<s>
In	O
computing	O
,	O
BIOS	B-Operating_System
(	O
,	O
;	O
Basic	B-Operating_System
Input/Output	I-Operating_System
System	I-Operating_System
,	O
also	O
known	O
as	O
the	O
System	B-Operating_System
BIOS	I-Operating_System
,	O
ROM	B-Operating_System
BIOS	I-Operating_System
,	O
BIOS	B-Operating_System
ROM	I-Operating_System
or	O
PC	B-Operating_System
BIOS	I-Operating_System
)	O
is	O
firmware	B-Application
used	O
to	O
provide	O
runtime	O
services	O
for	O
operating	B-General_Concept
systems	I-General_Concept
and	O
programs	O
and	O
to	O
perform	O
hardware	B-Architecture
initialization	O
during	O
the	O
booting	B-Operating_System
process	O
(	O
power-on	O
startup	O
)	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
firmware	B-Application
comes	O
pre-installed	O
on	O
an	O
IBM	B-Device
PC	I-Device
or	O
IBM	B-Device
PC	I-Device
compatible	O
's	O
system	B-Device
board	I-Device
and	O
exists	O
in	O
some	O
UEFI-based	O
systems	O
to	O
maintain	O
compatibility	O
with	O
operating	B-General_Concept
systems	I-General_Concept
that	O
do	O
not	O
support	O
UEFI	B-Architecture
native	O
operation	O
.	O
</s>
<s>
The	O
name	O
originates	O
from	O
the	O
Basic	B-Operating_System
Input/Output	I-Operating_System
System	I-Operating_System
used	O
in	O
the	O
CP/M	B-Application
operating	I-Application
system	I-Application
in	O
1975	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
originally	O
proprietary	B-Application
to	O
the	O
IBM	B-Device
PC	I-Device
has	O
been	O
reverse	O
engineered	O
by	O
some	O
companies	O
(	O
such	O
as	O
Phoenix	O
Technologies	O
)	O
looking	O
to	O
create	O
compatible	O
systems	O
.	O
</s>
<s>
The	O
interface	B-Application
of	O
that	O
original	O
system	O
serves	O
as	O
a	O
de	O
facto	O
standard	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
in	O
modern	O
PCs	O
initializes	O
and	O
tests	O
the	O
system	O
hardware	B-Architecture
components	O
(	O
Power-on	B-Device
self-test	I-Device
)	O
,	O
and	O
loads	O
a	O
boot	B-Application
loader	I-Application
from	O
a	O
mass	B-Device
storage	I-Device
device	I-Device
which	O
then	O
initializes	O
a	O
kernel	B-Operating_System
.	O
</s>
<s>
In	O
the	O
era	O
of	O
DOS	B-Device
,	O
the	O
BIOS	B-Operating_System
provided	O
BIOS	B-Device
interrupt	I-Device
calls	I-Device
for	O
the	O
keyboard	B-Device
,	O
display	O
,	O
storage	O
,	O
and	O
other	O
input/output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
devices	O
that	O
standardized	O
an	O
interface	B-Application
to	O
application	O
programs	O
and	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
More	O
recent	O
operating	B-General_Concept
systems	I-General_Concept
do	O
not	O
use	O
the	O
BIOS	B-Device
interrupt	I-Device
calls	I-Device
after	O
startup	O
.	O
</s>
<s>
Most	O
BIOS	B-Operating_System
implementations	O
are	O
specifically	O
designed	O
to	O
work	O
with	O
a	O
particular	O
computer	O
or	O
motherboard	B-Device
model	O
,	O
by	O
interfacing	O
with	O
various	O
devices	O
especially	O
system	O
chipset	B-Device
.	O
</s>
<s>
Originally	O
,	O
BIOS	B-Operating_System
firmware	B-Application
was	O
stored	O
in	O
a	O
ROM	B-Device
chip	I-Device
on	O
the	O
PC	B-Device
motherboard	I-Device
.	O
</s>
<s>
In	O
later	O
computer	O
systems	O
,	O
the	O
BIOS	B-Operating_System
contents	O
are	O
stored	O
on	O
flash	B-Device
memory	I-Device
so	O
it	O
can	O
be	O
rewritten	O
without	O
removing	O
the	O
chip	O
from	O
the	O
motherboard	B-Device
.	O
</s>
<s>
This	O
allows	O
easy	O
,	O
end-user	O
updates	O
to	O
the	O
BIOS	B-Operating_System
firmware	B-Application
so	O
new	O
features	O
can	O
be	O
added	O
or	O
bugs	B-Error_Name
can	O
be	O
fixed	O
,	O
but	O
it	O
also	O
creates	O
a	O
possibility	O
for	O
the	O
computer	O
to	O
become	O
infected	O
with	O
BIOS	B-Operating_System
rootkits	O
.	O
</s>
<s>
Furthermore	O
,	O
a	O
BIOS	B-Operating_System
upgrade	O
that	O
fails	O
could	O
brick	O
the	O
motherboard	B-Device
.	O
</s>
<s>
The	O
last	O
version	O
of	O
Microsoft	B-Application
Windows	I-Application
to	O
officially	O
support	O
running	O
on	O
PCs	O
which	O
use	O
legacy	B-Operating_System
BIOS	I-Operating_System
firmware	B-Application
is	O
Windows	B-Operating_System
10	I-Operating_System
as	O
Windows	B-Application
11	I-Application
requires	O
a	O
UEFI-compliant	O
system	O
.	O
</s>
<s>
Unified	B-Architecture
Extensible	I-Architecture
Firmware	I-Architecture
Interface	I-Architecture
(	O
UEFI	B-Architecture
)	O
is	O
a	O
successor	O
to	O
the	O
legacy	O
PC	B-Operating_System
BIOS	I-Operating_System
,	O
aiming	O
to	O
address	O
its	O
technical	O
limitations	O
.	O
</s>
<s>
The	O
term	O
BIOS	B-Operating_System
(	O
Basic	B-Operating_System
Input/Output	I-Operating_System
System	I-Operating_System
)	O
was	O
created	O
by	O
Gary	O
Kildall	O
and	O
first	O
appeared	O
in	O
the	O
CP/M	B-Application
operating	I-Application
system	I-Application
in	O
1975	O
,	O
describing	O
the	O
machine-specific	O
part	O
of	O
CP/M	B-Application
loaded	O
during	O
boot	B-Operating_System
time	I-Operating_System
that	O
interfaces	B-Application
directly	O
with	O
the	O
hardware	B-Architecture
.	O
</s>
<s>
(	O
A	O
CP/M	B-Application
machine	O
usually	O
has	O
only	O
a	O
simple	O
boot	B-Application
loader	I-Application
in	O
its	O
ROM	B-Device
.	O
)	O
</s>
<s>
Versions	O
of	O
MS-DOS	B-Application
,	O
PC	B-Operating_System
DOS	I-Operating_System
or	O
DR-DOS	B-Application
contain	O
a	O
file	O
called	O
variously	O
"	O
IO.SYS	O
"	O
,	O
"	O
IBMBIO.COM	O
"	O
,	O
"	O
IBMBIO.SYS	O
"	O
,	O
or	O
"	O
DRBIOS.SYS	O
"	O
;	O
this	O
file	O
is	O
known	O
as	O
the	O
"	O
DOS	B-Device
BIOS	B-Operating_System
"	O
(	O
also	O
known	O
as	O
the	O
"	O
DOS	B-Device
I/O	B-General_Concept
System	O
"	O
)	O
and	O
contains	O
the	O
lower-level	O
hardware-specific	O
part	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Together	O
with	O
the	O
underlying	O
hardware-specific	O
but	O
operating	O
system-independent	O
"	O
System	B-Operating_System
BIOS	I-Operating_System
"	O
,	O
which	O
resides	O
in	O
ROM	B-Device
,	O
it	O
represents	O
the	O
analogue	O
to	O
the	O
"	O
CP/M	B-Application
BIOS	B-Operating_System
"	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
originally	O
proprietary	B-Application
to	O
the	O
IBM	B-Device
PC	I-Device
has	O
been	O
reverse	O
engineered	O
by	O
some	O
companies	O
(	O
such	O
as	O
Phoenix	O
Technologies	O
)	O
looking	O
to	O
create	O
compatible	O
systems	O
.	O
</s>
<s>
With	O
the	O
introduction	O
of	O
PS/2	O
machines	O
,	O
IBM	O
divided	O
the	O
System	B-Operating_System
BIOS	I-Operating_System
into	O
real	O
-	O
and	O
protected-mode	B-Application
portions	O
.	O
</s>
<s>
The	O
real-mode	B-Application
portion	O
was	O
meant	O
to	O
provide	O
backward	O
compatibility	O
with	O
existing	O
operating	B-General_Concept
systems	I-General_Concept
such	O
as	O
DOS	B-Device
,	O
and	O
therefore	O
was	O
named	O
"	O
CBIOS	B-Operating_System
"	O
(	O
for	O
"	O
Compatibility	B-Operating_System
BIOS	I-Operating_System
"	O
)	O
,	O
whereas	O
the	O
"	O
ABIOS	B-Operating_System
"	O
(	O
for	O
"	O
Advanced	B-Operating_System
BIOS	I-Operating_System
"	O
)	O
provided	O
new	O
interfaces	B-Application
specifically	O
suited	O
for	O
multitasking	O
operating	B-General_Concept
systems	I-General_Concept
such	O
as	O
OS/2	B-Application
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
of	O
the	O
original	O
IBM	B-Device
PC	I-Device
and	O
XT	B-Device
had	O
no	O
interactive	O
user	O
interface	B-Application
.	O
</s>
<s>
Error	O
codes	O
or	O
messages	O
were	O
displayed	O
on	O
the	O
screen	O
,	O
or	O
coded	O
series	O
of	O
sounds	O
were	O
generated	O
to	O
signal	O
errors	O
when	O
the	O
power-on	B-Device
self-test	I-Device
(	O
POST	B-Device
)	O
had	O
not	O
proceeded	O
to	O
the	O
point	O
of	O
successfully	O
initializing	O
a	O
video	B-Device
display	I-Device
adapter	I-Device
.	O
</s>
<s>
Options	O
on	O
the	O
IBM	B-Device
PC	I-Device
and	O
XT	B-Device
were	O
set	O
by	O
switches	O
and	O
jumpers	O
on	O
the	O
main	B-Device
board	I-Device
and	O
on	O
expansion	B-Device
cards	I-Device
.	O
</s>
<s>
Starting	O
around	O
the	O
mid-1990s	O
,	O
it	O
became	O
typical	O
for	O
the	O
BIOS	B-Operating_System
ROM	I-Operating_System
to	O
include	O
a	O
"	O
BIOS	B-Operating_System
configuration	O
utility	O
"	O
(	O
BCU	O
)	O
or	O
"	O
BIOS	B-Operating_System
setup	O
utility	O
"	O
,	O
accessed	O
at	O
system	O
power-up	O
by	O
a	O
particular	O
key	O
sequence	O
.	O
</s>
<s>
This	O
program	O
allowed	O
the	O
user	O
to	O
set	O
system	O
configuration	O
options	O
,	O
of	O
the	O
type	O
formerly	O
set	O
using	O
DIP	O
switches	O
,	O
through	O
an	O
interactive	O
menu	O
system	O
controlled	O
through	O
the	O
keyboard	B-Device
.	O
</s>
<s>
In	O
the	O
interim	O
period	O
,	O
IBM-compatible	O
PCsincluding	O
the	O
IBM	O
ATheld	O
configuration	O
