<s>
A	O
volume	B-Device
boot	I-Device
record	I-Device
(	O
VBR	O
)	O
(	O
also	O
known	O
as	O
a	O
volume	B-Device
boot	I-Device
sector	I-Device
,	O
a	O
partition	B-Device
boot	I-Device
record	I-Device
or	O
a	O
partition	B-Device
boot	I-Device
sector	I-Device
)	O
is	O
a	O
type	O
of	O
boot	B-Device
sector	I-Device
introduced	O
by	O
the	O
IBM	B-Device
Personal	I-Device
Computer	I-Device
.	O
</s>
<s>
It	O
may	O
be	O
found	O
on	O
a	O
partitioned	B-Application
data	B-General_Concept
storage	I-General_Concept
device	I-General_Concept
,	O
such	O
as	O
a	O
hard	B-Device
disk	I-Device
,	O
or	O
an	O
unpartitioned	O
device	O
,	O
such	O
as	O
a	O
floppy	B-Device
disk	I-Device
,	O
and	O
contains	O
machine	B-Language
code	I-Language
for	O
bootstrapping	O
programs	O
(	O
usually	O
,	O
but	O
not	O
necessarily	O
,	O
operating	B-General_Concept
systems	I-General_Concept
)	O
stored	O
in	O
other	O
parts	O
of	O
the	O
device	O
.	O
</s>
<s>
On	O
non-partitioned	O
storage	O
devices	O
,	O
it	O
is	O
the	O
first	O
sector	B-Application
of	O
the	O
device	O
.	O
</s>
<s>
On	O
partitioned	B-Application
devices	O
,	O
it	O
is	O
the	O
first	O
sector	B-Application
of	O
an	O
individual	O
partition	B-Application
on	O
the	O
device	O
,	O
with	O
the	O
first	O
sector	B-Application
of	O
the	O
entire	O
device	O
being	O
a	O
Master	B-Device
Boot	I-Device
Record	I-Device
(	O
MBR	O
)	O
containing	O
the	O
partition	B-Application
table	O
.	O
</s>
<s>
The	O
code	O
in	O
volume	B-Device
boot	I-Device
records	I-Device
is	O
invoked	O
either	O
directly	O
by	O
the	O
machine	O
's	O
firmware	O
or	O
indirectly	O
by	O
code	O
in	O
the	O
master	B-Device
boot	I-Device
record	I-Device
or	O
a	O
boot	B-Application
manager	I-Application
.	O
</s>
<s>
Invoking	O
a	O
VBR	O
via	O
a	O
boot	B-Application
manager	I-Application
is	O
known	O
as	O
chain	B-Operating_System
loading	I-Operating_System
.	O
</s>
<s>
Some	O
dual-boot	B-Operating_System
systems	O
,	O
such	O
as	O
NTLDR	B-Application
(	O
the	O
boot	B-Application
loader	I-Application
for	O
all	O
releases	O
of	O
Microsoft	O
's	O
Windows	O
NT-derived	O
operating	B-General_Concept
systems	I-General_Concept
up	O
to	O
and	O
including	O
Windows	B-Application
XP	I-Application
and	O
Windows	B-Application
Server	I-Application
2003	I-Application
)	O
,	O
take	O
copies	O
of	O
the	O
bootstrap	O
code	O
that	O
individual	O
operating	B-General_Concept
systems	I-General_Concept
install	O
into	O
a	O
single	O
partition	B-Application
's	O
VBR	O
and	O
store	O
them	O
in	O
disc	O
files	O
,	O
loading	O
the	O
relevant	O
VBR	O
content	O
from	O
file	O
after	O
the	O
boot	B-Application
