<s>
In	O
computing	O
and	O
in	O
systems	O
theory	O
,	O
FIFO	B-Operating_System
is	O
an	O
acronym	O
for	O
first	B-Operating_System
in	I-Operating_System
,	I-Operating_System
first	I-Operating_System
out	I-Operating_System
(	O
the	O
first	O
in	O
is	O
the	O
first	O
out	O
)	O
,	O
a	O
method	O
for	O
organizing	O
the	O
manipulation	O
of	O
a	O
data	B-General_Concept
structure	I-General_Concept
(	O
often	O
,	O
specifically	O
a	O
data	B-General_Concept
buffer	I-General_Concept
)	O
where	O
the	O
oldest	O
(	O
first	O
)	O
entry	O
,	O
or	O
"	O
head	O
"	O
of	O
the	O
queue	B-Application
,	O
is	O
processed	O
first	O
.	O
</s>
<s>
Such	O
processing	O
is	O
analogous	O
to	O
servicing	O
people	O
in	O
a	O
queue	B-Application
area	O
on	O
a	O
first-come	O
,	O
first-served	O
(	O
FCFS	B-Operating_System
)	O
basis	O
,	O
i.e.	O
</s>
<s>
in	O
the	O
same	O
sequence	O
in	O
which	O
they	O
arrive	O
at	O
the	O
queue	B-Application
's	O
tail	O
.	O
</s>
<s>
FCFS	B-Operating_System
is	O
also	O
the	O
jargon	O
term	O
for	O
the	O
FIFO	B-Operating_System
operating	O
system	O
scheduling	O
algorithm	O
,	O
which	O
gives	O
every	O
process	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
time	O
in	O
the	O
order	O
in	O
which	O
it	O
is	O
demanded	O
.	O
</s>
<s>
FIFO	B-Operating_System
's	O
opposite	O
is	O
LIFO	B-Application
,	O
last-in-first-out	O
,	O
where	O
the	O
youngest	O
entry	O
or	O
"	O
top	O
of	O
the	O
stack	B-Application
"	O
is	O
processed	O
first	O
.	O
</s>
<s>
A	O
priority	B-Application
queue	I-Application
is	O
neither	O
FIFO	B-Operating_System
or	O
LIFO	B-Application
but	O
may	O
adopt	O
similar	O
behaviour	O
temporarily	O
or	O
by	O
default	O
.	O
</s>
<s>
Queueing	O
theory	O
encompasses	O
these	O
methods	O
for	O
processing	O
data	B-General_Concept
structures	I-General_Concept
,	O
as	O
well	O
as	O
interactions	O
between	O
strict-FIFO	O
queues	B-Application
.	O
</s>
<s>
thumb|300px|Representation	O
of	O
a	O
FIFO	B-Operating_System
queue	B-Application
with	O
enqueue	O
and	O
dequeue	O
operations	O
.	O
</s>
<s>
Depending	O
on	O
the	O
application	O
,	O
a	O
FIFO	B-Operating_System
could	O
be	O
implemented	O
as	O
a	O
hardware	O
shift	O
register	O
,	O
or	O
using	O
different	O
memory	O
structures	O
,	O
typically	O
a	O
circular	B-Data_Structure
buffer	I-Data_Structure
or	O
a	O
kind	O
of	O
list	O
.	O
</s>
<s>
For	O
information	O
on	O
the	O
abstract	O
data	B-General_Concept
structure	I-General_Concept
,	O
see	O
Queue	B-Application
(	O
data	B-General_Concept
structure	I-General_Concept
)	O
.	O
</s>
<s>
Most	O
software	O
implementations	O
of	O
a	O
FIFO	B-Operating_System
queue	B-Application
are	O
not	O
thread	B-Operating_System
safe	I-Operating_System
and	O
require	O
a	O
locking	O
mechanism	O
to	O
verify	O
the	O
data	B-General_Concept
structure	I-General_Concept
chain	O
is	O
being	O
manipulated	O
by	O
only	O
one	O
thread	O
at	O
a	O
time	O
.	O
</s>
<s>
The	O
following	O
code	O
shows	O
a	O
linked	B-Data_Structure
list	I-Data_Structure
FIFO	B-Operating_System
C++	B-Language
language	I-Language
implementation	O
.	O
</s>
<s>
In	O
practice	O
,	O
a	O
number	O
of	O
list	O
implementations	O
exist	O
,	O
including	O
popular	O
Unix	O
systems	O
C	O
sys/queue.h	O
macros	O
or	O
the	O
C++	B-Language
standard	B-Application
library	I-Application
std::list	O
template	O
,	O
avoiding	O
the	O
need	O
for	O
implementing	O
the	O
data	B-General_Concept
structure	I-General_Concept
from	O
scratch	O
.	O
</s>
<s>
In	O
computing	O
environments	O
that	O
support	O
the	O
pipes-and-filters	B-Operating_System
model	O
for	O
interprocess	B-Operating_System
communication	I-Operating_System
,	O
a	O
FIFO	B-Operating_System
is	O
another	O
name	O
for	O
a	O
named	B-Operating_System
pipe	I-Operating_System
.	O
</s>
<s>
Disk	O
controllers	O
can	O
use	O
the	O
FIFO	B-Operating_System
as	O
a	O
