<s>
Adeos	O
(	O
Adaptive	B-Operating_System
Domain	I-Operating_System
Environment	I-Operating_System
for	I-Operating_System
Operating	I-Operating_System
Systems	I-Operating_System
)	O
is	O
a	O
nanokernel	O
hardware	B-Operating_System
abstraction	I-Operating_System
layer	I-Operating_System
(	O
HAL	B-Application
)	O
,	O
or	O
hypervisor	B-Operating_System
,	O
that	O
operates	O
between	O
computer	B-Architecture
hardware	I-Architecture
and	O
the	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
that	O
runs	O
on	O
it	O
.	O
</s>
<s>
It	O
is	O
distinct	O
from	O
other	O
nanokernels	O
in	O
that	O
it	O
is	O
not	O
only	O
a	O
low	O
level	O
layer	O
for	O
an	O
outer	O
kernel	B-Operating_System
.	O
</s>
<s>
Instead	O
,	O
it	O
is	O
intended	O
to	O
run	O
several	O
kernels	B-Operating_System
together	O
,	O
which	O
makes	O
it	O
similar	O
to	O
full	B-Architecture
virtualization	I-Architecture
technologies	O
.	O
</s>
<s>
It	O
is	O
free	B-License
and	I-License
open-source	I-License
software	I-License
released	O
under	O
a	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
(	O
GPL	B-License
)	O
.	O
</s>
<s>
Adeos	O
provides	O
a	O
flexible	O
environment	O
for	O
sharing	O
hardware	B-Architecture
resources	O
among	O
multiple	O
operating	B-General_Concept
systems	I-General_Concept
,	O
or	O
among	O
multiple	O
instances	O
of	O
one	O
OS	O
,	O
thereby	O
enabling	O
multiple	O
prioritized	O
domains	O
to	O
exist	O
simultaneously	O
on	O
the	O
same	O
hardware	B-Architecture
.	O
</s>
<s>
Adeos	O
has	O
been	O
successfully	O
inserted	O
beneath	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
opening	O
a	O
range	O
of	O
possibilities	O
,	O
such	O
as	O
symmetric	B-Operating_System
multiprocessing	I-Operating_System
(	O
SMP	O
)	O
clustering	O
,	O
more	O
efficient	O
virtualization	O
,	O
patchless	O
kernel	B-Operating_System
debugging	O
,	O
and	O
real-time	B-General_Concept
computing	I-General_Concept
(	O
RT	O
)	O
systems	O
for	O
Linux	B-Application
.	O
</s>
<s>
Unusually	O
among	O
HALs	B-Application
,	O
Adeos	O
can	O
be	O
loaded	O
as	O
a	O
Linux	B-Application
loadable	B-Application
kernel	I-Application
module	I-Application
to	O
allow	O
another	O
OS	O
to	O
run	O
along	O
with	O
it	O
.	O
</s>
<s>
Adeos	O
was	O
developed	O
in	O
the	O
context	O
of	O
real-time	B-Application
application	I-Application
interface	I-Application
(	O
RTAI	B-Application
)	O
to	O
modularize	O
it	O
and	O
separate	O
the	O
HAL	B-Application
from	O
the	O
real-time	O
kernel	B-Operating_System
.	O
</s>
<s>
Two	O
categories	O
of	O
methods	O
exist	O
to	O
enable	O
multiple	O
operating	B-General_Concept
systems	I-General_Concept
to	O
run	O
on	O
the	O
same	O
system	O
.	O
</s>
<s>
The	O
first	O
is	O
simulation-based	O
and	O
provides	O
a	O
virtual	O
environment	O
for	O
which	O
to	O
run	O
additional	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
The	O
second	O
suggests	O
the	O
use	O
of	O
a	O
nanokernel	O
layer	O
to	O
enable	O
hardware	B-Architecture
