<s>
In	O
computing	B-Device
and	O
in	O
embedded	B-Architecture
systems	I-Architecture
,	O
a	O
programmable	B-Device
interval	I-Device
timer	I-Device
(	O
PIT	O
)	O
is	O
a	O
counter	O
that	O
generates	O
an	O
output	O
signal	O
when	O
it	O
reaches	O
a	O
programmed	O
count	O
.	O
</s>
<s>
The	O
output	O
signal	O
may	O
trigger	O
an	O
interrupt	B-Application
.	O
</s>
<s>
The	O
Intel	B-Device
8253	I-Device
PIT	O
was	O
the	O
original	O
timing	O
device	O
used	O
on	O
IBM	O
PC	O
compatibles	O
.	O
</s>
<s>
It	O
used	O
a	O
1.193182	O
MHz	O
clock	O
signal	O
(	O
one	O
third	O
of	O
the	O
color	O
burst	O
frequency	O
used	O
by	O
NTSC	O
,	O
one	O
twelfth	O
of	O
the	O
system	O
clock	O
crystal	O
oscillator	O
,	O
therefore	O
one	O
quarter	O
of	O
the	O
4.77	O
MHz	O
CPU	B-Device
clock	O
)	O
and	O
contains	O
three	O
timers	O
.	O
</s>
<s>
Timer	O
0	O
is	O
used	O
by	O
Microsoft	B-Application
Windows	I-Application
(	O
uniprocessor	O
)	O
and	O
Linux	B-Operating_System
as	O
a	O
system	B-Device
timer	I-Device
,	O
timer	O
1	O
was	O
historically	O
used	O
for	O
dynamic	O
random	O
access	O
memory	O
refreshes	O
and	O
timer	O
2	O
for	O
the	O
PC	B-Device
speaker	I-Device
.	O
</s>
<s>
The	O
LAPIC	B-Device
in	O
newer	O
Intel	O
systems	O
offers	O
a	O
higher-resolution	O
(	O
one	O
microsecond	O
)	O
timer	O
.	O
</s>
<s>
This	O
is	O
used	O
in	O
preference	O
to	O
the	O
PIT	O
timer	O
in	O
Linux	B-Operating_System
kernels	I-Operating_System
starting	O
with	O
2.6.18	O
.	O
</s>