loader	I-Application
has	O
asked	O
the	O
user	O
which	O
operating	B-General_Concept
system	I-General_Concept
to	O
bootstrap	O
.	O
</s>
<s>
In	O
Windows	B-Application
Vista	I-Application
,	O
Windows	B-Device
Server	I-Device
2008	I-Device
and	O
newer	O
versions	O
,	O
NTLDR	B-Application
was	O
replaced	O
;	O
the	O
boot-loader	O
functionality	O
is	O
instead	O
provided	O
by	O
two	O
new	O
components	O
:	O
WINLOAD.EXE	O
and	O
the	O
Windows	B-Operating_System
Boot	I-Operating_System
Manager	I-Operating_System
.	O
</s>
<s>
In	O
file	B-Application
systems	I-Application
such	O
as	O
FAT12	B-Application
(	O
except	O
for	O
in	O
DOS	O
1.x	O
)	O
,	O
FAT16	B-Application
,	O
FAT32	B-Application
,	O
HPFS	B-Application
and	O
NTFS	B-Application
,	O
the	O
VBR	O
also	O
contains	O
a	O
BIOS	B-Device
Parameter	I-Device
Block	I-Device
(	O
BPB	O
)	O
that	O
specifies	O
the	O
location	O
and	O
layout	O
of	O
the	O
principal	O
on-disk	O
data	O
structures	O
for	O
the	O
file	B-Application
system	I-Application
.	O
</s>
<s>
(	O
A	O
detailed	O
discussion	O
of	O
the	O
sector	B-Application
layout	O
of	O
FAT	B-Application
VBRs	O
,	O
the	O
various	O
FAT	B-Application
BPB	O
versions	O
and	O
their	O
entries	O
can	O
be	O
found	O
in	O
the	O
FAT	B-Application
article	O
.	O
)	O
</s>
<s>
The	O
presence	O
of	O
an	O
IBM	B-Device
PC	I-Device
compatible	O
boot	B-Application
loader	I-Application
for	O
x86-CPUs	O
in	O
the	O
boot	B-Device
sector	I-Device
is	O
by	O
convention	O
indicated	O
by	O
a	O
two-byte	O
hexadecimal	O
sequence	O
called	O
the	O
boot	B-Device
sector	I-Device
signature	O
(	O
at	O
fixed	O
offset	O
and	O
at	O
)	O
for	O
sector	B-Application
sizes	O
of	O
512	B-Application
bytes	I-Application
or	O
more	O
.	O
</s>
<s>
For	O
512	O
byte	O
sectors	O
,	O
the	O
boot	B-Device
sector	I-Device
signature	O
also	O
marks	O
the	O
end	O
of	O
the	O
sector	B-Application
.	O
</s>
<s>
VBRs	O
on	O
smaller	O
and	O
larger	O
sectors	O
may	O
show	O
signatures	O
at	O
the	O
end	O
of	O
the	O
actual	O
sector	B-Application
size	O
as	O
well	O
,	O
however	O
,	O
the	O
semantics	O
described	O
herein	O
apply	O
to	O
the	O
16-bit	O
signature	O
at	O
only	O
.	O
</s>
<s>
This	O
signature	O
indicates	O
the	O
presence	O
of	O
at	O
least	O
a	O
dummy	O
boot	B-Application
loader	I-Application
which	O
is	O
safe	O
to	O
be	O
executed	O
,	O
even	O
if	O
it	O
may	O
not	O
be	O
able	O
to	O
actually	O
load	O
an	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
It	O
does	O
not	O
indicate	O
the	O
presence	O
of	O
a	O
(	O
or	O
even	O
a	O
particular	O
)	O
file	B-Application
system	I-Application
or	O
operating	B-General_Concept
