<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
an	O
abstract	B-Application
machine	I-Application
is	O
a	O
theoretical	O
model	O
that	O
allows	O
for	O
a	O
detailed	O
and	O
precise	O
analysis	O
of	O
how	O
a	O
computer	O
system	O
functions	O
.	O
</s>
<s>
Abstract	B-Application
machines	I-Application
vary	O
from	O
literal	O
machines	O
in	O
that	O
they	O
are	O
expected	O
to	O
perform	O
correctly	O
and	O
independently	O
of	O
hardware	B-Architecture
.	O
</s>
<s>
Abstract	B-Application
machines	I-Application
are	O
"	O
machines	O
"	O
because	O
they	O
allow	O
step-by-step	O
execution	O
of	O
programmes	O
;	O
they	O
are	O
"	O
abstract	B-Application
"	O
because	O
they	O
ignore	O
many	O
aspects	O
of	O
actual	O
(	O
hardware	B-Architecture
)	O
machines	O
.	O
</s>
<s>
A	O
typical	O
abstract	B-Application
machine	I-Application
consists	O
of	O
a	O
definition	O
in	O
terms	O
of	O
input	O
,	O
output	O
,	O
and	O
the	O
set	O
of	O
allowable	O
operations	O
used	O
to	O
turn	O
the	O
former	O
into	O
the	O
latter	O
.	O
</s>
<s>
In	O
the	O
theory	O
of	O
computation	O
,	O
abstract	B-Application
machines	I-Application
are	O
often	O
used	O
in	O
thought	O
experiments	O
regarding	O
computability	O
or	O
to	O
analyse	O
the	O
complexity	O
of	O
algorithms	O
.	O
</s>
<s>
This	O
use	O
of	O
abstract	B-Application
machines	I-Application
is	O
fundamental	O
to	O
the	O
field	O
of	O
computational	O
complexity	O
theory	O
,	O
such	O
as	O
finite	B-Architecture
state	I-Architecture
machines	I-Architecture
,	O
Mealy	B-General_Concept
machines	I-General_Concept
,	O
push-down	B-Application
automata	I-Application
,	O
and	O
Turing	B-Architecture
machines	I-Architecture
.	O
</s>
<s>
Abstract	B-Application
machines	I-Application
are	O
generally	O
classified	O
into	O
two	O
types	O
depending	O
on	O
the	O
number	O
of	O
operations	O
they	O
are	O
allowed	O
to	O
undertake	O
at	O
any	O
one	O
time	O
:	O
deterministic	B-General_Concept
abstract	I-General_Concept
machines	I-General_Concept
and	O
non-deterministic	O
abstract	B-Application
machines	I-Application
.	O
</s>
<s>
A	O
deterministic	B-General_Concept
abstract	B-Application
machine	I-Application
is	O
a	O
system	O
in	O
which	O
a	O
particular	O
beginning	O
state	O
or	O
condition	O
always	O
yields	O
the	O
same	O
outputs	O
.	O
</s>
<s>
In	O
contrast	O
,	O
a	O
non-deterministic	O
abstract	B-Application
machine	I-Application
can	O
provide	O
various	O
outputs	O
for	O
the	O
same	O
input	O
on	O
different	O
executions	O
.	O
</s>
<s>
Unlike	O
a	O
deterministic	B-General_Concept
algorithm	I-General_Concept
,	O
which	O
gives	O
the	O
same	O
result	O
for	O
the	O
same	O
input	O
regardless	O
of	O
the	O
number	O
of	O
iterations	O
,	O
a	O
non-deterministic	O
algorithm	O
takes	O
various	O
paths	O
to	O
arrive	O
to	O
different	O
outputs	O
.	O
</s>
<s>
Non-deterministic	O
algorithms	O
are	O
helpful	O
for	O
obtaining	O
approximate	O
answers	O
when	O
deriving	O
a	O
precise	O
solution	O
using	O
a	O
