<s>
In	O
the	O
jargon	O
of	O
computer	B-General_Concept
programming	I-General_Concept
,	O
a	O
source	B-General_Concept
upgrade	I-General_Concept
is	O
a	O
modification	O
of	O
a	O
computer	B-Application
program	I-Application
's	O
source	O
code	O
,	O
which	O
adds	O
new	O
features	O
and	O
options	O
to	O
it	O
,	O
improves	O
performance	O
and	O
stability	O
,	O
or	O
fixes	O
bugs	B-Error_Name
and	O
errors	O
from	O
the	O
previous	O
version	O
.	O
</s>
<s>
There	O
are	O
two	O
popular	O
types	O
of	O
source	B-General_Concept
upgrades	I-General_Concept
,	O
which	O
are	O
listed	O
here	O
:	O
</s>
<s>
Patch	O
:	O
source	B-General_Concept
upgrade	I-General_Concept
,	O
where	O
the	O
modification	O
of	O
original	O
source	O
is	O
provided	O
in	O
a	O
special	O
patch	O
program	O
.	O
</s>
<s>
Generally	O
,	O
this	O
type	O
of	O
source	B-General_Concept
upgrade	I-General_Concept
focus	O
on	O
bug	B-Error_Name
and	O
error	O
fixing	O
,	O
not	O
on	O
adding	O
new	O
features	O
to	O
the	O
engine	O
.	O
</s>
<s>
Most	O
of	O
source	O
ports	O
are	O
created	O
for	O
old	O
,	O
classic	O
games	O
—	O
for	O
example	O
:	O
Doom	B-Application
,	O
Quake	O
I/II	O
,	O
Strife	B-Application
,	O
Duke	B-Application
Nukem	I-Application
3D	I-Application
,	O
Rise	B-Application
of	I-Application
the	I-Application
Triad	I-Application
.	O
</s>
<s>
The	O
source	O
code	O
of	O
those	O
applications	O
can	O
be	O
easily	O
and	O
legally	O
downloaded	O
from	O
the	O
Internet	O
under	O
GPL	B-License
license	I-License
–	O
they	O
were	O
released	O
by	O
the	O
developers	O
.	O
</s>