system	I-General_Concept
,	O
although	O
some	O
old	O
versions	O
of	O
DOS	O
prior	O
to	O
3.3	O
relied	O
on	O
it	O
in	O
their	O
process	O
to	O
detect	O
FAT-formatted	O
media	O
(	O
newer	O
versions	O
do	O
not	O
)	O
.	O
</s>
<s>
Boot	O
code	O
for	O
other	O
platforms	O
or	O
CPUs	B-Device
should	O
not	O
use	O
this	O
signature	O
,	O
since	O
this	O
may	O
lead	O
to	O
a	O
crash	O
when	O
the	O
BIOS	O
passes	O
execution	O
to	O
the	O
boot	B-Device
sector	I-Device
assuming	O
that	O
it	O
contains	O
valid	O
executable	O
code	O
.	O
</s>
<s>
This	O
implies	O
that	O
FAT12/FAT16	O
media	O
to	O
be	O
used	O
also	O
by	O
very	O
old	O
versions	O
of	O
DOS	O
must	O
maintain	O
the	O
signature	O
even	O
if	O
they	O
do	O
not	O
contain	O
an	O
operating	B-General_Concept
system	I-General_Concept
or	O
are	O
meant	O
to	O
be	O
bootable	O
on	O
other	O
platforms	O
only	O
;	O
consequently	O
they	O
must	O
contain	O
at	O
least	O
an	O
x86	B-Operating_System
compatible	I-Operating_System
(	O
dummy	O
)	O
loader	O
as	O
well	O
(	O
for	O
comparison	O
,	O
see	O
examples	O
of	O
FAT	B-Application
on	O
the	O
Atari	O
ST	O
and	O
with	O
MSX-DOS	O
)	O
.	O
</s>
<s>
Nevertheless	O
,	O
some	O
media	O
for	O
other	O
platforms	O
erroneously	O
contain	O
the	O
signature	O
even	O
without	O
a	O
x86	B-Operating_System
compatible	I-Operating_System
dummy	O
loader	O
,	O
making	O
the	O
check	O
not	O
100%	O
reliable	O
in	O
practice	O
.	O
</s>
<s>
The	O
signature	O
is	O
tested	O
for	O
by	O
most	O
System	O
BIOSes	O
since	O
(	O
at	O
least	O
)	O
the	O
IBM	B-Operating_System
PC/AT	I-Operating_System
(	O
but	O
not	O
by	O
the	O
original	O
IBM	B-Device
PC	I-Device
and	O
some	O
other	O
machines	O
)	O
.	O
</s>
<s>
Even	O
more	O
so	O
,	O
it	O
is	O
also	O
checked	O
by	O
most	O
MBR	O
boot	B-Application
loaders	I-Application
before	O
passing	O
control	O
to	O
the	O
boot	B-Device
sector	I-Device
.	O
</s>
<s>
Some	O
BIOSes	O
(	O
like	O
the	O
IBM	B-Operating_System
PC/AT	I-Operating_System
)	O
perform	O
the	O
check	O
only	O
for	O
fixed	B-Device
disk	I-Device
/	O
removable	B-Device
drives	I-Device
,	O
while	O
for	O
floppies	B-Device
and	O
superfloppies	O
it	O
is	O
enough	O
to	O
start	O
with	O
a	O
byte	O
greater	O
or	O
equal	O
to	O
and	O
the	O
first	O
nine	O
words	O
not	O
to	O
contain	O
the	O
same	O
value	O
,	O
before	O
the	O
boot	B-Device
sector	I-Device
is	O
accepted	O
as	O
valid	O
,	O
thereby	O
avoiding	O
the	O
explicit	O
test	O
for	O
,	O
on	O
floppies	B-Device
.	O
</s>
<s>
Since	O
old	O
boot	B-Device
sectors	I-Device
(	O
i.e.	O
</s>
<s>
very	O
old	O
CP/M	B-Operating_System
-86	I-Operating_System
and	O
