
driver	O
for	O
a	O
microkernel	B-Operating_System
in	O
a	O
completely	O
separate	O
memory	O
space	O
and	O
test	O
it	O
before	O
"	O
going	O
"	O
live	O
.	O
</s>
<s>
When	O
a	O
kernel	B-Application
module	I-Application
is	O
loaded	O
,	O
it	O
accesses	O
the	O
monolithic	O
portion	O
's	O
memory	O
space	O
by	O
adding	O
to	O
it	O
what	O
it	O
needs	O
,	O
therefore	O
,	O
opening	O
the	O
doorway	O
to	O
possible	O
pollution	O
.	O
</s>
<s>
A	O
few	O
advantages	O
to	O
the	O
modular	B-Architecture
(	O
or	O
)	O
Hybrid	B-Operating_System
kernel	I-Operating_System
are	O
:	O
</s>
<s>
Faster	O
development	O
time	O
for	O
drivers	O
that	O
can	O
operate	O
from	O
within	O
modules	B-Application
.	O
</s>
<s>
No	O
reboot	O
required	O
for	O
testing	O
(	O
provided	O
the	O
kernel	B-Operating_System
is	O
not	O
destabilized	O
)	O
.	O
</s>
<s>
On	O
demand	O
capability	O
versus	O
spending	O
time	O
recompiling	O
a	O
whole	O
kernel	B-Operating_System
for	O
things	O
like	O
new	O
drivers	O
or	O
subsystems	O
.	O
</s>
<s>
Modules	B-Application
,	O
generally	O
,	O
communicate	O
with	O
the	O
kernel	B-Operating_System
using	O
a	O
module	B-Application
interface	B-General_Concept
of	O
some	O
sort	O
.	O
</s>
<s>
The	O
interface	B-General_Concept
is	O
generalized	O
(	O
although	O
particular	O
to	O
a	O
given	O
operating	B-General_Concept
system	I-General_Concept
)	O
so	O
it	O
is	O
not	O
always	O
possible	O
to	O
use	O
modules	B-Application
.	O
</s>
<s>
Often	O
the	O
device	B-Application
drivers	I-Application
may	O
need	O
more	O
flexibility	O
than	O
the	O
module	B-Application
interface	B-General_Concept
affords	O
.	O
</s>
<s>
Essentially	O
,	O
it	O
is	O
two	O
system	B-Operating_System
calls	I-Operating_System
and	O
often	O
the	O
safety	O
checks	O
that	O
only	O
have	O
to	O
be	O
done	O
once	O
in	O
the	O
monolithic	B-Operating_System
kernel	I-Operating_System
now	O
may	O
be	O
done	O
twice	O
.	O
</s>
<s>
Some	O
of	O
the	O
disadvantages	O
of	O
the	O
modular	B-Architecture
approach	O
are	O
:	O
</s>
<s>
Maintaining	O
modules	B-Application
can	O
be	O
confusing	O
for	O
some	O
administrators	O
when	O
dealing	O
with	O
problems	O
like	O
symbol	O
differences	O
.	O
</s>
<s>
A	O
nanokernel	O
delegates	O
virtually	O
all	O
services	O
including	O
even	O
the	O
most	O
basic	O
ones	O
like	O
interrupt	B-Architecture
controllers	I-Architecture
or	O
the	O
timer	O
to	O
device	B-Application
drivers	I-Application
to	O
make	O
the	O
kernel	B-Operating_System
memory	O
requirement	O
even	O
smaller	O
than	O
a	O
traditional	O
microkernel	B-Operating_System
.	O
</s>
<s>
Exokernels	B-Operating_System
are	O
a	O
still-experimental	O
approach	O
to	O
operating	B-General_Concept
system	I-General_Concept
design	O
.	O
</s>
<s>
They	O
differ	O
from	O
other	O
types	O
of	O
kernels	B-Operating_System
in	O
limiting	O
their	O
functionality	O
to	O
the	O
protection	O
and	O
multiplexing	O
of	O
the	O
raw	O
hardware	O
,	O
providing	O
no	O
hardware	O
abstractions	O
