<s>
A	O
master	B-Device
boot	I-Device
record	I-Device
(	O
MBR	O
)	O
is	O
a	O
special	O
type	O
of	O
boot	B-Device
sector	I-Device
at	O
the	O
very	O
beginning	O
of	O
partitioned	B-Application
computer	O
mass	B-Device
storage	I-Device
devices	I-Device
like	O
fixed	B-Device
disks	I-Device
or	O
removable	B-Device
drives	I-Device
intended	O
for	O
use	O
with	O
IBM	O
PC-compatible	O
systems	O
and	O
beyond	O
.	O
</s>
<s>
The	O
concept	O
of	O
MBRs	O
was	O
publicly	O
introduced	O
in	O
1983	O
with	O
PC	B-Operating_System
DOS	I-Operating_System
2.0	O
.	O
</s>
<s>
The	O
MBR	O
holds	O
the	O
information	O
on	O
how	O
the	O
disc	O
's	O
sectors	B-Application
(	O
aka	O
“	O
blocks	O
”	O
)	O
are	O
divided	O
into	O
partitions	B-Application
,	O
each	O
partition	B-Application
notionally	O
containing	O
a	O
file	B-Application
system	I-Application
.	O
</s>
<s>
The	O
MBR	O
also	O
contains	O
executable	O
code	O
to	O
function	O
as	O
a	O
loader	O
for	O
the	O
installed	O
operating	B-General_Concept
system	I-General_Concept
—	O
usually	O
by	O
passing	O
control	O
over	O
to	O
the	O
loader	O
's	O
second	O
stage	O
,	O
or	O
in	O
conjunction	O
with	O
each	O
partition	B-Application
's	O
volume	B-Device
boot	I-Device
record	I-Device
(	O
VBR	B-Device
)	O
.	O
</s>
<s>
This	O
MBR	O
code	O
is	O
usually	O
referred	O
to	O
as	O
a	O
boot	B-Application
loader	I-Application
.	O
</s>
<s>
The	O
organization	O
of	O
the	O
partition	B-Application
table	O
in	O
the	O
MBR	O
limits	O
the	O
maximum	O
addressable	O
storage	O
space	O
of	O
a	O
partitioned	B-Application
disk	O
to	O
2TiB	O
.	O
</s>
<s>
Approaches	O
to	O
slightly	O
raise	O
this	O
limit	O
utilizing	O
32-bit	O
arithmetic	O
or	O
4096-byte	O
sectors	B-Application
are	O
not	O
officially	O
supported	O
,	O
as	O
they	O
fatally	O
break	O
compatibility	O
with	O
existing	O
boot	B-Application
loaders	I-Application
,	O
most	O
MBR-compliant	O
operating	B-General_Concept
systems	I-General_Concept
and	O
associated	O
system	O
tools	O
,	O
and	O
may	O
cause	O
serious	O
data	O
corruption	O
when	O
used	O
outside	O
of	O
narrowly	O
controlled	O
system	O
environments	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
MBR-based	O
partitioning	B-Application
scheme	O
is	O
in	O
the	O
process	O
of	O
being	O
superseded	O
by	O
the	O
GUID	B-Application
Partition	I-Application
Table	I-Application
(	O
GPT	O
)	O
scheme	O
in	O
new	O
computers	O
.	O
</s>
<s>
MBRs	O
are	O
not	O
present	O
on	O
non-partitioned	O
media	O
such	O
as	O
floppies	B-Device
,	O
superfloppies	O
or	O
other	O
storage	O
devices	O
configured	O
to	O
behave	O
as	O
such	O
,	O
nor	O
are	O
they	O
necessarily	O
present	O
on	O
drives	O
used	O
in	O
non-PC	O
platforms	O
.	O
</s>
<s>
Support	O
for	O
partitioned	B-Application
media	O
,	O
and	O
thereby	O
the	O
master	B-Device
boot	I-Device
record	I-Device
(	O
MBR	O
)	O
,	O
was	O
introduced	O
with	O
IBM	B-Device
PC	I-Device
DOS	B-Device
2.0	O
in	O
March	O
1983	O
in	O
order	O
to	O
support	O
the	O
10MB	O
hard	B-Device
disk	I-Device
of	O
the	O
then-new	O
IBM	B-Device
Personal	I-Device
Computer	I-Device
XT	I-Device
,	O
still	O
using	O
the	O
FAT12	B-Application
file	B-Application
system	I-Application
.	O
</s>
<s>
The	O
partition	B-Application
table	O
supported	O
up	O
to	O
four	O
primary	O
partitions	B-Application
,	O
of	O
which	O
DOS	B-Device
could	O
only	O
use	O
one	O
.	O
</s>
<s>
This	O
did	O
not	O
change	O
when	O
FAT16	B-Application
was	O
introduced	O
as	O
a	O
new	O
file	B-Application
system	I-Application
with	O
DOS	B-Device
3.0	O
.	O
</s>
<s>
Support	O
for	O
an	O
extended	B-Application
partition	I-Application
,	O
a	O
special	O
primary	O
partition	B-Application
type	I-Application
used	O
as	O
a	O
container	O
to	O
hold	O
other	O
partitions	B-Application
,	O
was	O
added	O
with	O
DOS	B-Device
3.2	O
,	O
and	O
nested	O
logical	B-Application
drives	I-Application
inside	O
an	O
extended	B-Application
partition	I-Application
came	O
with	O
DOS	B-Device
3.30	O
.	O
</s>
<s>
Since	O
MS-DOS	B-Application
,	O
PCDOS	B-Operating_System
,	O
OS/2	O
and	O
Windows	O
were	O
never	O
enabled	O
to	O
boot	O
off	O
them	O
,	O
the	O
MBR	O
format	O
and	O
boot	O
code	O
remained	O
almost	O
unchanged	O
in	O
functionality	O
,	O
except	O
for	O
in	O
some	O
third-party	O
implementations	O
,	O
throughout	O
the	O
eras	O
of	O
DOS	B-Device
and	O
OS/2	O
up	O
to	O
1996	O
.	O
</s>
<s>
In	O
1996	O
,	O
support	O
for	O
logical	B-Device
block	I-Device
addressing	I-Device
(	O
LBA	B-Device
)	O
was	O
introduced	O
in	O
Windows	O
95B	O
and	O
DOS	B-Device
7.10	O
in	O
order	O
to	O
support	O
disks	O
larger	O
than	O
8GB	O
.	O
</s>
<s>
This	O
also	O
reflected	O
the	O
idea	O
that	O
the	O
MBR	O
is	O
meant	O
to	O
be	O
operating	B-General_Concept
system	I-General_Concept
and	O
file	B-Application
system	I-Application
independent	O
.	O
</s>
<s>
However	O
,	O
this	O
design	O
rule	O
was	O
partially	O
compromised	O
in	O
more	O
recent	O
Microsoft	O
implementations	O
of	O
the	O
MBR	O
,	O
which	O
enforce	O
CHS	B-Device
access	O
for	O
FAT16B	B-Application
and	O
FAT32	B-Application
partition	B-Application
types	I-Application
/	O
,	O
whereas	O
LBA	B-Device
is	O
used	O
for	O
/	O
.	O
</s>
<s>
This	O
was	O
even	O
to	O
the	O
extent	O
of	O
being	O
supported	O
by	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
for	O
other	O
platforms	O
.	O
</s>
<s>
Sometimes	O
this	O
was	O
in	O
addition	O
to	O
other	O
pre-existing	O
or	O
cross-platform	B-Operating_System
standards	O
for	O
bootstrapping	B-Application
and	O
partitioning	B-Application
.	O
</s>
<s>
MBR	O
partition	B-Application
entries	O
and	O
the	O
MBR	O
boot	O
code	O
used	O
in	O
commercial	O
operating	B-General_Concept
systems	I-General_Concept
,	O
however	O
,	O
are	O
limited	O
to	O
32	O
bits	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
maximum	O
disk	O
size	O
supported	O
on	O
disks	O
using	O
512-byte	O
sectors	B-Application
(	O
whether	O
real	O
or	O
emulated	O
)	O
by	O
the	O
MBR	O
partitioning	B-Application
scheme	O
(	O
without	O
32-bit	O
arithmetic	O
)	O
is	O
limited	O
to	O
2TiB	O
.	O
</s>
<s>
Consequently	O
,	O
a	O
different	O
partitioning	B-Application
scheme	O
must	O
be	O
used	O
for	O
larger	O
disks	O
,	O
as	O
they	O
have	O
become	O
widely	O
available	O
since	O
2010	O
.	O
</s>
<s>
The	O
MBR	O
partitioning	B-Application
scheme	O
is	O
therefore	O
in	O
the	O
process	O
of	O
being	O
superseded	O
by	O
the	O
GUID	B-Application
Partition	I-Application
Table	I-Application
(	O
GPT	O
)	O
.	O
</s>
<s>
Specifically	O
,	O
it	O
does	O
not	O
provide	O
backward	O
compatibility	O
with	O
operating	B-General_Concept
systems	I-General_Concept
that	O
do	O
not	O
support	O
the	O
GPT	O
scheme	O
as	O
well	O
.	O
</s>
<s>
Meanwhile	O
,	O
multiple	O
forms	O
of	O
hybrid	O
MBRs	O
have	O
been	O
designed	O
and	O
implemented	O
by	O
third	O
parties	O
in	O
order	O
to	O
maintain	O
partitions	B-Application
located	O
in	O
the	O
first	O
physical	O
2TiB	O
of	O
a	O
disk	O
in	O
both	O
partitioning	B-Application
schemes	O
"	O
in	O
parallel	O
"	O
and/or	O
to	O
allow	O
older	O
operating	B-General_Concept
systems	I-General_Concept
to	O
boot	O
off	O
GPT	B-Application