DOS	O
media	O
)	O
sometimes	O
do	O
not	O
feature	O
this	O
signature	O
despite	O
the	O
fact	O
that	O
they	O
can	O
be	O
booted	O
successfully	O
,	O
the	O
check	O
can	O
be	O
disabled	O
in	O
some	O
environments	O
.	O
</s>
<s>
This	O
also	O
reflects	O
the	O
fact	O
that	O
floppies	B-Device
can	O
be	O
formatted	O
to	O
use	O
smaller	O
sector	B-Application
sizes	O
than	O
512	B-Application
bytes	I-Application
.	O
</s>
<s>
If	O
the	O
BIOS	O
or	O
MBR	O
code	O
does	O
not	O
detect	O
a	O
valid	O
boot	B-Device
sector	I-Device
and	O
therefore	O
cannot	O
pass	O
execution	O
to	O
the	O
boot	B-Device
sector	I-Device
code	O
,	O
it	O
will	O
try	O
the	O
next	O
boot	O
device	O
in	O
the	O
row	O
.	O
</s>
<s>
From	O
decimal	O
offset	O
72	O
to	O
79	O
,	O
an	O
NTFS	B-Application
PBR	O
contains	O
the	O
partition	B-Application
UUID	O
volume	O
ID	O
serial	O
number	O
.	O
</s>
<s>
BIOS	O
extension	O
overlays	O
,	O
encryption	O
systems	O
,	O
or	O
remote	O
bootstrap	O
loaders	O
)	O
may	O
be	O
present	O
elsewhere	O
in	O
memory	O
as	O
well	O
(	O
and	O
would	O
typically	O
hide	O
themselves	O
from	O
the	O
boot	B-Device
sector	I-Device
by	O
reducing	O
the	O
reported	O
INT	O
12h	O
memory	O
accordingly	O
,	O
so	O
that	O
they	O
do	O
not	O
get	O
overwritten	O
by	O
the	O
MBR	O
's	O
and	O
VBR	O
's	O
actions	O
)	O
.	O
</s>
<s>
The	O
boot	O
code	O
must	O
not	O
assume	O
better	O
CPUs	B-Device
than	O
the	O
original	O
Intel	B-Device
8088	I-Device
or	O
8086	B-General_Concept
(	O
used	O
in	O
the	O
original	O
PC	O
)	O
and	O
make	O
no	O
assumptions	O
in	O
regard	O
to	O
the	O
exact	O
state	O
of	O
the	O
hardware	O
,	O
the	O
interrupt	O
system	O
(	O
interrupts	O
can	O
be	O
enabled	O
or	O
disabled	O
)	O
or	O
the	O
location	O
and	O
size	O
of	O
the	O
stack	O
.	O
</s>
<s>
Although	O
the	O
original	O
IBM	O
BIOS	O
initializes	O
the	O
DS	O
,	O
ES	O
,	O
and	O
SS	O
CPU	B-Device
registers	O
to	O
segment	O
and	O
maintains	O
the	O
initial	O
stack	O
at	O
SS	O
:	O
SP	O
=	O
,	O
this	O
is	O
not	O
a	O
condition	O
to	O
rely	O
on	O
,	O
as	O
not	O
all	O
BIOSes	O
and	O
MBR	O
codes	O
follow	O
this	O
convention	O
.	O
</s>
<s>
Some	O
conditions	O
can	O
be	O
relaxed	O
in	O
controlled	O
environments	O
,	O
for	O
example	O
,	O
some	O
boot	B-Application
loaders	I-Application
today	O
assume	O
to	O
have	O
up	O
to	O
128	O
KB	O
of	O
memory	O
to	O
work	O
with	O
in	O
normal	O
operation	O
(	O
without	O
querying	O
for	O
more	O
)	O
,	O
and	O
some	O
boot	B-Application
loaders	I-Application
using	O
LBA	B-Device
access	O
assume	O
at	O
least	O
an	O
Intel	B-Device
80188	I-Device
or	O
80186	B-Device
CPU	B-Device
.	O
</s>
<s>
The	O
