<s>
An	O
Option	B-Device
ROM	I-Device
for	O
the	O
PC	O
platform	O
(	O
i.e.	O
</s>
<s>
the	O
IBM	B-Device
PC	I-Device
and	O
derived	O
successor	O
computer	O
systems	O
)	O
is	O
a	O
piece	O
of	O
firmware	B-Application
that	O
resides	O
in	O
ROM	B-Device
on	O
an	O
expansion	O
card	O
(	O
or	O
stored	O
along	O
with	O
the	O
main	O
system	B-Operating_System
BIOS	I-Operating_System
)	O
,	O
which	O
gets	O
executed	O
to	O
initialize	O
the	O
device	O
and	O
(	O
optionally	O
)	O
add	O
support	O
for	O
the	O
device	O
to	O
the	O
BIOS	B-Operating_System
.	O
</s>
<s>
In	O
its	O
usual	O
use	O
,	O
it	O
is	O
essentially	O
a	O
driver	B-Application
that	O
interfaces	O
between	O
the	O
BIOS	B-Operating_System
API	O
and	O
hardware	O
.	O
</s>
<s>
Technically	O
,	O
an	O
option	B-Device
ROM	I-Device
is	O
firmware	B-Application
that	O
is	O
executed	O
by	O
the	O
BIOS	B-Operating_System
after	O
POST	B-Device
(	O
the	O
testing	O
and	O
initialization	O
of	O
basic	O
system	O
hardware	O
)	O
and	O
before	O
the	O
BIOS	B-Operating_System
boot	B-Operating_System
process	I-Operating_System
,	O
gaining	O
complete	O
control	O
of	O
the	O
system	O
and	O
being	O
generally	O
unrestricted	O
in	O
what	O
it	O
can	O
do	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
relies	O
on	O
each	O
option	B-Device
ROM	I-Device
to	O
return	O
control	O
to	O
the	O
BIOS	B-Operating_System
so	O
that	O
it	O
can	O
either	O
call	O
the	O
next	O
option	B-Device
ROM	I-Device
or	O
commence	O
the	O
boot	B-Operating_System
process	I-Operating_System
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
it	O
is	O
possible	O
(	O
but	O
not	O
usual	O
)	O
for	O
an	O
option	B-Device
ROM	I-Device
to	O
keep	O
control	O
and	O
preempt	O
the	O
BIOS	B-Operating_System
boot	B-Operating_System
process	I-Operating_System
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
(	O
at	O
least	O
as	O
originally	O
designed	O
by	O
IBM	O
)	O
generally	O
scans	O
for	O
and	O
initializes	O
(	O
by	O
executing	O
)	O
option	O
ROMs	O
in	O
ascending	O
address	O
order	O
at	O
2	O
KB	O
address	O
intervals	O
within	O
two	O
different	O
address	O
ranges	O
above	O
address	O
C0000h	O
in	O
the	O
conventional	O
(	O
20-bit	O
)	O
memory	O
address	O
space	O
;	O
later	O
systems	O
may	O
also	O
scan	O
additional	O
address	O
ranges	O
in	O
the	O
24-bit	O
or	O
32-bit	O
extended	O
address	O
space	O
.	O
</s>
<s>
Option	O
ROMs	O
are	O
necessary	O
to	O
enable	O
non-Plug	O
and	O
Play	O
peripheral	O
devices	O
to	O
boot	B-Operating_System
and	O
to	O
extend	O
the	O
BIOS	B-Operating_System
to	O
provide	O
support	O
for	O
any	O
non-Plug	O
and	O
Play	O
peripheral	O
device	O
in	O
the	O
same	O
way	O
that	O
standard	O
and	O
motherboard-integrated	O
peripherals	O
are	O
supported	O
.	O
</s>
<s>
Option	O
ROMs	O
are	O
also	O
used	O
to	O
extend	O
the	O
BIOS	B-Operating_System
or	O
to	O
add	O
other	O
