<s>
Logical	B-Device
block	I-Device
addressing	I-Device
(	O
LBA	O
)	O
is	O
a	O
common	O
scheme	O
used	O
for	O
specifying	O
the	O
location	O
of	O
blocks	B-General_Concept
of	I-General_Concept
data	I-General_Concept
stored	O
on	O
computer	B-General_Concept
storage	I-General_Concept
devices	I-General_Concept
,	O
generally	O
secondary	O
storage	B-General_Concept
systems	I-General_Concept
such	O
as	O
hard	B-Device
disk	I-Device
drives	I-Device
.	O
</s>
<s>
LBA	O
is	O
a	O
particularly	O
simple	O
linear	B-General_Concept
addressing	I-General_Concept
scheme	O
;	O
blocks	O
are	O
located	O
by	O
an	O
integer	O
index	O
,	O
with	O
the	O
first	O
block	O
being	O
LBA	O
0	O
,	O
the	O
second	O
LBA	O
1	O
,	O
and	O
so	O
on	O
.	O
</s>
<s>
The	O
IDE	B-Protocol
standard	O
included	O
22-bit	O
LBA	O
as	O
an	O
option	O
,	O
which	O
was	O
further	O
extended	O
to	O
28-bit	O
with	O
the	O
release	O
of	O
ATA-1	O
(	O
1994	O
)	O
and	O
to	O
48-bit	O
with	O
the	O
release	O
of	O
ATA-6	B-Protocol
(	O
2003	O
)	O
,	O
whereas	O
the	O
size	O
of	O
entries	O
in	O
on-disk	O
and	O
in-memory	O
data	O
structures	O
holding	O
the	O
address	O
is	O
typically	O
32	O
or	O
64	O
bits	O
.	O
</s>
<s>
Most	O
hard	B-Device
disk	I-Device
drives	I-Device
released	O
after	O
1996	O
implement	O
logical	B-Device
block	I-Device
addressing	I-Device
.	O
</s>
<s>
In	O
logical	B-Device
block	I-Device
addressing	I-Device
,	O
only	O
one	O
number	O
is	O
used	O
to	O
address	O
data	O
,	O
and	O
each	O
linear	O
base	O
address	O
describes	O
a	O
single	O
block	O
.	O
</s>
<s>
Chief	O
among	O
these	O
was	O
the	O
cylinder-head-sector	B-Device
(	O
CHS	O
)	O
scheme	O
,	O
where	O
blocks	O
were	O
addressed	O
by	O
means	O
of	O
a	O
tuple	B-Application
which	O
defined	O
the	O
cylinder	O
,	O
head	O
,	O
and	O
sector	O
at	O
which	O
they	O
appeared	O
on	O
the	O
hard	B-Device
disk	I-Device
.	O
</s>
<s>
CHS	O
did	O
not	O
map	O
well	O
to	O
devices	O
other	O
than	O
hard	B-Device
disks	I-Device
(	O
such	O
as	O
tapes	O
and	O
networked	O
storage	O
)	O
,	O
and	O
was	O
generally	O
not	O
used	O
for	O
them	O
.	O
</s>
<s>
CHS	O
was	O
used	O
in	O
early	O
MFM	B-Protocol
and	O
RLL	B-Protocol
drives	O
,	O
and	O
both	O
it	O
and	O
its	O
successor	O
,	O
extended	O
cylinder-head-sector	B-Device
(	O
ECHS	O
)	O
,	O
were	O
used	O
in	O
the	O
first	O
ATA	B-Protocol
drives	I-Protocol
.	O
</s>
<s>
However	O
,	O
current	O
disk	B-Device
drives	I-Device
use	O
zone	B-Architecture
bit	I-Architecture
recording	I-Architecture
,	O
where	O
the	O
number	O
of	O
sectors	O
per	O
track	O
depends	O
on	O
the	O
track	O
number	O
.	O
</s>
<s>
Even	O
though	O
the	O
disk	B-Device
drive	I-Device
will	O