VBR	O
is	O
loaded	O
at	O
memory	O
location	O
0000h:7C00h	O
and	O
with	O
the	O
following	O
CPU	B-Device
registers	O
set	O
up	O
when	O
the	O
prior	O
bootstrap	O
loader	O
(	O
that	O
is	O
,	O
typically	O
the	O
BIOS	O
or	O
MBR	O
,	O
but	O
possibly	O
another	O
boot	B-Application
loader	I-Application
)	O
passes	O
execution	O
to	O
it	O
by	O
jumping	O
to	O
0000h:7C00h	O
in	O
the	O
CPU	B-Device
's	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
While	O
this	O
resolves	O
to	O
the	O
same	O
location	O
in	O
real	B-Application
mode	I-Application
memory	O
,	O
it	O
is	O
non-standard	O
and	O
should	O
be	O
avoided	O
,	O
since	O
VBR	O
code	O
assuming	O
certain	O
register	O
values	O
or	O
not	O
written	O
to	O
be	O
relocatible	O
may	O
not	O
work	O
otherwise	O
.	O
</s>
<s>
DL	O
=	O
boot	O
drive	O
unit	O
(	O
floppies	B-Device
/	O
superfloppies	O
:	O
00h	O
=	O
first	O
,	O
01h	O
=	O
second	O
,	O
...	O
,	O
7Eh	O
;	O
fixed	B-Device
disks	I-Device
/	O
removable	B-Device
drives	I-Device
:	O
80h	O
=	O
first	O
,	O
81h	O
=	O
second	O
,	O
...	O
,	O
FEh	O
;	O
values	O
7Fh	O
and	O
FFh	O
are	O
reserved	O
for	O
ROM	O
/	O
remote	O
drives	O
and	O
must	O
not	O
be	O
used	O
on	O
disk	O
)	O
.	O
</s>
<s>
The	O
Toshiba	O
T1000	O
BIOS	O
is	O
known	O
to	O
not	O
support	O
this	O
properly	O
,	O
and	O
some	O
old	O
Wyse	O
286	O
BIOSes	O
use	O
DL	O
values	O
greater	O
or	O
equal	O
to	O
2	O
for	O
hard	B-Device
disks	I-Device
.	O
</s>
<s>
USB	O
sticks	O
configured	O
as	O
superfloppies	O
typically	O
get	O
an	O
assignment	O
of	O
DL	O
=	O
00h	O
or	O
01h	O
,	O
however	O
,	O
some	O
rare	O
BIOSes	O
erroneously	O
present	O
USB	O
sticks	O
configured	O
as	O
removable	B-Device
drives	I-Device
as	O
DL	O
=	O
01h	O
as	O
well	O
,	O
instead	O
of	O
using	O
DL	O
=	O
80h	O
.	O
</s>
<s>
Traditionally	O
,	O
only	O
values	O
00h	O
and	O
80h	O
were	O
passed	O
on	O
by	O
the	O
BIOS	O
during	O
boot	O
,	O
and	O
many	O
boot	B-Device
sectors	I-Device
were	O
hard-wired	O
to	O
work	O
with	O
fixed	O
values	O
,	O
anyway	O
.	O
</s>
<s>
The	O
Plug	B-Device
and	I-Device
Play	I-Device
BIOS	I-Device
Specification	I-Device
and	O
BIOS	O
Boot	O
Specification	O
(	O
BBS	O
)	O
allow	O
other	O
devices	O
to	O
become	O
bootable	O
as	O
well	O
.	O
</s>
<s>
MS-DOS/PC	O
DOS	O
and	O
OS/2	B-Application
VBRs	O
ignore	O
the	O
provided	O
DL	O
value	O
and	O
instead	O
retrieve	O
the	O
value	O
stored	O
at	O
offset	O
+19h	O
in	O
the	O
Extended	B-Device
BIOS	I-Device
Parameter	I-Device
Block	I-Device
(	O
EBPB	B-Device
)	O
or	O
at	O
sector	B-Application
offset	O
+1FDh	O
in	O
DOS	O
versions	O
3.2	O
to	O
3.31	O
.	O
</s>
<s>
Some	O
versions	O
