<s>
In	O
programming	B-General_Concept
,	O
a	O
tag	B-Operating_System
is	O
an	O
argument	O
to	O
a	O
subroutine	O
that	O
determines	O
other	O
arguments	O
passed	O
to	O
it	O
,	O
which	O
is	O
used	O
as	O
a	O
way	O
to	O
pass	O
indefinite	O
number	O
of	O
tagged	O
parameters	O
to	O
the	O
subroutine	O
;	O
notably	O
,	O
tags	B-Operating_System
are	O
used	O
for	O
a	O
number	O
of	O
system	O
calls	O
in	O
AmigaOS	B-Application
v2.0	O
and	O
onwards	O
.	O
</s>
<s>
In	O
earlier	O
versions	O
of	O
AmigaOS	B-Application
,	O
if	O
a	O
system	O
call	O
required	O
setting	O
a	O
large	O
number	O
of	O
parameters	O
,	O
instead	O
of	O
passing	O
them	O
as	O
function	O
arguments	O
,	O
the	O
function	O
would	O
require	O
a	O
pointer	O
to	O
a	O
structure	B-General_Concept
that	O
holds	O
the	O
arguments	O
(	O
for	O
example	O
,	O
intuition.library	B-Application
'	O
s	O
OpenWindow( )	O
required	O
struct	O
NewWindow	O
with	O
17	O
different	O
parameters	O
)	O
.	O
</s>
<s>
Tags	B-Operating_System
were	O
introduced	O
in	O
AmigaOS	B-Application
2.0	O
because	O
they	O
"	O
make	O
it	O
possible	O
to	O
add	O
new	O
parameters	O
to	O
system	O
functions	O
without	O
interfering	O
with	O
the	O
original	O
parameters	O
.	O
</s>
<s>
A	O
number	O
of	O
third-party	B-General_Concept
software	I-General_Concept
libraries	B-Library
for	O
AmigaOS	B-Application
also	O
use	O
tags	B-Operating_System
extensively	O
.	O
</s>
<s>
AmigaOS	B-Application
1.3AmigaOS	O
2.0	O
+struct	O
Window	O
*	O
wnd	O
;	O
</s>
<s>
The	O
code	O
without	O
tags	B-Operating_System
is	O
obscure	O
(	O
for	O
example	O
,	O
0	O
,	O
1	O
define	O
window	O
colors	O
)	O
while	O
the	O
code	O
with	O
tags	B-Operating_System
is	O
self-documenting	O
.	O
</s>
<s>
Fewer	O
parameters	O
have	O
to	O
be	O
defined	O
with	O
tags	B-Operating_System
than	O
are	O
in	O
the	O
structure	B-General_Concept
,	O
as	O
OpenWindowTags	O
will	O
fall	O
back	O
to	O
default	O
parameters	O
.	O
</s>
<s>
AmigaOS	B-Application
provides	O
functions	O
for	O
tag	B-Operating_System
handling	O
in	O
its	O
utility.library	O
.	O
</s>
<s>
An	O
advantage	O
of	O
tags	B-Operating_System
is	O
that	O
they	O
ease	O
the	O
work	O
with	O
default	B-General_Concept
arguments	I-General_Concept
since	O
the	O
programmer	O
does	O
n't	O
have	O
to	O
specify	O
them	O
or	O
their	O
substitutes	O
.	O
</s>
<s>
From	O
this	O
follows	O
another	O
advantage	O
,	O
ease	O
of	O
achieving	O
of	O
both	O
forward	B-General_Concept
and	O
backward	B-General_Concept
compatibility	I-General_Concept
with	O
external	O
libraries	B-Library
:	O
a	O
program	O
written	O
for	O
an	O
older	O
version	O
of	O
the	O
library	O
will	O
work	O
with	O
a	O
newer	O
one	O
,	O
since	O
the	O
newer	O
library	O
will	O
simply	O
set	O
all	O
the	O
parameters	O
not	O
provided	O
by	O
the	O
program	O
to	O
their	O
default	O
values	O
;	O
and	O
a	O
program	O
written	O
for	O
a	O
newer	O
version	O
of	O
the	O
library	O
will	O
still	O
work	O
with	O
the	O
older	O
version	O
,	O
since	O
the	O
older	O
library	O
will	O
simply	O
pay	O
no	O
attention	O
to	O
the	O
newly	O
introduced	O
tags	B-Operating_System
.	O
</s>
<s>
A	O
disadvantage	O
of	O
tags	B-Operating_System
is	O
that	O
their	O
processing	O
is	O
slower	O
than	O
simply	O
reading	O
data	O
from	O
a	O
structure	B-General_Concept
or	O
the	O
stack	B-Application
.	O
</s>
<s>
Additionally	O
,	O
compile	B-Application
time	I-Application
type	O
checking	O
is	O
lost	O
.	O
</s>
