<s>
L4	B-Operating_System
is	O
a	O
family	O
of	O
second-generation	O
microkernels	B-Operating_System
,	O
used	O
to	O
implement	O
a	O
variety	O
of	O
types	O
of	O
operating	B-General_Concept
systems	I-General_Concept
(	O
OS	O
)	O
,	O
though	O
mostly	O
for	O
Unix-like	B-Operating_System
,	O
Portable	O
Operating	B-General_Concept
System	I-General_Concept
Interface	B-Application
(	O
POSIX	O
)	O
compliant	O
types	O
.	O
</s>
<s>
L4	B-Operating_System
,	O
like	O
its	O
predecessor	O
microkernel	B-Operating_System
L3	B-Operating_System
,	O
was	O
created	O
by	O
German	O
computer	O
scientist	O
Jochen	O
Liedtke	O
as	O
a	O
response	O
to	O
the	O
poor	O
performance	O
of	O
earlier	O
microkernel-based	O
OSes	O
.	O
</s>
<s>
Liedtke	O
felt	O
that	O
a	O
system	O
designed	O
from	O
the	O
start	O
for	O
high	O
performance	O
,	O
rather	O
than	O
other	O
goals	O
,	O
could	O
produce	O
a	O
microkernel	B-Operating_System
of	O
practical	O
use	O
.	O
</s>
<s>
His	O
original	O
implementation	O
in	O
hand-coded	O
Intel	O
i386-specific	O
assembly	B-Language
language	I-Language
code	O
in	O
1993	O
sparked	O
intense	O
interest	O
in	O
the	O
computer	O
industry	O
.	O
</s>
<s>
Since	O
its	O
introduction	O
,	O
L4	B-Operating_System
has	O
been	O
developed	O
to	O
be	O
cross-platform	B-Operating_System
and	O
to	O
improve	O
security	O
,	O
isolation	O
,	O
and	O
robustness	B-Application
.	O
</s>
<s>
There	O
have	O
been	O
various	O
re-implementations	O
of	O
the	O
original	O
binary	O
L4	B-Operating_System
kernel	I-Operating_System
application	B-Operating_System
binary	I-Operating_System
interface	I-Operating_System
(	O
ABI	O
)	O
and	O
its	O
successors	O
,	O
including	O
L4Ka::Pistachio	O
(	O
implemented	O
by	O
Liedtke	O
and	O
his	O
students	O
at	O
Karlsruhe	O
Institute	O
of	O
Technology	O
)	O
,	O
L4/MIPS	O
(	O
University	O
of	O
New	O
South	O
Wales	O
(	O
UNSW	O
)	O
)	O
,	O
Fiasco	O
(	O
Dresden	O
University	O
of	O
Technology	O
(	O
TU	O
Dresden	O
)	O
)	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
the	O
name	O
L4	B-Operating_System
has	O
been	O
generalized	O
and	O
no	O
longer	O
refers	O
to	O
only	O
Liedtke	O
's	O
original	O
implementation	O
.	O
</s>
<s>
It	O
now	O
applies	O
to	O
the	O
whole	O
microkernel	B-Operating_System
family	O
including	O
the	O
L4	B-Operating_System
kernel	I-Operating_System
interface	B-Application
and	O
its	O
different	O
versions	O
.	O
</s>
<s>
L4	B-Operating_System
is	O
widely	O
deployed	O
.	O
</s>
<s>
One	O
variant	O
,	O
OKL4	B-Operating_System
from	O
Open	O
Kernel	B-Operating_System
Labs	O
,	O
shipped	O
in	O
billions	O
of	O
mobile	O
devices	O
.	O
</s>
<s>
Specifying	O
the	O
general	O
idea	O
of	O
a	O
microkernel	B-Operating_System
,	O
Liedtke	O
states	O
:	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>
In	O
this	O
spirit	O
,	O
the	O
L4	B-Operating_System
microkernel	I-Operating_System
provides	O
few	O
basic	O
mechanisms	O
:	O
address	B-General_Concept
spaces	I-General_Concept
(	O
abstracting	O
page	O
tables	O
and	O
providing	O
memory	O
protection	O
)	O
,	O
threads	B-Operating_System
and	O
scheduling	O
(	O
abstracting	O
execution	O
and	O
providing	O
temporal	O
protection	O
)	O
,	O
