<s>
Execution	O
in	O
computer	O
and	O
software	O
engineering	O
is	O
the	O
process	O
by	O
which	O
a	O
computer	O
or	O
virtual	B-Architecture
machine	I-Architecture
reads	O
and	O
acts	O
on	O
the	O
instructions	O
of	O
a	O
computer	B-Application
program	I-Application
.	O
</s>
<s>
Execution	O
involves	O
repeatedly	O
following	O
a	O
'	O
fetch–decode–execute	O
 '	O
cycle	O
for	O
each	O
instruction	O
done	O
by	O
control	B-General_Concept
unit	I-General_Concept
.	O
</s>
<s>
As	O
the	O
executing	O
machine	O
follows	O
the	O
instructions	O
,	O
specific	O
effects	O
are	O
produced	O
in	O
accordance	O
with	O
the	O
semantics	B-Application
of	O
those	O
instructions	O
.	O
</s>
<s>
Programs	O
for	O
a	O
computer	O
may	O
be	O
executed	O
in	O
a	O
batch	B-General_Concept
process	I-General_Concept
without	O
human	O
interaction	O
or	O
a	O
user	O
may	O
type	O
commands	B-Application
in	O
an	O
interactive	B-Protocol
session	I-Protocol
of	O
an	O
interpreter	B-Application
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
"	O
commands	B-Application
"	O
are	O
simply	O
program	O
instructions	O
,	O
whose	O
execution	O
is	O
chained	O
together	O
.	O
</s>
<s>
This	O
is	O
generally	O
done	O
in	O
source	O
code	O
,	O
which	O
is	O
then	O
compiled	B-Language
at	O
compile	B-Application
time	I-Application
(	O
and	O
statically	O
linked	O
at	O
link	B-Application
time	I-Application
)	O
to	O
produce	O
an	O
executable	B-Application
.	O
</s>
<s>
This	O
executable	B-Application
is	O
then	O
invoked	O
,	O
most	O
often	O
by	O
an	O
operating	B-General_Concept
system	I-General_Concept
,	O
which	O
loads	O
the	O
program	O
into	O
memory	B-General_Concept
(	O
load	B-Operating_System
time	I-Operating_System
)	O
,	O
possibly	O
performs	O
dynamic	B-Application
linking	I-Application
,	O
and	O
then	O
begins	O
execution	O
by	O
moving	O
control	O
to	O
the	O
entry	B-Language
point	I-Language
of	O
the	O
program	O
;	O
all	O
these	O
steps	O
depend	O
on	O
the	O
Application	B-Operating_System
Binary	I-Operating_System
Interface	I-Operating_System
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
At	O
this	O
point	O
execution	O
begins	O
and	O
the	O
program	O
enters	O
run	B-Library
time	I-Library
.	O
</s>
<s>
The	O
program	O
then	O
runs	O
until	O
it	O
ends	O
,	O
either	O
normal	O
termination	B-Application
or	O
a	O
crash	B-General_Concept
.	O
</s>
<s>
binary	O
,	O
is	O
a	O
list	O
of	O
instructions	O
and	O
data	O
to	O
cause	O
a	O
computer	O
"	O
to	O
perform	O
indicated	O
tasks	O
according	O
to	O
encoded	O
instructions	O
"	O
,	O
as	O
opposed	O
to	O
a	O
data	B-General_Concept
file	I-General_Concept
that	O
must	O
be	O
interpreted	O
(	O
parsed	B-Language
)	O
by	O
a	O
program	O
to	O
be	O
meaningful	O
.	O
</s>
<s>
"	O
Instructions	O
"	O
is	O
traditionally	O
taken	O
to	O
mean	O
machine	B-Language
code	I-Language
instructions	O
for	O
a	O
physical	O
CPU	B-Device
.	O
</s>
<s>
In	O
some	O
contexts	O
,	O
a	O
file	O
containing	O
scripting	O
instructions	O
(	O
such	O
as	O
bytecode	O
)	O
may	O
also	O
be	O
considered	O
executable	B-Application
.	O
</s>
<s>
Most	O
programs	O
execute	O
within	O
multitasking	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
and	O
run-time	B-Library
libraries	I-Library
specific	O
to	O
the	O
source	O
language	O
that	O
provide	O
crucial	O
services	O
not	O
supplied	O
directly	O
by	O
the	O
computer	O
itself	O
.	O
</s>
<s>
In	O
order	O
for	O
programs	O
and	O
interrupt	B-General_Concept
handlers	I-General_Concept
to	O
work	O
without	O
interference	O
and	O
share	O