partitions	I-Application
as	O
well	O
.	O
</s>
<s>
The	O
MBR	O
consists	O
of	O
512	O
or	O
more	O
bytes	B-Application
located	O
in	O
the	O
first	O
sector	B-Application
of	O
the	O
drive	O
.	O
</s>
<s>
A	O
partition	B-Application
table	O
describing	O
the	O
partitions	B-Application
of	O
a	O
storage	O
device	O
.	O
</s>
<s>
In	O
this	O
context	O
the	O
boot	B-Device
sector	I-Device
may	O
also	O
be	O
called	O
a	O
partition	B-Device
sector	I-Device
.	O
</s>
<s>
Bootstrap	O
code	O
:	O
Instructions	O
to	O
identify	O
the	O
configured	O
bootable	O
partition	B-Application
,	O
then	O
load	O
and	O
execute	O
its	O
volume	B-Device
boot	I-Device
record	I-Device
(	O
VBR	B-Device
)	O
as	O
a	O
chain	B-Operating_System
loader	I-Operating_System
.	O
</s>
<s>
IBM	B-Operating_System
PC	I-Operating_System
DOS	I-Operating_System
2.0	O
introduced	O
the	O
FDISK	B-Application
utility	O
to	O
set	O
up	O
and	O
maintain	O
MBR	O
partitions	B-Application
.	O
</s>
<s>
When	O
a	O
storage	O
device	O
has	O
been	O
partitioned	B-Application
according	O
to	O
this	O
scheme	O
,	O
its	O
MBR	O
contains	O
a	O
partition	B-Application
table	O
describing	O
the	O
locations	O
,	O
sizes	O
,	O
and	O
other	O
attributes	O
of	O
linear	O
regions	O
referred	O
to	O
as	O
partitions	B-Application
.	O
</s>
<s>
The	O
partitions	B-Application
themselves	O
may	O
also	O
contain	O
data	O
to	O
describe	O
more	O
complex	O
partitioning	B-Application
schemes	O
,	O
such	O
as	O
extended	B-Application
boot	I-Application
records	I-Application
(	O
EBRs	O
)	O
,	O
BSD	B-Application
disklabels	I-Application
,	O
or	O
Logical	O
Disk	B-Application
Manager	I-Application
metadata	O
partitions	B-Application
.	O
</s>
<s>
The	O
MBR	O
is	O
not	O
located	O
in	O
a	O
partition	B-Application
;	O
it	O
is	O
located	O
at	O
a	O
first	O
sector	B-Application
of	O
the	O
device	O
(	O
physical	O
offset	O
0	O
)	O
,	O
preceding	O
the	O
first	O
partition	B-Application
.	O
</s>
<s>
(	O
The	O
boot	B-Device
sector	I-Device
present	O
on	O
a	O
non-partitioned	O
device	O
or	O
within	O
an	O
individual	O
partition	B-Application
is	O
called	O
a	O
volume	B-Device
boot	I-Device
record	I-Device
instead	O
.	O
)	O
</s>
<s>
In	O
cases	O
where	O
the	O
computer	O
is	O
running	O
a	O
DDO	B-Device
BIOS	I-Device
overlay	I-Device
or	O
boot	B-Application
manager	I-Application
,	O
the	O
partition	B-Application
table	O
may	O
be	O
moved	O
to	O
some	O
other	O
physical	O
location	O
on	O
the	O
device	O
;	O
e.g.	O
,	O
Ontrack	B-Application
Disk	I-Application
Manager	I-Application
often	O
placed	O
a	O
copy	O
of	O
the	O
original	O
MBR	O
contents	O
in	O
the	O
second	O
sector	B-Application
,	O
then	O
hid	O
itself	O
from	O
any	O
subsequently	O
booted	O
OS	O
or	O
application	O
,	O
so	O
the	O
MBR	O
copy	O
was	O
treated	O
as	O
if	O
it	O
were	O
still	O
residing	O
in	O
the	O
first	O
sector	B-Application
.	O
</s>
<s>
By	O
convention	O
,	O
there	O
are	O
exactly	O
four	O
primary	O
partition	B-Application
table	O
entries	O
in	O
the	O
MBR	O
partition	B-Application
table	O
scheme	O
,	O
although	O
some	O
operating	B-General_Concept
systems	I-General_Concept
and	O
system	O
tools	O
extended	O
this	O
to	O
five	O
(	O
Advanced	O
Active	O
Partitions	B-Application
(	O
AAP	O
)	O
with	O
PTS-DOS	B-Operating_System
6.60	O
and	O
DR-DOS	B-Application
7.07	O
)	O
,	O
eight	O
(	O
AST	O
and	O
NEC	O
MS-DOS	B-Application
3.x	O
as	O
well	O
as	O
Storage	O
Dimensions	O
SpeedStor	O
)	O
,	O
or	O
even	O
sixteen	O
entries	O
(	O
with	O
Ontrack	B-Application
Disk	I-Application
Manager	I-Application
)	O
.	O
</s>
<s>
+	O
Structure	O
of	O
a	O
modern	O
standard	O
MBR	O
Address	B-General_Concept
Description	O
Size(bytes )	O
0x0000	O
(	O
0	O
)	O
Bootstrap	O
code	O
area	O
(	O
part	O
1	O
)	O
218	O
0x00DA	O
(	O
218	O
)	O
0x0000	O
Disk	O
timestamp	O
(	O
optional	O
;	O
Windows	O
95B/98/98SE/ME	O
(	O
MS-DOS	B-Application
7.1	O
–	O
8.0	O
)	O
.	O
</s>
<s>
+	O
Structure	O
of	O
NEWLDR	O
MBR	O
Address	B-General_Concept
Description	O
Size(bytes )	O
0x0000	O
(	O
0	O
)	O
JMPS	O
(	O
EBhex	O
)	O
/	O
NEWLDR	O
record	O
size	O
(	O
often	O
0x0A/0x16/0x1C	O
for	O
code	O
start	O
at	O
0x000C/0x0018/0x001E	O
)	O
NEWLDR	O
record	O
(	O
optional	O
)	O
2	O
0x0002	O
(	O
2	O
)	O
"	O
NEWLDR	O
"	O
signature	O
6	O
0x0008	O
(	O
8	O
)	O
LOADER	O
physical	O
drive	O
and	O
boot	O
flag	O
(	O
0x80-0xFE	O
,	O
0x00-0x7E	O
,	O
0xFF	O
,	O
0x7F	O
)	O
(	O
if	O
not	O
used	O
,	O
this	O
and	O
following	O
3	O
bytes	B-Application
must	O
be	O
all	O
0	O
)	O
1	O
0x0009	O
(	O
9	O
)	O
CHS	B-Device
address	I-Device
of	O
LOADER	O
boot	B-Device
sector	I-Device
or	O
image	O
file	O
(	O
f.e.	O
</s>
<s>
+	O
Layout	O
of	O
one	O
16-byte	O
partition	B-Application
entry	O
(	O
all	O
multi-byte	O
fields	O
are	O
little-endian	O
)	O
Offset(bytes )	O
Fieldlength	O
Description	O
1	O
byte	B-Application
Status	O
physical	O
drive	O
(	O
bit	O
7	O
set	O
is	O
for	O
active	O
or	O
bootable	O
,	O
old	O
MBRs	O
only	O
accept	O
,	O
means	O
inactive	O
,	O
and	O
–	O
stand	O
for	O
invalid	O
)	O
3	O
bytes	B-Application
CHS	B-Device
address	I-Device
of	O
first	O
absolute	O
sector	B-Application
in	O
partition	B-Application
.	O
</s>
<s>
The	O
format	O
is	O
described	O
by	O
three	O
bytes	B-Application
,	O
see	O
the	O
next	O
three	O
rows	O
.	O
</s>
<s>
1	O
byte	B-Application
h7	O
–	O
0	O
head	O
x	O
x	O
x	O
x	O
x	O
x	O
x	O
x	O
1	O
byte	B-Application
c9	O
–	O
8	O
s5	O
–	O
0	O
sector	B-Application
in	O
bits	O
5	O
–	O
0	O
;	O
bits	O
7	O
–	O
6	O
are	O
high	O
bits	O
of	O
cylinder	O
x	O
x	O
x	O
x	O
x	O
x	O
x	O
x	O
1	O
byte	B-Application
c7	O
–	O
0	O
bits	O
7	O
–	O
0	O
of	O
cylinder	O
x	O
x	O
x	O
x	O
x	O
x	O
x	O
x	O
1	O
byte	B-Application
Partition	B-Application
type	I-Application
3	O
bytes	B-Application
CHS	B-Device
address	I-Device
of	O
last	O
absolute	O
sector	B-Application
in	O
partition	B-Application
.	O
</s>
<s>
The	O
format	O
is	O
described	O
by	O
3	O
bytes	B-Application
,	O
see	O
the	O
next	O
3	O
rows	O
.	O
</s>
<s>
An	O
artifact	O
of	O
hard	B-Device
disk	I-Device
technology	O
from	O
the	O
era	O
of	O
the	O
PC	B-Device
XT	I-Device
,	O
the	O
partition	B-Application
table	O
subdivides	O
a	O
storage	O
medium	O
using	O
units	O
of	O
cylinders	O
,	O
heads	B-Device
,	O
and	O
sectors	B-Application
(	O
CHS	B-Device
addressing	O
)	O
.	O
</s>
<s>
These	O
values	O
no	O
longer	O
correspond	O
to	O
their	O
namesakes	O
in	O
modern	O
disk	O
drives	O
,	O
as	O
well	O
as	O
being	O
irrelevant	O
in	O
other	O
devices	O
such	O
as	O
solid-state	B-Device
drives	I-Device
,	O
which	O
do	O
not	O
physically	O
have	O
cylinders	O
or	O
heads	B-Device
.	O
</s>
<s>
In	O
the	O
CHS	B-Device
scheme	O
,	O
sector	B-Application
indices	O
have	O
(	O
almost	O
)	O
always	O
begun	O
with	O
sector	B-Application
1	O
rather	O
than	O
sector	B-Application
0	O
by	O
convention	O
,	O
and	O
due	O
to	O
an	O
error	O
in	O
all	O
versions	O
of	O
MS-DOS/PC	O
DOS	O
up	O
to	O
including	O
7.10	O
,	O
the	O
number	O
of	O
heads	B-Device
is	O
generally	O
limited	O
to	O
255	O
instead	O
of	O
256	O
.	O
</s>
<s>
When	O
a	O
CHS	B-Device
address	I-Device
is	O
too	O
large	O
to	O
fit	O
into	O
these	O
fields	O
,	O
the	O
tuple	B-Application
(	O
