<s>
The	O
Advanced	B-Device
Disc	I-Device
Filing	I-Device
System	I-Device
(	O
ADFS	O
)	O
is	O
a	O
computing	O
file	B-Application
system	I-Application
unique	O
to	O
the	O
Acorn	O
computer	O
range	O
and	O
RISC	O
OS-based	O
successors	O
.	O
</s>
<s>
Initially	O
based	O
on	O
the	O
rare	O
Acorn	O
Winchester	O
Filing	O
System	O
,	O
it	O
was	O
renamed	O
to	O
the	O
Advanced	B-Device
Disc	I-Device
Filing	I-Device
System	I-Device
when	O
support	O
for	O
floppy	B-Device
discs	I-Device
was	O
added	O
(	O
using	O
a	O
WD1770	B-Device
floppy	I-Device
disc	I-Device
controller	I-Device
)	O
and	O
on	O
later	O
32-bit	O
systems	O
a	O
variant	O
of	O
a	O
PC-style	O
floppy	B-Device
controller	O
.	O
</s>
<s>
Acorn	O
's	O
original	O
Disc	B-Application
Filing	I-Application
System	I-Application
was	O
limited	O
to	O
31	O
files	O
per	O
disk	O
surface	O
,	O
7	O
characters	O
per	O
file	O
name	O
and	O
a	O
single	O
character	O
for	O
directory	O
names	O
,	O
a	O
format	O
inherited	O
from	O
the	O
earlier	O
Atom	O
and	O
System	O
3	O
–	O
5	O
Eurocard	O
computers	O
.	O
</s>
<s>
It	O
retained	O
some	O
superficial	O
attributes	O
from	O
DFS	O
;	O
the	O
directory	O
separator	O
continued	O
to	O
be	O
a	O
dot	O
and	O
$	O
now	O
indicated	O
the	O
hierarchical	O
root	O
of	O
the	O
filesystem	B-Application
.	O
</s>
<s>
ADFS	O
on	O
8-bit	O
systems	O
required	O
a	O
WD1770	B-Device
or	O
later	O
1772-series	O
floppy	B-Device
controller	O
,	O
owing	O
to	O
the	O
inability	O
of	O
the	O
original	O
Intel	O
8271	O
chip	O
to	O
cope	O
with	O
the	O
double-density	O
format	O
ADFS	O
required	O
.	O
</s>
<s>
ADFS	O
could	O
however	O
be	O
used	O
to	O
support	O
hard	O
discs	O
without	O
a	O
1770	O
controller	O
present	O
;	O
in	O
development	O
the	O
use	O
of	O
hard	O
discs	O
was	O
the	O
primary	O
goal	O
,	O
extension	O
to	O
handle	O
floppies	B-Device
came	O
later	O
.	O
</s>
<s>
The	O
1770	O
floppy	B-Device
controller	O
was	O
directly	O
incorporated	O
into	O
the	O
design	O
of	O
the	O
Master	O
Series	O
and	O
B+	O
models	O
,	O
and	O
was	O
available	O
as	O
an	O
upgrade	O
board	O
for	O
the	O
earlier	O
Model	O
B	O
.	O
ADFS	O
could	O
be	O
added	O
to	O
Model	O
B	O
and	O
B+	O
systems	O
with	O
an	O
additional	O
upgrade	O
.	O
</s>
<s>
Acorn	O
's	O
original	O
ADFS	O
implementation	O
on	O
the	O
Electron	O
was	O
unreliable	O
when	O
writing	O
to	O
the	O
first	O
few	O
tracks	O
of	O
a	O
floppy	B-Device
disc	I-Device
,	O
so	O
this	O
was	O
a	O
"	O
fix	O
"	O
and	O
simply	O
involved	O
writing	O
a	O
file	O
full	O
of	O
garbage	O
to	O
the	O
suspect	O
part	O
.	O
</s>
<s>
ADFS	O
supported	O
hard	O
discs	O
,	O
and	O
5¼	O
"	O
floppy	B-Device
discs	I-Device
formatted	O
up	O
to	O
640KB	O
capacity	O
using	O
double	O
density	O
MFM	B-Protocol
encoding	I-Protocol
(	O
L	O
format	O
;	O
single-sided	O
disks	O
were	O
supported	O
with	O
the	O
S	O
format	O
(	O
160KB	O
)	O
and	O
M	O
format	O
(	O
320KB	O
)	O
)	O
.	O
</s>
<s>
ADFS	O
as	O
implemented	O
in	O
the	O
BBC	O
microcomputer	O
system	O
(	O
and	O
later	O
RISC	B-Operating_System
OS	I-Operating_System
)	O
never	O
had	O
support	O
for	O
single-density	O
floppies	B-Device
.	O
</s>
<s>
only	O
the	O
underlying	O
arrangement	O
of	O
tracks	O
and	O
sectors	O
differed	O
depending	O
on	O
the	O
actual	O
drive	O
used	O
,	O
but	O
this	O
was	O
managed	O
by	O
the	O
SCSI	B-Architecture
controller	O
.	O
</s>
<s>
It	O
interfaced	O
to	O
a	O
ST506/ST412	B-Device
-based	O
Winchester	O
unit	O
via	O
the	O
BBC	O
Micro	O
's	O
1MHz	O
Bus	O
,	O
an	O
Acorn-designed	O
interface	O
card	O
(	O
1MHz	O
Bus	O
to	O
SCSI	B-Architecture
adapter	O
)	O
and	O
an	O
off-the-shelf	O
Adaptec	O
SCSI	B-Architecture
controller	O
(	O
SCSI	B-Architecture
to	O
ST-506	B-Device
adapter	O
)	O
.	O
</s>
<s>
Support	O
for	O
IDE/ATAPI	O
style	O
drives	O
has	O
been	O
