<s>
Verve	B-Operating_System
is	O
a	O
research	O
operating	B-General_Concept
system	I-General_Concept
developed	O
by	O
Microsoft	O
Research	O
.	O
</s>
<s>
Verve	B-Operating_System
is	O
verified	O
end-to-end	O
for	O
type	B-Language
safety	I-Language
and	O
memory	B-Application
safety	I-Application
.	O
</s>
<s>
Because	O
of	O
their	O
complexity	O
,	O
a	O
holy	O
grail	O
of	O
software	O
verification	O
has	O
been	O
to	O
verify	O
properties	O
of	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
are	O
usually	O
written	O
in	O
low-level	O
languages	O
,	O
such	O
as	O
C	B-Language
,	O
that	O
provide	O
very	O
few	O
guarantees	O
.	O
</s>
<s>
The	O
took	O
the	O
approach	O
of	O
writing	O
an	O
operating	B-General_Concept
system	I-General_Concept
in	O
C#	B-Application
,	O
a	O
type-safe	B-Language
,	O
memory-safe	B-Application
language	O
.	O
</s>
<s>
A	O
weakness	O
of	O
this	O
approach	O
is	O
that	O
operating	B-General_Concept
systems	I-General_Concept
necessarily	O
need	O
to	O
call	O
lower-level	O
code	O
to	O
,	O
for	O
instance	O
,	O
move	O
the	O
stack	O
pointer	O
.	O
</s>
<s>
Verve	B-Operating_System
addresses	O
this	O
problem	O
by	O
partitioning	O
the	O
operating	B-General_Concept
system	I-General_Concept
into	O
verified	O
assembly	B-Language
language	I-Language
that	O
is	O
required	O
to	O
be	O
low-level	O
and	O
a	O
trusted	O
interface	O
to	O
rest	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
written	O
in	O
C#	B-Application
.	O
</s>
<s>
There	O
is	O
a	O
trusted	O
specification	O
that	O
guarantees	O
the	O
low-level	O
assembly	B-Language
code	I-Language
does	O
not	O
modify	O
the	O
heap	O
and	O
that	O
the	O
high-level	O
C#	B-Application
code	O
does	O
not	O
modify	O
the	O
stacks	O
.	O
</s>
<s>
Verve	B-Operating_System
consists	O
of	O
a	O
small	O
Nucleus	O
,	O
which	O
acts	O
as	O
a	O
minimal	O
hardware	O
abstraction	O
layer	O
,	O
and	O
a	O
Kernel	O
,	O
which	O
uses	O
primitives	O
provided	O
by	O
the	O
Nucleus	O
to	O
expose	O
a	O
more	O
traditional	O
interface	O
to	O
applications	O
.	O
</s>
<s>
All	O
components	O
of	O
the	O
system	O
other	O
than	O
the	O
Nucleus	O
are	O
written	O
in	O
managed	O
code	O
C#	B-Application
and	O
compiled	O
by	O
Bartok	B-Application
(	O
originally	O
developed	O
for	O
the	O
Singularity	B-Operating_System
project	O
)	O
into	O
typed	B-Language
assembly	I-Language
language	I-Language
(	O
TAL	O
)	O
,	O
which	O
is	O
verified	O
by	O
a	O
TAL	O
checker	O
.	O
</s>
<s>
It	O
is	O
written	O
in	O
BoogiePL	O
,	O
which	O
serves	O
as	O
input	O
to	O
MSR	O
's	O
Boogie	O
verifier	O
,	O
which	O
proves	O
the	O
Nucleus	O
correct	O
using	O
the	O
Z3	B-Language
Theorem	I-Language
Prover	I-Language
satisfiability	B-Application
modulo	I-Application
theories	I-Application
(	O
SMT	O
)	O
automated	B-Application
theorem	I-Application
prover	I-Application
(	O
solver	O
)	O
.	O
</s>
<s>
Even	O
though	O
the	O
Kernel	O
is	O
not	O
formally	O
verified	O
,	O
so	O
,	O
for	O
example	O
,	O
a	O
bug	O
in	O
scheduling	O
could	O
cause	O
the	O
system	O
to	O
hang	O
,	O
it	O
cannot	O
violate	O
type	O
or	O
memory	B-Application
safety	I-Application
,	O
and	O
thus	O
cannot	O
directly	O
cause	O
undefined	O
behavior	O
.	O
</s>
<s>
If	O
it	O
attempts	O
to	O
make	O
invalid	O
requests	O
to	O
the	O
Nucleus	O
,	O
formal	O
verification	O
guarantees	O
that	O
the	O
Nucleus	O
handles	O
the	O
situation	O
in	O
a	O
controlled	B-Operating_System
manner	I-Operating_System
.	O
</s>
<s>
Verve	B-Operating_System
's	O
trusted	O
computing	O
base	O
(	O
TCB	O
)	O
is	O
limited	O
to	O
:	O
Boogie/Z3	O
for	O
verifying	O
the	O
Nucleus	O
's	O
correctness	O
;	O
BoogieASM	O
for	O
translating	O
it	O
into	O
x86	B-Operating_System
assembly	O
;	O
the	O
BoogiePL	O
specification	O
of	O
how	O
the	O
Nucleus	O
should	O
behave	O
;	O
the	O
TAL	O
verifier	O
;	O
the	O
assembler	B-Language
and	O
linker	O
;	O
and	O
the	O
bootloader	O
.	O
</s>
<s>
Notably	O
,	O
neither	O
the	O
C#	B-Application
compiler/runtime	O
nor	O
the	O
Bartok	B-Application
compiler	O
are	O
part	O
of	O
the	O
TCB	O
.	O
</s>
