<s>
Polling	B-General_Concept
,	O
or	O
polled	O
operation	O
,	O
in	O
computer	B-General_Concept
science	I-General_Concept
,	O
refers	O
to	O
actively	O
sampling	O
the	O
status	O
of	O
an	O
external	B-Device
device	I-Device
by	O
a	O
client	B-Protocol
program	I-Protocol
as	O
a	O
synchronous	O
activity	O
.	O
</s>
<s>
Polling	B-General_Concept
is	O
most	O
often	O
used	O
in	O
terms	O
of	O
input/output	B-General_Concept
(	O
)	O
,	O
and	O
is	O
also	O
referred	O
to	O
as	O
polled	O
or	O
software-driven	O
.	O
</s>
<s>
A	O
good	O
example	O
of	O
hardware	O
implementation	O
is	O
a	O
watchdog	B-Application
timer	I-Application
.	O
</s>
<s>
Polling	B-General_Concept
is	O
the	O
process	O
where	O
the	O
computer	O
or	O
controlling	O
device	O
waits	O
for	O
an	O
external	B-Device
device	I-Device
to	O
check	O
for	O
its	O
readiness	O
or	O
state	O
,	O
often	O
with	O
low-level	B-Language
hardware	I-Language
.	O
</s>
<s>
These	O
processes	O
can	O
be	O
as	O
minute	O
as	O
only	O
reading	O
one	B-General_Concept
bit	I-General_Concept
.	O
</s>
<s>
This	O
is	O
sometimes	O
used	O
synonymously	O
with	O
'	O
busy-wait	B-Operating_System
'	O
polling	B-General_Concept
.	O
</s>
<s>
Polling	B-General_Concept
also	O
refers	O
to	O
the	O
situation	O
where	O
a	O
device	O
is	O
repeatedly	O
checked	O
for	O
readiness	O
,	O
and	O
if	O
it	O
is	O
not	O
,	O
the	O
computer	O
returns	O
to	O
a	O
different	O
task	O
.	O
</s>
<s>
Although	O
not	O
as	O
wasteful	O
of	O
CPU	B-Device
cycles	O
as	O
busy	B-Operating_System
waiting	I-Operating_System
,	O
this	O
is	O
generally	O
not	O
as	O
efficient	O
as	O
the	O
alternative	O
to	O
polling	B-General_Concept
,	O
interrupt-driven	O
.	O
</s>
<s>
In	O
a	O
simple	O
single-purpose	O
system	O
,	O
even	O
busy-wait	B-Operating_System
is	O
perfectly	O
appropriate	O
if	O
no	O
action	O
is	O
possible	O
until	O
the	O
access	O
,	O
but	O
more	O
often	O
than	O
not	O
this	O
was	O
traditionally	O
a	O
consequence	O
of	O
simple	O
hardware	O
or	O
non-multitasking	O
operating	O
systems	O
.	O
</s>
<s>
Polling	B-General_Concept
is	O
often	O
intimately	O
involved	O
with	O
very	O
low-level	B-Language
hardware	I-Language
.	O
</s>
<s>
For	O
example	O
,	O
polling	B-General_Concept
a	O
parallel	O
printer	O
port	O
to	O
check	O
whether	O
it	O
is	O
ready	O
for	O
another	O
character	O
involves	O
examining	O
as	O
little	O
as	O
one	B-General_Concept
bit	I-General_Concept
of	O
a	O
byte	B-Application
.	O
</s>
<s>
The	O
instruction	O
that	O
reads	O
this	O
byte	B-Application
directly	O
transfers	O
the	O
voltage	O
state	O
of	O
eight	O
real	O
world	O
wires	O
to	O
the	O
eight	O
circuits	O
(	O
flip	B-General_Concept
flops	I-General_Concept
)	O
that	O
make	O
up	O
one	O
byte	B-Application
of	O
a	O
CPU	B-Device
register	B-General_Concept
.	O
</s>
<s>
Polling	B-General_Concept
has	O
the	O
disadvantage	O
that	O
if	O
there	O
