<s>
An	O
active	B-Language
object	I-Language
framework	O
is	O
a	O
callback-based	O
form	O
of	O
multitasking	B-Operating_System
for	O
computer	O
systems	O
.	O
</s>
<s>
Specifically	O
,	O
it	O
is	O
a	O
form	O
of	O
cooperative	B-Operating_System
multitasking	I-Operating_System
and	O
is	O
an	O
important	O
feature	O
of	O
the	O
Symbian	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
Within	O
the	O
framework	O
,	O
active	B-Language
objects	I-Language
may	O
make	O
requests	O
of	O
asynchronous	B-Architecture
services	O
(	O
e.g.	O
</s>
<s>
When	O
an	O
asynchronous	B-Architecture
request	O
is	O
made	O
,	O
control	O
is	O
returned	O
to	O
the	O
calling	O
object	O
immediately	O
(	O
i.e.	O
</s>
<s>
The	O
caller	O
may	O
choose	O
to	O
do	O
other	O
things	O
before	O
it	O
returns	O
control	O
back	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
which	O
typically	O
schedules	O
other	O
tasks	O
or	O
puts	O
the	O
machine	O
to	O
sleep	O
.	O
</s>
<s>
When	O
the	O
asynchronous	B-Architecture
task	O
completes	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
identifies	O
the	O
thread	B-Operating_System
containing	O
the	O
requesting	O
active	B-Language
object	I-Language
,	O
and	O
wakes	O
it	O
up	O
.	O
</s>
<s>
An	O
"	O
active	O
scheduler	O
"	O
in	O
the	O
thread	B-Operating_System
identifies	O
the	O
object	O
that	O
made	O
the	O
request	O
,	O
and	O
passes	O
control	O
back	O
to	O
that	O
object	O
.	O
</s>
<s>
The	O
implementation	O
of	O
active	B-Language
objects	I-Language
in	O
Symbian	B-Operating_System
is	O
based	O
around	O
each	O
thread	B-Operating_System
having	O
a	O
"	O
request	O
semaphore	B-Operating_System
"	O
.	O
</s>
<s>
This	O
is	O
incremented	O
when	O
a	O
thread	B-Operating_System
makes	O
an	O
asynchronous	B-Architecture
request	O
,	O
and	O
decremented	O
when	O
the	O
request	O
is	O
completed	O
.	O
</s>
<s>
When	O
there	O
are	O
no	O
outstanding	O
requests	O
,	O
the	O
thread	B-Operating_System
is	O
put	O
to	O
sleep	O
.	O
</s>
<s>
In	O
practice	O
there	O
may	O
be	O
many	O
active	B-Language
objects	I-Language
in	O
a	O
thread	B-Operating_System
,	O
each	O
doing	O
its	O
own	O
task	O
.	O
</s>
<s>
They	O
can	O
interact	O
by	O
requesting	O
things	O
of	O
each	O
other	O
,	O
and	O
of	O
active	B-Language
objects	I-Language
in	O
other	O
threads	B-Operating_System
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
was	O
acting	O
as	O
the	O
first	O
object	O
and	O
the	O
peripheral	O
as	O
the	O
second	O
one	O
.	O
</s>