deterministic	B-General_Concept
approach	O
is	O
difficult	O
or	O
costly	O
.	O
</s>
<s>
Turing	B-Architecture
machines	I-Architecture
,	O
for	O
example	O
,	O
are	O
some	O
of	O
the	O
most	O
fundamental	O
abstract	B-Application
machines	I-Application
in	O
computer	B-General_Concept
science	I-General_Concept
.	O
</s>
<s>
Their	O
instructions	B-Language
provide	O
for	O
both	O
modifying	O
the	O
symbols	O
and	O
changing	O
the	O
symbol	O
that	O
the	O
machine	O
’s	O
pointer	O
is	O
currently	O
at	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
rudimentary	O
Turing	B-Architecture
machine	I-Architecture
could	O
have	O
a	O
single	O
command	O
,	O
"	O
convert	O
symbol	O
to	O
1	O
then	O
move	O
right	O
"	O
,	O
and	O
this	O
machine	O
would	O
only	O
produce	O
a	O
string	O
of	O
1s	O
.	O
</s>
<s>
This	O
basic	O
Turing	B-Architecture
machine	I-Architecture
is	O
deterministic	B-General_Concept
;	O
however	O
,	O
nondeterministic	O
Turing	B-Architecture
machines	I-Architecture
that	O
can	O
execute	O
several	O
actions	O
given	O
the	O
same	O
input	O
may	O
also	O
be	O
built	O
.	O
</s>
<s>
Any	O
implementation	O
of	O
an	O
abstract	B-Application
machine	I-Application
in	O
the	O
case	O
of	O
physical	O
implementation	O
(	O
in	O
hardware	B-Architecture
)	O
use	O
some	O
kind	O
of	O
physical	O
device	O
(	O
mechanical	O
or	O
electronic	O
)	O
to	O
execute	O
the	O
instructions	B-Language
of	O
a	O
programming	O
language	O
.	O
</s>
<s>
Abstract	B-Application
machine	I-Application
however	O
can	O
also	O
be	O
implemented	O
in	O
software	O
or	O
firmware	B-Application
at	O
levels	O
between	O
the	O
abstract	B-Application
machine	I-Application
and	O
underlying	O
physical	O
device	O
.	O
</s>
<s>
Implementation	O
in	O
hardware	B-Architecture
:	O
The	O
direct	O
implementation	O
of	O
abstract	B-Application
machine	I-Application
in	O
hardware	B-Architecture
is	O
a	O
matter	O
of	O
using	O
physical	O
devices	O
such	O
as	O
memory	B-Algorithm
,	O
arithmetic	B-General_Concept
and	O
logic	O
circuits	O
,	O
buses	O
,	O
etc.	O
,	O
to	O
implement	O
a	O
physical	O
machine	O
whose	O
machine	B-Language
language	I-Language
coincides	O
with	O
the	O
programming	O
language	O
.	O
</s>
<s>
A	O
CPU	B-General_Concept
may	O
be	O
thought	O
of	O
as	O
a	O
concrete	O
hardware	B-Architecture
realisation	O
of	O
an	O
abstract	B-Application
machine	I-Application
,	O
particularly	O
the	O
processor	O
's	O
design	O
.	O
</s>
<s>
Simulation	O
using	O
software	O
:	O
Implementing	O
an	O
abstract	B-Application
machine	I-Application
with	O
software	O
entails	O
writing	O
programmes	O
in	O
a	O
different	O
language	O
to	O
implement	O
the	O
data	B-General_Concept
structures	I-General_Concept
and	O
algorithms	O
needed	O
by	O
the	O
abstract	B-Application
machine	I-Application
.	O
</s>
<s>
This	O
provides	O
the	O
most	O
flexibility	O
since	O
programmes	O
implementing	O
abstract	B-Application
machine	I-Application
constructs	O
can	O
be	O
easily	O
changed	O
.	O
