<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
machine	B-Language
code	I-Language
is	O
computer	B-Application
code	I-Application
consisting	O
of	O
machine	B-Language
language	I-Language
instructions	B-General_Concept
,	O
which	O
are	O
used	O
to	O
control	O
a	O
computer	O
's	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
.	O
</s>
<s>
Each	O
instruction	O
causes	O
the	O
CPU	O
to	O
perform	O
a	O
very	O
specific	O
task	O
,	O
such	O
as	O
a	O
load	O
,	O
a	O
store	O
,	O
a	O
jump	B-General_Concept
,	O
or	O
an	O
arithmetic	B-General_Concept
logic	I-General_Concept
unit	I-General_Concept
(	O
ALU	O
)	O
operation	O
on	O
one	O
or	O
more	O
units	O
of	O
data	O
in	O
the	O
CPU	O
's	O
registers	B-General_Concept
or	O
memory	B-General_Concept
.	O
</s>
<s>
Early	O
CPUs	O
had	O
specific	O
machine	B-Language
code	I-Language
that	O
might	O
break	O
backwards	B-General_Concept
compatibility	I-General_Concept
with	O
each	O
new	O
CPU	O
released	O
.	O
</s>
<s>
The	O
notion	O
of	O
an	O
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
(	O
ISA	O
)	O
defines	O
and	O
specifies	O
the	O
behavior	O
and	O
encoding	O
in	O
memory	B-General_Concept
of	O
the	O
instruction	B-General_Concept
set	I-General_Concept
of	O
the	O
system	O
,	O
without	O
specifying	O
its	O
exact	O
implementation	O
.	O
</s>
<s>
This	O
acts	O
as	O
an	O
abstraction	O
layer	O
,	O
enabling	O
compatibility	O
within	O
the	O
same	O
family	O
of	O
CPUs	O
,	O
so	O
that	O
machine	B-Language
code	I-Language
written	O
or	O
generated	O
according	O
to	O
the	O
ISA	O
for	O
the	O
family	O
will	O
run	O
on	O
all	O
CPUs	O
in	O
the	O
family	O
,	O
including	O
future	O
CPUs	O
.	O
</s>
<s>
In	O
general	O
,	O
each	O
architecture	B-General_Concept
family	O
(	O
e.g.	O
</s>
<s>
x86	B-Operating_System
,	O
ARM	B-Architecture
)	O
has	O
its	O
own	O
ISA	O
,	O
and	O
hence	O
its	O
own	O
specific	O
machine	B-Language
code	I-Language
language	O
.	O
</s>
<s>
the	O
IA-64	B-General_Concept
can	O
emulate	O
x86	B-Operating_System
.	O
</s>
<s>
Machine	B-Language
code	I-Language
is	O
a	O
strictly	O
numerical	O
language	O
,	O
and	O
is	O
the	O
lowest-level	O
interface	O
to	O
the	O
CPU	O
intended	O
for	O
a	O
programmer	O
.	O
</s>
<s>
There	O
is	O
,	O
on	O
some	O
CPUs	O
,	O
a	O
lower	O
level	O
interface	O
in	O
the	O
form	O
of	O
(	O
modifiable	O
)	O
microcode	B-Device
that	O
implement	O
the	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
However	O
,	O
microcode	B-Device
is	O
not	O
intended	O
to	O
be	O
changed	O
by	O
the	O
end	O
user	O
on	O
normal	O
commercial	O
CPUs	O
.	O
</s>
<s>
Assembly	B-Language
language	I-Language
provides	O
a	O
direct	O
mapping	O
between	O
the	O
numerical	O
machine	B-Language
code	I-Language
and	O
a	O
human	O
readable	O
version	O
where	O
numerical	O
opcodes	B-Language
and	O
operands	O
are	O
replaced	O
by	O
readable	O
strings	O
(	O
e.g.	O
</s>
<s>
0x90	O
is	O
the	O
NOP	O
instruction	O
on	O
x86	B-Operating_System
)	O
.	O
</s>
<s>
While	O
it	O
is	O
possible	O
to	O
write	O
programs	O
directly	O
in	O
machine	B-Language
code	I-Language
,	O
managing	O
individual	O
bits	O
