<s>
In	O
a	O
computer	O
,	O
an	O
interrupt	B-General_Concept
request	I-General_Concept
(	O
or	O
IRQ	B-General_Concept
)	O
is	O
a	O
hardware	O
signal	O
sent	O
to	O
the	O
processor	O
that	O
temporarily	O
stops	O
a	O
running	O
program	O
and	O
allows	O
a	O
special	O
program	O
,	O
an	O
interrupt	B-General_Concept
handler	I-General_Concept
,	O
to	O
run	O
instead	O
.	O
</s>
<s>
Hardware	O
interrupts	B-Application
are	O
used	O
to	O
handle	O
events	O
such	O
as	O
receiving	O
data	O
from	O
a	O
modem	B-Application
or	O
network	B-Protocol
card	I-Protocol
,	O
key	O
presses	O
,	O
or	O
mouse	B-Device
movements	O
.	O
</s>
<s>
Interrupt	B-Application
lines	I-Application
are	O
often	O
identified	O
by	O
an	O
index	O
with	O
the	O
format	O
of	O
IRQ	B-General_Concept
followed	O
by	O
a	O
number	O
.	O
</s>
<s>
For	O
example	O
,	O
on	O
the	O
Intel	B-Device
8259	I-Device
family	O
of	O
programmable	B-Architecture
interrupt	I-Architecture
controllers	I-Architecture
(	O
PICs	B-Architecture
)	O
there	O
are	O
eight	O
interrupt	B-Application
inputs	O
commonly	O
referred	O
to	O
as	O
IRQ0	O
through	O
IRQ7	O
.	O
</s>
<s>
In	O
x86	B-Operating_System
based	I-Operating_System
computer	O
systems	O
that	O
use	O
two	O
of	O
these	O
PICs	B-Architecture
,	O
the	O
combined	O
set	O
of	O
lines	O
are	O
referred	O
to	O
as	O
IRQ0	O
through	O
IRQ15	O
.	O
</s>
<s>
Technically	O
these	O
lines	O
are	O
named	O
IR0	O
through	O
IR7	O
,	O
and	O
the	O
lines	O
on	O
the	O
ISA	B-Architecture
bus	I-Architecture
to	O
which	O
they	O
were	O
historically	O
attached	O
are	O
named	O
IRQ0	O
through	O
IRQ15	O
(	O
although	O
historically	O
as	O
the	O
number	O
of	O
hardware	O
devices	O
increased	O
,	O
the	O
total	O
possible	O
number	O
of	O
interrupts	B-Application
was	O
increased	O
by	O
means	O
of	O
cascading	O
requests	O
,	O
by	O
making	O
one	O
of	O
the	O
IRQ	B-General_Concept
numbers	O
cascade	O
to	O
another	O
set	O
or	O
sets	O
of	O
numbered	O
IRQs	B-General_Concept
,	O
handled	O
by	O
one	O
or	O
more	O
subsequent	O
controllers	O
)	O
.	O
</s>
<s>
Newer	O
x86	B-Operating_System
systems	O
integrate	O
an	O
Advanced	B-Device
Programmable	I-Device
Interrupt	I-Device
Controller	I-Device
(	O
APIC	O
)	O
that	O
conforms	O
to	O
the	O
Intel	B-Device
APIC	I-Device
Architecture	I-Device
.	O
</s>
<s>
These	O
APICs	O
support	O
a	O
programming	O
interface	O
for	O
up	O
to	O
255	O
physical	O
hardware	O
IRQ	B-General_Concept
lines	O
per	O
APIC	O
,	O
with	O
a	O
typical	O
system	O
implementing	O
support	O
for	O
only	O
around	O
24	O
total	O
hardware	O
lines	O
.	O
</s>
<s>
During	O
the	O
early	O
years	O
of	O
personal	B-Device
computing	I-Device
,	O
IRQ	B-General_Concept
management	O
was	O
often	O
of	O
user	O
concern	O
.	O
</s>
<s>
With	O
the	O
introduction	O
of	O
plug	B-Device
and	I-Device
play	I-Device
devices	O
this	O
has	O
been	O
alleviated	O
through	O
automatic	O
configuration	O
.	O
</s>
<s>
When	O
working	O
with	O
personal	B-Device
computer	I-Device
hardware	O
,	O
installing	O
and	O
removing	O
devices	O
,	O
the	O
system	O
relies	O
on	O
interrupt	B-General_Concept
requests	I-General_Concept
.	O
</s>
<s>
There	O
are	O
default	O
settings	O
that	O
are	O
configured	O
in	O
the	O
system	B-Operating_System
BIOS	I-Operating_System
and	O
