<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
microkernel	B-Operating_System
(	O
often	O
abbreviated	O
as	O
μ-kernel	O
)	O
is	O
the	O
near-minimum	O
amount	O
of	O
software	O
that	O
can	O
provide	O
the	O
mechanisms	O
needed	O
to	O
implement	O
an	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
.	O
</s>
<s>
These	O
mechanisms	O
include	O
low-level	O
address	B-General_Concept
space	I-General_Concept
management	O
,	O
thread	B-Operating_System
management	I-Operating_System
,	O
and	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	B-Operating_System
)	O
.	O
</s>
<s>
If	O
the	O
hardware	O
provides	O
multiple	O
rings	B-Operating_System
or	O
CPU	B-General_Concept
modes	I-General_Concept
,	O
the	O
microkernel	B-Operating_System
may	O
be	O
the	O
only	O
software	O
executing	O
at	O
the	O
most	O
privileged	O
level	O
,	O
which	O
is	O
generally	O
referred	O
to	O
as	O
supervisor	O
or	O
kernel	B-Operating_System
mode	O
.	O
</s>
<s>
Traditional	O
operating	B-General_Concept
system	I-General_Concept
functions	O
,	O
such	O
as	O
device	B-Application
drivers	I-Application
,	O
protocol	B-Protocol
stacks	I-Protocol
and	O
file	B-Application
systems	I-Application
,	O
are	O
typically	O
removed	O
from	O
the	O
microkernel	B-Operating_System
itself	O
and	O
are	O
instead	O
run	O
in	O
user	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
In	O
terms	O
of	O
the	O
source	O
code	O
size	O
,	O
microkernels	B-Operating_System
are	O
often	O
smaller	O
than	O
monolithic	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
The	O
MINIX	B-Operating_System
3	O
microkernel	B-Operating_System
,	O
for	O
example	O
,	O
has	O
only	O
approximately	O
12,000	O
lines	O
of	O
code	O
.	O
</s>
<s>
Microkernels	B-Operating_System
trace	O
their	O
roots	O
back	O
to	O
Danish	O
computer	O
pioneer	O
Per	O
Brinch	O
Hansen	O
and	O
his	O
tenure	O
in	O
Danish	O
computer	O
company	O
Regnecentralen	O
where	O
he	O
led	O
software	O
development	O
efforts	O
for	O
the	O
RC	O
4000	O
computer	O
.	O
</s>
<s>
The	O
computer	O
used	O
a	O
small	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
tailored	O
for	O
the	O
needs	O
of	O
the	O
plant	O
.	O
</s>
<s>
They	O
feared	O
that	O
each	O
installation	O
would	O
require	O
a	O
different	O
operating	B-General_Concept
system	I-General_Concept
so	O
they	O
started	O
to	O
investigate	O
novel	O
and	O
more	O
general	O
ways	O
of	O
creating	O
software	O
for	O
the	O
RC	O
4000	O
.	O
</s>
<s>
In	O
1969	O
,	O
their	O
effort	O
resulted	O
in	O
the	O
completion	O
of	O
the	O
RC	B-Operating_System
4000	I-Operating_System
Multiprogramming	I-Operating_System
System	I-Operating_System
.	O
</s>
<s>
Its	O
nucleus	B-Operating_System
provided	O
inter-process	B-Operating_System
communication	I-Operating_System
based	O
on	O
message-passing	B-Architecture
for	O
up	O
to	O
23	O
unprivileged	O
processes	O
,	O
out	O
of	O
which	O
8	O
at	O
a	O
time	O
were	O
protected	O
from	O
one	O
another	O
.	O
</s>
<s>
This	O
strategy	O
was	O
to	O
be	O
implemented	O
by	O
a	O
hierarchy	O
of	O
running	O
programs	O
in	O
which	O
parent	O
processes	O
had	O
complete	O
control	O
over	O
child	O
processes	O
and	O
acted	O
as	O
their	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Following	O
Brinch	O
Hansen	O
's	O
work	O
,	O
microkernels	B-Operating_System
have	O
been	O
developed	O
since	O
the	O
1970s	O
.	O
</s>
<s>
The	O
term	O
microkernel	B-Operating_System
itself	O
first	O
appeared	O
no	O
later	O
than	O
1981	O
.	O
</s>
<s>
Microkernels	B-Operating_System
were	O
meant	O
as	O
a	O
response	O
to	O
changes	O
in	O
the	O
computer	O
world	O
,	O
and	O
to	O
several	O
challenges	O
adapting	O
existing	O
"	O
mono-kernels	B-Operating_System
"	O
to	O
these	O
new	O
systems	O
.	O
</s>
<s>
New	O
device	B-Application
drivers	I-Application
,	O
protocol	B-Protocol
stacks	I-Protocol
,	O
file	B-Application
systems	I-Application
and	O
other	O
low-level	O
systems	O
were	O
being	O
developed	O
all	O
the	O
time	O
.	O
</s>
<s>
This	O
code	O
was	O
normally	O
located	O
in	O
the	O
monolithic	B-Operating_System
kernel	I-Operating_System
,	O
and	O
thus	O
required	O
considerable	O
work	O
and	O
careful	O
code	O
management	O
to	O
work	O
on	O
.	O
</s>
<s>
Microkernels	B-Operating_System
were	O
developed	O
with	O
the	O
idea	O
that	O
all	O
of	O
these	O
services	O
would	O
be	O
implemented	O
as	O
user-space	B-Operating_System
programs	O
,	O
like	O
any	O
other	O
,	O
allowing	O
them	O
to	O
be	O
worked	O
on	O
monolithically	O
and	O
started	O
and	O
stopped	O
like	O
any	O
other	O
program	O
.	O
</s>
<s>
This	O
would	O
not	O
only	O
allow	O
these	O
services	O
to	O
be	O
more	O
easily	O
worked	O
on	O
,	O
but	O
also	O
separated	O
the	O
kernel	B-Operating_System
code	O
to	O
allow	O
it	O
to	O
be	O
finely	O
tuned	O
without	O
worrying	O
about	O
unintended	O
side	O
effects	O
.	O
</s>
<s>
Moreover	O
,	O
it	O
would	O
allow	O
entirely	O
new	O
operating	B-General_Concept
systems	I-General_Concept
to	O
be	O
"	O
built	O
up	O
"	O
on	O
a	O
common	O
core	O
,	O
aiding	O
OS	O