and	O
calculating	O
numerical	O
addresses	B-General_Concept
and	O
constants	O
manually	O
is	O
tedious	O
and	O
error-prone	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
programs	O
are	O
very	O
rarely	O
written	O
directly	O
in	O
machine	B-Language
code	I-Language
in	O
modern	O
contexts	O
,	O
but	O
may	O
be	O
done	O
for	O
low	O
level	O
debugging	O
,	O
program	O
patching	O
(	O
especially	O
when	O
assembler	B-Language
source	O
is	O
not	O
available	O
)	O
and	O
assembly	B-Language
language	I-Language
disassembly	O
.	O
</s>
<s>
The	O
majority	O
of	O
practical	O
programs	O
today	O
are	O
written	O
in	O
higher-level	B-Language
languages	I-Language
or	O
assembly	B-Language
language	I-Language
.	O
</s>
<s>
The	O
source	O
code	O
is	O
then	O
translated	O
to	O
executable	O
machine	B-Language
code	I-Language
by	O
utilities	O
such	O
as	O
compilers	B-Language
,	O
assemblers	B-Language
,	O
and	O
linkers	B-Application
,	O
with	O
the	O
important	O
exception	O
of	O
interpreted	B-Application
programs	O
,	O
which	O
are	O
not	O
translated	O
into	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
However	O
,	O
the	O
interpreter	B-Application
itself	O
,	O
which	O
may	O
be	O
seen	O
as	O
an	O
executor	O
or	O
processor	O
performing	O
the	O
instructions	B-General_Concept
of	O
the	O
source	O
code	O
,	O
typically	O
consists	O
of	O
directly	O
executable	O
machine	B-Language
code	I-Language
(	O
generated	O
from	O
assembly	O
or	O
high-level	B-Language
language	I-Language
source	O
code	O
)	O
.	O
</s>
<s>
Machine	B-Language
code	I-Language
is	O
by	O
definition	O
the	O
lowest	O
level	O
of	O
programming	O
detail	O
visible	O
to	O
the	O
programmer	O
,	O
but	O
internally	O
many	O
processors	O
use	O
microcode	B-Device
or	O
optimise	O
and	O
transform	O
machine	B-Language
code	I-Language
instructions	B-General_Concept
into	O
sequences	O
of	O
micro-ops	B-General_Concept
.	O
</s>
<s>
This	O
is	O
not	O
generally	O
considered	O
to	O
be	O
a	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
Every	O
processor	O
or	O
processor	O
family	O
has	O
its	O
own	O
instruction	B-General_Concept
set	I-General_Concept
.	O
</s>
<s>
Instructions	B-General_Concept
are	O
patterns	O
of	O
bits	O
,	O
digits	O
,	O
or	O
characters	O
that	O
correspond	O
to	O
machine	O
commands	O
.	O
</s>
<s>
Thus	O
,	O
the	O
instruction	B-General_Concept
set	I-General_Concept
is	O
specific	O
to	O
a	O
class	O
of	O
processors	O
using	O
(	O
mostly	O
)	O
the	O
same	O
architecture	B-General_Concept
.	O
</s>
<s>
Successor	O
or	O
derivative	O
processor	O
designs	O
often	O
include	O
instructions	B-General_Concept
of	O
a	O
predecessor	O
and	O
may	O
add	O
new	O
additional	O
instructions	B-General_Concept
.	O
</s>
<s>
Occasionally	O
,	O
a	O
successor	O
design	O
will	O
discontinue	O
or	O
alter	O
the	O
meaning	O
of	O
some	O
instruction	B-Language
code	I-Language
(	O
typically	O
because	O
it	O
is	O
needed	O
for	O
new	O
purposes	O
)	O
,	O
affecting	O
code	O
compatibility	O
to	O
some	O
extent	O
;	O
even	O
compatible	O
processors	O
may	O
show	O
slightly	O
different	O
behavior	O
for	O