settings	O
in	O
battery-backed	O
RAM	B-Architecture
and	O
used	O
a	O
bootable	B-Operating_System
configuration	O
program	O
on	O
floppy	B-Device
disk	I-Device
,	O
not	O
in	O
the	O
ROM	B-Device
,	O
to	O
set	O
the	O
configuration	O
options	O
contained	O
in	O
this	O
memory	O
.	O
</s>
<s>
The	O
floppy	B-Device
disk	I-Device
was	O
supplied	O
with	O
the	O
computer	O
,	O
and	O
if	O
it	O
was	O
lost	O
the	O
system	O
settings	O
could	O
not	O
be	O
changed	O
.	O
</s>
<s>
The	O
same	O
applied	O
in	O
general	O
to	O
computers	O
with	O
an	O
EISA	B-Device
bus	I-Device
,	O
for	O
which	O
the	O
configuration	O
program	O
was	O
called	O
an	O
EISA	B-Device
Configuration	O
Utility	O
(	O
ECU	O
)	O
.	O
</s>
<s>
A	O
modern	O
Wintel-compatible	O
computer	O
provides	O
a	O
setup	O
routine	O
essentially	O
unchanged	O
in	O
nature	O
from	O
the	O
ROM-resident	O
BIOS	B-Operating_System
setup	O
utilities	O
of	O
the	O
late	O
1990s	O
;	O
the	O
user	O
can	O
configure	O
hardware	B-Architecture
options	O
using	O
the	O
keyboard	B-Device
and	O
video	B-Device
display	I-Device
.	O
</s>
<s>
The	O
modern	O
Wintel	B-Device
machine	O
may	O
store	O
the	O
BIOS	B-Operating_System
configuration	O
settings	O
in	O
flash	B-Device
ROM	I-Device
,	O
perhaps	O
the	O
same	O
flash	B-Device
ROM	I-Device
that	O
holds	O
the	O
BIOS	B-Operating_System
itself	O
.	O
</s>
<s>
Systems	O
with	O
later	O
processors	O
provide	O
logic	O
to	O
start	O
running	O
the	O
BIOS	B-Operating_System
from	O
the	O
system	O
ROM	B-Device
.	O
</s>
<s>
If	O
the	O
system	O
has	O
just	O
been	O
powered	O
up	O
or	O
the	O
reset	O
button	O
was	O
pressed	O
(	O
"	O
cold	B-Operating_System
boot	I-Operating_System
"	O
)	O
,	O
the	O
full	O
power-on	B-Device
self-test	I-Device
(	O
POST	B-Device
)	O
is	O
run	O
.	O
</s>
<s>
If	O
Ctrl+Alt+Delete	O
was	O
pressed	O
(	O
"	O
warm	O
boot	B-Operating_System
"	O
)	O
,	O
a	O
special	O
flag	O
value	O
stored	O
in	O
nonvolatile	B-Device
BIOS	I-Device
memory	I-Device
(	O
"	O
CMOS	B-Device
"	O
)	O
tested	O
by	O
the	O
BIOS	B-Operating_System
allows	O
bypass	O
of	O
the	O
lengthy	O
POST	B-Device
and	O
memory	O
detection	O
.	O
</s>
<s>
The	O
POST	B-Device
identifies	O
,	O
tests	O
and	O
initializes	O
system	O
devices	O
such	O
as	O
the	O
CPU	B-General_Concept
,	O
chipset	B-Device
,	O
RAM	B-Architecture
,	O
motherboard	B-Device
,	O
video	B-Device
card	I-Device
,	O
keyboard	B-Device
,	O
mouse	B-Device
,	O
hard	B-Device
disk	I-Device
drive	I-Device
,	O
optical	O
disc	O
drive	O
and	O
other	O
hardware	B-Architecture
,	O
including	O
integrated	O
peripherals	O
.	O
</s>
<s>
Early	O
IBM	B-Device
PCs	I-Device
had	O
a	O
routine	O
in	O
the	O
POST	B-Device
that	O
would	O
download	O
a	O
program	O
into	O
RAM	B-Architecture
through	O
the	O
keyboard	B-Device
port	O
and	O
run	O
it	O
.	O
</s>
<s>
After	O
the	O
option	B-Device
ROM	I-Device
scan	O
is	O
completed	O
and	O
all	O
detected	O
ROM	B-Device
modules	O
with	O
valid	O
checksums	B-Algorithm
have	O
been	O
called	O
,	O
or	O
immediately	O
after	O
POST	B-Device
in	O
a	O
BIOS	B-Operating_System
version	O
that	O
does	O
not	O
scan	O
for	O
option	O
ROMs	O
,	O
the	O
BIOS	B-Device
calls	I-Device
INT	O
19h	O
to	O
start	O
boot	B-Operating_System
processing	O
.	O
</s>
<s>
Post-boot	O
,	O
programs	O
loaded	O
can	O
also	O
call	O
INT	O
19h	O
to	O
reboot	O
the	O
system	O
,	O
but	O
they	O
must	O
be	O
careful	O
to	O
disable	O
interrupts	O
and	O
other	O
asynchronous	O
hardware	B-Architecture
processes	O
that	O
may	O
interfere	O
with	O
the	O
BIOS	B-Operating_System
rebooting	O
process	O
,	O
or	O
else	O
the	O
system	O
may	O
hang	O
or	O
crash	O
while	O
it	O
is	O
rebooting	O
.	O
</s>
<s>
When	O
INT	O
19h	O
is	O
called	O
,	O
the	O
BIOS	B-Operating_System
attempts	O
to	O
locate	O
boot	B-Application
loader	I-Application
software	O
on	O
a	O
"	O
boot	B-Operating_System
device	O
"	O
,	O
such	O
as	O
a	O
hard	B-Device
disk	I-Device
,	O
a	O
floppy	B-Device
disk	I-Device
,	O
CD	B-Device
,	O
or	O
DVD	O
.	O
</s>
<s>
It	O
loads	O
and	O
executes	O
the	O
first	O
boot	B-Operating_System
software	I-Operating_System
it	O
finds	O
,	O
giving	O
it	O
control	O
of	O
the	O
PC	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
uses	O
the	O
boot	B-Operating_System
devices	O
set	O
in	O
Nonvolatile	B-Device
BIOS	I-Device
memory	I-Device
(	O
CMOS	B-Device
)	O
,	O
or	O
,	O
in	O
the	O
earliest	O
PCs	O
,	O
DIP	O
switches	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
checks	O
each	O
device	O
in	O
order	O
to	O
see	O
if	O
it	O
is	O
bootable	B-Operating_System
by	O
attempting	O
to	O
load	O
the	O
first	O
sector	O
(	O
boot	B-Device
sector	I-Device
)	O
.	O
</s>
<s>
If	O
the	O
sector	O
cannot	O
be	O
read	O
,	O
the	O
BIOS	B-Operating_System
proceeds	O
to	O
the	O
next	O
device	O
.	O
</s>
<s>
If	O
the	O
sector	O
is	O
read	O
successfully	O
,	O
some	O
BIOSes	O
will	O
also	O
check	O
for	O
the	O
boot	B-Device
sector	I-Device
signature	O
0x55	O
0xAA	O
in	O
the	O
last	O
two	O
bytes	O
of	O
the	O
sector	O
(	O
which	O
is	O
512	O
bytes	O
long	O
)	O
,	O
before	O
accepting	O
a	O
boot	B-Device
sector	I-Device
and	O
considering	O
the	O
device	O
bootable	B-Operating_System
.	O
</s>
<s>
When	O
a	O
bootable	B-Operating_System
device	O
is	O
found	O
,	O
the	O
BIOS	B-Operating_System
transfers	O
control	O
to	O
the	O
loaded	O
sector	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
does	O
not	O
interpret	O
the	O
contents	O
of	O
the	O
boot	B-Device
sector	I-Device
other	O
than	O
to	O
possibly	O
check	O
for	O
the	O
boot	B-Device
sector	I-Device
signature	O
in	O
the	O
last	O
two	O
bytes	O
.	O
</s>
<s>
Interpretation	O
of	O
data	O
structures	O
like	O
partition	O
tables	O
and	O
BIOS	B-Operating_System
Parameter	O
Blocks	O
is	O
done	O
by	O
the	O
boot	B-Operating_System
program	I-Operating_System
in	O
the	O
boot	B-Device
sector	I-Device
itself	O
or	O
by	O
other	O
programs	O
loaded	O
through	O
the	O
boot	B-Operating_System
process	I-Operating_System
.	O
</s>
<s>
A	O
non-disk	O
device	O
such	O
as	O
a	O
network	B-Protocol
adapter	I-Protocol
attempts	O
booting	B-Operating_System
by	O
a	O
procedure	O
that	O
is	O
defined	O
by	O
its	O
option	B-Device
ROM	I-Device
or	O
the	O
equivalent	O
integrated	O
into	O
the	O
motherboard	B-Device
BIOS	B-Operating_System
ROM	I-Operating_System
.	O
</s>
<s>
As	O
such	O
,	O
option	O
ROMs	O
may	O
also	O
influence	O
or	O
supplant	O
the	O
boot	B-Operating_System
process	I-Operating_System
defined	O
by	O
the	O
motherboard	B-Device
BIOS	B-Operating_System
ROM	I-Operating_System
.	O
</s>
<s>
With	O
the	O
El	B-Device
Torito	I-Device
optical	I-Device
media	I-Device
boot	I-Device
standard	I-Device
,	O
the	O
optical	O
drive	O
actually	O
emulates	O
a	O
3.5	B-Device
"	I-Device
high-density	O
floppy	B-Device
disk	I-Device
to	O
the	O
BIOS	B-Operating_System
for	O
boot	B-Operating_System
purposes	O
.	O
</s>
<s>
Reading	O
the	O
"	O
first	O
sector	O
"	O
of	O
a	O
CD-ROM	O
or	O
DVD-ROM	O
is	O
not	O
a	O
simply	O
defined	O
operation	O
like	O
it	O
is	O
on	O
a	O
floppy	B-Device
disk	I-Device
or	O
a	O
hard	B-Device
disk	I-Device
.	O
</s>
<s>
Furthermore	O
,	O
the	O
complexity	O
of	O
the	O
medium	O
makes	O
it	O
difficult	O
to	O
write	O
a	O
useful	O
boot	B-Operating_System
program	I-Operating_System
in	O
one	O
sector	O
.	O
</s>
<s>
The	O
bootable	B-Operating_System
virtual	O
floppy	B-Device
disk	I-Device
can	O
contain	O
software	O
that	O
provides	O
access	O
to	O
the	O
optical	O
medium	O
in	O
its	O
native	O
format	O
.	O
</s>
<s>
The	O
user	O
can	O
select	O
the	O
boot	B-Operating_System
priority	O
implemented	O
by	O
the	O
BIOS	B-Operating_System
.	O
</s>
<s>
For	O
example	O
,	O
most	O
computers	O
have	O
a	O
hard	B-Device
disk	I-Device
that	O
is	O
bootable	B-Operating_System
,	O
but	O
sometimes	O
there	O
is	O
a	O
removable-media	O
drive	O
that	O
has	O
higher	O
boot	B-Operating_System
priority	O
,	O
so	O
the	O
user	O
can	O
cause	O
a	O
removable	B-Device
disk	I-Device
to	O
be	O
booted	O
.	O
</s>
<s>
In	O
most	O
modern	O
BIOSes	O
,	O
the	O
boot	B-Operating_System
priority	O
order	O
can	O
be	O
configured	O
by	O
the	O
user	O
.	O
</s>
<s>
In	O
older	O
BIOSes	O
,	O
limited	O
boot	B-Operating_System
priority	O
options	O
are	O
selectable	O
;	O
in	O
the	O
earliest	O
BIOSes	O
,	O
a	O
fixed	O
priority	O
scheme	O
was	O
implemented	O
,	O
with	O
floppy	B-Device
disk	I-Device
drives	I-Device
first	O
,	O
fixed	B-Device
disks	I-Device
(	O
i.e.	O
</s>
<s>
hard	B-Device
disks	I-Device
)	O
second	O
,	O
and	O
typically	O
no	O
other	O
boot	B-Operating_System
devices	O
supported	O
,	O
subject	O
to	O
modification	O
of	O
these	O
rules	O
by	O
installed	O
option	O
ROMs	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
in	O
an	O
early	O
PC	O
also	O
usually	O
would	O
only	O
boot	B-Operating_System
from	O