research	O
.	O
</s>
<s>
Microkernels	B-Operating_System
were	O
a	O
very	O
hot	O
topic	O
in	O
the	O
1980s	O
when	O
the	O
first	O
usable	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
were	O
being	O
introduced	O
..	O
</s>
<s>
The	O
AmigaOS	O
Exec	B-Operating_System
kernel	B-Operating_System
was	O
an	O
early	O
example	O
,	O
introduced	O
in	O
1986	O
and	O
used	O
in	O
a	O
PC	O
with	O
relative	O
commercial	O
success	O
.	O
</s>
<s>
The	O
lack	O
of	O
memory	O
protection	O
,	O
considered	O
in	O
other	O
respects	O
a	O
flaw	O
,	O
allowed	O
this	O
kernel	B-Operating_System
to	O
have	O
very	O
high	O
message-passing	B-Architecture
performance	O
because	O
it	O
did	O
not	O
need	O
to	O
copy	O
data	O
while	O
exchanging	O
messages	B-Architecture
between	O
user-space	B-Operating_System
programs	O
.	O
</s>
<s>
The	O
same	O
mechanisms	O
that	O
allowed	O
the	O
kernel	B-Operating_System
to	O
be	O
distributed	O
into	O
user	B-Operating_System
space	I-Operating_System
also	O
allowed	O
the	O
system	O
to	O
be	O
distributed	O
across	O
network	O
links	O
.	O
</s>
<s>
The	O
first	O
microkernels	B-Operating_System
,	O
notably	O
Mach	B-Operating_System
created	O
by	O
Richard	O
Rashid	O
,	O
proved	O
to	O
have	O
disappointing	O
performance	O
,	O
but	O
the	O
inherent	O
advantages	O
appeared	O
so	O
great	O
that	O
it	O
was	O
a	O
major	O
line	O
of	O
research	O
into	O
the	O
late	O
1990s	O
.	O
</s>
<s>
Many	O
attempts	O
were	O
made	O
to	O
adapt	O
the	O
existing	O
systems	O
to	O
have	O
better	O
performance	O
,	O
but	O
the	O
overhead	O
was	O
always	O
considerable	O
and	O
most	O
of	O
these	O
efforts	O
required	O
the	O
user-space	B-Operating_System
programs	O
to	O
be	O
moved	O
back	O
into	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
By	O
2000	O
,	O
most	O
large-scale	O
Mach	B-Operating_System
kernel	I-Operating_System
efforts	O
had	O
ended	O
,	O
although	O
Apple	O
's	O
macOS	B-Application
,	O
released	O
in	O
2001	O
,	O
still	O
uses	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
called	O
XNU	B-Operating_System
,	O
which	O
combines	O
a	O
heavily	O
modified	O
(	O
hybrid	O
)	O
OSF/1	B-Operating_System
'	O
s	O
Mach	B-Operating_System
kernel	I-Operating_System
(	O
OSFMK	O
7.3	O
kernel	B-Operating_System
)	O
with	O
code	O
from	O
BSD	B-Operating_System
UNIX	I-Operating_System
,	O
and	O
this	O
kernel	B-Operating_System
is	O
also	O
used	O
in	O
iOS	B-Application
,	O
tvOS	B-Operating_System
,	O
and	O
watchOS	B-Device
.	O
</s>
<s>
Windows	B-Device
NT	I-Device
,	O
starting	O
with	O
NT	B-Device
3.1	I-Device
and	O
continuing	O
with	O
Windows	B-Application
11	I-Application
,	O
uses	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
design	O
.	O
</s>
<s>
,	O
the	O
Mach-based	O
GNU	B-Application
Hurd	I-Application
is	O
also	O
functional	O
and	O
included	O
in	O
testing	O
versions	O
of	O
Arch	O
Linux	B-Application
and	O
Debian	O
.	O
</s>
<s>
Although	O
major	O
work	O
on	O
microkernels	B-Operating_System
had	O
largely	O
ended	O
,	O
experimenters	O
continued	O
development	O
.	O
</s>
<s>
Using	O
a	O
more	O
pragmatic	O
approach	O
to	O
the	O
problem	O
,	O
including	O
assembly	B-Language
code	I-Language
and	O
relying	O
on	O
the	O
processor	O
to	O
enforce	O
concepts	O
normally	O
supported	O
in	O
software	O
led	O
to	O
a	O
new	O
series	O
of	O
microkernels	B-Operating_System
with	O
dramatically	O
improved	O
performance	O
.	O
</s>
<s>
Microkernels	B-Operating_System
are	O
closely	O
related	O
to	O
exokernels	B-Operating_System
.	O
</s>
<s>
They	O
also	O
have	O
much	O
in	O
common	O
with	O
hypervisors	B-Operating_System
,	O
but	O
the	O
latter	O
make	O
no	O
claim	O
to	O
minimality	O
and	O
are	O
specialized	O
to	O
supporting	O
virtual	B-Architecture
machines	I-Architecture
;	O
the	O
L4	B-Operating_System
microkernel	I-Operating_System
frequently	O
finds	O
use	O
in	O
a	O
hypervisor	B-Operating_System
capacity	O
.	O
</s>
<s>
Early	O
operating	B-Operating_System
system	I-Operating_System
kernels	I-Operating_System
were	O
rather	O
small	O
,	O
partly	O
because	O
computer	O
memory	O
was	O
limited	O
.	O
</s>
<s>
As	O
the	O
capability	O
of	O
computers	O
grew	O
,	O
the	O
number	O
of	O
devices	O
the	O
kernel	B-Operating_System
had	O
to	O
control	O
also	O
grew	O
.	O
</s>
<s>
Throughout	O
the	O
early	O
history	O
of	O
Unix	B-Application
,	O
kernels	B-Operating_System
were	O
generally	O
small	O
,	O
even	O
though	O
they	O
contained	O
various	O
device	B-Application
drivers	I-Application
and	O
file	B-Application
system	I-Application
implementations	O
.	O
</s>
<s>
When	O
address	B-General_Concept
spaces	I-General_Concept
increased	O
from	O
16	O
to	O
32	O
bits	O
,	O
kernel	B-Operating_System
design	I-Operating_System
was	O
no	O
longer	O
constrained	O
by	O
the	O
hardware	O
architecture	O
,	O
and	O
kernels	B-Operating_System
began	O
to	O
grow	O
larger	O
.	O
</s>
<s>
The	O
Berkeley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
(	O
BSD	B-Operating_System
)	O
of	O
Unix	B-Application
began	O
the	O
era	O
of	O
larger	O
