<s>
A	O
computer	B-Application
program	I-Application
is	O
said	O
to	O
be	O
portable	O
if	O
there	O
is	O
very	O
low	O
effort	O
required	O
to	O
make	O
it	O
run	O
on	O
different	O
platforms	B-Device
.	O
</s>
<s>
The	O
pre-requirement	O
for	O
portability	B-Architecture
is	O
the	O
generalized	O
abstraction	B-Application
between	O
the	O
application	O
logic	O
and	O
system	B-Application
interfaces	I-Application
.	O
</s>
<s>
When	O
software	O
with	O
the	O
same	O
functionality	O
is	O
produced	O
for	O
several	O
computing	B-Device
platforms	I-Device
,	O
portability	B-Architecture
is	O
the	O
key	O
issue	O
for	O
development	O
cost	O
reduction	O
.	O
</s>
<s>
Software	B-Architecture
portability	I-Architecture
may	O
involve	O
:	O
</s>
<s>
Transferring	O
installed	B-Application
program	B-Application
files	I-Application
to	O
another	O
computer	O
of	O
basically	O
the	O
same	O
architecture	O
.	O
</s>
<s>
Building	B-Language
executable	B-Application
programs	I-Application
for	O
different	O
platforms	B-Device
from	O
source	O
code	O
;	O
this	O
is	O
what	O
is	O
usually	O
understood	O
by	O
"	O
porting	O
"	O
.	O
</s>
<s>
When	O
operating	B-General_Concept
systems	I-General_Concept
of	O
the	O
same	O
family	O
are	O
installed	B-Application
on	O
two	O
computers	O
with	O
processors	B-Device
with	O
similar	O
instruction	B-General_Concept
sets	I-General_Concept
it	O
is	O
often	O
possible	O
to	O
transfer	O
the	O
files	O
implementing	O
program	B-Application
files	I-Application
between	O
them	O
.	O
</s>
<s>
However	O
,	O
in	O
many	O
cases	O
,	O
the	O
software	O
is	O
installed	B-Application
on	O
a	O
computer	O
in	O
a	O
way	O
which	O
depends	O
upon	O
its	O
detailed	O
hardware	O
,	O
software	O
,	O
and	O
setup	O
,	O
with	O
device	B-Application
drivers	I-Application
for	O
particular	O
devices	O
,	O
using	O
installed	B-Application
operating	B-General_Concept
system	I-General_Concept
and	O
supporting	O
software	O
components	O
,	O
and	O
using	O
different	O
drives	B-Application
or	O
directories	B-Application
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
software	O
,	O
usually	O
described	O
as	O
"	O
portable	B-Application
software	I-Application
"	O
,	O
is	O
specifically	O
designed	O
to	O
run	O
on	O
different	O
computers	O
with	O
compatible	O
operating	B-General_Concept
systems	I-General_Concept
and	O
processors	B-Device
,	O
without	O
any	O
machine-dependent	O
installation	B-Application
.	O
</s>
<s>
Porting	O
is	O
no	O
more	O
than	O
transferring	O
specified	O
directories	B-Application
and	O
their	O
contents	O
.	O
</s>
<s>
Software	O
installed	B-Application
on	O
portable	O
mass	B-Device
storage	I-Device
devices	I-Device
such	O
as	O
USB	O
sticks	O
can	O
be	O
used	O
on	O
any	O
compatible	O
computer	O
on	O
simply	O
plugging	O
the	O
storage	O
device	O
in	O
,	O
and	O
stores	O
all	O
configuration	O
information	O
on	O
the	O
removable	O
device	O
.	O
</s>
<s>
Hardware	O
-	O
and	O
software-specific	O
information	O
is	O
often	O
stored	O
in	O
configuration	B-Protocol
files	I-Protocol
in	O
specified	O
locations	O
(	O
e.g.	O
</s>
<s>
the	O
registry	B-Protocol
on	O
machines	O
running	O
Microsoft	B-Application
Windows	I-Application
)	O
.	O
</s>
<s>
the	O
majority	O
of	O
desktop	O
and	O
laptop	O
computers	O
used	O
microprocessors	B-Architecture
compatible	O
with	O
the	O
32	O
-	O
and	O
64-bit	O
x86	B-Operating_System
instruction	B-General_Concept
sets	I-General_Concept
.	O
</s>
<s>
Smaller	O
portable	O
devices	O
use	O
processors	B-Device
with	O
different	O
and	O
incompatible	O
instruction	B-General_Concept