some	O
instructions	B-General_Concept
,	O
but	O
this	O
is	O
rarely	O
a	O
problem	O
.	O
</s>
<s>
Systems	O
may	O
also	O
differ	O
in	O
other	O
details	O
,	O
such	O
as	O
memory	B-General_Concept
arrangement	O
,	O
operating	O
systems	O
,	O
or	O
peripheral	O
devices	O
.	O
</s>
<s>
Because	O
a	O
program	O
normally	O
relies	O
on	O
such	O
factors	O
,	O
different	O
systems	O
will	O
typically	O
not	O
run	O
the	O
same	O
machine	B-Language
code	I-Language
,	O
even	O
when	O
the	O
same	O
type	O
of	O
processor	O
is	O
used	O
.	O
</s>
<s>
A	O
processor	O
's	O
instruction	B-General_Concept
set	I-General_Concept
may	O
have	O
fixed-length	O
or	O
variable-length	O
instructions	B-General_Concept
.	O
</s>
<s>
How	O
the	O
patterns	O
are	O
organized	O
varies	O
with	O
the	O
particular	O
architecture	B-General_Concept
and	O
type	O
of	O
instruction	O
.	O
</s>
<s>
Most	O
instructions	B-General_Concept
have	O
one	O
or	O
more	O
opcode	B-Language
fields	O
that	O
specify	O
the	O
basic	O
instruction	O
type	O
(	O
such	O
as	O
arithmetic	O
,	O
logical	O
,	O
jump	B-General_Concept
,	O
etc	O
.	O
</s>
<s>
Not	O
all	O
machines	O
or	O
individual	O
instructions	B-General_Concept
have	O
explicit	O
operands	O
.	O
</s>
<s>
On	O
a	O
machine	O
with	O
a	O
single	O
accumulator	B-General_Concept
,	O
the	O
accumulator	B-General_Concept
is	O
implicitly	O
both	O
the	O
left	O
operand	O
and	O
result	O
of	O
most	O
arithmetic	O
instructions	B-General_Concept
.	O
</s>
<s>
Some	O
other	O
architectures	O
,	O
such	O
as	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
,	O
have	O
accumulator	B-General_Concept
versions	O
of	O
common	O
instructions	B-General_Concept
,	O
with	O
the	O
accumulator	B-General_Concept
regarded	O
as	O
one	O
of	O
the	O
general	O
registers	B-General_Concept
by	O
longer	O
instructions	B-General_Concept
.	O
</s>
<s>
A	O
stack	B-Application
machine	I-Application
has	O
most	O
or	O
all	O
of	O
its	O
operands	O
on	O
an	O
implicit	O
stack	O
.	O
</s>
<s>
Special	O
purpose	O
instructions	B-General_Concept
also	O
often	O
lack	O
explicit	O
operands	O
;	O
for	O
example	O
,	O
CPUID	O
in	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
writes	O
values	O
into	O
four	O
implicit	O
destination	O
registers	B-General_Concept
.	O
</s>
<s>
This	O
distinction	O
between	O
explicit	O
and	O
implicit	O
operands	O
is	O
important	O
in	O
code	O
generators	O
,	O
especially	O
in	O
the	O
register	B-General_Concept
allocation	O
and	O
live	O
range	O
tracking	O
parts	O
.	O
</s>
<s>
A	O
good	O
code	O
optimizer	O
can	O
track	O
implicit	O
as	O
well	O
as	O
explicit	O
operands	O
which	O
may	O
allow	O
more	O
frequent	O
constant	O
propagation	O
,	O
constant	O
folding	O
of	O
registers	B-General_Concept
(	O
a	O
register	B-General_Concept
assigned	O
the	O
result	O
of	O
a	O
constant	O
expression	O
freed	O
up	O
by	O
replacing	O
it	O
by	O
that	O
constant	O
)	O
and	O
other	O
code	O
enhancements	O
.	O
</s>
<s>
A	O
computer	B-Application
program	I-Application
is	O
a	O
list	O
of	O
instructions	B-General_Concept
that	O
can	O
be	O
executed	O
by	O
a	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