kernels	B-Operating_System
.	O
</s>
<s>
In	O
addition	O
to	O
operating	O
a	O
basic	O
system	O
consisting	O
of	O
the	O
CPU	O
,	O
disks	O
and	O
printers	O
,	O
BSD	B-Operating_System
added	O
a	O
complete	O
TCP/IP	B-Protocol
networking	I-Protocol
system	I-Protocol
and	O
a	O
number	O
of	O
"	O
virtual	O
"	O
devices	O
that	O
allowed	O
the	O
existing	O
programs	O
to	O
work	O
'	O
invisibly	O
 '	O
over	O
the	O
network	O
.	O
</s>
<s>
This	O
growth	O
continued	O
for	O
many	O
years	O
,	O
resulting	O
in	O
kernels	B-Operating_System
with	O
millions	O
of	O
lines	O
of	O
source	O
code	O
.	O
</s>
<s>
As	O
a	O
result	O
of	O
this	O
growth	O
,	O
kernels	B-Operating_System
were	O
prone	O
to	O
bugs	O
and	O
became	O
increasingly	O
difficult	O
to	O
maintain	O
.	O
</s>
<s>
The	O
microkernel	B-Operating_System
was	O
intended	O
to	O
address	O
this	O
growth	O
of	O
kernels	B-Operating_System
and	O
the	O
difficulties	O
that	O
resulted	O
.	O
</s>
<s>
In	O
theory	O
,	O
the	O
microkernel	B-Operating_System
design	O
allows	O
for	O
easier	O
management	O
of	O
code	O
due	O
to	O
its	O
division	O
into	O
user	B-Operating_System
space	I-Operating_System
services	O
.	O
</s>
<s>
This	O
also	O
allows	O
for	O
increased	O
security	O
and	O
stability	O
resulting	O
from	O
the	O
reduced	O
amount	O
of	O
code	O
running	O
in	O
kernel	B-Operating_System
mode	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
networking	O
service	O
crashed	O
due	O
to	O
buffer	B-General_Concept
overflow	I-General_Concept
,	O
only	O
the	O
networking	O
service	O
's	O
memory	O
would	O
be	O
corrupted	O
,	O
leaving	O
the	O
rest	O
of	O
the	O
system	O
still	O
functional	O
.	O
</s>
<s>
Inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	B-Operating_System
)	O
is	O
any	O
mechanism	O
which	O
allows	O
separate	O
processes	O
to	O
communicate	O
with	O
each	O
other	O
,	O
usually	O
by	O
sending	O
messages	B-Architecture
.	O
</s>
<s>
Shared	B-Operating_System
memory	I-Operating_System
is	O
,	O
strictly	O
defined	O
,	O
also	O
an	O
inter-process	B-Operating_System
communication	I-Operating_System
mechanism	O
,	O
but	O
the	O
abbreviation	O
IPC	B-Operating_System
usually	O
refers	O
to	O
message	B-Architecture
passing	I-Architecture
only	O
,	O
and	O
it	O
is	O
the	O
latter	O
that	O
is	O
particularly	O
relevant	O
to	O
microkernels	B-Operating_System
.	O
</s>
<s>
IPC	B-Operating_System
allows	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
be	O
built	O
from	O
a	O
number	O
of	O
smaller	O
programs	O
called	O
servers	O
,	O
which	O
are	O
used	O
by	O
other	O
programs	O
on	O
the	O
system	O
,	O
invoked	O
via	O
IPC	B-Operating_System
.	O
</s>
<s>
Most	O
or	O
all	O
support	O
for	O
peripheral	O
hardware	O
is	O
handled	O
in	O
this	O
fashion	O
,	O
with	O
servers	O
for	O
device	B-Application
drivers	I-Application
,	O
network	B-Protocol
protocol	I-Protocol
stacks	I-Protocol
,	O
file	B-Application
systems	I-Application
,	O
graphics	O
,	O
etc	O
.	O
</s>
<s>
IPC	B-Operating_System
can	O
be	O
synchronous	O
or	O
asynchronous	O
.	O
</s>
<s>
Asynchronous	O
IPC	B-Operating_System
is	O
analogous	O
to	O
network	O
communication	O
:	O
the	O
sender	O
dispatches	O
a	O
message	O
and	O
continues	O
executing	O
.	O
</s>
<s>
The	O
receiver	O
checks	O
(	O
polls	B-General_Concept
)	O
for	O
the	O
availability	O
of	O
the	O
message	O
,	O
or	O
is	O
alerted	O
to	O
it	O
via	O
some	O
notification	O
mechanism	O
.	O
</s>
<s>
Asynchronous	O
IPC	B-Operating_System
requires	O
that	O
the	O
kernel	B-Operating_System
maintains	O
buffers	O
and	O
queues	O
for	O
messages	B-Architecture
,	O
and	O
deals	O
with	O
buffer	B-General_Concept
overflows	I-General_Concept
;	O
it	O
also	O
requires	O
double	O
copying	O
of	O
messages	B-Architecture
(	O
sender	O
to	O
kernel	B-Operating_System
and	O
kernel	B-Operating_System
to	O
receiver	O
)	O
.	O
</s>
<s>
In	O
synchronous	O
IPC	B-Operating_System
,	O
the	O
first	O
party	O
(	O
sender	O
or	O
receiver	O
)	O
blocks	O
until	O
the	O
other	O
party	O
is	O
ready	O
to	O
perform	O
the	O
IPC	B-Operating_System
.	O
</s>
<s>
First-generation	O
microkernels	B-Operating_System
typically	O
supported	O
synchronous	O
as	O
well	O
as	O
asynchronous	O
IPC	B-Operating_System
,	O
and	O
suffered	O
from	O
poor	O
IPC	B-Operating_System
performance	O
.	O
</s>
<s>
Jochen	O
Liedtke	O
assumed	O
the	O
design	O
and	O
implementation	O
of	O
the	O
IPC	B-Operating_System
mechanisms	O
to	O
be	O
the	O
underlying	O
reason	O
for	O
this	O
poor	O
performance	O
.	O
</s>
<s>
In	O
his	O
L4	B-Operating_System
microkernel	I-Operating_System
he	O
pioneered	O
methods	O
that	O
lowered	O
IPC	B-Operating_System
costs	O
by	O
an	O
order	O
of	O
magnitude	O
.	O
</s>
<s>
These	O
include	O
an	O
IPC	B-Operating_System
system	O
call	O
that	O
supports	O
a	O
send	O
as	O
well	O
as	O
a	O
receive	O
operation	O
,	O
making	O
all	O
IPC	B-Operating_System
synchronous	O
,	O
and	O
passing	O
as	O
much	O
data	O
as	O
possible	O
in	O
registers	O
.	O
</s>
<s>
Furthermore	O
,	O
Liedtke	O
introduced	O
the	O
concept	O
of	O
the	O
direct	O
