<s>
DriveSpace	B-General_Concept
(	O
initially	O
known	O
as	O
DoubleSpace	B-General_Concept
)	O
is	O
a	O
disk	B-General_Concept
compression	I-General_Concept
utility	O
supplied	O
with	O
MS-DOS	B-Application
starting	O
from	O
version	O
6.0	O
in	O
1993	O
and	O
ending	O
in	O
2000	O
with	O
the	O
release	O
of	O
Windows	B-Device
Me	I-Device
.	O
</s>
<s>
The	O
purpose	O
of	O
DriveSpace	B-General_Concept
is	O
to	O
increase	O
the	O
amount	O
of	O
data	O
the	O
user	O
could	O
store	O
on	O
disks	O
by	O
transparently	O
compressing	O
and	O
decompressing	O
data	O
on-the-fly	O
.	O
</s>
<s>
It	O
is	O
primarily	O
intended	O
for	O
use	O
with	O
hard	B-Device
drives	I-Device
,	O
but	O
use	O
for	O
floppy	B-Device
disks	I-Device
is	O
also	O
supported	O
.	O
</s>
<s>
This	O
feature	O
was	O
removed	O
in	O
Windows	B-Application
XP	I-Application
and	O
later	O
.	O
</s>
<s>
In	O
the	O
most	O
common	O
usage	O
scenario	O
,	O
the	O
user	O
would	O
have	O
one	O
hard	B-Device
drive	I-Device
in	O
the	O
computer	O
,	O
with	O
all	O
the	O
space	O
allocated	O
to	O
one	O
partition	O
(	O
usually	O
as	O
drive	O
C	O
:	O
)	O
.	O
</s>
<s>
Microsoft	O
's	O
decision	O
to	O
add	O
disk	B-General_Concept
compression	I-General_Concept
to	O
MS-DOS	B-Application
6.0	O
was	O
influenced	O
by	O
the	O
fact	O
that	O
the	O
competing	O
DR	O
DOS	O
had	O
earlier	O
started	O
to	O
include	O
disk	B-General_Concept
compression	I-General_Concept
software	O
since	O
version	O
6.0	O
in	O
1991	O
.	O
</s>
<s>
Instead	O
of	O
developing	O
its	O
own	O
product	O
from	O
scratch	O
,	O
Microsoft	O
licensed	O
the	O
technology	O
for	O
the	O
DoubleDisk	O
product	O
developed	O
by	O
Vertisoft	O
and	O
adapted	O
it	O
to	O
become	O
DoubleSpace	B-General_Concept
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
loading	O
of	O
the	O
driver	O
controlling	O
the	O
compression/decompression	O
(	O
DBLSPACE.BIN	O
)	O
became	O
more	O
deeply	O
integrated	O
into	O
the	O
operating	O
system	O
(	O
being	O
loaded	O
through	O
the	O
undocumented	O
pre-load	O
API	O
even	O
before	O
the	O
CONFIG.SYS	B-Device
file	O
)	O
.	O
</s>
<s>
A	O
few	O
computer	O
programs	O
,	O
particularly	O
games	O
,	O
were	O
incompatible	O
with	O
DoubleSpace	B-General_Concept
because	O
they	O
effectively	O
bypassed	O
the	O
DoubleSpace	B-General_Concept
driver	O
.	O
</s>
<s>
DoubleSpace	B-General_Concept
also	O
consumed	O
a	O
significant	O
amount	O
of	O
conventional	B-Device
memory	I-Device
,	O
making	O
it	O
difficult	O
to	O
run	O
memory-intensive	O
programs	O
.	O
</s>
<s>
Microsoft	O
's	O
position	O
was	O
that	O
the	O
error	O
only	O
occurred	O
under	O
unlikely	O
conditions	O
,	O
but	O
fixed	O
the	O
problem	O
in	O
MS-DOS	B-Application
6.2	O
.	O
</s>
<s>
The	O
fragmentation	O
condition	O
was	O
related	O
to	O
the	O
way	O
DoubleSpace	B-General_Concept
compresses	O
individual	O
clusters	O
(	O
of	O