</s>
<s>
An	O
abstract	B-Application
machine	I-Application
implemented	O
as	O
a	O
software	O
simulation	O
,	O
or	O
for	O
which	O
an	O
interpreter	B-Application
exists	O
,	O
is	O
called	O
a	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Emulation	B-Application
using	I-Application
firmware	I-Application
:	O
Firmware	B-Application
implementation	O
sits	O
between	O
hardware	B-Architecture
and	O
software	O
implementation	O
.	O
</s>
<s>
It	O
consists	O
of	O
microcode	B-Device
simulations	O
of	O
data	B-General_Concept
structures	I-General_Concept
and	O
algorithms	O
for	O
abstract	B-Application
machines	I-Application
.	O
</s>
<s>
Microcode	B-Device
allows	O
a	O
computer	O
programmer	O
to	O
write	O
machine	B-Language
instructions	I-Language
without	O
needing	O
to	O
fabricate	O
electrical	O
circuitry	O
.	O
</s>
<s>
An	O
abstract	B-Application
machine	I-Application
is	O
,	O
intuitively	O
,	O
just	O
an	O
abstraction	B-Application
of	O
the	O
idea	O
of	O
a	O
physical	O
computer	O
.	O
</s>
<s>
This	O
implies	O
that	O
the	O
algorithms	O
to	O
be	O
executed	O
must	O
be	O
expressed	O
using	O
programming	O
language	O
instructions	B-Language
.	O
</s>
<s>
The	O
syntax	O
of	O
a	O
programming	O
language	O
enables	O
the	O
construction	O
of	O
programs	B-Application
using	O
a	O
finite	O
set	O
of	O
constructs	O
known	O
as	O
instructions	B-Language
.	O
</s>
<s>
Most	O
abstract	B-Application
machines	I-Application
share	O
a	O
program	O
store	O
and	O
a	B-Application
state	I-Application
,	O
which	O
often	O
includes	O
a	B-Application
stack	I-Application
and	O
registers	O
.	O
</s>
<s>
In	O
digital	O
computers	O
,	O
the	O
stack	B-Application
is	O
simply	O
a	O
memory	B-Algorithm
unit	I-Algorithm
with	O
an	O
address	B-Language
register	O
that	O
can	O
count	O
only	O
positive	O
integers	O
(	O
after	O
an	O
initial	O
value	O
is	O
loaded	O
into	O
it	O
)	O
.	O
</s>
<s>
The	O
address	B-Language
register	O
for	O
the	O
stack	B-Application
is	O
known	O
as	O
a	B-General_Concept
stack	I-General_Concept
pointer	I-General_Concept
because	O
its	O
value	O
always	O
refers	O
to	O
the	O
top	O
item	O
on	O
the	O
stack	B-Application
.	O
</s>
<s>
The	O
program	O
consists	O
of	O
a	O
series	O
of	O
instructions	B-Language
,	O
with	O
a	B-General_Concept
stack	I-General_Concept
pointer	I-General_Concept
indicating	O
the	O
next	B-General_Concept
instruction	I-General_Concept
to	O
be	O
performed	O
.	O
</s>
<s>
When	O
the	O
instruction	O
is	O
completed	O
,	O
a	B-General_Concept
stack	I-General_Concept
pointer	I-General_Concept
is	O
advanced	O
.	O
</s>
<s>
This	O
fundamental	O
control	O
mechanism	O
of	O
an	O
abstract	B-Application
machine	I-Application
is	O
also	O
known	O
as	O
its	O
execution	B-General_Concept
loop	I-General_Concept
.	O
</s>
<s>
Thus	O
,	O
an	O
abstract	B-Application
machine	I-Application
for	O
programming	O
language	O
is	O
any	O
collection	O
of	O
data	B-General_Concept
structures	I-General_Concept
and	O
algorithms	O
