<s>
The	O
Disc	B-Application
Filing	I-Application
System	I-Application
(	O
DFS	O
)	O
is	O
a	O
computer	B-Application
file	I-Application
system	I-Application
developed	O
by	O
Acorn	O
Computers	O
,	O
initially	O
as	O
an	O
add-on	O
to	O
the	O
Eurocard-based	O
Acorn	B-Device
System	I-Device
2	I-Device
.	O
</s>
<s>
Acorn	O
's	O
Australian	O
computer	O
distributor	O
,	O
Barson	O
Computers	O
,	O
convinced	O
Joint	O
Managing	O
Directors	O
Hermann	O
Hauser	O
and	O
Chris	O
Curry	O
to	O
allow	O
the	O
soon	O
to	O
be	O
released	O
Acorn	B-Device
BBC	I-Device
Microcomputer	I-Device
to	O
be	O
offered	O
with	O
disk	O
storage	O
as	O
part	O
of	O
the	O
bundle	O
.	O
</s>
<s>
They	O
agreed	O
on	O
condition	O
that	O
Barson	O
adapted	O
the	O
Acorn	B-Application
DFS	I-Application
from	O
the	O
System	O
2	O
without	O
assistance	O
from	O
Acorn	O
as	O
they	O
had	O
no	O
resources	O
available	O
.	O
</s>
<s>
This	O
required	O
some	O
minor	O
hardware	O
and	O
software	O
changes	O
to	O
make	O
the	O
DFS	O
compatible	O
with	O
the	O
BBC	B-Device
Micro	I-Device
.	O
</s>
<s>
It	O
was	O
this	O
early	O
initiative	O
that	O
resulted	O
in	O
the	O
BBC	B-Device
Micro	I-Device
being	O
more	O
heavily	O
focused	O
on	O
the	O
education	O
market	O
in	O
Australia	O
,	O
with	O
very	O
little	O
penetration	O
of	O
the	O
home	O
computer	O
market	O
until	O
the	O
arrival	O
of	O
the	O
Acorn	B-Device
Electron	I-Device
.	O
</s>
<s>
The	O
DFS	O
shipped	O
as	O
a	O
ROM	B-Device
and	O
Disk	O
Controller	O
Chip	O
fitted	O
to	O
the	O
BBC	B-Device
Micro	I-Device
's	O
motherboard	O
.	O
</s>
<s>
Other	O
features	O
in	O
third-party	O
implementations	O
included	O
being	O
able	O
to	O
review	O
free	O
space	O
,	O
and	O
built-in	O
FORMAT	O
and	O
VERIFY	O
commands	O
,	O
which	O
were	O
shipped	O
on	O
a	O
utility	O
disc	O
with	O
the	O
original	O
Acorn	B-Application
DFS	I-Application
.	O
</s>
<s>
Acorn	O
followed	O
up	O
their	O
original	O
DFS	O
series	O
with	O
the	O
Acorn	O
1770	O
DFS	O
,	O
which	O
used	O
the	O
same	O
disc	O
format	O
as	O
the	O
earlier	O
version	O
but	O
added	O
a	O
set	O
of	O
extra	O
commands	O
and	O
supported	O
the	O
improved	O
WD1770	B-Device
floppy	B-Device
drive	I-Device
controller	O
chip	O
.	O
</s>
<s>
DFS	O
conventionally	O
uses	O
one	O
side	O
of	O
a	O
double-density	O
5¼	O
"	O
floppy	B-Device
disc	I-Device
.	O
</s>
<s>
Discs	O
are	O
formatted	O
as	O
either	O
40	O
or	O
80	O
track	O
,	O
giving	O
a	O
capacity	O
of	O
100	O
or	O
200	O
KB	O
per	O
side	O
(	O
ten	O
256-byte	O
sectors	O
per	O
track	O
,	O
with	O
FM	B-Protocol
encoding	O
)	O
.	O
</s>
<s>
The	O
capacity	O
is	O
limited	O
by	O
the	O
choice	O
of	O
the	O
Intel	O
8271	O
controller	O
in	O
the	O
original	O
BBC	B-Device
Micro	I-Device
,	O
which	O
only	O
supports	O
FM	B-Protocol
encoding	O
,	O
not	O
the	O
MFM	B-Protocol
encoding	I-Protocol
which	O
was	O
already	O
in	O
common	O
use	O