the	O
first	O
floppy	B-Device
disk	I-Device
drive	I-Device
or	O
the	O
first	O
hard	B-Device
disk	I-Device
drive	I-Device
,	O
even	O
if	O
there	O
were	O
two	O
drives	O
installed	O
.	O
</s>
<s>
On	O
the	O
original	O
IBM	B-Device
PC	I-Device
and	O
XT	B-Device
,	O
if	O
no	O
bootable	B-Operating_System
disk	O
was	O
found	O
,	O
ROM	B-Language
BASIC	I-Language
was	O
started	O
by	O
calling	O
INT	O
18h	O
.	O
</s>
<s>
Since	O
few	O
programs	O
used	O
BASIC	O
in	O
ROM	B-Device
,	O
clone	O
PC	O
makers	O
left	O
it	O
out	O
;	O
then	O
a	O
computer	O
that	O
failed	O
to	O
boot	B-Operating_System
from	O
a	O
disk	O
would	O
display	O
"	O
No	O
ROM	B-Language
BASIC	I-Language
"	O
and	O
halt	O
(	O
in	O
response	O
to	O
INT	O
18h	O
)	O
.	O
</s>
<s>
Later	O
computers	O
would	O
display	O
a	O
message	O
like	O
"	O
No	O
bootable	B-Operating_System
disk	O
found	O
"	O
;	O
some	O
would	O
prompt	O
for	O
a	O
disk	O
to	O
be	O
inserted	O
and	O
a	O
key	O
to	O
be	O
pressed	O
to	O
retry	O
the	O
boot	B-Operating_System
process	I-Operating_System
.	O
</s>
<s>
A	O
modern	O
BIOS	B-Operating_System
may	O
display	O
nothing	O
or	O
may	O
automatically	O
enter	O
the	O
BIOS	B-Operating_System
configuration	O
utility	O
when	O
the	O
boot	B-Operating_System
process	I-Operating_System
fails	O
.	O
</s>
<s>
The	O
environment	O
for	O
the	O
boot	B-Operating_System
program	I-Operating_System
is	O
very	O
simple	O
:	O
the	O
CPU	B-General_Concept
is	O
in	O
real	B-Application
mode	I-Application
and	O
the	O
general-purpose	O
and	O
segment	O
registers	O
are	O
undefined	O
,	O
except	O
SS	O
,	O
SP	O
,	O
CS	B-Language
,	O
and	O
DL	O
.	O
</s>
<s>
CS:IP	O
always	O
points	O
to	O
physical	O
address	O
0x07C00	O
.	O
</s>
<s>
What	O
values	O
CS	B-Language
and	O
IP	O
actually	O
have	O
is	O
not	O
well	O
defined	O
.	O
</s>
<s>
Some	O
BIOSes	O
use	O
a	O
CS:IP	O
of	O
0x0000:0x7C00	O
while	O
others	O
may	O
use	O
0x07C0:0x0000	O
.	O
</s>
<s>
Because	O
boot	B-Operating_System
programs	I-Operating_System
are	O
always	O
loaded	O
at	O
this	O
fixed	O
address	O
,	O
there	O
is	O
no	O
need	O
for	O
a	O
boot	B-Operating_System
program	I-Operating_System
to	O
be	O
relocatable	O
.	O
</s>
<s>
DL	O
may	O
contain	O
the	O
drive	O
number	O
,	O
as	O
used	O
with	O
INT	B-Device
13h	I-Device
,	O
of	O
the	O
boot	B-Operating_System
device	O
.	O
</s>
<s>
SS	O
:	O
SP	O
points	O
to	O
a	O
valid	O
stack	O
that	O
is	O
presumably	O
large	O
enough	O
to	O
support	O
hardware	B-Architecture
interrupts	O
,	O
but	O
otherwise	O
SS	O
and	O
SP	O
are	O
undefined	O
.	O
</s>
<s>
(	O
A	O
stack	O
must	O
be	O
already	O
set	O
up	O
in	O
order	O
for	O
interrupts	O
to	O
be	O
serviced	O
,	O
and	O
interrupts	O
must	O
be	O
enabled	O
in	O
order	O
for	O
the	O
system	O
timer-tick	O
interrupt	O
,	O
which	O
BIOS	B-Operating_System
always	O
uses	O
at	O
least	O
to	O
maintain	O
the	O
time-of-day	O
count	O
and	O
which	O
it	O
initializes	O
during	O
POST	B-Device
,	O
to	O
be	O
active	O
and	O
for	O
the	O
keyboard	B-Device
to	O
work	O
.	O
</s>
<s>
The	O
keyboard	B-Device
works	O
even	O
if	O
the	O
BIOS	B-Operating_System
keyboard	B-Device
service	O
is	O
not	O
called	O
;	O
keystrokes	O
are	O
received	O
and	O
placed	O
in	O
the	O
15-character	O
type-ahead	O
buffer	O
maintained	O
by	O
BIOS	B-Operating_System
.	O
)	O
</s>
<s>
The	O
boot	B-Operating_System
program	I-Operating_System
must	O
set	O
up	O
its	O
own	O
stack	O
,	O
because	O
the	O
size	O
of	O
the	O
stack	O
set	O
up	O
by	O
BIOS	B-Operating_System
is	O
unknown	O
and	O
its	O
location	O
is	O
likewise	O
variable	O
;	O
although	O
the	O
boot	B-Operating_System
program	I-Operating_System
can	O
investigate	O
the	O
default	O
stack	O
by	O
examining	O
SS	O
:	O
SP	O
,	O
it	O
is	O
easier	O
and	O
shorter	O
to	O
just	O
unconditionally	O
set	O
up	O
a	O
new	O
stack	O
.	O
</s>
<s>
At	O
boot	B-Operating_System
time	I-Operating_System
,	O
all	O
BIOS	B-Operating_System
services	O
are	O
available	O
,	O
and	O
the	O
memory	O
below	O
address	O
0x00400	O
contains	O
the	O
interrupt	B-General_Concept
vector	I-General_Concept
table	I-General_Concept
.	O
</s>
<s>
BIOS	B-Device
POST	I-Device
has	O
initialized	O
the	O
system	O
timers	O
,	O
interrupt	O
controller(s )	O
,	O
DMA	O
controller(s )	O
,	O
and	O
other	O
motherboard/chipset	O
hardware	B-Architecture
as	O
necessary	O
to	O
bring	O
all	O
BIOS	B-Operating_System
services	O
to	O
ready	O
status	O
.	O
</s>
<s>
The	O
interrupt	B-General_Concept
vectors	I-General_Concept
corresponding	O
to	O
the	O
BIOS	B-Device
interrupts	I-Device
have	O
been	O
set	O
to	O
point	O
at	O
the	O
appropriate	O
entry	B-Language
points	I-Language
in	O
the	O
BIOS	B-Operating_System
,	O
hardware	B-Architecture
interrupt	B-General_Concept
vectors	I-General_Concept
for	O
devices	O
initialized	O
by	O
the	O
BIOS	B-Operating_System
have	O
been	O
set	O
to	O
point	O
to	O
the	O
BIOS-provided	O
ISRs	O
,	O
and	O
some	O
other	O
interrupts	O
,	O
including	O
ones	O
that	O
BIOS	B-Operating_System
generates	O
for	O
programs	O
to	O
hook	O
,	O
have	O
been	O
set	O
to	O
a	O
default	O
dummy	O
ISR	O
that	O
immediately	O
returns	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
maintains	O
a	O
reserved	O
block	O
of	O
system	O
RAM	B-Architecture
at	O
addresses	O
0x00400	O
–	O
0x004FF	O
with	O
various	O
parameters	O
initialized	O
during	O
the	O
POST	B-Device
.	O
</s>
<s>
All	O
memory	O
at	O
and	O
above	O
address	O
0x00500	O
can	O
be	O
used	O
by	O
the	O
boot	B-Operating_System
program	I-Operating_System
;	O
it	O
may	O
even	O
overwrite	O
itself	O
.	O
</s>
<s>
Peripheral	O
cards	O
such	O
as	O
hard	B-Device
disk	I-Device
drive	I-Device
host	B-Architecture
bus	I-Architecture
adapters	I-Architecture
and	O
video	B-Device
cards	I-Device
have	O
their	O
own	O
firmware	B-Application
,	O
and	O
BIOS	B-Device
extension	I-Device
option	B-Device
ROM	I-Device
may	O
be	O
a	O
part	O
of	O
the	O
expansion	B-Device
card	I-Device
firmware	B-Application
,	O
which	O
provide	O
additional	O
functionality	O
to	O
BIOS	B-Operating_System
.	O
</s>
<s>
Code	O
in	O
option	O
ROMs	O
runs	O
before	O
the	O
BIOS	B-Operating_System
boots	O
the	O
operating	B-General_Concept
system	I-General_Concept
from	O
mass	B-Device
storage	I-Device
.	O
</s>
<s>
These	O
ROMs	O
typically	O
test	O
and	O
initialize	O
hardware	B-Architecture
,	O
add	O
new	O
BIOS	B-Operating_System
services	O
,	O
or	O
replace	O
existing	O
BIOS	B-Operating_System
services	O
with	O
their	O
own	O
services	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
SCSI	B-Architecture
controller	O
usually	O
has	O
a	O
BIOS	B-Device
extension	I-Device
ROM	B-Device
that	O
adds	O
support	O
for	O
hard	B-Device
drives	I-Device
connected	O
through	O
that	O
controller	O
.	O
</s>
<s>
An	O
extension	O
ROM	B-Device
could	O
in	O
principle	O
contain	O
operating	B-General_Concept
system	I-General_Concept
,	O
or	O
it	O
could	O
implement	O
an	O
entirely	O
different	O
boot	B-Operating_System
process	I-Operating_System
such	O
as	O
network	B-Device
booting	I-Device
.	O
</s>
<s>
Operation	O
of	O
an	O
IBM-compatible	O
computer	O
system	O
can	O
be	O
completely	O
changed	O
by	O
removing	O
or	O
inserting	O
an	O
adapter	B-Device
card	I-Device
(	O
or	O
a	O
ROM	B-Device
chip	I-Device
)	O
that	O
contains	O
a	O
BIOS	B-Device
extension	I-Device
ROM	B-Device
.	O
</s>
<s>
The	O
motherboard	B-Device
BIOS	B-Operating_System
typically	O
contains	O
code	O
for	O
initializing	O
and	O
bootstrapping	O
integrated	O
display	O
and	O
integrated	O
storage	O
.	O
</s>
<s>
In	O
addition	O
,	O
plug-in	O
adapter	B-Device
cards	I-Device
such	O
as	O
SCSI	B-Architecture
,	O
RAID	B-Architecture
,	O
network	B-Protocol
interface	I-Protocol
cards	I-Protocol
,	O
and	O
video	B-Device
cards	I-Device
often	O
include	O
their	O
own	O
BIOS	B-Operating_System
(	O
e.g.	O
</s>
<s>
Video	B-Device
BIOS	I-Device
)	O
,	O
complementing	O
or	O
replacing	O
the	O
system	B-Operating_System
BIOS	I-Operating_System
code	O
for	O
the	O
given	O
component	O
.	O
</s>
<s>
Even	O
devices	O
built	O
into	O
the	O
motherboard	B-Device
can	O
behave	O
in	O
this	O
way	O
;	O
their	O
option	O
ROMs	O
can	O
be	O
a	O
part	O
of	O
the	O
motherboard	B-Device
BIOS	B-Operating_System
.	O
</s>
<s>
An	O
add-in	B-Device
card	I-Device
requires	O
an	O
option	B-Device
ROM	I-Device
if	O
the	O
card	O
is	O
not	O
supported	O
by	O
the	O
motherboard	B-Device
BIOS	B-Operating_System
and	O
the	O
card	O
needs	O
to	O
be	O
initialized	O
or	O
made	O
accessible	O
through	O
BIOS	B-Operating_System
services	O
before	O
the	O
operating	B-General_Concept
system	I-General_Concept
can	O
be	O
loaded	O
(	O
usually	O
this	O
means	O
it	O
is	O
required	O
in	O
the	O
boot	B-Operating_System
process	I-Operating_System
)	O
.	O
</s>
<s>
An	O
additional	O
advantage	O
of	O
ROM	B-Device
on	O
some	O