1023	O
,	O
254	O
,	O
63	O
)	O
is	O
typically	O
used	O
today	O
,	O
although	O
on	O
older	O
systems	O
,	O
and	O
with	O
older	O
disk	O
tools	O
,	O
the	O
cylinder	O
value	O
often	O
wrapped	O
around	O
modulo	O
the	O
CHS	B-Device
barrier	O
near	O
8	O
GB	O
,	O
causing	O
ambiguity	O
and	O
risks	O
of	O
data	O
corruption	O
.	O
</s>
<s>
(	O
If	O
the	O
situation	O
involves	O
a	O
"	O
protective	O
"	O
MBR	O
on	O
a	O
disk	O
with	O
a	O
GPT	O
,	O
Intel	O
's	O
Extensible	B-Architecture
Firmware	I-Architecture
Interface	I-Architecture
specification	O
requires	O
that	O
the	O
tuple	B-Application
(	O
1023	O
,	O
255	O
,	O
63	O
)	O
be	O
used	O
.	O
)	O
</s>
<s>
The	O
10-bit	O
cylinder	O
value	O
is	O
recorded	O
within	O
two	O
bytes	B-Application
in	O
order	O
to	O
facilitate	O
making	O
calls	O
to	O
the	O
original/legacy	O
INT	B-Device
13h	I-Device
BIOS	B-Operating_System
disk	O
access	O
routines	O
,	O
where	O
16	O
bits	O
were	O
divided	O
into	O
sector	B-Application
and	O
cylinder	O
parts	O
,	O
and	O
not	O
on	O
byte	B-Application
boundaries	O
.	O
</s>
<s>
Due	O
to	O
the	O
limits	O
of	O
CHS	B-Device
addressing	O
,	O
a	O
transition	O
was	O
made	O
to	O
using	O
LBA	B-Device
,	O
or	O
logical	B-Device
block	I-Device
addressing	I-Device
.	O
</s>
<s>
Both	O
the	O
partition	B-Application
length	O
and	O
partition	B-Application
start	O
address	B-General_Concept
are	O
sector	B-Application
values	O
stored	O
in	O
the	O
partition	B-Application
table	O
entries	O
as	O
32-bit	O
quantities	O
.	O
</s>
<s>
The	O
sector	B-Application
size	O
used	O
to	O
be	O
considered	O
fixed	O
at	O
512	O
(	O
29	O
)	O
bytes	B-Application
,	O
and	O
a	O
broad	O
range	O
of	O
important	O
components	O
including	O
chipsets	B-Device
,	O
boot	B-Device
sectors	I-Device
,	O
operating	B-General_Concept
systems	I-General_Concept
,	O
database	B-General_Concept
engines	I-General_Concept
,	O
partitioning	B-Application
tools	O
,	O
backup	B-Protocol
and	O
file	B-Application
system	I-Application
utilities	O
and	O
other	O
software	O
had	O
this	O
value	O
hard-coded	O
.	O
</s>
<s>
Since	O
the	O
end	O
of	O
2009	O
,	O
disk	O
drives	O
employing	O
4096-byte	O
sectors	B-Application
(	O
4Kn	O
or	O
Advanced	B-Device
Format	I-Device
)	O
have	O
been	O
available	O
,	O
although	O
the	O
size	O
of	O
the	O
sector	B-Application
for	O
some	O
of	O
these	O
drives	O
was	O
still	O
reported	O
as	O
512	B-Application
bytes	I-Application
to	O
the	O
host	O
system	O
through	O
conversion	O
in	O
the	O
hard-drive	B-Device
firmware	B-Application
and	O
referred	O
to	O
as	O
512	O
emulation	O
drives	O
(	O
512e	O
)	O
.	O
</s>
<s>
Since	O
block	O
addresses	O
and	O
sizes	O
are	O
stored	O
in	O
the	O
partition	B-Application
table	O
of	O
an	O
MBR	O
using	O
32	O
bits	O
,	O
the	O
maximum	O
size	O
,	O
as	O
well	O
as	O
the	O
highest	O
start	O
address	B-General_Concept
,	O
of	O
a	O
partition	B-Application
using	O
drives	O
that	O
have	O
512-byte	O
sectors	B-Application
(	O
actual	O
or	O
emulated	O
)	O
cannot	O
exceed	O
2	O
TiB−512	O
bytes	B-Application
(	O
bytes	B-Application
or	O
(	O
232−1	O
)	O
sectors×	O
512	O
(	O
29	O
)	O
bytes	B-Application
per	O
sector	B-Application
)	O
.	O
</s>
<s>
Since	O
partitioning	B-Application
information	O
is	O
stored	O
in	O
the	O
MBR	O
partition	B-Application
table	O
using	O
a	O
beginning	O
block	O
address	B-General_Concept
and	O
a	O
length	O
,	O
it	O
may	O
in	O
theory	O
be	O
possible	O
to	O
define	O
partitions	B-Application
in	O
such	O
a	O
way	O
that	O
the	O
allocated	O
space	O
for	O
a	O
disk	O
with	O
512-byte	O
sectors	B-Application
gives	O
a	O
total	O
size	O
approaching	O
4	O
TiB	O
,	O
if	O
all	O
but	O
one	O
partition	B-Application
are	O
located	O
below	O
the	O
2TiB	O
limit	O
and	O
the	O
last	O
one	O
is	O
assigned	O
as	O
starting	O
at	O
or	O
close	O
to	O
block	O
232−1	O
and	O
specify	O
the	O
size	O
as	O
up	O
to	O
232−1	O
,	O
thereby	O
defining	O
a	O
partition	B-Application
that	O
requires	O
33	O
rather	O
than	O
32	O
bits	O
for	O
the	O
sector	B-Application
address	B-General_Concept
to	O
be	O
accessed	O
.	O
</s>
<s>
However	O
,	O
in	O
practice	O
,	O
only	O
certain	O
LBA-48-enabled	O
operating	B-General_Concept
systems	I-General_Concept
,	O
including	O
Linux	B-Application
,	O
FreeBSD	O
and	O
Windows	B-Device
7	I-Device
that	O
use	O
64-bit	O
sector	B-Application
addresses	O
internally	O
actually	O
support	O
this	O
.	O
</s>
<s>
Due	O
to	O
code	O
space	O
constraints	O
and	O
the	O
nature	O
of	O
the	O
MBR	O
partition	B-Application
table	O
to	O
only	O
support	O
32	O
bits	O
,	O
boot	B-Device
sectors	I-Device
,	O
even	O
if	O
enabled	O
to	O
support	O
LBA-48	O
rather	O
than	O
LBA-28	B-Device
,	O
often	O
use	O
32-bit	O
calculations	O
,	O
unless	O
they	O
are	O
specifically	O
designed	O
to	O
support	O
the	O
full	O
address	B-General_Concept
range	O
of	O
LBA-48	O
or	O
are	O
intended	O
to	O
run	O
on	O
64-bit	O
platforms	O
only	O
.	O
</s>
<s>
Any	O
boot	O
code	O
or	O
operating	B-General_Concept
system	I-General_Concept
using	O
32-bit	O
sector	B-Application
addresses	O
internally	O
would	O
cause	O
addresses	O
to	O
wrap	O
around	O
accessing	O
this	O
partition	B-Application
and	O
thereby	O
result	O
in	O
serious	O
data	O
corruption	O
over	O
all	O
partitions	B-Application
.	O
</s>
<s>
For	O
disks	O
that	O
present	O
a	O
sector	B-Application
size	O
other	O
than	O
512	B-Application
bytes	I-Application
,	O
such	O
as	O
USB	B-Protocol
external	B-Device
drives	I-Device
,	O
there	O
are	O
limitations	O
as	O
well	O
.	O
</s>
<s>
A	O
sector	B-Application
size	O
of	O
4096	O
results	O
in	O
an	O
eight-fold	O
increase	O
in	O
the	O
size	O
of	O
a	O
partition	B-Application
that	O
can	O
be	O
defined	O
using	O
MBR	O
,	O
allowing	O
partitions	B-Application
up	O
to	O
16	O
TiB	O
(	O
232×4096bytes	O
)	O
in	O
size	O
.	O
</s>
<s>
Versions	O
of	O
Windows	O
more	O
recent	O
than	O
Windows	B-Application
XP	I-Application
support	O
the	O
larger	O
sector	B-Application
sizes	O
,	O
as	O
well	O
as	O
Mac	O
OS	O
X	O
,	O
and	O
Linux	B-Application
has	O
supported	O
larger	O
sector	B-Application
sizes	O
since	O
2.6.31	O
or	O
2.6.32	O
,	O
but	O
issues	O
with	O
boot	B-Application
loaders	I-Application
,	O
partitioning	B-Application
tools	O
and	O
computer	O
BIOS	B-Operating_System
implementations	O
present	O
certain	O
limitations	O
,	O
since	O
they	O
are	O
often	O
hard-wired	O
to	O
reserve	O
only	O
512	B-Application
bytes	I-Application
for	O
sector	B-Application
buffers	O
,	O
causing	O
memory	O
to	O
become	O
overwritten	O
for	O
larger	O
sector	B-Application
sizes	O
.	O
</s>
<s>
Where	O
a	O
data	O
storage	O
device	O
has	O
been	O
partitioned	B-Application
with	O
the	O
GPT	O
scheme	O
,	O
the	O
master	B-Device
boot	I-Device
record	I-Device
will	O
still	O
contain	O
a	O
partition	B-Application
table	O
,	O
but	O
its	O
only	O
purpose	O
is	O
to	O
indicate	O
the	O
existence	O
of	O
the	O
GPT	O
and	O
to	O
prevent	O
utility	O
programs	O
that	O
understand	O
only	O
the	O
MBR	O
partition	B-Application
table	O
scheme	O
from	O
creating	O
any	O
partitions	B-Application
in	O
what	O
they	O
would	O
otherwise	O
see	O
as	O
free	O
space	O
on	O
the	O
disk	O
,	O
thereby	O