firmware	B-Application
services	O
to	O
the	O
BIOS	B-Operating_System
.	O
</s>
<s>
In	O
principle	O
,	O
an	O
option	B-Device
ROM	I-Device
could	O
provide	O
any	O
sort	O
of	O
firmware	B-Application
extension	O
,	O
such	O
as	O
a	O
library	O
of	O
video	O
graphics	O
subroutines	O
,	O
or	O
a	O
set	O
of	O
PCM	O
audio	O
processing	O
services	O
,	O
and	O
cause	O
it	O
to	O
be	O
installed	O
into	O
the	O
system	O
RAM	O
and	O
optionally	O
the	O
CPU	O
interrupt	O
system	O
before	O
boot	B-Operating_System
time	I-Operating_System
.	O
</s>
<s>
A	O
common	O
option	B-Device
ROM	I-Device
is	O
the	O
video	B-Device
BIOS	I-Device
which	O
gets	O
loaded	O
very	O
early	O
on	O
in	O
the	O
boot	B-Operating_System
process	I-Operating_System
and	O
hooks	O
INT	B-Device
10h	I-Device
so	O
that	O
output	O
from	O
the	O
power-on	B-Device
self-test	I-Device
(	O
POST	B-Device
)	O
can	O
be	O
displayed	O
.	O
</s>
<s>
The	O
video	B-Device
BIOS	I-Device
is	O
almost	O
always	O
located	O
in	O
the	O
memory	O
segment	O
beginning	O
at	O
C0000h	O
,	O
the	O
start	O
of	O
the	O
memory	O
area	O
reserved	O
for	O
option	O
ROMs	O
;	O
this	O
is	O
because	O
when	O
the	O
motherboard	O
has	O
a	O
built-in	O
VGA	O
controller	O
,	O
the	O
option	B-Device
ROM	I-Device
will	O
reside	O
in	O
the	O
BIOS	B-Operating_System
–	O
the	O
BIOS	B-Operating_System
knows	O
where	O
it	O
is	O
and	O
shadows	O
it	O
into	O
RAM	O
at	O
a	O
fixed	O
time	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
Boot	B-Operating_System
Specification	O
requires	O
that	O
option	O
ROMs	O
be	O
aligned	O
to	O
2	O
kB	O
boundaries	O
(	O
e.g.	O
</s>
<s>
The	O
first	O
two	O
bytes	O
of	O
the	O
ROM	B-Device
must	O
be	O
55	O
AA	O
.	O
</s>
<s>
The	O
third	O
byte	O
indicates	O
the	O
ROM	B-Device
size	O
,	O
and	O
the	O
fourth	O
byte	O
is	O
where	O
the	O
BIOS	B-Operating_System
begins	O
execution	O
of	O
the	O
option	B-Device
ROM	I-Device
to	O
initialize	O
it	O
before	O
the	O
system	B-Operating_System
boots	I-Operating_System
.	O
</s>
<s>
Often	O
this	O
initialization	O
is	O
done	O
by	O
a	O
3	O
byte	O
jump	B-General_Concept
instruction	I-General_Concept
starting	O
with	O
hexadecimal	O
value	O
E9	O
.	O
</s>
<s>
Prior	O
to	O
the	O
development	O
and	O
ubiquitous	O
adoption	O
of	O
the	O
Plug	B-Device
and	I-Device
Play	I-Device
BIOS	I-Device
standard	O
,	O
an	O
add-on	O
device	O
such	O
as	O
a	O
hard	O
disk	O
controller	O
or	O
a	O
network	O
adapter	O
card	O
(	O
NIC	O
)	O
was	O
generally	O
required	O
to	O
include	O
an	O
option	B-Device
ROM	I-Device
in	O
order	O
to	O
be	O
bootable	B-Operating_System
,	O
as	O
the	O
motherboard	O
BIOS	B-Operating_System
did	O
not	O
include	O
any	O
support	O
for	O
the	O
device	O
and	O
so	O
could	O
not	O
incorporate	O
it	O
into	O
the	O
BIOS	B-Operating_System
's	O
boot	B-Operating_System
protocol	O
.	O
</s>
<s>
Such	O
an	O
option	B-Device
ROM	I-Device
would	O