of	O
DR-DOS	B-Application
use	O
DL	O
starting	O
with	O
7.02	O
.	O
</s>
<s>
DR-DOS	B-Application
7.07	O
VBRs	O
default	O
to	O
use	O
DL	O
and	O
ignore	O
the	O
BPB	O
value	O
,	O
but	O
SYS	O
/O[:nnn]	O
can	O
be	O
used	O
to	O
reinvoke	O
the	O
old	O
behaviour	O
of	O
using	O
the	O
BPB	O
value	O
or	O
even	O
enforce	O
a	O
particular	O
boot	O
drive	O
unit	O
nnn	O
to	O
be	O
stored	O
there	O
.	O
</s>
<s>
On	O
FAT32	B-Application
volumes	O
,	O
FreeDOS	B-Application
takes	O
advantage	O
of	O
the	O
DL	O
value	O
as	O
well	O
,	O
whereas	O
it	O
does	O
so	O
on	O
FAT12/FAT16	O
volumes	O
only	O
,	O
if	O
the	O
BPB	O
value	O
is	O
set	O
to	O
FFh	O
.	O
)	O
</s>
<s>
DH	O
bit	O
5	O
=	O
0	O
:	O
device	O
supported	O
through	O
INT	B-Device
13h	I-Device
;	O
else	O
:	O
do	O
n't	O
care	O
(	O
should	O
be	O
zero	O
)	O
.	O
</s>
<s>
Systems	O
with	O
Plug-and-Play	B-Device
BIOS	O
or	O
BBS	O
support	O
will	O
provide	O
a	O
pointer	O
to	O
PnP	O
data	O
in	O
addition	O
to	O
DL	O
:	O
</s>
<s>
This	O
information	O
allows	O
the	O
boot	B-Application
loader	I-Application
(	O
in	O
the	O
MBR	O
or	O
VBR	O
)	O
to	O
actively	O
interact	O
with	O
the	O
BIOS	O
or	O
a	O
resident	O
PnP	O
/	O
BBS	O
overlay	O
in	O
memory	O
in	O
order	O
to	O
configure	O
the	O
boot	O
order	O
etc.	O
,	O
however	O
,	O
this	O
information	O
is	O
ignored	O
by	O
most	O
standard	O
MBRs	O
and	O
VBRs	O
.	O
</s>
<s>
Ideally	O
,	O
ES:DI	O
is	O
passed	O
on	O
,	O
but	O
PnP-enabled	O
operating	B-General_Concept
systems	I-General_Concept
typically	O
also	O
have	O
fallback	O
methods	O
to	O
retrieve	O
the	O
PnP	O
BIOS	O
entry	O
point	O
later	O
on	O
so	O
that	O
most	O
operating	B-General_Concept
systems	I-General_Concept
do	O
not	O
rely	O
on	O
this	O
.	O
</s>
<s>
With	O
partitioned	B-Application
media	O
,	O
when	O
the	O
VBR	O
is	O
launched	O
by	O
the	O
MBR	O
(	O
or	O
different	O
boot	B-Application
loader	I-Application
)	O
rather	O
than	O
the	O
BIOS	O
,	O
many	O
implementation	O
pass	O
additional	O
information	O
to	O
the	O
VBR	O
besides	O
just	O
DL	O
(	O
and	O
sometimes	O
DH	O
and	O
ES:DI	O
as	O
well	O
)	O
:	O
</s>
<s>
DS:SI	O
=	O
points	O
to	O
the	O
16-byte	O
MBR	O
partition	B-Application
table	O
entry	O
(	O
in	O
the	O
relocated	O
MBR	O
)	O
corresponding	O
with	O
the	O
activated	O
VBR	O
.	O
</s>
<s>
PC-MOS	O
5.1	O
depends	O
on	O
this	O
to	O
boot	O
if	O
no	O
partition	B-Application
in	O
the	O
partition	B-Application
table	O
is	O
flagged	O
as	O
bootable	O
.	O
</s>
<s>
In	O
conjunction	O
with	O
LOADER	O
,	O
Multiuser	B-Operating_System
DOS	I-Operating_System
and	O
REAL/32	B-Operating_System
boot	B-Device
sectors	I-Device