early	O
PC	O
systems	O
(	O
notably	O
including	O
the	O
IBM	B-Device
PCjr	I-Device
)	O
was	O
that	O
ROM	B-Device
was	O
faster	O
than	O
main	O
system	O
RAM	B-Architecture
.	O
</s>
<s>
(	O
On	O
modern	O
systems	O
,	O
the	O
case	O
is	O
very	O
much	O
the	O
reverse	O
of	O
this	O
,	O
and	O
BIOS	B-Operating_System
ROM	I-Operating_System
code	O
is	O
usually	O
copied	O
(	O
"	O
shadowed	O
"	O
)	O
into	O
RAM	B-Architecture
so	O
it	O
will	O
run	O
faster	O
.	O
)	O
</s>
<s>
If	O
an	O
expansion	B-Device
ROM	I-Device
wishes	O
to	O
change	O
the	O
way	O
the	O
system	B-Operating_System
boots	I-Operating_System
(	O
such	O
as	O
from	O
a	O
network	O
device	O
or	O
a	O
SCSI	B-Architecture
adapter	O
)	O
in	O
a	O
cooperative	O
way	O
,	O
it	O
can	O
use	O
the	O
BIOS	B-Operating_System
Boot	B-Operating_System
Specification	O
(	O
BBS	O
)	O
API	B-Application
to	O
register	O
its	O
ability	O
to	O
do	O
so	O
.	O
</s>
<s>
Once	O
the	O
expansion	O
ROMs	O
have	O
registered	O
using	O
the	O
BBS	O
APIs	B-Application
,	O
the	O
user	O
can	O
select	O
among	O
the	O
available	O
boot	B-Operating_System
options	O
from	O
within	O
the	O
BIOS	B-Operating_System
's	O
user	O
interface	B-Application
.	O
</s>
<s>
This	O
is	O
why	O
most	O
BBS	O
compliant	O
PC	B-Operating_System
BIOS	I-Operating_System
implementations	O
will	O
not	O
allow	O
the	O
user	O
to	O
enter	O
the	O
BIOS	B-Operating_System
's	O
user	O
interface	B-Application
until	O
the	O
expansion	O
ROMs	O
have	O
finished	O
executing	O
and	O
registering	O
themselves	O
with	O
the	O
BBS	O
API	B-Application
.	O
</s>
<s>
Also	O
,	O
if	O
an	O
expansion	B-Device
ROM	I-Device
wishes	O
to	O
change	O
the	O
way	O
the	O
system	B-Operating_System
boots	I-Operating_System
unilaterally	O
,	O
it	O
can	O
simply	O
hook	O
INT	O
19h	O
or	O
other	O
interrupts	O
normally	O
called	O
from	O
interrupt	O
19h	O
,	O
such	O
as	O
INT	B-Device
13h	I-Device
,	O
the	O
BIOS	B-Operating_System
disk	O
service	O
,	O
to	O
intercept	O
the	O
BIOS	B-Operating_System
boot	B-Operating_System
process	I-Operating_System
.	O
</s>
<s>
Then	O
it	O
can	O
replace	O
the	O
BIOS	B-Operating_System
boot	B-Operating_System
process	I-Operating_System
with	O
one	O
of	O
its	O
own	O
,	O
or	O
it	O
can	O
merely	O
modify	O
the	O
boot	B-Operating_System
sequence	I-Operating_System
by	O
inserting	O
its	O
own	O
boot	B-Operating_System
actions	O
into	O
it	O
,	O
by	O
preventing	O
the	O
BIOS	B-Operating_System
from	O
detecting	O
certain	O
devices	O
as	O
bootable	B-Operating_System
,	O
or	O
both	O
.	O
</s>
<s>
Before	O
the	O
BIOS	B-Operating_System
Boot	B-Operating_System
Specification	O
was	O
promulgated	O
,	O
this	O
was	O
the	O
only	O
way	O
for	O
expansion	O
ROMs	O
to	O
implement	O
boot	B-Operating_System
capability	O
for	O
devices	O
not	O
supported	O
for	O
booting	B-Operating_System
by	O
the	O
native	O
BIOS	B-Operating_System
of	O
the	O
motherboard	B-Device
.	O
</s>
<s>
After	O
the	O
motherboard	B-Device
BIOS	B-Operating_System
completes	O
its	O
POST	B-Device
,	O
most	O
BIOS	B-Operating_System
versions	O
search	O
for	O
option	B-Device
ROM	I-Device
modules	O
,	O
also	O
called	O
BIOS	B-Device
extension	I-Device
ROMs	O
,	O
and	O
execute	O
them	O
.	O
</s>
<s>
The	O
motherboard	B-Device
BIOS	B-Operating_System
scans	O
for	O
extension	O
ROMs	O
in	O
a	O
portion	O
of	O
the	O
"	O
upper	B-Device
memory	I-Device
area	I-Device
"	O
(	O
the	O
part	O
of	O
the	O
x86	B-Operating_System
real-mode	B-Application
address	O
space	O
at	O
and	O
above	O
address	O
0xA0000	O
)	O
and	O
runs	O
each	O
ROM	B-Device
found	O
,	O
in	O
order	O
.	O
</s>
<s>
To	O
discover	O
memory-mapped	O
option	O
ROMs	O
,	O
a	O
BIOS	B-Operating_System
implementation	O
scans	O
the	O
real-mode	B-Application
address	O
space	O
from	O
0x0C0000	O
to	O
0x0F0000	O
on	O
2KB	O
(	O
2,048	O
bytes	O
)	O
boundaries	O
,	O
looking	O
for	O
a	O
two-byte	O
ROM	B-Device
signature	O
:	O
0x55	O
followed	O
by	O
0xAA	O
.	O
</s>
<s>
In	O
a	O
valid	O
expansion	B-Device
ROM	I-Device
,	O
this	O
signature	O
is	O
followed	O
by	O
a	O
single	O
byte	O
indicating	O
the	O
number	O
of	O
512-byte	O
blocks	O
the	O
expansion	B-Device
ROM	I-Device
occupies	O
in	O
real	O
memory	O
,	O
and	O
the	O
next	O
byte	O
is	O
the	O
option	B-Device
ROM	I-Device
's	O
entry	B-Language
point	I-Language
(	O
also	O
known	O
as	O
its	O
"	O
entry	O
offset	O
"	O
)	O
.	O
</s>
<s>
If	O
the	O
ROM	B-Device
has	O
a	O
valid	O
checksum	B-Algorithm
,	O
the	O
BIOS	B-Operating_System
transfers	O
control	O
to	O
the	O
entry	O
address	O
,	O
which	O
in	O
a	O
normal	O
BIOS	B-Device
extension	I-Device
ROM	B-Device
should	O
be	O
the	O
beginning	O
of	O
the	O
extension	O
's	O
initialization	O
routine	O
.	O
</s>
<s>
At	O
this	O
point	O
,	O
the	O
extension	O
ROM	B-Device
code	O
takes	O
over	O
,	O
typically	O
testing	O
and	O
initializing	O
the	O
hardware	B-Architecture
it	O
controls	O
and	O
registering	O
interrupt	B-General_Concept
vectors	I-General_Concept
for	O
use	O
by	O
post-boot	O
applications	O
.	O
</s>
<s>
It	O
may	O
use	O
BIOS	B-Operating_System
services	O
(	O
including	O
those	O
provided	O
by	O
previously	O
initialized	O
option	O
ROMs	O
)	O
to	O
provide	O
a	O
user	O
configuration	O
interface	B-Application
,	O
to	O
display	O
diagnostic	O
information	O
,	O
or	O
to	O
do	O
anything	O
else	O
that	O
it	O
requires	O
.	O
</s>
<s>
It	O
is	O
possible	O
that	O
an	O
option	B-Device
ROM	I-Device
will	O
not	O
return	O
to	O
BIOS	B-Operating_System
,	O
pre-empting	O
the	O
BIOS	B-Operating_System
's	O
boot	B-Operating_System
sequence	I-Operating_System
altogether	O
.	O
</s>
<s>
An	O
option	B-Device
ROM	I-Device
should	O
normally	O
return	O
to	O
the	O
BIOS	B-Operating_System
after	O
completing	O
its	O
initialization	O
process	O
.	O
</s>
<s>
Once	O
(	O
and	O
if	O
)	O
an	O
option	B-Device
ROM	I-Device
returns	O
,	O
the	O
BIOS	B-Operating_System
continues	O
searching	O
for	O
more	O
option	O
ROMs	O
,	O
calling	O
each	O
as	O
it	O
is	O
found	O
,	O
until	O
the	O
entire	O
option	B-Device
ROM	I-Device
area	O
in	O
the	O
memory	O
space	O
has	O
been	O
scanned	O
.	O
</s>
<s>
Option	O
ROMs	O
normally	O
reside	O
on	O
adapter	B-Device
cards	I-Device
.	O
</s>
<s>
However	O
,	O
the	O
original	O
PC	O
,	O
and	O
perhaps	O
also	O
the	O
PC	B-Device
XT	I-Device
,	O
have	O
a	O
spare	O
ROM	B-Device
socket	O
on	O
the	O
motherboard	B-Device
(	O
the	O
"	O
system	B-Device
board	I-Device
"	O
in	O
IBM	O
's	O
terms	O
)	O
into	O
which	O
an	O
option	B-Device
ROM	I-Device
can	O
be	O
inserted	O
,	O
and	O
the	O
four	O
ROMs	O
that	O
contain	O
the	O
BASIC	O
interpreter	O
can	O
also	O
be	O
removed	O
and	O
replaced	O
with	O
custom	O
ROMs	O
which	O
can	O
be	O
option	O
ROMs	O
.	O
</s>
<s>
The	O
IBM	B-Device
PCjr	I-Device
is	O
unique	O
among	O
PCs	O
in	O
having	O
two	O
ROM	B-Device
cartridge	O
slots	O
on	O
the	O
front	O
.	O
</s>
<s>
Cartridges	O
in	O
these	O
slots	O
map	O
into	O
the	O
same	O
region	O
of	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
used	O
for	O
option	O
ROMs	O
,	O
and	O
the	O
cartridges	O
can	O
contain	O
option	B-Device
ROM	I-Device
modules	O
that	O
the	O
BIOS	B-Operating_System
would	O
recognize	O
.	O
</s>
<s>
The	O
cartridges	O
can	O
also	O
contain	O
other	O
types	O
of	O
ROM	B-Device
modules	O
,	O
such	O
as	O
BASIC	O
programs	O
,	O
that	O
are	O
handled	O
differently	O
.	O
</s>
<s>
One	O
PCjr	B-Device
cartridge	O
can	O
contain	O
several	O
ROM	B-Device
modules	O
of	O
different	O
types	O
,	O
possibly	O
stored	O
together	O
in	O
one	O
ROM	B-Device
chip	I-Device
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
ROM	I-Operating_System
is	O
customized	O
to	O
the	O
particular	O
manufacturer	O
's	O
hardware	B-Architecture
,	O
allowing	O
low-level	O
services	O
(	O
such	O
as	O
reading	O
a	O
keystroke	O
or	O
writing	O
a	O
sector	O
of	O
data	O
to	O
diskette	B-Device
)	O
to	O
be	O
provided	O
in	O
a	O
standardized	O
way	O
to	O
programs	O
,	O
including	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
an	O
IBM	B-Device
PC	I-Device
might	O
have	O
either	O
a	O
monochrome	O
or	O
a	O
color	B-Device
display	I-Device
adapter	O
(	O
using	O
different	O
display	O
memory	O
addresses	O
and	O
hardware	B-Architecture
)	O
,	O
but	O
a	O
single	O
,	O
standard	O
,	O
BIOS	B-Operating_System
system	B-Operating_System
call	I-Operating_System
may	O
be	O
invoked	O
to	O
display	O
a	O
character	O
at	O
a	O
specified	O
position	O
on	O
the	O
screen	O
in	O
text	B-Application
mode	I-Application
or	O
graphics	O
mode	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
provides	O
a	O
small	O
library	B-Library
of	O
basic	O
input/output	B-General_Concept
functions	O
to	O
operate	O
peripherals	O
(	O
such	O
as	O
the	O
keyboard	B-Device
,	O
rudimentary	O
text	O
and	O
graphics	O
display	O
functions	O
and	O
so	O
forth	B-Application