on	O
top	O
of	O
which	O
to	O
develop	O
applications	O
.	O
</s>
<s>
Exokernels	B-Operating_System
in	O
themselves	O
are	O
extremely	O
small	O
.	O
</s>
<s>
However	O
,	O
they	O
are	O
accompanied	O
by	O
library	O
operating	B-General_Concept
systems	I-General_Concept
(	O
see	O
also	O
unikernel	B-Operating_System
)	O
,	O
providing	O
application	O
developers	O
with	O
the	O
functionalities	O
of	O
a	O
conventional	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
This	O
comes	O
down	O
to	O
every	O
user	O
writing	O
their	O
own	O
rest-of-the	O
kernel	B-Operating_System
from	O
near	O
scratch	O
,	O
which	O
is	O
a	O
very-risky	O
,	O
complex	O
and	O
quite	O
a	O
daunting	O
assignment	O
-	O
particularly	O
in	O
a	O
time-constrained	O
production-oriented	O
environment	O
,	O
which	O
is	O
why	O
exokernels	B-Operating_System
have	O
never	O
caught	O
on	O
.	O
</s>
<s>
A	O
major	O
advantage	O
of	O
exokernel-based	O
systems	O
is	O
that	O
they	O
can	O
incorporate	O
multiple	O
library	O
operating	B-General_Concept
systems	I-General_Concept
,	O
each	O
exporting	O
a	O
different	O
API	B-General_Concept
,	O
for	O
example	O
one	O
for	O
high	O
level	O
UI	B-Application
development	O
and	O
one	O
for	O
real-time	B-General_Concept
control	I-General_Concept
.	O
</s>
<s>
A	O
multikernel	O
operating	B-General_Concept
system	I-General_Concept
treats	O
a	O
multi-core	B-Architecture
machine	O
as	O
a	O
network	O
of	O
independent	O
cores	O
,	O
as	O
if	O
it	O
were	O
a	O
distributed	B-Architecture
system	I-Architecture
.	O
</s>
<s>
It	O
does	O
not	O
assume	O
shared	B-Operating_System
memory	I-Operating_System
but	O
rather	O
implements	O
inter-process	B-Operating_System
communications	I-Operating_System
as	O
message	B-Architecture
passing	I-Architecture
.	O
</s>
<s>
Barrelfish	O
was	O
the	O
first	O
operating	B-General_Concept
system	I-General_Concept
to	O
be	O
described	O
as	O
a	O
multikernel	O
.	O
</s>
<s>
Strictly	O
speaking	O
,	O
an	O
operating	B-General_Concept
system	I-General_Concept
(	O
and	O
thus	O
,	O
a	O
kernel	B-Operating_System
)	O
is	O
not	O
required	O
to	O
run	O
a	O
computer	O
.	O
</s>
<s>
Programs	O
can	O
be	O
directly	O
loaded	O
and	O
executed	O
on	O
the	O
"	O
bare	O
metal	O
"	O
machine	O
,	O
provided	O
that	O
the	O
authors	O
of	O
those	O
programs	O
are	O
willing	O
to	O
work	O
without	O
any	O
hardware	O
abstraction	O
or	O
operating	B-General_Concept
system	I-General_Concept
support	O
.	O
</s>
<s>
Eventually	O
,	O
small	O
ancillary	O
programs	O
such	O
as	O
program	B-Operating_System
loaders	I-Operating_System
and	O
debuggers	B-Application
were	O
left	O
in	O
memory	O
between	O
runs	O
,	O
or	O
loaded	O
from	O
ROM	B-Device
.	O
</s>
<s>
As	O
these	O
were	O
developed	O
,	O
they	O
formed	O
the	O
basis	O
of	O
what	O
became	O
early	O
operating	B-Operating_System
system	I-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
The	O
"	O
bare	O
metal	O
"	O
approach	O
is	O
still	O
used	O
today	O
on	O
some	O