process	B-Operating_System
switch	I-Operating_System
,	O
where	O
during	O
an	O
IPC	B-Operating_System
execution	O
an	O
(	O
incomplete	O
)	O
context	B-Operating_System
switch	I-Operating_System
is	O
performed	O
from	O
the	O
sender	O
directly	O
to	O
the	O
receiver	O
.	O
</s>
<s>
If	O
,	O
as	O
in	O
L4	B-Operating_System
,	O
part	O
or	O
all	O
of	O
the	O
message	O
is	O
passed	O
in	O
registers	O
,	O
this	O
transfers	O
the	O
in-register	O
part	O
of	O
the	O
message	O
without	O
any	O
copying	O
at	O
all	O
.	O
</s>
<s>
Furthermore	O
,	O
the	O
overhead	O
of	O
invoking	O
the	O
scheduler	O
is	O
avoided	O
;	O
this	O
is	O
especially	O
beneficial	O
in	O
the	O
common	O
case	O
where	O
IPC	B-Operating_System
is	O
used	O
in	O
a	O
remote	B-Operating_System
procedure	I-Operating_System
call	I-Operating_System
(	O
RPC	O
)	O
type	O
fashion	O
by	O
a	O
client	O
invoking	O
a	O
server	O
.	O
</s>
<s>
Another	O
optimization	O
,	O
called	O
lazy	O
scheduling	O
,	O
avoids	O
traversing	O
scheduling	O
queues	O
during	O
IPC	B-Operating_System
by	O
leaving	O
threads	B-Operating_System
that	O
block	O
during	O
IPC	B-Operating_System
in	O
the	O
ready	O
queue	O
.	O
</s>
<s>
Once	O
the	O
scheduler	O
is	O
invoked	O
,	O
it	O
moves	O
such	O
threads	B-Operating_System
to	O
the	O
appropriate	O
waiting	O
queue	O
.	O
</s>
<s>
As	O
in	O
many	O
cases	O
a	O
thread	B-Operating_System
gets	O
unblocked	O
before	O
the	O
next	O
scheduler	O
invocation	O
,	O
this	O
approach	O
saves	O
significant	O
work	O
.	O
</s>
<s>
Similar	O
approaches	O
have	O
since	O
been	O
adopted	O
by	O
QNX	B-Operating_System
and	O
MINIX	B-Application
3	I-Application
.	O
</s>
<s>
In	O
a	O
series	O
of	O
experiments	O
,	O
Chen	O
and	O
Bershad	O
compared	O
memory	O
cycles	O
per	O
instruction	O
(	O
MCPI	O
)	O
of	O
monolithic	O
Ultrix	B-Operating_System
with	O
those	O
of	O
microkernel	B-Operating_System
Mach	B-Operating_System
combined	O
with	O
a	O
4.3BSD	B-Operating_System
Unix	B-Application
server	O
running	O
in	O
user	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
Their	O
results	O
explained	O
Mach	B-Operating_System
's	O
poorer	O
performance	O
by	O
higher	O
MCPI	O
and	O
demonstrated	O
that	O
IPC	B-Operating_System
alone	O
is	O
not	O
responsible	O
for	O
much	O
of	O
the	O
system	O
overhead	O
,	O
suggesting	O
that	O
optimizations	O
focused	O
exclusively	O
on	O
IPC	B-Operating_System
will	O
have	O
a	O
limited	O
effect	O
.	O
</s>
<s>
Liedtke	O
later	O
refined	O
Chen	O
and	O
Bershad	O
's	O
results	O
by	O
making	O
an	O
observation	O
that	O
the	O
bulk	O
of	O
the	O
difference	O
between	O
Ultrix	B-Operating_System
and	O
Mach	B-Operating_System
MCPI	O
was	O
caused	O
by	O
capacity	O
cache-misses	O
and	O
concluding	O
that	O
drastically	O
reducing	O
the	O
cache	B-General_Concept
working	O
set	O
of	O
a	O
microkernel	B-Operating_System
will	O
solve	O
the	O
problem	O
.	O
</s>
<s>
As	O
it	O
also	O
lends	O
itself	O
to	O
more	O
efficient	O
implementation	O
,	O
most	O
microkernels	B-Operating_System
generally	O
followed	O
L4	B-Operating_System
's	O
lead	O
and	O
only	O
provided	O
a	O
synchronous	O
IPC	B-Operating_System
primitive	O
.	O
</s>
<s>
Asynchronous	O
IPC	B-Operating_System
could	O
be	O
implemented	O
on	O
top	O
by	O
using	O
helper	O
threads	B-Operating_System
.	O
</s>
<s>
However	O
,	O
experience	O
has	O
shown	O
that	O
the	O
utility	O
of	O
synchronous	O
IPC	B-Operating_System
is	O
dubious	O
:	O
synchronous	O
IPC	B-Operating_System
forces	O
a	O
multi-threaded	B-Operating_System
design	O
onto	O
otherwise	O
simple	O
systems	O
,	O
with	O
the	O
resulting	O
synchronization	O
complexities	O
.	O
</s>
<s>
Versions	O
of	O
L4	B-Operating_System
deployed	O
in	O
commercial	O
products	O
have	O
therefore	O
found	O
it	O
necessary	O
to	O
add	O
an	O
asynchronous	O
notification	O
mechanism	O
to	O
better	O
support	O
asynchronous	O
communication	O
.	O
</s>
<s>
This	O
signal-like	O
mechanism	O
does	O
not	O
carry	O
data	O
and	O
therefore	O
does	O
not	O
require	O
buffering	O
by	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
By	O
having	O
two	O
forms	O
of	O
IPC	B-Operating_System
,	O
they	O
have	O
nonetheless	O
violated	O
the	O
principle	O
of	O
minimality	O
.	O
</s>
<s>
Other	O
versions	O
of	O
L4	B-Operating_System
have	O
switched	O
to	O
asynchronous	O
IPC	B-Operating_System
completely	O
.	O
</s>
<s>
As	O
synchronous	O
IPC	B-Operating_System
blocks	O
the	O
first	O
party	O
until	O
the	O
other	O
is	O
ready	O
,	O
unrestricted	O
use	O
could	O
easily	O
lead	O
to	O
deadlocks	O
.	O
</s>
<s>
Therefore	O
,	O
synchronous	O
IPC	B-Operating_System
must	O
provide	O
a	O
means	O
to	O
prevent	O
indefinite	O
blocking	O
.	O
</s>
<s>
Many	O
microkernels	B-Operating_System
provide	O
timeouts	O
on	O
IPC	B-Operating_System
calls	O
,	O
which	O
limit	O
the	O
blocking	O
time	O
.	O
</s>
<s>
As	O
a	O
consequence	O
,	O
the	O
trend	O
is	O
towards	O
not	O
providing	O
arbitrary	O
timeouts	O
,	O
but	O
only	O
a	O
flag	O
which	O
indicates	O
that	O
the	O
IPC	B-Operating_System
should	O
fail	O
