<s>
A	O
disk	O
compression	B-General_Concept
software	I-General_Concept
utility	O
increases	O
the	O
amount	O
of	O
information	O
that	O
can	O
be	O
stored	O
on	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
of	O
given	O
size	O
.	O
</s>
<s>
Unlike	O
a	O
file	B-General_Concept
compression	B-General_Concept
utility	I-General_Concept
,	O
which	O
compresses	B-General_Concept
only	O
specified	O
files	O
—	O
and	O
which	O
requires	O
the	O
user	O
to	O
designate	O
the	O
files	O
to	O
be	O
compressed	O
—	O
an	O
on-the-fly	O
disk	O
compression	B-General_Concept
utility	I-General_Concept
works	O
automatically	O
through	O
resident	O
software	O
without	O
the	O
user	O
needing	O
to	O
be	O
aware	O
of	O
its	O
existence	O
.	O
</s>
<s>
When	O
information	O
needs	O
to	O
be	O
stored	O
to	O
the	O
hard	B-Device
disk	I-Device
,	O
the	O
utility	O
compresses	B-General_Concept
the	O
information	O
.	O
</s>
<s>
A	O
disk	O
compression	B-General_Concept
utility	I-General_Concept
overrides	O
the	O
standard	O
operating	B-General_Concept
system	I-General_Concept
routines	O
.	O
</s>
<s>
Since	O
all	O
software	B-Application
applications	I-Application
access	O
the	O
hard	B-Device
disk	I-Device
using	O
these	O
routines	O
,	O
they	O
continue	O
to	O
work	O
after	O
disk	O
compression	O
has	O
been	O
installed	O
.	O
</s>
<s>
Disk	O
compression	B-General_Concept
utilities	I-General_Concept
were	O
popular	O
especially	O
in	O
the	O
early	O
1990s	O
,	O
when	O
microcomputer	B-Architecture
hard	B-Device
disks	I-Device
were	O
still	O
relatively	O
small	O
(	O
20	O
to	O
80	O
megabytes	O
)	O
.	O
</s>
<s>
Hard	B-Device
drives	I-Device
were	O
also	O
rather	O
expensive	O
at	O
the	O
time	O
,	O
costing	O
roughly	O
10	O
USD	O
per	O
megabyte	O
.	O
</s>
<s>
A	O
good	O
disk	O
compression	B-General_Concept
utility	I-General_Concept
could	O
,	O
on	O
average	O
,	O
double	O
the	O
available	O
space	O
with	O
negligible	O
speed	O
loss	O
.	O
</s>
<s>
Disk	O
compression	O
fell	O
into	O
disuse	O
by	O
the	O
late	O
1990s	O
,	O
as	O
advances	O
in	O
hard	B-Device
drive	I-Device
technology	O
and	O
manufacturing	O
led	O
to	O
increased	O
capacities	O
and	O
lower	O
prices	O
.	O
</s>
<s>
Some	O
of	O
the	O
initial	O
disk	O
compression	O
solutions	O
were	O
hardware-assisted	O
and	O
utilized	O
add-on	O
compressor/decompressor	O
coprocessor	B-General_Concept
cards	O
in	O
addition	O
to	O
a	O
software	B-Application
driver	I-Application
.	O
</s>
<s>
These	O
compression	B-General_Concept
utilities	I-General_Concept
were	O
sold	O
independently	O
.	O
</s>
<s>
DR	O
DOS	B-Device
6.0	O
(	O
1991	O
)	O
,	O
from	O
Digital	O
Research	O
,	O
included	O
a	O
custom	O
version	O
of	O
AddStor	O
's	O
SuperStor	O
.	O
</s>
<s>
MS-DOS	B-Application
6.0	O