and	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
for	O
controlled	O
communication	O
across	O
isolation	O
boundaries	O
)	O
.	O
</s>
<s>
An	O
operating	B-General_Concept
system	I-General_Concept
based	O
on	O
a	O
microkernel	B-Operating_System
like	O
L4	B-Operating_System
provides	O
services	O
as	O
servers	O
in	O
user	B-Operating_System
space	I-Operating_System
that	O
monolithic	B-Operating_System
kernels	I-Operating_System
like	O
Linux	B-Application
or	O
older	O
generation	O
microkernels	B-Operating_System
include	O
internally	O
.	O
</s>
<s>
For	O
example	O
,	O
to	O
implement	O
a	O
secure	O
Unix-like	B-Operating_System
system	I-Operating_System
,	O
servers	O
must	O
provide	O
the	O
rights	O
management	O
that	O
Mach	B-Operating_System
included	O
inside	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
poor	O
performance	O
of	O
first-generation	O
microkernels	B-Operating_System
,	O
such	O
as	O
Mach	B-Operating_System
,	O
led	O
a	O
number	O
of	O
developers	O
to	O
re-examine	O
the	O
entire	O
microkernel	B-Operating_System
concept	O
in	O
the	O
mid-1990s	O
.	O
</s>
<s>
The	O
asynchronous	O
in-kernel-buffering	O
process	B-Operating_System
communication	I-Operating_System
concept	O
used	O
in	O
Mach	B-Operating_System
turned	O
out	O
to	O
be	O
one	O
of	O
the	O
main	O
reasons	O
for	O
its	O
poor	O
performance	O
.	O
</s>
<s>
This	O
induced	O
developers	O
of	O
Mach-based	O
operating	B-General_Concept
systems	I-General_Concept
to	O
move	O
some	O
time-critical	O
components	O
,	O
like	O
file	O
systems	O
or	O
drivers	O
,	O
back	O
inside	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
While	O
this	O
somewhat	O
ameliorated	O
the	O
performance	O
issues	O
,	O
it	O
plainly	O
violates	O
the	O
minimality	O
concept	O
of	O
a	O
true	O
microkernel	B-Operating_System
(	O
and	O
squanders	O
their	O
major	O
advantages	O
)	O
.	O
</s>
<s>
Detailed	O
analysis	O
of	O
the	O
Mach	B-Operating_System
bottleneck	O
indicated	O
that	O
,	O
among	O
other	O
things	O
,	O
its	O
working	B-General_Concept
set	I-General_Concept
is	O
too	O
large	O
:	O
the	O
IPC	O
code	O
expresses	O
poor	O
spatial	O
locality	O
;	O
that	O
is	O
,	O
it	O
results	O
in	O
too	O
many	O
cache	B-General_Concept
misses	O
,	O
of	O
which	O
most	O
are	O
in-kernel	O
.	O
</s>
<s>
This	O
analysis	O
gave	O
rise	O
to	O
the	O
principle	O
that	O
an	O
efficient	O
microkernel	B-Operating_System
should	O
be	O
small	O
enough	O
that	O
the	O
majority	O
of	O
performance-critical	O
code	O
fits	O
into	O
the	O
(	O
first-level	O
)	O
cache	B-General_Concept
(	O
preferably	O
a	O
small	O
fraction	O
of	O
said	O
cache	B-General_Concept
)	O
.	O
</s>
<s>
Jochen	O
Liedtke	O
set	O
out	O
to	O
prove	O
that	O
a	O
well	O
designed	O
thinner	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
layer	O
,	O
with	O
careful	O
attention	O
to	O
performance	O
and	O
machine-specific	O
(	O
in	O
contrast	O
to	O
cross-platform	B-Operating_System
software	I-Operating_System
)	O
design	O
could	O
yield	O
large	O
real-world	O
performance	O
improvements	O
.	O
</s>
<s>
Instead	O
of	O
Mach	B-Operating_System
's	O
complex	O
IPC	O
system	O
,	O
his	O
L3	B-Operating_System
microkernel	B-Operating_System