immediately	O
if	O
the	O
partner	O
is	O
not	O
ready	O
.	O
</s>
<s>
Recent	O
versions	O
of	O
L4	B-Operating_System
and	O
MINIX	B-Operating_System
have	O
gone	O
down	O
this	O
path	O
(	O
older	O
versions	O
of	O
L4	B-Operating_System
used	O
timeouts	O
)	O
.	O
</s>
<s>
QNX	B-Operating_System
avoids	O
the	O
problem	O
by	O
requiring	O
the	O
client	O
to	O
specify	O
the	O
reply	O
buffer	O
as	O
part	O
of	O
the	O
message	O
send	O
call	O
.	O
</s>
<s>
When	O
the	O
server	O
replies	O
the	O
kernel	B-Operating_System
copies	O
the	O
data	O
to	O
the	O
client	O
's	O
buffer	O
,	O
without	O
having	O
to	O
wait	O
for	O
the	O
client	O
to	O
receive	O
the	O
response	O
explicitly	O
.	O
</s>
<s>
Microkernel	B-Operating_System
servers	O
are	O
essentially	O
daemon	B-Operating_System
programs	O
like	O
any	O
others	O
,	O
except	O
that	O
the	O
kernel	B-Operating_System
grants	O
some	O
of	O
them	O
privileges	O
to	O
interact	O
with	O
parts	O
of	O
physical	O
memory	O
that	O
are	O
otherwise	O
off	O
limits	O
to	O
most	O
programs	O
.	O
</s>
<s>
This	O
allows	O
some	O
servers	O
,	O
particularly	O
device	B-Application
drivers	I-Application
,	O
to	O
interact	O
directly	O
with	O
hardware	O
.	O
</s>
<s>
A	O
basic	O
set	O
of	O
servers	O
for	O
a	O
general-purpose	O
microkernel	B-Operating_System
includes	O
file	B-Application
system	I-Application
servers	O
,	O
device	B-Application
driver	I-Application
servers	O
,	O
networking	O
servers	O
,	O
display	O
servers	O
,	O
and	O
user	O
interface	O
device	O
servers	O
.	O
</s>
<s>
This	O
set	O
of	O
servers	O
(	O
drawn	O
from	O
QNX	B-Operating_System
)	O
provides	O
roughly	O
the	O
set	O
of	O
services	O
offered	O
by	O
a	O
Unix	B-Application
monolithic	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
The	O
necessary	O
servers	O
are	O
started	O
at	O
system	B-Operating_System
startup	I-Operating_System
and	O
provide	O
services	O
,	O
such	O
as	O
file	O
,	O
network	O
,	O
and	O
device	O
access	O
,	O
to	O
ordinary	O
application	O
programs	O
.	O
</s>
<s>
With	O
such	O
servers	O
running	O
in	O
the	O
environment	O
of	O
a	O
user	O
application	O
,	O
server	O
development	O
is	O
similar	O
to	O
ordinary	O
application	O
development	O
,	O
rather	O
than	O
the	O
build-and-boot	O
process	O
needed	O
for	O
kernel	B-Operating_System
development	O
.	O
</s>
<s>
Additionally	O
,	O
many	O
"	O
crashes	O
"	O
can	O
be	O
corrected	O
by	O
simply	O
stopping	B-Operating_System
and	I-Operating_System
restarting	I-Operating_System
the	I-Operating_System
server	I-Operating_System
.	O
</s>
<s>
A	O
good	O
example	O
is	O
a	O
server	O
responsible	O
for	O
TCP/IP	B-Protocol
connections	O
:	O
If	O
this	O
server	O
is	O
restarted	O
,	O
applications	O
will	O
experience	O
a	O
"	O
lost	O
"	O
connection	O
,	O
a	O
normal	O
occurrence	O
in	O
a	O
networked	O
system	O
.	O
</s>
<s>
For	O
QNX	B-Operating_System
,	O
restart	O
capability	O
is	O
offered	O
as	O
the	O
QNX	B-Operating_System
High	O
Availability	O
Toolkit	O
.	O
</s>
<s>
Device	B-Application
drivers	I-Application
frequently	O
perform	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
(	O
DMA	O
)	O
,	O
and	O
therefore	O
can	O
write	O
to	O
arbitrary	O
locations	O
of	O
physical	O
memory	O
,	O
including	O
various	O
kernel	B-Operating_System
data	O
structures	O
.	O
</s>
<s>
It	O
is	O
a	O
common	O
misconception	O
that	O
this	O
means	O
that	O
they	O
must	O
be	O
part	O
of	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
In	O
fact	O
,	O
a	O
driver	O
is	O
not	O
inherently	O
more	O
or	O
less	O
trustworthy	O
by	O
being	O
part	O
of	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
While	O
running	O
a	O
device	B-Application
driver	I-Application
in	O
user	B-Operating_System
space	I-Operating_System
does	O
not	O
necessarily	O
reduce	O
the	O
damage	O
a	O
misbehaving	O
driver	O
can	O
cause	O
,	O
in	O
practice	O
it	O
is	O
beneficial	O
for	O
system	O
stability	O
in	O
the	O
presence	O
of	O
buggy	O
(	O
rather	O
than	O
malicious	O
)	O
drivers	O
:	O
memory-access	O
violations	O
by	O
the	O
driver	O
code	O
itself	O
(	O
as	O
opposed	O
to	O
the	O
device	O
)	O
may	O
still	O
be	O
caught	O
by	O
the	O
memory-management	O
hardware	O
.	O
</s>
<s>
Furthermore	O
,	O
many	O
devices	O
are	O
not	O
DMA-capable	O
,	O
their	O
drivers	O
can	O
be	O
made	O
untrusted	O
by	O
running	O
them	O
in	O
user	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
Recently	O
,	O
an	O
increasing	O
number	O
of	O
computers	O
feature	O
IOMMUs	B-General_Concept
,	O
many	O
of	O
which	O
can	O
be	O
used	O
to	O
restrict	O
a	O
device	O
's	O
access	O
to	O
physical	O
memory	O
.	O
</s>
<s>
This	O
also	O
allows	O
user-mode	B-Operating_System
drivers	O
to	O
become	O
untrusted	O
.	O
</s>
<s>
User-mode	B-Operating_System
drivers	O
actually	O
predate	O
microkernels	B-Operating_System
.	O
</s>
<s>
The	O
Michigan	B-Application
Terminal	I-Application
System	I-Application
(	O
MTS	O
)	O
,	O
in	O
1967	O
,	O
supported	O
user	B-Operating_System
space	I-Operating_System
drivers	O
(	O
including	O
its	O
file	B-Application