report	O
some	O
CHS	O
values	O
as	O
sectors	O
per	O
track	O
(	O
SPT	O
)	O
and	O
heads	O
per	O
cylinder	O
(	O
HPC	O
)	O
,	O
they	O
have	O
little	O
to	O
do	O
with	O
the	O
disk	B-Device
drive	I-Device
's	O
true	O
geometry	O
.	O
</s>
<s>
LBA	O
was	O
first	O
introduced	O
in	O
SCSI	B-Architecture
as	O
an	O
abstraction	O
.	O
</s>
<s>
While	O
the	O
drive	O
controller	O
still	O
addresses	O
data	O
blocks	O
by	O
their	O
CHS	O
address	O
,	O
this	O
information	O
is	O
generally	O
not	O
used	O
by	O
the	O
SCSI	B-Architecture
device	O
driver	O
,	O
the	O
OS	O
,	O
filesystem	O
code	O
,	O
or	O
any	O
applications	O
(	O
such	O
as	O
databases	O
)	O
that	O
access	O
the	O
"	O
raw	O
"	O
disk	B-Device
.	O
</s>
<s>
In	O
redundant	B-Architecture
array	I-Architecture
of	I-Architecture
independent	I-Architecture
disks	I-Architecture
(	O
RAID	B-Architecture
)	O
devices	O
and	O
storage	O
area	O
networks	O
(	O
SANs	O
)	O
and	O
where	O
logical	B-Application
drives	I-Application
(	O
logical	O
unit	O
numbers	O
,	O
LUNs	O
)	O
are	O
composed	O
via	O
LUN	O
virtualization	O
and	O
aggregation	O
,	O
LBA	O
addressing	O
of	O
individual	O
disk	B-Device
should	O
be	O
translated	O
by	O
a	O
software	O
layer	O
to	O
provide	O
uniform	O
LBA	O
addressing	O
for	O
the	O
entire	O
storage	O
device	O
.	O
</s>
<s>
The	O
earlier	O
IDE	B-Protocol
standard	O
from	O
Western	O
Digital	O
introduced	O
22-bit	O
LBA	O
;	O
in	O
1994	O
,	O
the	O
ATA-1	B-Protocol
standard	I-Protocol
allowed	O
for	O
28	O
bit	O
addresses	O
in	O
both	O
LBA	O
and	O
CHS	O
modes	O
.	O
</s>
<s>
This	O
means	O
the	O
reported	O
number	O
of	O
heads	O
never	O
exceeds	O
16	O
(	O
0	O
–	O
15	O
)	O
,	O
the	O
number	O
of	O
sectors	O
can	O
be	O
255	O
(	O
1	O
–	O
255	O
;	O
though	O
63	O
is	O
often	O
the	O
largest	O
used	O
)	O
and	O
the	O
number	O
of	O
cylinders	O
can	O
be	O
as	O
large	O
as	O
65,536	O
(	O
0	O
–	O
65535	O
)	O
,	O
limiting	O
disk	B-Device
size	O
to	O
128	O
GiB	O
( ≈	O
137.4	O
GB	O
)	O
,	O
assuming	O
512	O
byte	O
sectors	O
.	O
</s>
<s>
These	O
values	O
can	O
be	O
accessed	O
by	O
issuing	O
the	O
ATA	B-Protocol
command	O
"	O
Identify	O
Device	O
"	O
(	O
ECh	O
)	O
to	O
the	O
drive	O
.	O
</s>
<s>
However	O
,	O
the	O
IBM	B-Operating_System
BIOS	I-Operating_System
implementation	O
defined	O
in	O
the	O
INT	O
13h	O
disk	B-Device
access	O
routines	O
used	O
quite	O
a	O
different	O
24-bit	O
scheme	O
for	O
CHS	O
addressing	O
,	O
with	O
10	O
bits	O
for	O
cylinder	O
,	O
8	O
bits	O
for	O
head	O
,	O
and	O
6	O
bits	O
for	O
sector	O
,	O
or	O
1024	O
cylinders	O
,	O
256	O
heads	O
,	O
and	O
63	O
sectors	O
.	O
</s>
<s>
This	O
INT13h	O
implementation	O
had	O
pre-dated	O
the	O