capable	O
of	O
storing	O
and	O
running	O
programs	B-Application
written	O
in	O
the	O
programming	O
language	O
.	O
</s>
<s>
It	O
bridges	O
the	O
gap	O
between	O
the	O
high	B-Language
level	I-Language
of	I-Language
a	I-Language
programming	I-Language
language	I-Language
and	O
the	O
low	B-Language
level	I-Language
of	I-Language
an	I-Language
actual	I-Language
machine	I-Language
by	O
providing	O
an	O
intermediate	O
language	O
step	O
for	O
compilation	B-Language
.	O
</s>
<s>
An	O
abstract	B-Application
machine	I-Application
's	O
instructions	B-Language
are	O
adapted	O
to	O
the	O
unique	O
operations	O
necessary	O
to	O
implement	O
operations	O
of	O
a	O
certain	O
source	O
language	O
or	O
set	O
of	O
source	O
languages	O
.	O
</s>
<s>
In	O
the	O
late	O
1950s	O
,	O
the	O
Association	O
for	O
Computing	O
Machinery	O
(	O
ACM	O
)	O
and	O
other	O
allied	O
organisations	O
developed	O
many	O
proposals	O
for	O
Universal	B-Architecture
Computer	I-Architecture
Oriented	O
Language	O
(	O
UNCOL	B-Application
)	O
,	O
such	O
as	O
Conway	O
's	O
machine	O
.	O
</s>
<s>
The	O
UNCOL	B-Application
concept	O
is	O
good	O
,	O
but	O
it	O
has	O
not	O
been	O
widely	O
used	O
due	O
to	O
the	O
poor	O
performance	O
of	O
the	O
generated	O
code	O
.	O
</s>
<s>
In	O
many	O
areas	O
of	O
computing	O
,	O
its	O
performance	O
will	O
continue	O
to	O
be	O
an	O
issue	O
despite	O
the	O
development	O
of	O
the	O
Java	B-Language
Virtual	I-Language
Machine	I-Language
in	O
the	O
late	O
1990s	O
.	O
</s>
<s>
Algol	B-Language
Object	I-Language
Code	I-Language
(	O
1964	O
)	O
,	O
P4-machine	O
(	O
1976	O
)	O
,	O
UCSD	B-Language
P-machine	I-Language
(	O
1977	O
)	O
,	O
and	O
Forth	B-Application
(	O
1970	O
)	O
are	O
some	O
successful	O
abstract	B-Application
machines	I-Application
of	O
this	O
kind	O
.	O
</s>
<s>
Abstract	B-Application
machines	I-Application
for	O
object-oriented	B-Language
programming	I-Language
languages	I-Language
are	O
often	O
stack-based	B-General_Concept
and	O
have	O
special	O
access	O
instructions	B-Language
for	O
object	O
fields	O
and	O
methods	B-Language
.	O
</s>
<s>
In	O
these	O
machines	O
,	O
memory	B-General_Concept
management	I-General_Concept
is	O
often	O
implicit	O
performed	O
by	O
a	O
garbage	B-General_Concept
collector	I-General_Concept
(	O
memory	B-Algorithm
recovery	O
feature	O
built	O
into	O
programming	O
languages	O
)	O
.	O
</s>
<s>
Smalltalk-80	B-Application
(	O
1980	O
)	O
,	O
Self	B-Operating_System
(	O
1989	O
)	O
,	O
and	O
Java	B-Language
(	O
1994	O
)	O
are	O
examples	O
of	O
this	O
implementation	O
.	O
</s>
<s>
There	O
have	O
been	O
string	O
processing	O
languages	O
in	O
the	O
form	O
of	O
command	B-Operating_System
shells	I-Operating_System
,	O
programming	B-Application
tools	I-Application
,	O
macro	B-Application
processors	I-Application
,	O
and	O
scripting	B-Language
languages	I-Language
for	O
decades	O
.	O
</s>
<s>
Using	O
a	O
suitable	O
abstract	B-Application
machine	I-Application
has	O
two	O