by	O
the	O
time	O
of	O
the	O
BBC	B-Device
Micro	I-Device
's	O
launch	O
.	O
</s>
<s>
FM	B-Protocol
encoding	O
gives	O
half	O
the	O
recording	O
capacity	O
of	O
MFM	B-Protocol
for	O
a	O
given	O
physical	O
disc	O
density	O
.	O
</s>
<s>
FM	B-Protocol
and	O
MFM	B-Protocol
encoding	I-Protocol
are	O
commonly	O
referred	O
to	O
as	O
"	O
single	O
density	O
"	O
and	O
"	O
double	O
density	O
"	O
,	O
although	O
the	O
discs	O
and	O
drives	O
are	O
the	O
same	O
,	O
unlike	O
"	O
high	O
density	O
"	O
,	O
which	O
uses	O
different	O
drives	O
and	O
discs	O
.	O
</s>
<s>
Double-density	O
3½	O
"	O
discs	O
can	O
be	O
formatted	O
and	O
used	O
with	O
1770	O
DFS	O
(	O
the	O
Intel	O
8271-based	O
DFS	O
has	O
problems	O
with	O
many	O
3½	O
"	O
drives	O
)	O
,	O
giving	O
the	O
same	O
"	O
single-density	O
"	O
capacity	O
with	O
FM	B-Protocol
encoding	O
,	O
but	O
this	O
was	O
not	O
originally	O
standard	O
practice	O
.	O
</s>
<s>
3½	O
"	O
discs	O
were	O
normally	O
formatted	O
as	O
MFM	B-Protocol
"	O
double	O
density	O
"	O
using	O
the	O
later	O
Advanced	B-Device
Disc	I-Device
Filing	I-Device
System	I-Device
,	O
as	O
this	O
is	O
present	O
in	O
all	O
Acorn	O
machines	O
supplied	O
with	O
3½	O
"	O
drives	O
.	O
</s>
<s>
As	O
of	O
2009	O
,	O
3½	O
"	O
drives	O
are	O
more	O
commonly	O
used	O
with	O
BBC	B-Device
Micros	I-Device
than	O
in	O
the	O
past	O
,	O
including	O
use	O
with	O
DFS	O
,	O
due	O
to	O
their	O
greater	O
availability	O
and	O
easier	O
data	O
interchange	O
with	O
more	O
recent	O
computers	O
.	O
</s>
<s>
Track	O
zero	O
is	O
located	O
in	O
the	O
same	O
place	O
on	O
both	O
40	O
-	O
and	O
80-track	O
discs	O
,	O
allowing	O
a	O
disc	O
file	B-Application
system	I-Application
to	O
set	O
the	O
motor	O
stepping	O
accordingly	O
.	O
</s>
<s>
However	O
,	O
the	O
Intel	O
8271-based	O
Acorn	B-Application
DFS	I-Application
does	O
not	O
do	O
so	O
,	O
and	O
so	O
dual-format	O
capability	O
was	O
addressed	O
in	O
a	O
number	O
of	O
ways	O
:	O
</s>
<s>
by	O
simply	O
attaching	O
both	O
a	O
40-track	O
drive	O
and	O
an	O
80-track	O
drive	O
to	O
the	O
BBC	B-Device
Micro	I-Device
,	O
although	O
this	O
was	O
costly	O
for	O
the	O
home	O
user	O
;	O
</s>
<s>
the	O
user	O
could	O
upgrade	O
to	O
a	O
WD1770	B-Device
or	O
similar	O
controller	O
.	O
</s>
<s>
Acorn	O
1770	O
DFS	O
and	O
some	O
third-party	O
controller	O
systems	O
provided	O
dual-format	O
capability	O
in	O
software	O
by	O
reprogramming	O
the	O
controller	O
during	O
track	O
seeks	O
;	O
as	O
a	O
bonus	O
,	O
third-party	O
systems	O
offered	O
proprietary	O
MFM	B-Protocol
(	O
so-called	O
"	O
double-density	O
"	O
)	O
formats	O
for	O
even	O
greater	O
disc	O
capacity	O
.	O
</s>
<s>
The	O
prevalence	O
of	O
all-capitals	O
filenames	O
is	O
most	O
likely	O
due	O
to	O
the	O
BBC	B-Device
Micro	I-Device
defaulting	O
to	O
caps	O
lock	O
being	O
enabled	O
after	O
a	O
hard	O
or	O
soft	O
reset	O
