<s>
UPX	B-Language
(	O
Ultimate	O
Packer	O
for	O
Executables	O
)	O
is	O
a	O
free	B-Application
and	I-Application
open	I-Application
source	I-Application
executable	B-Application
packer	I-Application
supporting	O
a	O
number	O
of	O
file	O
formats	O
from	O
different	O
operating	O
systems	O
.	O
</s>
<s>
UPX	B-Language
uses	O
a	O
data	B-General_Concept
compression	I-General_Concept
algorithm	I-General_Concept
called	O
UCL	O
,	O
which	O
is	O
an	O
open-source	B-Application
implementation	O
of	O
portions	O
of	O
the	O
proprietary	B-Application
NRV	O
(	O
Not	O
Really	O
Vanished	O
)	O
algorithm	O
.	O
</s>
<s>
UCL	O
has	O
been	O
designed	O
to	O
be	O
simple	O
enough	O
that	O
a	O
decompressor	O
can	O
be	O
implemented	O
in	O
just	O
a	O
few	O
hundred	O
bytes	B-Application
of	O
code	O
.	O
</s>
<s>
UCL	O
requires	O
no	O
additional	O
memory	O
to	O
be	O
allocated	O
for	O
decompression	O
,	O
a	O
considerable	O
advantage	O
that	O
means	O
that	O
a	O
UPX	B-Language
packed	B-Application
executable	I-Application
usually	O
requires	O
no	O
additional	O
memory	O
.	O
</s>
<s>
UPX	B-Language
(	O
since	O
2.90	O
beta	O
)	O
can	O
use	O
LZMA	B-General_Concept
on	O
most	O
platforms	O
;	O
however	O
,	O
this	O
is	O
disabled	O
by	O
default	O
for	O
16-bit	O
due	O
to	O
slow	O
decompression	O
speed	O
on	O
older	O
computers	O
(	O
use	O
--lzma	O
to	O
force	O
it	O
on	O
)	O
.	O
</s>
<s>
Starting	O
with	O
version	O
3.91	O
,	O
UPX	B-Language
also	O
supports	O
64-Bit	O
(	O
x64	B-Device
)	O
PE	O
files	O
on	O
the	O
Windows	O
platform	O
.	O
</s>
<s>
UPX	B-Language
supports	O
two	O
mechanisms	O
for	O
decompression	O
:	O
an	O
in-place	O
technique	O
and	O
extraction	O
to	O
temporary	B-Application
file	I-Application
.	O
</s>
<s>
The	O
rest	O
uses	O
extraction	O
to	O
temporary	B-Application
file	I-Application
.	O
</s>
<s>
The	O
extraction	O
to	O
temporary	B-Application
file	I-Application
method	O
has	O
several	O
disadvantages	O
:	O
</s>
<s>
Unmodified	O
UPX	B-Language
packing	O
is	O
often	O
detected	O
and	O
unpacked	O
by	O
antivirus	B-Application
software	I-Application
scanners	O
.	O
</s>
<s>
UPX	B-Language
also	O
has	O
a	O
built-in	O
feature	O
for	O
unpacking	O
unmodified	O
executables	O
packed	O
with	O
itself	O
.	O
</s>
<s>
UPX	B-Language
supports	O
the	O
following	O
formats	O
:	O
</s>
<s>
Raw	O
8086/DOS	O
files	O
:	O
</s>
<s>
UPX	B-Language
does	O
not	O
currently	O
support	O
PE	O
files	O
containing	O
CIL	O
code	O
intended	O
to	O
run	O
on	O
the	O
.NET	O
Framework	O
.	O
</s>