sharing	O
.	O
</s>
<s>
In	O
the	O
simulation	O
category	O
,	O
there	O
are	O
tools	O
such	O
as	O
Xen	B-Operating_System
,	O
VMware	O
,	O
Plex86	B-General_Concept
,	O
VirtualPC	B-Application
and	O
SimOS	B-Application
.	O
</s>
<s>
There	O
is	O
also	O
Kernel-based	B-Application
Virtual	I-Application
Machine	I-Application
(	O
KVM	O
)	O
which	O
is	O
more	O
similar	O
to	O
Adeos	O
,	O
but	O
is	O
not	O
RT	O
and	O
requires	O
specific	O
virtualization	O
hardware	B-Architecture
support	O
.	O
</s>
<s>
In	O
the	O
nanokernel	O
category	O
there	O
are	O
tools	O
such	O
as	O
SPACE	O
,	O
cache	O
kernel	B-Operating_System
and	O
Exokernel	B-Operating_System
.	O
</s>
<s>
All	O
of	O
these	O
suggest	O
building	O
miniature	O
hardware	B-Architecture
management	O
facilities	O
which	O
can	O
thereafter	O
be	O
used	O
to	O
build	O
production	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
The	O
problem	O
of	O
this	O
approach	O
is	O
that	O
it	O
does	O
not	O
address	O
the	O
issue	O
of	O
extant	O
operating	B-General_Concept
systems	I-General_Concept
and	O
their	O
user	O
base	O
.	O
</s>
<s>
Adeos	O
addresses	O
the	O
requirements	O
of	O
both	O
categories	O
of	O
application	O
by	O
providing	O
a	O
simple	O
layer	O
that	O
is	O
inserted	O
under	O
an	O
unmodified	O
running	O
OS	O
and	O
thereafter	O
provides	O
the	O
required	O
primitives	O
and	O
mechanisms	O
to	O
allow	O
multiple	O
OSes	O
to	O
share	O
the	O
same	O
hardware	B-Architecture
environment	O
.	O
</s>
<s>
Adeos	O
does	O
not	O
attempt	O
to	O
impose	O
any	O
restrictions	O
on	O
the	O
hardware	B-Architecture
’s	O
use	O
,	O
by	O
the	O
different	O
OSes	O
,	O
more	O
than	O
is	O
necessary	O
for	O
Adeos’	O
own	O
operation	O
.	O
</s>
<s>
Instead	O
,	O
such	O
restriction	O
is	O
to	O
be	O
imposed	O
by	O
the	O
system	B-Application
administrator	I-Application
or	O
the	O
system	O
programmer	O
.	O
</s>
<s>
This	O
exposes	O
the	O
system	O
to	O
mismanagement	O
,	O
but	O
the	O
idea	O
behind	O
Adeos	O
is	O
to	O
give	O
back	O
control	O
to	O
system	B-Application
administrators	I-Application
and	O
programmers	O
.	O
</s>
<s>
Adeos	O
implements	O
a	O
queue	B-Application
of	O
signals	B-Operating_System
.	O
</s>
<s>
Each	O
time	O
that	O
a	O
peripheral	O
sends	O
a	O
signal	O
,	O
the	O
different	O
operating	B-General_Concept
systems	I-General_Concept
that	O
are	O
running	O
in	O
the	O
machine	O
are	O
awakened	O
,	O
in	O
turn	O
,	O
and	O
must	O
decide	O
if	O
they	O
will	O
accept	O
(	O
handle	O
)	O
,	O
ignore	O
,	O
discard	O
,	O
or	O
terminate	O
the	O
signal	O
.	O
</s>
<s>
Signals	B-Operating_System
not	O
handled	O
(	O
or	O
discarded	O
)	O
by	O
an	O
OS	O
are	O
passed	O
to	O
the	O
next	O
OS	O
in	O
the	O
chain	O
.	O
</s>
<s>
Signals	B-Operating_System
that	O
are	O
terminated	O
are	O
not	O
propagated	O
to	O
latter	O
stages	O
.	O
</s>
<s>
As	O
Adeos	O
has	O
to	O
ensure	O
equal	O
and	O
trusted	O
access	O
to	O
the	O
hardware	B-Architecture
,	O
it	O
takes	O
control	O
of	O
some	O
hardware	B-Architecture