accidentally	O
erasing	O
the	O
GPT	O
.	O
</s>
<s>
On	O
IBM	O
PC-compatible	O
computers	O
,	O
the	O
bootstrapping	B-Application
firmware	B-Application
(	O
contained	O
within	O
the	O
ROM	B-Operating_System
BIOS	I-Operating_System
)	O
loads	O
and	O
executes	O
the	O
master	B-Device
boot	I-Device
record	I-Device
.	O
</s>
<s>
The	O
PC/XT	B-Device
(	O
type	O
5160	O
)	O
used	O
an	O
Intel	B-Device
8088	I-Device
microprocessor	O
.	O
</s>
<s>
In	O
order	O
to	O
remain	O
compatible	O
,	O
all	O
x86	B-Operating_System
architecture	I-Operating_System
systems	O
start	O
with	O
the	O
microprocessor	O
in	O
an	O
operating	O
mode	O
referred	O
to	O
as	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
The	O
BIOS	B-Operating_System
reads	O
the	O
MBR	O
from	O
the	O
storage	O
device	O
into	O
physical	O
memory	O
,	O
and	O
then	O
it	O
directs	O
the	O
microprocessor	O
to	O
the	O
start	O
of	O
the	O
boot	O
code	O
.	O
</s>
<s>
Since	O
the	O
BIOS	B-Operating_System
runs	O
in	O
real	B-Application
mode	I-Application
,	O
the	O
processor	O
is	O
in	O
real	B-Application
mode	I-Application
when	O
the	O
MBR	O
program	O
begins	O
to	O
execute	O
,	O
and	O
so	O
the	O
beginning	O
of	O
the	O
MBR	O
is	O
expected	O
to	O
contain	O
real-mode	B-Application
machine	B-Language
code	I-Language
.	O
</s>
<s>
Since	O
the	O
BIOS	B-Operating_System
bootstrap	O
routine	O
loads	O
and	O
runs	O
exactly	O
one	O
sector	B-Application
from	O
the	O
physical	O
disk	O
,	O
having	O
the	O
partition	B-Application
table	O
in	O
the	O
MBR	O
with	O
the	O
boot	O
code	O
simplifies	O
the	O
design	O
of	O
the	O
MBR	O
program	O
.	O
</s>
<s>
It	O
contains	O
a	O
small	O
program	O
that	O
loads	O
the	O
Volume	B-Device
Boot	I-Device
Record	I-Device
(	O
VBR	B-Device
)	O
of	O
the	O
targeted	O
partition	B-Application
.	O
</s>
<s>
Control	O
is	O
then	O
passed	O
to	O
this	O
code	O
,	O
which	O
is	O
responsible	O
for	O
loading	O
the	O
actual	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
This	O
process	O
is	O
known	O
as	O
chain	B-Operating_System
loading	I-Operating_System
.	O
</s>
<s>
Popular	O
MBR	O
code	O
programs	O
were	O
created	O
for	O
booting	O
PC	B-Operating_System
DOS	I-Operating_System
and	O
MS-DOS	B-Application
,	O
and	O
similar	O
boot	O
code	O
remains	O
in	O
wide	O
use	O
.	O
</s>
<s>
These	O
boot	B-Device
sectors	I-Device
expect	O
the	O
FDISK	B-Application
partition	B-Application
table	O
scheme	O
to	O
be	O
in	O
use	O
and	O
scans	O
the	O
list	O
of	O
partitions	B-Application
in	O
the	O
MBR	O
's	O
embedded	O
partition	B-Application
table	O
to	O
find	O
the	O
only	O
one	O
that	O
is	O
marked	O
with	O
the	O
active	O
flag	O
.	O
</s>
<s>
It	O
then	O
loads	O
and	O
runs	O
the	O
volume	B-Device
boot	I-Device
record	I-Device
(	O
VBR	B-Device
)	O
of	O
the	O
active	O
partition	B-Application
.	O
</s>
<s>
There	O
are	O
alternative	O
boot	O
code	O
implementations	O
,	O
some	O
of	O
which	O
are	O
installed	O
by	O
boot	B-Application
managers	I-Application
,	O
which	O
operate	O
in	O
a	O
variety	O
of	O
ways	O
.	O
</s>
<s>
Some	O
MBR	O
code	O
loads	O
additional	O
code	O
for	O
a	O
boot	B-Application
manager	I-Application
from	O
the	O
first	O
track	O
of	O
the	O
disk	O
,	O
which	O
it	O
assumes	O
to	O
be	O
"	O
free	O
"	O
space	O
that	O
is	O
not	O
allocated	O
to	O
any	O
disk	B-Application
partition	I-Application
,	O
and	O
executes	O
it	O
.	O
</s>
<s>
A	O
MBR	O
program	O
may	O
interact	O
with	O
the	O
user	O
to	O
determine	O
which	O
partition	B-Application
on	O
which	O
drive	O
should	O
boot	O
,	O
and	O
may	O
transfer	O
control	O
to	O
the	O
MBR	O
of	O
a	O
different	O
drive	O
.	O
</s>
<s>
Other	O
MBR	O
code	O
contains	O
a	O
list	O
of	O
disk	O
locations	O
(	O
often	O
corresponding	O
to	O
the	O
contents	O
of	O
files	B-Operating_System
in	O
a	O
filesystem	B-Application
)	O
of	O
the	O
remainder	O
of	O
the	O
boot	B-Application
manager	I-Application
code	O
to	O
load	O
and	O
to	O
execute	O
.	O
</s>
<s>
(	O
The	O
first	O
relies	O
on	O
behavior	O
that	O
is	O
not	O
universal	O
across	O
all	O
disk	B-Application
partitioning	I-Application
utilities	O
,	O
most	O
notably	O
those	O
that	O
read	O
and	O
write	O
GPTs	O
.	O
</s>
<s>
On	O
machines	O
that	O
do	O
not	O
use	O
x86	B-Operating_System
processors	O
,	O
or	O
on	O
x86	B-Operating_System
machines	O
with	O
non-BIOS	O
firmware	B-Application
such	O
as	O
Open	B-Architecture
Firmware	I-Architecture
or	O
Extensible	B-Architecture
Firmware	I-Architecture
Interface	I-Architecture
(	O
EFI	O
)	O
firmware	B-Application
,	O
this	O
design	O
is	O
unsuitable	O
,	O
and	O
the	O
MBR	O
is	O
not	O
used	O
as	O
part	O
of	O
the	O
system	O
bootstrap	O
.	O
</s>
<s>
EFI	O
firmware	B-Application
is	O
instead	O
capable	O
of	O
directly	O
understanding	O
the	O
GPT	O
partitioning	B-Application
scheme	O
and	O
the	O
FAT	B-Application
filesystem	I-Application
format	O
,	O
and	O
loads	O
and	O
runs	O
programs	O
held	O
as	O
files	B-Operating_System
in	O
the	O
EFI	B-Device
System	I-Device
partition	I-Device
.	O
</s>
<s>
The	O
MBR	O
will	O
be	O
involved	O
only	O
insofar	O
as	O
it	O
might	O
contain	O
a	O
partition	B-Application
table	O
for	O
compatibility	O
purposes	O
if	O
the	O
GPT	B-Application
partition	I-Application
table	O
scheme	O
has	O
been	O
used	O
.	O
</s>
<s>
There	O
is	O
some	O
MBR	O
replacement	O
code	O
that	O
emulates	O
EFI	O
firmware	B-Application
's	O
bootstrap	O
,	O
which	O
makes	O
non-EFI	O
machines	O
capable	O
of	O
booting	O
from	O
disks	O
using	O
the	O
GPT	O
partitioning	B-Application
scheme	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
bootstrap	O
code	O
and	O
a	O
partition	B-Application
table	O
,	O
master	B-Device
boot	I-Device
records	I-Device
may	O
contain	O
a	O
disk	O
signature	O
.	O
</s>
<s>
This	O
is	O
a	O
32-bit	O
value	O
that	O
is	O
intended	O
to	O
identify	O
uniquely	O
the	O
disk	O
medium	O
(	O
as	O
opposed	O
to	O
the	O
disk	O
unit	O
—	O
the	O
two	O
not	O
necessarily	O
being	O
the	O
same	O
for	O
removable	O
hard	B-Device
disks	I-Device
)	O
.	O
</s>
<s>
The	O
disk	O
signature	O
was	O
introduced	O
by	O
Windows	B-Device
NT	I-Device
version	O
3.5	O
,	O
but	O
it	O
is	O
now	O
used	O
by	O
several	O
operating	B-General_Concept
systems	I-General_Concept
,	O
including	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
version	O
2.6	O
and	O
later	O
.	O
</s>
<s>
Linux	B-Application
tools	O
can	O
use	O
the	O
NT	O
disk	O
signature	O
to	O
determine	O
which	O
disk	O
the	O
machine	O
booted	O
from	O
.	O
</s>
<s>
Windows	B-Device
NT	I-Device
(	O
and	O
later	O
Microsoft	O
operating	B-General_Concept
systems	I-General_Concept
)	O
uses	O
the	O
disk	O
signature	O
as	O
an	O
index	O
to	O
all	O
the	O
partitions	B-Application
on	O
any	O
disk	O
ever	O
connected	O
to	O
the	O
computer	O
under	O
that	O
OS	O
;	O
these	O
signatures	O
are	O
kept	O
in	O
Windows	B-Protocol
Registry	I-Protocol
keys	O
,	O
primarily	O
for	O
storing	O
the	O
persistent	O
mappings	O
between	O
disk	B-Application
partitions	I-Application
and	O
drive	O
letters	O
.	O
</s>
<s>
It	O
may	O
also	O
be	O
used	O
in	O
Windows	B-Device
NT	I-Device
BOOT.INI	O
files	B-Operating_System
(	O
though	O
most	O
do	O
not	O
)	O
,	O
to	O
