<s>
A	O
DOS	B-Device
extender	I-Device
is	O
a	O
computer	B-Application
software	I-Application
program	O
running	O
under	O
DOS	B-Device
that	O
enables	O
software	O
to	O
run	O
in	O
a	O
protected	B-Application
mode	I-Application
environment	O
even	O
though	O
the	O
host	O
operating	O
system	O
is	O
only	O
capable	O
of	O
operating	O
in	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
DOS	B-Device
extenders	I-Device
were	O
initially	O
developed	O
in	O
the	O
1980s	O
following	O
the	O
introduction	O
of	O
the	O
Intel	B-General_Concept
80286	I-General_Concept
processor	O
(	O
and	O
later	O
expanded	O
upon	O
with	O
the	O
Intel	B-General_Concept
80386	I-General_Concept
)	O
,	O
to	O
cope	O
with	O
the	O
memory	O
limitations	O
of	O
DOS	B-Device
.	O
</s>
<s>
A	O
DOS	B-Device
extender	I-Device
is	O
a	O
program	O
that	O
"	O
extends	O
"	O
DOS	B-Device
so	O
that	O
programs	O
running	O
in	O
protected	B-Application
mode	I-Application
can	O
transparently	O
interface	O
with	O
the	O
underlying	O
DOS	B-Application
API	I-Application
.	O
</s>
<s>
This	O
was	O
necessary	O
because	O
many	O
of	O
the	O
functions	O
provided	O
by	O
DOS	B-Device
require	O
16-bit	O
segment	O
and	O
offset	O
addresses	O
pointing	O
to	O
memory	O
locations	O
within	O
the	O
first	B-Device
640	I-Device
kilobytes	I-Device
of	I-Device
memory	I-Device
.	O
</s>
<s>
Protected	B-Application
mode	I-Application
,	O
however	O
,	O
uses	O
an	O
incompatible	O
addressing	O
method	O
where	O
the	O
segment	O
registers	O
(	O
now	O
called	O
selectors	O
)	O
are	O
used	O
to	O
point	O
to	O
an	O
entry	O
in	O
the	O
Global	B-General_Concept
Descriptor	I-General_Concept
Table	I-General_Concept
which	O
describes	O
the	O
characteristics	O
of	O
the	O
segment	O
.	O
</s>
<s>
The	O
two	O
methods	O
of	O
addressing	O
are	O
mutually	O
exclusive	O
,	O
with	O
the	O
processor	O
having	O
to	O
make	O
costly	O
switches	O
to	O
real	O
(	O
or	O
V86	B-Application
)	O
mode	O
to	O
service	O
non-protected	O
mode	O
requests	O
.	O
</s>
<s>
In	O
addition	O
to	O
setting	O
up	O
the	O
environment	O
and	O
loading	O
the	O
actual	O
program	O
to	O
be	O
executed	O
,	O
the	O
DOS	B-Device
extender	I-Device
also	O
provides	O
(	O
amongst	O
other	O
things	O
)	O
a	O
translation	O
layer	O
that	O
maintains	O
buffers	B-General_Concept
allocated	O
below	O
the	O
1MB	O
real	B-Application
mode	I-Application
memory	O
barrier	O
.	O
</s>
<s>
These	O
buffers	B-General_Concept
are	O
used	O
to	O
transfer	O
data	O
between	O
the	O
underlying	O
real	B-Application
mode	I-Application
operating	O
system	O
and	O
the	O
protected	B-Application
mode	I-Application
program	O
.	O
</s>
<s>
Since	O
switching	O
between	O
real/V86	O
mode	O
and	O
protected	B-Application
mode	I-Application