(	O
1993	O
)	O
,	O
from	O
Microsoft	O
,	O
included	O
DoubleSpace	B-General_Concept
,	O
which	O
was	O
based	O
in	O
part	O
on	O
Vertisoft	O
Systems	O
 '	O
DoubleDisk	O
.	O
</s>
<s>
MS-DOS	B-Application
6.0	O
introduced	O
the	O
so-called	O
DOS	B-Device
preload	O
API	O
,	O
an	O
undocumented	O
challenge	O
–	O
response	O
type	O
interface	O
which	O
allowed	O
the	O
disk	O
compression	B-General_Concept
software	I-General_Concept
(	O
DBLSPACE.BIN	O
)	O
to	O
be	O
loaded	O
automatically	O
even	O
before	O
CONFIG.SYS	B-Device
was	O
processed	O
.	O
</s>
<s>
PC	O
DOS	B-Device
6.1	O
(	O
1993	O
)	O
,	O
from	O
IBM	O
,	O
while	O
the	O
original	O
release	O
did	O
not	O
include	O
a	O
disk	O
compressor	O
,	O
a	O
custom	O
version	O
of	O
SuperStor/DS	O
(	O
after	O
acquiring	O
AddStor	O
)	O
was	O
included	O
in	O
a	O
later	O
release	O
,	O
also	O
introducing	O
the	O
preload	O
API	O
in	O
the	O
PC	O
DOS	B-Device
line	O
.	O
</s>
<s>
MS-DOS	B-Application
6.2	O
(	O
1993	O
)	O
,	O
from	O
Microsoft	O
,	O
included	O
an	O
improved	O
version	O
of	O
DoubleSpace	B-General_Concept
.	O
</s>
<s>
Novell	O
DOS	B-Device
7	O
(	O
1993	O
)	O
,	O
from	O
Novell	O
,	O
included	O
a	O
DPMS-enabled	O
version	O
of	O
Stacker	O
3.12	O
(	O
STACKER.BIN	O
)	O
from	O
Stac	O
Electronics	O
,	O
which	O
also	O
supported	O
compressed	B-General_Concept
data	I-General_Concept
exchange	O
with	O
remote	O
NetWare	B-Operating_System
and	O
PNW	B-Application
servers	O
also	O
running	O
Stacker	O
in	O
order	O
to	O
avoid	O
unnecessary	O
recompression	O
when	O
copying	O
files	O
.	O
</s>
<s>
It	O
also	O
introduced	O
support	O
for	O
the	O
DOS	B-Device
6	O
preload	O
API	O
in	O
order	O
to	O
maintain	O
compatibility	O
with	O
Microsoft	O
's	O
DoubleSpace	B-General_Concept
in	O
multi-boot	O
scenarios	O
.	O
</s>
<s>
PC	O
DOS	B-Device
6.3	O
(	O
1994	O
)	O
,	O
from	O
IBM	O
,	O
included	O
a	O
version	O
of	O
SuperStor/DS	O
.	O
</s>
<s>
MS-DOS	B-Application
6.22	O
(	O
1994	O
)	O
,	O
from	O
Microsoft	O
,	O
included	O
DriveSpace	B-General_Concept
,	O
which	O
was	O
DoubleSpace	B-General_Concept
with	O
a	O
different	O
compression	B-General_Concept
algorithm	I-General_Concept
(	O
DRVSPACE.BIN	O
)	O
,	O
after	O
MS-DOS	B-Application
6.21	O
had	O
to	O
ship	O
without	O
any	O
disk	O
compression	B-General_Concept
software	I-General_Concept
at	O
all	O
for	O
legal	O
reasons	O
.	O
</s>
<s>
PC	O
DOS	B-Device
7.0	O
(	O
1995	O
)	O
,	O
from	O
IBM	O
,	O
included	O
a	O
DPMS-enabled	O
version	O
of	O
Stacker	O
4.02	O
from	O
Stac	O
Electronics	O
.	O
</s>
<s>
Windows	B-Application
95	I-Application
(	O
1995	O
)	O
,	O
from	O
Microsoft	O
,	O
included	O
an	O
updated	O
version	O
of	O
DriveSpace	B-General_Concept
2	O
.	O
</s>
<s>
Windows	B-Application
95	I-Application
