<s>
DiskDoubler	B-Application
(	O
DD	O
)	O
is	O
a	O
data	B-General_Concept
compression	I-General_Concept
utility	O
for	O
compressing	O
files	O
on	O
the	O
Apple	B-Device
Macintosh	I-Device
platform	O
.	O
</s>
<s>
Unlike	O
most	O
such	O
programs	O
,	O
which	O
compresses	O
numerous	O
files	O
into	O
a	O
single	O
archive	O
for	O
transmission	O
,	O
DiskDoubler	B-Application
is	O
intended	O
to	O
compress	B-Device
single	O
files	O
"	O
in	O
place	O
"	O
to	O
save	O
space	O
on	O
the	O
drive	O
.	O
</s>
<s>
When	O
such	O
a	O
file	B-Operating_System
is	O
opened	O
,	O
DiskDoubler	B-Application
decompresses	O
the	O
file	B-Operating_System
before	O
handing	O
it	O
off	O
to	O
the	O
application	O
for	O
use	O
.	O
</s>
<s>
A	O
later	O
addition	O
,	O
AutoDoubler	B-Application
,	O
adds	O
background	O
compression	O
,	O
finding	O
and	O
compressing	O
files	O
automatically	O
when	O
the	O
computer	O
was	O
idle	O
.	O
</s>
<s>
DiskDoubler	B-Application
was	O
created	O
by	O
Terry	O
Morse	O
and	O
Lloyd	O
Chambers	O
,	O
fellow	O
employees	O
at	O
a	O
small	O
software	O
firm	O
that	O
went	O
out	O
of	O
business	O
in	O
1989	O
.	O
</s>
<s>
Chambers	O
had	O
already	O
released	O
a	O
version	O
of	O
the	O
Unix	B-Application
Compress	B-Device
utility	O
on	O
the	O
Mac	B-Device
as	O
MacCompress	O
,	O
and	O
while	O
working	O
on	O
another	O
"	O
real	O
"	O
project	O
,	O
Chambers	O
wrote	O
DiskDoubler	B-Application
in	O
his	O
spare	O
time	O
.	O
</s>
<s>
When	O
demonstrating	O
their	O
new	O
product	O
at	O
a	O
local	O
Mac	B-Device
store	O
,	O
they	O
noticed	O
that	O
it	O
was	O
DiskDoubler	B-Application
that	O
got	O
all	O
of	O
the	O
attention	O
.	O
</s>
<s>
Realizing	O
they	O
needed	O
real	O
marketing	O
muscle	O
,	O
they	O
approached	O
Symantec	O
,	O
who	O
agreed	O
to	O
include	O
it	O
in	O
their	O
Symantec	O
Utilities	O
for	O
Macintosh	B-Device
(	O
SUM	O
)	O
package	O
for	O
a	O
pittance	O
.	O
</s>
<s>
When	O
System	B-Operating_System
7	I-Operating_System
shipped	O
in	O
June	O
1991	O
sales	O
took	O
off	O
,	O
as	O
the	O
new	O
system	O
was	O
rather	O
hungry	O
for	O
drive	O
space	O
.	O
</s>
<s>
They	O
also	O
repackaged	O
it	O
in	O
a	O
suite	O
as	O
SuperDoubler	O
4.0	O
,	O
including	O
AutoDoubler	B-Application
,	O
DiskDoubler	B-Application
,	O
and	O
a	O
file-copy	O
speedup	O
known	O
as	O
CopyDoubler	O
.	O
</s>
<s>
For	O
some	O
time	O
,	O
DiskDoubler	B-Application
was	O
the	O
second-best	O
selling	O
product	O
on	O
the	O
Mac	B-Device
,	O
second	O
only	O
to	O
After	B-Operating_System
Dark	I-Operating_System
,	O
the	O
popular	O
screen	O
saver	O
.	O
</s>
<s>
Fifth	O
Generation	O
was	O
later	O
sold	O
,	O
somewhat	O
ironically	O
,	O
to	O
Symantec	O
,	O
who	O
re-released	O
it	O
as	O
a	O
fat	O
binary	O
as	O
Norton	O
DiskDoubler	B-Application
Pro	O
1.1	O
.	O
</s>
<s>
DiskDoubler	B-Application
concentrated	O
on	O
speed	O
,	O
originally	O
supporting	O
only	O
a	O
single	O
variety	O
of	O
the	O
LZ78	O
compression	B-General_Concept
algorithm	I-General_Concept
used	O
in	O
Compress/MacCompress	O
to	O
avoid	O
having	O
to	O
try	O
different	O
settings	O
.	O
</s>
<s>
Better	O
yet	O
,	O
DiskDoubler	B-Application
was	O
extremely	O
fast	O