are	O
too	O
many	O
devices	O
to	O
check	O
,	O
the	O
time	O
required	O
to	O
poll	O
them	O
can	O
exceed	O
the	O
time	O
available	O
to	O
service	O
the	O
I/O	B-General_Concept
device	I-General_Concept
.	O
</s>
<s>
Polling	B-General_Concept
can	O
be	O
described	O
in	O
the	O
following	O
steps	O
:	O
</s>
<s>
The	O
host	O
repeatedly	O
reads	O
the	O
busy	B-General_Concept
bit	I-General_Concept
of	O
the	O
controller	O
until	O
it	O
becomes	O
clear	O
(	O
with	O
a	O
value	O
of	O
0	O
)	O
.	O
</s>
<s>
When	O
clear	O
,	O
the	O
host	O
writes	O
the	O
command	O
into	O
the	O
command	O
register	B-General_Concept
.	O
</s>
<s>
If	O
the	O
host	O
is	O
sending	O
output	O
,	O
it	O
sets	O
the	O
write	O
bit	O
and	O
writes	O
a	O
byte	B-Application
into	O
the	O
data-out	O
register	B-General_Concept
.	O
</s>
<s>
If	O
the	O
host	O
is	O
receiving	O
input	O
,	O
it	O
reads	O
the	O
controller-written	O
data	O
from	O
the	O
data-in	O
register	B-General_Concept
,	O
and	O
sets	O
the	O
read	O
bit	O
to	O
0	O
as	O
the	O
next	O
command	O
.	O
</s>
<s>
When	O
the	O
controller	O
notices	O
that	O
the	O
command-ready	O
bit	O
is	O
set	O
,	O
it	O
sets	O
the	O
busy	B-General_Concept
bit	I-General_Concept
to	O
1	O
.	O
</s>
<s>
The	O
controller	O
reads	O
the	O
command	O
register	B-General_Concept
.	O
</s>
<s>
If	O
the	O
write	O
bit	O
inside	O
is	O
set	O
,	O
it	O
reads	O
from	O
the	O
data-out	O
register	B-General_Concept
and	O
performs	O
the	O
necessary	O
operations	O
on	O
the	O
device	O
.	O
</s>
<s>
If	O
the	O
read	O
bit	O
is	O
set	O
,	O
data	O
from	O
the	O
device	O
is	O
loaded	O
into	O
the	O
data-in	O
register	B-General_Concept
for	O
the	O
host	O
to	O
read	O
.	O
</s>
<s>
Once	O
the	O
operations	O
are	O
over	O
,	O
the	O
controller	O
clears	O
the	O
command-ready	O
bit	O
,	O
clears	O
the	O
error	O
bit	O
to	O
show	O
the	O
operation	O
was	O
successful	O
,	O
and	O
clears	O
the	O
busy	B-General_Concept
bit	I-General_Concept
.	O
</s>
<s>
A	O
polling	B-General_Concept
cycle	O
is	O
the	O
time	O
in	O
which	O
each	O
element	O
is	O
monitored	O
once	O
.	O
</s>
<s>
The	O
optimal	O
polling	B-General_Concept
cycle	O
will	O
vary	O
according	O
to	O
several	O
factors	O
,	O
including	O
the	O
desired	O
speed	O
of	O
response	O
and	O
the	O
overhead	O
(	O
e.g.	O
,	O
processor	O
time	O
and	O
bandwidth	O
)	O
of	O
the	O
polling	B-General_Concept
.	O
</s>
<s>
In	O
roll	O
call	O
polling	B-General_Concept
,	O
the	O
polling	B-General_Concept
device	O
or	O
process	O
queries	O
each	O
element	O
on	O
a	O
list	O
in	O
a	O
fixed	O
sequence	O
.	O
</s>
<s>
Roll	O
call	O
polling	B-General_Concept
can	O
be	O
inefficient	O
if	O
the	O
overhead	O
for	O
the	O
polling	B-General_Concept
messages	O
is	O
high	O
,	O
there	O
are	O
numerous	O
elements	O
to	O
be	O
polled	O
in	O
each	O
polling	B-General_Concept
cycle	O
and	O
only	O
a	O
few	O
elements	O
are	O
