<s>
Exokernel	B-Operating_System
is	O
an	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
developed	O
by	O
the	O
MIT	O
Parallel	O
and	O
Distributed	O
Operating	B-General_Concept
Systems	I-General_Concept
group	O
,	O
and	O
also	O
a	O
class	O
of	O
similar	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
generally	O
present	O
hardware	O
resources	O
to	O
applications	O
through	O
high-level	O
abstractions	B-Application
such	O
as	O
(	O
virtual	O
)	O
file	O
systems	O
.	O
</s>
<s>
The	O
idea	O
behind	O
exokernels	B-Operating_System
is	O
to	O
force	O
as	O
few	O
abstractions	B-Application
as	O
possible	O
on	O
application	O
developers	O
,	O
enabling	O
them	O
to	O
make	O
as	O
many	O
decisions	O
as	O
possible	O
about	O
hardware	O
abstractions	B-Application
.	O
</s>
<s>
Exokernels	B-Operating_System
are	O
tiny	O
,	O
since	O
functionality	O
is	O
limited	O
to	O
ensuring	O
protection	O
and	O
multiplexing	B-Architecture
of	O
resources	O
,	O
which	O
is	O
considerably	O
simpler	O
than	O
conventional	O
microkernels	B-Operating_System
 '	O
implementation	O
of	O
message	O
passing	O
and	O
monolithic	B-Operating_System
kernels	I-Operating_System
 '	O
implementation	O
of	O
high-level	O
abstractions	B-Application
.	O
</s>
<s>
Implemented	O
abstractions	B-Application
are	O
called	O
library	O
operating	B-General_Concept
systems	I-General_Concept
;	O
they	O
may	O
request	O
specific	O
memory	O
addresses	O
,	O
disk	B-Device
blocks	O
,	O
etc	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
only	O
ensures	O
that	O
the	O
requested	O
resource	O
is	O
free	O
,	O
and	O
the	O
application	O
is	O
allowed	O
to	O
access	O
it	O
.	O
</s>
<s>
This	O
low-level	O
hardware	O
access	O
allows	O
the	O
programmer	O
to	O
implement	O
custom	O
abstractions	B-Application
,	O
and	O
omit	O
unnecessary	O
ones	O
,	O
most	O
commonly	O
to	O
improve	O
a	O
program	O
's	O
performance	O
.	O
</s>
<s>
It	O
also	O
allows	O
programmers	O
to	O
choose	O
what	O
level	O
of	O
abstraction	B-Application
they	O
want	O
,	O
high	O
,	O
or	O
low	O
.	O
</s>
<s>
Exokernels	B-Operating_System
can	O
be	O
seen	O
as	O
an	O
application	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
to	O
operating	B-General_Concept
systems	I-General_Concept
,	O
in	O
that	O
they	O
do	O
not	O
force	O
an	O
application	O
program	O
to	O
layer	O
its	O
abstractions	B-Application
on	O
top	O
of	O
other	O
abstractions	B-Application
that	O
were	O
designed	O
with	O
different	O
requirements	O
in	O
mind	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
MIT	O
Exokernel	B-Operating_System
project	O
,	O
the	O
Cheetah	O
web	B-Application
server	I-Application
stores	O
preformatted	O
Internet	B-Protocol
Protocol	I-Protocol
packets	O
on	O
the	O
disk	B-Device
,	O
the	O
kernel	B-Operating_System
provides	O
safe	O
access	O
to	O
the	O
disk	B-Device
by	O
preventing	O
unauthorized	O
reading	O
and	O
writing	O
,	O
but	O
how	O
the	O
disk	B-Device
is	O
abstracted	O
is	O
up	O
to	O
the	O
application	O
or	O
the	O
libraries	O
the	O
application	O
uses	O
.	O
</s>
<s>
Traditionally	O
kernel	B-Operating_System
designers	O
have	O
sought	O
to	O