)	O
.	O
</s>
<s>
When	O
using	O
MS-DOS	B-Application
,	O
BIOS	B-Operating_System
services	O
could	O
be	O
accessed	O
by	O
an	O
application	O
program	O
(	O
or	O
by	O
MS-DOS	B-Application
)	O
by	O
executing	O
an	O
INT	B-Device
13h	I-Device
interrupt	O
instruction	O
to	O
access	O
disk	O
functions	O
,	O
or	O
by	O
executing	O
one	O
of	O
a	O
number	O
of	O
other	O
documented	O
BIOS	B-Device
interrupt	I-Device
calls	I-Device
to	O
access	O
video	B-Device
display	I-Device
,	O
keyboard	B-Device
,	O
cassette	O
,	O
and	O
other	O
device	O
functions	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
and	O
executive	O
software	O
that	O
are	O
designed	O
to	O
supersede	O
this	O
basic	O
firmware	B-Application
functionality	O
provide	O
replacement	O
software	O
interfaces	B-Application
to	O
application	O
software	O
.	O
</s>
<s>
This	O
began	O
even	O
in	O
the	O
1980s	O
under	O
MS-DOS	B-Application
,	O
when	O
programmers	O
observed	O
that	O
using	O
the	O
BIOS	B-Operating_System
video	O
services	O
for	O
graphics	O
display	O
were	O
very	O
slow	O
.	O
</s>
<s>
To	O
increase	O
the	O
speed	O
of	O
screen	O
output	O
,	O
many	O
programs	O
bypassed	O
the	O
BIOS	B-Operating_System
and	O
programmed	O
the	O
video	B-Device
display	I-Device
hardware	B-Architecture
directly	O
.	O
</s>
<s>
Other	O
graphics	O
programmers	O
,	O
particularly	O
but	O
not	O
exclusively	O
in	O
the	O
demoscene	O
,	O
observed	O
that	O
there	O
were	O
technical	O
capabilities	O
of	O
the	O
PC	O
display	B-Device
adapters	I-Device
that	O
were	O
not	O
supported	O
by	O
the	O
IBM	B-Operating_System
BIOS	I-Operating_System
and	O
could	O
not	O
be	O
taken	O
advantage	O
of	O
without	O
circumventing	O
it	O
.	O
</s>
<s>
Since	O
the	O
AT-compatible	O
BIOS	B-Operating_System
ran	O
in	O
Intel	O
real	B-Application
mode	I-Application
,	O
operating	B-General_Concept
systems	I-General_Concept
that	O
ran	O
in	O
protected	B-Application
mode	I-Application
on	O
286	O
and	O
later	O
processors	O
required	O
hardware	B-Architecture
device	O
drivers	O
compatible	O
with	O
protected	B-Application
mode	I-Application
operation	O
to	O
replace	O
BIOS	B-Operating_System
services	O
.	O
</s>
<s>
In	O
modern	O
PCs	O
running	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
(	O
such	O
as	O
Windows	B-Application
and	O
Linux	B-Operating_System
)	O
the	O
BIOS	B-Device
interrupt	I-Device
calls	I-Device
is	O
used	O
only	O
during	O
booting	B-Operating_System
and	O
initial	O
loading	O
of	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Before	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
first	O
graphical	O
screen	O
is	O
displayed	O
,	O
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
are	O
typically	O
handled	O
through	O
BIOS	B-Operating_System
.	O
</s>
<s>
A	O
boot	B-Operating_System
menu	I-Operating_System
such	O
as	O
the	O
textual	O
menu	O
of	O
Windows	B-Application
,	O
which	O
allows	O
users	O
to	O
choose	O
an	O
operating	B-General_Concept
system	I-General_Concept
to	O
boot	B-Operating_System
,	O
to	O
boot	B-Operating_System
into	O
the	O
safe	B-Operating_System
mode	I-Operating_System
,	O
or	O
to	O
use	O
the	O
last	O
known	O
good	O
configuration	O
,	O
is	O
displayed	O
through	O
BIOS	B-Operating_System
and	O
receives	O
keyboard	B-Device
input	O
through	O
BIOS	B-Operating_System
.	O
</s>
<s>
Many	O
modern	O
PCs	O
can	O
still	O
boot	B-Operating_System
and	O
run	O
legacy	O
operating	B-General_Concept
systems	I-General_Concept
such	O
as	O
MS-DOS	B-Application
or	O
DR-DOS	B-Application
that	O
rely	O
heavily	O
on	O
BIOS	B-Operating_System
for	O
their	O
console	O
and	O
disk	O
I/O	B-General_Concept
,	O
providing	O
that	O
the	O
system	O
has	O
a	O
BIOS	B-Operating_System
,	O
or	O
a	O
CSM-capable	O
UEFI	B-Architecture
firmware	I-Architecture
.	O
</s>
<s>
Intel	O
processors	O
have	O
reprogrammable	O
microcode	B-Device
since	O
the	O
P6	B-Device
microarchitecture	I-Device
.	O
</s>
<s>
AMD	O
processors	O
have	O
reprogrammable	O
microcode	B-Device
since	O
the	O
K7	B-Architecture
microarchitecture	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
contain	O
patches	O
to	O
the	O
processor	O
microcode	B-Device
that	O
fix	O
errors	O
in	O
the	O
initial	O
processor	O
microcode	B-Device
;	O
microcode	B-Device
is	O
loaded	O
into	O
processor	O
's	O
SRAM	B-Architecture
so	O
reprogramming	O
is	O
not	O
persistent	O
,	O
thus	O
loading	O
of	O
microcode	B-Device
updates	O
is	O
performed	O
each	O
time	O
the	O
system	O
is	O
powered	O
up	O
.	O
</s>
<s>
Without	O
reprogrammable	O
microcode	B-Device
,	O
an	O
expensive	O
processor	O
swap	O
would	O
be	O
required	O
;	O
for	O
example	O
,	O
the	O
Pentium	B-Device
FDIV	I-Device
bug	I-Device
became	O
an	O
expensive	O
fiasco	O
for	O
Intel	O
as	O
it	O
required	O
a	O
product	O
recall	O
because	O
the	O
original	O
Pentium	O
processor	O
's	O
defective	O
microcode	B-Device
could	O
not	O
be	O
reprogrammed	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
can	O
update	O
main	O
processor	O
microcode	B-Device
also	O
.	O
</s>
<s>
Some	O
BIOSes	O
contain	O
a	O
software	O
licensing	O
description	O
table	O
(	O
SLIC	O
)	O
,	O
a	O
digital	O
signature	O
placed	O
inside	O
the	O
BIOS	B-Operating_System
by	O
the	O
original	O
equipment	O
manufacturer	O
(	O
OEM	O
)	O
,	O
for	O
example	O
Dell	O
.	O
</s>
<s>
The	O
SLIC	O
is	O
inserted	O
into	O
the	O
ACPI	B-Device
data	O
table	O
and	O
contains	O
no	O
active	O
code	O
.	O
</s>
<s>
Computer	O
manufacturers	O
that	O
distribute	O
OEM	O
versions	O
of	O
Microsoft	B-Application
Windows	I-Application
and	O
Microsoft	O
application	O
software	O
can	O
use	O
the	O
SLIC	O
to	O
authenticate	O
licensing	O
to	O
the	O
OEM	O
Windows	B-Application
Installation	O
disk	O
and	O
system	O
recovery	B-Operating_System
disc	I-Operating_System
containing	O
Windows	B-Application
software	O
.	O
</s>
<s>
Systems	O
with	O
a	O
SLIC	O
can	O
be	O
preactivated	O
with	O
an	O
OEM	O
product	O
key	O
,	O
and	O
they	O
verify	O
an	O
XML	O
formatted	O
OEM	O
certificate	O
against	O
the	O
SLIC	O
in	O
the	O
BIOS	B-Operating_System
as	O
a	O
means	O
of	O
self-activating	O
(	O
see	O
System	O
Locked	O
Preinstallation	O
,	O
SLP	O
)	O
.	O
</s>
<s>
If	O
a	O
user	O
performs	O
a	O
fresh	O
install	O
of	O
Windows	B-Application
,	O
they	O
will	O
need	O
to	O
have	O
possession	O
of	O
both	O
the	O
OEM	O
key	O
(	O
either	O
SLP	O
or	O
COA	O
)	O
and	O
the	O
digital	O
certificate	O
for	O
their	O
SLIC	O
in	O
order	O
to	O
bypass	O
activation	O
.	O
</s>
<s>
Cracks	O
for	O
non-genuine	O
Windows	B-Application
distributions	O
usually	O
edit	O
the	O
SLIC	O
or	O
emulate	O
it	O
in	O
order	O
to	O
bypass	O
Windows	B-Application
activation	O
.	O
</s>
<s>
Some	O
BIOS	B-Operating_System
implementations	O
allow	O
overclocking	B-Application
,	O
an	O
action	O
in	O
which	O
the	O
CPU	B-General_Concept
is	O
adjusted	O
to	O
a	O
higher	O
clock	O
rate	O
than	O
its	O
manufacturer	O
rating	O
for	O
guaranteed	O
capability	O
.	O
</s>
<s>
Overclocking	B-Application
may	O
,	O
however	O
,	O
seriously	O
compromise	O
system	O
reliability	O
in	O
insufficiently	O
cooled	O
computers	O
and	O
generally	O
shorten	O
component	O
lifespan	O
.	O
</s>
<s>
Overclocking	B-Application
,	O
when	O
incorrectly	O
performed	O
,	O
may	O
also	O
cause	O
components	O
to	O
overheat	O
so	O
quickly	O
that	O
they	O
mechanically	O
destroy	O
themselves	O
.	O
</s>
<s>
Some	O
older	O
operating	B-General_Concept
systems	I-General_Concept
,	O
for	O
example	O
MS-DOS	B-Application
,	O
rely	O
on	O
the	O
BIOS	B-Operating_System
to	O
carry	O
out	O
most	O
input/output	B-General_Concept
tasks	O
within	O
the	O
PC	O
.	O
</s>
<s>
Calling	O
real	B-Application
mode	I-Application
BIOS	B-Operating_System
services	O
directly	O
is	O
inefficient	O
for	O
protected	B-Application
mode	I-Application
(	O
and	O
long	B-Application
mode	I-Application
)	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
BIOS	B-Device
interrupt	I-Device
calls	I-Device
are	O
not	O
used	O
by	O
modern	O
multitasking	O
operating	B-General_Concept
systems	I-General_Concept
after	O
they	O
initially	O
load	O
.	O
</s>
<s>
In	O
1990s	O
,	O
BIOS	B-Operating_System
provided	O
some	O
protected	B-Application
mode	I-Application
interfaces	B-Application
for	O
Microsoft	B-Application
Windows	I-Application
and	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
such	O
as	O
Advanced	B-Device
Power	I-Device
Management	I-Device
(	O
APM	O
)	O
,	O
Plug	B-Device
and	I-Device
Play	I-Device
BIOS	I-Device
,	O
Desktop	B-Device
Management	I-Device
Interface	I-Device
(	O
DMI	O
)	O
,	O
VESA	B-Device
BIOS	I-Device
Extensions	I-Device
(	O
VBE	B-Device
)	O
,	O
e820	B-Device
and	O
MultiProcessor	B-Device
Specification	I-Device
(	O
MPS	O
)	O
.	O
</s>
<s>
Starting	O
from	O
the	O
2000	O
,	O
most	O