.	O
</s>
<s>
While	O
simple	O
processors	O
are	O
able	O
to	O
execute	O
instructions	B-General_Concept
one	O
after	O
another	O
,	O
superscalar	B-General_Concept
processors	I-General_Concept
are	O
able	O
under	O
certain	O
circumstances	O
(	O
when	O
the	O
pipeline	O
is	O
full	O
)	O
of	O
executing	O
two	O
or	O
more	O
instructions	B-General_Concept
simultaneously	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
the	O
original	B-General_Concept
Intel	I-General_Concept
Pentium	I-General_Concept
from	O
1993	O
can	O
execute	O
at	O
most	O
two	O
instructions	B-General_Concept
per	O
clock	O
cycle	O
when	O
its	O
pipeline	O
is	O
full	O
.	O
</s>
<s>
Program	O
flow	O
may	O
be	O
influenced	O
by	O
special	O
'	O
jump	B-General_Concept
 '	O
instructions	B-General_Concept
that	O
transfer	O
execution	O
to	O
an	O
address	O
(	O
and	O
hence	O
instruction	O
)	O
other	O
than	O
the	O
next	O
numerically	O
sequential	O
address	O
.	O
</s>
<s>
Whether	O
these	O
conditional	B-General_Concept
jumps	I-General_Concept
occur	O
is	O
dependent	O
upon	O
a	O
condition	O
such	O
as	O
a	O
value	O
being	O
greater	O
than	O
,	O
less	O
than	O
,	O
or	O
equal	O
to	O
another	O
value	O
.	O
</s>
<s>
A	O
much	O
more	O
human	O
friendly	O
rendition	O
of	O
machine	B-Language
language	I-Language
,	O
called	O
assembly	B-Language
language	I-Language
,	O
uses	O
mnemonic	O
codes	O
to	O
refer	O
to	O
machine	B-Language
code	I-Language
instructions	B-General_Concept
,	O
rather	O
than	O
using	O
the	O
instructions	B-General_Concept
 '	O
numeric	O
values	O
directly	O
,	O
and	O
uses	O
symbolic	B-Application
names	I-Application
to	O
refer	O
to	O
storage	O
locations	O
and	O
sometimes	O
registers	B-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
on	O
the	O
Zilog	B-General_Concept
Z80	I-General_Concept
processor	O
,	O
the	O
machine	B-Language
code	I-Language
00000101	O
,	O
which	O
causes	O
the	O
CPU	O
to	O
decrement	O
the	O
B	O
general-purpose	O
register	B-General_Concept
,	O
would	O
be	O
represented	O
in	O
assembly	B-Language
language	I-Language
as	O
DEC	O
B	O
.	O
</s>
<s>
The	O
MIPS	B-Device
architecture	I-Device
provides	O
a	O
specific	O
example	O
for	O
a	O
machine	B-Language
code	I-Language
whose	O
instructions	B-General_Concept
are	O
always	O
32	O
bits	O
long	O
.	O
</s>
<s>
J-type	O
(	O
jump	B-General_Concept
)	O
and	O
I-type	O
(	O
immediate	O
)	O
instructions	B-General_Concept
are	O
fully	O
specified	O
by	O
op	O
.	O
</s>
<s>
R-type	O
(	O
register	B-General_Concept
)	O
instructions	B-General_Concept
include	O
an	O
additional	O
field	O
funct	O
to	O
determine	O
the	O
exact	O
operation	O
.	O
</s>
<s>
rs	O
,	O
rt	O
,	O
and	O
rd	O
indicate	O
register	B-General_Concept
operands	O
;	O
shamt	O
gives	O
a	O
shift	O
amount	O
;	O
and	O
the	O
address	O
or	O
immediate	O
fields	O
contain	O
an	O
operand	O
directly	O
.	O
</s>
<s>
For	O
example	O
,	O
adding	O
the	O
registers	B-General_Concept
1	O
and	O
2	O
and	O
placing	O
the	O
result	O
in	O
register	B-General_Concept
6	O
is	O
encoded	O
:	O
</s>
<s>
Load	O
a	O
value	O
into	O
register	B-General_Concept