size	O
,	O
say	O
,	O
8	O
K	O
)	O
,	O
and	O
fits	O
them	O
on	O
the	O
disk	O
,	O
occupying	O
fewer	O
sectors	O
(	O
size	O
512	O
bytes	O
)	O
than	O
the	O
fixed	O
number	O
required	O
without	O
DoubleSpace	B-General_Concept
(	O
16	O
sectors	O
in	O
this	O
example	O
)	O
.	O
</s>
<s>
This	O
created	O
the	O
possibility	O
of	O
a	O
kind	O
of	O
internal	O
fragmentation	O
issue	O
,	O
where	O
DoubleSpace	B-General_Concept
would	O
be	O
unable	O
to	O
find	O
enough	O
consecutive	O
sectors	O
for	O
storing	O
a	O
compressed	O
cluster	O
even	O
if	O
plenty	O
of	O
space	O
was	O
available	O
.	O
</s>
<s>
Other	O
potential	O
causes	O
of	O
data	O
loss	O
included	O
the	O
corruption	O
of	O
DoubleSpace	B-General_Concept
's	O
memory	O
areas	O
by	O
other	O
programs	O
,	O
DoubleSpace	B-General_Concept
's	O
memory	O
areas	O
were	O
not	O
protected	O
,	O
because	O
MS-DOS	B-Application
ran	O
in	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
Microsoft	O
attempted	O
to	O
remedy	O
this	O
in	O
the	O
MS-DOS	B-Application
6.2	O
version	O
of	O
DoubleSpace	B-General_Concept
(	O
via	O
a	O
feature	O
called	O
DoubleGuard	O
that	O
would	O
check	O
for	O
such	O
corruption	O
)	O
.	O
</s>
<s>
Turning	O
off	O
the	O
computer	O
before	O
DoubleSpace	B-General_Concept
could	O
finish	O
updating	O
its	O
data	O
structures	O
could	O
also	O
result	O
in	O
data	O
loss	O
.	O
</s>
<s>
This	O
problem	O
was	O
compounded	O
by	O
Microsoft	O
making	O
write	O
caching	O
enabled	O
by	O
default	O
in	O
the	O
SMARTDRV	O
disk	O
cache	O
software	O
that	O
came	O
with	O
MS-DOS	B-Application
6.0	O
.	O
</s>
<s>
Because	O
of	O
this	O
change	O
,	O
after	O
exiting	O
an	O
application	O
,	O
the	O
MS-DOS	B-Application
prompt	O
might	O
appear	O
before	O
all	O
data	O
had	O
been	O
written	O
to	O
the	O
disk	O
.	O
</s>
<s>
However	O
,	O
due	O
to	O
the	O
lack	O
of	O
a	O
controlled	O
shutdown	O
procedure	O
(	O
as	O
found	O
in	O
modern	O
operating	O
systems	O
)	O
,	O
many	O
users	O
saw	O
the	O
appearance	O
of	O
the	O
MS-DOS	B-Application
prompt	O
as	O
an	O
indication	O
that	O
it	O
was	O
safe	O
to	O
switch	O
off	O
the	O
computer	O
,	O
which	O
was	O
typically	O
the	O
case	O
prior	O
to	O
MS-DOS	B-Application
6.0	O
.	O
</s>
<s>
Microsoft	O
addressed	O
this	O
issue	O
in	O
MS-DOS	B-Application
6.2	O
where	O
the	O
write	O
caching	O
was	O
still	O
enabled	O
by	O
default	O
,	O
but	O
where	O
the	O
cache	O
would	O
be	O
flushed	O
before	O
allowing	O
the	O
command	B-Application
prompt	O
to	O
reappear	O
.	O
</s>
<s>
AddStor	O
,	O
Inc	O
.	O
offered	O
an	O
add-on	O
product	O
called	O
Double	B-Application
Tools	I-Application
for	I-Application
DoubleSpace	I-Application
.	O
</s>
<s>
It	O
contained	O
a	O
number	O
of	O
tools	O
to	O
enhance	O
the	O
functions	O
of	O
the	O
version	O
of	O
DoubleSpace	B-General_Concept
that	O
came	O
with	O
MS-DOS	B-Application
6.0	O
.	O
</s>