BIOSes	O
provide	O
ACPI	B-Device
,	O
SMBIOS	O
,	O
VBE	B-Device
and	O
e820	B-Device
interfaces	B-Application
for	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
After	O
operating	B-General_Concept
systems	I-General_Concept
load	O
,	O
the	O
System	B-Architecture
Management	I-Architecture
Mode	I-Architecture
code	O
is	O
still	O
running	O
in	O
SMRAM	B-Architecture
.	O
</s>
<s>
Since	O
2010	O
,	O
BIOS	B-Operating_System
technology	O
is	O
in	O
a	O
transitional	O
process	O
toward	O
UEFI	B-Architecture
.	O
</s>
<s>
Historically	O
,	O
the	O
BIOS	B-Operating_System
in	O
the	O
IBM	B-Device
PC	I-Device
and	O
XT	B-Device
had	O
no	O
built-in	O
user	O
interface	B-Application
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
versions	O
in	O
earlier	O
PCs	O
(	O
XT-class	O
)	O
were	O
not	O
software	O
configurable	O
;	O
instead	O
,	O
users	O
set	O
the	O
options	O
via	O
DIP	O
switches	O
on	O
the	O
motherboard	B-Device
.	O
</s>
<s>
Later	O
computers	O
,	O
including	O
all	O
IBM-compatibles	O
with	O
80286	O
CPUs	O
,	O
had	O
a	O
battery-backed	O
nonvolatile	B-Device
BIOS	I-Device
memory	I-Device
(	O
CMOS	B-Device
RAM	I-Device
chip	O
)	O
that	O
held	O
BIOS	B-Operating_System
settings	I-Operating_System
.	O
</s>
<s>
These	O
settings	O
,	O
such	O
as	O
video-adapter	O
type	O
,	O
memory	O
size	O
,	O
and	O
hard-disk	B-Device
parameters	O
,	O
could	O
only	O
be	O
configured	O
by	O
running	O
a	O
configuration	O
program	O
from	O
a	O
disk	O
,	O
not	O
built	O
into	O
the	O
ROM	B-Device
.	O
</s>
<s>
A	O
special	O
"	O
reference	O
diskette	B-Device
"	O
was	O
inserted	O
in	O
an	O
IBM	B-Operating_System
AT	I-Operating_System
to	O
configure	O
settings	O
such	O
as	O
memory	O
size	O
.	O
</s>
<s>
Early	O
BIOS	B-Operating_System
versions	O
did	O
not	O
have	O
passwords	O
or	O
boot-device	O
selection	O
options	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
was	O
hard-coded	O
to	O
boot	B-Operating_System
from	O
the	O
first	O
floppy	B-Device
drive	I-Device
,	O
or	O
,	O
if	O
that	O
failed	O
,	O
the	O
first	O
hard	B-Device
disk	I-Device
.	O
</s>
<s>
Access	O
control	O
in	O
early	O
AT-class	O
machines	O
was	O
by	O
a	O
physical	O
keylock	O
switch	O
(	O
which	O
was	O
not	O
hard	O
to	O
defeat	O
if	O
the	O
computer	B-General_Concept
case	I-General_Concept
could	O
be	O
opened	O
)	O
.	O
</s>
<s>
Anyone	O
who	O
could	O
switch	O
on	O
the	O
computer	O
could	O
boot	B-Operating_System
it	O
.	O
</s>
<s>
Later	O
,	O
386-class	O
computers	O
started	O
integrating	O
the	O
BIOS	B-Operating_System
setup	O
utility	O
in	O
the	O
ROM	B-Device
itself	O
,	O
alongside	O
the	O
BIOS	B-Operating_System
code	O
;	O
these	O
computers	O
usually	O
boot	B-Operating_System
into	O
the	O
BIOS	B-Operating_System
setup	O
utility	O
if	O
a	O
certain	O
key	O
or	O
key	O
combination	O
is	O
pressed	O
,	O
otherwise	O
the	O
BIOS	B-Device
POST	I-Device
and	O
boot	B-Operating_System
process	I-Operating_System
are	O
executed	O
.	O
</s>
<s>
A	O
modern	O
BIOS	B-Operating_System
setup	O
utility	O
has	O
a	O
text	B-Application
user	I-Application
interface	I-Application
(	O
TUI	O
)	O
or	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
accessed	O
by	O
pressing	O
a	O
certain	O
key	O
on	O
the	O
keyboard	B-Device
when	O
the	O
PC	O
starts	O
.	O
</s>
<s>
The	O
actual	O
key	O
depends	O
on	O
specific	O
hardware	B-Architecture
.	O
</s>
<s>
Features	O
present	O
in	O
the	O
BIOS	B-Operating_System
setup	O
utility	O
typically	O
include	O
:	O
</s>
<s>
A	O
modern	O
BIOS	B-Operating_System
setup	O
screen	O
often	O
features	O
a	O
PC	O
Health	O
Status	O
or	O
a	O
Hardware	B-Architecture
Monitoring	O
tab	O
,	O
which	O
directly	O
interfaces	B-Application
with	O
a	O
Hardware	B-Architecture
Monitor	O
chip	O
of	O
the	O
mainboard	B-Device
.	O
</s>
<s>
This	O
makes	O
it	O
possible	O
to	O
monitor	O
CPU	B-General_Concept
and	O
chassis	B-General_Concept
temperature	O
,	O
the	O
voltage	O
provided	O
by	O
the	O
power	O
supply	O
unit	O
,	O
as	O
well	O
as	O
monitor	O
and	O
control	B-Device
the	I-Device
speed	I-Device
of	I-Device
the	I-Device
fans	I-Device
connected	O
to	O
the	O
motherboard	B-Device
.	O
</s>
<s>
Once	O
the	O
system	O
is	O
booted	O
,	O
hardware	B-Architecture
monitoring	O
and	O
computer	B-Device
fan	I-Device
control	I-Device
is	O
normally	O
done	O
directly	O
by	O
the	O
Hardware	B-Architecture
Monitor	O
chip	O
itself	O
,	O
which	O
can	O
be	O
a	O
separate	O
chip	O
,	O
interfaced	O
through	O
I2C	O
or	O
SMBus	B-Algorithm
,	O
or	O
come	O
as	O
a	O
part	O
of	O
a	O
Super	B-Device
I/O	I-Device
solution	O
,	O
interfaced	O
through	O
Industry	B-Architecture
Standard	I-Architecture
Architecture	I-Architecture
(	O
ISA	O
)	O
or	O
Low	O
Pin	O
Count	O
(	O
LPC	O
)	O
.	O
</s>
<s>
Some	O
operating	B-General_Concept
systems	I-General_Concept
,	O
like	O
NetBSD	B-Device
with	O
envsys	B-Device
and	O
OpenBSD	B-Operating_System
with	O
sysctl	O
hw.sensors	B-Application
,	O
feature	O
integrated	O
interfacing	O
with	O
hardware	B-Architecture
monitors	O
.	O
</s>
<s>
However	O
,	O
in	O
some	O
circumstances	O
,	O
the	O
BIOS	B-Operating_System
also	O
provides	O
the	O
underlying	O
information	O
about	O
hardware	B-Architecture
monitoring	O
through	O
ACPI	B-Device
,	O
in	O
which	O
case	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
may	O
be	O
using	O
ACPI	B-Device
to	O
perform	O
hardware	B-Architecture
monitoring	O
.	O
</s>
<s>
In	O
modern	O
PCs	O
the	O
BIOS	B-Operating_System
is	O
stored	O
in	O
rewritable	O
EEPROM	B-General_Concept
or	O
NOR	O
flash	B-Device
memory	I-Device
,	O
allowing	O
the	O
contents	O
to	O
be	O
replaced	O
and	O
modified	O
.	O
</s>
<s>
It	O
can	O
be	O
done	O
by	O
a	O
special	O
program	O
,	O
usually	O
provided	O
by	O
the	O
system	O
's	O
manufacturer	O
,	O
or	O
at	O
POST	B-Device
,	O
with	O
a	O
BIOS	B-Operating_System
image	O
in	O
a	O
hard	B-Device
drive	I-Device
or	O
USB	B-Protocol
flash	O
drive	O
.	O
</s>
<s>
A	O
file	O
containing	O
such	O
contents	O
is	O
sometimes	O
termed	O
"	O
a	O
BIOS	B-Operating_System
image	O
"	O
.	O
</s>
<s>
A	O
BIOS	B-Operating_System
might	O
be	O
reflashed	O
in	O
order	O
to	O
upgrade	O
to	O
a	O
newer	O
version	O
to	O
fix	O
bugs	B-Error_Name
or	O
provide	O
improved	O
performance	O
or	O
to	O
support	O
newer	O
hardware	B-Architecture
.	O
</s>
<s>
The	O
original	O
IBM	B-Operating_System
PC	I-Operating_System
BIOS	I-Operating_System
(	O
and	O
cassette	O
BASIC	O
)	O
was	O
stored	O
on	O
mask-programmed	O
read-only	B-Device
memory	I-Device
(	O
ROM	B-Device
)	O
chips	O
in	O
sockets	O
on	O
the	O
motherboard	B-Device
.	O
</s>
<s>
To	O
allow	O
for	O
updates	O
,	O
many	O
compatible	O
computers	O
used	O
re-programmable	O
BIOS	B-Operating_System
memory	O
devices	O
such	O
as	O
EPROM	B-General_Concept
,	O
EEPROM	B-General_Concept
and	O
later	O
flash	B-Device
memory	I-Device
(	O
usually	O
NOR	O
flash	O
)	O
devices	O
.	O
</s>
<s>
According	O
to	O
Robert	O
Braver	O
,	O
the	O
president	O
of	O
the	O
BIOS	B-Operating_System
manufacturer	O
Micro	O
Firmware	B-Application
,	O
Flash	B-Operating_System
BIOS	I-Operating_System
chips	O
became	O
common	O
around	O
1995	O
because	O
the	O
electrically	O
erasable	O
PROM	O
(	O
EEPROM	B-General_Concept
)	O
chips	O
are	O
cheaper	O
and	O
easier	O
to	O
program	O
than	O
standard	O
ultraviolet	B-Application
erasable	O
PROM	O
(	O
EPROM	B-General_Concept
)	O
chips	O
.	O
</s>
<s>
Flash	B-Device
chips	I-Device
are	O
programmed	O
(	O
and	O
re-programmed	O
)	O
in-circuit	O
,	O
while	O
EPROM	B-General_Concept
chips	O
need	O
to	O
be	O
removed	O
from	O
the	O
motherboard	B-Device
for	O
re-programming	O
.	O
</s>
<s>
BIOS	B-Operating_System
versions	O
are	O
upgraded	O
to	O
take	O
advantage	O
of	O
newer	O
versions	O
of	O
hardware	B-Architecture
and	O
to	O
correct	O
bugs	B-Error_Name
in	O
previous	O
revisions	O
of	O
BIOSes	O
.	O
</s>
<s>
Beginning	O
with	O
the	O
IBM	B-Operating_System
AT	I-Operating_System
,	O
PCs	O
supported	O
a	O
hardware	B-Architecture
clock	O
settable	O
through	O
BIOS	B-Operating_System
.	O
</s>
<s>
It	O
had	O
a	O
century	O
bit	O
which	O
allowed	O
for	O
manually	O
changing	O
the	O
century	O
when	O
the	B-Error_Name
year	I-Error_Name
2000	I-Error_Name
happened	O
.	O
</s>
<s>
Most	O
BIOS	B-Operating_System
revisions	O
created	O
in	O
1995	O
and	O
nearly	O
all	O
BIOS	B-Operating_System
revisions	O
in	O
1997	O
supported	O
the	B-Error_Name
year	I-Error_Name
2000	I-Error_Name
by	O
setting	O
the	O
century	O
bit	O
automatically	O
when	O
the	O
clock	O
rolled	O
past	O
midnight	O
,	O
31	O
December	O
1999	O
.	O
</s>
<s>
The	O
first	O
flash	B-Device
chips	I-Device
were	O
attached	O
to	O
the	O
ISA	B-Architecture
bus	I-Architecture
.	O
</s>
<s>
Starting	O
in	O
1998	O
,	O
the	O
BIOS	B-Operating_System
flash	O
moved	O
to	O
the	O
LPC	O
bus	O
,	O