system	I-Application
support	O
)	O
,	O
the	O
first	O
operating	B-General_Concept
system	I-General_Concept
to	O
be	O
designed	O
with	O
that	O
capability	O
.	O
</s>
<s>
Historically	O
,	O
drivers	O
were	O
less	O
of	O
a	O
problem	O
,	O
as	O
the	O
number	O
of	O
devices	O
was	O
small	O
and	O
trusted	O
anyway	O
,	O
so	O
having	O
them	O
in	O
the	O
kernel	B-Operating_System
simplified	O
the	O
design	O
and	O
avoided	O
potential	O
performance	O
problems	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
traditional	O
driver-in-the-kernel	O
style	O
of	O
Unix	B-Application
,	O
Linux	B-Application
,	O
and	O
Windows	B-Device
NT	I-Device
.	O
</s>
<s>
With	O
the	O
proliferation	O
of	O
various	O
kinds	O
of	O
peripherals	O
,	O
the	O
amount	O
of	O
driver	O
code	O
escalated	O
and	O
in	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
dominates	O
the	O
kernel	B-Operating_System
in	O
code	O
size	O
.	O
</s>
<s>
As	O
a	O
microkernel	B-Operating_System
must	O
allow	O
building	O
arbitrary	O
operating	B-Operating_System
system	I-Operating_System
services	I-Operating_System
on	O
top	O
,	O
it	O
must	O
provide	O
some	O
core	O
functionality	O
.	O
</s>
<s>
This	O
minimal	O
design	O
was	O
pioneered	O
by	O
Brinch	O
Hansen	O
's	O
Nucleus	B-Operating_System
and	O
the	O
hypervisor	B-Operating_System
of	O
IBM	O
's	O
VM	B-Application
.	O
</s>
<s>
A	O
concept	O
is	O
tolerated	O
inside	O
the	O
microkernel	B-Operating_System
only	O
if	O
moving	O
it	O
outside	O
the	O
kernel	B-Operating_System
,	O
i.e.	O
,	O
permitting	O
competing	O
implementations	O
,	O
would	O
prevent	O
the	O
implementation	O
of	O
the	O
system	O
's	O
required	O
functionality	O
.	O
</s>
<s>
Everything	O
else	O
can	O
be	O
done	O
in	O
a	O
usermode	B-Operating_System
program	O
,	O
although	O
device	B-Application
drivers	I-Application
implemented	O
as	O
user	O
programs	O
may	O
on	O
some	O
processor	O
architectures	O
require	O
special	O
privileges	O
to	O
access	O
I/O	O
hardware	O
.	O
</s>
<s>
Related	O
to	O
the	O
minimality	O
principle	O
,	O
and	O
equally	O
important	O
for	O
microkernel	B-Operating_System
design	O
,	O
is	O
the	O
separation	O
of	O
mechanism	O
and	O
policy	O
,	O
it	O
is	O
what	O
enables	O
the	O
construction	O
of	O
arbitrary	O
systems	O
on	O
top	O
of	O
a	O
minimal	O
kernel	B-Operating_System
.	O
</s>
<s>
Any	O
policy	O
built	O
into	O
the	O
kernel	B-Operating_System
cannot	O
be	O
overwritten	O
at	O
user	O
level	O
and	O
therefore	O
limits	O
the	O
generality	O
of	O
the	O
microkernel	B-Operating_System
.	O
</s>
<s>
Policy	O
implemented	O
in	O
user-level	B-Operating_System
servers	O
can	O
be	O
changed	O
by	O
replacing	O
the	O
servers	O
(	O
or	O
letting	O
the	O
application	O
choose	O
between	O
competing	O
servers	O
offering	O
similar	O
services	O
)	O
.	O
</s>
<s>
For	O
efficiency	O
,	O
most	O
microkernels	B-Operating_System
contain	O
schedulers	O
and	O
manage	O
timers	O
,	O
in	O
violation	O
of	O
the	O
minimality	O
principle	O
and	O
the	O
principle	O
of	O
policy-mechanism	O
separation	O
.	O
</s>
<s>
Start	O
up	O
(	O
booting	B-Operating_System
)	O
of	O
a	O
microkernel-based	O
system	O
requires	O
device	B-Application
drivers	I-Application
,	O
which	O
are	O
not	O
part	O
of	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
Typically	O
this	O
means	O
that	O
they	O
are	O
packaged	O
with	O
the	O
kernel	B-Operating_System
in	O
the	O
boot	B-Operating_System
image	O
,	O
and	O
the	O
kernel	B-Operating_System
supports	O
a	O
bootstrap	O
protocol	O
that	O
defines	O
how	O
the	O
drivers	O
are	O
located	O
and	O
started	O
;	O
this	O
is	O
the	O
traditional	O
bootstrap	O
procedure	O
of	O
L4	B-Operating_System
microkernels	I-Operating_System
.	O
</s>
<s>
Some	O
microkernels	B-Operating_System
simplify	O
this	O
by	O
placing	O
some	O
key	O
drivers	O
inside	O
the	O
kernel	B-Operating_System
(	O
in	O
violation	O
of	O
the	O
minimality	O
principle	O
)	O
,	O
LynxOS	B-Operating_System
and	O
the	O
original	O
Minix	B-Operating_System
are	O
examples	O
.	O
</s>
<s>
Some	O
even	O
include	O
a	O
file	B-Application
system	I-Application
in	O
the	O
kernel	B-Operating_System
to	O
simplify	O
booting	B-Operating_System
.	O
</s>
<s>
A	O
microkernel-based	O
system	O
may	O
boot	B-Operating_System
via	O
multiboot	O
compatible	O
boot	B-Operating_System
loader	O
.	O
</s>
<s>
A	O
key	O
component	O
of	O
a	O
microkernel	B-Operating_System
is	O
a	O
good	O
IPC	B-Operating_System
system	O
and	O
virtual-memory-manager	O
design	O
that	O
allows	O
implementing	O
page-fault	O
handling	O
and	O
swapping	O
in	O
usermode	B-Operating_System
servers	O
in	O
a	O
safe	O
way	O
.	O
</s>
<s>
Since	O
all	O
services	O
are	O
performed	O
by	O
usermode	B-Operating_System
programs	O
,	O
efficient	O
means	O
of	O
communication	O
between	O
programs	O
are	O
essential	O
,	O
far	O
more	O
so	O
than	O
in	O
monolithic	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
The	O
design	O
of	O
the	O
IPC	B-Operating_System
system	O
makes	O
or	O
breaks	O
a	O
microkernel	B-Operating_System
.	O
</s>
<s>