video	B-Device
game	I-Device
consoles	I-Device
and	O
embedded	B-Architecture
systems	I-Architecture
,	O
but	O
in	O
general	O
,	O
newer	O
computers	O
use	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
and	O
kernels	B-Operating_System
.	O
</s>
<s>
In	O
1969	O
,	O
the	O
RC	B-Operating_System
4000	I-Operating_System
Multiprogramming	I-Operating_System
System	I-Operating_System
introduced	O
the	O
system	O
design	O
philosophy	O
of	O
a	O
small	O
nucleus	B-Operating_System
"	O
upon	O
which	O
operating	B-General_Concept
systems	I-General_Concept
for	O
different	O
purposes	O
could	O
be	O
built	O
in	O
an	O
orderly	O
manner	O
"	O
,	O
what	O
would	O
be	O
called	O
the	O
microkernel	B-Operating_System
approach	O
.	O
</s>
<s>
In	O
the	O
decade	O
preceding	O
Unix	B-Application
,	O
computers	O
had	O
grown	O
enormously	O
in	O
power	O
to	O
the	O
point	O
where	O
computer	O
operators	O
were	O
looking	O
for	O
new	O
ways	O
to	O
get	O
people	O
to	O
use	O
their	O
spare	O
time	O
on	O
their	O
machines	O
.	O
</s>
<s>
One	O
of	O
the	O
major	O
developments	O
during	O
this	O
era	O
was	O
time-sharing	B-General_Concept
,	O
whereby	O
a	O
number	O
of	O
users	O
would	O
get	O
small	O
slices	O
of	O
computer	O
time	O
,	O
at	O
a	O
rate	O
at	O
which	O
it	O
appeared	O
they	O
were	O
each	O
connected	O
to	O
their	O
own	O
,	O
slower	O
,	O
machine	O
.	O
</s>
<s>
The	O
development	O
of	O
time-sharing	B-General_Concept
systems	I-General_Concept
led	O
to	O
a	O
number	O
of	O
problems	O
.	O
</s>
<s>
One	O
was	O
that	O
users	O
,	O
particularly	O
at	O
universities	O
where	O
the	O
systems	O
were	O
being	O
developed	O
,	O
seemed	O
to	O
want	O
to	O
hack	O
the	O
system	O
to	O
get	O
more	O
CPU	B-General_Concept
time	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
security	O
and	O
access	O
control	O
became	O
a	O
major	O
focus	O
of	O
the	O
Multics	B-Application
project	O
in	O
1965	O
.	O
</s>
<s>
Another	O
ongoing	O
issue	O
was	O
properly	O
handling	O
computing	O
resources	O
:	O
users	O
spent	O
most	O
of	O
their	O
time	O
staring	O
at	O
the	O
terminal	B-General_Concept
and	O
thinking	O
about	O
what	O
to	O
input	O
instead	O
of	O
actually	O
using	O
the	O
resources	O
of	O
the	O
computer	O
,	O
and	O
a	O
time-sharing	B-General_Concept
system	I-General_Concept
should	O
give	O
the	O
CPU	B-General_Concept
time	O
to	O
an	O
active	O
user	O
during	O
these	O
periods	O
.	O
</s>
<s>
Finally	O
,	O
the	O
systems	O
typically	O
offered	O
a	O
memory	B-General_Concept
hierarchy	I-General_Concept
several	O
layers	O
deep	O
,	O
and	O
partitioning	O
this	O
expensive	O
resource	O
led	O
to	O
major	O
developments	O
in	O
virtual	B-Architecture
memory	I-Architecture
systems	O
.	O
</s>
<s>
The	O
Commodore	B-Device
Amiga	I-Device
was	O
released	O
in	O
1985	O
,	O
and	O
was	O
among	O
the	O
first	O
and	O
certainly	O
most	O
successful	O
home	O
computers	O
to	O
feature	O
an	O
advanced	O
kernel	B-Operating_System
architecture	O
.	O
</s>
<s>
The	O
AmigaOS	O