is	O
a	O
relatively	O
time	O
consuming	O
operation	O
,	O
the	O
extender	O
attempts	O
to	O
minimize	O
the	O
number	O
of	O
switches	O
by	O
duplicating	O
the	O
functionality	O
of	O
many	O
real	B-Application
mode	I-Application
operations	O
within	O
its	O
own	O
protected	B-Application
mode	I-Application
environment	O
.	O
</s>
<s>
As	O
DOS	B-Device
uses	O
interrupts	B-Application
extensively	O
for	O
communication	O
between	O
the	O
operating	O
system	O
and	O
user	O
level	O
software	O
,	O
DOS	B-Device
extenders	I-Device
intercept	O
many	O
of	O
the	O
common	O
hardware	O
(	O
e.g.	O
</s>
<s>
DOS	B-Device
itself	O
and	O
the	O
mouse	O
API	O
)	O
interrupts	B-Application
.	O
</s>
<s>
Some	O
extenders	O
also	O
handle	O
other	O
common	O
interrupt	B-Application
functions	O
,	O
such	O
as	O
video	O
BIOS	O
routines	O
.	O
</s>
<s>
Essentially	O
,	O
a	O
DOS	B-Device
extender	I-Device
is	O
like	O
a	O
miniature	O
operating	O
system	O
,	O
handling	O
much	O
of	O
the	O
functionality	O
of	O
the	O
underlying	O
operating	O
system	O
itself	O
.	O
</s>
<s>
The	O
DOS	B-Device
extender	I-Device
was	O
arguably	O
invented	O
by	O
Phar	O
Lap	O
,	O
but	O
it	O
was	O
Tenberry	O
Software	O
's	O
(	O
formerly	O
Rational	O
Systems	O
)	O
386	B-General_Concept
extender	O
DOS/4GW	B-Application
that	O
brought	O
protected	B-Application
mode	I-Application
DOS	B-Device
programs	O
to	O
a	O
mass	O
market	O
.	O
</s>
<s>
Included	O
with	O
Watcom	O
's	O
C	B-Language
,	O
C++	B-Language
,	O
and	O
Fortran	B-Application
compilers	B-Language
for	O
386	B-General_Concept
class	O
processors	O
,	O
it	O
soon	O
became	O
a	O
ubiquitous	O
mainstay	O
of	O
PC	O
applications	O
and	O
games	O
such	O
as	O
id	O
Software	O
's	O
successful	O
Doom	B-Application
.	O
</s>
<s>
While	O
initially	O
it	O
was	O
the	O
memory-hungry	O
business	O
applications	O
that	O
drove	O
the	O
development	O
of	O
DOS	B-Device
extenders	I-Device
,	O
it	O
would	O
be	O
PC	O
games	O
that	O
truly	O
brought	O
them	O
into	O
the	O
spotlight	O
.	O
</s>
<s>
As	O
a	O
result	O
of	O
the	O
development	O
of	O
DOS	B-Device
extenders	I-Device
,	O
two	O
new	O
software	O
interfaces	O
were	O
created	O
to	O
take	O
care	O
of	O
the	O
many	O
potential	O
conflicts	O
that	O
could	O
arise	O
from	O
the	O
varied	O
methods	O
of	O
memory	O
management	O
that	O
already	O
existed	O
,	O
as	O
well	O
as	O
provide	O
a	O
uniform	O
interface	O
for	O
client	O
programs	O
.	O
</s>
<s>
The	O
first	O
of	O
these	O
interfaces	O
was	O
the	O
Virtual	O
Control	O
Program	O
Interface	O
(	O
VCPI	O
)	O
,	O
but	O
this	O
was	O
rapidly	O
overshadowed	O
by	O
the	O
DOS	B-Device
Protected	B-Application
Mode	I-Application
Interface	O
(	O
DPMI	O
)	O
specification	O
,	O
which	O
grew	O
from	O
the	O