ATA	B-Protocol
standard	O
,	O
as	O
it	O
was	O
introduced	O
when	O
the	O
IBM	B-Device
PC	I-Device
had	O
only	O
floppy	B-Device
disk	I-Device
storage	O
,	O
and	O
when	O
hard	B-Device
disk	I-Device
drives	I-Device
were	O
introduced	O
on	O
the	O
IBM	B-Device
PC/XT	I-Device
,	O
INT13h	O
interface	O
could	O
not	O
be	O
practically	O
redesigned	O
due	O
to	O
backward	B-General_Concept
compatibility	I-General_Concept
issues	O
.	O
</s>
<s>
Overlapping	O
ATA	B-Protocol
CHS	O
mapping	O
with	O
BIOS	B-Operating_System
CHS	O
mapping	O
produced	O
the	O
lowest	O
common	O
denominator	O
of	O
10:4:6	O
bits	O
,	O
or	O
1024	O
cylinders	O
,	O
16	O
heads	O
,	O
and	O
63	O
sectors	O
,	O
which	O
gave	O
the	O
practical	O
limit	O
of	O
1024×16×63	O
sectors	O
and	O
528MB	O
(	O
504MiB	O
)	O
,	O
assuming	O
512	O
byte	O
sectors	O
.	O
</s>
<s>
In	O
order	O
for	O
the	O
BIOS	B-Operating_System
to	O
overcome	O
this	O
limit	O
and	O
successfully	O
work	O
with	O
larger	O
hard	B-Device
drives	I-Device
,	O
a	O
CHS	O
translation	O
scheme	O
had	O
to	O
be	O
implemented	O
in	O
the	O
BIOS	B-Operating_System
disk	B-Device
I/O	O
routines	O
which	O
would	O
convert	O
between	O
24-bit	O
CHS	O
used	O
by	O
INT13h	O
and	O
28-bit	O
CHS	O
numbering	O
used	O
by	O
ATA	B-Protocol
.	O
</s>
<s>
This	O
method	O
would	O
remap	O
16:4:8	O
bit	O
ATA	B-Protocol
cylinders	O
and	O
heads	O
to	O
10:8:6	O
bit	O
scheme	O
used	O
by	O
INT13h	O
,	O
generating	O
much	O
more	O
"	O
virtual	O
"	O
drive	O
heads	O
than	O
the	O
physical	O
disk	B-Device
reported	O
.	O
</s>
<s>
To	O
further	O
overcome	O
this	O
limit	O
,	O
INT	O
13h	O
Extensions	O
were	O
introduced	O
with	O
the	O
BIOS	B-Operating_System
Enhanced	O
Disk	B-Device
Drive	I-Device
Services	O
,	O
which	O
removed	O
practical	O
limits	O
on	O
disk	B-Device
size	O
for	O
operating	O
systems	O
which	O
are	O
aware	O
of	O
this	O
new	O
interface	O
,	O
such	O
as	O
the	O
DOS	B-Device
7.0	O
component	O
in	O
Windows	B-Application
95	I-Application
.	O
</s>
<s>
This	O
enhanced	O
BIOS	B-Operating_System
subsystem	O
supports	O
LBA	O
addressing	O
with	O
LBA	O
or	O
LBA-assisted	O
method	O
,	O
which	O
uses	O
native	O
28-bit	O
LBA	O
for	O
addressing	O
ATA	B-Protocol
disks	B-Device
and	O
performs	O
CHS	O
conversion	O
as	O
needed	O
.	O
</s>
<s>
Until	O
the	O
release	O
of	O
ATA-2	B-Protocol
standard	O
in	O
1996	O
,	O
there	O
were	O
a	O
handful	O
of	O
large	O
hard	B-Device
drives	I-Device
which	O
did	O
not	O
support	O
LBA	O
addressing	O
,	O
so	O
only	O
large	O
or	O
normal	O
methods	O
could	O
be	O
used	O
.	O
</s>
<s>
However	O
,	O
using	O
the	O
large	O
method	O
also	O
introduced	O
portability	O
problems	O
,	O
as	O