kernel	B-Operating_System
's	O
executive	O
component	O
,	O
exec.library	O
,	O
uses	O
a	O
microkernel	B-Operating_System
message-passing	B-Architecture
design	O
,	O
but	O
there	O
are	O
other	O
kernel	B-Operating_System
components	O
,	O
like	O
graphics.library	O
,	O
that	O
have	O
direct	O
access	O
to	O
the	O
hardware	O
.	O
</s>
<s>
There	O
is	O
no	O
memory	B-General_Concept
protection	I-General_Concept
,	O
and	O
the	O
kernel	B-Operating_System
is	O
almost	O
always	O
running	O
in	O
user	B-Operating_System
mode	I-Operating_System
.	O
</s>
<s>
Only	O
special	O
actions	O
are	O
executed	O
in	O
kernel	B-Operating_System
mode	O
,	O
and	O
user-mode	B-Operating_System
applications	O
can	O
ask	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
execute	O
their	O
code	O
in	O
kernel	B-Operating_System
mode	O
.	O
</s>
<s>
During	O
the	O
design	O
phase	O
of	O
Unix	B-Application
,	O
programmers	O
decided	O
to	O
model	O
every	O
high-level	O
device	B-Application
as	I-Application
a	I-Application
file	I-Application
,	O
because	O
they	O
believed	O
the	O
purpose	O
of	O
computation	O
was	O
data	O
transformation	O
.	O
</s>
<s>
Other	O
systems	O
,	O
to	O
provide	O
a	O
similar	O
functionality	O
,	O
tended	O
to	O
virtualize	B-General_Concept
devices	O
at	O
a	O
lower	B-Application
level	I-Application
that	O
is	O
,	O
both	O
devices	O
and	O
files	O
would	O
be	O
instances	O
of	O
some	O
lower	B-Application
level	I-Application
concept	O
.	O
</s>
<s>
Virtualizing	B-General_Concept
the	O
system	O
at	O
the	O
file	O
level	O
allowed	O
users	O
to	O
manipulate	O
the	O
entire	O
system	O
using	O
their	O
existing	O
file	B-Application
management	I-Application
utilities	O
and	O
concepts	O
,	O
dramatically	O
simplifying	O
operation	O
.	O
</s>
<s>
As	O
an	O
extension	O
of	O
the	O
same	O
paradigm	O
,	O
Unix	B-Application
allows	O
programmers	O
to	O
manipulate	O
files	O
using	O
a	O
series	O
of	O
small	O
programs	O
,	O
using	O
the	O
concept	O
of	O
pipes	B-Operating_System
,	O
which	O
allowed	O
users	O
to	O
complete	O
operations	O
in	O
stages	O
,	O
feeding	O
a	O
file	O
through	O
a	O
chain	O
of	O
single-purpose	O
tools	O
.	O
</s>
<s>
In	O
the	O
Unix	B-Application
model	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
consists	O
of	O
two	O
parts	O
:	O
first	O
,	O
the	O
huge	O
collection	O
of	O
utility	O
programs	O
that	O
drive	O
most	O
operations	O
;	O
second	O
,	O
the	O
kernel	B-Operating_System
that	O
runs	O
the	O
programs	O
.	O
</s>
<s>
Under	O
Unix	B-Application
,	O
from	O
a	O
programming	O
standpoint	O
,	O
the	O
distinction	O
between	O
the	O
two	O
is	O
fairly	O
thin	O
;	O
the	O
kernel	B-Operating_System
is	O
a	O
program	O
,	O
running	O
in	O
supervisor	B-Operating_System
mode	I-Operating_System
,	O
that	O
acts	O
as	O
a	O
program	B-Operating_System
loader	I-Operating_System
and	O
supervisor	O
for	O
the	O
small	O
utility	O
programs	O
making	O
up	O
the	O
rest	O
of	O
the	O
system	O
,	O
and	O
to	O
provide	O