simply	O
passed	O
the	O
message	O
with	O
no	O
added	O
overhead	O
.	O
</s>
<s>
Defining	O
and	O
implementing	O
the	O
required	O
security	O
policies	O
were	O
considered	O
to	O
be	O
duties	O
of	O
the	O
user	B-Operating_System
space	I-Operating_System
servers	O
.	O
</s>
<s>
The	O
role	O
of	O
the	O
kernel	B-Operating_System
was	O
only	O
to	O
provide	O
the	O
needed	O
mechanism	O
to	O
enable	O
the	O
user-level	B-Operating_System
servers	O
to	O
enforce	O
the	O
policies	O
.	O
</s>
<s>
L3	B-Operating_System
,	O
developed	O
in	O
1988	O
,	O
proved	O
itself	O
a	O
safe	O
and	O
robust	O
operating	B-General_Concept
system	I-General_Concept
,	O
used	O
for	O
many	O
years	O
for	O
example	O
by	O
Technischer	O
Überwachungsverein	O
(	O
Technical	O
Inspection	O
Association	O
)	O
.	O
</s>
<s>
After	O
some	O
experience	O
using	O
L3	B-Operating_System
,	O
Liedtke	O
came	O
to	O
the	O
conclusion	O
that	O
several	O
other	O
Mach	B-Operating_System
concepts	O
were	O
also	O
misplaced	O
.	O
</s>
<s>
By	O
simplifying	O
the	O
microkernel	B-Operating_System
concepts	O
even	O
further	O
he	O
developed	O
the	O
first	O
L4	B-Operating_System
kernel	I-Operating_System
which	O
was	O
primarily	O
designed	O
for	O
high	O
performance	O
.	O
</s>
<s>
To	O
maximise	O
performance	O
,	O
the	O
whole	O
kernel	B-Operating_System
was	O
written	O
in	O
assembly	B-Language
language	I-Language
,	O
and	O
its	O
IPC	O
was	O
20	O
times	O
faster	O
than	O
Mach	B-Operating_System
's	O
.	O
</s>
<s>
Such	O
dramatic	O
performance	O
increases	O
are	O
a	O
rare	O
event	O
in	O
operating	B-General_Concept
systems	I-General_Concept
,	O
and	O
Liedtke	O
's	O
work	O
triggered	O
new	O
L4	B-Operating_System
implementations	O
and	O
work	O
on	O
L4-based	O
systems	O
at	O
a	O
number	O
of	O
universities	O
and	O
research	O
institutes	O
,	O
including	O
IBM	O
,	O
where	O
Liedtke	O
started	O
to	O
work	O
in	O
1996	O
,	O
TU	O
Dresden	O
and	O
UNSW	O
.	O
</s>
<s>
At	O
IBM	O
's	O
Thomas	O
J	O
.	O
Watson	O
Research	O
Center	O
Liedtke	O
and	O
his	O
colleagues	O
continued	O
research	O
on	O
L4	B-Operating_System
and	O
microkernel	B-Operating_System
based	O
systems	O
in	O
general	O
,	O
especially	O
the	O
Sawmill	O
OS	O
.	O
</s>
<s>
In	O
1999	O
,	O
Liedtke	O
took	O
over	O
the	O
Systems	O
Architecture	O
Group	O
at	O
the	O
University	O
of	O
Karlsruhe	O
,	O
where	O
he	O
continued	O
the	O
research	O
into	O
microkernel	B-Operating_System
systems	O
.	O
</s>
<s>
As	O
a	O
proof	O
of	O
concept	O
that	O
a	O
high	O
performance	O
microkernel	B-Operating_System
could	O
also	O
be	O
constructed	O
in	O
a	O
higher	O
level	O
language	O
,	O
the	O
group	O
developed	O
L4Ka::Hazelnut	O
,	O
a	O
C++	B-Language
version	O
of	O
the	O
kernel	B-Operating_System
that	O
ran	O
on	O
IA-32	B-Device
-	O
and	O
ARM-based	O
machines	O
.	O
</s>
<s>
The	O
effort	O
was	O
a	O
success	O
,	O
performance	O
was	O
still	O
acceptable	O
,	O
and	O
with	O
its	O
release	O
,	O
the	O
pure	O
assembly	B-Language
language	I-Language
versions	O
of	O
the	O
kernels	B-Operating_System
were	O
effectively	O
discontinued	O
.	O
</s>
<s>
In	O
parallel	O
to	O
the	O
development	O
of	O
L4Ka::Hazelnut	O
,	O
in	O
1998	O
the	O
