<s>
A	O
power-on	B-Device
self-test	I-Device
(	O
POST	O
)	O
is	O
a	O
process	O
performed	O
by	O
firmware	B-Application
or	O
software	O
routines	O
immediately	O
after	O
a	O
computer	O
or	O
other	O
digital	O
electronic	O
device	O
is	O
powered	O
on	O
.	O
</s>
<s>
This	O
article	O
mainly	O
deals	O
with	O
POSTs	O
on	O
personal	O
computers	O
,	O
but	O
many	O
other	O
embedded	B-Architecture
systems	I-Architecture
such	O
as	O
those	O
in	O
major	O
appliances	O
,	O
avionics	O
,	O
communications	O
,	O
or	O
medical	O
equipment	O
also	O
have	O
self-test	O
routines	O
which	O
are	O
automatically	O
invoked	O
at	O
power-on	O
.	O
</s>
<s>
The	O
results	O
of	O
the	O
POST	O
may	O
be	O
displayed	O
on	O
a	O
panel	O
that	O
is	O
part	O
of	O
the	O
device	O
,	O
output	O
to	O
an	O
external	B-Device
device	I-Device
,	O
or	O
stored	O
for	O
future	O
retrieval	O
by	O
a	O
diagnostic	O
tool	O
.	O
</s>
<s>
Since	O
a	O
self-test	O
might	O
detect	O
that	O
the	O
system	O
's	O
usual	O
human-readable	O
display	O
is	O
non-functional	O
,	O
an	O
indicator	O
lamp	O
or	O
a	O
speaker	B-Device
may	O
be	O
provided	O
to	O
show	O
error	O
codes	O
as	O
a	O
sequence	O
of	O
flashes	O
or	O
beeps	O
.	O
</s>
<s>
In	O
addition	O
to	O
running	O
tests	O
,	O
the	O
POST	O
process	O
may	O
also	O
set	O
the	O
initial	O
state	O
of	O
the	O
device	O
from	O
firmware	B-Application
.	O
</s>
<s>
In	O
the	O
case	O
of	O
a	O
computer	O
,	O
the	O
POST	O
routines	O
are	O
part	O
of	O
a	O
device	O
's	O
pre-boot	O
sequence	O
;	O
if	O
they	O
complete	O
successfully	O
,	O
the	O
bootstrap	B-Operating_System
loader	I-Operating_System
code	O
is	O
invoked	O
to	O
load	O
an	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
In	O
IBM	O
PC	O
compatible	O
computers	O
,	O
the	O
main	O
duties	O
of	O
POST	O
are	O
handled	O
by	O
the	O
BIOS/UEFI	O
,	O
which	O
may	O
hand	O
some	O
of	O
these	O
duties	O
to	O
other	O
programs	O
designed	O
to	O
initialize	O
very	O
specific	O
peripheral	B-Device
devices	I-Device
,	O
notably	O
for	O
video	O
and	O
SCSI	B-Architecture
initialization	O
.	O
</s>
<s>
These	O
other	O
duty-specific	O
programs	O
are	O
generally	O
known	O
collectively	O
as	O
option	O
ROMs	O
or	O
individually	O
as	O
the	O
video	B-Device
BIOS	I-Device
,	O
SCSI	B-Architecture
BIOS	B-Operating_System
,	O
etc	O
.	O
</s>
<s>
The	O
principal	O
duties	O
of	O
the	O
main	O
BIOS	B-Operating_System
during	O
POST	O
are	O
as	O
follows	O
:	O
</s>
<s>
The	O
functions	O
above	O
are	O
served	O
by	O
the	O
POST	O
in	O
all	O
BIOS	B-Operating_System
versions	O
back	O
to	O
the	O
very	O
first	O
.	O
</s>
<s>
In	O
later	O
BIOS	B-Operating_System
versions	O