locking	B-Operating_System
and	O
I/O	B-General_Concept
services	O
for	O
these	O
programs	O
;	O
beyond	O
that	O
,	O
the	O
kernel	B-Operating_System
did	O
n't	O
intervene	O
at	O
all	O
in	O
user	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
Over	O
the	O
years	O
the	O
computing	O
model	O
changed	O
,	O
and	O
Unix	B-Application
's	O
treatment	O
of	O
everything	B-Application
as	I-Application
a	I-Application
file	I-Application
or	O
byte	O
stream	O
no	O
longer	O
was	O
as	O
universally	O
applicable	O
as	O
it	O
was	O
before	O
.	O
</s>
<s>
Although	O
a	O
terminal	B-General_Concept
could	O
be	O
treated	O
as	O
a	O
file	O
or	O
a	O
byte	O
stream	O
,	O
which	O
is	O
printed	O
to	O
or	O
read	O
from	O
,	O
the	O
same	O
did	O
not	O
seem	O
to	O
be	O
true	O
for	O
a	O
graphical	B-Application
user	I-Application
interface	I-Application
.	O
</s>
<s>
Networking	B-Architecture
posed	O
another	O
problem	O
.	O
</s>
<s>
Even	O
if	O
network	O
communication	O
can	O
be	O
compared	O
to	O
file	B-Application
access	I-Application
,	O
the	O
low-level	O
packet-oriented	O
architecture	O
dealt	O
with	O
discrete	O
chunks	O
of	O
data	O
and	O
not	O
with	O
whole	O
files	O
.	O
</s>
<s>
As	O
the	O
capability	O
of	O
computers	O
grew	O
,	O
Unix	B-Application
became	O
increasingly	O
cluttered	O
with	O
code	O
.	O
</s>
<s>
It	O
is	O
also	O
because	O
the	O
modularity	B-Architecture
of	O
the	O
Unix	B-Application
kernel	B-Operating_System
is	O
extensively	O
scalable	O
.	O
</s>
<s>
While	O
kernels	B-Operating_System
might	O
have	O
had	O
100,000	O
lines	O
of	O
code	O
in	O
the	O
seventies	O
and	O
eighties	O
,	O
kernels	B-Operating_System
like	O
Linux	B-Application
,	O
of	O
modern	O
Unix	B-Application
successors	O
like	O
GNU	B-Operating_System
,	O
have	O
more	O
than	O
13	O
million	O
lines	O
.	O
</s>
<s>
Modern	O
Unix-derivatives	O
are	O
generally	O
based	O
on	O
module-loading	O
monolithic	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
Examples	O
of	O
this	O
are	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
in	O
the	O
many	O
distributions	B-Application
of	O
GNU	B-Operating_System
,	O
IBM	B-Application
AIX	I-Application
,	O
as	O
well	O
as	O
the	O
Berkeley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
variant	O
kernels	B-Operating_System
such	O
as	O
FreeBSD	B-Operating_System
,	O
DragonFly	B-Application
BSD	I-Application
,	O
OpenBSD	B-Operating_System
,	O
NetBSD	B-Device
,	O
and	O
macOS	B-Application
.	O
</s>
<s>
Apart	O
from	O
these	O
alternatives	O
,	O
amateur	O
developers	O
maintain	O
an	O
active	O
operating	B-Operating_System
system	I-Operating_System
development	I-Operating_System
community	I-Operating_System
,	O
populated	O
by	O
self-written	O
hobby	O
kernels	B-Operating_System
which	O
mostly	O
end	O
up	O
sharing	O
many	O
features	O
with	O
Linux	B-Application
,	O
FreeBSD	B-Operating_System
,	O
DragonflyBSD	B-Application
,	O
OpenBSD	B-Operating_System
or	O
NetBSD	B-Device
kernels	I-Device
and/or	O