To	O
be	O
effective	O
,	O
the	O
IPC	B-Operating_System
system	O
must	O
not	O
only	O
have	O
low	O
overhead	O
,	O
but	O
also	O
interact	O
well	O
with	O
CPU	O
scheduling	O
.	O
</s>
<s>
On	O
most	O
mainstream	O
processors	O
,	O
obtaining	O
a	O
service	O
is	O
inherently	O
more	O
expensive	O
in	O
a	O
microkernel-based	O
system	O
than	O
a	O
monolithic	O
system	O
.	O
</s>
<s>
In	O
the	O
monolithic	O
system	O
,	O
the	O
service	O
is	O
obtained	O
by	O
a	O
single	O
system	O
call	O
,	O
which	O
requires	O
two	O
mode	O
switches	O
(	O
changes	O
of	O
the	O
processor	O
's	O
ring	B-Operating_System
or	O
CPU	B-General_Concept
mode	I-General_Concept
)	O
.	O
</s>
<s>
In	O
the	O
microkernel-based	O
system	O
,	O
the	O
service	O
is	O
obtained	O
by	O
sending	O
an	O
IPC	B-Operating_System
message	O
to	O
a	O
server	O
,	O
and	O
obtaining	O
the	O
result	O
in	O
another	O
IPC	B-Operating_System
message	O
from	O
the	O
server	O
.	O
</s>
<s>
This	O
requires	O
a	O
context	B-Operating_System
switch	I-Operating_System
if	O
the	O
drivers	O
are	O
implemented	O
as	O
processes	O
,	O
or	O
a	O
function	O
call	O
if	O
they	O
are	O
implemented	O
as	O
procedures	O
.	O
</s>
<s>
In	O
addition	O
,	O
passing	O
actual	O
data	O
to	O
the	O
server	O
and	O
back	O
may	O
incur	O
extra	O
copying	O
overhead	O
,	O
while	O
in	O
a	O
monolithic	O
system	O
the	O
kernel	B-Operating_System
can	O
directly	O
access	O
the	O
data	O
in	O
the	O
client	O
's	O
buffers	O
.	O
</s>
<s>
Performance	O
is	O
therefore	O
a	O
potential	O
issue	O
in	O
microkernel	B-Operating_System
systems	O
.	O
</s>
<s>
The	O
experience	O
of	O
first-generation	O
microkernels	B-Operating_System
such	O
as	O
Mach	B-Operating_System
and	O
ChorusOS	B-Operating_System
showed	O
that	O
systems	O
based	O
on	O
them	O
performed	O
very	O
poorly	O
.	O
</s>
<s>
However	O
,	O
Jochen	O
Liedtke	O
showed	O
that	O
Mach	B-Operating_System
's	O
performance	O
problems	O
were	O
the	O
result	O
of	O
poor	O
design	O
and	O
implementation	O
,	O
specifically	O
Mach	B-Operating_System
's	O
excessive	O
cache	B-General_Concept
footprint	O
.	O
</s>
<s>
Liedtke	O
demonstrated	O
with	O
his	O
own	O
L4	B-Operating_System
microkernel	I-Operating_System
that	O
through	O
careful	O
design	O
and	O
implementation	O
,	O
and	O
especially	O
by	O
following	O
the	O
minimality	O
principle	O
,	O
IPC	B-Operating_System
costs	O
could	O
be	O
reduced	O
by	O
more	O
than	O
an	O
order	O
of	O
magnitude	O
compared	O
to	O
Mach	B-Operating_System
.	O
</s>
<s>
L4	B-Operating_System
's	O
IPC	B-Operating_System
performance	O
is	O
still	O
unbeaten	O
across	O
a	O
range	O
of	O
architectures	O
.	O
</s>
<s>
While	O
these	O
results	O
demonstrate	O
that	O
the	O
poor	O
performance	O
of	O
systems	O
based	O
on	O
first-generation	O
microkernels	B-Operating_System
is	O
not	O
representative	O
for	O
second-generation	O
kernels	B-Operating_System
such	O
as	O
L4	B-Operating_System
,	O
this	O
constitutes	O
no	O
proof	O
that	O
microkernel-based	O
systems	O
can	O
be	O
built	O
with	O
good	O
performance	O
.	O
</s>
<s>
It	O
has	O
been	O
shown	O
that	O
a	O
monolithic	O
Linux	B-Application
server	O
ported	O
to	O
L4	B-Operating_System
exhibits	O
only	O
a	O
few	O
percent	O
overhead	O
over	O
native	O
Linux	B-Application
.	O
</s>
<s>
However	O
,	O
such	O
a	O
single-server	O
system	O
exhibits	O
few	O
,	O
if	O
any	O
,	O
of	O
the	O
advantages	O
microkernels	B-Operating_System
are	O
supposed	O
to	O
provide	O
by	O
structuring	O
operating	B-General_Concept
system	I-General_Concept
functionality	O
into	O
separate	O
servers	O
.	O
</s>
<s>
A	O
number	O
of	O
commercial	O
multi-server	O
systems	O
exist	O
,	O
in	O
particular	O
the	O
real-time	B-General_Concept
systems	I-General_Concept
QNX	B-Operating_System
and	O
Integrity	B-Operating_System
.	O
</s>
<s>
Furthermore	O
,	O
performance	O
does	O
not	O
seem	O
to	O
be	O
the	O
overriding	O
concern	O
for	O
those	O
commercial	O
systems	O
,	O
which	O
instead	O
emphasize	O
reliably	O
quick	O
interrupt	O
handling	O
response	O
times	O
(	O
QNX	B-Operating_System
)	O
and	O
simplicity	O
for	O
the	O
sake	O
of	O
robustness	O
.	O
</s>
<s>
An	O
attempt	O
to	O
build	O
a	O
high-performance	O
multiserver	O
operating	B-General_Concept
system	I-General_Concept
was	O
the	O
IBM	O
Sawmill	O
Linux	B-Application
project	O
.	O
</s>
<s>
It	O
has	O
been	O
shown	O
in	O
the	O
meantime	O
that	O
user-level	B-Operating_System
device	B-Application
drivers	I-Application
can	O
come	O
close	O
to	O
the	O
performance	O
of	O
in-kernel	O
drivers	O
even	O
for	O
such	O
high-throughput	O
,	O
high-interrupt	O
devices	O
as	O
Gigabit	O
Ethernet	O
.	O
</s>
<s>
The	O
security	O
benefits	O
of	O
microkernels	B-Operating_System
have	O
been	O
frequently	O
discussed	O
.	O
</s>
<s>
In	O
the	O
context	O
of	O
security	O
the	O
minimality	O
principle	O
of	O
microkernels	B-Operating_System
is	O
,	O
some	O
have	O
argued	O
,	O
a	O
direct	O
consequence	O
of	O
the	O
principle	O
of	O
least	O
privilege	O
,	O
according	O
to	O
which	O
all	O
code	O
should	O
have	O
only	O
the	O