,	O
POST	O
will	O
also	O
:	O
</s>
<s>
(	O
In	O
early	O
BIOSes	O
,	O
POST	O
did	O
not	O
organize	O
or	O
select	O
boot	B-Operating_System
devices	O
,	O
it	O
simply	O
identified	O
floppy	O
or	O
hard	O
disks	O
,	O
which	O
the	O
system	O
would	O
try	O
to	O
boot	B-Operating_System
in	O
that	O
order	O
,	O
always	O
.	O
)	O
</s>
<s>
The	O
BIOS	B-Operating_System
begins	O
its	O
POST	O
when	O
the	O
CPU	B-Device
is	O
reset	O
.	O
</s>
<s>
The	O
first	O
memory	O
location	O
the	O
CPU	B-Device
tries	O
to	O
execute	O
is	O
known	O
as	O
the	O
reset	B-General_Concept
vector	I-General_Concept
.	O
</s>
<s>
In	O
the	O
case	O
of	O
a	O
hard	O
reboot	O
,	O
the	O
northbridge	B-Device
will	O
direct	O
this	O
code	O
fetch	O
(	O
request	O
)	O
to	O
the	O
BIOS	B-Operating_System
located	O
on	O
the	O
system	O
flash	B-Device
memory	I-Device
.	O
</s>
<s>
For	O
a	O
warm	O
boot	B-Operating_System
,	O
the	O
BIOS	B-Operating_System
will	O
be	O
located	O
in	O
the	O
proper	O
place	O
in	O
RAM	B-Architecture
and	O
the	O
northbridge	B-Device
will	O
direct	O
the	O
reset	B-General_Concept
vector	I-General_Concept
call	O
to	O
the	O
RAM	B-Architecture
.	O
</s>
<s>
(	O
In	O
earlier	O
PC	O
systems	O
,	O
before	O
chipsets	B-Device
were	O
standard	O
,	O
the	O
BIOS	B-Operating_System
ROM	I-Operating_System
would	O
be	O
located	O
at	O
an	O
address	O
range	O
that	O
included	O
the	O
reset	B-General_Concept
vector	I-General_Concept
,	O
and	O
BIOS	B-Operating_System
ran	O
directly	O
out	O
of	O
ROM	O
.	O
</s>
<s>
This	O
is	O
why	O
the	O
motherboard	B-Device
BIOS	B-Operating_System
ROM	I-Operating_System
is	O
in	O
segment	O
F000	O
in	O
the	O
conventional	B-Device
memory	I-Device
map	O
.	O
)	O
</s>
<s>
During	O
the	O
POST	O
flow	O
of	O
a	O
contemporary	O
BIOS	B-Operating_System
,	O
one	O
of	O
the	O
first	O
things	O
a	O
BIOS	B-Operating_System
should	O
do	O
is	O
determine	O
the	O
reason	O
it	O
is	O
executing	O
.	O
</s>
<s>
For	O
a	O
cold	B-Operating_System
boot	I-Operating_System
,	O
for	O
example	O
,	O
it	O
may	O
need	O
to	O
execute	O
all	O
of	O
its	O
functionality	O
.	O
</s>
<s>
If	O
,	O
however	O
,	O
the	O
system	O
supports	O
power	O
saving	O
or	O
quick	O
boot	B-Operating_System
methods	O
,	O
the	O
BIOS	B-Operating_System
may	O
be	O
able	O
to	O
circumvent	O
the	O
standard	O
POST	O
device	O
discovery	O
,	O
and	O
simply	O
program	O
the	O
devices	O
from	O
a	O
preloaded	O
system	O
device	O
table	O
.	O
</s>
<s>
During	O
the	O
POST	O
,	O
the	O
BIOS	B-Operating_System
must	O
integrate	O
a	O
plethora	O
of	O
competing	O
,	O
evolving	O
,	O
and	O
even	O
mutually	O
exclusive	O
standards	O
and	O
initiatives	O
for	O