benefits	O
:	O
increased	O
execution	B-General_Concept
speed	I-General_Concept
and	O
enhanced	O
portability	O
.	O
</s>
<s>
Snobol4	B-Application
and	O
ML/I	B-Language
are	O
two	O
notable	O
instances	O
of	O
early	O
string	O
processing	O
languages	O
that	O
use	O
an	O
abstract	B-Application
machine	I-Application
to	O
gain	O
machine	O
independence	O
.	O
</s>
<s>
The	O
early	O
abstract	B-Application
machines	I-Application
for	O
functional	B-Language
languages	I-Language
,	O
including	O
the	O
SECD	B-Application
machine	I-Application
(	O
1964	O
)	O
and	O
Cardelli	O
's	O
Functional	O
Abstract	B-Application
Machine	I-Application
(	O
1983	O
)	O
,	O
defined	O
strict	O
evaluation	O
,	O
also	O
known	O
as	O
eager	B-Language
or	I-Language
call-by-value	I-Language
evaluation	I-Language
,	O
in	O
which	O
function	O
arguments	O
are	O
evaluated	O
before	O
to	O
the	O
call	O
and	O
precisely	O
once	O
.	O
</s>
<s>
In	O
recent	O
years	O
,	O
the	O
majority	O
of	O
research	O
has	O
been	O
on	O
lazy	O
(	O
or	O
call-by-need	O
)	O
evaluation	O
,	O
such	O
as	O
the	O
G-machine	O
(	O
1984	O
)	O
,	O
Krivine	B-Application
machine	I-Application
(	O
1985	O
)	O
,	O
and	O
Three	O
Instruction	O
Machine	O
(	O
1986	O
)	O
,	O
in	O
which	O
function	O
arguments	O
are	O
evaluated	O
only	O
if	O
necessary	O
and	O
at	O
most	O
once	O
.	O
</s>
<s>
One	O
reason	O
is	O
because	O
effective	O
implementation	O
of	O
strict	O
evaluation	O
is	O
now	O
well-understood	O
,	O
therefore	O
the	O
necessity	O
for	O
an	O
abstract	B-Application
machine	I-Application
has	O
diminished	O
.	O
</s>
<s>
Predicate	O
calculus	O
(	O
first	O
order	O
logic	O
)	O
is	O
the	O
foundation	O
of	O
logic	B-Language
programming	I-Language
languages	I-Language
.	O
</s>
<s>
The	O
most	O
well-known	O
logic	B-Language
programming	I-Language
language	I-Language
is	O
Prolog	B-Language
.	O
</s>
<s>
The	O
rules	O
in	O
Prolog	B-Language
are	O
written	O
in	O
a	O
uniform	O
format	O
known	O
as	O
universally	O
quantified	O
'	O
Horn	O
clauses	O
 '	O
,	O
which	O
means	O
to	O
begin	O
the	O
calculation	O
that	O
attempts	O
to	O
discover	O
a	O
proof	O
of	O
the	O
objective	O
.	O
</s>
<s>
The	O
Warren	B-Application
Abstract	I-Application
Machine	I-Application
WAM	I-Application
(	O
1983	O
)	O
,	O
which	O
has	O
become	O
the	O
de	O
facto	O
standard	O
in	O
Prolog	B-Language
program	B-Language
compilation	I-Language
,	O
has	O
been	O
the	O
focus	O
of	O
most	O
study	O
.	O
</s>
<s>
It	O
provides	O
special	O
purpose	O
instructions	B-Language
such	O
as	O
data	O
unification	O
instructions	B-Language
and	O
control	O
flow	O
instructions	B-Language
to	O
support	O
backtracking	O
(	O
searching	O
algorithm	O
)	O
.	O
</s>
<s>
A	O
generic	O
abstract	B-Application
machine	I-Application
is	O
made	O
up	O
of	O
a	O
memory	B-Algorithm
and	O
an	O
interpreter	B-Application
.	O
</s>
<s>
The	O
memory	B-Algorithm
is	O
used	O
to	O
store	O
data	O
and	O
programs	B-Application
,	O
while	O
the	O
interpreter	B-Application