the	O
same	O
hardware	O
memory	B-General_Concept
and	O
access	O
to	O
the	O
I/O	O
system	O
,	O
in	O
a	O
multitasking	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
running	O
on	O
a	O
digital	O
system	O
with	O
a	O
single	O
CPU/MCU	O
it	O
is	O
required	O
to	O
have	O
some	O
sort	O
of	O
software	O
and	O
hardware	O
facilities	O
to	O
keep	O
track	O
of	O
an	O
executing	O
processes	O
data	O
(	O
memory	B-General_Concept
page	O
addresses	O
,	O
registers	B-General_Concept
etc	O
.	O
)	O
</s>
<s>
In	O
Linux-based	O
operating	B-General_Concept
systems	I-General_Concept
,	O
a	O
set	O
of	O
data	O
stored	O
in	O
registers	B-General_Concept
is	O
usually	O
saved	O
into	O
a	O
process	O
descriptor	O
in	O
memory	B-General_Concept
to	O
implement	O
switching	O
of	O
context	O
.	O
</s>
<s>
A	O
runtime	B-Device
system	I-Device
,	O
also	O
called	O
runtime	B-Device
environment	I-Device
,	O
primarily	O
implements	O
portions	O
of	O
an	O
execution	B-Device
model	I-Device
.	O
</s>
<s>
This	O
is	O
not	O
to	O
be	O
confused	O
with	O
the	O
runtime	B-Library
lifecycle	O
phase	O
of	O
a	O
program	O
,	O
during	O
which	O
the	O
runtime	B-Device
system	I-Device
is	O
in	O
operation	O
.	O
</s>
<s>
When	O
treating	O
the	O
runtime	B-Device
system	I-Device
as	O
distinct	O
from	O
the	O
runtime	B-Device
environment	I-Device
(	O
RTE	O
)	O
,	O
the	O
first	O
may	O
be	O
defined	O
as	O
a	O
specific	O
part	O
of	O
the	O
application	O
software	O
(	O
IDE	O
)	O
used	O
for	O
programming	O
,	O
a	O
piece	O
of	O
software	O
that	O
provides	O
the	O
programmer	O
a	O
more	O
convenient	O
environment	O
for	O
running	O
programs	O
during	O
their	O
production	O
(	O
testing	O
and	O
similar	O
)	O
,	O
while	O
the	O
second	O
(	O
RTE	O
)	O
would	O
be	O
the	O
very	O
instance	O
of	O
an	O
execution	B-Device
model	I-Device
being	O
applied	O
to	O
the	O
developed	O
program	O
which	O
is	O
itself	O
then	O
run	O
in	O
the	O
aforementioned	O
runtime	B-Device
system	I-Device
.	O
</s>
<s>
Most	O
programming	O
languages	O
have	O
some	O
form	O
of	O
runtime	B-Device
system	I-Device
that	O
provides	O
an	O
environment	O
in	O
which	O
programs	O
run	O
.	O
</s>
<s>
This	O
environment	O
may	O
address	O
a	O
number	O
of	O
issues	O
including	O
the	O
management	B-General_Concept
of	O
application	O
memory	B-General_Concept
,	O
how	O
the	O
program	O
accesses	O
variables	O
,	O
mechanisms	O
for	O
passing	O
parameters	O
between	O
procedures	O
,	O
interfacing	O
with	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
and	O
otherwise	O
.	O
</s>
<s>
The	O
compiler	B-Language
makes	O
assumptions	O
depending	O
on	O
the	O
specific	O
runtime	B-Device
system	I-Device
to	O
generate	O
correct	O
code	O
.	O
</s>
<s>
Typically	O
the	O
runtime	B-Device
system	I-Device
will	O
have	O
some	O
responsibility	O
for	O
setting	O
up	O
and	O
managing	O
the	O
stack	B-General_Concept
and	I-General_Concept
heap	I-General_Concept
,	O
and	O
may	O
include	O
features	O
such	O
as	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
threads	B-Operating_System
or	O
other	O
dynamic	B-Language
features	O
built	O
into	O
the	O
language	O
.	O
</s>
<s>
The	O
instruction	B-General_Concept
cycle	I-General_Concept
(	O
also	O
known	O
as	O
the	O
fetch	B-General_Concept
–	I-General_Concept
decode	I-General_Concept
–	I-General_Concept
execute	I-General_Concept
cycle	O
,	O
or	O
simply	O
the	O
fetch-execute	B-General_Concept
cycle	I-General_Concept
)	O
is	O
the	O
cycle	O
that	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	B-Device
)	O
follows	O