<s>
This	O
included	O
various	O
diagnostic	O
features	O
,	O
the	O
ability	O
to	O
have	O
compressed	O
removable	O
media	O
auto-mounted	O
as	O
they	O
were	O
used	O
,	O
as	O
well	O
as	O
support	O
for	O
background	O
defragmentation	O
of	O
DoubleSpace	B-General_Concept
compressed	O
drives	O
.	O
</s>
<s>
To	O
defragment	O
files	O
in	O
the	O
background	O
,	O
it	O
was	O
possible	O
to	O
let	O
DoubleTools	B-Application
replace	O
the	O
low-level	O
DoubleSpace	B-General_Concept
driver	O
(	O
DBLSPACE.BIN	O
)	O
with	O
one	O
supplied	O
by	O
DoubleTools	B-Application
.	O
</s>
<s>
Replacing	O
the	O
driver	O
also	O
enabled	O
other	O
enhanced	O
functionality	O
of	O
the	O
product	O
,	O
such	O
as	O
the	O
use	O
of	O
32-bit	O
code	O
paths	O
when	O
it	O
detected	O
an	O
Intel	B-General_Concept
80386	I-General_Concept
or	O
higher	O
CPU	B-Device
,	O
caching	O
capabilities	O
and	O
in	O
addition	O
to	O
its	O
supporting	O
the	O
use	O
of	O
the	O
Upper	B-Device
Memory	I-Device
Area	I-Device
also	O
permitted	O
the	O
use	O
of	O
Extended	B-Device
Memory	I-Device
for	O
some	O
of	O
its	O
buffers	O
(	O
reducing	O
the	O
driver	O
's	O
total	O
footprint	O
in	O
conventional	B-Device
and	O
upper	B-Device
memory	I-Device
,	O
albeit	O
at	O
the	O
cost	O
of	O
somewhat	O
reduced	O
speed	O
)	O
.	O
</s>
<s>
Another	O
function	O
was	O
the	O
ability	O
to	O
split	O
a	O
compressed	O
volume	O
over	O
multiple	O
floppy	B-Device
disks	I-Device
,	O
being	O
able	O
to	O
see	O
the	O
entire	O
volume	O
with	O
only	O
the	O
first	O
disk	O
inserted	O
(	O
and	O
being	O
prompted	O
to	O
change	O
discs	O
as	O
necessary	O
)	O
.	O
</s>
<s>
Double	O
Tools	O
also	O
had	O
the	O
capability	O
to	O
put	O
a	O
special	O
utility	O
on	O
compressed	O
floppy	B-Device
disks	I-Device
that	O
made	O
it	O
possible	O
to	O
access	O
the	O
compressed	O
data	O
even	O
on	O
computers	O
that	O
did	O
n't	O
have	O
DoubleSpace	B-General_Concept
(	O
or	O
Double	O
Tools	O
)	O
.	O
</s>
<s>
Vertisoft	O
,	O
the	O
company	O
who	O
developed	O
the	O
DoubleDisk	O
program	O
that	O
Microsoft	O
subsequently	O
licensed	O
and	O
turned	O
into	O
DoubleSpace	B-General_Concept
,	O
developed	O
and	O
sold	O
a	O
DoubleSpace	B-General_Concept
add-on	O
program	O
called	O
SpaceManager	O
,	O
which	O
contained	O
a	O
number	O
of	O
usability	O
enhancements	O
.	O
</s>
<s>
Other	O
products	O
,	O
like	O
later	O
versions	O
of	O
Stacker	O
from	O
Stac	O
Electronics	O
,	O
were	O
capable	O
of	O
converting	O
existing	O
DoubleSpace	B-General_Concept
compressed	O
drives	O
into	O
their	O
own	O
format	O
.	O
</s>
<s>
MS-DOS	B-Application
6.2	O
featured	O
a	O
new	O
and	O
improved	O
version	O
of	O
DoubleSpace	B-General_Concept
.	O
</s>
<s>
The	O
ability	O
to	O
remove	O
DoubleSpace	B-General_Concept
was	O
added	O
.	O
</s>
<s>
The	O
program	O
SCANDISK	O
introduced	O
in	O
this	O
release	O
was	O
able	O
to	O