is	O
the	O
component	O
that	O
executes	O
the	O
instructions	B-Language
included	O
in	O
programs	B-Application
.	O
</s>
<s>
The	O
interpreter	B-Application
must	O
carry	O
out	O
the	O
operations	O
that	O
are	O
unique	O
to	O
the	O
language	O
it	O
is	O
interpreting	O
.	O
</s>
<s>
However	O
,	O
given	O
the	O
variety	O
of	O
languages	O
,	O
it	O
is	O
conceivable	O
to	O
identify	O
categories	O
of	O
operations	O
and	O
an	O
"	O
execution	B-General_Concept
mechanism	I-General_Concept
"	O
shared	O
by	O
all	O
interpreters	B-Application
.	O
</s>
<s>
The	O
interpreter	B-Application
's	O
operations	O
and	O
accompanying	O
data	B-General_Concept
structures	I-General_Concept
are	O
divided	O
into	O
the	O
following	O
categories	O
:	O
</s>
<s>
Operations	O
and	O
data	B-General_Concept
structures	I-General_Concept
for	O
controlling	O
the	O
sequence	O
of	O
execution	B-General_Concept
of	I-General_Concept
operations	I-General_Concept
;	O
</s>
<s>
Operations	O
and	O
data	B-General_Concept
structures	I-General_Concept
for	O
controlling	O
data	O
transfers	O
;	O
</s>
<s>
Operations	O
and	O
data	B-General_Concept
structures	I-General_Concept
for	O
memory	B-General_Concept
management	I-General_Concept
.	O
</s>
<s>
An	O
abstract	B-Application
machine	I-Application
must	O
contain	O
operations	O
for	O
manipulating	O
primitive	O
data	O
types	O
such	O
as	O
strings	O
and	O
integers	O
.	O
</s>
<s>
For	O
example	O
,	O
integers	O
are	O
nearly	O
universally	O
considered	O
a	O
basic	O
data	O
type	O
for	O
both	O
physical	O
abstract	B-Application
machines	I-Application
and	O
the	O
abstract	B-Application
machines	I-Application
used	O
by	O
many	O
programming	O
languages	O
.	O
</s>
<s>
The	O
machine	O
carries	O
out	O
the	O
arithmetic	B-General_Concept
operations	O
necessary	O
,	O
such	O
as	O
addition	O
and	O
multiplication	O
,	O
within	O
a	O
single	O
time	O
step	O
.	O
</s>
<s>
Operations	O
and	O
structures	O
for	O
"	O
sequence	O
control	O
"	O
allow	O
controlling	O
the	O
execution	B-General_Concept
flow	I-General_Concept
of	O
program	O
instructions	B-Language
.	O
</s>
<s>
When	O
certain	O
conditions	B-Language
are	O
met	O
,	O
it	O
is	O
necessary	O
to	O
change	O
the	O
typical	O
sequential	O
execution	O
of	O
a	O
program	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
interpreter	B-Application
employs	O
data	B-General_Concept
structures	I-General_Concept
(	O
such	O
as	O
those	O
used	O
to	O
store	O
the	O
address	B-Language
of	O
the	O
next	B-General_Concept
instruction	I-General_Concept
to	O
execute	O
)	O
that	O
are	O
modified	O
by	O
operations	O
distinct	O
from	O
those	O
used	O
for	O
data	O
manipulation	O
(	O
for	O
example	O
,	O
operations	O
to	O
update	O
the	O
address	B-Language
of	O
the	O
next	B-General_Concept
instruction	I-General_Concept
to	O
execute	O
)	O
.	O
</s>
<s>
Data	O
transfer	O
operations	O
are	O
used	O
to	O
control	O
how	O
operands	O
and	O
data	O
are	O
transported	O
from	O
memory	B-Algorithm