hook	O
INT	O
19h	O
,	O
the	O
BIOS	B-Operating_System
boot	B-Operating_System
interrupt	O
,	O
to	O
preempt	O
the	O
BIOS	B-Operating_System
boot	B-Operating_System
loader	O
and	O
substitute	O
their	O
own	O
boot	B-Operating_System
loader	O
.	O
</s>
<s>
The	O
boot	B-Operating_System
loader	O
on	O
the	O
option	B-Device
ROM	I-Device
would	O
attempt	O
to	O
boot	B-Operating_System
from	O
a	O
disk	O
,	O
network	O
,	O
or	O
other	O
boot	B-Operating_System
program	I-Operating_System
source	O
attached	O
to	O
or	O
installed	O
on	O
the	O
adapter	O
card	O
;	O
if	O
that	O
boot	B-Operating_System
attempt	O
failed	O
,	O
it	O
would	O
pass	O
control	O
to	O
the	O
previous	O
boot	B-Operating_System
loader	O
(	O
to	O
which	O
INT	O
19h	O
pointed	O
before	O
the	O
option	B-Device
ROM	I-Device
hooked	O
it	O
)	O
,	O
allowing	O
the	O
system	O
to	O
boot	B-Operating_System
from	O
another	O
device	O
as	O
a	O
fallback	O
strategy	O
.	O
</s>
<s>
Some	O
adapters	O
cards	O
,	O
such	O
as	O
certain	O
SCSI	B-Architecture
adapters	O
(	O
e.g.	O
</s>
<s>
some	O
made	O
by	O
Adaptec	O
)	O
,	O
were	O
available	O
in	O
versions	O
that	O
differed	O
only	O
in	O
the	O
presence	O
or	O
absence	O
of	O
the	O
option	B-Device
ROM	I-Device
to	O
enable	O
booting	B-Operating_System
from	O
attached	O
SCSI	B-Architecture
devices	O
.	O
</s>
<s>
As	O
a	O
result	O
of	O
the	O
option	B-Device
ROM	I-Device
scanning	O
protocol	O
,	O
the	O
highest-addressed	O
option	B-Device
ROM	I-Device
is	O
the	O
last	O
one	O
to	O
be	O
initialized	O
and	O
so	O
the	O
last	O
one	O
to	O
hook	O
any	O
interrupts	O
and	O
the	O
first	O
one	O
in	O
those	O
interrupt	O
service	O
routine	O
(	O
ISR	O
)	O
chains	O
;	O
thus	O
the	O
addresses	O
of	O
the	O
option	O
ROMs	O
completely	O
determine	O
the	O
boot	B-Operating_System
priority	O
between	O
adapter	O
cards	O
that	O
are	O
enabled	O
for	O
booting	B-Operating_System
,	O
and	O
the	O
boot	B-Operating_System
devices	O
supported	O
by	O
the	O
motherboard	O
BIOS	B-Operating_System
collectively	O
have	O
lowest	O
priority	O
,	O
i.e.	O
</s>
<s>
the	O
system	O
will	O
attempt	O
to	O
boot	B-Operating_System
from	O
them	O
only	O
after	O
attempting	O
to	O
boot	B-Operating_System
from	O
all	O
boot-enabled	O
adapter	O
cards	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
Boot	B-Operating_System
Specification	O
(	O
BBS	O
)	O
was	O
developed	O
by	O
a	O
consortium	O
comprising	O
Compaq	O
,	O
Intel	O
and	O
Phoenix	O
Technologies	O
to	O
standardize	O
the	O
initialization	O
sequence	O
of	O
Plug	O
and	O
Play	O
(	O
PnP	O
)	O
BIOS	B-Operating_System
and	O
Option	O
ROMs	O
.	O
</s>
<s>
The	O
standard	O
presents	O
the	O
notion	O
of	O
a	O
Boot	B-Operating_System
Connection	O
Vector	O
(	O
BCV	O
)	O
table	O
and	O
BCV	O
priority	O
.	O
</s>
<s>
The	O
core	O
principles	O
of	O
the	O
standard	O
make	O
behaviour	O
more	O
defined	O
and	O
debuggable	O
