<s>
Fixed-block	B-Device
architecture	I-Device
(	O
FBA	O
)	O
is	O
an	O
IBM	O
term	O
for	O
the	O
hard	B-Device
disk	I-Device
drive	I-Device
(	O
HDD	O
)	O
layout	O
in	O
which	O
each	O
addressable	O
block	B-General_Concept
(	O
more	O
commonly	O
,	O
sector	B-Application
)	O
on	O
the	O
disk	O
has	O
the	O
same	O
size	O
,	O
utilizing	O
4	O
byte	O
block	B-General_Concept
numbers	O
and	O
a	O
new	O
set	O
of	O
command	O
codes	O
.	O
</s>
<s>
FBA	O
as	O
a	O
term	O
was	O
created	O
and	O
used	O
by	O
IBM	O
for	O
its	O
3310	B-Device
and	O
3370	B-Device
HDDs	O
beginning	O
in	O
1979	O
to	O
distinguish	O
such	O
drives	O
as	O
IBM	O
transitioned	O
away	O
from	O
their	O
variable	O
record	O
size	O
format	O
used	O
on	O
IBM	B-Device
's	I-Device
mainframe	I-Device
hard	B-Device
disk	I-Device
drives	I-Device
beginning	O
in	O
1964	O
with	O
its	O
System/360	B-Application
.	O
</s>
<s>
From	O
RAMAC	O
until	O
the	O
early	O
1960s	O
most	O
hard	B-Device
disk	I-Device
drive	I-Device
data	O
were	O
addressed	O
in	O
the	O
form	O
of	O
a	O
three	O
number	O
block	B-General_Concept
addressing	O
scheme	O
Cylinder	O
,	O
Head	O
&	O
Sector	B-Application
(	O
CHS	O
)	O
;	O
the	O
cylinder	O
number	O
,	O
which	O
positioned	O
the	O
head	O
access	O
mechanism	O
;	O
the	O
head	O
number	O
,	O
which	O
selected	O
the	O
read-write	O
head	O
;	O
and	O
the	O
sector	B-Application
number	O
,	O
which	O
specified	O
the	O
rotational	O
position	O
of	O
a	O
fixed	O
size	O
block	B-General_Concept
.	O
</s>
<s>
On	O
June	O
2	O
,	O
1961	O
,	O
IBM	O
introduced	O
the	O
1301	B-Device
,	O
which	O
had	O
variable	O
length	O
records	O
,	O
and	O
the	O
market	O
for	O
sector-oriented	O
disks	O
was	O
eclipsed	O
for	O
decades	O
.	O
</s>
<s>
IBM	O
's	O
1964	O
System/360	B-Application
introduced	O
their	O
new	O
self-formatting	O
variable-length	B-Application
record	I-Application
format	I-Application
for	O
disk	O
and	O
other	O
random-access	O
drives	O
,	O
wherein	O
each	O
record	O
had	O
an	O
optional	O
variable	O
length	O
key	O
field	O
and	O
a	O
variable	O
length	O
data	O
field	O
.	O
</s>
<s>
IBM	B-Device
mainframe	I-Device
disk	O
,	O
drum	O
and	O
mass	O
storage	O
devices	O
,	O
called	O
direct	B-Application
access	I-Application
storage	I-Application
devices	I-Application
(	O
DASD	B-Application
)	O
are	O
addressed	O
using	O
a	O
six	O
byte	O
seek	O
address	O
for	O
seek	O
commands	O
and	O
a	O
five	O
byte	O
CCHHR	O
for	O
search	O
count	O
commands	O
.	O
</s>
<s>
OS/360	B-Application
and	O
other	O
S/360	B-Application
operating	O
systems	O
used	O
an	O
8-byte	O
address	O
structured	O
as	O
MBBCCHHR	O
(	O
Extent	O
(	O
M	O
)	O
-Bin	O
(	O
BB	O
)	O
-Cylinder	O
(	O
CC	O
)	O
-Head	O
(	O
HH	O
)	O
-Record	O
(	O
R	O
)	O
,	O
which	O
was	O
capable	O
of	O
storing	O
records	O
of	O
varying	O
size	O
,	O
up	O
to	O
255	O
such	O
records	O
per	O
track	B-Device
,	O
with	O
the	O
zeroth	O
record	O
(	O
R0	O
)	O
being	O
reserved	O
for	O
certain	O
error	O
correction	O
information	O
,	O
such	O
as	O
skip	O
defects	O
)	O
.	O
</s>
<s>
In	O
addition	O
to	O
addressing	O
records	O
by	O
number	O
,	O
it	O
was	O
possible	O
to	O
search	O
disks	O
by	O
key	O
,	O
