<s>
The	O
disk	B-General_Concept
controller	I-General_Concept
is	O
the	O
controller	O
circuit	O
which	O
enables	O
the	O
CPU	B-Device
to	O
communicate	O
with	O
a	O
hard	B-Device
disk	I-Device
,	O
floppy	B-Device
disk	I-Device
or	O
other	O
kind	O
of	O
disk	B-Device
drive	I-Device
.	O
</s>
<s>
It	O
also	O
provides	O
an	O
interface	O
between	O
the	O
disk	B-Device
drive	I-Device
and	O
the	O
bus	O
connecting	O
it	O
to	O
the	O
rest	O
of	O
the	O
system	O
.	O
</s>
<s>
Early	O
disk	B-General_Concept
controllers	I-General_Concept
were	O
identified	O
by	O
their	O
storage	O
methods	O
and	O
data	O
encoding	O
.	O
</s>
<s>
Modified	B-Protocol
frequency	I-Protocol
modulation	I-Protocol
(	O
MFM	B-Protocol
)	O
controllers	O
were	O
the	O
most	O
common	O
type	O
in	O
small	O
computers	O
,	O
used	O
for	O
both	O
floppy	B-Device
disk	I-Device
and	O
hard	B-Device
disk	I-Device
drives	I-Device
.	O
</s>
<s>
Run	B-Protocol
length	I-Protocol
limited	I-Protocol
(	O
RLL	B-Protocol
)	O
controllers	O
used	O
data	O
compression	O
to	O
increase	O
storage	O
capacity	O
by	O
about	O
50%	O
.	O
</s>
<s>
Priam	O
created	O
a	O
proprietary	O
storage	O
algorithm	O
that	O
could	O
double	O
the	O
disk	B-Device
storage	I-Device
.	O
</s>
<s>
Shugart	O
Associates	O
Systems	O
Interface	O
(	O
SASI	O
)	O
was	O
a	O
predecessor	O
to	O
SCSI	B-Architecture
.	O
</s>
<s>
Modern	O
disk	B-General_Concept
controllers	I-General_Concept
are	O
integrated	O
into	O
the	O
disk	B-Device
drive	I-Device
as	O
peripheral	O
controllers	O
.	O
</s>
<s>
For	O
example	O
,	O
disks	B-Device
called	O
"	O
SCSI	B-Architecture
disks	B-Device
"	O
have	O
built-in	O
SCSI	B-Architecture
controllers	O
.	O
</s>
<s>
In	O
the	O
past	O
,	O
before	O
most	O
SCSI	B-Architecture
controller	O
functionality	O
was	O
implemented	O
in	O
a	O
single	O
chip	O
,	O
separate	O
SCSI	B-Architecture
controllers	O
interfaced	O
disks	B-Device
to	O
the	O
SCSI	B-Architecture
bus	O
.	O
</s>
<s>
These	O
integrated	O
peripheral	O
controllers	O
communicate	O
with	O
a	O
host	B-Architecture
adapter	I-Architecture
in	O
the	O
host	O
system	O
over	O
a	O
standardized	O
,	O
high-level	O
storage	O
bus	O
interface	O
.	O
</s>
<s>
The	O
most	O
common	O
types	O
of	O
interfaces	O
provided	O
nowadays	O
by	O
host	B-Architecture
controllers	I-Architecture
are	O
PATA	B-Protocol
(	O
IDE	O
)	O
and	O
Serial	O
ATA	O
for	O
home	O
use	O
.	O
</s>
<s>
High-end	O
disks	B-Device
use	O
Parallel	O
SCSI	B-Architecture
,	O
Fibre	B-Architecture
Channel	I-Architecture
or	O
Serial	O
Attached	O
SCSI	B-Architecture
.	O
</s>
<s>
Disk	B-General_Concept
controllers	I-General_Concept
can	O
also	O
control	O
the	O
timing	O
of	O
access	O
to	O
flash	B-Device
memory	I-Device
which	O
is	O
not	O
mechanical	O
in	O
nature	O
(	O
i.e.	O
</s>
<s>
no	O
physical	O
disk	B-Device
)	O
.	O
</s>
<s>
The	O
component	O
that	O
allows	O
a	O
computer	O
to	O
talk	O
to	O
a	O
peripheral	O
bus	O
is	O
host	B-Architecture
adapter	I-Architecture
or	O
host	B-Architecture
bus	I-Architecture
adapter	I-Architecture
(	O
HBA	O
,	O
e.g.	O
</s>
<s>
Advanced	O
Host	B-Architecture
Controller	I-Architecture
Interface	O
or	O
AHDC	O
)	O
.	O
</s>
<s>
A	O
disk	B-General_Concept
controller	I-General_Concept
allows	O
a	O
disk	B-Device
to	O
talk	O
to	O
the	O
same	O
bus	O
.	O
</s>
<s>
Signals	O
read	O
by	O
a	O
disk	B-Device
read-and-write	I-Device
head	I-Device
are	O
converted	O
by	O
a	O
disk	B-General_Concept
controller	I-General_Concept
,	O
then	O
transmitted	O
over	O
the	O
peripheral	O
bus	O
,	O
then	O
converted	O
again	O
by	O
the	O
host	B-Architecture
adapter	I-Architecture
into	O
the	O
suitable	O
format	O
for	O
the	O
motherboard	B-Device
's	O
bus	O
,	O
and	O
then	O
read	O
by	O
the	O
CPU	B-Device
into	O
the	O
memory	O
(	O
RAM	O
)	O
.	O
</s>
<s>
Sometimes	O
there	O
may	O
be	O
yet	O
another	O
controller	O
between	O
a	O
host	B-Architecture
adapter	I-Architecture
and	O
a	O
disk	B-General_Concept
controller	I-General_Concept
-	O
a	O
disk	B-Device
array	I-Device
controller	I-Device
that	O
allows	O
hardware	O
RAID	O
to	O
be	O
formed	O
.	O
</s>