added	O
'	O
unofficially	O
 '	O
by	O
third	O
parties	O
in	O
recent	O
years	O
.	O
</s>
<s>
The	O
ADFS	O
file	O
format	O
used	O
the	O
ASCII	B-Protocol
bytes	O
to	O
delimit	O
the	O
directory	O
names	O
on	O
the	O
disc	O
,	O
named	O
after	O
ADFS	O
author	O
Hugo	O
Tyson	O
.	O
</s>
<s>
On	O
32-bit	O
systems	O
,	O
a	O
WD	O
1770	O
or	O
1772	O
was	O
initially	O
used	O
as	O
a	O
floppy	B-Device
controller	O
on	O
the	O
early	O
machines	O
of	O
the	O
range	O
.	O
</s>
<s>
In	O
addition	O
to	O
legacy	O
support	O
for	O
the	O
'	O
L'-type	O
format	O
,	O
Arthur	O
and	O
later	O
RISC	B-Operating_System
OS	I-Operating_System
provided	O
enhanced	O
formats	O
which	O
overcome	O
the	O
limitations	O
of	O
the	O
BBC	O
Micro	O
.	O
</s>
<s>
Arthur	O
added	O
D	O
format	O
with	O
77	O
entries	O
per	O
directory	O
as	O
opposed	O
to	O
the	O
previous	O
47	O
,	O
also	O
usable	O
on	O
hard	O
discs	O
and	O
a	O
new	O
800KB	O
double-density	O
floppy	B-Device
format	O
.	O
</s>
<s>
This	O
is	O
similar	O
to	O
the	O
32-bit	O
type	O
attributes	O
stored	O
in	O
Apple	O
's	O
HFS	B-Application
file	B-Application
system	I-Application
,	O
and	O
conceptually	O
comparable	O
to	O
the	O
more	O
general	O
use	O
of	O
MIME	O
Types	O
by	O
the	O
Be	B-Application
Operating	I-Application
System	I-Application
(	O
BeOS	B-Application
)	O
,	O
or	O
magic	O
numbers	O
in	O
Unix	O
systems	O
(	O
though	O
the	O
latter	O
is	O
stored	O
as	O
part	O
of	O
the	O
file	O
,	O
not	O
as	O
metadata	O
)	O
.	O
</s>
<s>
RISC	B-Operating_System
OS	I-Operating_System
brought	O
in	O
E	O
and	O
F	O
format	O
for	O
double-density	O
discs	O
and	O
high-density	O
discs/hard	O
disc	O
drives	O
respectively	O
.	O
</s>
<s>
The	O
allocation	O
strategy	O
is	O
optimised	O
to	O
minimise	O
fragmentation	O
,	O
and	O
sometimes	O
performs	O
defragmentation	O
as	O
part	O
of	O
a	O
file	B-Application
storage	I-Application
operation	O
.	O
</s>
<s>
RISC	B-Operating_System
OS	I-Operating_System
4	O
added	O
E+	O
/F	O
+	O
format	O
which	O
allowed	O
for	O
long	O
filenames	O
and	O
more	O
than	O
77	O
files	O
per	O
directory	O
.	O
</s>
<s>
More	O
recent	O
versions	O
of	O
RISC	B-Operating_System
OS	I-Operating_System
,	O
including	O
those	O
for	O
Iyonix	B-Device
,	O
continue	O
to	O
provide	O
ADFS	O
,	O
and	O
have	O
further	O
extended	O
it	O
to	O
cope	O
with	O
larger	O
hard	O
disc	O
sizes	O
.	O
</s>
<s>
Unlike	O
the	O
8-bit	O
implementation	O
,	O
ADFS	O
as	O
implemented	O
on	O
RISC	B-Operating_System
OS	I-Operating_System
is	O
not	O
monolithic	O
.	O
</s>
<s>
A	O
system	O
module	O
called	O
"	O
ADFS	O
"	O
provides	O
no	O
more	O
than	O
the	O
block	O
driver	O
and	O
user	O
interfaces	O
,	O
where	O
the	O
"	O
FileCore	O
"	O
module	O
contains	O
the	O
actual	O
file	B-Application
system	I-Application
implementation	O
,	O
and	O
FileSwitch	O
contains	O
the	O
VFS	O
and	O
high-level	O
file-access	O
API	O
implementations	O
.	O
</s>
<s>
This	O
allows	O
for	O
other	O
hardware	O
to	O
use	O
the	O
ADFS	O
format	O
easily	O
,	O
such	O
as	O
IDEFS	O
(	O
commonly	O
used	O
for	O
IDE	B-Protocol
add-on	O
cards	O
)	O
,	O
SCSIFS	O
,	O
and	O
the	O
network-aware	O
AppFS	O
.	O
</s>
<s>
This	O
flexibility	O
has	O
allowed	O
other	O
filing	O
systems	O
to	O
be	O
implemented	O
into	O
RISC	B-Operating_System
OS	I-Operating_System
relatively	O
easily	O
.	O
</s>
<s>
The	O
Linux	B-Application
kernel	O
has	O
ADFS	O
support	O
for	O
E	O
format	O
and	O
later	O
.	O
</s>
<s>
NetBSD	B-Device
has	O
filecore	O
support	O
in	O
NetBSD	B-Device
1.4	O
onwards	O
.	O
</s>
<s>
Tools	O
such	O
as	O
Omniflop	O
(	O
in	O
Windows	O
2000	O
and	O
later	O
)	O
,	O
and	O
Libdsk	O
support	O
permit	O
the	O
'	O
physical	O
 '	O
layout	O
of	O
ADFS	O
floppies	B-Device
to	O
be	O
read	O
on	O
PC	O
systems	O
utilising	O
an	O
internal	O
drive	O
.	O
</s>