.	O
</s>
<s>
The	O
character	O
set	O
is	O
quite	O
permissive	O
,	O
and	O
all	O
printable	O
characters	O
of	O
7-bit	B-Protocol
ASCII	I-Protocol
are	O
allowed	O
,	O
including	O
spaces	O
,	O
but	O
excluding	O
:	O
</s>
<s>
Control	O
codes	O
generated	O
by	O
the	O
shell	O
escape	B-Algorithm
character	I-Algorithm
|	O
,	O
although	O
the	O
sequence	O
||	O
can	O
be	O
used	O
to	O
represent	O
a	O
single	O
|	O
character	O
in	O
the	O
filename	O
.	O
</s>
<s>
Quotation	O
marks	O
are	O
allowed	O
,	O
although	O
BBC	B-Language
BASIC	I-Language
requires	O
them	O
to	O
be	O
escaped	O
twice	O
:	O
</s>
<s>
commands	O
)	O
,	O
files	O
in	O
the	O
current	B-Application
directory	I-Application
are	O
shown	O
with	O
no	O
directory	O
prefix	O
in	O
one	O
block	O
,	O
and	O
below	O
that	O
are	O
listed	O
all	O
other	O
files	O
in	O
a	O
second	O
block	O
,	O
with	O
their	O
directory	O
prefixes	O
visible	O
.	O
</s>
<s>
For	O
example	O
,	O
(	O
from	O
Acorn	B-Application
DFS	I-Application
-	O
third	O
party	O
DFS	O
implementations	O
may	O
vary	O
slightly	O
)	O
:	O
</s>
<s>
The	O
top	O
seven	O
files	O
are	O
all	O
in	O
the	O
current	B-Application
directory	I-Application
which	O
is	O
$	O
on	O
drive	O
0	O
.	O
</s>
<s>
The	O
"	O
directory	O
"	O
is	O
the	O
working	B-Application
directory	I-Application
on	O
the	O
current	O
volume	O
,	O
much	O
like	O
the	O
working	B-Application
directory	I-Application
on	O
any	O
other	O
command	O
line	O
system	O
.	O
</s>
<s>
The	O
"	O
library	O
"	O
is	O
a	O
second	O
,	O
alternative	O
working	B-Application
directory	I-Application
that	O
functions	O
more	O
like	O
PATH	B-Application
and	O
had	O
the	O
benefit	O
of	O
being	O
able	O
to	O
be	O
on	O
any	O
volume	O
.	O
</s>
<s>
Requests	O
to	O
open	O
files	O
with	O
unqualified	O
names	O
,	O
will	O
first	O
be	O
searched	O
for	O
in	O
the	O
working	B-Application
directory	I-Application
;	O
failing	O
this	O
,	O
the	O
library	O
directory	O
will	O
also	O
be	O
searched	O
.	O
</s>
<s>
The	O
DFS	O
does	O
not	O
support	O
data	B-Architecture
fragmentation	I-Architecture
,	O
meaning	O
a	O
file	O
's	O
data	O
must	O
be	O
stored	O
in	O
a	O
single	O
run	O
of	O
consecutive	O
sectors	O
,	O
but	O
free	O
space	O
is	O
prone	O
to	O
becoming	O
fragmented	O
.	O
</s>
<s>
In	O
such	O
cases	O
the	O
DFS	O
aborts	O
with	O
a	O
Ca	B-Device
n't	I-Device
extend	I-Device
error	O
.	O
</s>
<s>
In	O
contrast	O
the	O
*	O
COMPACT	O
command	O
uses	O
program	B-Device
memory	I-Device
as	O
a	O
buffer	O
to	O
relocate	O
the	O
files	O
,	O
overwriting	O
any	O
program	O
and	O
data	O
in	O
memory	O
.	O
</s>
<s>
Like	O
the	O
cassette	O
filing	O
system	O
,	O
the	O
Acorn	B-Application
DFS	I-Application
supports	O
the	O
BBC	B-Device
Micro	I-Device
's	O
standard	O
file	O
metadata	O
:	O
load	O
address	O
and	O
execution	O
address	O
,	O
required	O
because	O
Acorn	B-Operating_System
MOS	I-Operating_System