Windows	O
3.0	O
development	O
.	O
</s>
<s>
They	O
provided	O
an	O
API	O
through	O
which	O
an	O
extended	O
program	O
could	O
interface	O
with	O
real	B-Application
mode	I-Application
software	O
,	O
allocate	O
memory	O
,	O
and	O
handle	O
interrupt	B-Application
services	O
.	O
</s>
<s>
They	O
also	O
provided	O
an	O
easy	O
method	O
for	O
the	O
extender	O
to	O
set	O
up	O
the	O
switch	O
to	O
protected	B-Application
mode	I-Application
,	O
and	O
allowed	O
multiple	O
protected	B-Application
mode	I-Application
programs	O
to	O
coexist	O
peacefully	O
.	O
</s>
<s>
DOS/4G	B-Application
and	I-Application
DOS/4GW	I-Application
and	O
DOS/16M	O
by	O
Tenberry	O
Software	O
,	O
Inc	O
.	O
</s>
<s>
286|DOS	O
Extender	O
and	O
386|DOS	O
Extender	O
by	O
Phar	O
Lap	O
.	O
</s>
<s>
Later	O
superseded	O
by	O
the	O
TNT	O
Dos	B-Device
Extender	I-Device
.	O
</s>
<s>
PROT	O
by	O
Al	O
Williams	O
,	O
a	O
32-bit	O
DOS	B-Device
extender	I-Device
published	O
in	O
Dr.	O
Dobb	O
's	O
Journal	O
and	O
in	O
two	O
books	O
.	O
</s>
<s>
This	O
extender	O
had	O
the	O
virtue	O
of	O
running	O
DOS	B-Device
and	O
BIOS	O
calls	O
in	O
emulated	O
mode	O
instead	O
of	O
switching	O
back	O
to	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
CauseWay	O
was	O
a	O
formerly	O
proprietary	O
extender	O
that	O
competed	O
with	O
DOS4G	B-Application
.	O
</s>
<s>
A	O
few	O
rare	O
games	O
such	O
as	O
Daggerfall	B-Application
use	O
it	O
.	O
</s>
<s>
DOS/32	B-Device
as	O
an	O
alternative	O
to	O
DOS/4G	B-Application
by	O
Narech	O
K	O
.	O
</s>
<s>
386Power	O
32-bit	O
DOS	B-Device
Extender	I-Device
is	O
an	O
extender	O
for	O
32-bit	O
Assembly	B-Language
apps	O
.	O
</s>
<s>
all	O
Microsoft	B-Application
Windows	I-Application
versions	O
since	O
1990	O
,	O
except	O
NT	O
branch	O
,	O
include	O
both	O
a	O
DPMI	O
server	O
and	O
DOS	B-Device
extender	I-Device
.	O
</s>
<s>
HX	B-Device
DOS	I-Device
Extender	I-Device
provides	O
limited	O
Win32	O
support	O
to	O
allow	O
Windows	O
console	O
and	O
some	O
Win32	O
GUI	O
applications	O
to	O
run	O
under	O
DOS	B-Device
.	O
</s>
<s>
WDosX	O
was	O
an	O
early	O
implementation	O
of	O
limited	O
Win32	O
support	O
,	O
used	O
by	O
the	O
TMT	O
Pascal	O
compiler	B-Language
.	O
</s>
<s>
Borland	B-Application
Power	I-Application
Pack	I-Application
was	O
an	O
extender	O
included	O
with	O
some	O
of	O
their	O
development	O
suites	O
that	O
could	O
access	O
a	O
limited	O
portion	O
of	O
the	O
Win32	O
API	O
.	O
</s>
<s>
TASM	B-Application
,	O
again	O
from	O
Borland	O
,	O
included	O
32RTM	O
with	O
DPMI32VM	O
and	O
RTM	O
with	O
DPMI16BI	O
,	O
two	O
DPMI	O
hosts	O
.	O
</s>
<s>
CWSDPMI	B-Application
by	O
Charles	B-Application
W	I-Application
.	I-Application
Sandmann	I-Application
,	O
a	O
DPMI	O
server	O