commands	O
issued	O
by	O
the	O
different	O
OSes	O
;	O
but	O
,	O
it	O
also	O
must	O
not	O
intrude	O
too	O
much	O
on	O
the	O
different	O
OSes’	O
normal	O
behavior	O
.	O
</s>
<s>
Adeos	O
does	O
not	O
attempt	O
to	O
impose	O
any	O
policy	O
of	O
use	O
of	O
the	O
hardware	B-Architecture
except	O
as	O
needed	O
for	O
its	O
operation	O
.	O
</s>
<s>
Adeos	O
uses	O
an	O
interrupt	B-Application
pipe	B-Operating_System
to	O
propagate	O
interrupts	B-Application
through	O
the	O
different	O
domains	O
running	O
on	O
the	O
hardware	B-Architecture
.	O
</s>
<s>
As	O
some	O
domains	O
may	O
prefer	O
to	O
be	O
the	O
first	O
to	O
receive	O
hardware	B-Architecture
interrupts	B-Application
,	O
Adeos	O
provides	O
a	O
mechanism	O
for	O
domains	O
to	O
have	O
access	O
to	O
priority	O
interrupt	B-Application
dispatching	O
.	O
</s>
<s>
In	O
effect	O
,	O
Adeos	O
places	O
the	O
requesting	O
domain	O
's	O
interrupt	B-Application
handler	O
and	O
accompanying	O
tables	O
,	O
which	O
may	O
be	O
called	O
as	O
an	O
interrupt	B-Application
mechanism	O
in	O
SPACE	O
terminology	O
,	O
at	O
the	O
first	O
stages	O
of	O
the	O
interrupt	B-Application
pipeline	O
.	O
</s>
<s>
Domains	O
can	O
control	O
whether	O
they	O
accept	O
,	O
ignore	O
,	O
discard	O
or	O
terminate	O
interrupts	B-Application
.	O
</s>
<s>
Accepting	O
interrupts	B-Application
is	O
the	O
normal	O
state	O
of	O
a	O
domain	O
's	O
interrupt	B-Application
mechanism	O
.	O
</s>
<s>
When	O
Adeos	O
encounters	O
a	O
domain	O
that	O
is	O
accepting	O
interrupts	B-Application
it	O
summons	O
its	O
interrupt	B-Application
handler	O
after	O
having	O
set	O
the	O
required	O
CPU	O
environment	O
and	O
stack	O
content	O
for	O
the	O
interrupt	B-Application
handler	O
to	O
operate	O
correctly	O
.	O
</s>
<s>
Once	O
the	O
OS	O
is	O
done	O
,	O
the	O
pipeline	O
proceeds	O
as	O
planned	O
by	O
propagating	O
interrupts	B-Application
down	O
the	O
pipeline	O
.	O
</s>
<s>
When	O
an	O
OS	O
in	O
a	O
domain	O
does	O
not	O
want	O
to	O
be	O
interrupted	O
,	O
for	O
any	O
reason	O
,	O
it	O
asks	O
Adeos	O
to	O
stall	O
the	O
stage	O
its	O
domain	O
occupies	O
in	O
the	O
interrupt	B-Application
pipeline	O
.	O
</s>
<s>
By	O
doing	O
so	O
,	O
interrupts	B-Application
go	O
no	O
further	O
in	O
the	O
pipeline	O
and	O
are	O
stalled	O
at	O
the	O
stage	O
occupied	O
by	O
the	O
domain	O
.	O
</s>
<s>
When	O
the	O
OS	O
is	O
done	O
wanting	O
to	O
be	O
uninterrupted	O
,	O
it	O
asks	O
Adeos	O
to	O
install	O
the	O
pipeline	O
and	O
thereafter	O
all	O
the	O
interrupts	B-Application
that	O
were	O
stalled	O
at	O
the	O
corresponding	O
stage	O
follow	O
their	O
route	O
to	O
the	O
other	O
stages	O
of	O
the	O
pipeline	O
.	O
</s>
<s>
When	O
a	O
domain	O
is	O
discarding	O
interrupts	B-Application
,	O
the	O
interrupt	B-Application
passes	O
over	O
the	O
stage	O
occupied	O
by	O
the	O
domain	O
and	O
continues	O
onto	O
the	O
other	O
stages	O
.	O
</s>
<s>
When	O
a	O
domain	O
terminates	O
interrupts	B-Application
then	O
the	O
interrupts	B-Application