(	O
the	O
operating	B-General_Concept
system	I-General_Concept
used	O
by	O
the	O
BBC	B-Device
Micro	I-Device
)	O
does	O
not	O
support	O
relocation	O
of	O
binary	O
code	O
.	O
</s>
<s>
DFS	O
discs	O
do	O
not	O
track	O
any	O
dates	O
(	O
because	O
Acorn	B-Operating_System
MOS	I-Operating_System
prior	O
to	O
version	O
3	O
did	O
not	O
maintain	O
a	O
real-time	O
clock	O
)	O
but	O
instead	O
offer	O
a	O
peculiar	O
feature	O
:	O
a	O
modification	O
count	O
.	O
</s>
<s>
Option	O
3	O
reads	O
"	O
EXEC	O
"	O
files	O
,	O
text	O
macro	O
files	O
used	O
as	O
primitive	O
shell	B-Language
scripts	I-Language
.	O
</s>
<s>
These	O
are	O
not	O
true	O
shell	B-Language
scripts	I-Language
but	O
simply	O
a	O
series	O
of	O
keys	O
to	O
be	O
typed	O
,	O
like	O
a	O
recording	O
to	O
play	O
back	O
.	O
</s>
<s>
As	O
well	O
as	O
being	O
used	O
during	O
a	O
reset	O
,	O
they	O
can	O
be	O
executed	O
at	O
any	O
time	O
with	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
*	O
EXEC	O
command	O
.	O
</s>
<s>
EXEC	O
files	O
are	O
file	B-Application
system	I-Application
independent	O
.	O
</s>
<s>
There	O
was	O
a	O
variant	O
of	O
the	O
DFS	O
called	O
the	O
DNFS	O
,	O
or	O
Disc/Network	O
Filing	O
System	O
,	O
that	O
contained	O
the	O
Econet	O
Network	O
Filing	O
System	O
(	O
NFS	O
)	O
,	O
standard	O
Disc	B-Application
Filing	I-Application
System	I-Application
and	O
Tube	O
co-processor	O
support	O
software	O
on	O
a	O
single	O
ROM	B-Device
;	O
this	O
ROM	B-Device
installed	O
two	O
filing	O
systems	O
into	O
the	O
OS	O
at	O
once	O
.	O
</s>
<s>
The	O
initial	O
design	O
for	O
the	O
DFS	O
was	O
based	O
around	O
an	O
Intel	O
Corporation	O
FDC	O
8271	O
disc	O
drive	O
controller	O
,	O
the	O
immediate	O
predecessor	O
of	O
the	O
8272	O
design	O
found	O
in	O
the	O
IBM	B-Device
Personal	I-Device
Computer	I-Device
.	O
</s>
<s>
The	O
8271	O
controller	O
was	O
of	O
limited	O
functionality	O
and	O
obsolete	O
,	O
and	O
later	O
versions	O
of	O
the	O
file	B-Application
system	I-Application
from	O
various	O
vendors	O
including	O
Solidisk	O
,	O
Acorn	O
and	O
Watford	O
Electronics	O
were	O
based	O
on	O
the	O
later	O
WD1770	B-Device
and	O
WD1772	O
drive	O
controllers	O
.	O
</s>
<s>
The	O
DFS	O
was	O
superseded	O
by	O
the	O
Advanced	B-Device
Disc	I-Device
Filing	I-Device
System	I-Device
(	O
ADFS	O
)	O
which	O
was	O
fully	O
hierarchical	O
and	O
was	O
suitable	O
for	O
running	O
hard	O
drives	O
on	O
the	O
BBC	B-Device
Micro	I-Device
.	O
</s>
<s>
The	O
ADFS	O
was	O
the	O
default	O
filing	O
system	O
on	O
the	O
BBC	B-Device
Master	I-Device
but	O
most	O
users	O
are	O
likely	O
to	O
have	O
reverted	O
to	O
the	O
DFS	O
for	O
compatibility	O
reasons	O
;	O
ADFS	O
could	O
not	O
read	O
DFS	O
discs	O
.	O
</s>
<s>
The	O
ADFS	O
was	O
also	O
chosen	O
as	O
the	O
standard	O
filing	O
system	O
for	O
RISC	B-Operating_System
OS	I-Operating_System
.	O
</s>