recognized	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Modern	O
plug	B-Device
and	I-Device
play	I-Device
technology	O
has	O
not	O
only	O
reduced	O
the	O
need	O
for	O
concern	O
for	O
these	O
settings	O
,	O
but	O
has	O
also	O
virtually	O
eliminated	O
manual	O
configuration	O
.	O
</s>
<s>
Early	O
PCs	B-Device
using	O
the	O
Intel	O
8086/8088	O
processors	O
only	O
had	O
a	O
single	O
PIC	B-Architecture
,	O
and	O
are	O
therefore	O
limited	O
to	O
eight	O
interrupts	B-Application
.	O
</s>
<s>
This	O
was	O
expanded	O
to	O
two	O
PICs	B-Architecture
with	O
the	O
introduction	O
of	O
the	O
286	O
based	O
PCs	B-Device
.	O
</s>
<s>
Typically	O
,	O
on	O
systems	O
using	O
the	O
Intel	B-Device
8259	I-Device
PIC	B-Architecture
,	O
16	O
IRQs	B-General_Concept
are	O
used	O
.	O
</s>
<s>
IRQs	B-General_Concept
0	O
to	O
7	O
are	O
managed	O
by	O
one	O
Intel	B-Device
8259	I-Device
PIC	B-Architecture
,	O
and	O
IRQs	B-General_Concept
8	O
to	O
15	O
by	O
a	O
second	O
Intel	B-Device
8259	I-Device
PIC	B-Architecture
.	O
</s>
<s>
The	O
first	O
PIC	B-Architecture
,	O
the	O
master	O
,	O
is	O
the	O
only	O
one	O
that	O
directly	O
signals	O
the	O
CPU	O
.	O
</s>
<s>
The	O
second	O
PIC	B-Architecture
,	O
the	O
slave	O
,	O
instead	O
signals	O
to	O
the	O
master	O
on	O
its	O
IRQ	B-General_Concept
2	O
line	O
,	O
and	O
the	O
master	O
passes	O
the	O
signal	O
on	O
to	O
the	O
CPU	O
.	O
</s>
<s>
There	O
are	O
therefore	O
only	O
15	O
interrupt	B-General_Concept
request	I-General_Concept
lines	O
available	O
for	O
hardware	O
.	O
</s>
<s>
On	O
APIC	O
with	O
IOAPIC	B-Device
systems	O
,	O
typically	O
there	O
are	O
24	O
IRQs	B-General_Concept
available	O
,	O
and	O
the	O
extra	O
8	O
IRQs	B-General_Concept
are	O
used	O
to	O
route	O
PCI	O
interrupts	B-Application
,	O
avoiding	O
conflict	O
between	O
dynamically	O
configured	O
PCI	O
interrupts	B-Application
and	O
statically	O
configured	O
ISA	B-Architecture
interrupts	B-Application
.	O
</s>
<s>
On	O
early	O
APIC	O
systems	O
with	O
only	O
16	O
IRQs	B-General_Concept
or	O
with	O
only	O
Intel	B-Device
8259	I-Device
interrupt	B-Architecture
controllers	I-Architecture
,	O
PCI	O
interrupt	B-Application
lines	I-Application
were	O
routed	O
to	O
the	O
16	O
IRQs	B-General_Concept
using	O
a	O
PIR	O
(	O
PCI	O
interrupt	B-Application
routing	O
)	O
integrated	O
into	O
the	O
BIOS	B-Operating_System
.	O
</s>
<s>
On	O
APIC	O
with	O
MSI	B-Architecture
systems	O
,	O
typically	O
there	O
are	O
224	O
interrupts	B-Application
available	O
.	O
</s>
<s>
The	O
easiest	O
way	O
of	O
viewing	O
this	O
information	O
on	O
Windows	B-Application
is	O
to	O
use	O
Device	O
Manager	O
or	O
System	B-Application
Information	I-Application
(	O
msinfo32.exe	B-Application
)	O
.	O
</s>
<s>
On	O
Linux	B-Application
,	O
IRQ	B-General_Concept
mappings	O
can	O
be	O
viewed	O
by	O
executing	O
cat	O
/proc/interrupts	O
or	O
using	O
the	O
procinfo	O
utility	O
.	O
</s>
<s>
IRQ	B-General_Concept
7	O
parallel	B-Device
port	I-Device
1	O
(	O
shared	O
with	O
parallel	B-Device
port	I-Device
2	O
,	O
if	O
present	O
)	O
.	O
</s>
<s>
It	O
is	O
used	O
for	O
printers	O
or	O
for	O
any	O
parallel	B-Device
port	I-Device
if	O
a	O
printer	O
is	O
not	O
present	O
.	O
</s>
<s>
It	O
can	O
also	O
be	O
potentially	O
be	O