Operating	B-General_Concept
Systems	I-General_Concept
Group	O
TUD:OS	O
of	O
the	O
TU	O
Dresden	O
started	O
to	O
develop	O
their	O
own	O
C++	B-Language
implementation	O
of	O
the	O
L4	B-Operating_System
kernel	I-Operating_System
interface	B-Application
,	O
named	O
L4/Fiasco	O
.	O
</s>
<s>
In	O
contrast	O
to	O
L4Ka::Hazelnut	O
,	O
which	O
allows	O
no	O
concurrency	O
in	O
the	O
kernel	B-Operating_System
,	O
and	O
its	O
successor	O
L4Ka::Pistachio	O
,	O
which	O
allows	O
interrupts	O
in	O
the	O
kernel	B-Operating_System
only	O
at	O
specific	O
preemption	O
points	O
,	O
L4/Fiasco	O
was	O
fully	O
preemptible	O
(	O
with	O
the	O
exception	O
of	O
extremely	O
short	O
atomic	O
operations	O
)	O
to	O
achieve	O
a	O
low	O
interrupt	B-General_Concept
latency	I-General_Concept
.	O
</s>
<s>
This	O
was	O
considered	O
necessary	O
because	O
L4/Fiasco	O
is	O
used	O
as	O
the	O
basis	O
of	O
DROPS	O
,	O
a	O
hard	O
real-time	B-General_Concept
computing	I-General_Concept
capable	O
operating	B-General_Concept
system	I-General_Concept
,	O
also	O
developed	O
at	O
the	O
TU	O
Dresden	O
.	O
</s>
<s>
However	O
,	O
the	O
complexities	O
of	O
a	O
fully	O
preemptible	O
design	O
prompted	O
later	O
versions	O
of	O
Fiasco	O
to	O
return	O
to	O
the	O
traditional	O
L4	B-Operating_System
approach	O
of	O
running	O
the	O
kernel	B-Operating_System
with	O
interrupts	O
disabled	O
,	O
except	O
for	O
a	O
limited	O
number	O
of	O
preemption	O
points	O
.	O
</s>
<s>
Up	O
until	O
the	O
release	O
of	O
L4Ka::Pistachio	O
and	O
newer	O
versions	O
of	O
Fiasco	O
,	O
all	O
L4	B-Operating_System
microkernels	I-Operating_System
had	O
been	O
inherently	O
tied	O
close	O
to	O
the	O
underlying	O
CPU	O
architecture	O
.	O
</s>
<s>
The	O
next	O
big	O
shift	O
in	O
L4	B-Operating_System
development	O
was	O
the	O
development	O
of	O
a	O
cross-platform	B-Operating_System
(	O
platform-independent	B-Operating_System
)	O
application	O
programming	O
interface	B-Application
(	O
API	B-General_Concept
)	O
that	O
still	O
retained	O
the	O
high	O
performance	O
characteristics	O
despite	O
its	O
higher	O
level	O
of	O
portability	O
.	O
</s>
<s>
Although	O
the	O
underlying	O
concepts	O
of	O
the	O
kernel	B-Operating_System
were	O
the	O
same	O
,	O
the	O
new	O
API	B-General_Concept
provided	O
many	O
significant	O
changes	O
relative	O
to	O
prior	O
L4	B-Operating_System
versions	O
,	O
including	O
better	O
support	O
for	O
multi-processor	O
systems	O
,	O
looser	O
ties	O
between	O
threads	B-Operating_System
and	O
address	B-General_Concept
spaces	I-General_Concept
,	O
and	O
the	O
introduction	O
of	O
user-level	B-Operating_System
thread	B-Operating_System
control	O
blocks	O
(	O
UTCBs	O
)	O
and	O
virtual	O
registers	O
.	O
</s>
<s>
After	O
releasing	O
the	O
new	O
L4	B-Operating_System
API	B-General_Concept
(	O
version	O
X.2	O
a.k.a.	O
</s>
<s>
version	O
4	O
)	O
in	O
early	O
2001	O
,	O
the	O
System	O
Architecture	O
Group	O
at	O
the	O
University	O
of	O
Karlsruhe	O
implemented	O
a	O
new	O
kernel	B-Operating_System
,	O
L4Ka::Pistachio	O
,	O
completely	O
from	O
scratch	O
,	O
now	O
with	O
focus	O
on	O
both	O
high	O
performance	O
and	O
portability	O
.	O
</s>
<s>
The	O
L4/Fiasco	O
