<s>
Frequency	B-Protocol
modulation	I-Protocol
encoding	I-Protocol
,	O
or	O
simply	O
FM	B-Protocol
,	O
is	O
a	O
method	O
of	O
storing	O
data	O
that	O
saw	O
widespread	O
use	O
in	O
early	O
floppy	B-Device
disk	I-Device
drives	I-Device
and	O
hard	B-Device
disk	I-Device
drives	I-Device
.	O
</s>
<s>
The	O
data	O
is	O
modified	O
using	O
differential	B-Protocol
Manchester	I-Protocol
encoding	I-Protocol
when	O
written	O
to	O
allow	O
clock	O
recovery	O
to	O
address	O
timing	O
effects	O
known	O
as	O
"	O
jitter	O
"	O
seen	O
on	O
disk	O
media	O
.	O
</s>
<s>
It	O
was	O
introduced	O
on	O
IBM	O
mainframe	O
drives	O
and	O
was	O
almost	O
universal	O
among	O
early	O
minicomputer	B-Architecture
and	O
microcomputer	B-Architecture
floppies	B-Device
.	O
</s>
<s>
In	O
the	O
case	O
of	O
floppies	B-Device
,	O
FM	B-Protocol
encoding	I-Protocol
allowed	O
about	O
80kB	O
of	O
data	O
to	O
be	O
stored	O
on	O
a	O
-inch	O
disk	O
.	O
</s>
<s>
IBM	O
began	O
introducing	O
the	O
more	O
efficient	O
modified	B-Protocol
frequency	I-Protocol
modulation	I-Protocol
,	O
or	O
MFM	B-Protocol
,	O
starting	O
in	O
1970	O
.	O
</s>
<s>
They	O
referred	O
to	O
this	O
format	O
as	O
"	O
double	O
density	O
"	O
,	O
with	O
the	O
original	O
FM	B-Protocol
retroactively	O
becoming	O
"	O
single	O
density	O
"	O
.	O
</s>
<s>
MFM	B-Protocol
was	O
more	O
difficult	O
to	O
implement	O
and	O
it	O
was	O
not	O
until	O
the	O
early	O
1980s	O
that	O
low-cost	O
all-in-one	O
MFM	B-Protocol
floppy	B-Device
drive	I-Device
controllers	O
like	O
the	O
WD1770	B-Device
emerged	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
rapid	O
demise	O
of	O
FM	B-Protocol
encoding	I-Protocol
in	O
favor	O
of	O
MFM	B-Protocol
by	O
the	O
mid-1980s	O
.	O
</s>
<s>
The	O
letter	O
"	O
A	O
"	O
in	O
ASCII	B-Protocol
is	O
represented	O
as	O
01000001	O
in	O
binary	O
,	O
which	O
might	O
be	O
stored	O
in	O
a	O
typical	O
late-1970s	O
DRAM	O
like	O
the	O
Mostek	O
MK4116	O
as	O
a	O
series	O
of	O
0	O
and	O
5V	O
voltages	O
in	O
the	O
individual	O
capacitors	O
making	O
up	O
the	O
memory	O
.	O
</s>
<s>
In	O
contrast	O
,	O
magnetic	B-Architecture
recording	I-Architecture
systems	O
like	O
floppy	B-Device
disks	I-Device
record	O
this	O
data	O
as	O
a	O
change	O
in	O
magnetic	O
polarity	O
.	O
</s>
<s>
During	O
reading	O
,	O
the	O
disk	O
is	O
rotating	O
so	O
its	O
surface	O
moves	O
rapidly	O
past	O
the	O
read/write	B-Device
head	I-Device
,	O
a	O
small	O
electromagnet	O
.	O
</s>
<s>
One	O
form	O
of	O
unavoidable	O
jitter	O
is	O
due	O
to	O
the	O
hysteresis	O
of	O
the	O
magnetic	B-Architecture
media	I-Architecture
,	O
which	O
can	O
lead	O
to	O
an	O
effect	O
known	O
as	O
bit	O
shift	O
that	O
causes	O
the	O
strings	O
of	O
magnetic	O
transition	O
to	O
be	O
stretched	O
out	O
in	O
time	O
.	O
</s>
<s>
FM	B-Protocol
encoding	I-Protocol
uses	O
a	O
simple	O
system	O
to	O
encode	O
the	O
original	O
data	O
in	O
such	O
a	O
way	O
that	O
every	O
bit	O
of	O
data	O
will	O
contain	O
at	O
least	O
one	O
transition	O
,	O
ensuring	O
there	O
are	O
enough	O
transitions	O
during	O
a	O
given	O
period	O
for	O
a	O
successful	O
clock	O
recovery	O
.	O
</s>
<s>
Since	O
each	O
bit	O
of	O
data	O
requires	O
two	O
minimum	O
times	O
,	O
FM	B-Protocol
encoding	I-Protocol
stores	O
about	O
half	O
the	O
amount	O
that	O
is	O
theoretically	O
possible	O
on	O
that	O
media	O
.	O
</s>
<s>
FM	B-Protocol
uses	O
an	O
implementation	O
of	O
the	O
differential	B-Protocol
Manchester	I-Protocol
encoding	I-Protocol
.	O
</s>
<s>
For	O
instance	O
,	O
if	O
a	O
byte	O
of	O
data	O
from	O
the	O
original	O
system	O
contains	O
the	O
bits	O
01000001	O
,	O
the	O
floppy	B-Device
disk	I-Device