and	O
gives	O
BIOS	B-Operating_System
manufacturers	O
room	O
to	O
further	O
dynamise	O
boot	B-Operating_System
device	O
selection	O
for	O
the	O
user	O
,	O
beyond	O
the	O
suggestions	O
of	O
the	O
standard	O
.	O
</s>
<s>
After	O
the	O
basic	O
POST	B-Device
checks	O
are	O
complete	O
,	O
the	O
BBS	O
specifies	O
that	O
the	O
BIOS	B-Operating_System
will	O
detect	O
and	O
shadow	O
all	O
option	O
ROMs	O
that	O
reside	O
in	O
the	O
BIOS	B-Operating_System
into	O
the	O
aforementioned	O
region	O
and	O
it	O
will	O
traverse	O
the	O
PCI	B-Architecture
configuration	I-Architecture
space	I-Architecture
,	O
filling	O
in	O
XROMBARs	O
and	O
copying	O
the	O
expansion	O
card	O
option	O
ROMs	O
from	O
MMIO	B-Architecture
space	O
to	O
the	O
region	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
then	O
scans	O
the	O
region	O
,	O
and	O
if	O
the	O
option	B-Device
ROM	I-Device
has	O
a	O
PnP	O
Expansion	O
header	O
,	O
it	O
does	O
a	O
far	O
call	O
to	O
offset	O
+03h	O
in	O
the	O
option	B-Device
ROM	I-Device
header	O
to	O
initialize	O
it	O
.	O
</s>
<s>
It	O
then	O
rescans	O
the	O
region	O
after	O
all	O
the	O
PnP	O
option	O
ROMs	O
have	O
been	O
initialized	O
(	O
because	O
,	O
as	O
appendix	O
E	O
states	O
,	O
the	O
option	B-Device
ROM	I-Device
initialization	O
routine	O
may	O
have	O
chained	O
more	O
PnP	O
expansion	O
headers	O
for	O
individual	O
disks	O
the	O
device	O
owns	O
)	O
.	O
</s>
<s>
The	O
BCV	O
entries	O
in	O
the	O
BCV	O
table	O
are	O
then	O
called	O
according	O
to	O
priority	O
settable	O
in	O
NVRAM	B-Device
.	O
</s>
<s>
The	O
BCV	O
table	O
is	O
full	O
of	O
BCV	O
function	O
pointers	O
but	O
has	O
a	O
fixed	O
entry	O
representing	O
legacy	O
option	O
ROMs	O
which	O
is	O
a	O
pointer	O
to	O
a	O
BIOS	B-Operating_System
routine	O
which	O
calls	O
+03h	O
in	O
all	O
the	O
remaining	O
option	O
ROMs	O
that	O
do	O
n't	O
have	O
a	O
PnP	O
Expansion	O
header	O
.	O
</s>
<s>
The	O
BCV	O
function	O
initializes	O
the	O
INT	B-Device
13h	I-Device
and	O
INT	O
19h	O
hooks	O
,	O
which	O
the	O
BBS	O
stipulates	O
must	O
not	O
be	O
done	O
in	O
the	O
initialization	O
routine	O
at	O
+03h	O
.	O
</s>
<s>
In	O
the	O
initial	O
initialization	O
routine	O
,	O
as	O
the	O
Option	B-Device
ROM	I-Device
points	O
to	O
a	O
PCI	O
data	O
structure	O
(	O
not	O
the	O
same	O
as	O
the	O
configuration	O
space	O
)	O
,	O
the	O
option	B-Device
ROM	I-Device
code	O
knows	O
the	O
device	O
and	O
vendor	O
ID	O
is	O
at	O
a	O
fixed	O
offset	O
from	O
RIP	B-General_Concept
.	O
</s>
<s>
This	O
allows	O
it	O
to	O
scan	O
the	O
PCI	B-Architecture
configuration	I-Architecture
space	I-Architecture
to	O
find	O
the	O
correct	O
device	O
and	O
BARs	O
it	O
needs	O
to	O
use	O
.	O
</s>
<s>
To	O
prevent	O
this	O
scan	O
,	O
and	O
in	O
case	O
of	O
two	O
identical	O
cards	O
in	O
the	O
system	O
,	O
the	O
BIOS	B-Operating_System
passes	O
the	O
PFA	O
(	O