shared	O
with	O
a	O
secondary	O
sound	B-Device
card	I-Device
with	O
careful	O
management	O
of	O
the	O
port	O
.	O
</s>
<s>
IRQ	B-General_Concept
9	O
Advanced	B-Device
Configuration	I-Device
and	I-Device
Power	I-Device
Interface	I-Device
(	O
ACPI	B-Device
)	O
system	O
control	O
interrupt	B-Application
on	O
Intel	O
chipsets	O
.	O
</s>
<s>
In	O
early	O
IBM-compatible	O
personal	B-Device
computers	I-Device
,	O
an	O
IRQ	B-General_Concept
conflict	I-General_Concept
is	O
a	O
once	O
common	O
hardware	O
error	O
,	O
received	O
when	O
two	O
devices	O
were	O
trying	O
to	O
use	O
the	O
same	O
interrupt	B-General_Concept
request	I-General_Concept
(	O
or	O
IRQ	B-General_Concept
)	O
to	O
signal	O
an	O
interrupt	B-Application
to	O
the	O
Programmable	B-Architecture
Interrupt	I-Architecture
Controller	I-Architecture
(	O
PIC	B-Architecture
)	O
.	O
</s>
<s>
The	O
PIC	B-Architecture
expects	O
interrupt	B-General_Concept
requests	I-General_Concept
from	O
only	O
one	O
device	O
per	O
line	O
,	O
thus	O
more	O
than	O
one	O
device	O
sending	O
IRQ	B-General_Concept
signals	O
along	O
the	O
same	O
line	O
will	O
generally	O
cause	O
an	O
IRQ	B-General_Concept
conflict	I-General_Concept
that	O
can	O
freeze	O
a	O
computer	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
modem	B-Application
expansion	B-Device
card	I-Device
is	O
added	O
into	O
a	O
system	O
and	O
assigned	O
to	O
IRQ4	O
,	O
which	O
is	O
traditionally	O
assigned	O
to	O
the	O
serial	O
port1	O
,	O
it	O
will	O
likely	O
cause	O
an	O
IRQ	B-General_Concept
conflict	I-General_Concept
.	O
</s>
<s>
Initially	O
,	O
IRQ7	O
was	O
a	O
common	O
choice	O
for	O
the	O
use	O
of	O
a	O
sound	B-Device
card	I-Device
,	O
but	O
later	O
IRQ5	O
was	O
used	O
when	O
it	O
was	O
found	O
that	O
IRQ7	O
would	O
interfere	O
with	O
the	O
printer	B-Device
port	I-Device
(	O
LPT1	O
)	O
.	O
</s>
<s>
The	O
serial	B-Protocol
ports	I-Protocol
are	O
frequently	O
disabled	O
to	O
free	O
an	O
IRQ	B-General_Concept
line	O
for	O
another	O
device	O
.	O
</s>
<s>
IRQ2/9	O
is	O
the	O
traditional	O
interrupt	B-Application
line	I-Application
for	O
an	O
MPU-401	O
MIDI	O
port	O
,	O
but	O
this	O
conflicts	O
with	O
the	O
ACPI	B-Device
system	O
control	O
interrupt	B-Application
(	O
SCI	O
is	O
hardwired	O
to	O
IRQ9	O
on	O
Intel	O
chipsets	O
)	O
;	O
this	O
means	O
ISA	B-Architecture
MPU-401	O
cards	O
with	O
a	O
hardwired	O
IRQ2/9	O
,	O
and	O
MPU-401	O
device	O
drivers	O
with	O
a	O
hardcoded	O
IRQ2/9	O
,	O
cannot	O
be	O
used	O
in	O
interrupt-driven	O
mode	O
on	O
a	O
system	O
with	O
ACPI	B-Device
enabled	O
.	O
</s>
<s>
In	O
some	O
conditions	O
,	O
two	O
ISA	B-Architecture
devices	O
could	O
share	O
the	O
same	O
IRQ	B-General_Concept
as	O
long	O
as	O
they	O
were	O
not	O
used	O
simultaneously	O
.	O
</s>
<s>
To	O
solve	O
this	O
problem	O
,	O
the	O
later	O
ISA	B-Device
PnP	I-Device
and	O
PCI	B-Architecture
bus	I-Architecture
allows	O
for	O
IRQ	B-General_Concept
sharing	O
.	O
</s>
<s>
PCI	O
Express	O
does	O
not	O
have	O
physical	O
interrupt	B-Application
lines	I-Application
,	O
and	O
uses	O
Message	B-Architecture
Signaled	I-Architecture
Interrupts	I-Architecture
(	O
MSI	B-Architecture
)	O
to	O
the	O
operating	B-General_Concept
systems	I-General_Concept
if	O
available	O
.	O
</s>
