<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
an	O
abstract	B-Application
state	I-Application
machine	I-Application
(	O
ASM	O
)	O
is	O
a	O
state	B-Architecture
machine	I-Architecture
operating	O
on	O
states	B-Application
that	O
are	O
arbitrary	O
data	O
structures	O
(	O
structure	O
in	O
the	O
sense	O
of	O
mathematical	O
logic	O
,	O
that	O
is	O
a	O
nonempty	O
set	O
together	O
with	O
a	O
number	O
of	O
functions	O
(	O
operations	O
)	O
and	O
relations	B-Algorithm
over	O
the	O
set	O
)	O
.	O
</s>
<s>
refinement	O
:	O
a	O
most	O
general	O
scheme	O
for	O
stepwise	O
instantiations	O
of	O
model	O
abstractions	B-Application
to	O
concrete	O
system	O
elements	O
,	O
providing	O
controllable	O
links	O
between	O
the	O
more	O
and	O
more	O
detailed	O
descriptions	O
at	O
the	O
successive	O
stages	O
of	O
system	O
development	O
.	O
</s>
<s>
In	O
the	O
original	O
conception	O
of	O
ASMs	O
,	O
a	O
single	O
agent	B-General_Concept
executes	O
a	O
program	O
in	O
a	O
sequence	O
of	O
steps	O
,	O
possibly	O
interacting	O
with	O
its	O
environment	O
.	O
</s>
<s>
This	O
notion	O
was	O
extended	O
to	O
capture	O
distributed	B-Architecture
computations	I-Architecture
,	O
in	O
which	O
multiple	O
agents	O
execute	O
their	O
programs	O
concurrently	O
.	O
</s>
<s>
Since	O
ASMs	O
model	O
algorithms	O
at	O
arbitrary	O
levels	O
of	O
abstraction	B-Application
,	O
they	O
can	O
provide	O
high-level	O
,	O
low-level	O
and	O
mid-level	O
views	O
of	O
a	O
hardware	B-Architecture
or	O
software	O
design	O
.	O
</s>
<s>
ASM	O
specifications	B-Application
often	O
consist	O
of	O
a	O
series	O
of	O
ASM	O
models	O
,	O
starting	O
with	O
an	O
abstract	B-Application
ground	O
model	O
and	O
proceeding	O
to	O
greater	O
levels	O
of	O
detail	O
in	O
successive	O
refinements	O
or	O
coarsenings	O
.	O
</s>
<s>
Due	O
to	O
the	O
algorithmic	O
and	O
mathematical	O
nature	O
of	O
these	O
three	O
concepts	O
,	O
ASM	O
models	O
and	O
their	O
properties	O
of	O
interest	O
can	O
be	O
analyzed	O
using	O
any	O
rigorous	O
form	O
of	O
verification	B-Application
(	O
by	O
reasoning	O
)	O
or	O
validation	B-Application
(	O
by	O
experimentation	O
,	O
testing	O
model	O
executions	O
)	O
.	O
</s>
<s>
The	O
concept	O
of	O
ASMs	O
is	O
due	O
to	O
Yuri	O
Gurevich	O
,	O
who	O
first	O
proposed	O
it	O
in	O
the	O
mid-1980s	O
as	O
a	O
way	O
of	O
improving	O
on	O
Turing	O
's	O
thesis	O
that	O
every	O
algorithm	O
is	O
simulated	O
by	O
an	O
appropriate	O
Turing	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
He	O
formulated	O
the	O
ASM	O
Thesis	O
:	O
every	O
algorithm	O
,	O
no	O
matter	O
how	O
abstract	B-Application
,	O
is	O
step-for-step	O
emulated	B-Application
by	O
an	O
appropriate	O
ASM	O
.	O
</s>
<s>
Roughly	O
stated	O
,	O
the	O
axioms	O
are	O
as	O
follows	O
:	O
states	B-Application
are	O
structures	O
,	O
the	O
state	B-Application
transition	I-Application
involves	O
only	O
a	O
bounded	O
part	O
of	O
the	O
state	O
,	O
and	O
everything	O
is	O
invariant	O
under	O
isomorphisms	O
of	O
structures	O
.	O
</s>
<s>
(	O
Structures	O
can	O
be	O
viewed	O
as	O
algebras	O
,	O
which	O
explains	O
the	O
original	O
name	O
evolving	B-Application
algebras	I-Application
for	O
ASMs	O
.	O
)	O
</s>
<s>
The	O
axiomatization	O
and	O
characterization	O
of	O
sequential	O
algorithms	O
have	O
been	O
extended	O
to	O
parallel	B-Operating_System
and	O
interactive	O
algorithms	O
.	O
</s>
<s>
In	O
the	O
1990s	O
,	O
through	O
a	O
community	O
effort	O
,	O
the	O
ASM	O
method	O
was	O
developed	O
,	O
using	O
ASMs	O
for	O
the	O
formal	B-Application
specification	I-Application
and	O
analysis	O
(	O
verification	B-Application
and	I-Application
validation	I-Application
)	O
of	O
computer	B-Architecture
hardware	I-Architecture
and	O
software	O
.	O
</s>
<s>
Comprehensive	O
ASM	O
specifications	B-Application
of	O
programming	O
languages	O
(	O
including	O
Prolog	B-Language
,	O
C	B-Language
,	O
and	O
Java	B-Language
)	O
and	O
design	O
languages	O
(	O
UML	B-Language
and	O
SDL	B-Application
)	O
have	O
been	O
developed	O
.	O
</s>