describe	O
the	O
location	O
of	O
bootable	O
Windows	B-Device
NT	I-Device
(	O
or	O
later	O
)	O
partitions	B-Application
.	O
</s>
<s>
One	O
key	O
(	O
among	O
many	O
)	O
,	O
where	O
NT	O
disk	O
signatures	O
appear	O
in	O
a	O
Windows	O
2000/XP	O
registry	O
,	O
is	O
:	O
</s>
<s>
If	O
a	O
disk	O
's	O
signature	O
stored	O
in	O
the	O
MBR	O
was	O
(	O
in	O
that	O
order	O
)	O
and	O
its	O
first	O
partition	B-Application
corresponded	O
with	O
logical	B-Application
drive	I-Application
C	O
:	O
under	O
Windows	O
,	O
then	O
the	O
REG_BINARY	O
data	O
under	O
the	O
key	O
value	O
\DosDevices\C	O
:	O
would	O
be	O
:	O
</s>
<s>
The	O
first	O
four	O
bytes	B-Application
are	O
said	O
disk	O
signature	O
.	O
</s>
<s>
(	O
In	O
other	O
keys	O
,	O
these	O
bytes	B-Application
may	O
appear	O
in	O
reverse	O
order	O
from	O
that	O
found	O
in	O
the	O
MBR	O
sector	B-Application
.	O
)	O
</s>
<s>
These	O
are	O
followed	O
by	O
eight	O
more	O
bytes	B-Application
,	O
forming	O
a	O
64-bit	O
integer	O
,	O
in	O
little-endian	O
notation	O
,	O
which	O
are	O
used	O
to	O
locate	O
the	O
byte	B-Application
offset	O
of	O
this	O
partition	B-Application
.	O
</s>
<s>
Under	O
the	O
assumption	O
that	O
the	O
drive	O
in	O
question	O
reports	O
a	O
sector	B-Application
size	O
of	O
512	B-Application
bytes	I-Application
,	O
then	O
dividing	O
this	O
byte	B-Application
offset	O
by	O
512	O
results	O
in	O
63	O
,	O
which	O
is	O
the	O
physical	O
sector	B-Application
number	O
(	O
or	O
LBA	B-Device
)	O
containing	O
the	O
first	O
sector	B-Application
of	O
the	O
partition	B-Application
(	O
unlike	O
the	O
sector	B-Application
count	O
used	O
in	O
the	O
sectors	B-Application
value	O
of	O
CHS	B-Device
tuples	B-Application
,	O
which	O
counts	O
from	O
one	O
,	O
the	O
absolute	O
or	O
LBA	B-Device
sector	B-Application
value	O
starts	O
counting	O
from	O
zero	O
)	O
.	O
</s>
<s>
If	O
this	O
disk	O
had	O
another	O
partition	B-Application
with	O
the	O
values	O
following	O
the	O
disk	O
signature	O
(	O
under	O
,	O
e.g.	O
,	O
the	O
key	O
value	O
\DosDevices\D	O
:	O
)	O
,	O
it	O
would	O
begin	O
at	O
byte	B-Application
offset	O
(	O
)	O
,	O
which	O
is	O
also	O
the	O
first	O
byte	B-Application
of	O
physical	O
sector	B-Application
.	O
</s>
<s>
Starting	O
with	O
Windows	B-Application
Vista	I-Application
,	O
the	O
disk	O
signature	O
is	O
also	O
stored	O
in	O
the	O
Boot	O
Configuration	O
Data	O
(	O
BCD	O
)	O
store	O
,	O
and	O
the	O
boot	O
process	O
depends	O
on	O
it	O
.	O
</s>
<s>
Unless	O
Windows	O
is	O
forced	O
to	O
use	O
the	O
overlapping	O
part	O
of	O
the	O
LBA	B-Device
address	B-General_Concept
of	O
the	O
Advanced	O
Active	O
Partition	B-Application
entry	O
as	O
pseudo-disk	O
signature	O
,	O
Windows	O
 '	O
usage	O
is	O
conflictive	O
with	O
the	O
Advanced	O
Active	O
Partition	B-Application
feature	O
of	O
PTS-DOS	B-Operating_System
7	O
and	O
DR-DOS	B-Application
7.07	O
,	O
in	O
particular	O
if	O
their	O
boot	O
code	O
is	O
located	O
outside	O
the	O
first	O
8GB	O
of	O
the	O
disk	O
,	O
so	O
that	O
LBA	B-Device
addressing	O
must	O
be	O
used	O
.	O
</s>
<s>
The	O
MBR	O
originated	O
in	O
the	O
PC	B-Device
XT	I-Device
.	O
</s>
<s>
IBM	O
PC-compatible	O
computers	O
are	O
little-endian	O
,	O
which	O
means	O
the	O
processor	O
stores	O
numeric	O
values	O
spanning	O
two	O
or	O
more	O
bytes	B-Application
in	O
memory	O
least	O
significant	O
byte	B-Application
first	O
.	O
</s>
<s>
Thus	O
,	O
the	O
MBR	O
signature	O
will	O
appear	O
in	O
a	O
disk	B-Application
editor	I-Application
as	O
the	O
sequence	O
55	O
AA	O
.	O
</s>
<s>
The	O
bootstrap	O
sequence	O
in	O
the	O
BIOS	B-Operating_System
will	O
load	O
the	O
first	O
valid	O
MBR	O
that	O
it	O
finds	O
into	O
the	O
computer	O
's	O
physical	O
memory	O
at	O
address	B-General_Concept
:	O
.	O
</s>
<s>
The	O
last	O
instruction	O
executed	O
in	O
the	O
BIOS	B-Operating_System
code	O
will	O
be	O
a	O
"	O
jump	O
"	O
to	O
that	O
address	B-General_Concept
in	O
order	O
to	O
direct	O
execution	O
to	O
the	O
beginning	O
of	O
the	O
MBR	O
copy	O
.	O
</s>
<s>
The	O
primary	O
validation	O
for	O
most	O
BIOSes	O
is	O
the	O
signature	O
at	O
offset	O
,	O
although	O
a	O
BIOS	B-Operating_System
implementer	O
may	O
choose	O
to	O
include	O
other	O
checks	O
,	O
such	O
as	O
verifying	O
that	O
the	O
MBR	O
contains	O
a	O
valid	O
partition	B-Application
table	O
without	O
entries	O
referring	O
to	O
sectors	B-Application
beyond	O
the	O
reported	O
capacity	O
of	O
the	O
disk	O
.	O
</s>
<s>
To	O
the	O
BIOS	B-Operating_System
,	O
removable	O
(	O
e.g.	O
</s>
<s>
floppy	B-Device
)	O
and	O
fixed	B-Device
disks	I-Device
are	O
essentially	O
the	O
same	O
.	O
</s>
<s>
For	O
either	O
,	O
the	O
BIOS	B-Operating_System
reads	O
the	O
first	O
physical	O
sector	B-Application
of	O
the	O
media	O
into	O
RAM	O
at	O
absolute	O
address	B-General_Concept
,	O
checks	O
the	O
signature	O
in	O
the	O
last	O
two	O
bytes	B-Application
of	O
the	O
loaded	O
sector	B-Application
,	O
and	O
then	O
,	O
if	O
the	O
correct	O
signature	O
is	O
found	O
,	O
transfers	O
control	O
to	O
the	O
first	O
byte	B-Application
of	O
the	O
sector	B-Application
with	O
a	O
jump	O
(	O
JMP	O
)	O
instruction	O
.	O
</s>
<s>
The	O
only	O
real	O
distinction	O
that	O
the	O
BIOS	B-Operating_System
makes	O
is	O
that	O
(	O
by	O
default	O
,	O
or	O
if	O
the	O
boot	O
order	O
is	O
not	O
configurable	O
)	O
it	O
attempts	O
to	O
boot	O
from	O
the	O
first	O
removable	O
disk	O
before	O
trying	O
to	O
boot	O
from	O
the	O
first	O
fixed	B-Device
disk	I-Device
.	O
</s>
<s>
From	O
the	O
perspective	O
of	O
the	O
BIOS	B-Operating_System
,	O
the	O
action	O
of	O
the	O
MBR	O
loading	O
a	O
volume	B-Device
boot	I-Device
record	I-Device
into	O
RAM	O
is	O
exactly	O
the	O
same	O
as	O
the	O
action	O
of	O
a	O
floppy	B-Device
disk	I-Device
volume	B-Device
boot	I-Device
record	I-Device
loading	O
the	O
object	O
code	O
of	O
an	O
operating	B-General_Concept
system	I-General_Concept
loader	O
into	O
RAM	O
.	O
</s>
<s>
In	O
either	O
case	O
,	O
the	O
program	O
that	O
the	O
BIOS	B-Operating_System
loaded	O
is	O
going	O
about	O
the	O
work	O
of	O
chain	B-Operating_System
loading	I-Operating_System
an	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
While	O
the	O
MBR	O
boot	B-Device
sector	I-Device
code	O
expects	O
to	O
be	O
loaded	O
at	O
physical	O
address	B-General_Concept
:	O
,	O
all	O
the	O
memory	O
from	O
physical	O
address	B-General_Concept
:	O
(	O
address	B-General_Concept
:	O
is	O
the	O
last	O
one	O
used	O
by	O
a	O
Phoenix	O
BIOS	B-Operating_System
)	O
to	O
:	O
,	O
later	O
relaxed	O
to	O
:	O
(	O
and	O
sometimes	O
up	O
to	O
:	O
)	O
the	O
end	O
of	O
the	O
first	O
640	O
KBis	O
available	O
in	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
The	O
INT	O
12h	O
BIOS	B-Device
interrupt	I-Device
call	I-Device
may	O
help	O
in	O
determining	O
how	O
much	O
memory	O
can	O
be	O
allocated	O
safely	O
(	O
by	O
default	O
,	O
it	O
simply	O
reads	O
the	O
base	O
memory	O
size	O
in	O
KB	O
from	O
segment:offset	B-General_Concept
location	O
:	O
,	O
but	O
it	O
may	O
be	O
hooked	O
by	O
other	O
resident	O
pre-boot	O
software	O
like	O
BIOS	B-Operating_System
overlays	O
,	O
RPL	O
code	O
or	O
viruses	O