the	O
matrix	O
of	O
hardware	O
and	O
OSes	O
the	O
PC	O
is	O
expected	O
to	O
support	O
,	O
although	O
at	O
most	O
only	O
simple	O
memory	O
tests	O
and	O
the	O
setup	O
screen	O
are	O
displayed	O
.	O
</s>
<s>
As	O
the	O
PC	O
platform	O
evolved	O
into	O
more	O
of	O
a	O
commodity	O
consumer	O
device	O
,	O
the	O
mainframe	O
-	O
and	O
minicomputer-inspired	O
high-reliability	O
features	O
such	O
as	O
parity	B-Error_Name
memory	O
and	O
the	O
thorough	O
memory	O
test	O
in	O
every	O
POST	O
were	O
dropped	O
from	O
most	O
models	O
.	O
</s>
<s>
The	O
exponential	O
growth	O
of	O
PC	O
memory	O
sizes	O
,	O
driven	O
by	O
the	O
equally	O
exponential	O
drop	O
in	O
memory	O
prices	O
,	O
was	O
also	O
a	O
factor	O
in	O
this	O
,	O
as	O
the	O
duration	O
of	O
a	O
memory	O
test	O
using	O
a	O
given	O
CPU	B-Device
is	O
directly	O
proportional	O
to	O
the	O
memory	O
size	O
.	O
</s>
<s>
The	O
original	O
IBM	O
PC	O
could	O
be	O
equipped	O
with	O
as	O
little	O
as	O
16	O
KB	O
of	O
RAM	B-Architecture
and	O
typically	O
had	O
between	O
64	O
and	O
640	O
KB	O
;	O
depending	O
on	O
the	O
amount	O
of	O
equipped	O
memory	O
,	O
the	O
computer	O
's	O
4.77MHz	O
8088	O
required	O
between	O
5	O
seconds	O
and	O
1.5	O
minutes	O
to	O
complete	O
the	O
POST	O
and	O
there	O
was	O
no	O
way	O
to	O
skip	O
it	O
.	O
</s>
<s>
With	O
people	O
being	O
more	O
concerned	O
with	O
boot	B-Operating_System
times	I-Operating_System
now	O
than	O
in	O
the	O
1980s	O
,	O
the	O
30	O
-	O
to	O
60-second	O
memory	O
test	O
adds	O
undesirable	O
delay	O
for	O
a	O
benefit	O
of	O
confidence	O
that	O
is	O
not	O
perceived	O
to	O
be	O
worth	O
that	O
cost	O
by	O
most	O
users	O
.	O
</s>
<s>
Most	O
clone	O
PC	O
BIOSes	O
allowed	O
the	O
user	O
to	O
skip	O
the	O
POST	O
RAM	B-Architecture
check	O
by	O
pressing	O
a	O
key	O
,	O
and	O
more	O
modern	O
machines	O
often	O
performed	O
no	O
RAM	B-Architecture
test	O
at	O
all	O
unless	O
it	O
was	O
enabled	O
via	O
the	O
BIOS	B-Operating_System
setup	O
.	O
</s>
<s>
As	O
part	O
of	O
the	O
starting	O
sequence	O
the	O
POST	O
routines	O
may	O
display	O
a	O
prompt	O
to	O
the	O
user	O
for	O
a	O
key	O
press	O
to	O
access	O
built-in	O
setup	O
functions	O
of	O
the	O
BIOS	B-Operating_System
.	O
</s>
<s>
This	O
allows	O
the	O
user	O
to	O
set	O
various	O
options	O
particular	O
to	O
the	O
mother	B-Device
board	I-Device
before	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
loaded	O
.	O
</s>
<s>
If	O
no	O
key	O
is	O
pressed	O
,	O
the	O
POST	O
will	O
proceed	O
on	O
to	O
the	O
boot	B-Operating_System
sequence	I-Operating_System
required	O
to	O
load	O
the	O
installed	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Many	O