microkernel	B-Operating_System
has	O
also	O
been	O
extensively	O
improved	O
over	O
the	O
years	O
.	O
</s>
<s>
It	O
now	O
supports	O
several	O
hardware	O
platforms	O
ranging	O
from	O
x86	B-Operating_System
through	O
AMD64	B-Device
to	O
several	O
ARM	B-Architecture
platforms	O
.	O
</s>
<s>
Notably	O
,	O
a	O
version	O
of	O
Fiasco	O
(	O
Fiasco-UX	O
)	O
can	O
run	O
as	O
a	O
user-level	B-Operating_System
application	O
on	O
Linux	B-Application
.	O
</s>
<s>
L4/Fiasco	O
implements	O
several	O
extensions	O
to	O
the	O
L4v2	O
API	B-General_Concept
.	O
</s>
<s>
Exception	O
IPC	O
enables	O
the	O
kernel	B-Operating_System
to	O
send	O
CPU	O
exceptions	O
to	O
user-level	B-Operating_System
handler	O
applications	O
.	O
</s>
<s>
With	O
the	O
help	O
of	O
alien	B-Operating_System
threads	I-Operating_System
,	O
it	O
is	O
possible	O
to	O
perform	O
fine-grained	O
control	O
over	O
system	O
calls	O
.	O
</s>
<s>
Also	O
,	O
Fiasco	O
contains	O
mechanisms	O
for	O
controlling	O
communication	O
rights	O
and	O
kernel-level	O
resource	O
use	O
.	O
</s>
<s>
On	O
Fiasco	O
,	O
a	O
collection	O
of	O
basic	O
user	O
level	O
services	O
are	O
developed	O
(	O
named	O
L4Env	O
)	O
that	O
among	O
others	O
are	O
used	O
to	O
para-virtualise	O
the	O
current	O
Linux	B-Application
version	O
(	O
4.19	O
)	O
(	O
named	O
L4Linux	B-Application
)	O
.	O
</s>
<s>
Development	O
also	O
occurred	O
at	O
the	O
University	O
of	O
New	O
South	O
Wales	O
(	O
UNSW	O
)	O
,	O
where	O
developers	O
implemented	O
L4	B-Operating_System
on	O
several	O
64-bit	O
platforms	O
.	O
</s>
<s>
Their	O
work	O
resulted	O
in	O
L4/MIPS	O
and	O
L4/Alpha	O
,	O
resulting	O
in	O
Liedtke	O
's	O
original	O
version	O
being	O
retrospectively	O
named	O
L4/x86	O
.	O
</s>
<s>
Like	O
Liedtke	O
's	O
original	O
kernels	B-Operating_System
,	O
the	O
UNSW	O
kernels	B-Operating_System
(	O
written	O
in	O
a	O
mix	O
of	O
assembly	O
and	O
C	B-Language
)	O
were	O
unportable	O
and	O
each	O
implemented	O
from	O
scratch	O
.	O
</s>
<s>
With	O
the	O
release	O
of	O
the	O
highly	O
portable	O
L4Ka::Pistachio	O
,	O
the	O
UNSW	O
group	O
abandoned	O
their	O
own	O
kernels	B-Operating_System
in	O
favor	O
of	O
producing	O
highly	O
tuned	O
ports	O
of	O
L4Ka::Pistachio	O
,	O
including	O
the	O
fastest-ever	O
reported	O
implementation	O
of	O
message	O
passing	O
(	O
36	O
cycles	O
on	O
the	O
Itanium	B-General_Concept
architecture	O
)	O
.	O
</s>
<s>
The	O
group	O
has	O
also	O
demonstrated	O
that	O
device	B-Application
drivers	I-Application
can	O
perform	O
equally	O
well	O
at	O
user-level	B-Operating_System
as	O
in-kernel	O
,	O
and	O
developed	O
Wombat	B-Operating_System
,	O
a	O
highly	O
portable	O
version	O
of	O
Linux	B-Application
on	O
L4	B-Operating_System
that	O
runs	O
on	O
x86	B-Operating_System
,	O
ARM	B-Architecture
,	O
and	O
MIPS	B-Device
processors	O
.	O
</s>
<s>
On	O
XScale	B-Application
processors	O
,	O
Wombat	B-Operating_System
context-switching	O
costs	O
are	O
up	O
to	O
50	O
times	O
lower	O
than	O
in	O
native	O
Linux	B-Application
.	O
</s>
<s>
Later	O
the	O
UNSW	O
group	O
,	O
at	O
their	O
new	O
home	O
at	O
NICTA	O
(	O
formerly	O
National	O
ICT	O
Australia	O
,	O
Ltd	O
.	O
)	O
,	O
forked	O