to	O
reduce	O
the	O
reported	O
amount	O
of	O
available	O
memory	O
in	O
order	O
to	O
keep	O
other	O
boot	O
stage	O
software	O
like	O
boot	B-Device
sectors	I-Device
from	O
overwriting	O
them	O
)	O
.	O
</s>
<s>
The	O
last	O
66bytes	O
of	O
the	O
512-byte	O
MBR	O
are	O
reserved	O
for	O
the	O
partition	B-Application
table	O
and	O
other	O
information	O
,	O
so	O
the	O
MBR	O
boot	B-Device
sector	I-Device
program	O
must	O
be	O
small	O
enough	O
to	O
fit	O
within	O
446bytes	O
of	O
memory	O
or	O
less	O
.	O
</s>
<s>
The	O
MBR	O
code	O
examines	O
the	O
partition	B-Application
table	O
,	O
selects	O
a	O
suitable	O
partition	B-Application
and	O
loads	O
the	O
program	O
that	O
will	O
perform	O
the	O
next	O
stage	O
of	O
the	O
boot	O
process	O
,	O
usually	O
by	O
making	O
use	O
of	O
INT13h	O
BIOS	B-Device
calls	I-Device
.	O
</s>
<s>
The	O
MBR	O
bootstrap	O
code	O
loads	O
and	O
runs	O
(	O
a	O
boot	B-Application
loader	I-Application
-	O
or	O
operating	O
system-dependent	O
)	O
volume	B-Device
boot	I-Device
record	I-Device
code	O
that	O
is	O
located	O
at	O
the	O
beginning	O
of	O
the	O
"	O
active	O
"	O
partition	B-Application
.	O
</s>
<s>
The	O
volume	B-Device
boot	I-Device
record	I-Device
will	O
fit	O
within	O
a	O
512-byte	O
sector	B-Application
,	O
but	O
it	O
is	O
safe	O
for	O
the	O
MBR	O
code	O
to	O
load	O
additional	O
sectors	B-Application
to	O
accommodate	O
boot	B-Application
loaders	I-Application
longer	O
than	O
one	O
sector	B-Application
,	O
provided	O
they	O
do	O
not	O
make	O
any	O
assumptions	O
on	O
what	O
the	O
sector	B-Application
size	O
is	O
.	O
</s>
<s>
In	O
fact	O
,	O
at	O
least	O
1	O
KB	O
of	O
RAM	O
is	O
available	O
at	O
address	B-General_Concept
in	O
every	O
IBM	B-Device
XT	I-Device
-	O
and	O
AT-class	O
machine	O
,	O
so	O
a	O
1KB	O
sector	B-Application
could	O
be	O
used	O
with	O
no	O
problem	O
.	O
</s>
<s>
Like	O
the	O
MBR	O
,	O
a	O
volume	B-Device
boot	I-Device
record	I-Device
normally	O
expects	O
to	O
be	O
loaded	O
at	O
address	B-General_Concept
:	O
.	O
</s>
<s>
This	O
derives	O
from	O
the	O
fact	O
that	O
the	O
volume	B-Device
boot	I-Device
record	I-Device
design	O
originated	O
on	O
unpartitioned	O
media	O
,	O
where	O
a	O
volume	B-Device
boot	I-Device
record	I-Device
would	O
be	O
directly	O
loaded	O
by	O
the	O
BIOS	B-Operating_System
boot	O
procedure	O
;	O
as	O
mentioned	O
above	O
,	O
the	O
BIOS	B-Operating_System
treats	O
MBRs	O
and	O
volume	B-Device
boot	I-Device
records	I-Device
(	O
VBRs	B-Device
)	O
exactly	O
alike	O
.	O
</s>
<s>
The	O
relocation	O
address	B-General_Concept
is	O
determined	O
by	O
the	O
MBR	O
,	O
but	O
it	O
is	O
most	O
often	O
:	O
(	O
for	O
MS-DOS/PCDOS	O
,	O
OS/2	O
and	O
Windows	O
MBR	O
code	O
)	O
or	O
:	O
(	O
most	O
DR-DOS	B-Application
MBRs	O
)	O
.	O
</s>
<s>
(	O
Even	O
though	O
both	O
of	O
these	O
segmented	O
addresses	O
resolve	O
to	O
the	O
same	O
physical	O
memory	B-General_Concept
address	I-General_Concept
in	O
real	B-Application
mode	I-Application
,	O
for	O
Apple	B-Operating_System
Darwin	I-Operating_System
to	O
boot	O
,	O
the	O
MBR	O
must	O
be	O
relocated	O
to	O
:	O
instead	O
of	O
:	O
,	O
since	O
the	O
code	O
depends	O
on	O
the	O
DS:SI	O
pointer	O
to	O
the	O
partition	B-Application
entry	O
provided	O
by	O
the	O
MBR	O
,	O
but	O
it	O
erroneously	O
refers	O
to	O
it	O
via	O
:SI	O
only	O
.	O
)	O
</s>
<s>
It	O
is	O
important	O
not	O
to	O
relocate	O
to	O
other	O
addresses	O
in	O
memory	O
because	O
many	O
VBRs	B-Device
will	O
assume	O
a	O
certain	O
standard	O
memory	O
layout	O
when	O
loading	O
their	O
boot	O
file	O
.	O
</s>
<s>
The	O
Status	O
field	O
in	O
a	O
partition	B-Application
table	O
record	O
is	O
used	O
to	O
indicate	O
an	O
active	O
partition	B-Application
.	O
</s>
<s>
Standard-conformant	O
MBRs	O
will	O
allow	O
only	O
one	O
partition	B-Application
marked	O
active	O
and	O
use	O
this	O
as	O
part	O
of	O
a	O
sanity-check	O
to	O
determine	O
the	O
existence	O
of	O
a	O
valid	O
partition	B-Application
table	O
.	O
</s>
<s>
They	O
will	O
display	O
an	O
error	O
message	O
,	O
if	O
more	O
than	O
one	O
partition	B-Application
has	O
been	O
marked	O
active	O
.	O
</s>
<s>
Some	O
non-standard	O
MBRs	O
will	O
not	O
treat	O
this	O
as	O
an	O
error	O
condition	O
and	O
just	O
use	O
the	O
first	O
marked	O
partition	B-Application
in	O
the	O
row	O
.	O
</s>
<s>
However	O
,	O
the	O
Plug	B-Device
and	I-Device
Play	I-Device
BIOS	I-Device
Specification	I-Device
and	O
BIOS	B-Operating_System
Boot	O
Specification	O
(	O
BBS	O
)	O
allowed	O
other	O
devices	O
to	O
become	O
bootable	O
as	O
well	O
since	O
1994	O
.	O
</s>
<s>
Consequently	O
,	O
with	O
the	O
introduction	O
of	O
MS-DOS	B-Application
7.10	O
(	O
Windows	O
95B	O
)	O
and	O
higher	O
,	O
the	O
MBR	O
started	O
to	O
treat	O
a	O
set	O
bit	O
7	O
as	O
active	O
flag	O
and	O
showed	O
an	O
error	O
message	O
for	O
values	O
..	O
only	O
.	O
</s>
<s>
It	O
continued	O
to	O
treat	O
the	O
entry	O
as	O
physical	O
drive	O
unit	O
to	O
be	O
used	O
when	O
loading	O
the	O
corresponding	O
partition	B-Application
's	O
VBR	B-Device
later	O
on	O
,	O
thereby	O
now	O
also	O
accepting	O
other	O
boot	O
drives	O
than	O
as	O
valid	O
,	O
however	O
,	O
MS-DOS	B-Application
did	O
not	O
make	O
use	O
of	O
this	O
extension	O
by	O
itself	O
.	O
</s>
<s>
Storing	O
the	O
actual	O
physical	O
drive	O
number	O
in	O
the	O
partition	B-Application
table	O
does	O
not	O
normally	O
cause	O
backward	O
compatibility	O
problems	O
,	O
since	O
the	O
value	O
will	O
differ	O
from	O
only	O
on	O
drives	O
other	O
than	O
the	O
first	O
one	O
(	O
which	O
have	O
not	O
been	O
bootable	O
before	O
,	O
anyway	O
)	O
.	O
</s>
<s>
However	O
,	O
even	O
with	O
systems	O
enabled	O
to	O
boot	O
off	O
other	O
drives	O
,	O
the	O
extension	O
may	O
still	O
not	O
work	O
universally	O
,	O
for	O
example	O
,	O
after	O
the	O
BIOS	B-Operating_System
assignment	O
of	O
physical	O
drives	O
has	O
changed	O
when	O
drives	O
are	O
removed	O
,	O
added	O
or	O
swapped	O
.	O
</s>
<s>
Therefore	O
,	O
per	O
the	O
BIOS	B-Operating_System
Boot	O
Specification	O
(	O
BBS	O
)	O
,	O
it	O
is	O
best	O
practice	O
for	O
a	O
modern	O
MBR	O
accepting	O
bit	O
7	O
as	O
active	O
flag	O
to	O
pass	O
on	O
the	O
DL	O
value	O
originally	O
provided	O
by	O
the	O
BIOS	B-Operating_System
instead	O
of	O
using	O
the	O
entry	O
in	O
the	O
partition	B-Application
table	O
.	O
</s>
<s>
The	O
MBR	O
is	O
loaded	O
at	O
memory	B-General_Concept
location	I-General_Concept
:	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
normally	O
the	O
IPL	O
in	O
the	O
BIOS	B-Operating_System
)	O
passes	O
execution	O
to	O
it	O
by	O
jumping	O
to	O
:	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
MBR	O
code	O
assuming	O
certain	O
register	O
values	O
or	O
not	O
written	O
to	O
be	O
relocatable	O
may	O
not	O
work	O
otherwise	O
.	O
</s>
<s>
The	O
Toshiba	O
T1000	O
BIOS	B-Operating_System
is	O
known	O
not	O
to	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
fixed	B-Device
disks	I-Device
(	O
thereby	O
reflecting	O
the	O
logical	B-Application
drive	I-Application
numbers	O
under	O
DOS	B-Device
rather	O
than	O
the	O
physical	O
drive	O
numbers	O
of	O
the	O
BIOS	B-Operating_System