8	O
,	O
taken	O
from	O
the	O
memory	B-General_Concept
cell	O
68	O
cells	O
after	O
the	O
location	O
listed	O
in	O
register	B-General_Concept
3	O
:	O
</s>
<s>
On	O
processor	O
architectures	O
with	O
variable-length	O
instruction	B-General_Concept
sets	I-General_Concept
(	O
such	O
as	O
Intel	O
's	O
x86	B-Operating_System
processor	O
family	O
)	O
it	O
is	O
,	O
within	O
the	O
limits	O
of	O
the	O
control-flow	O
resynchronizing	O
phenomenon	O
known	O
as	O
the	O
Kruskal	O
Count	O
,	O
sometimes	O
possible	O
through	O
opcode-level	B-Language
programming	I-Language
to	O
deliberately	O
arrange	O
the	O
resulting	O
code	O
so	O
that	O
two	O
code	O
paths	O
share	O
a	O
common	O
fragment	O
of	O
opcode	B-Language
sequences	I-Language
.	O
</s>
<s>
These	O
are	O
called	O
overlapping	O
instructions	B-General_Concept
,	O
overlapping	O
opcodes	B-Language
,	O
overlapping	O
code	O
,	O
overlapped	O
code	O
,	O
instruction	O
scission	O
,	O
or	O
jump	B-General_Concept
into	O
the	O
middle	O
of	O
an	O
instruction	O
,	O
and	O
represent	O
a	O
form	O
of	O
superposition	O
.	O
</s>
<s>
In	O
the	O
1970s	O
and	O
1980s	O
,	O
overlapping	O
instructions	B-General_Concept
were	O
sometimes	O
used	O
to	O
preserve	O
memory	B-General_Concept
space	O
.	O
</s>
<s>
One	O
example	O
were	O
in	O
the	O
implementation	O
of	O
error	O
tables	O
in	O
Microsoft	O
's	O
Altair	B-Language
BASIC	I-Language
,	O
where	O
interleaved	O
instructions	B-General_Concept
mutually	O
shared	O
their	O
instruction	B-Language
bytes	I-Language
.	O
</s>
<s>
The	O
technique	O
is	O
rarely	O
used	O
today	O
,	O
but	O
might	O
still	O
be	O
necessary	O
to	O
resort	O
to	O
in	O
areas	O
where	O
extreme	O
optimization	O
for	O
size	O
is	O
necessary	O
on	O
byte-level	O
such	O
as	O
in	O
the	O
implementation	O
of	O
boot	B-Application
loaders	I-Application
which	O
have	O
to	O
fit	O
into	O
boot	B-Device
sectors	I-Device
.	O
</s>
<s>
This	O
property	O
is	O
also	O
used	O
to	O
find	O
unintended	B-Language
instructions	I-Language
called	O
gadgets	O
in	O
existing	O
code	O
repositories	O
and	O
is	O
utilized	O
in	O
return-oriented	O
programming	O
as	O
alternative	O
to	O
code	O
injection	O
for	O
exploits	O
such	O
as	O
return-to-libc	B-Language
attacks	O
.	O
</s>
<s>
In	O
some	O
computers	O
,	O
the	O
machine	B-Language
code	I-Language
of	O
the	O
architecture	B-General_Concept
is	O
implemented	O
by	O
an	O
even	O
more	O
fundamental	O
underlying	O
layer	O
called	O
microcode	B-Device
,	O
providing	O
a	O
common	O
machine	B-Language
language	I-Language
interface	O
across	O
a	O
line	O
or	O
family	O
of	O
different	O
models	O
of	O
computer	O
with	O
widely	O
different	O
underlying	O
dataflows	B-Application
.	O
</s>
<s>
This	O
is	O
done	O
to	O
facilitate	O
porting	O
of	O
machine	B-Language
language	I-Language
programs	O
between	O
different	O
models	O
.	O
</s>
<s>
An	O
example	O
of	O
this	O
use	O
is	O
the	O
IBM	B-Application
System/360	I-Application
family	O
of	O
computers	O
and	O
their	O
successors	O
.	O
</s>
<s>
With	O
dataflow	B-Application
path	O
widths	O
of	O
8bits	O
to	O
64bits	O
and	O
beyond	O
,	O
they	O