modern	O
BIOS	B-Operating_System
and	O
UEFI	B-Architecture
implementations	O
show	O
a	O
manufacturers	O
logo	O
during	O
POST	O
and	O
hide	O
the	O
classic	O
text	O
screens	O
unless	O
an	O
error	O
occurs	O
.	O
</s>
<s>
The	O
text	O
screen	O
can	O
often	O
be	O
enabled	O
in	O
the	O
BIOS	B-Operating_System
settings	I-Operating_System
by	O
disabling	O
the	O
"	O
Quiet	O
Boot	B-Operating_System
"	O
option	O
.	O
</s>
<s>
The	O
original	O
IBM	B-Operating_System
BIOS	I-Operating_System
made	O
POST	O
diagnostic	O
information	O
available	O
by	O
outputting	O
a	O
number	O
to	O
I/O	B-Architecture
port	I-Architecture
0x80	O
(	O
a	O
screen	O
display	O
was	O
not	O
possible	O
with	O
some	O
failure	O
modes	O
)	O
.	O
</s>
<s>
Once	O
an	O
operating	B-General_Concept
system	I-General_Concept
is	O
running	O
on	O
the	O
computer	O
the	O
code	O
displayed	O
by	O
such	O
a	O
board	O
may	O
become	O
meaningless	O
,	O
since	O
some	O
OSes	O
,	O
e.g.	O
</s>
<s>
Linux	B-Application
,	O
use	O
port	O
0x80	O
for	O
I/O	O
timing	O
operations	O
.	O
</s>
<s>
The	O
actual	O
numeric	O
codes	O
for	O
the	O
possible	O
stages	O
and	O
error	O
conditions	O
differ	O
from	O
one	O
BIOS	B-Operating_System
supplier	O
to	O
another	O
.	O
</s>
<s>
Codes	O
for	O
different	O
BIOS	B-Operating_System
versions	O
from	O
a	O
single	O
supplier	O
may	O
also	O
vary	O
,	O
although	O
many	O
codes	O
remain	O
unchanged	O
in	O
different	O
versions	O
.	O
</s>
<s>
Later	O
BIOSes	O
used	O
a	O
sequence	O
of	O
beeps	O
from	O
the	O
motherboard-attached	O
PC	B-Device
speaker	I-Device
(	O
if	O
present	O
and	O
working	O
)	O
to	O
signal	O
error	O
codes	O
.	O
</s>
<s>
POST	O
beep	B-Device
codes	I-Device
vary	O
from	O
manufacturer	O
to	O
manufacturer	O
.	O
</s>
<s>
Information	O
on	O
numeric	O
and	O
beep	B-Device
codes	I-Device
is	O
available	O
from	O
manufacturers	O
of	O
BIOSes	O
and	O
motherboards	B-Device
.	O
</s>
<s>
These	O
POST	O
beep	B-Device
codes	I-Device
are	O
covered	O
specifically	O
on	O
the	O
CompTIA	O
A+	O
Exam	O
:	O
</s>
<s>
Apple	O
's	O
Macintosh	B-Device
computers	O
also	O
perform	O
a	O
POST	O
after	O
a	O
cold	B-Operating_System
boot	I-Operating_System
.	O
</s>
<s>
When	O
Apple	O
introduced	O
the	O
iMac	B-Device
in	O
1998	O
,	O
it	O
was	O
a	O
radical	O
departure	O
from	O
other	O
Macs	O
of	O
the	O
time	O
.	O
</s>
<s>
The	O
iMac	B-Device
began	O
the	O
production	O
of	O
New	B-Device
World	I-Device
Macs	I-Device
,	O
as	O
they	O
are	O
called	O
;	O
New	B-Device
World	I-Device
Macs	I-Device
,	O
such	O
as	O
the	O
iMac	B-Device
G3	I-Device
,	O
Power	B-Device
Macintosh	I-Device
G3	I-Device
(	O