privileges	O
needed	O
to	O
provide	O
required	O
functionality	O
.	O
</s>
<s>
As	O
the	O
kernel	B-Operating_System
(	O
the	O
code	O
that	O
executes	O
in	O
the	O
privileged	B-Operating_System
mode	I-Operating_System
of	O
the	O
hardware	O
)	O
has	O
unvetted	O
access	O
to	O
any	O
data	O
and	O
can	O
thus	O
violate	O
its	O
integrity	B-Operating_System
or	O
confidentiality	O
,	O
the	O
kernel	B-Operating_System
is	O
always	O
part	O
of	O
the	O
TCB	O
.	O
</s>
<s>
Consequently	O
,	O
microkernel	B-Operating_System
designs	O
have	O
been	O
used	O
for	O
systems	O
designed	O
for	O
high-security	O
applications	O
,	O
including	O
KeyKOS	B-Operating_System
,	O
EROS	B-Operating_System
and	O
military	O
systems	O
.	O
</s>
<s>
In	O
2018	O
,	O
a	O
paper	O
presented	O
at	O
the	O
Asia-Pacific	O
Systems	O
Conference	O
claimed	O
that	O
microkernels	B-Operating_System
were	O
demonstrably	O
safer	O
than	O
monolithic	B-Operating_System
kernels	I-Operating_System
by	O
investigating	O
all	O
published	O
critical	O
CVEs	O
for	O
the	O
Linux	B-Application
kernel	B-Operating_System
at	O
the	O
time	O
.	O
</s>
<s>
The	O
study	O
concluded	O
that	O
40%	O
of	O
the	O
issues	O
could	O
not	O
occur	O
at	O
all	O
in	O
a	O
formally	O
verified	O
microkernel	B-Operating_System
,	O
and	O
only	O
4%	O
of	O
the	O
issues	O
would	O
remain	O
entirely	O
unmitigated	O
in	O
such	O
a	O
system	O
.	O
</s>
<s>
More	O
recent	O
work	O
on	O
microkernels	B-Operating_System
has	O
been	O
focusing	O
on	O
formal	O
specifications	O
of	O
the	O
kernel	B-Operating_System
API	O
,	O
and	O
formal	O
proofs	O
of	O
the	O
API	O
's	O
security	O
properties	O
and	O
implementation	O
correctness	O
.	O
</s>
<s>
The	O
first	O
example	O
of	O
this	O
is	O
a	O
mathematical	O
proof	O
of	O
the	O
confinement	O
mechanisms	O
in	O
EROS	B-Operating_System
,	O
based	O
on	O
a	O
simplified	O
model	O
of	O
the	O
EROS	B-Operating_System
API	O
.	O
</s>
<s>
More	O
recently	O
(	O
in	O
2007	O
)	O
a	O
comprehensive	O
set	O
of	O
machine-checked	O
proofs	O
was	O
performed	O
of	O
the	O
properties	O
of	O
the	O
protection	O
model	O
of	O
seL4	O
,	O
a	O
version	O
of	O
L4	B-Operating_System
.	O
</s>
<s>
This	O
has	O
led	O
to	O
what	O
is	O
referred	O
to	O
as	O
third-generation	O
microkernels	B-Operating_System
,	O
characterised	O
by	O
a	O
security-oriented	O
API	O
with	O
resource	O
access	O
controlled	O
by	O
capabilities	O
,	O
virtualization	O
as	O
a	O
first-class	O
concern	O
,	O
novel	O
approaches	O
to	O
kernel	B-Operating_System
resource	O
management	O
,	O
and	O
a	O
design	O
goal	O
of	O
suitability	O
for	O
formal	B-Architecture
analysis	I-Architecture
,	O
besides	O
the	O
usual	O
goal	O
of	O
high	O
performance	O
.	O
</s>
<s>
Examples	O
are	O
Coyotos	O
,	O
seL4	O
,	O
Nova	O
,	O
Redox	B-Operating_System
and	O
Fiasco.OC	O
.	O
</s>
<s>
a	O
mathematical	O
proof	O
that	O
the	O
kernel	B-Operating_System
's	O
implementation	O
is	O
consistent	O
with	O
its	O
formal	O
specification	O
.	O
</s>
<s>
This	O
provides	O
a	O
guarantee	O
that	O
the	O
properties	O
proved	O
about	O
the	O
API	O
actually	O
hold	O
for	O
the	O
real	O
kernel	B-Operating_System
,	O
a	O
degree	O
of	O
assurance	O
which	O
goes	O
beyond	O
even	O
CC	O
EAL7	O
.	O
</s>
<s>
Taken	O
together	O
,	O
these	O
proofs	O
establish	O
an	O
end-to-end	O
proof	O
of	O
security	O
properties	O
of	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
Some	O
examples	O
of	O
microkernels	B-Operating_System
are	O
:	O
</s>
<s>
A	O
kernel	B-Operating_System
where	O
the	O
total	O
amount	O
of	O
kernel	B-Operating_System
code	O
,	O
i.e.	O
</s>
<s>
code	O
executing	O
in	O
the	O
privileged	B-Operating_System
mode	I-Operating_System
of	O
the	O
hardware	O
,	O
is	O
very	O
small	O
.	O
</s>
<s>
It	O
was	O
a	O
sardonic	O
response	O
to	O
Mach	B-Operating_System
,	O
which	O
claimed	O
to	O
be	O
a	O
microkernel	B-Operating_System
while	O
Shapiro	O
considered	O
it	O
monolithic	O
,	O
essentially	O
unstructured	O
,	O
and	O
slower	O
than	O
the	O
systems	O
it	O
sought	O
to	O
replace	O
.	O
</s>
<s>
Both	O
nanokernel	O
and	O
picokernel	O
have	O
subsequently	O
come	O
to	O
have	O
the	O
same	O
meaning	O
expressed	O
by	O
the	O
term	O
microkernel	B-Operating_System
.	O
</s>
<s>
A	O
virtualization	O
layer	O
underneath	O
an	O
operating	B-General_Concept
system	I-General_Concept
,	O
which	O
is	O
more	O
correctly	O
referred	O
to	O
as	O
a	O
hypervisor	B-Operating_System
.	O
</s>
<s>
A	O
hardware	B-Operating_System
abstraction	I-Operating_System
layer	I-Operating_System
that	O
forms	O
the	O
lowest-level	O
part	O
of	O
a	O
kernel	B-Operating_System
,	O
sometimes	O
used	O
to	O
provide	O
real-time	B-General_Concept
functionality	O
to	O
normal	O
operating	B-General_Concept
systems	I-General_Concept
,	O
like	O
Adeos	B-Operating_System
.	O
</s>
<s>
There	O
is	O
also	O
at	O
least	O
one	O
case	O
where	O
the	O
term	O
nanokernel	O
is	O
used	O
to	O
refer	O
not	O
to	O
a	O
small	O
kernel	B-Operating_System
,	O
but	O
one	O
that	O
supports	O
a	O
nanosecond	O
clock	O
resolution	O
.	O
</s>