controller	I-Device
will	O
translate	O
this	O
into	O
the	O
series	O
1011101010101011	O
,	O
inserting	O
additional	O
signals	O
in	O
front	O
of	O
every	O
bit	O
to	O
represent	O
the	O
clock	O
.	O
</s>
<s>
When	O
this	O
signal	O
is	O
then	O
sent	O
to	O
the	O
read/write	B-Device
head	I-Device
,	O
the	O
polarity	O
will	O
be	O
flipped	O
every	O
time	O
there	O
is	O
a	O
pulse	O
.	O
</s>
<s>
Encoding	O
these	O
transitions	O
requires	O
the	O
system	O
to	O
accept	O
digital	O
data	O
from	O
the	O
host	O
computer	O
and	O
then	O
re-code	O
it	O
into	O
the	O
underlying	O
FM	B-Protocol
format	O
.	O
</s>
<s>
Because	O
the	O
FM	B-Protocol
system	O
is	O
so	O
simple	O
,	O
it	O
could	O
be	O
implemented	O
in	O
single-chip	O
forms	O
using	O
late	O
1970	O
's	O
semiconductor	B-Architecture
fabrication	I-Architecture
techniques	O
.	O
</s>
<s>
Especially	O
popular	O
was	O
the	O
Western	B-Device
Digital	I-Device
FD1771	I-Device
and	O
its	O
variations	O
.	O
</s>
<s>
In	O
most	O
disks	O
,	O
the	O
only	O
unit	O
of	O
data	O
is	O
the	O
sector	B-Application
,	O
and	O
the	O
individual	O
bytes	O
within	O
it	O
have	O
no	O
meaning	O
to	O
the	O
controller	O
.	O
</s>
<s>
When	O
data	O
is	O
written	O
,	O
the	O
controller	O
is	O
handed	O
a	O
full	O
sector	B-Application
's	O
worth	O
of	O
data	O
and	O
told	O
to	O
write	O
it	O
as	O
a	O
single	O
atomic	O
operation	O
as	O
a	O
series	O
of	O
bits	O
.	O
</s>
<s>
The	O
controller	O
cannot	O
align	O
the	O
bits	O
with	O
the	O
bytes	O
based	O
solely	O
on	O
the	O
FM	B-Protocol
information	O
.	O
</s>
<s>
Thus	O
it	O
is	O
not	O
only	O
the	O
bits	O
within	O
the	O
data	O
that	O
have	O
to	O
be	O
aligned	O
on	O
reading	O
,	O
but	O
the	O
starting	O
point	O
of	O
the	O
sector	B-Application
's	O
data	O
as	O
a	O
whole	O
.	O
</s>
<s>
This	O
is	O
not	O
accomplished	O
with	O
the	O
encoding	O
scheme	O
,	O
but	O
the	O
disk	B-Application
format	I-Application
instead	O
.	O
</s>
<s>
When	O
the	O
controller	O
writes	O
a	O
sector	B-Application
of	O
data	O
,	O
it	O
adds	O
a	O
header	O
section	O
containing	O
information	O
about	O
the	O
data	O
that	O
follows	O
,	O
as	O
well	O
as	O
the	O
address	O
of	O
the	O
sector	B-Application
so	O
it	O
can	O
be	O
found	O
in	O
the	O
future	O
.	O
</s>
<s>
These	O
are	O
not	O
FM	B-Protocol
encoded	O
,	O
so	O
the	O
controller	O
can	O
easily	O
identify	O
them	O
on-the-fly	O
.	O
</s>
<s>
As	O
each	O
bit	O
of	O
data	O
requires	O
two	O
transition	O
periods	O
in	O
the	O
FM	B-Protocol
system	O
,	O
it	O
makes	O
use	O
of	O
only	O
half	O
the	O
potential	O
storage	O
capacity	O
of	O
the	O
disk	O
.	O
</s>
<s>
The	O
most	O
widely	O
used	O
replacement	O
was	O
modified	B-Protocol
frequency	I-Protocol
modulation	I-Protocol
,	O
or	O
MFM	B-Protocol
.	O
</s>
<s>
MFM	B-Protocol
requires	O
a	O
more	O
complex	O
solution	O
to	O
recovering	O
the	O
clock	O
signal	O
.	O
</s>
<s>
This	O
was	O
beyond	O
the	O
capabilities	O
of	O
low-cost	O
ICs	O
from	O
the	O
late	O
1970s	O
,	O
which	O
is	O
why	O
FM	B-Protocol
remained	O
popular	O
during	O
the	O
early	O
home	O
computer	O
era	O
in	O
the	O
early	O
1980s	O
.	O
</s>
<s>
MFM	B-Protocol
IC	O
's	O
were	O
available	O
,	O
and	O
were	O
used	O
on	O
more	O
expensive	O
platforms	O
like	O
the	O
IBM	B-Device
PC	I-Device
,	O
but	O
using	O
them	O
required	O
the	O
clock	O
recovery	O
to	O
be	O
performed	O
by	O
external	O
hardware	O
,	O
the	O
"	O
data	O
separator	O
"	O
.	O
</s>
<s>
IC	O
manufacturing	O
was	O
advancing	O
rapidly	O
during	O
this	O
period	O
,	O
and	O
by	O
the	O
mid-1980s	O
all-in-one	O
MFM	B-Protocol
controllers	O
appeared	O
and	O
the	O
market	O
rapidly	O
moved	O
to	O
the	O
double-density	O
format	O
.	O
</s>