OSR2	O
(	O
1996	O
)	O
,	O
from	O
Microsoft	O
,	O
included	O
a	O
stripped-down	O
version	O
of	O
DriveSpace	B-General_Concept
3	O
.	O
</s>
<s>
Windows	B-Application
95	I-Application
Plus	I-Application
!	I-Application
</s>
<s>
(	O
1996	O
)	O
,	O
from	O
Microsoft	O
,	O
included	O
the	O
full	O
version	O
of	O
DriveSpace	B-General_Concept
3	O
.	O
</s>
<s>
OpenDOS	O
7.01	O
(	O
1997	O
)	O
,	O
from	O
Caldera	O
(	O
after	O
acquiring	O
Novell	O
DOS	B-Device
)	O
,	O
included	O
a	O
DPMS-enabled	O
version	O
of	O
Stacker	O
3.12	O
.	O
</s>
<s>
DR-DOS	O
7.02	O
(	O
1998	O
)	O
,	O
from	O
Caldera	O
,	O
included	O
a	O
DPMS-enabled	O
version	O
of	O
Stacker	O
3.12	O
.	O
</s>
<s>
DR-DOS	O
7.02	O
also	O
added	O
support	O
for	O
Microsoft	O
's	O
DriveSpace	B-General_Concept
.	O
</s>
<s>
PC	O
DOS	B-Device
2000	O
(	O
1998	O
)	O
,	O
from	O
IBM	O
,	O
included	O
a	O
DPMS-enabled	O
version	O
of	O
Stacker	O
4.02	O
.	O
</s>
<s>
Windows	B-Device
98	I-Device
(	O
1998	O
)	O
,	O
from	O
Microsoft	O
,	O
included	O
the	O
full	O
version	O
of	O
DriveSpace	B-General_Concept
3	O
.	O
</s>
<s>
DR-DOS	O
7.03	O
(	O
1999	O
)	O
,	O
from	O
Caldera	O
,	O
included	O
a	O
DPMS-enabled	O
version	O
of	O
Stacker	O
3.12	O
.	O
</s>
<s>
Windows	B-Device
98	I-Device
SE	O
(	O
1999	O
)	O
,	O
from	O
Microsoft	O
,	O
included	O
the	O
full	O
version	O
of	O
DriveSpace	B-General_Concept
3	O
.	O
</s>
<s>
PTS-DOS	B-Operating_System
Pro	I-Operating_System
2000	I-Operating_System
(	O
1999	O
)	O
,	O
from	O
Paragon	O
Technology	O
Software	O
,	O
included	O
a	O
disk	O
compression	O
component	O
named	O
DATACOMP	O
.	O
</s>
<s>
Squish	O
Plus	O
ROM	B-Device
from	O
Sundog	O
Software	O
,	O
but	O
distributed	O
exclusively	O
through	O
Personalized	O
Software	O
(	O
now	O
Thaddeus	O
Computing	O
)	O
,	O
was	O
a	O
special	O
ROM	B-Device
chip	I-Device
containing	O
a	O
custom	O
version	O
of	O
SquishPlus	O
with	O
only	O
36KB	O
footprint	O
adapted	O
for	O
the	O
HP	O
Portable	O
Plus	O
.	O
</s>
<s>
Double	B-Application
Tools	I-Application
for	I-Application
DoubleSpace	I-Application
from	O
Addstor	O
was	O
an	O
add-on	O
product	O
,	O
enhancing	O
the	O
functionality	O
of	O
the	O
DoubleSpace	B-General_Concept
bundled	O
with	O
MS-DOS	B-Application
6.0	O
.	O
</s>
<s>
Multimedia	O
Stacker	O
from	O
Helix	O
Software	O
was	O
a	O
bundle	O
of	O
Stac	O
's	O
DPMS-enabled	O
Stacker	O
4.01	O
combined	O
with	O
Helix	O
Cloaking	O
and	O
utilities	O
,	O
utilizing	O
Cloaking	O
's	O
built-in	O
DPMS	O
server	O
to	O
relocate	O
and	O
run	O
in	O
protected	B-Application
mode	I-Application
.	O
</s>
<s>
THS	O
filesystem	O
(	O
1994	O
–	O
1996	O
by	O
Thomas	O
Scheuermann	O
)	O
,	O
a	O
compressed	O
file	O
system	O
driver	O
for	O
Linux	B-Application
,	O
which	O
could	O
read	O