using	O
the	O
underlying	O
count	B-Application
key	I-Application
data	I-Application
(	O
CKD	O
)	O
structure	O
.	O
</s>
<s>
The	O
term	O
fixed-block	B-Device
architecture	I-Device
was	O
created	O
by	O
IBM	O
in	O
1979	O
to	O
distinguish	O
this	O
format	O
from	O
its	O
variable-length	B-Application
record	I-Application
format	I-Application
.	O
</s>
<s>
Each	O
track	B-Device
is	O
divided	O
into	O
fixed-length	O
blocks	O
,	O
consisting	O
of	O
an	O
ID	O
field	O
and	O
a	O
data	O
field	O
.	O
</s>
<s>
Application	O
programs	O
refer	O
to	O
blocks	O
by	O
relative	O
block	B-General_Concept
number	O
,	O
and	O
cannot	O
address	O
them	O
by	O
cylinder	O
,	O
head	O
and	O
record	O
.	O
</s>
<s>
Although	O
the	O
FBA	O
commands	O
allowed	O
a	O
query	O
to	O
determine	O
the	O
data	O
area	O
size	O
,	O
the	O
3310	B-Device
and	O
3370	B-Device
have	O
the	O
same	O
size	O
data	O
fields	O
,	O
512	B-Application
bytes	I-Application
.	O
</s>
<s>
Fixed-block	B-Device
architecture	I-Device
was	O
adopted	O
for	O
a	O
few	O
mainframe	O
HDDs	O
produced	O
by	O
IBM	O
beginning	O
in	O
the	O
1970s	O
,	O
and	O
contemporary	O
DASD	B-Application
systems	O
continue	O
to	O
support	O
3310	B-Device
and	O
3370	B-Device
compatibility	O
.	O
</s>
<s>
MVS	B-Application
continues	O
to	O
require	O
CKD	O
DASD	B-Application
,	O
although	O
by	O
the	O
1990s	O
all	O
new	O
IBM	O
HDDs	O
used	O
fixed	O
sectors	O
internally	O
.	O
</s>
<s>
Blocks	O
are	O
typically	O
separated	O
on	O
the	O
track	B-Device
by	O
inter-record	O
gaps	O
.	O
</s>
<s>
Together	O
,	O
the	O
block	B-General_Concept
size	I-General_Concept
and	O
the	O
size	O
of	O
the	O
inter-record	O
gap	O
determine	O
how	O
many	O
blocks	O
can	O
fit	O
in	O
each	O
track	B-Device
.	O
</s>
<s>
A	O
later	O
development	O
in	O
disk	O
addressing	O
was	O
logical	B-Device
block	I-Device
addressing	I-Device
(	O
LBA	B-Device
)	O
,	O
in	O
which	O
the	O
cylinder-head-sector	B-Device
triplet	O
was	O
replaced	O
by	O
a	O
single	O
number	O
,	O
called	O
the	O
block	B-General_Concept
number	O
.	O
</s>
<s>
Within	O
the	O
disk	O
drive	O
,	O
this	O
linear	O
block	B-General_Concept
number	O
was	O
translated	O
into	O
a	O
cylinder	O
number	O
,	O
head	O
number	O
and	O
sector	B-Application
number	O
.	O
</s>
<s>
Moving	O
the	O
translation	O
into	O
the	O
disk	O
drive	O
allowed	O
drive	O
manufacturers	O
to	O
place	O
a	O
different	O
number	O
of	O
blocks	O
on	O
each	O
track	B-Device
transparently	O
to	O
the	O
accessing	O
software	O
.	O
</s>
<s>
Still	O
later	O
,	O
magnetic	O
hard	B-Device
disks	I-Device
employed	O
an	O
evolution	O
of	O
LBA	B-Device
where	O
the	O
size	O
of	O
the	O
addressable	O
disk	B-Application
sectors	I-Application
can	O
differ	O
from	O
the	O
physical	O
block	B-General_Concept
size	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
Advanced	B-Device
Format	I-Device
(	O
AF	O
)	O
512e	O
HDDs	O
use	O
4096-byte	O
physical	O
sectors	O
,	O
while	O
their	O
firmware	O
provides	O
emulation	O
for	O
a	O
virtual	O
sector	B-Application
size	O
of	O
512	B-Application
bytes	I-Application
;	O
thus	O
,	O
"	O
512e	O
"	O
stands	O
for	O
"	O
512-byte	O
emulation	O
"	O
.	O
</s>