for	O
use	O
with	O
32-bit	O
protected	B-Application
mode	I-Application
DOS	B-Device
DJGPP	B-Application
programs	O
.	O
</s>
<s>
QDPMI	O
by	O
Quarterdeck	O
Office	O
Systems	O
,	O
was	O
a	O
DPMI	O
host	O
included	O
with	O
QEMM	B-Device
.	O
</s>
<s>
D3X	O
is	O
an	O
DPMI	O
sever	O
written	O
entirely	O
in	O
Assembly	B-Language
.	O
</s>
<s>
Originally	O
developed	O
for	O
32	O
bit	O
programs	O
generated	O
by	O
Borland	B-Application
C++	I-Application
and	O
Delphi	O
.	O
</s>
<s>
BLINKER	O
by	O
Blink	O
Inc	O
Version	O
3	O
and	O
above	O
provided	O
a	O
286	B-General_Concept
DOS	B-Device
Extender	I-Device
for	O
several	O
16	O
bit	O
DOS	B-Device
compilers	B-Language
including	O
CA-Clipper	O
,	O
Microsoft	O
C/C	O
++	O
,	O
PASCAL	O
,	O
FORTRAN	B-Application
and	O
Borland	O
C/C	O
++	O
.	O
</s>
<s>
Supported	O
unique	O
'	O
Dual	O
Mode	O
 '	O
executables	O
capable	O
of	O
running	O
in	O
either	O
real	O
or	O
protected	B-Application
mode	I-Application
depending	O
on	O
the	O
run	O
time	O
environment	O
.	O
</s>
<s>
ATT	O
Graphics	O
Software	O
Labs	O
'	O
RIO	O
 '	O
--	O
Resolution	O
Independent	O
Objects	O
 '	O
graphics	O
software	B-Application
.	I-Application
</s>
<s>
Major	B-Application
BBS	I-Application
,	O
a	O
1980s	O
BBS	B-Application
software	O
package	O
that	O
utilized	O
the	O
Phar	O
Lap	O
DOS	B-Device
extender	I-Device
.	O
</s>
<s>
Countless	O
DOS	B-Device
games	O
from	O
the	O
early	O
to	O
mid	O
1990s	O
,	O
mostly	O
using	O
DOS/4GW	B-Application
,	O
including	O
:	O
</s>
<s>
DMA	O
Design	O
(	O
now	O
Rockstar	O
North	O
)	O
'	O
s	O
Grand	B-Application
Theft	I-Application
Auto	I-Application
.	O
</s>
<s>
Comanche	B-Application
:	I-Application
Maximum	I-Application
Overkill	I-Application
by	O
NovaLogic	O
used	O
a	O
custom	O
Unreal	B-Device
mode	I-Device
memory	B-General_Concept
manager	I-General_Concept
which	O
required	O
a	O
80386	B-General_Concept
processor	O
and	O
was	O
incompatible	O
with	O
memory	O
managers	O
and	O
virtual	O
DOS	B-Device
boxes	O
,	O
requiring	O
a	O
complicated	O
DOS	B-Device
boot	O
menu	O
configuration	O
in	O
CONFIG.SYS	B-Device
.	O
</s>
<s>
Later	O
revisions	O
included	O
a	O
DOS	B-Device
extender	I-Device
which	O
solved	O
the	O
problem	O
.	O
</s>
<s>
Ultima	B-Application
VII	I-Application
and	O
Ultima	B-Application
VII	I-Application
Part	I-Application
Two	I-Application
:	I-Application
Serpent	I-Application
Isle	I-Application
by	O
Origin	O
Systems	O
also	O
used	O
an	O
custom	O
Unreal	B-Device
mode	I-Device
memory	B-General_Concept
manager	I-General_Concept
called	O
the	O
Voodoo	B-Device
Memory	I-Device
Manager	I-Device
which	O
was	O
incompatible	O
with	O
EMS	B-Device
memory	O
and	O
memory	O
managers	O
such	O
as	O
EMM386	B-Device
.	O
</s>