make	O
individual	O
hardware	O
resources	O
invisible	O
to	O
application	O
programs	O
by	O
requiring	O
the	O
programs	O
to	O
interact	O
with	O
the	O
hardware	O
via	O
some	O
abstraction	B-Application
model	O
.	O
</s>
<s>
These	O
models	O
include	O
file	O
systems	O
for	O
disk	B-Device
storage	I-Device
,	O
virtual	O
address	O
spaces	O
for	O
memory	O
,	O
schedulers	O
for	O
task	O
management	O
,	O
and	O
sockets	B-Protocol
for	O
network	O
communication	O
.	O
</s>
<s>
These	O
abstractions	B-Application
of	O
the	O
hardware	O
make	O
it	O
easier	O
to	O
write	O
programs	O
in	O
general	O
,	O
but	O
limit	O
performance	O
and	O
stifle	O
experimentation	O
in	O
new	O
abstractions	B-Application
.	O
</s>
<s>
A	O
security-oriented	O
application	O
might	O
need	O
a	O
file	O
system	O
that	O
does	O
not	O
leave	O
old	O
data	O
on	O
the	O
disk	B-Device
,	O
while	O
a	O
reliability-oriented	O
application	O
might	O
need	O
a	O
file	O
system	O
that	O
keeps	O
such	O
data	O
for	O
failure	O
recovery	O
.	O
</s>
<s>
One	O
option	O
is	O
to	O
remove	O
the	O
kernel	B-Operating_System
completely	O
and	O
program	O
directly	O
to	O
the	O
hardware	O
,	O
but	O
then	O
the	O
entire	O
machine	O
would	O
be	O
dedicated	O
to	O
the	O
application	O
being	O
written	O
(	O
and	O
,	O
conversely	O
,	O
the	O
entire	O
application	O
codebase	O
would	O
be	O
dedicated	O
to	O
that	O
machine	O
)	O
.	O
</s>
<s>
The	O
exokernel	B-Operating_System
concept	O
is	O
a	O
compromise	O
:	O
let	O
the	O
kernel	B-Operating_System
allocate	O
the	O
basic	O
physical	O
resources	O
of	O
the	O
machine	O
(	O
e.g.	O
</s>
<s>
disk	B-Device
blocks	O
,	O
memory	B-General_Concept
pages	I-General_Concept
,	O
and	O
processor	O
time	O
)	O
to	O
multiple	O
application	O
programs	O
,	O
and	O
let	O
each	O
program	O
decide	O
what	O
to	O
do	O
with	O
these	O
resources	O
.	O
</s>
<s>
The	O
program	O
can	O
then	O
link	O
to	O
a	O
support	O
library	O
that	O
implements	O
the	O
abstractions	B-Application
it	O
needs	O
(	O
or	O
it	O
can	O
implement	O
its	O
own	O
)	O
.	O
</s>
<s>
MIT	O
developed	O
two	O
exokernel-based	O
operating	B-General_Concept
systems	I-General_Concept
,	O
using	O
two	O
kernels	B-Operating_System
:	O
Aegis	O
,	O
a	O
proof	O
of	O
concept	O
with	O
limited	O
support	O
for	O
storage	O
,	O
and	O
XOK	O
,	O
which	O
applied	O
the	O
exokernel	B-Operating_System
concept	O
more	O
thoroughly	O
.	O
</s>
<s>
An	O
essential	O
idea	O
of	O
the	O
MIT	O
exokernel	B-Operating_System
system	O
is	O
that	O
the	O
operating	B-General_Concept
system	I-General_Concept
should	O
act	O
as	O
an	O
executive	O
for	O
small	O
programs	O
provided	O
by	O
the	O
application	O
software	O
,	O
which	O
are	O
constrained	O
only	O
by	O
the	O
requirement	O
that	O
the	O
exokernel	B-Operating_System
must	O
be	O
able	O
to	O
guarantee	O
that	O
they	O
use	O
the	O
hardware	O
safely	O
.	O
</s>
<s>
The	O
MIT	O
exokernel	B-Operating_System
manages	O
hardware	O
resources	O
as	O
follows	O
:	O
</s>
<s>
The	O
kernel	B-Operating_System
represents	O
the	O
processor	O
resources	O
as	O
a	O
timeline	O
from	O
which	O
programs	O
can	O
allocate	O
intervals	O
of	O