nevertheless	O
present	O
a	O
common	O
architecture	B-General_Concept
at	O
the	O
machine	B-Language
language	I-Language
level	O
across	O
the	O
entire	O
line	O
.	O
</s>
<s>
Using	O
microcode	B-Device
to	O
implement	O
an	O
emulator	B-Application
enables	O
the	O
computer	O
to	O
present	O
the	O
architecture	B-General_Concept
of	O
an	O
entirely	O
different	O
computer	O
.	O
</s>
<s>
The	O
System/360	B-Application
line	O
used	O
this	O
to	O
allow	O
porting	O
programs	O
from	O
earlier	O
IBM	O
machines	O
to	O
the	O
new	O
family	O
of	O
computers	O
,	O
e.g.	O
</s>
<s>
an	O
IBM	B-Device
1401/1440/1460	I-Device
emulator	B-Application
on	O
the	O
IBM	B-Application
S/360	I-Application
model	O
40	O
.	O
</s>
<s>
Machine	B-Language
code	I-Language
is	O
generally	O
different	O
from	O
bytecode	O
(	O
also	O
known	O
as	O
p-code	O
)	O
,	O
which	O
is	O
either	O
executed	O
by	O
an	O
interpreter	B-Application
or	O
itself	O
compiled	B-Language
into	O
machine	B-Language
code	I-Language
for	O
faster	O
(	O
direct	O
)	O
execution	O
.	O
</s>
<s>
An	O
exception	O
is	O
when	O
a	O
processor	O
is	O
designed	O
to	O
use	O
a	O
particular	O
bytecode	O
directly	O
as	O
its	O
machine	B-Language
code	I-Language
,	O
such	O
as	O
is	O
the	O
case	O
with	O
Java	B-Language
processors	I-Language
.	O
</s>
<s>
Machine	B-Language
code	I-Language
and	O
assembly	B-Language
code	I-Language
are	O
sometimes	O
called	O
native	B-Language
code	I-Language
when	O
referring	O
to	O
platform-dependent	O
parts	O
of	O
language	O
features	O
or	O
libraries	O
.	O
</s>
<s>
From	O
the	O
point	O
of	O
view	O
of	O
the	O
CPU	O
,	O
machine	B-Language
code	I-Language
is	O
stored	O
in	O
RAM	O
,	O
but	O
is	O
typically	O
also	O
kept	O
in	O
a	O
set	O
of	O
caches	O
for	O
performance	O
reasons	O
.	O
</s>
<s>
There	O
may	O
be	O
different	O
caches	O
for	O
instructions	B-General_Concept
and	O
data	O
,	O
depending	O
on	O
the	O
architecture	B-General_Concept
.	O
</s>
<s>
The	O
CPU	O
knows	O
what	O
machine	B-Language
code	I-Language
to	O
execute	O
,	O
based	O
on	O
its	O
internal	O
program	O
counter	O
.	O
</s>
<s>
The	O
program	O
counter	O
points	O
to	O
a	O
memory	B-General_Concept
address	I-General_Concept
and	O
is	O
changed	O
based	O
on	O
special	O
instructions	B-General_Concept
which	O
may	O
cause	O
programmatic	O
branches	O
.	O
</s>
<s>
The	O
program	O
counter	O
is	O
typically	O
set	O
to	O
a	O
hard	O
coded	O
value	O
when	O
the	O
CPU	O
is	O
first	O
powered	O
on	O
,	O
and	O
will	O
hence	O
execute	O
whatever	O
machine	B-Language
code	I-Language
happens	O
to	O
be	O
at	O
this	O
address	O
.	O
</s>
<s>
Similarly	O
,	O
the	O
program	O
counter	O
can	O
be	O
set	O
to	O
execute	O
whatever	O
machine	B-Language
code	I-Language
is	O
at	O
some	O
arbitrary	O
address	O
,	O
even	O
if	O
this	O
is	O
n't	O
valid	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
This	O
will	O
typically	O
trigger	O
an	O
architecture	B-General_Concept
specific	O
protection	O
fault	O
.	O
</s>
<s>
The	O
CPU	O
is	O
oftentimes	O
told	O
,	O
by	O
page	O
permissions	O
in	O
a	O
paging	O
based	O
system	O
,	O
if	O
the	O
current	O