bus/device/function	O
)	O
to	O
the	O
initialization	O
routine	O
in	O
AX	O
,	O
and	O
the	O
card	O
select	O
number	O
(	O
CSN	O
)	O
for	O
ISA	O
option	O
ROMs	O
is	O
passed	O
in	O
BX	O
.	O
</s>
<s>
It	O
can	O
then	O
interact	O
with	O
the	O
device	O
using	O
PMIO	B-Architecture
/	O
MMIO	B-Architecture
to	O
see	O
how	O
many	O
disks	O
it	O
has	O
and	O
which	O
ones	O
are	O
bootable	B-Operating_System
by	O
reading	O
the	O
MBR	B-Device
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
will	O
have	O
already	O
combed	O
the	O
configuration	O
space	O
,	O
allocated	O
the	O
BARs	O
and	O
filled	O
in	O
the	O
ACPI	O
table	O
prior	O
to	O
the	O
initialization	O
routine	O
call	O
,	O
so	O
the	O
option	B-Device
ROM	I-Device
would	O
use	O
the	O
addresses	O
allocated	O
to	O
its	O
BARs	O
.	O
</s>
<s>
The	O
BCV	O
,	O
however	O
,	O
hooks	O
interrupt	O
routines	O
which	O
interact	O
with	O
the	O
device	O
which	O
are	O
adjusted	O
based	O
on	O
a	O
base	O
MMIO	B-Architecture
address	O
location	O
,	O
disk	O
information	O
ascertained	O
in	O
the	O
option	B-Device
ROM	I-Device
initialization	O
routine	O
and	O
the	O
current	O
disk	O
number	O
in	O
the	O
BDA	O
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
INT	O
19h	O
procedure	O
then	O
uses	O
the	O
IPL	O
table	O
priority	O
in	O
NVRAM	B-Device
to	O
decide	O
whether	O
to	O
call	O
an	O
entry	O
containing	O
a	O
boot	B-Operating_System
handler	O
which	O
will	O
read	O
the	O
MBR	B-Device
of	O
00h	O
(	O
floppy	O
disk	O
BAID	O
;	O
the	O
first	O
device	O
in	O
the	O
BCV	O
table	O
to	O
register	O
disk	O
00h	O
)	O
,	O
an	O
entry	O
containing	O
a	O
boot	B-Operating_System
handler	O
which	O
will	O
read	O
the	O
MBR	B-Device
of	O
80h	O
(	O
the	O
hard	O
drive	O
BAID	O
;	O
the	O
first	O
device	O
in	O
the	O
BCV	O
Table	O
to	O
register	O
disk	O
80h	O
)	O
or	O
one	O
of	O
the	O
BEV	O
entries	O
in	O
the	O
table	O
.	O
</s>
<s>
A	O
device	O
only	O
has	O
a	O
BEV	O
or	O
a	O
BCV	O
if	O
it	O
is	O
a	O
bootable	B-Operating_System
device	O
.	O
</s>
<s>
A	O
SCSI	B-Architecture
controller	O
card	O
may	O
hook	O
INT	B-Device
13h	I-Device
which	O
is	O
responsible	O
for	O
providing	O
disk	O
services	O
.	O
</s>
<s>
Once	O
it	O
has	O
done	O
this	O
,	O
any	O
subsequent	O
calls	O
to	O
INT	B-Device
13h	I-Device
will	O
be	O
"	O
caught	O
"	O
by	O
the	O
SCSI	B-Architecture
option	B-Device
ROM	I-Device
(	O
or	O
"	O
SCSI	B-Architecture
BIOS	B-Operating_System
"	O
)	O
,	O
allowing	O
it	O
to	O
respond	O
for	O
disks	O
that	O
may	O
exist	O
on	O
the	O
SCSI	B-Architecture
bus	O
.	O
</s>
<s>
Before	O
it	O
had	O
hooked	O
the	O
interrupt	O
there	O
may	O
have	O
been	O
no	O
disks	O
on	O
the	O
system	O
,	O
but	O
by	O
intercepting	O
the	O
interrupt	O
and	O
altering	O
the	O
values	O
returned	O
,	O
the	O
SCSI	B-Architecture
BIOS	B-Operating_System
can	O
make	O
all	O
the	O
disks	O
on	O
the	O
SCSI	B-Architecture
bus	O