to	O
the	O
interpreter	B-Application
and	O
vice	O
versa	O
.	O
</s>
<s>
Memory	B-General_Concept
management	I-General_Concept
is	O
concerned	O
with	O
the	O
operations	O
performed	O
in	O
memory	B-Algorithm
to	O
allocate	O
data	O
and	O
applications	O
.	O
</s>
<s>
In	O
the	O
abstract	B-Application
machine	I-Application
,	O
data	O
and	O
programmes	O
can	O
be	O
held	O
indefinitely	O
,	O
or	O
in	O
the	O
case	O
of	O
programming	O
languages	O
,	O
memory	B-Algorithm
can	O
be	O
allocated	O
or	O
deallocated	O
using	O
a	O
more	O
complex	O
mechanism	O
.	O
</s>
<s>
Abstract	B-Application
machine	I-Application
hierarchies	O
are	O
often	O
employed	O
,	O
in	O
which	O
each	O
machine	O
uses	O
the	O
functionality	O
of	O
the	O
level	O
immediately	O
below	O
and	O
adds	O
additional	O
functionality	O
of	O
its	O
own	O
to	O
meet	O
the	O
level	O
immediately	O
above	O
.	O
</s>
<s>
A	O
hardware	B-Architecture
computer	I-Architecture
,	O
constructed	O
with	O
physical	O
electronic	O
devices	O
,	O
can	O
be	O
added	O
at	O
the	O
most	O
basic	O
level	O
.	O
</s>
<s>
Above	O
this	O
level	O
,	O
the	O
abstract	B-Application
microprogrammed	B-Device
machine	I-Device
level	I-Device
may	O
be	O
introduced	O
.	O
</s>
<s>
The	O
abstract	B-Application
machine	I-Application
supplied	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
which	O
is	O
implemented	O
by	O
program	O
written	O
in	O
machine	B-Language
language	I-Language
,	O
is	O
located	O
immediately	O
above	O
(	O
or	O
directly	O
above	O
the	O
hardware	B-Architecture
if	O
the	O
firmware	B-Application
level	I-Application
is	O
not	O
there	O
)	O
.	O
</s>
<s>
On	O
the	O
one	O
hand	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
extends	O
the	O
capability	O
of	O
the	O
physical	O
machine	O
by	O
providing	O
higher-level	O
primitives	O
that	O
are	O
not	O
available	O
on	O
the	O
physical	O
machine	O
(	O
for	O
example	O
,	O
primitives	O
that	O
act	O
on	O
files	O
)	O
.	O
</s>
<s>
The	O
host	B-Operating_System
machine	I-Operating_System
is	O
formed	O
by	O
the	O
abstract	B-Application
machine	I-Application
given	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
on	O
which	O
a	O
high-level	B-Language
programming	I-Language
language	I-Language
is	O
implemented	O
using	O
an	O
intermediary	B-Architecture
machine	I-Architecture
,	O
such	O
as	O
the	O
Java	B-Language
Virtual	I-Language
machine	I-Language
and	O
its	O
bytecode	O
language	O
.	O
</s>
<s>
The	O
level	O
given	O
by	O
the	O
abstract	B-Application
machine	I-Application
for	O
the	O
high-level	B-Language
language	I-Language
(	O
for	O
example	O
,	O
Java	B-Language
)	O
is	O
not	O
usually	O
the	O
final	O
level	O
of	O
hierarchy	O
.	O
</s>
<s>
A	O
"	O
web	O
machine	O
"	O
level	O
,	O
for	O
example	O
,	O
can	O
be	O
added	O
to	O
implement	O
the	O
functionalities	O
necessary	O
to	O
handle	O
Web	O
communications	O
(	O
communications	O
protocols	O
or	O
HTML	B-Language
code	I-Language
presentation	I-Language
)	O
.	O
</s>