)	O
.	O
</s>
<s>
USB	B-Protocol
sticks	O
configured	O
as	O
removable	B-Device
drives	I-Device
typically	O
get	O
an	O
assignment	O
of	O
DL	O
=	O
,	O
,	O
etc	O
.	O
</s>
<s>
A	O
standard	O
conformant	O
BIOS	B-Operating_System
assigns	O
numbers	O
greater	O
or	O
equal	O
to	O
exclusively	O
to	O
fixed	B-Device
disk	I-Device
/	O
removable	B-Device
drives	I-Device
,	O
and	O
traditionally	O
only	O
values	O
and	O
were	O
passed	O
on	O
as	O
physical	O
drive	O
units	O
during	O
boot	O
.	O
</s>
<s>
By	O
convention	O
,	O
only	O
fixed	B-Device
disks	I-Device
/	O
removable	B-Device
drives	I-Device
are	O
partitioned	B-Application
,	O
therefore	O
,	O
the	O
only	O
DL	O
value	O
a	O
MBR	O
could	O
see	O
traditionally	O
was	O
.	O
</s>
<s>
The	O
Plug	B-Device
and	I-Device
Play	I-Device
BIOS	I-Device
Specification	I-Device
and	O
BIOS	B-Operating_System
Boot	O
Specification	O
(	O
BBS	O
)	O
allow	O
other	O
devices	O
to	O
become	O
bootable	O
as	O
well	O
since	O
1994	O
.	O
</s>
<s>
The	O
later	O
recommends	O
that	O
MBR	O
and	O
VBR	B-Device
code	O
should	O
use	O
DL	O
rather	O
than	O
internally	O
hardwired	O
defaults	O
.	O
</s>
<s>
Bootable	O
CD-ROMs	O
following	O
the	O
El	B-Device
Torito	I-Device
specification	O
may	O
contain	O
disk	O
images	O
mounted	O
by	O
the	O
BIOS	B-Operating_System
to	O
occur	O
as	O
floppy	B-Device
or	O
superfloppies	O
on	O
this	O
interface	O
.	O
</s>
<s>
DL	O
values	O
of	O
and	O
may	O
also	O
be	O
used	O
by	O
Protected	O
Area	O
Run	O
Time	O
Interface	O
Extension	O
Services	O
(	O
PARTIES	O
)	O
and	O
Trusted	O
Computing	O
Group	O
(	O
TCG	O
)	O
BIOS	B-Operating_System
extensions	O
in	O
Trusted	O
mode	O
to	O
access	O
otherwise	O
invisible	O
PARTIES	O
partitions	B-Application
,	O
disk	O
image	O
files	B-Operating_System
located	O
via	O
the	O
Boot	O
Engineering	O
Extension	O
Record	O
(	O
BEER	O
)	O
in	O
the	O
last	O
physical	O
sector	B-Application
of	O
a	O
hard	B-Device
disk	I-Device
's	O
Host	O
Protected	O
Area	O
(	O
HPA	O
)	O
.	O
</s>
<s>
While	O
designed	O
to	O
emulate	O
floppies	B-Device
or	O
superfloppies	O
,	O
MBR	O
code	O
accepting	O
these	O
non-standard	O
DL	O
values	O
allows	O
to	O
use	O
images	O
of	O
partitioned	B-Application
media	O
at	O
least	O
in	O
the	O
boot	O
stage	O
of	O
operating	B-General_Concept
systems	I-General_Concept
.	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>
Some	O
of	O
the	O
other	O
registers	O
may	O
typically	O
also	O
hold	O
certain	O
register	O
values	O
(	O
DS	O
,	O
ES	O
,	O
SS	O
=	O
;	O
SP	O
=	O
)	O
with	O
original	O
IBM	O
ROM	B-Device
BIOSes	O
,	O
but	O
this	O
is	O
nothing	O
to	O
rely	O
on	O
,	O
as	O
other	O
BIOSes	O
may	O
use	O
other	O
values	O
.	O
</s>
<s>
Relying	O
on	O
these	O
register	O
values	O
in	O
boot	B-Device
sectors	I-Device
may	O
also	O
cause	O
problems	O
in	O
chain-boot	O
scenarios	O
.	O
</s>
<s>
Systems	O
with	O
Plug-and-Play	B-Device
BIOS	B-Operating_System
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
in	O
the	O
MBR	O
(	O
or	O
VBR	B-Device
,	O
if	O
passed	O
on	O
)	O
to	O
actively	O
interact	O
with	O
the	O
BIOS	B-Operating_System
or	O
a	O
resident	O
PnP	O
/	O
BBS	O
BIOS	B-Operating_System
overlay	O
in	O
memory	O
in	O
order	O
to	O
configure	O
the	O
boot	O
order	O
,	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	B-Device
.	O
</s>
<s>
Ideally	O
,	O
ES:DI	O
is	O
passed	O
on	O
to	O
the	O
VBR	B-Device
for	O
later	O
use	O
by	O
the	O
loaded	O
operating	B-General_Concept
system	I-General_Concept
,	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	B-Operating_System
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>
By	O
convention	O
,	O
a	O
standard	O
conformant	O
MBR	O
passes	O
execution	O
to	O
a	O
successfully	O
loaded	O
VBR	B-Device
,	O
loaded	O
at	O
memory	B-General_Concept
location	I-General_Concept
:	O
,	O
by	O
jumping	O
to	O
:	O
in	O
the	O
CPU	B-Device
's	O
real	B-Application
mode	I-Application
with	O
the	O
following	O
registers	O
maintained	O
or	O
specifically	O
set	O
up	O
:	O
</s>
<s>
MS-DOS	B-Application
2.0-7.0	O
/	O
PC	B-Operating_System
DOS	I-Operating_System
2.0-6.3	O
MBRs	O
do	O
not	O
pass	O
on	O
the	O
DL	O
value	O
received	O
on	O
entry	O
,	O
but	O
they	O
rather	O
use	O
the	O
boot	O
status	O
entry	O
in	O
the	O
partition	B-Application
table	O
entry	O
of	O
the	O
selected	O
primary	O
partition	B-Application
as	O
physical	O
boot	O
drive	O
unit	O
.	O
</s>
<s>
Since	O
this	O
is	O
,	O
by	O
convention	O
,	O
in	O
most	O
MBR	O
partition	B-Application
tables	O
,	O
it	O
wo	O
n't	O
change	O
things	O
unless	O
the	O
BIOS	B-Operating_System
attempted	O
to	O
boot	O
off	O
a	O
physical	O
device	O
other	O
than	O
the	O
first	O
fixed	B-Device
disk	I-Device
/	O
removable	B-Device
drive	I-Device
in	O
the	O
row	O
.	O
</s>
<s>
This	O
is	O
also	O
the	O
reason	O
why	O
these	O
operating	B-General_Concept
systems	I-General_Concept
cannot	O
boot	O
off	O
a	O
second	O
hard	B-Device
disk	I-Device
,	O
etc	O
.	O
</s>
<s>
Some	O
FDISK	B-Application
tools	O
allow	O
to	O
mark	O
partitions	B-Application
on	O
secondary	O
disks	O
as	O
"	O
active	O
"	O
as	O
well	O
.	O
</s>
<s>
In	O
this	O
situation	O
,	O
knowing	O
that	O
these	O
operating	B-General_Concept
systems	I-General_Concept
cannot	O
boot	O
off	O
other	O
drives	O
anyway	O
,	O
some	O
of	O
them	O
continue	O
to	O
use	O
the	O
traditionally	O
fixed	O
value	O
of	O
as	O
active	O
marker	O
,	O
whereas	O
others	O
use	O
values	O
corresponding	O
with	O
the	O
currently	O
assigned	O
physical	O
drive	O
unit	O
(	O
,	O
)	O
,	O
thereby	O
allowing	O
booting	O
from	O
other	O
drives	O
,	O
at	O
least	O
in	O
theory	O
.	O
</s>
<s>
In	O
fact	O
,	O
this	O
will	O
work	O
with	O
many	O
MBR	O
codes	O
,	O
which	O
take	O
a	O
set	O
bit	O
7	O
of	O
the	O
boot	O
status	O
entry	O
as	O
active	O
flag	O
rather	O
than	O
insisting	O
on	O
,	O
however	O
,	O
MS-DOS/PC	O
DOS	O
MBRs	O
are	O
hard-wired	O
to	O
accept	O
the	O
fixed	O
value	O
of	O
only	O
.	O
</s>
<s>
Storing	O
the	O
actual	O
physical	O
drive	O
number	O
in	O
the	O
partition	B-Application
table	O
will	O
also	O
cause	O
problems	O
,	O
when	O
the	O
BIOS	B-Operating_System
assignment	O
of	O
physical	O
drives	O
changes	O
,	O
for	O
example	O
when	O
drives	O
are	O
removed	O
,	O
added	O
or	O
swapped	O
.	O
</s>
<s>
Therefore	O
,	O
for	O
a	O
normal	O
MBR	O
accepting	O
bit	O
7	O
as	O
active	O
flag	O
and	O
otherwise	O
just	O
using	O
and	O
passing	O
on	O
to	O
the	O
VBR	B-Device
the	O
DL	O
value	O
originally	O
provided	O
by	O
the	O
BIOS	B-Operating_System
allows	O
for	O
maximum	O
flexibility	O
.	O
</s>
<s>
MS-DOS	B-Application
7.1	O
-	O
8.0	O
MBRs	O
have	O
changed	O
to	O
treat	O
bit	O
7	O
as	O
active	O
flag	O
and	O
any	O
values	O
..	O
as	O
invalid	O
,	O
but	O
they	O
still	O
take	O
the	O
physical	O
drive	O
unit	O
from	O
the	O
partition	B-Application
table	O
rather	O
than	O
using	O
the	O
DL	O
value	O
provided	O
by	O
the	O
BIOS	B-Operating_System
.	O
</s>
<s>
DR-DOS	B-Application
7.07	O
extended	O
MBRs	O
treat	O
bit	O
7	O
as	O
active	O
flag	O
and	O
use	O
and	O
pass	O
on	O
the	O
BIOS	B-Operating_System
DL	O
value	O
by	O
default	O
(	O
including	O
non-standard	O
values	O
..	O
used	O
by	O
some	O
BIOSes	O
also	O