following	O
a	O
new	O
standard	O
implementation	O
known	O
as	O
"	O
firmware	B-Application
hub	O
"	O
(	O
FWH	O
)	O
.	O
</s>
<s>
In	O
2005	O
,	O
the	O
BIOS	B-Operating_System
flash	B-Device
memory	I-Device
moved	O
to	O
the	O
SPI	B-Architecture
bus	I-Architecture
.	O
</s>
<s>
The	O
size	O
of	O
the	O
BIOS	B-Operating_System
,	O
and	O
the	O
capacity	O
of	O
the	O
ROM	B-Device
,	O
EEPROM	B-General_Concept
,	O
or	O
other	O
media	O
it	O
may	O
be	O
stored	O
on	O
,	O
has	O
increased	O
over	O
time	O
as	O
new	O
features	O
have	O
been	O
added	O
to	O
the	O
code	O
;	O
BIOS	B-Operating_System
versions	O
now	O
exist	O
with	O
sizes	O
up	O
to	O
32	O
megabytes	O
.	O
</s>
<s>
For	O
contrast	O
,	O
the	O
original	O
IBM	B-Operating_System
PC	I-Operating_System
BIOS	I-Operating_System
was	O
contained	O
in	O
an	O
8KB	O
mask	B-Device
ROM	I-Device
.	O
</s>
<s>
Some	O
modern	O
motherboards	B-Device
are	O
including	O
even	O
bigger	O
NAND	O
flash	B-Device
memory	I-Device
ICs	O
on	O
board	O
which	O
are	O
capable	O
of	O
storing	O
whole	O
compact	O
operating	B-General_Concept
systems	I-General_Concept
,	O
such	O
as	O
some	O
Linux	B-Application
distributions	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
some	O
ASUS	O
notebooks	O
included	O
Splashtop	B-Device
OS	I-Device
embedded	O
into	O
their	O
NAND	O
flash	B-Device
memory	I-Device
ICs	O
.	O
</s>
<s>
However	O
,	O
the	O
idea	O
of	O
including	O
an	O
operating	B-General_Concept
system	I-General_Concept
along	O
with	O
BIOS	B-Operating_System
in	O
the	O
ROM	B-Device
of	O
a	O
PC	O
is	O
not	O
new	O
;	O
in	O
the	O
1980s	O
,	O
Microsoft	O
offered	O
a	O
ROM	B-Device
option	O
for	O
MS-DOS	B-Application
,	O
and	O
it	O
was	O
included	O
in	O
the	O
ROMs	O
of	O
some	O
PC	O
clones	O
such	O
as	O
the	O
Tandy	O
1000	O
HX	O
.	O
</s>
<s>
Another	O
type	O
of	O
firmware	B-Application
chip	O
was	O
found	O
on	O
the	O
IBM	B-Operating_System
PC	I-Operating_System
AT	I-Operating_System
and	O
early	O
compatibles	O
.	O
</s>
<s>
In	O
the	O
AT	O
,	O
the	O
keyboard	B-Device
interface	I-Device
was	O
controlled	O
by	O
a	O
microcontroller	B-Architecture
with	O
its	O
own	O
programmable	O
memory	O
.	O
</s>
<s>
On	O
the	O
IBM	B-Operating_System
AT	I-Operating_System
,	O
that	O
was	O
a	O
40-pin	O
socketed	O
device	O
,	O
while	O
some	O
manufacturers	O
used	O
an	O
EPROM	B-General_Concept
version	O
of	O
this	O
chip	O
which	O
resembled	O
an	O
EPROM	B-General_Concept
.	O
</s>
<s>
This	O
controller	O
was	O
also	O
assigned	O
the	O
A20	O
gate	O
function	O
to	O
manage	O
memory	O
above	O
the	O
one-megabyte	O
range	O
;	O
occasionally	O
an	O
upgrade	O
of	O
this	O
"	O
keyboard	B-Device
BIOS	B-Operating_System
"	O
was	O
necessary	O
to	O
take	O
advantage	O
of	O
software	O
that	O
could	O
use	O
upper	B-Device
memory	I-Device
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
may	O
contain	O
components	O
such	O
as	O
the	O
Memory	B-Device
Reference	I-Device
Code	I-Device
(	O
MRC	O
)	O
,	O
which	O
is	O
responsible	O
for	O
the	O
memory	O
initialization	O
(	O
e.g.	O
</s>
<s>
SPD	B-General_Concept
and	O
memory	B-General_Concept
timings	I-General_Concept
initialization	O
)	O
.	O
</s>
<s>
Intel	B-Device
Management	I-Device
Engine	I-Device
or	O
AMD	B-Device
Platform	I-Device
Security	I-Device
Processor	I-Device
firmware	B-Application
.	O
</s>
<s>
IBM	O
published	O
the	O
entire	O
listings	O
of	O
the	O
BIOS	B-Operating_System
for	O
its	O
original	O
PC	O
,	O
PC	B-Device
XT	I-Device
,	O
PC	B-Operating_System
AT	I-Operating_System
,	O
and	O
other	O
contemporary	O
PC	O
models	O
,	O
in	O
an	O
appendix	O
of	O
the	O
IBM	B-Device
PC	I-Device
Technical	O
Reference	O
Manual	O
for	O
each	O
machine	O
type	O
.	O
</s>
<s>
The	O
effect	O
of	O
the	O
publication	O
of	O
the	O
BIOS	B-Operating_System
listings	O
is	O
that	O
anyone	O
can	O
see	O
exactly	O
what	O
a	O
definitive	O
BIOS	B-Operating_System
does	O
and	O
how	O
it	O
does	O
it	O
.	O
</s>
<s>
In	O
May	O
1984	O
Phoenix	O
Software	O
Associates	O
released	O
its	O
first	O
ROM-BIOS	B-Operating_System
,	O
which	O
enabled	O
OEMs	O
to	O
build	O
essentially	O
fully	O
compatible	O
clones	O
without	O
having	O
to	O
reverse-engineer	O
the	O
IBM	B-Operating_System
PC	I-Operating_System
BIOS	I-Operating_System
themselves	O
,	O
as	O
Compaq	O
had	O
done	O
for	O
the	O
Portable	B-Device
,	O
helping	O
fuel	O
the	O
growth	O
in	O
the	O
PC-compatibles	O
industry	O
and	O
sales	O
of	O
non-IBM	O
versions	O
of	O
DOS	B-Device
.	O
</s>
<s>
And	O
the	O
first	O
American	O
Megatrends	O
(	O
AMI	O
)	O
BIOS	B-Operating_System
was	O
released	O
on	O
1986	O
.	O
</s>
<s>
New	O
standards	O
grafted	O
onto	O
the	O
BIOS	B-Operating_System
are	O
usually	O
without	O
complete	O
public	O
documentation	O
or	O
any	O
BIOS	B-Operating_System
listings	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
it	O
is	O
not	O
as	O
easy	O
to	O
learn	O
the	O
intimate	O
details	O
about	O
the	O
many	O
non-IBM	O
additions	O
to	O
BIOS	B-Operating_System
as	O
about	O
the	O
core	O
BIOS	B-Operating_System
services	O
.	O
</s>
<s>
Most	O
PC	B-Device
motherboard	I-Device
suppliers	O
licensed	O
a	O
BIOS	B-Operating_System
"	O
core	O
"	O
and	O
toolkit	O
from	O
a	O
commercial	O
third	O
party	O
,	O
known	O
as	O
an	O
"	O
independent	O
BIOS	B-Operating_System
vendor	O
"	O
or	O
IBV	O
.	O
</s>
<s>
The	O
motherboard	B-Device
manufacturer	O
then	O
customized	O
this	O
BIOS	B-Operating_System
to	O
suit	O
its	O
own	O
hardware	B-Architecture
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
updated	O
BIOSes	O
are	O
normally	O
obtained	O
directly	O
from	O
the	O
motherboard	B-Device
manufacturer	O
.	O
</s>
<s>
General	O
Software	O
,	O
which	O
was	O
also	O
acquired	O
by	O
Phoenix	O
in	O
2007	O
,	O
sold	O
BIOS	B-Operating_System
for	O
embedded	O
systems	O
based	O
on	O
Intel	O
processors	O
.	O
</s>
<s>
The	O
open-source	O
community	O
increased	O
their	O
effort	O
to	O
develop	O
a	O
replacement	O
for	O
proprietary	B-Application
BIOSes	O
and	O
their	O
future	O
incarnations	O
with	O
an	O
open-sourced	O
counterpart	O
through	O
the	O
libreboot	B-Device
,	O
coreboot	B-Device
and	O
OpenBIOS/Open	O
Firmware	O
projects	O
.	O
</s>
<s>
AMD	O
provided	O
product	O
specifications	O
for	O
some	O
chipsets	B-Device
,	O
and	O
Google	B-Application
is	O
sponsoring	O
the	O
project	O
.	O
</s>
<s>
Motherboard	B-Device
manufacturer	O
Tyan	O
offers	O
coreboot	B-Device
next	O
to	O
the	O
standard	B-Operating_System
BIOS	I-Operating_System
with	O
their	O
Opteron	B-General_Concept
line	O
of	O
motherboards	B-Device
.	O
</s>
<s>
EEPROM	B-General_Concept
and	O
Flash	B-Device
memory	I-Device
chips	O
are	O
advantageous	O
because	O
they	O
can	O
be	O
easily	O
updated	O
by	O
the	O
user	O
;	O
it	O
is	O
customary	O
for	O
hardware	B-Architecture
manufacturers	O
to	O
issue	O
BIOS	B-Operating_System
updates	O
to	O
upgrade	O
their	O
products	O
,	O
improve	O
compatibility	O
and	O
remove	O
bugs	B-Error_Name
.	O
</s>
<s>
However	O
,	O
this	O
advantage	O
had	O
the	O
risk	O
that	O
an	O
improperly	O
executed	O
or	O
aborted	O
BIOS	B-Operating_System
update	O
could	O
render	O
the	O
computer	O
or	O
device	O
unusable	O
.	O
</s>
<s>
To	O
avoid	O
these	O
situations	O
,	O
more	O
recent	O
BIOSes	O
use	O
a	O
"	O
boot	B-Device
block	I-Device
"	O
;	O
a	O
portion	O
of	O
the	O
BIOS	B-Operating_System
which	O
runs	O
first	O
and	O
must	O
be	O
updated	O
separately	O
.	O
</s>
<s>
This	O
code	O
verifies	O
if	O
the	O
rest	O
of	O
the	O
BIOS	B-Operating_System
is	O
intact	O
(	O
using	O
hash	B-Error_Name
checksums	B-Algorithm
or	O
other	O
methods	O
)	O
before	O
transferring	O
control	O
to	O
it	O
.	O
</s>
<s>
If	O
the	O
boot	B-Device
block	I-Device
detects	O
any	O
corruption	O
in	O
the	O
main	O
BIOS	B-Operating_System
,	O
it	O
will	O
typically	O
warn	O
the	O
user	O
that	O
a	O
recovery	O
process	O
must	O
be	O
initiated	O
by	O
booting	B-Operating_System
from	O
removable	B-Device
media	I-Device
(	O
floppy	B-Device
,	O
CD	B-Device
or	O
USB	B-Protocol
flash	O
drive	O
)	O
so	O
the	O
user	O
can	O
try	O
flashing	O
the	O
BIOS	B-Operating_System
again	O
.	O
</s>
<s>
Some	O
motherboards	B-Device
have	O
a	O
backup	O
BIOS	B-Operating_System
(	O
sometimes	O
referred	O
to	O
as	O
DualBIOS	O
boards	O
)	O
to	O
recover	O
from	O
BIOS	B-Operating_System
corruptions	O
.	O
</s>
<s>
There	O
are	O
at	O
least	O
five	O
known	O
BIOS	B-Operating_System
attack	O
viruses	O
,	O
two	O
of	O
which	O
were	O
for	O
demonstration	O
purposes	O
.	O
</s>
<s>
The	O
first	O
BIOS	B-Operating_System
virus	O
was	O
BIOS	B-Operating_System
Meningitis	O
,	O
which	O
instead	O
of	O
erasing	O
BIOS	B-Operating_System
chips	I-Operating_System
it	O
infected	O
them	O
.	O
</s>
<s>
BIOS	B-Operating_System
Meningitis	O
has	O
relatively	O
harmless	O
,	O
compared	O
to	O
a	O
virus	O