L4Ka::Pistachio	O
into	O
a	O
new	O
L4	B-Operating_System
version	O
named	O
NICTA::L4-embedded	O
.	O
</s>
<s>
As	O
the	O
name	O
implies	O
,	O
it	O
was	O
for	O
use	O
in	O
commercial	O
embedded	B-Architecture
systems	I-Architecture
,	O
and	O
consequently	O
the	O
implementation	O
trade-offs	O
favored	O
small	O
memory	O
size	O
and	O
reduced	O
complexity	O
.	O
</s>
<s>
The	O
API	B-General_Concept
was	O
modified	O
to	O
keep	O
almost	O
all	O
system	O
calls	O
short	O
enough	O
that	O
they	O
need	O
no	O
preemption	O
points	O
to	O
ensure	O
high	O
real-time	O
responsiveness	O
.	O
</s>
<s>
In	O
November	O
2005	O
,	O
NICTA	O
announced	O
that	O
Qualcomm	O
was	O
deploying	O
NICTA	O
's	O
L4	B-Operating_System
version	O
on	O
their	O
Mobile	B-Architecture
Station	I-Architecture
Modem	I-Architecture
chipsets	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
use	O
of	O
L4	B-Operating_System
in	O
mobile	O
phone	O
handsets	O
on	O
sale	O
from	O
late	O
2006	O
.	O
</s>
<s>
In	O
August	O
2006	O
,	O
ERTOS	O
leader	O
and	O
UNSW	O
professor	O
Gernot	O
Heiser	O
spun	O
out	O
a	O
company	O
named	O
Open	O
Kernel	B-Operating_System
Labs	O
(	O
OK	O
Labs	O
)	O
to	O
support	O
commercial	O
L4	B-Operating_System
users	O
and	O
further	O
develop	O
L4	B-Operating_System
for	O
commercial	O
use	O
under	O
the	O
brand	O
name	O
OKL4	B-Operating_System
,	O
in	O
close	O
collaboration	O
with	O
NICTA	O
.	O
</s>
<s>
OKL4	B-Operating_System
Version	O
2.1	O
,	O
released	O
in	O
April	O
2008	O
,	O
was	O
the	O
first	O
generally	O
available	O
version	O
of	O
L4	B-Operating_System
which	O
featured	O
capability-based	O
security	O
.	O
</s>
<s>
OKL4	B-Operating_System
3.0	O
,	O
released	O
in	O
October	O
2008	O
,	O
was	O
the	O
last	O
open-source	B-Application
version	O
of	O
OKL4	B-Operating_System
.	O
</s>
<s>
More	O
recent	O
versions	O
are	O
closed	B-Application
source	I-Application
and	O
based	O
on	O
a	O
rewrite	O
to	O
support	O
a	O
native	O
hypervisor	B-Operating_System
variant	O
named	O
the	O
OKL4	B-Operating_System
Microvisor	O
.	O
</s>
<s>
OK	O
Labs	O
also	O
distributed	O
a	O
paravirtualized	O
Linux	B-Application
named	O
OK:Linux	O
,	O
a	O
descendant	O
of	O
Wombat	B-Operating_System
,	O
and	O
paravirtualized	O
versions	O
of	O
SymbianOS	B-Operating_System
and	O
Android	B-Application
.	O
</s>
<s>
OKL4	B-Operating_System
shipments	O
exceeded	O
1.5	O
billion	O
in	O
early	O
2012	O
,	O
mostly	O
on	O
Qualcomm	O
wireless	O
modem	O
chips	O
.	O
</s>
<s>
based	O
on	O
the	O
L4-embedded	O
kernel	B-Operating_System
developed	O
at	O
NICTA	O
in	O
2006	O
.	O
</s>
<s>
This	O
implies	O
that	O
L4	B-Operating_System
is	O
now	O
shipping	O
on	O
all	O
iOS	O
devices	O
,	O
the	O
total	O
shipment	O
of	O
which	O
is	O
estimated	O
at	O
310	O
million	O
for	O
the	O
year	O
2015	O
.	O
</s>
<s>
In	O
2006	O
,	O
the	O
NICTA	O
group	O
commenced	O
a	O
from-scratch	O
design	O
of	O
a	O
third-generation	O
microkernel	B-Operating_System
,	O
named	O
seL4	O
,	O
with	O
the	O
aim	O
of	O
providing	O
a	O
basis	O
for	O
highly	O
secure	O
and	O
reliable	O
systems	O
,	O
suitable	O
for	O
satisfying	O
security	O
requirements	O
such	O
as	O
those	O
of	O
Common	O
Criteria	O
and	O