disk	B-General_Concept
scheduling	I-General_Concept
algorithm	O
to	O
determine	O
the	O
order	O
in	O
which	O
to	O
service	O
disk	O
I/O	B-General_Concept
requests	O
,	O
where	O
it	O
is	O
also	O
known	O
by	O
the	O
same	O
FCFS	B-Operating_System
initialism	O
as	O
for	O
CPU	O
scheduling	O
mentioned	O
before	O
.	O
</s>
<s>
Communication	O
network	B-Protocol
bridges	I-Protocol
,	O
switches	B-Protocol
and	O
routers	B-Protocol
used	O
in	O
computer	B-Architecture
networks	I-Architecture
use	O
FIFOs	O
to	O
hold	O
data	O
packets	O
in	O
route	O
to	O
their	O
next	O
destination	O
.	O
</s>
<s>
Typically	O
at	O
least	O
one	O
FIFO	B-Operating_System
structure	O
is	O
used	O
per	O
network	O
connection	O
.	O
</s>
<s>
In	O
its	O
hardware	O
form	O
,	O
a	O
FIFO	B-Operating_System
primarily	O
consists	O
of	O
a	O
set	O
of	O
read	O
and	O
write	O
pointers	O
,	O
storage	O
and	O
control	O
logic	O
.	O
</s>
<s>
Storage	O
may	O
be	O
static	B-Architecture
random	I-Architecture
access	I-Architecture
memory	I-Architecture
(	O
SRAM	O
)	O
,	O
flip-flops	B-General_Concept
,	O
latches	B-General_Concept
or	O
any	O
other	O
suitable	O
form	O
of	O
storage	O
.	O
</s>
<s>
The	O
first	O
known	O
FIFO	B-Operating_System
implemented	O
in	O
electronics	O
was	O
by	O
Peter	O
Alfke	O
in	O
1969	O
at	O
Fairchild	O
Semiconductor	O
.	O
</s>
<s>
A	O
synchronous	O
FIFO	B-Operating_System
is	O
a	O
FIFO	B-Operating_System
where	O
the	O
same	O
clock	O
is	O
used	O
for	O
both	O
reading	O
and	O
writing	O
.	O
</s>
<s>
An	O
asynchronous	O
FIFO	B-Operating_System
uses	O
different	O
clocks	O
for	O
reading	O
and	O
writing	O
and	O
they	O
can	O
introduce	O
metastability	O
issues	O
.	O
</s>
<s>
A	O
common	O
implementation	O
of	O
an	O
asynchronous	O
FIFO	B-Operating_System
uses	O
a	O
Gray	B-Device
code	I-Device
(	O
or	O
any	O
unit	O
distance	O
code	O
)	O
for	O
the	O
read	O
and	O
write	O
pointers	O
to	O
ensure	O
reliable	O
flag	O
generation	O
.	O
</s>
<s>
One	O
further	O
note	O
concerning	O
flag	O
generation	O
is	O
that	O
one	O
must	O
necessarily	O
use	O
pointer	O
arithmetic	O
to	O
generate	O
flags	O
for	O
asynchronous	O
FIFO	B-Operating_System
implementations	O
.	O
</s>
<s>
Conversely	O
,	O
one	O
may	O
use	O
either	O
a	O
leaky	B-Algorithm
bucket	I-Algorithm
approach	O
or	O
pointer	O
arithmetic	O
to	O
generate	O
flags	O
in	O
synchronous	O
FIFO	B-Operating_System
implementations	O
.	O
</s>
<s>
A	O
hardware	O
FIFO	B-Operating_System
is	O
used	O
for	O
synchronization	O
purposes	O
.	O
</s>
<s>
It	O
is	O
often	O
implemented	O
as	O
a	O
circular	B-Data_Structure
queue	I-Data_Structure
,	O
and	O
thus	O
has	O
two	O
pointers	O
:	O
</s>
<s>
Examples	O
of	O
FIFO	B-Operating_System
status	O
flags	O
include	O
:	O
full	O
,	O
empty	O
,	O
almost	O
full	O
,	O
and	O
almost	O
empty	O
.	O
</s>
<s>
A	O
FIFO	B-Operating_System
is	O
empty	O
when	O
the	O
read	O
address	B-General_Concept
register	I-General_Concept
reaches	O
the	O
write	O
address	B-General_Concept
register	I-General_Concept
.	O
</s>
<s>
A	O
FIFO	B-Operating_System
is	O
full	O
when	O
the	O
write	O
address	B-General_Concept
register	I-General_Concept
reaches	O
the	O
read	O
address	B-General_Concept
register	I-General_Concept
.	O
</s>
<s>
Read	O
and	O
write	O
addresses	O
are	O
initially	O
both	O
at	O
the	O
first	O
memory	O
location	O
and	O
the	O
FIFO	B-Operating_System
queue	B-Application
is	O
empty	O
.	O
</s>
<s>
When	O
the	O
read	O
address	B-General_Concept
register	I-General_Concept
equals	O
the	O
write	O
address	B-General_Concept
register	I-General_Concept
,	O
the	O
FIFO	B-Operating_System
is	O
empty	O
.	O
</s>
<s>
When	O
the	O
read	O
and	O
write	O
address	B-General_Concept
registers	I-General_Concept
differ	O
only	O
in	O
the	O
extra	O
most	O
significant	O
bit	O
and	O
the	O
rest	O
are	O
equal	O
,	O
the	O
FIFO	B-Operating_System
is	O
full	O
.	O
</s>