visible	O
to	O
the	O
system	O
.	O
</s>
<s>
In	O
this	O
particular	O
case	O
,	O
the	O
BIOS	B-Operating_System
itself	O
may	O
call	O
INT	B-Device
13h	I-Device
to	O
provide	O
a	O
list	O
of	O
possible	O
boot	B-Operating_System
devices	O
to	O
the	O
user	O
,	O
and	O
because	O
the	O
SCSI	B-Architecture
BIOS	B-Operating_System
has	O
hooked	O
the	O
interrupt	O
the	O
user	O
will	O
be	O
able	O
to	O
choose	O
not	O
only	O
which	O
standard	O
system	O
devices	O
to	O
boot	B-Operating_System
from	O
,	O
but	O
also	O
which	O
SCSI	B-Architecture
disks	O
as	O
well	O
.	O
</s>
<s>
This	O
is	O
because	O
,	O
as	O
suggested	O
in	O
Appendix	O
D	O
of	O
the	O
Boot	B-Operating_System
BIOS	B-Operating_System
Specification	O
,	O
the	O
BIOS	B-Operating_System
could	O
populate	O
the	O
IPL	O
table	O
with	O
device	O
and	O
vendor	O
information	O
from	O
INT	B-Device
13h	I-Device
calls	O
to	O
the	O
different	O
disks	O
,	O
paired	O
with	O
the	O
Hard	O
Disk	O
Number	O
(	O
80h	O
,	O
81h	O
...	O
)	O
,	O
to	O
allow	O
any	O
hard	O
disk	O
device	O
to	O
be	O
booted	O
from	O
,	O
rather	O
than	O
just	O
the	O
first	O
disk	O
of	O
the	O
first	O
controller	O
to	O
hook	O
INT	B-Device
13h	I-Device
(	O
the	O
highest	O
priority	O
item	O
in	O
the	O
BCV	O
table	O
)	O
,	O
referred	O
to	O
as	O
a	O
BIOS	B-Operating_System
Aware	O
IPL	O
Device	O
(	O
BAID	O
)	O
in	O
the	O
specification	O
.	O
</s>
<s>
Multiple	O
controllers	O
can	O
hook	O
INT	B-Device
13h	I-Device
at	O
once	O
.	O
</s>
<s>
For	O
instance	O
,	O
after	O
the	O
SCSI	B-Architecture
controller	O
,	O
an	O
AHCI	O
controller	O
can	O
also	O
hook	O
INT	B-Device
13h	I-Device
by	O
putting	O
a	O
call	O
to	O
the	O
previous	O
handler	O
,	O
which	O
was	O
stored	O
in	O
the	O
IDT	O
at	O
entry	O
13h	O
by	O
the	O
SCSI	B-Architecture
controller	O
,	O
at	O
the	O
end	O
of	O
its	O
own	O
handler	O
,	O
before	O
it	O
puts	O
the	O
address	O
of	O
its	O
own	O
handler	O
into	O
the	O
IDT	O
at	O
entry	O
13h	O
.	O
</s>
<s>
The	O
first	O
controller	O
to	O
hook	O
INT	B-Device
13h	I-Device
will	O
see	O
that	O
0	O
disks	O
have	O
been	O
installed	O
by	O
checking	O
the	O
byte	O
at	O
0040:0075	O
,	O
which	O
resides	O
in	O
the	O
BIOS	B-Operating_System
Data	O
Area	O
(	O
BDA	O
)	O
,	O
and	O
if	O
it	O
has	O
4	O
disks	O
to	O
enumerate	O
,	O
it	O
will	O
assign	O
the	O
range	O
of	O
disk	O
numbers	O
80h	O
–	O
83h	O
and	O
store	O
'	O
4	O
 '	O
in	O
the	O
BDA	O
.	O
</s>
<s>
If	O
the	O
second	O
controller	O
to	O
hook	O
INT	B-Device
13h	I-Device
has	O
2	O
disks	O
,	O
it	O
will	O
read	O
'	O
4	O
 '	O
from	O
the	O
BDA	O
,	O
assign	O
the	O
disk	O
numbers	O
84h	O
and	O
85h	O
,	O
and	O
store	O
'	O
6	O
 '	O
in	O
place	O
of	O
the	O
'	O
4	O
 '	O
.	O
</s>
<s>
Now	O
if	O
INT	B-Device
13h	I-Device
is	O
called	O
with	O
DL	O
=	O
83h	O
,	O
then	O
the	O
handler	O
of	O
the	O
second	O
controller	O
,	O
which	O
did	O
not	O
assign	O
disk	O