being	O
compatible	O
with	O
them	O
.	O
</s>
<s>
Apple	O
first	O
launched	O
its	O
classic	B-Application
Mac	I-Application
OS	I-Application
in	O
1984	O
,	O
bundled	O
with	O
its	O
Macintosh	B-Device
personal	B-Device
computer	I-Device
.	O
</s>
<s>
Apple	O
moved	O
to	O
a	O
nanokernel	O
design	O
in	O
Mac	B-Application
OS	I-Application
8.6	O
.	O
</s>
<s>
Against	O
this	O
,	O
the	O
modern	O
macOS	B-Application
(	O
originally	O
named	O
Mac	B-Application
OS	I-Application
X	O
)	O
is	O
based	O
on	O
Darwin	B-Operating_System
,	O
which	O
uses	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
called	O
XNU	B-Operating_System
,	O
which	O
was	O
created	O
by	O
combining	O
the	O
4.3BSD	O
kernel	B-Operating_System
and	O
the	O
Mach	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
was	O
first	O
released	O
in	O
1985	O
as	O
an	O
add-on	O
to	O
MS-DOS	B-Application
.	O
</s>
<s>
Because	O
of	O
its	O
dependence	O
on	O
another	O
operating	B-General_Concept
system	I-General_Concept
,	O
initial	O
releases	O
of	O
Windows	O
,	O
prior	O
to	O
Windows	O
95	O
,	O
were	O
considered	O
an	O
operating	B-Application
environment	I-Application
(	O
not	O
to	O
be	O
confused	O
with	O
an	O
operating	B-General_Concept
system	I-General_Concept
)	O
.	O
</s>
<s>
This	O
product	O
line	O
continued	O
to	O
evolve	O
through	O
the	O
1980s	O
and	O
1990s	O
,	O
with	O
the	O
Windows	O
9x	O
series	O
adding	O
32-bit	O
addressing	O
and	O
pre-emptive	O
multitasking	B-Operating_System
;	O
but	O
ended	O
with	O
the	O
release	O
of	O
Windows	B-Device
Me	I-Device
in	O
2000	O
.	O
</s>
<s>
Microsoft	O
also	O
developed	O
Windows	B-Device
NT	I-Device
,	O
an	O
operating	B-General_Concept
system	I-General_Concept
with	O
a	O
very	O
similar	O
interface	B-General_Concept
,	O
but	O
intended	O
for	O
high-end	O
and	O
business	O
users	O
.	O
</s>
<s>
This	O
line	O
started	O
with	O
the	O
release	O
of	O
Windows	B-Device
NT	I-Device
3.1	I-Device
in	O
1993	O
,	O
and	O
was	O
introduced	O
to	O
general	O
users	O
with	O
the	O
release	O
of	O
Windows	B-Application
XP	I-Application
in	O
October	O
2001	O
—	O
replacing	O
Windows	O
9x	O
with	O
a	O
completely	O
different	O
,	O
much	O
more	O
sophisticated	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
This	O
is	O
the	O
line	O
that	O
continues	O
with	O
Windows	B-Application
11	I-Application
.	O
</s>
<s>
The	O
architecture	O
of	O
Windows	B-Device
NT	I-Device
's	O
kernel	B-Operating_System
is	O
considered	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
because	O
the	O
kernel	B-Operating_System
itself	O
contains	O
tasks	O
such	O
as	O
the	O
Window	O
Manager	O
and	O
the	O
IPC	O
Managers	O
,	O
with	O
a	O
client/server	O
layered	O
subsystem	O
model	O
.	O
</s>
<s>
It	O
was	O
designed	O
as	O
a	O
modified	O
microkernel	B-Operating_System
,	O
as	O
the	O
Windows	B-Device
NT	I-Device
kernel	B-Operating_System
was	O
influenced	O
by	O
the	O
Mach	B-Operating_System
microkernel	I-Operating_System
but	O
does	O
not	O