,	O
generally	O
twice	O
as	O
fast	O
as	O
StuffIt	B-Application
,	O
and	O
50%	O
faster	O
than	O
Compact	B-Operating_System
Pro	I-Operating_System
,	O
the	O
two	O
main	O
archivers	O
in	O
use	O
on	O
the	O
Mac	B-Device
in	O
the	O
1990s	O
.	O
</s>
<s>
DiskDoubler	B-Application
also	O
had	O
the	O
capability	O
of	O
decompressing	O
StuffIt	B-Application
and	O
PackIt	B-Operating_System
files	O
,	O
which	O
it	O
did	O
much	O
faster	O
than	O
those	O
programs	O
could	O
.	O
</s>
<s>
Over	O
the	O
years	O
DiskDoubler	B-Application
eventually	O
ended	O
up	O
with	O
four	O
different	O
algorithms	O
,	O
typically	O
using	O
the	O
fastest	O
compressor	O
,	O
DD1	O
,	O
for	O
a	O
first	O
pass	O
,	O
and	O
then	O
running	O
the	O
most	O
effective	O
,	O
DD3+	O
,	O
when	O
the	O
machine	O
was	O
idle	O
.	O
</s>
<s>
DiD3+	O
provided	O
the	O
highest	O
compression	O
of	O
any	O
Mac-based	O
compression	B-General_Concept
software	I-General_Concept
,	O
using	O
knowledge	O
of	O
specific	O
file	B-Operating_System
types	O
to	O
improve	O
over	O
a	O
"	O
generic	O
"	O
LZW	B-Algorithm
scheme	O
.	O
</s>
<s>
Users	O
typically	O
interacted	O
with	O
DiskDoubler	B-Application
via	O
an	O
additional	O
menu	O
placed	O
in	O
the	O
Macintosh	B-Application
Finder	I-Application
.	O
</s>
<s>
Selecting	O
a	O
file	B-Operating_System
,	O
or	O
group	O
of	O
them	O
,	O
the	O
user	O
selects	O
Compress	B-Device
from	O
the	O
DD	O
menu	O
.	O
</s>
<s>
The	O
file	B-Operating_System
in	O
question	O
is	O
quickly	O
compressed	O
and	O
replaced	O
by	O
a	O
similar	O
icon	O
stamped	O
with	O
a	O
small	O
"	O
DD	O
"	O
tag	O
to	O
indicate	O
it	O
was	O
compressed	O
.	O
</s>
<s>
The	O
original	O
Classic	B-Application
Mac	I-Application
OS	I-Application
did	O
not	O
include	O
any	O
sort	O
of	O
composite	O
icon	O
support	O
,	O
so	O
DiskDoubler	B-Application
had	O
to	O
copy	O
and	O
modify	O
every	O
icon	O
it	O
found	O
and	O
then	O
hand	O
those	O
modified	O
icons	O
back	O
to	O
the	O
Finder	B-Application
with	O
a	O
new	O
file	B-Operating_System
type	O
.	O
</s>
<s>
When	O
a	O
file	B-Operating_System
was	O
compressed	O
,	O
its	O
(	O
hidden	O
)	O
file	B-Operating_System
type	O
flag	O
was	O
changed	O
to	O
the	O
one	O
DiskDoubler	B-Application
"	O
made	O
up	O
"	O
,	O
making	O
the	O
Finder	B-Application
display	O
the	O
modified	O
icon	O
.	O
</s>
<s>
AutoDoubler	B-Application
(	O
AD	O
)	O
was	O
a	O
small	O
software	B-Operating_System
daemon	I-Operating_System
for	O
which	O
speed	O
was	O
the	O
main	O
concern	O
,	O
since	O
AD	O
was	O
intended	O
to	O
be	O
used	O
"	O
invisibly	O
"	O
.	O
</s>
<s>
For	O
this	O
reason	O
it	O
first	O
used	O
the	O
AD1/DD1	O
"	O
fastest	O
"	O
method	O
to	O
compress	B-Device
as	O
many	O
files	O
as	O
possible	O
as	O
quickly	O
as	O
it	O
could	O
,	O
and	O
then	O
when	O
that	O
was	O
complete	O
it	O
would	O
go	O
back	O
and	O
re-compress	O
with	O
DD3+	O
if	O
the	O
machine	O
was	O
still	O
idle	O
.	O
</s>
<s>
DD/AD	O
was	O
so	O
invisible	O
that	O
it	O
would	O
compress	B-Device
anything	O
outside	O
the	O
System	O
folder	O
,	O
including	O
applications	O
and	O
various	O
resources	O
.	O
</s>
<s>
The	O
main	O
reason	O
for	O
better	O
performance	O