sets	I-General_Concept
,	O
such	O
as	O
ARM	B-Architecture
.	O
</s>
<s>
Web	B-Application
applications	I-Application
are	O
required	O
to	O
be	O
processor	O
independent	O
,	O
so	O
portability	B-Architecture
can	O
be	O
achieved	O
by	O
using	O
web	O
programming	O
techniques	O
,	O
writing	O
in	O
JavaScript	B-Language
.	O
</s>
<s>
Such	O
web	B-Application
applications	I-Application
must	O
,	O
for	O
security	O
reasons	O
,	O
have	O
limited	O
control	O
over	O
the	O
host	O
computer	O
,	O
especially	O
regarding	O
reading	O
and	O
writing	O
files	O
.	O
</s>
<s>
Non-web	O
programs	O
,	O
installed	B-Application
upon	O
a	O
computer	O
in	O
the	O
normal	O
manner	O
,	O
can	O
have	O
more	O
control	O
,	O
and	O
yet	O
achieve	O
system	O
portability	B-Architecture
by	O
linking	B-Application
to	O
portable	O
libraries	O
providing	O
the	O
same	O
interface	O
on	O
different	O
systems	O
.	O
</s>
<s>
Software	O
can	O
be	O
compiled	B-Language
and	O
linked	B-Application
from	O
source	O
code	O
for	O
different	O
operating	B-General_Concept
systems	I-General_Concept
and	O
processors	B-Device
if	O
written	O
in	O
a	O
programming	O
language	O
supporting	O
compilation	B-Language
for	O
the	O
platforms	B-Device
.	O
</s>
<s>
In	O
open-source	B-Application
environments	O
such	O
as	O
Linux	O
the	O
source	O
code	O
is	O
available	O
to	O
all	O
.	O
</s>
<s>
In	O
earlier	O
days	O
source	O
code	O
was	O
often	O
distributed	O
in	O
a	O
standardised	O
format	O
,	O
and	O
could	O
be	O
built	O
into	O
executable	B-Application
code	I-Application
with	O
a	O
standard	O
Make	B-Application
tool	I-Application
for	O
any	O
particular	O
system	O
by	O
moderately	O
knowledgeable	O
users	O
if	O
no	O
errors	O
occurred	O
during	O
the	O
build	O
.	O
</s>
<s>
Some	O
Linux	B-Application
distributions	I-Application
distribute	O
software	O
to	O
users	O
in	O
source	O
form	O
.	O
</s>
<s>
In	O
these	O
cases	O
there	O
is	O
usually	O
no	O
need	O
for	O
detailed	O
adaptation	O
of	O
the	O
software	O
for	O
the	O
system	O
;	O
it	O
is	O
distributed	O
in	O
a	O
way	O
which	O
modifies	B-Application
the	I-Application
compilation	I-Application
process	I-Application
to	I-Application
match	I-Application
the	I-Application
system	I-Application
.	O
</s>
<s>
Even	O
with	O
seemingly	O
portable	O
languages	O
like	O
C	B-Language
and	O
C++	B-Language
,	O
the	O
effort	O
to	O
port	O
source	O
code	O
can	O
vary	O
considerably	O
.	O
</s>
<s>
The	O
authors	O
of	O
UNIX/32V	B-Operating_System
(	O
1979	O
)	O
reported	O
that	O
"[t]he	O
(	O
Bourne	O
)	O
shell	O
 [ ... ] 	O
required	O
by	O
far	O
the	O
largest	O
conversion	O
effort	O
of	O
any	O
supposedly	O
portable	O
program	O
,	O
for	O
the	O
simple	O
reason	O
that	O
it	O
is	O
not	O
portable.	O
"	O
</s>
<s>
right	O
shifting	O
a	O
signed	O
integer	O
in	O
C	B-Language
can	O
do	O
a	O
logical	O
or	O
an	O
arithmetic	O
shift	O
)	O
.	O
</s>
<s>
Operating	B-General_Concept
system	I-General_Concept
functions	O
or	O
third	O
party	O
libraries	O
might	O
not	O
be	O
available	O
on	O
the	O
target	O
system	O
.	O
</s>
<s>
:	O
utime( )	O
fails	O
under	O
Windows	O
with	O
EACCES	O
,	O
when	O
it	O
is	O
called	O
for	O
a	O
directory	B-Application
)	O
.	O
</s>
<s>
Drive	O
letters	O
and	O
the	O
backslash	O
as	O
path	O
delimiter	O
are	O
not	O
accepted	O
on	O
all	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
In	O
practice	O
the	O
claim	O
of	O
languages	O
,	O
like	O
C	B-Language
and	O
C++	B-Language
,	O
to	O
have	O
the	O
WOCA	O
(	O
write	O
once	O
,	O
compile	B-Language
anywhere	O
)	O
is	O
arguable	O
.	O
</s>