scan	O
the	O
non-compressed	O
and	O
compressed	O
drives	O
,	O
including	O
checks	O
of	O
the	O
internal	O
DoubleSpace	B-General_Concept
structures	O
.	O
</s>
<s>
The	O
memory	O
footprint	O
of	O
the	O
DoubleSpace	B-General_Concept
driver	O
was	O
reduced	O
compared	O
to	O
the	O
version	O
shipped	O
in	O
MS-DOS	B-Application
6.0	O
.	O
</s>
<s>
Following	O
a	O
successful	O
lawsuit	O
by	O
Stac	O
Electronics	O
regarding	O
demonstrated	O
patent	O
infringement	O
,	O
Microsoft	O
released	O
MS-DOS	B-Application
6.21	O
without	O
DoubleSpace	B-General_Concept
.	O
</s>
<s>
A	O
court	O
injunction	O
also	O
prevented	O
any	O
further	O
distribution	O
of	O
the	O
previous	O
versions	O
of	O
MS-DOS	B-Application
that	O
included	O
DoubleSpace	B-General_Concept
.	O
</s>
<s>
MS-DOS	B-Application
6.22	O
contained	O
a	O
reimplemented	O
version	O
of	O
the	O
disk	B-General_Concept
compression	I-General_Concept
software	O
,	O
but	O
this	O
time	O
released	O
under	O
the	O
name	O
DriveSpace	B-General_Concept
.	O
</s>
<s>
The	O
software	O
was	O
essentially	O
identical	O
to	O
the	O
MS-DOS	B-Application
6.2	O
version	O
of	O
DoubleSpace	B-General_Concept
from	O
a	O
user	O
point	O
of	O
view	O
,	O
and	O
was	O
compatible	O
with	O
previous	O
versions	O
.	O
</s>
<s>
Windows	B-Application
95	I-Application
had	O
full	O
support	O
of	O
DoubleSpace/DriveSpace	O
via	O
a	O
native	O
32-bit	O
driver	O
for	O
accessing	O
the	O
compressed	O
drives	O
,	O
along	O
with	O
a	O
graphical	O
version	O
of	O
the	O
software	O
tools	O
.	O
</s>
<s>
MS-DOS	B-Application
DriveSpace	B-General_Concept
users	O
could	O
upgrade	O
to	O
Windows	B-Application
95	I-Application
without	O
any	O
troubles	O
.	O
</s>
<s>
Furthermore	O
,	O
the	O
Microsoft	B-Application
Plus	I-Application
!	I-Application
</s>
<s>
for	O
Windows	B-Application
95	I-Application
pack	O
contained	O
version	O
3	O
of	O
DriveSpace	B-General_Concept
.	O
</s>
<s>
One	O
could	O
upgrade	O
from	O
DriveSpace	B-General_Concept
2	O
to	O
DriveSpace	B-General_Concept
3	O
,	O
but	O
there	O
was	O
no	O
downgrade	O
path	O
back	O
to	O
DriveSpace	B-General_Concept
2	O
.	O
</s>
<s>
One	O
could	O
,	O
however	O
,	O
decompress	O
a	O
DriveSpace	B-General_Concept
3	O
drive	O
.	O
</s>
<s>
The	O
DOS	O
device	O
driver	O
of	O
DriveSpace	B-General_Concept
3	O
had	O
a	O
memory	O
footprint	O
of	O
around	O
150	O
KB	O
because	O
of	O
all	O
these	O
new	O
features	O
.	O
</s>
<s>
This	O
caused	O
difficulty	O
for	O
users	O
rebooting	O
into	O
the	O
MS-DOS	B-Application
mode	O
of	O
Windows	B-Application
95	I-Application
for	O
running	O
games	O
,	O
because	O
of	O
the	O
reduced	O
amount	O
of	O
conventional	B-Device
memory	I-Device
that	O
was	O
available	O
.	O
</s>
<s>
DriveSpace	B-General_Concept
3	O
also	O
shipped	O
with	O
Windows	B-Application
95	I-Application
OSR2	O
but	O
many	O
features	O
were	O
disabled	O