like	O
CIH	O
.	O
</s>
<s>
The	O
second	O
BIOS	B-Operating_System
virus	O
was	O
CIH	O
,	O
also	O
known	O
as	O
the	O
"	O
Chernobyl	O
Virus	O
"	O
,	O
which	O
was	O
able	O
to	O
erase	O
flash	B-Device
ROM	I-Device
BIOS	B-Operating_System
content	O
on	O
compatible	O
chipsets	B-Device
.	O
</s>
<s>
Often	O
,	O
infected	O
computers	O
could	O
no	O
longer	O
boot	B-Operating_System
,	O
and	O
people	O
had	O
to	O
remove	O
the	O
flash	B-Device
ROM	I-Device
IC	O
from	O
the	O
motherboard	B-Device
and	O
reprogram	O
it	O
.	O
</s>
<s>
CIH	O
targeted	O
the	O
then-widespread	O
Intel	O
i430TX	O
motherboard	B-Device
chipset	B-Device
and	O
took	O
advantage	O
of	O
the	O
fact	O
that	O
the	O
Windows	B-Application
9x	O
operating	B-General_Concept
systems	I-General_Concept
,	O
also	O
widespread	O
at	O
the	O
time	O
,	O
allowed	O
direct	O
hardware	B-Architecture
access	O
to	O
all	O
programs	O
.	O
</s>
<s>
Modern	O
systems	O
are	O
not	O
vulnerable	O
to	O
CIH	O
because	O
of	O
a	O
variety	O
of	O
chipsets	B-Device
being	O
used	O
which	O
are	O
incompatible	O
with	O
the	O
Intel	O
i430TX	O
chipset	B-Device
,	O
and	O
also	O
other	O
flash	B-Device
ROM	I-Device
IC	O
types	O
.	O
</s>
<s>
There	O
is	O
also	O
extra	O
protection	O
from	O
accidental	O
BIOS	B-Operating_System
rewrites	O
in	O
the	O
form	O
of	O
boot	B-Device
blocks	I-Device
which	O
are	O
protected	O
from	O
accidental	O
overwrite	O
or	O
dual	O
and	O
quad	O
BIOS	B-Operating_System
equipped	O
systems	O
which	O
may	O
,	O
in	O
the	O
event	O
of	O
a	O
crash	O
,	O
use	O
a	O
backup	O
BIOS	B-Operating_System
.	O
</s>
<s>
Also	O
,	O
all	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
such	O
as	O
FreeBSD	B-Operating_System
,	O
Linux	B-Operating_System
,	O
macOS	B-Application
,	O
Windows	O
NT-based	O
Windows	B-Application
OS	I-Application
like	O
Windows	B-Application
2000	I-Application
,	O
Windows	B-Application
XP	I-Application
and	O
newer	O
,	O
do	O
not	O
allow	O
user-mode	B-Operating_System
programs	O
to	O
have	O
direct	O
hardware	B-Architecture
access	O
using	O
a	O
hardware	B-Operating_System
abstraction	I-Operating_System
layer	I-Operating_System
.	O
</s>
<s>
As	O
a	O
result	O
,	O
as	O
of	O
2008	O
,	O
CIH	O
has	O
become	O
essentially	O
harmless	O
,	O
at	O
worst	O
causing	O
annoyance	O
by	O
infecting	O
executable	O
files	O
and	O
triggering	O
antivirus	B-Application
software	I-Application
.	O
</s>
<s>
Other	O
BIOS	B-Operating_System
viruses	O
remain	O
possible	O
,	O
however	O
;	O
since	O
most	O
Windows	B-Application
home	O
users	O
without	O
Windows	B-Application
Vista/7	O
'	O
s	O
UAC	O
run	O
all	O
applications	O
with	O
administrative	O
privileges	O
,	O
a	O
modern	O
CIH-like	O
virus	O
could	O
in	O
principle	O
still	O
gain	O
access	O
to	O
hardware	B-Architecture
without	O
first	O
using	O
an	O
exploit	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
OpenBSD	B-Operating_System
prevents	O
all	O
users	O
from	O
having	O
this	O
access	O
and	O
the	O
grsecurity	O
patch	O
for	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
also	O
prevents	O
this	O
direct	O
hardware	B-Architecture
access	O
by	O
default	O
,	O
the	O
difference	O
being	O
an	O
attacker	O
requiring	O
a	O
much	O
more	O
difficult	O
kernel	B-Operating_System
level	O
exploit	O
or	O
reboot	O
of	O
the	O
machine	O
.	O
</s>
<s>
The	O
third	O
BIOS	B-Operating_System
virus	O
was	O
a	O
technique	O
presented	O
by	O
John	O
Heasman	O
,	O
principal	O
security	O
consultant	O
for	O
UK-based	O
Next-Generation	O
Security	O
Software	O
.	O
</s>
<s>
In	O
2006	O
,	O
at	O
the	O
Black	O
Hat	O
Security	O
Conference	O
,	O
he	O
showed	O
how	O
to	O
elevate	O
privileges	O
and	O
read	O
physical	O
memory	O
,	O
using	O
malicious	O
procedures	O
that	O
replaced	O
normal	O
ACPI	B-Device
functions	O
stored	O
in	O
flash	B-Device
memory	I-Device
.	O
</s>
<s>
The	O
fourth	O
BIOS	B-Operating_System
virus	O
was	O
a	O
technique	O
called	O
"	O
Persistent	O
BIOS	B-Operating_System
infection.	O
"	O
</s>
<s>
Researchers	O
Anibal	O
Sacco	O
and	O
Alfredo	O
Ortega	O
,	O
from	O
Core	O
Security	O
Technologies	O
,	O
demonstrated	O
how	O
to	O
insert	O
malicious	O
code	O
into	O
the	O
decompression	O
routines	O
in	O
the	O
BIOS	B-Operating_System
,	O
allowing	O
for	O
nearly	O
full	O
control	O
of	O
the	O
PC	B-Operating_System
at	I-Operating_System
start-up	O
,	O
even	O
before	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
booted	O
.	O
</s>
<s>
The	O
proof-of-concept	O
does	O
not	O
exploit	O
a	O
flaw	O
in	O
the	O
BIOS	B-Operating_System
implementation	O
,	O
but	O
only	O
involves	O
the	O
normal	O
BIOS	B-Operating_System
flashing	O
procedures	O
.	O
</s>
<s>
We	O
even	O
have	O
a	O
little	O
code	O
that	O
can	O
remove	O
or	O
disable	O
antivirus.	O
"	O
</s>
<s>
Mebromi	O
is	O
a	O
trojan	O
which	O
targets	O
computers	O
with	O
AwardBIOS	O
,	O
Microsoft	B-Application
Windows	I-Application
,	O
and	O
antivirus	B-Application
software	I-Application
from	O
two	O
Chinese	O
companies	O
:	O
Rising	O
Antivirus	B-Application
and	O
Jiangmin	O
KV	O
Antivirus	B-Application
.	O
</s>
<s>
Mebromi	O
installs	O
a	O
rootkit	O
which	O
infects	O
the	O
Master	B-Device
boot	I-Device
record	I-Device
.	O
</s>
<s>
In	O
a	O
December	O
2013	O
interview	O
with	O
60	O
Minutes	O
,	O
Deborah	O
Plunkett	O
,	O
Information	O
Assurance	O
Director	O
for	O
the	O
US	O
National	O
Security	O
Agency	O
claimed	O
the	O
NSA	O
had	O
uncovered	O
and	O
thwarted	O
a	O
possible	O
BIOS	B-Operating_System
attack	O
by	O
a	O
foreign	O
nation	O
state	O
,	O
targeting	O
the	O
US	O
financial	O
system	O
.	O
</s>
<s>
Newer	O
Intel	O
platforms	O
have	O
Intel	O
Boot	B-Operating_System
Guard	O
(	O
IBG	O
)	O
technology	O
enabled	O
,	O
this	O
technology	O
will	O
check	O
the	O
BIOS	B-Operating_System
digital	O
signature	O
at	O
startup	O
,	O
and	O
the	O
IBG	O
public	O
key	O
is	O
fused	O
into	O
the	O
PCH	B-Device
.	O
</s>
<s>
Unified	B-Architecture
Extensible	I-Architecture
Firmware	I-Architecture
Interface	I-Architecture
(	O
UEFI	B-Architecture
)	O
supplements	O
the	O
BIOS	B-Operating_System
in	O
many	O
new	O
machines	O
.	O
</s>
<s>
Initially	O
written	O
for	O
the	O
Intel	B-General_Concept
Itanium	I-General_Concept
architecture	I-General_Concept
,	O
UEFI	B-Architecture
is	O
now	O
available	O
for	O
x86	B-Operating_System
and	O
ARM	B-Architecture
architecture	I-Architecture
platforms	O
;	O
the	O
specification	O
development	O
is	O
driven	O
by	O
the	O
Unified	B-Device
EFI	I-Device
Forum	I-Device
,	O
an	O
industry	O
Special	O
Interest	O
Group	O
.	O
</s>
<s>
EFI	O
booting	B-Operating_System
has	O
been	O
supported	O
in	O
only	O
Microsoft	B-Application
Windows	I-Application
versions	O
supporting	O
GPT	B-Application
,	O
the	O
Linux	O
kernel	O
2.6.1	O
and	O
later	O
,	O
and	O
macOS	B-Application
on	O
Intel-based	B-Device
Macs	I-Device
.	O
</s>
<s>
,	O
new	O
PC	B-Architecture
hardware	I-Architecture
predominantly	O
ships	O
with	O
UEFI	B-Architecture
firmware	I-Architecture
.	O
</s>
<s>
The	O
architecture	O
of	O
the	O
rootkit	O
safeguard	O
can	O
also	O
prevent	O
the	O
system	O
from	O
running	O
the	O
user	O
's	O
own	O
software	O
changes	O
,	O
which	O
makes	O
UEFI	B-Architecture
controversial	O
as	O
a	O
legacy	B-Operating_System
BIOS	I-Operating_System
replacement	O
in	O
the	O
open	O
hardware	B-Architecture
community	O
.	O
</s>
<s>
Also	O
,	O
Windows	B-Application
11	I-Application
requires	O
UEFI	B-Architecture
to	O
boot	B-Operating_System
.	O
</s>
<s>
Other	O
alternatives	O
to	O
the	O
functionality	O
of	O
the	O
"	O
Legacy	B-Operating_System
BIOS	I-Operating_System
"	O
in	O
the	O
x86	B-Operating_System
world	O
include	O
coreboot	B-Device
and	O
libreboot	B-Device
.	O
</s>
<s>
Some	O
servers	O
and	O
workstations	O
use	O
a	O
platform-independent	O
Open	B-Architecture
Firmware	I-Architecture
(	O
IEEE-1275	B-Architecture
)	O
based	O
on	O
the	O
Forth	B-Application
programming	I-Application
language	I-Application
;	O
it	O
is	O
included	O
with	O
Sun	O
's	O
SPARC	B-Architecture
computers	O
,	O
IBM	O
's	O
RS/6000	B-Device
line	O
,	O
and	O
other	O
PowerPC	B-Architecture
systems	O
such	O
as	O
the	O
CHRP	B-Device
motherboards	B-Device
,	O
along	O
with	O
the	O
x86-based	O
OLPC	B-Application
XO-1	I-Application
.	O
</s>
<s>
As	O
of	O
at	O
least	O
2015	O
,	O
Apple	O
has	O
removed	O
legacy	B-Operating_System
BIOS	I-Operating_System
support	O
from	O
MacBook	B-Device
Pro	I-Device
computers	O
.	O
</s>
<s>
As	O
such	O
the	O
BIOS	B-Operating_System
utility	O
no	O
longer	O
supports	O
the	O
legacy	O
option	O
,	O
and	O
prints	O
"	O
Legacy	O
mode	O
not	O
supported	O
on	O
this	O
system	O
"	O
.	O
</s>
<s>
In	O
2017	O
,	O
Intel	O
announced	O
that	O
it	O
would	O
remove	O
legacy	B-Operating_System
BIOS	I-Operating_System
support	O
by	O
2020	O
.	O
</s>