that	O
are	O
terminated	O
by	O
it	O
are	O
not	O
propagated	O
to	O
latter	O
stages	O
.	O
</s>
<s>
Interrupt	B-Application
discarding	O
and	O
termination	O
is	O
only	O
possible	O
when	O
the	O
OS	O
in	O
a	O
domain	O
recognizes	O
Adeos	O
.	O
</s>
<s>
Hence	O
,	O
in	O
the	O
interrupt	B-Application
pipeline	O
,	O
this	O
stage	O
always	O
precedes	O
the	O
handled	O
domain	O
's	O
stage	O
and	O
may	O
take	O
actions	O
for	O
that	O
domain	O
with	O
Adeos	O
in	O
order	O
to	O
provide	O
the	O
handled	O
domain	O
's	O
OS	O
with	O
the	O
illusion	O
of	O
normal	O
system	O
operation	O
.	O
</s>
<s>
This	O
task	O
remains	O
active	O
until	O
the	O
occurrence	O
of	O
the	O
next	O
interrupt	B-Application
.	O
</s>
<s>
If	O
all	O
the	O
domains	O
are	O
n't	O
dormant	O
it	O
restores	O
the	O
processor	O
to	O
the	O
state	O
it	O
had	O
prior	O
the	O
interrupt	B-Application
entering	O
the	O
pipeline	O
and	O
execution	O
continues	O
where	O
it	O
had	O
left	O
.	O
</s>
<s>
Since	O
Adeos	O
is	O
very	O
much	O
hardware	B-Architecture
dependent	O
,	O
many	O
details	O
are	O
specific	O
to	O
one	O
of	O
its	O
particular	O
implementations	O
.	O
</s>
<s>
General-purpose	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
resource	O
sharing	O
is	O
one	O
of	O
the	O
main	O
objectives	O
of	O
Adeos	O
,	O
to	O
provide	O
an	O
environment	O
which	O
enables	O
multiple	O
general	O
purpose	O
OSes	O
to	O
share	O
the	O
same	O
hardware	B-Architecture
.	O
</s>
<s>
Developing	O
OSes	O
is	O
usually	O
a	O
complicated	O
process	O
which	O
sometimes	O
requires	O
extra	O
hardware	B-Architecture
such	O
as	O
in-circuit	B-Application
emulators	I-Application
to	O
probe	O
the	O
hardware	B-Architecture
on	O
which	O
an	O
OS	O
is	O
running	O
.	O
</s>
<s>
It	O
can	O
also	O
provide	O
a	O
default	O
domain	O
handler	O
for	O
OS	O
development	O
under	O
which	O
developers	O
may	O
have	O
controlled	O
direct	O
access	O
to	O
the	O
hardware	B-Architecture
they	O
are	O
meant	O
to	O
control	O
.	O
</s>
<s>
As	O
Adeos	O
is	O
itself	O
a	O
kernel-module	O
,	O
such	O
development	O
domain	O
handlers	O
may	O
be	O
developed	O
independently	O
from	O
Adeos	O
.	O
</s>
<s>
Adeos	O
provides	O
for	O
a	O
way	O
for	O
kernel	B-Operating_System
debuggers	O
and	O
probers	O
to	O
take	O
control	O
of	O
Linux	B-Application
without	O
modifying	O
Linux	B-Application
.	O
</s>
<s>
As	O
with	O
other	O
Adeos	O
domains	O
,	O
these	O
facilities	O
would	O
load	O
as	O
normal	O
kernel	B-Application
modules	I-Application
and	O
would	O
thereafter	O
request	O
a	O
ring-zero	O
domain	O
from	O
Adeos	O
.	O
</s>
<s>
Once	O
that	O
is	O
done	O
,	O
they	O
may	O
request	O
priority	O
interrupt	B-Application
dispatching	O
in	O
the	O
interrupt	B-Application
pipeline	O
.	O
</s>
<s>
Hence	O
,	O
before	O
Linux	B-Application
gets	O
to	O
handle	O
any	O
interrupts	B-Application
,	O
they	O
will	O
be	O
able	O
to	O
intercept	O
those	O
interrupts	B-Application
and	O
carry	O
out	O
the	O
requested	O
debugging	O
tasks	O
.	O
</s>