unless	O
Plus	B-Application
!	I-Application
</s>
<s>
DriveSpace	B-General_Concept
could	O
also	O
not	O
be	O
used	O
with	O
FAT32	O
,	O
making	O
it	O
of	O
little	O
use	O
on	O
PCs	O
with	O
large	O
hard	B-Device
drives	I-Device
.	O
</s>
<s>
Windows	B-Device
98	I-Device
shipped	O
with	O
DriveSpace	B-General_Concept
3	O
as	O
part	O
of	O
the	O
operating	O
system	O
.	O
</s>
<s>
Functionality	O
was	O
the	O
same	O
as	O
in	O
Windows	B-Application
95	I-Application
with	O
Plus	B-Application
!	I-Application
.	O
</s>
<s>
Because	O
of	O
the	O
removal	O
of	O
real	B-Application
mode	I-Application
support	O
,	O
FAT32	O
going	O
mainstream	O
and	O
the	O
decreasing	O
popularity	O
of	O
DriveSpace	B-General_Concept
,	O
DriveSpace	B-General_Concept
in	O
Windows	B-Device
ME	I-Device
had	O
only	O
limited	O
support	O
.	O
</s>
<s>
DriveSpace	B-General_Concept
no	O
longer	O
supported	O
hard	B-General_Concept
disk	I-General_Concept
compression	I-General_Concept
,	O
but	O
still	O
supported	O
reading	O
and	O
writing	O
compressed	O
removable	O
media	O
,	O
although	O
the	O
only	O
DriveSpace	B-General_Concept
operation	O
supported	O
beside	O
that	O
was	O
deleting	O
and	O
reallocating	O
compressed	O
drives	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
restore	O
full	O
function	O
of	O
DriveSpace	B-General_Concept
3	O
(	O
unofficially	O
)	O
in	O
Windows	B-Device
ME	I-Device
,	O
copying	O
the	O
executable	O
file	O
from	O
a	O
Windows	B-Device
98	I-Device
installation	O
and	O
using	O
it	O
to	O
replace	O
the	O
executable	O
included	O
with	O
Windows	B-Device
ME	I-Device
.	O
</s>
<s>
After	O
that	O
,	O
one	O
could	O
compress	O
new	O
drives	O
as	O
they	O
could	O
do	O
on	O
Windows	B-Device
98	I-Device
.	O
</s>
<s>
DMSDOS	O
,	O
a	O
Linux	O
kernel	O
driver	O
,	O
was	O
developed	O
in	O
the	O
late	O
1990s	O
to	O
support	O
both	O
the	O
reading	O
and	O
writing	O
of	O
DoubleSpace/DriveSpace	O
disks	O
.	O
</s>
<s>
While	O
DR-DOS	O
supported	O
its	O
own	O
disk	B-General_Concept
compression	I-General_Concept
technology	O
(	O
originally	O
based	O
on	O
SuperStor	O
,	O
later	O
on	O
Stacker	O
)	O
,	O
Novell	O
DOS	O
7	O
in	O
1993	O
and	O
higher	O
introduced	O
an	O
emulation	O
of	O
the	O
undocumented	O
pre-load	O
API	O
in	O
order	O
to	O
provide	O
seamless	O
support	O
for	O
DoubleSpace	B-General_Concept
as	O
well	O
.	O
</s>
<s>
Since	O
the	O
DR-DOS	O
drivers	O
were	O
DPMS-enabled	O
whereas	O
the	O
MS-DOS	B-Application
ones	O
were	O
not	O
,	O
this	O
did	O
not	O
offer	O
any	O
advantages	O
for	O
DR-DOS	O
users	O
,	O
but	O
allowed	O
easier	O
coexistence	O
or	O
migration	O
due	O
to	O
the	O
possibility	O
of	O
shared	O
use	O
of	O
already	O
existing	O
compressed	O
volumes	O
in	O
multi-boot	O
scenarios	O
.	O
</s>
<s>
DR-DOS	O
7.02	O
and	O
higher	O
also	O
added	O
support	O
for	O
DriveSpace	B-General_Concept
in	O
1998	O
.	O
</s>