different	O
BIOSes	O
often	O
used	O
different	O
and	O
incompatible	O
translation	O
methods	O
,	O
and	O
hard	B-Device
drives	I-Device
partitioned	O
on	O
a	O
computer	O
with	O
a	O
BIOS	B-Operating_System
from	O
a	O
particular	O
vendor	O
often	O
could	O
not	O
be	O
read	O
on	O
a	O
computer	O
with	O
a	O
different	O
make	O
of	O
BIOS	B-Operating_System
.	O
</s>
<s>
The	O
solution	O
was	O
to	O
use	O
conversion	O
software	O
such	O
as	O
OnTrack	B-Application
Disk	I-Application
Manager	I-Application
,	O
Micro	O
House	O
EZ-Drive/EZ	O
-BIOS	O
,	O
etc.	O
,	O
which	O
installed	O
to	O
the	O
disk	B-Device
's	O
OS	B-Device
loader	I-Device
and	O
replaced	O
INT13h	O
routines	O
at	O
boot	O
time	O
with	O
custom	O
code	O
.	O
</s>
<s>
When	O
the	O
BIOS	B-Operating_System
is	O
configured	O
to	O
use	O
a	O
disk	B-Device
in	O
LBA-assisted	O
translation	O
mode	O
,	O
the	O
BIOS	B-Operating_System
accesses	O
the	O
hardware	O
using	O
LBA	O
mode	O
,	O
but	O
also	O
presents	O
a	O
translated	O
CHS	O
geometry	O
via	O
the	O
INT13h	O
interface	O
.	O
</s>
<s>
The	O
number	O
of	O
cylinders	O
,	O
heads	O
,	O
and	O
sectors	O
in	O
the	O
translated	O
geometry	O
depends	O
on	O
the	O
total	O
size	O
of	O
the	O
disk	B-Device
,	O
as	O
shown	O
in	O
the	O
following	O
table	O
.	O
</s>
<s>
The	O
current	O
48-bit	O
LBA	O
scheme	O
was	O
introduced	O
in	O
2002	O
with	O
the	O
ATA-6	B-Protocol
standard	O
,	O
raising	O
the	O
addressing	O
limit	O
to	O
2×	O
512	O
bytes	O
,	O
which	O
is	O
exactly	O
128PiB	O
or	O
approximately	O
144PB	O
.	O
</s>
<s>
Current	O
PC-compatible	O
computers	O
support	O
INT13h	O
Extensions	O
,	O
which	O
use	O
64-bit	O
structures	O
for	O
LBA	O
addressing	O
and	O
should	O
encompass	O
any	O
future	O
extension	O
of	O
LBA	O
addressing	O
,	O
though	O
modern	O
operating	O
systems	O
implement	O
direct	O
disk	B-Device
access	O
and	O
do	O
not	O
use	O
the	O
BIOS	B-Operating_System
subsystems	O
,	O
except	O
at	O
boot	B-Application
load	I-Application
time	O
.	O
</s>
<s>
However	O
,	O
the	O
common	O
DOS	B-Device
style	O
Master	B-Device
Boot	I-Device
Record	I-Device
(	O
MBR	O
)	O
partition	B-Application
table	O
only	O
supports	O
disk	B-Application
partitions	I-Application
up	O
to	O
2TiB	O
in	O
size	O
.	O
</s>
<s>
For	O
larger	O
partitions	B-Application
this	O
needs	O
to	O
be	O
replaced	O
by	O
another	O
scheme	O
,	O
for	O
instance	O
the	O
GUID	B-Application
Partition	I-Application
Table	I-Application
(	O
GPT	O
)	O
which	O
has	O
the	O
same	O
64-bit	O
limit	O
as	O
the	O
current	O
INT13h	O
Extensions	O
.	O
</s>
<s>
In	O
the	O
LBA	O
addressing	O
scheme	O
,	O
sectors	O
are	O
numbered	O
as	O
integer	O
indexes	O
;	O