some	O
CVFs	O
.	O
</s>
<s>
DMSDOS	B-Application
(	O
1996	O
)	O
,	O
a	O
Linux	B-Application
kernel	B-Application
driver	I-Application
,	O
to	O
support	O
the	O
reading	O
and	O
writing	O
of	O
DoubleSpace	B-General_Concept
,	O
DriveSpace	B-General_Concept
2	O
&	O
3	O
,	O
as	O
well	O
as	O
Stacker	O
3	O
&	O
4	O
volumes	O
.	O
</s>
<s>
While	O
Windows	B-Application
XP	I-Application
,	O
from	O
Microsoft	O
,	O
included	O
both	O
a	O
native	O
support	O
and	O
a	O
command	B-Application
named	O
that	O
compresses	B-General_Concept
files	O
on	O
NTFS	B-Application
systems	O
,	O
that	O
is	O
not	O
implemented	O
as	O
a	O
separate	O
"	O
compressed	O
drive	O
"	O
like	O
those	O
above	O
.	O
</s>
<s>
Disk	O
compression	O
usually	O
creates	O
a	O
single	O
large	O
file	O
,	O
which	O
becomes	O
a	O
virtual	B-Application
hard	B-Device
drive	I-Device
.	O
</s>
<s>
This	O
is	O
similar	O
to	O
how	O
a	O
single	O
physical	O
hard	B-Device
drive	I-Device
can	O
be	O
partitioned	B-Application
into	O
multiple	O
virtual	B-Application
drives	O
.	O
</s>
<s>
The	O
compressed	O
drive	O
is	O
accessed	O
via	O
a	O
device	B-Application
driver	I-Application
.	O
</s>
<s>
Usually	O
certain	O
system	B-General_Concept
files	I-General_Concept
would	O
not	O
be	O
transferred	O
.	O
</s>
<s>
A	O
device	B-Application
driver	I-Application
had	O
to	O
be	O
loaded	O
to	O
access	O
the	O
compressed	O
drive	O
.	O
</s>
<s>
A	O
compressed	O
drive	O
C	O
:	O
required	O
changes	O
to	O
the	O
boot	B-Operating_System
process	I-Operating_System
as	O
follows	O
:	O
</s>
<s>
The	O
System	B-Operating_System
BIOS	I-Operating_System
loads	O
sector	O
0	O
of	O
the	O
first	O
physical	O
hard	B-Device
drive	I-Device
(	O
Master	B-Device
Boot	I-Device
Record	I-Device
)	O
.	O
</s>
<s>
The	O
partition	B-Device
sector	I-Device
loads	O
sector	O
0	O
of	O
the	O
bootable	B-Operating_System
partition	B-Application
(	O
Volume	B-Device
Boot	I-Device
Record	I-Device
)	O
.	O
</s>
<s>
Host	O
drive	O
sector	O
0	O
loads	O
(	O
in	O
the	O
case	O
of	O
DOS	B-Device
)	O
the	O
DOS	B-Device
BIOS	B-Operating_System
(	O
IO.SYS	O
,	O
IBMBIO.COM	O
etc	O
.	O
)	O
</s>
<s>
In	O
the	O
case	O
of	O
DR-DOS	O
,	O
the	O
BIOS	B-Operating_System
is	O
packed	O
and	O
will	O
decompress	O
itself	O
through	O
an	O
embedded	O
decompression	O
routine	O
.	O
</s>
<s>
The	O
DOS	B-Device
BIOS	B-Operating_System
then	O
loads	O
the	O
DOS	B-Device
kernel	O
(	O
MSDOS.SYS	O
,	O
IBMDOS.COM	O
etc	O
.	O
)	O
</s>
<s>
If	O
the	O
DOS	B-Device
supports	O
the	O
preload	O
API	O
,	O
the	O
DOS	B-Device
BIOS	B-Operating_System
attempts	O
to	O
load	O
a	O
preloadable	O
compression	O
driver	O
(	O
DBLSPACE.BIN	O
,	O
DRVSPACE.BIN	O
or	O
STACKER.BIN	O
)	O
from	O
the	O
root	O
of	O
the	O
host	O
drive	O
and	O