beyond	O
.	O
</s>
<s>
From	O
the	O
beginning	O
,	O
development	O
aimed	O
for	O
formal	O
verification	O
of	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
To	O
ease	O
meeting	O
the	O
sometimes	O
conflicting	O
requirements	O
of	O
performance	O
and	O
verification	O
,	O
the	O
team	O
used	O
a	O
middle-out	O
software	O
process	O
starting	O
from	O
an	O
executable	O
specification	O
written	O
in	O
Haskell	B-Language
.	O
</s>
<s>
The	O
proof	O
provides	O
a	O
guarantee	O
that	O
the	O
kernel	B-Operating_System
's	O
implementation	O
is	O
correct	O
against	O
its	O
specification	O
,	O
and	O
implies	O
that	O
it	O
is	O
free	O
of	O
implementation	O
bugs	O
such	O
as	O
deadlocks	B-Operating_System
,	O
livelocks	B-Application
,	O
buffer	B-General_Concept
overflows	I-General_Concept
,	O
arithmetic	O
exceptions	O
or	O
use	O
of	O
uninitialised	B-Error_Name
variables	I-Error_Name
.	O
</s>
<s>
seL4	O
is	O
claimed	O
to	O
be	O
the	O
first-ever	O
general-purpose	O
operating-system	O
kernel	B-Operating_System
that	O
has	O
been	O
verified	O
.	O
</s>
<s>
seL4	O
takes	O
a	O
novel	O
approach	O
to	O
kernel	B-Operating_System
resource	O
management	O
,	O
exporting	O
the	O
management	O
of	O
kernel	B-Operating_System
resources	O
to	O
user	O
level	O
and	O
subjects	O
them	O
to	O
the	O
same	O
capability-based	O
access	O
control	O
as	O
user	O
resources	O
.	O
</s>
<s>
This	O
model	O
,	O
which	O
was	O
also	O
adopted	O
by	O
Barrelfish	B-Operating_System
,	O
simplifies	O
reasoning	O
about	O
isolation	O
properties	O
,	O
and	O
was	O
an	O
enabler	O
for	O
later	O
proofs	O
that	O
seL4	O
enforces	O
the	O
core	O
security	O
properties	O
of	O
integrity	O
and	O
confidentiality	O
.	O
</s>
<s>
The	O
NICTA	O
team	O
also	O
proved	O
correctness	O
of	O
the	O
translation	O
from	O
the	O
programming	B-Language
language	I-Language
C	I-Language
to	O
executable	O
machine	B-Language
code	I-Language
,	O
taking	O
the	O
compiler	B-Language
out	O
of	O
the	O
trusted	O
computing	O
base	O
of	O
seL4	O
.	O
</s>
<s>
This	O
implies	O
that	O
the	O
high-level	O
security	O
proofs	O
hold	O
for	O
the	O
kernel	B-Operating_System
executable	O
.	O
</s>
<s>
seL4	O
is	O
also	O
the	O
first	O
published	O
protected-mode	O
OS	B-Operating_System
kernel	I-Operating_System
with	O
a	O
complete	O
and	O
sound	O
worst-case	B-General_Concept
execution	I-General_Concept
time	I-General_Concept
(	O
WCET	B-General_Concept
)	O
analysis	O
,	O
a	O
prerequisite	O
for	O
its	O
use	O
in	O
hard	O
real-time	B-General_Concept
computing	I-General_Concept
.	O
</s>
<s>
On	O
29	O
July	O
2014	O
,	O
NICTA	O
and	O
General	O
Dynamics	O
C4	O
Systems	O
announced	O
that	O
seL4	O
,	O
with	O
end	O
to	O
end	O
proofs	O
,	O
was	O
now	O
released	O
under	O
open-source	B-Application
licenses	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
source	O
code	O
and	O
proofs	O
are	O
licensed	B-License
under	O
GNU	O
General	O
Public	O
License	O
version	O
2	O
(	O
GPLv2	O
)	O
,	O
and	O
most	O
libraries	B-Library
and	O
tools	B-Application
are	O
under	O
the	O
BSD	O
2-clause	O
.	O
</s>
<s>
In	O
April	O
2020	O
,	O
it	O
was	O
announced	O
that	O
the	O
seL4	O
Foundation	O
was	O
created	O
under	O
the	O
umbrella	O