was	O
the	O
fact	O
that	O
compressing	O
a	O
file	B-Operating_System
and	O
writing	O
it	O
was	O
faster	O
than	O
writing	O
the	O
original	O
file	B-Operating_System
as	O
the	O
bottleneck	O
was	O
to	O
be	O
found	O
in	O
hard	B-Device
disk	I-Device
I/O	O
times	O
.	O
</s>
<s>
The	O
product	O
also	O
included	O
a	O
freeware	B-License
(	O
but	O
closed	B-Application
source	I-Application
)	O
decompressor	O
known	O
as	O
DDExpand	O
.	O
</s>
<s>
Since	O
DiskDoubler	B-Application
was	O
intended	O
to	O
compress	B-Device
"	O
in	O
place	O
"	O
and	O
generally	O
be	O
invisible	O
,	O
most	O
users	O
set	O
up	O
DiskDoubler	B-Application
to	O
decompress	O
automatically	O
when	O
copying	O
files	O
to	O
other	O
media	O
so	O
that	O
it	O
would	O
open	O
fine	O
on	O
other	O
people	O
's	O
machines	O
.	O
</s>
<s>
Additionally	O
,	O
since	O
the	O
program	O
decompressed	O
files	O
as	O
they	O
were	O
opened	O
,	O
the	O
simple	O
action	O
of	O
archiving	O
files	O
using	O
another	O
utility	O
like	O
StuffIt	B-Application
automatically	O
decompressed	O
the	O
files	O
before	O
they	O
were	O
inserted	O
into	O
the	O
new	O
archive	O
.	O
</s>
<s>
For	O
these	O
reasons	O
the	O
DiskDoubler	B-Application
format	O
was	O
rarely	O
seen	O
"	O
in	O
the	O
wild	O
"	O
,	O
and	O
DDExpand	O
was	O
rarely	O
needed	O
.	O
</s>
<s>
DiskDoubler	B-Application
did	O
include	O
an	O
option	O
for	O
this	O
,	O
however	O
,	O
which	O
would	O
combine	O
several	O
files	O
into	O
one	O
archive	O
.	O
</s>
<s>
DiskDoubler	B-Application
created	O
a	O
market	O
for	O
similar	O
products	O
.	O
</s>
<s>
The	O
first	O
attempt	O
to	O
produce	O
a	O
similar	O
product	O
resulted	O
in	O
SuperDisk	O
!,	O
which	O
,	O
when	O
released	O
,	O
was	O
faster	O
than	O
DiskDoubler	B-Application
but	O
offered	O
less	O
compression	O
.	O
</s>
<s>
also	O
offered	O
"	O
on	O
the	O
fly	O
"	O
compression	O
,	O
which	O
DiskDoubler	B-Application
had	O
not	O
added	O
at	O
that	O
point	O
.	O
</s>
<s>
An	O
updated	O
version	O
of	O
DiskDoubler	B-Application
fought	O
back	O
with	O
a	O
new	O
compression	O
scheme	O
that	O
ran	O
entirely	O
in	O
the	O
256-byte	O
cache	O
of	O
the	O
68020	B-Device
,	O
which	O
greatly	O
improved	O
performance	O
.	O
</s>
<s>
AutoDoubler	B-Application
was	O
also	O
included	O
as	O
a	O
new	O
feature	O
.	O
</s>
<s>
Now	O
Software	O
also	O
introduced	O
a	O
product	O
in	O
this	O
space	O
called	O
Now	O
Compress	B-Device
.	O
</s>
<s>
Aladdin	O
Software	O
eventually	O
brought	O
out	O
their	O
own	O
solution	O
as	O
well	O
,	O
as	O
StuffIt	B-Application
SpaceSaver	O
.	O
</s>
<s>
All	O
of	O
these	O
products	O
had	O
a	O
following	O
during	O
the	O
era	O
of	O
small	O
hard	B-Device
drives	I-Device
.	O
</s>
<s>
Eventually	O
,	O
a	O
combination	O
of	O
shrinking	O
Mac	B-Device
marketshare	O
,	O
changes	O
to	O
the	O
underlying	O
filesystem	O
and	O
ever-increasing	O
drive	O
space	O
killed	O
off	O
this	O
product	O
niche	O
.	O
</s>
<s>
With	O
the	O
introduction	O
of	O
Mac	B-Device
OS	I-Device
X	I-Device
Snow	I-Device
Leopard	I-Device
,	O
Apple	O
introduced	O
a	O
similar	O
technology	O
known	O
as	O
Transparent	O
Compression	O
into	O
HFS	B-Application
Plus	I-Application
.	O
</s>