from	O
boot-up	B-Operating_System
until	O
the	O
computer	O
has	O
shut	O
down	O
in	O
order	O
to	O
process	O
instructions	O
.	O
</s>
<s>
In	O
simpler	O
CPUs	B-Device
,	O
the	O
instruction	B-General_Concept
cycle	I-General_Concept
is	O
executed	O
sequentially	O
,	O
each	O
instruction	O
being	O
processed	O
before	O
the	O
next	O
one	O
is	O
started	O
.	O
</s>
<s>
In	O
most	O
modern	O
CPUs	B-Device
,	O
the	O
instruction	B-General_Concept
cycles	I-General_Concept
are	O
instead	O
executed	O
concurrently	B-Architecture
,	O
and	O
often	O
in	O
parallel	B-Operating_System
,	O
through	O
an	O
instruction	B-General_Concept
pipeline	I-General_Concept
:	O
the	O
next	B-General_Concept
instruction	I-General_Concept
starts	O
being	O
processed	O
before	O
the	O
previous	O
instruction	O
has	O
finished	O
,	O
which	O
is	O
possible	O
because	O
the	O
cycle	O
is	O
broken	O
up	O
into	O
separate	O
steps	O
.	O
</s>
<s>
A	O
system	O
that	O
executes	O
a	O
program	O
is	O
called	O
an	O
interpreter	B-Application
of	O
the	O
program	O
.	O
</s>
<s>
Loosely	O
speaking	O
,	O
an	O
interpreter	B-Application
directly	O
executes	O
a	O
program	O
.	O
</s>
<s>
This	O
contrasts	O
with	O
a	O
language	O
translator	B-Application
that	O
converts	O
a	O
program	O
from	O
one	O
language	O
to	O
another	O
before	O
it	O
is	O
executed	O
.	O
</s>
<s>
A	O
virtual	B-Architecture
machine	I-Architecture
(	O
VM	O
)	O
is	O
the	O
virtualization/emulation	O
of	O
a	O
computer	O
system	O
.	O
</s>
<s>
Virtual	B-Architecture
machines	I-Architecture
are	O
based	O
on	O
computer	B-General_Concept
architectures	I-General_Concept
and	O
provide	O
functionality	O
of	O
a	O
physical	O
computer	O
.	O
</s>
<s>
Virtual	B-Architecture
machines	I-Architecture
differ	O
and	O
are	O
organized	O
by	O
their	O
function	O
,	O
shown	O
here	O
:	O
</s>
<s>
System	B-Operating_System
virtual	I-Operating_System
machines	I-Operating_System
(	O
also	O
termed	O
full	B-Architecture
virtualization	I-Architecture
VMs	O
)	O
provide	O
a	O
substitute	O
for	O
a	O
real	O
machine	O
.	O
</s>
<s>
They	O
provide	O
functionality	O
needed	O
to	O
execute	O
entire	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
A	O
hypervisor	B-Operating_System
uses	O
native	B-Language
execution	I-Language
to	O
share	O
and	O
manage	O
hardware	O
,	O
allowing	O
for	O
multiple	O
environments	O
which	O
are	O
isolated	O
from	O
one	O
another	O
,	O
yet	O
exist	O
on	O
the	O
same	O
physical	O
machine	O
.	O
</s>
<s>
Modern	O
hypervisors	B-Operating_System
use	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
,	O
virtualization-specific	O
hardware	O
,	O
primarily	O
from	O
the	O
host	O
CPUs	B-Device
.	O
</s>
<s>
Process	O
virtual	B-Architecture
machines	I-Architecture
are	O
designed	O
to	O
execute	O
computer	B-Application
programs	I-Application
in	O
a	O
platform-independent	O
environment	O
.	O
</s>
<s>
Some	O
virtual	B-Architecture
machine	I-Architecture
emulators	B-Application
,	O
such	O
as	O
QEMU	B-Application
and	O
video	O
game	O
console	O
emulators	B-Application
,	O
are	O
designed	O
to	O
also	O
emulate	O
(	O
or	O
"	O
virtually	O
imitate	O
"	O
)	O
different	O
system	O
architectures	O
thus	O
allowing	O
execution	O
of	O
software	O
applications	O
and	O
operating	B-General_Concept
systems	I-General_Concept
written	O
for	O
another	O
CPU	B-Device
or	O
architecture	O
.	O
</s>
<s>
OS-level	B-Application
virtualization	I-Application
allows	O
the	O
resources	O
of	O
a	O
computer	O
to	O
be	O
partitioned	O
via	O
the	O
kernel	B-Operating_System
.	O
</s>