time	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
notifies	O
programs	O
of	O
processor	O
events	O
,	O
such	O
as	O
interrupts	B-Application
,	O
hardware	O
exceptions	O
,	O
and	O
the	O
beginning	O
or	O
end	O
of	O
a	O
time	O
slice	O
.	O
</s>
<s>
If	O
a	O
program	O
takes	O
a	O
long	O
time	O
to	O
handle	O
an	O
event	O
,	O
the	O
kernel	B-Operating_System
will	O
penalize	O
it	O
on	O
subsequent	O
time	O
slice	O
allocations	O
;	O
in	O
extreme	O
cases	O
the	O
kernel	B-Operating_System
can	O
abort	O
the	O
program	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
allocates	O
physical	O
memory	B-General_Concept
pages	I-General_Concept
to	O
programs	O
and	O
controls	O
the	O
translation	B-Architecture
lookaside	I-Architecture
buffer	I-Architecture
.	O
</s>
<s>
The	O
kernel	B-Operating_System
ensures	O
that	O
programs	O
access	O
only	O
pages	O
for	O
which	O
they	O
have	O
a	O
capability	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
identifies	O
disk	B-Device
blocks	O
to	O
the	O
application	O
program	O
by	O
their	O
physical	O
block	O
address	O
,	O
allowing	O
the	O
application	O
to	O
optimize	O
data	O
placement	O
.	O
</s>
<s>
When	O
the	O
program	O
initializes	O
its	O
use	O
of	O
the	O
disk	B-Device
,	O
it	O
provides	O
the	O
kernel	B-Operating_System
with	O
a	O
function	O
that	O
the	O
kernel	B-Operating_System
can	O
use	O
to	O
determine	O
which	O
blocks	O
the	O
program	O
controls	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
uses	O
this	O
callback	O
to	O
verify	O
that	O
when	O
it	O
allocates	O
a	O
new	O
block	O
,	O
the	O
program	O
claims	O
only	O
the	O
block	O
that	O
was	O
allocated	O
in	O
addition	O
to	O
those	O
it	O
already	O
controlled	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
implements	O
a	O
programmable	O
packet	B-Protocol
filter	I-Protocol
,	O
which	O
executes	O
programs	O
in	O
a	O
bytecode	O
language	O
designed	O
for	O
easy	O
security-checking	O
by	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
available	O
library	O
operating	B-General_Concept
systems	I-General_Concept
for	O
Exokernel	B-Operating_System
include	O
the	O
custom	O
ExOS	B-Operating_System
system	O
and	O
an	O
emulator	O
for	O
BSD	B-Operating_System
.	O
</s>
<s>
In	O
addition	O
to	O
these	O
,	O
the	O
exokernel	B-Operating_System
team	O
created	O
the	O
Cheetah	O
web	B-Application
server	I-Application
,	O
which	O
uses	O
the	O
kernel	B-Operating_System
directly	O
.	O
</s>
<s>
The	O
exokernel	B-Operating_System
concept	O
has	O
been	O
around	O
since	O
at	O
least	O
1994	O
,	O
but	O
exokernels	B-Operating_System
are	O
still	O
a	O
research	O
effort	O
and	O
have	O
not	O
been	O
used	O
in	O
any	O
major	O
commercial	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
A	O
concept	O
operating	O
exokernel	B-Operating_System
system	O
is	O
Nemesis	O
,	O
written	O
by	O
University	O
of	O
Cambridge	O
,	O
University	O
of	O
Glasgow	O
,	O
Citrix	B-Application
Systems	I-Application
,	O
and	O
the	O
Swedish	O
Institute	O
of	O
Computer	O
Science	O
.	O
</s>
<s>
MIT	O
has	O
also	O
built	O
several	O
exokernel-based	O
systems	O
,	O
including	O
ExOS	B-Operating_System
.	O
</s>