Blue	O
&	O
White	O
)	O
,	O
Power	O
Mac	O
G4	O
(	O
PCI	O
Graphics	O
)	O
,	O
PowerBook	O
G3	O
(	O
bronze	O
keyboard	B-Device
)	O
,	O
and	O
PowerBook	O
G3	O
(	O
FireWire	O
)	O
,	O
load	O
the	O
Mac	O
OS	O
ROM	O
from	O
the	O
hard	O
drive	O
.	O
</s>
<s>
The	O
beep	B-Device
codes	I-Device
were	O
revised	O
in	O
October	O
1999	O
.	O
</s>
<s>
With	O
the	B-Device
introduction	I-Device
of	I-Device
Intel-based	I-Device
Macs	I-Device
with	O
EFI-based	O
firmware	B-Application
,	O
the	O
startup	O
tones	O
were	O
changed	O
again	O
.	O
</s>
<s>
Macs	O
with	O
the	O
T2	B-Device
security	I-Device
chip	I-Device
do	O
n't	O
have	O
startup	O
tones	O
,	O
therefore	O
the	O
beep	B-Device
codes	I-Device
are	O
no	O
longer	O
heard	O
and	O
used	O
.	O
</s>
<s>
It	O
is	O
currently	O
unknown	O
whether	O
or	O
not	O
Apple	O
silicon-based	O
Macs	O
have	O
beep	B-Device
codes	I-Device
,	O
due	O
to	O
them	O
not	O
using	O
EFI-based	O
firmware	B-Application
.	O
</s>
<s>
Amiga	B-Device
historical	O
line	O
of	O
computers	O
,	O
from	O
A1000	B-Device
to	O
4000	B-Device
present	O
an	O
interesting	O
POST	O
sequence	O
that	O
prompts	O
the	O
user	O
with	O
a	O
sequence	O
of	O
flashing	O
screens	O
of	O
different	O
colors	O
(	O
rather	O
than	O
audible	O
beeps	O
as	O
in	O
other	O
systems	O
)	O
to	O
show	O
if	O
various	O
hardware	O
POST	O
tests	O
were	O
correct	O
or	O
else	O
if	O
they	O
failed	O
:	O
</s>
<s>
The	O
Amiga	B-Device
system	O
performs	O
the	O
following	O
steps	O
at	O
boot	B-Operating_System
:	O
</s>
<s>
Performs	O
checksum	B-Algorithm
test	O
on	O
ROMs	O
.	O
</s>
<s>
Almost	O
all	O
Amiga	B-Device
models	O
present	O
the	O
same	O
color	O
sequence	O
when	O
turned	O
on	O
:	O
black	O
screen	O
,	O
dark	O
gray	O
,	O
light	O
gray	O
color	O
screens	O
filling	O
all	O
monitor	O
screen	O
in	O
a	O
rapid	O
sequence	O
(	O
Amigas	B-Device
taken	O
up	O
usually	O
2	O
,	O
or	O
at	O
least	O
max	O
3	O
seconds	O
to	O
turn	O
on	O
and	O
boot	B-Operating_System
)	O
.	O
</s>
<s>
The	O
keyboards	B-Device
of	O
historical	O
Amiga	B-Device
models	O
are	O
not	O
proprietary	O
as	O
it	O
happened	O
in	O
early	O
computer	O
ages	O
,	O
but	O
more	O
pragmatically	O
it	O
was	O
based	O
on	O
international	O
standard	O
ANSI/ISO	O
8859-1	O
.	O
</s>
<s>
The	O
keyboard	B-Device
itself	O
was	O
an	O
intelligent	O
device	O
and	O
had	O
its	O
own	O
processor	O
and	O
4	O
kilobytes	O
of	O
RAM	B-Architecture
for	O
keeping	O
a	O
buffer	O
of	O
the	O
sequence	O
of	O
keys	O
that	O
were	O
being	O
pressed	O
,	O
thus	O
can	O
communicate	O
with	O
the	O
user	O
if	O
a	O
fault	O
is	O
found	O
by	O
flashing	O
its	O
main	O
LED	O
in	O
sequence	O
:	O
</s>