of	O
the	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
to	O
accelerate	O
development	O
and	O
deployment	O
of	O
seL4	O
.	O
</s>
<s>
Under	O
the	O
Defense	O
Advanced	O
Research	O
Projects	O
Agency	O
(	O
DARPA	O
)	O
High-Assurance	O
Cyber	O
Military	O
Systems	O
(	O
HACMS	O
)	O
program	O
,	O
NICTA	O
together	O
with	O
project	O
partners	O
Rockwell	O
Collins	O
,	O
Galois	O
Inc	O
,	O
the	O
University	O
of	O
Minnesota	O
and	O
Boeing	O
developed	O
a	O
high-assurance	O
drone	O
using	O
seL4	O
,	O
along	O
with	O
other	O
assurance	O
tools	B-Application
and	O
software	O
,	O
with	O
planned	O
technology	O
transfer	O
onto	O
the	O
optionally	O
piloted	O
autonomous	O
Boeing	O
AH-6	O
Unmanned	O
Little	O
Bird	O
helicopter	O
being	O
developed	O
by	O
Boeing	O
.	O
</s>
<s>
Osker	O
,	O
an	O
OS	O
written	O
in	O
Haskell	B-Language
,	O
targeted	O
the	O
L4	B-Operating_System
specification	O
;	O
although	O
this	O
project	O
focused	O
mainly	O
on	O
the	O
use	O
of	O
a	O
functional	B-Language
programming	I-Language
language	I-Language
for	O
OS	O
development	O
,	O
not	O
on	O
microkernel	B-Operating_System
research	O
.	O
</s>
<s>
CodeZero	O
is	O
an	O
L4	B-Operating_System
microkernel	I-Operating_System
for	O
embedded	B-Architecture
systems	I-Architecture
with	O
a	O
focus	O
on	O
virtualization	O
and	O
implementation	O
of	O
native	O
OS	O
services	O
.	O
</s>
<s>
There	O
is	O
a	O
GPL-licensed	O
version	O
,	O
and	O
a	O
version	O
that	O
was	O
relicensed	O
by	O
B	O
Labs	O
Ltd.	O
,	O
acquired	O
by	O
Nvidia	O
,	O
as	O
closed	B-Application
source	I-Application
and	O
forked	O
in	O
2010	O
.	O
</s>
<s>
F9	O
microkernel	B-Operating_System
,	O
a	O
BSD-licensed	O
L4	B-Operating_System
implementation	O
,	O
is	O
dedicated	O
to	O
ARM	B-Architecture
Cortex-M	O
processors	O
for	O
deeply	O
embedded	B-Architecture
devices	I-Architecture
with	O
memory	O
protection	O
.	O
</s>
<s>
NOVA	O
consists	O
of	O
a	O
microhypervisor	O
,	O
a	O
user	O
level	O
hypervisor	B-Operating_System
(	O
virtual	B-Operating_System
machine	I-Operating_System
monitor	I-Operating_System
)	O
,	O
and	O
an	O
unprivileged	O
componentised	O
multi-server	O
user	O
environment	O
running	O
on	O
it	O
named	O
NUL	O
.	O
</s>
<s>
NOVA	O
runs	O
on	O
ARMv8-A	O
and	O
x86-based	O
multi-core	O
systems	O
.	O
</s>
<s>
WrmOS	O
is	O
a	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
based	O
on	O
L4	B-Operating_System
microkernel	I-Operating_System
.	O
</s>
<s>
It	O
has	O
own	O
implementations	O
of	O
kernel	B-Operating_System
,	O
standard	O
libraries	B-Library
,	O
and	O
network	O
stack	O
,	O
supporting	O
ARM	B-Architecture
,	O
SPARC	B-Architecture
,	O
x86	B-Operating_System
,	O
and	O
x86-64	B-Device
architectures	O
.	O
</s>
<s>
There	O
is	O
the	O
paravirtualized	O
Linux	B-Application
kernel	B-Operating_System
(	O
w4linux	O
)	O
working	O
on	O
WrmOS	O
.	O
</s>
<s>
Helios	O
is	O
a	O
microkernel	B-Operating_System
inspired	O
by	O
seL4	O
.	O
</s>
<s>
It	O
is	O
part	O
of	O
the	O
Ares	O
operating	B-General_Concept
system	I-General_Concept
,	O
supports	O
x86-64	B-Device
and	O
aarch64	O
and	O
is	O
still	O
under	O
active	O
development	O
as	O
of	O
February	O
2023	O
.	O
</s>