use	O
this	O
to	O
locate	O
the	O
boot	B-Device
sector	I-Device
of	O
the	O
active	O
partition	B-Application
(	O
or	O
another	O
bootstrap	O
loader	O
like	O
IBMBIO.LDR	O
at	O
a	O
fixed	O
position	O
on	O
disk	O
)	O
if	O
the	O
boot	O
file	O
(	O
LOADER.SYS	O
)	O
could	O
not	O
be	O
found	O
.	O
</s>
<s>
PTS-DOS	B-Operating_System
6.5	O
and	O
S/DOS	B-Operating_System
1.0	O
use	O
this	O
in	O
conjunction	O
with	O
their	O
Advanced	O
Active	O
Partition	B-Application
(	O
AAP	O
)	O
feature	O
.	O
</s>
<s>
In	O
addition	O
to	O
support	O
for	O
LOADER	O
and	O
AAPs	O
,	O
DR-DOS	B-Application
7.07	O
can	O
use	O
this	O
to	O
determine	O
the	O
necessary	O
INT	B-Device
13h	I-Device
access	O
method	O
when	O
using	O
its	O
dual	O
CHS/LBA	O
VBR	O
code	O
.	O
</s>
<s>
The	O
MBR	O
code	O
of	O
OS/2	B-Application
,	O
MS-DOS	B-Application
(	O
prior	O
to	O
7.0	O
)	O
,	O
PC	B-Operating_System
DOS	I-Operating_System
(	O
up	O
to	O
7.10	O
)	O
,	O
and	O
Windows	B-Device
NT	I-Device
(	O
up	O
to	O
ca	O
.	O
</s>
<s>
The	O
MBR	O
installed	O
by	O
Windows	B-Device
NT	I-Device
6.0	O
(	O
and	O
higher	O
)	O
uses	O
other	O
registers	O
,	O
and	O
is	O
therefore	O
no	O
longer	O
compatible	O
with	O
these	O
extensions	O
.	O
</s>
<s>
While	O
some	O
extensions	O
only	O
depend	O
on	O
the	O
16-byte	O
partition	B-Application
table	O
entry	O
itself	O
,	O
other	O
extensions	O
may	O
require	O
the	O
whole	O
4	O
(	O
or	O
5	O
entry	O
)	O
partition	B-Application
table	O
to	O
be	O
present	O
as	O
well	O
.	O
</s>
<s>
Under	O
DR-DOS	B-Application
7.07	O
an	O
extended	O
interface	O
may	O
be	O
optionally	O
used	O
by	O
the	O
MBR	O
and	O
in	O
conjunction	O
with	O
LOADER	O
:	O
</s>
<s>
In	O
conjunction	O
with	O
GUID	B-Application
partition	I-Application
tables	I-Application
(	O
GPT	O
)	O
,	O
an	O
Enhanced	O
Disk	O
Drive	O
Specification	O
(	O
EDD	O
)	O
4	O
Hybrid	O
MBR	O
proposal	O
recommends	O
another	O
extension	O
to	O
the	O
MBR	O
to	O
VBR	O
interface	O
:	O
</s>
<s>
DS:SI	O
=	O
points	O
to	O
a	O
Hybrid	O
MBR	O
handover	O
structure	O
,	O
consisting	O
of	O
a	O
16-byte	O
dummy	O
MBR	O
partition	B-Application
table	O
entry	O
(	O
with	O
all	O
bits	O
set	O
except	O
for	O
the	O
boot	O
flag	O
at	O
offset	O
+0h	O
and	O
the	O
partition	B-Application
type	I-Application
at	O
offset	O
+4h	O
)	O
followed	O
by	O
additional	O
data	O
.	O
</s>
<s>
This	O
is	O
partially	O
compatible	O
with	O
the	O
older	O
DS:SI	O
extension	O
discussed	O
above	O
,	O
if	O
only	O
the	O
16-byte	O
partition	B-Application
entry	O
,	O
not	O
the	O
whole	O
partition	B-Application
table	O
is	O
required	O
by	O
these	O
older	O
extensions	O
.	O
</s>