number	O
83h	O
,	O
will	O
relay	O
the	O
call	O
to	O
the	O
previous	O
handler	O
;	O
that	O
handler	O
,	O
which	O
did	O
assign	O
disk	O
number	O
83h	O
,	O
will	O
handle	O
the	O
call	O
itself	O
.	O
</s>
<s>
With	O
any	O
number	O
of	O
controllers	O
 '	O
ISRs	O
hooked	O
into	O
INT	B-Device
13h	I-Device
,	O
the	O
ISRs	O
will	O
each	O
pass	O
control	O
to	O
the	O
next	O
one	O
until	O
the	O
one	O
that	O
assigned	O
the	O
specified	O
drive	O
number	O
recognizes	O
the	O
number	O
,	O
handles	O
the	O
call	O
,	O
and	O
returns	O
from	O
the	O
interrupt	O
.	O
</s>
<s>
Another	O
common	O
option	B-Device
ROM	I-Device
is	O
a	O
network	B-Device
boot	I-Device
ROM	B-Device
.	O
</s>
<s>
The	O
option	B-Device
ROM	I-Device
contains	O
the	O
program	O
required	O
to	O
download	O
the	O
boot	B-Operating_System
code	O
.	O
</s>
<s>
The	O
original	O
IBM	B-Device
Personal	I-Device
Computer	I-Device
ROMs	O
hooked	O
INT	O
18H	O
(	O
originally	O
to	O
invoke	O
Cassette	O
BASIC	O
)	O
and	O
INT	O
19H	O
,	O
as	O
these	O
two	O
interrupts	O
were	O
used	O
for	O
the	O
boot	B-Operating_System
process	I-Operating_System
.	O
</s>
<s>
INT	O
19h	O
is	O
called	O
to	O
initiate	O
the	O
boot	B-Operating_System
process	I-Operating_System
,	O
and	O
INT	O
18h	O
was	O
called	O
to	O
start	O
Cassette	O
BASIC	O
from	O
ROM	B-Device
when	O
the	O
boot	B-Operating_System
process	I-Operating_System
found	O
that	O
none	O
of	O
the	O
possible	O
boot	B-Operating_System
devices	O
was	O
bootable	B-Operating_System
.	O
</s>
<s>
Originally	O
,	O
by	O
hooking	O
INT	O
18h	O
,	O
the	O
network	O
adapter	O
ROM	B-Device
would	O
try	O
to	O
boot	B-Operating_System
from	O
the	O
network	O
when	O
all	O
other	O
boot	B-Operating_System
devices	O
(	O
floppy	O
drives	O
,	O
hard	O
drives	O
,	O
etc	O
.	O
)	O
</s>
<s>
By	O
hooking	O
INT	O
19H	O
,	O
the	O
network	O
adapter	O
ROM	B-Device
would	O
attempt	O
to	O
boot	B-Operating_System
from	O
the	O
network	O
before	O
any	O
other	O
devices	O
.	O
</s>
<s>
The	O
BBS	O
specifies	O
that	O
the	O
NIC	O
option	B-Device
ROM	I-Device
does	O
not	O
hook	O
INT	O
19h	O
,	O
but	O
instead	O
the	O
BIOS	B-Operating_System
19h	O
handler	O
should	O
call	O
the	O
BEV	O
,	O
which	O
will	O
then	O
download	O
the	O
boot	B-Operating_System
code	O
.	O
</s>
<s>
The	O
Video	B-Device
BIOS	I-Device
provides	O
some	O
basic	O
display	O
services	O
for	O
BIOS	B-Operating_System
and	O
operating	B-General_Concept
systems	I-General_Concept
,	O
for	O
example	O
INT	B-Device
10H	I-Device
(	O
Legacy	B-Operating_System
BIOS	I-Operating_System
)	O
,	O
VBE	B-Device
(	O
Legacy	B-Operating_System
BIOS	I-Operating_System
)	O
and	O
UEFI	B-Architecture
GOP	O
.	O
</s>
<s>
The	O
original	O
IBM	B-Operating_System
PC	I-Operating_System
BIOS	I-Operating_System
included	O
integrated	O
support	O
for	O
the	O
IBM	O
CGA	O
and	O
MDA	O
video	O
adapters	O
(	O
and	O
did	O
not	O