active	O
.	O
</s>
<s>
In	O
hub	B-Protocol
polling	B-General_Concept
,	O
also	O
referred	O
to	O
as	O
token	O
polling	B-General_Concept
,	O
each	O
element	O
polls	O
the	O
next	O
element	O
in	O
some	O
fixed	O
sequence	O
.	O
</s>
<s>
This	O
continues	O
until	O
the	O
first	O
element	O
is	O
reached	O
,	O
at	O
which	O
time	O
the	O
polling	B-General_Concept
cycle	O
starts	O
all	O
over	O
again	O
.	O
</s>
<s>
Polling	B-General_Concept
can	O
be	O
employed	O
in	O
various	O
computing	O
contexts	O
in	O
order	O
to	O
control	O
the	O
execution	O
or	O
transmission	O
sequence	O
of	O
the	O
elements	O
involved	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
multitasking	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
polling	B-General_Concept
can	O
be	O
used	O
to	O
allocate	O
processor	O
time	O
and	O
other	O
resources	O
to	O
the	O
various	O
competing	O
processes	O
.	O
</s>
<s>
In	O
networks	O
,	O
polling	B-General_Concept
is	O
used	O
to	O
determine	O
which	O
nodes	O
want	O
to	O
access	O
the	O
network	O
.	O
</s>
<s>
It	O
is	O
also	O
used	O
by	O
routing	O
protocols	O
to	O
retrieve	O
routing	O
information	O
,	O
as	O
is	O
the	O
case	O
with	O
EGP	O
(	O
exterior	B-Protocol
gateway	I-Protocol
protocol	I-Protocol
)	O
.	O
</s>
<s>
An	O
alternative	O
to	O
polling	B-General_Concept
is	O
the	O
use	O
of	O
interrupts	B-Application
,	O
which	O
are	O
signals	O
generated	O
by	O
devices	O
or	O
processes	O
to	O
indicate	O
that	O
they	O
need	O
attention	O
,	O
want	O
to	O
communicate	O
,	O
etc	O
.	O
</s>
<s>
Although	O
polling	B-General_Concept
can	O
be	O
very	O
simple	O
,	O
in	O
many	O
situations	O
(	O
e.g.	O
,	O
multitasking	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
)	O
it	O
is	O
more	O
efficient	O
to	O
use	O
interrupts	B-Application
because	O
it	O
can	O
reduce	O
processor	O
usage	O
and/or	O
bandwidth	O
consumption	O
.	O
</s>
<s>
In	O
a	O
multidrop	O
line	O
arrangement	O
(	O
a	O
central	O
computer	O
and	O
different	O
terminals	O
in	O
which	O
the	O
terminals	O
share	O
a	O
single	O
communication	O
line	O
to	O
and	O
from	O
the	O
computer	O
)	O
,	O
the	O
system	O
uses	O
a	O
master/slave	B-Operating_System
polling	B-General_Concept
arrangement	O
whereby	O
the	O
central	O
computer	O
sends	O
message	O
(	O
called	O
polling	B-General_Concept
message	O
)	O
to	O
a	O
specific	O
terminal	O
on	O
the	O
outgoing	O
line	O
.	O
</s>
<s>
In	O
star	B-Architecture
networks	I-Architecture
,	O
which	O
,	O
in	O
its	O
simplest	O
form	O
,	O
consists	O
of	O
one	O
central	O
switch	B-Protocol
,	O
hub	B-Protocol
,	O
or	O
computer	O
that	O
acts	O
as	O
a	O
conduit	O
to	O
transmit	O
messages	O
,	O
polling	B-General_Concept
is	O
not	O
required	O
to	O
avoid	O
chaos	O
on	O
the	O
lines	O
,	O
but	O
it	O
is	O
often	O
used	O
to	O
allow	O
the	O
master	O
to	O
acquire	O
input	O
in	O
an	O
orderly	O
fashion	O
.	O
</s>