for	O
partitioned	B-Application
media	O
)	O
,	O
but	O
they	O
also	O
provide	O
a	O
special	O
NEWLDR	O
configuration	O
block	O
in	O
order	O
to	O
support	O
alternative	O
boot	O
methods	O
in	O
conjunction	O
with	O
LOADER	O
and	O
REAL/32	B-Operating_System
as	O
well	O
as	O
to	O
change	O
the	O
detail	O
behaviour	O
of	O
the	O
MBR	O
,	O
so	O
that	O
it	O
can	O
also	O
work	O
with	O
drive	O
values	O
retrieved	O
from	O
the	O
partition	B-Application
table	O
(	O
important	O
in	O
conjunction	O
with	O
LOADER	O
and	O
AAPs	O
,	O
see	O
NEWLDR	O
offset	O
0x000C	O
)	O
,	O
translate	O
Wyse	O
non-standard	O
drive	O
units	O
..	O
to	O
..	O
,	O
and	O
optionally	O
fix	O
up	O
the	O
drive	O
value	O
(	O
stored	O
at	O
offset	O
0x019	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
0x01FD	O
)	O
in	O
loaded	O
VBRs	B-Device
before	O
passing	O
execution	O
to	O
them	O
(	O
see	O
NEWLDR	O
offset	O
0x0014	O
)	O
—	O
this	O
also	O
allows	O
other	O
boot	B-Application
loaders	I-Application
to	O
use	O
NEWLDR	O
as	O
a	O
chain-loader	O
,	O
configure	O
its	O
in-memory	O
image	O
on	O
the	O
fly	O
and	O
"	O
tunnel	O
"	O
the	O
loading	O
of	O
VBRs	B-Device
,	O
EBRs	O
,	O
or	O
AAPs	O
through	O
NEWLDR	O
.	O
</s>
<s>
The	O
contents	O
of	O
DH	O
and	O
ES:DI	O
should	O
be	O
preserved	O
by	O
the	O
MBR	O
for	O
full	O
Plug-and-Play	B-Device
support	O
(	O
see	O
above	O
)	O
,	O
however	O
,	O
many	O
MBRs	O
,	O
including	O
those	O
of	O
MS-DOS	B-Application
2.0	O
-	O
8.0	O
/	O
PC	B-Operating_System
DOS	I-Operating_System
2.0	O
-	O
6.3	O
and	O
Windows	O
NT/2000/XP	O
,	O
do	O
not	O
.	O
</s>
<s>
(	O
This	O
is	O
unsurprising	O
,	O
since	O
those	O
versions	O
of	O
DOS	B-Device
predate	O
the	O
Plug-and-Play	B-Device
BIOS	B-Operating_System
standard	O
,	O
and	O
previous	O
standards	O
and	O
conventions	O
indicated	O
no	O
requirements	O
to	O
preserve	O
any	O
register	O
other	O
than	O
DL	O
.	O
)	O
</s>
<s>
The	O
MBR	O
code	O
passes	O
additional	O
information	O
to	O
the	O
VBR	B-Device
in	O
many	O
implementations	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	B-Device
.	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
6.6	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	B-Device
code	O
and	O
it	O
will	O
update	O
the	O
boot	O
drive	O
/	O
status	O
flag	O
field	O
in	O
the	O
partition	B-Application
entry	O
according	O
to	O
the	O
effectively	O
used	O
DL	O
value	O
.	O
</s>
<s>
Darwin	B-Operating_System
bootloaders	B-Application
(	O
Apple	O
's	O
boot1h	O
,	O
boot1u	O
,	O
and	O
David	O
Elliott	O
's	O
boot1fat32	O
)	O
depend	O
on	O
this	O
pointer	O
as	O
well	O
,	O
but	O
additionally	O
they	O
do	O
n't	O
use	O
DS	O
,	O
but	O
assume	O
it	O
to	O
be	O
set	O
to	O
instead	O
.	O
</s>
<s>
The	O
MBR	O
code	O
of	O
OS/2	O
,	O
MS-DOS	B-Application
2.0	O
to	O
8.0	O
,	O
PC	B-Operating_System
DOS	I-Operating_System
2.0	O
to	O
7.10	O
and	O
Windows	O
NT/2000/XP	O
provides	O
this	O
same	O
interface	O
as	O
well	O
,	O
although	O
these	O
systems	O
do	O
not	O
use	O
it	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>
DS:BP	O
=	O
optionally	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	B-Device
.	O
</s>
<s>
This	O
is	O
identical	O
to	O
the	O
pointer	O
provided	O
by	O
DS:SI	O
(	O
see	O
above	O
)	O
and	O
is	O
provided	O
by	O
MS-DOS	B-Application
2.0-8.0	O
,	O
PC	B-Operating_System
DOS	I-Operating_System
2.0-7.10	O
,	O
Windows	O
NT/2000/XP/Vista/7	O
MBRs	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
provided	O
by	O
the	O
extended	O
MBR	O
and	O
in	O
conjunction	O
with	O
LOADER	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
and	O
the	O
partition	B-Application
type	I-Application
at	O
offset	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>
<s>
Since	O
older	O
operating	B-General_Concept
systems	I-General_Concept
(	O
including	O
their	O
VBRs	B-Device
)	O
do	O
not	O
support	O
this	O
extension	O
nor	O
are	O
they	O
able	O
to	O
address	B-General_Concept
sectors	B-Application
beyond	O
the	O
2TiB	O
barrier	O
,	O
a	O
GPT-enabled	O
hybrid	O
boot	B-Application
loader	I-Application
should	O
still	O
emulate	O
the	O
16-byte	O
dummy	O
MBR	O
partition	B-Application
table	O
entry	O
if	O
the	O
boot	O
partition	B-Application
is	O
located	O
within	O
the	O
first	O
2TiB	O
.	O
</s>
<s>
Though	O
it	O
is	O
possible	O
to	O
manipulate	O
the	O
bytes	B-Application
in	O
the	O
MBR	O
sector	B-Application
directly	O
using	O
various	O
disk	B-Application
editors	I-Application
,	O
there	O
are	O
tools	O
to	O
write	O
fixed	O
sets	O
of	O
functioning	O
code	O
to	O
the	O
MBR	O
.	O
</s>
<s>
Since	O
MS-DOS	B-Application
5.0	O
,	O
the	O
program	O
FDISK	B-Application
has	O
included	O
the	O
switch	O
/MBR	O
,	O
which	O
will	O
rewrite	O
the	O
MBR	O
code	O
.	O
</s>
<s>
Under	O
Windows	B-Application
2000	I-Application
and	O
Windows	B-Application
XP	I-Application
,	O
the	O
Recovery	O
Console	O
can	O
be	O
used	O
to	O
write	O
new	O
MBR	O
code	O
to	O
a	O
storage	O
device	O
using	O
its	O
fixmbr	O
command	O
.	O
</s>
<s>
Under	O
Windows	B-Application
Vista	I-Application
and	O
Windows	B-Device
7	I-Device
,	O
the	O
Recovery	O
Environment	O
can	O
be	O
used	O
to	O
write	O
new	O
MBR	O
code	O
using	O
the	O
BOOTREC	O
/FIXMBR	O
command	O
.	O
</s>
<s>
Some	O
third-party	O
utilities	O
may	O
also	O
be	O
used	O
for	O
directly	O
editing	O
the	O
contents	O
of	O
partition	B-Application
tables	O
(	O
without	O
requiring	O
any	O
knowledge	O
of	O
hexadecimal	O
or	O
disk/sector	O
editors	O
)	O
,	O
such	O
as	O
MBRWizard	B-Language
.	O
</s>
<s>
In	O
Linux	B-Application
,	O
ms-sys	O
may	O
be	O
used	O
to	O
install	O
a	O
Windows	O
MBR	O
.	O
</s>
<s>
The	O
GRUB	B-Application
and	O
LILO	B-Application
projects	O
have	O
tools	O
for	O
writing	O
code	O
to	O
the	O
MBR	O
sector	B-Application
,	O
namely	O
grub-install	O
and	O
lilo	B-Application
-mbr	O
.	O
</s>
<s>
The	O
GRUB	B-Application
Legacy	O
interactive	O
console	O
can	O
write	O
to	O
the	O
MBR	O
,	O
using	O
the	O
setup	O
and	O
embed	O
commands	O
,	O
but	O
GRUB2	O
currently	O
requires	O
grub-install	O
to	O
be	O
run	O
from	O
within	O
an	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Various	O
programs	O
are	O
able	O
to	O
create	O
a	O
"	O
backup	B-Protocol
"	O
of	O
both	O
the	O
primary	O
partition	B-Application
table	O
and	O
the	O
logical	O
partitions	B-Application
in	O
the	O
extended	B-Application
partition	I-Application
.	O
</s>
<s>
Linux	B-Application
sfdisk	O
(	O
on	O
a	O
SystemRescueCD	B-Application
)	O
is	O
able	O
to	O
save	O
a	O
backup	B-Protocol
of	O
the	O
primary	O
and	O
extended	B-Application
partition	I-Application
table	O
.	O
</s>
<s>
An	O
example	O
command	O
to	O
back	B-Protocol
up	I-Protocol
the	O
partition	B-Application
table	O
is	O
sfdisk	O
-d	O
/dev/hda	O
>	O
hda.out	O
and	O
to	O
restore	O
is	O
sfdisk	O
/dev/hda	O
<	O
hda.out	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
copy	O
the	O
partition	B-Application
table	O
from	O
one	O
disk	O
to	O
another	O
this	O
way	O
,	O
useful	O
for	O
setting	O
up	O
mirroring	O
,	O
but	O
sfdisk	O
executes	O
the	O
command	O
without	O
prompting/warnings	O
using	O
sfdisk	O
-d	O
/dev/sda	O
|	O
sfdisk	O
/dev/sdb	O
.	O
</s>