meet	O
all	O
of	O
the	O
criteria	O
of	O
a	O
pure	O
microkernel	B-Operating_System
.	O
</s>
<s>
Supervisory	O
program	O
or	O
supervisor	O
is	O
a	O
computer	B-Application
program	I-Application
,	O
usually	O
part	O
of	O
an	O
operating	B-General_Concept
system	I-General_Concept
,	O
that	O
controls	O
the	O
execution	O
of	O
other	O
routines	O
and	O
regulates	O
work	O
scheduling	O
,	O
input/output	B-General_Concept
operations	O
,	O
error	B-General_Concept
actions	I-General_Concept
,	O
and	O
similar	O
functions	O
and	O
regulates	O
the	O
flow	O
of	O
work	O
in	O
a	O
data	B-General_Concept
processing	I-General_Concept
system	O
.	O
</s>
<s>
Historically	O
,	O
this	O
term	O
was	O
essentially	O
associated	O
with	O
IBM	O
's	O
line	O
of	O
mainframe	B-Architecture
operating	B-General_Concept
systems	I-General_Concept
starting	O
with	O
OS/360	B-Application
.	O
</s>
<s>
In	O
other	O
operating	B-General_Concept
systems	I-General_Concept
,	O
the	O
supervisor	O
is	O
generally	O
called	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
In	O
the	O
1970s	O
,	O
IBM	O
further	O
abstracted	O
the	O
supervisor	O
state	B-Application
from	O
the	O
hardware	O
,	O
resulting	O
in	O
a	O
hypervisor	B-Operating_System
that	O
enabled	O
full	B-Architecture
virtualization	I-Architecture
,	O
i.e.	O
</s>
<s>
the	O
capacity	O
to	O
run	O
multiple	O
operating	B-General_Concept
systems	I-General_Concept
on	O
the	O
same	O
machine	O
totally	O
independently	O
from	O
each	O
other	O
.	O
</s>
<s>
Hence	O
the	O
first	O
such	O
system	O
was	O
called	O
Virtual	O
Machine	O
or	O
VM	B-Application
.	O
</s>
<s>
Although	O
Mach	B-Operating_System
,	O
developed	O
by	O
Richard	O
Rashid	O
at	O
Carnegie	O
Mellon	O
University	O
,	O
is	O
the	O
best-known	O
general-purpose	O
microkernel	B-Operating_System
,	O
other	O
microkernels	B-Operating_System
have	O
been	O
developed	O
with	O
more	O
specific	O
aims	O
.	O
</s>
<s>
The	O
L4	B-Operating_System
microkernel	I-Operating_System
family	I-Operating_System
(	O
mainly	O
the	O
L3	B-Operating_System
and	O
the	O
L4	B-Operating_System
kernel	I-Operating_System
)	O
was	O
created	O
to	O
demonstrate	O
that	O
microkernels	B-Operating_System
are	O
not	O
necessarily	O
slow	O
.	O
</s>
<s>
Newer	O
implementations	O
such	O
as	O
Fiasco	O
and	O
Pistachio	O
are	O
able	O
to	O
run	O
Linux	B-Application
next	O
to	O
other	O
L4	B-Operating_System
processes	O
in	O
separate	O
address	B-General_Concept
spaces	I-General_Concept
.	O
</s>
<s>
Additionally	O
,	O
QNX	B-Operating_System
is	O
a	O
microkernel	B-Operating_System
which	O
is	O
principally	O
used	O
in	O
embedded	B-Architecture
systems	I-Architecture
,	O
and	O
the	O
open-source	B-Application
software	I-Application
MINIX	B-Operating_System
,	O
while	O
originally	O
created	O
for	O
educational	O
purposes	O
,	O
is	O
now	O
focused	O
on	O
being	O
a	O
highly	O
reliable	O
and	O
self-healing	B-General_Concept
microkernel	B-Operating_System
OS	B-General_Concept
.	O
</s>