support	O
option	O
ROMs	O
at	O
all	O
)	O
,	O
so	O
those	O
video	O
cards	O
had	O
no	O
option	O
ROMs	O
.	O
</s>
<s>
The	O
CGA	O
and	O
MDA	O
support	O
in	O
the	O
BIOS	B-Operating_System
proper	O
was	O
maintained	O
through	O
the	O
IBM	B-Device
PC	I-Device
XT	O
and	O
PC	O
AT	O
product	O
lines	O
(	O
which	O
did	O
support	O
option	O
ROMs	O
)	O
,	O
so	O
that	O
those	O
cards	O
worked	O
(	O
with	O
full	O
BIOS	B-Operating_System
support	O
)	O
in	O
those	O
machines	O
.	O
</s>
<s>
The	O
first	O
PC	O
video	O
adapter	O
card	O
that	O
had	O
an	O
option	B-Device
ROM	I-Device
was	O
the	O
IBM	O
EGA	O
,	O
introduced	O
in	O
1984	O
with	O
the	O
IBM	B-Device
PC	I-Device
AT	O
.	O
</s>
<s>
(	O
The	O
Hercules	O
Graphics	O
Card	O
had	O
no	O
option	B-Device
ROM	I-Device
and	O
no	O
BIOS	B-Operating_System
support	O
except	O
for	O
its	O
MDA-compatible	O
features	O
,	O
for	O
which	O
it	O
relied	O
in	O
the	O
IBM-supplied	O
MDA	O
support	O
in	O
the	O
main	O
BIOS	B-Operating_System
.	O
)	O
</s>
<s>
Most	O
subsequent	O
PC	O
video	O
adapters	O
were	O
supported	O
by	O
option	O
ROMs	O
,	O
although	O
VGA	O
and	O
MCGA	O
integrated	O
onto	O
PS/2	O
motherboards	O
may	O
have	O
used	O
integrated	O
BIOS	B-Operating_System
support	O
.	O
</s>
<s>
Once	O
integrated	O
Super	O
VGAs	O
(	O
SVGAs	O
)	O
,	O
integrated	O
on	O
clone	O
PC	O
motherboards	O
,	O
were	O
being	O
provided	O
by	O
separate	O
companies	O
than	O
the	O
systems	O
themselves	O
,	O
it	O
became	O
common	O
for	O
the	O
SVGA	O
vendor-provided	O
video	B-Device
BIOS	I-Device
to	O
be	O
included	O
as	O
a	O
separate	O
option	B-Device
ROM	I-Device
module	O
on	O
the	O
same	O
BIOS	B-Operating_System
chip	I-Operating_System
as	O
the	O
main	O
system	B-Operating_System
BIOS	I-Operating_System
(	O
provided	O
by	O
a	O
third	O
separate	O
company	O
)	O
.	O
</s>
<s>
The	O
PCI	O
spec	O
allows	O
multiple	O
option	B-Device
ROM	I-Device
images	O
on	O
the	O
same	O
device	O
.	O
</s>
<s>
These	O
option	O
ROMs	O
could	O
be	O
Legacy	O
x86	O
&	O
UEFI	B-Architecture
.	O
</s>
<s>
If	O
the	O
Option	B-Device
ROM	I-Device
format	O
is	O
set	O
to	O
"	O
UEFI	B-Architecture
Compatible	O
"	O
in	O
the	O
UEFI	B-Architecture
Setup	O
,	O
the	O
DXE	O
stage	O
will	O
load	O
the	O
newer	O
UEFI	B-Architecture
Option	B-Device
ROM	I-Device
if	O
one	O
is	O
present	O
and	O
the	O
legacy	O
Option	B-Device
ROM	I-Device
if	O
one	O
is	O
not	O
.	O
</s>
<s>
UEFI	B-Architecture
can	O
use	O
legacy	O
option	O
ROMs	O
when	O
a	O
Compatibility	O
Support	O
Module	O
(	O
CSM	O
)	O
is	O
enabled	O
.	O
</s>
<s>
Note	O
that	O
when	O
Secure	O
Boot	B-Operating_System
is	O
enabled	O
,	O
execution	O
of	O
the	O
Compatibility	O
Support	O
Module	O
and	O
legacy	O
option	O
ROMs	O
is	O
prohibited	O
because	O
legacy	O
firmware	B-Application
drivers	O
do	O
not	O
support	O
authentication	O
,	O
which	O
is	O
a	O
security	O
hole	O
.	O
</s>