when	O
mapped	O
to	O
CHS	O
(	O
cylinder-head-sector	B-Device
)	O
tuples	B-Application
,	O
LBA	O
numbering	O
starts	O
with	O
the	O
first	O
cylinder	O
,	O
first	O
head	O
,	O
and	O
track	O
's	O
first	O
sector	O
.	O
</s>
<s>
Thus	O
,	O
the	O
lower	O
the	O
LBA	O
value	O
is	O
,	O
the	O
closer	O
the	O
physical	O
sector	O
is	O
to	O
the	O
hard	B-Device
drive	I-Device
's	O
first	O
(	O
that	O
is	O
,	O
outermost	O
)	O
cylinder	O
.	O
</s>
<s>
CHS	O
tuples	B-Application
can	O
be	O
mapped	O
to	O
LBA	O
address	O
with	O
the	O
following	O
formula	O
:	O
</s>
<s>
LBA	O
addresses	O
can	O
be	O
mapped	O
to	O
CHS	O
tuples	B-Application
with	O
the	O
following	O
formula	O
(	O
"	O
mod	O
"	O
is	O
the	O
modulo	O
operation	O
,	O
i.e.	O
</s>
<s>
According	O
to	O
the	O
ATA	B-Protocol
specifications	O
,	O
"	O
If	O
the	O
content	O
of	O
words	O
(	O
61:60	O
)	O
is	O
greater	O
than	O
or	O
equal	O
to	O
16	O
,	O
514	O
,	O
064	O
,	O
then	O
the	O
content	O
of	O
word	O
1	O
[	O
the	O
number	O
of	O
logical	O
cylinders ]	O
shall	O
be	O
equal	O
to	O
16,383	O
.	O
"	O
</s>
<s>
Therefore	O
,	O
for	O
LBA	O
16450559	O
,	O
an	O
ATA	B-Protocol
drive	I-Protocol
may	O
actually	O
respond	O
with	O
the	O
CHS	O
tuple	B-Application
(	O
16319	O
,	O
15	O
,	O
63	O
)	O
,	O
and	O
the	O
number	O
of	O
cylinders	O
in	O
this	O
scheme	O
must	O
be	O
much	O
larger	O
than	O
1024	O
allowed	O
by	O
INT13h	O
.	O
</s>
<s>
Operating	O
systems	O
that	O
are	O
sensitive	O
to	O
BIOS-reported	O
drive	O
geometry	O
include	O
Solaris	B-Application
,	O
DOS	B-Device
and	O
Windows	B-Device
NT	I-Device
family	I-Device
,	O
where	O
NTLDR	B-Application
(	O
NT	B-Device
,	O
2000	B-Application
,	O
XP	B-Application
,	O
Server	B-Application
2003	I-Application
)	O
or	O
WINLOAD	B-Operating_System
(	O
Vista	B-Application
,	O
Server	B-Device
2008	I-Device
,	O
Windows	B-Device
7	I-Device
and	O
Server	B-Device
2008	I-Device
R2	I-Device
)	O
use	O
Master	B-Device
boot	I-Device
record	I-Device
which	O
addresses	O
the	O
disk	B-Device
using	O
CHS	O
;	O
x86-64	B-Device
and	O
Itanium	B-General_Concept
versions	O
of	O
Windows	O
can	O
partition	B-Application
the	O
drive	O
with	O
GUID	B-Application
Partition	I-Application
Table	I-Application
which	O
uses	O
LBA	O
addressing	O
.	O
</s>
<s>
Some	O
operating	O
systems	O
do	O
not	O
require	O
any	O
translation	O
because	O
they	O
do	O
not	O
use	O
geometry	O
reported	O
by	O
BIOS	B-Operating_System
in	O
their	O
boot	B-Application
loaders	I-Application
.	O
</s>
<s>
Among	O
these	O
operating	O
systems	O
are	O
BSD	B-Operating_System
,	O
Linux	B-Application
,	O
macOS	B-Application
,	O
OS/2	B-Application
and	O
ReactOS	B-Application
.	O
</s>