attempts	O
to	O
communicate	O
with	O
it	O
through	O
the	O
DOS	B-Device
preload	O
API	O
.	O
</s>
<s>
If	O
the	O
driver	O
responds	O
as	O
expected	O
,	O
the	O
DOS	B-Device
BIOS	B-Operating_System
will	O
keep	O
the	O
driver	O
,	O
if	O
not	O
,	O
it	O
will	O
get	O
discarded	O
.	O
</s>
<s>
The	O
DOS	B-Device
BIOS	B-Operating_System
starts	O
CONFIG.SYS	B-Device
processing	O
by	O
looking	O
for	O
the	O
file	O
in	O
the	O
root	O
of	O
the	O
boot	B-Operating_System
drive	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
DR-DOS	O
,	O
DCONFIG.SYS	B-Device
takes	O
precedence	O
over	O
CONFIG.SYS	B-Device
,	O
if	O
present	O
.	O
</s>
<s>
Compression	O
drivers	O
not	O
loaded	O
through	O
the	O
preload	O
API	O
may	O
be	O
loaded	O
through	O
DEVICE	B-Device
statements	O
.	O
</s>
<s>
On	O
systems	O
with	O
slower	O
hard	B-Device
drives	I-Device
,	O
disk	O
compression	O
could	O
actually	O
increase	O
system	O
performance	O
.	O
</s>
<s>
If	O
the	O
system	O
had	O
to	O
wait	O
frequently	O
for	O
hard	B-Device
drive	I-Device
access	O
to	O
complete	O
(	O
I/O	B-General_Concept
bound	I-General_Concept
)	O
converting	O
the	O
hard	B-Device
drive	I-Device
to	O
compressed	O
drives	O
could	O
speed	O
up	O
the	O
system	O
significantly	O
.	O
</s>
<s>
Not	O
all	O
compression	B-General_Concept
utilities	I-General_Concept
would	O
confirm	O
the	O
absence	O
of	O
errors	O
in	O
the	O
FAT	B-Application
file	I-Application
system	I-Application
before	O
compressing	O
a	O
disk	O
in	O
place	O
.	O
</s>
<s>
The	O
compressed	O
drive	O
is	O
only	O
visible	O
if	O
the	O
device	B-Application
driver	I-Application
is	O
loaded	O
and	O
the	O
compressed	O
drive	O
is	O
mounted	O
.	O
</s>
<s>
A	O
boot	B-Operating_System
disk	I-Operating_System
,	O
for	O
example	O
,	O
might	O
not	O
contain	O
the	O
driver	O
.	O
</s>
<s>
(	O
This	O
was	O
not	O
normally	O
an	O
issue	O
for	O
solutions	O
that	O
came	O
bundled	O
with	O
the	O
operation	B-General_Concept
systems	I-General_Concept
unless	O
booting	B-Operating_System
foreign	O
operating	B-General_Concept
systems	I-General_Concept
,	O
as	O
system	O
utilities	O
like	O
SYS	B-Language
were	O
modified	O
to	O
copy	O
the	O
compression	O
drivers	O
to	O
boot	B-Operating_System
disks	I-Operating_System
by	O
default	O
,	O
and	O
the	O
corresponding	O
operation	B-General_Concept
systems	I-General_Concept
would	O
also	O
fall	O
back	O
to	O
look	O
for	O
them	O
on	O
drives	O
other	O
than	O
the	O
boot	B-Operating_System
disk	I-Operating_System
if	O
they	O
could	O
n't	O
be	O
found	O
there	O
.	O
)	O
</s>
<s>
While	O
it	O
was	O
usually	O
"	O
hidden	O
"	O
by	O
default	O
,	O
users	O
who	O
did	O
find	O
the	O
large	O
file	O
curious	O
or	O
suspicious	O
were	O
able	O
to	O
delete	B-Device
it	O
.	O
</s>