page	O
actually	O
holds	O
machine	B-Language
code	I-Language
by	O
an	O
execute	O
bit	O
—	O
pages	O
have	O
multiple	O
such	O
permission	O
bits	O
(	O
readable	O
,	O
writable	O
,	O
etc	O
.	O
)	O
</s>
<s>
on	O
Unix-like	B-Operating_System
systems	I-Operating_System
memory	B-General_Concept
pages	O
can	O
be	O
toggled	O
to	O
be	O
executable	O
with	O
the	O
system	O
call	O
,	O
and	O
on	O
Windows	O
,	O
can	O
be	O
used	O
to	O
achieve	O
a	O
similar	O
result	O
.	O
</s>
<s>
If	O
an	O
attempt	O
is	O
made	O
to	O
execute	O
machine	B-Language
code	I-Language
on	O
a	O
non-executable	O
page	O
,	O
an	O
architecture	B-General_Concept
specific	O
fault	O
will	O
typically	O
occur	O
.	O
</s>
<s>
Treating	O
data	O
as	O
machine	B-Language
code	I-Language
,	O
or	O
finding	O
new	O
ways	O
to	O
use	O
existing	O
machine	B-Language
code	I-Language
,	O
by	O
various	O
techniques	O
,	O
is	O
the	O
basis	O
of	O
some	O
security	O
vulnerabilities	O
.	O
</s>
<s>
From	O
the	O
point	O
of	O
view	O
of	O
a	O
process	B-Operating_System
,	O
the	O
code	O
space	O
is	O
the	O
part	O
of	O
its	O
address	B-General_Concept
space	I-General_Concept
where	O
the	O
code	O
in	O
execution	O
is	O
stored	O
.	O
</s>
<s>
In	O
multitasking	B-Operating_System
systems	O
this	O
comprises	O
the	O
program	O
's	O
code	B-General_Concept
segment	I-General_Concept
and	O
usually	O
shared	O
libraries	O
.	O
</s>
<s>
In	O
multi-threading	B-Operating_System
environment	O
,	O
different	O
threads	B-Operating_System
of	O
one	O
process	B-Operating_System
share	O
code	O
space	O
along	O
with	O
data	O
space	O
,	O
which	O
reduces	O
the	O
overhead	O
of	O
context	B-Operating_System
switching	I-Operating_System
considerably	O
as	O
compared	O
to	O
process	B-Operating_System
switching	O
.	O
</s>
<s>
Pamela	O
Samuelson	O
wrote	O
that	O
machine	B-Language
code	I-Language
is	O
so	O
unreadable	O
that	O
the	O
United	O
States	O
Copyright	O
Office	O
cannot	O
identify	O
whether	O
a	O
particular	O
encoded	O
program	O
is	O
an	O
original	O
work	O
of	O
authorship	O
;	O
however	O
,	O
the	O
US	O
Copyright	O
Office	O
allow	O
for	O
copyright	O
registration	O
of	O
computer	B-Application
programs	I-Application
and	O
a	O
program	O
's	O
machine	B-Language
code	I-Language
can	O
sometimes	O
be	O
decompiled	B-Application
in	O
order	O
to	O
make	O
its	O
functioning	O
more	O
easily	O
understandable	O
to	O
humans	O
.	O
</s>
<s>
However	O
,	O
the	O
output	O
of	O
a	O
decompiler	B-Application
or	O
disassembler	O
will	O
be	O
missing	O
the	O
comments	O
and	O
symbolic	O
references	O
,	O
so	O
while	O
the	O
output	O
may	O
be	O
easier	O
to	O
read	O
than	O
the	O
object	B-Language
code	I-Language
,	O
it	O
will	O
still	O
be	O
more	O
difficult	O
than	O
the	O
original	O
source	O
code	O
.	O
</s>
<s>
Cognitive	O
science	O
professor	O
Douglas	O
Hofstadter	O
has	O
compared	O
machine	B-Language
code	I-Language
to	O
genetic	O
code	O
,	O
saying	O
that	O
"	O
Looking	O
at	O
a	O
program	O
written	O
in	O
machine	B-Language
language	I-Language
is	O
vaguely	O
comparable	O
to	O
looking	O
at	O
a	O
DNA	O
molecule	O
atom	O
by	O
atom.	O
"	O
</s>
