<s>
DOS/4G	B-Application
is	O
a	O
32-bit	O
DOS	B-Device
extender	I-Device
developed	O
by	O
Rational	O
Systems	O
(	O
later	O
Tenberry	O
Software	O
)	O
.	O
</s>
<s>
It	O
allows	O
DOS	B-Device
programs	O
to	O
eliminate	O
the	O
640	O
KB	O
conventional	B-Device
memory	I-Device
limit	O
by	O
addressing	O
up	O
to	O
64	O
MB	O
of	O
extended	B-Device
memory	I-Device
on	O
Intel	B-General_Concept
80386	I-General_Concept
and	O
above	O
machines	O
.	O
</s>
<s>
Functioning	O
as	O
a	O
highly	O
flexible	O
and	O
reusable	O
memory	O
extension	O
library	O
,	O
DOS/4G	B-Application
allowed	O
programmers	O
to	O
access	O
extended	B-Device
memory	I-Device
without	O
writing	O
specialized	O
code	O
.	O
</s>
<s>
It	O
embeds	O
itself	O
in	O
the	O
executable	O
file	O
at	O
linking	B-Application
time	O
and	O
executes	O
before	O
main	O
application	O
code	O
,	O
so	O
usually	O
DOS/4G	B-Application
initialization	O
messages	O
show	O
up	O
at	O
launch	O
.	O
</s>
<s>
It	O
can	O
in	O
principle	O
operate	O
within	O
MS-DOS	B-Application
,	O
PC	B-Operating_System
DOS	I-Operating_System
,	O
DR-DOS	B-Application
and	O
other	O
DOS	B-Device
clones	O
,	O
the	O
DOS	B-Application
boxes	I-Application
of	O
OS/2	B-Application
,	O
Microsoft	B-Application
Windows	I-Application
,	O
Windows	B-Device
NT	I-Device
and	O
Windows	B-Application
95	I-Application
,	O
and	O
DOS	B-Device
emulators	O
such	O
as	O
DOSBox	B-Application
.	O
</s>
<s>
However	O
,	O
in	O
practice	O
few	O
DOS/4G	B-Application
games	O
or	O
other	O
applications	O
will	O
run	O
on	O
non-DOS-based	O
versions	O
of	O
Windows	O
,	O
including	O
Windows	B-Device
NT	I-Device
,	O
Windows	B-Application
2000	I-Application
and	O
Windows	B-Application
XP	I-Application
,	O
since	O
none	O
of	O
these	O
allow	O
direct	O
access	O
to	O
the	O
hardware	O
as	O
was	O
used	O
for	O
display	O
rendering	O
in	O
those	O
days	O
.	O
</s>
<s>
DOS/4GW	B-Application
1.95	O
was	O
a	O
free	O
limited	O
edition	O
of	O
DOS/4G	B-Application
and	O
was	O
included	O
with	O
the	O
Watcom	B-Application
C	I-Application
compiler	I-Application
with	O
a	O
commercial	O
re-distribution	O
license	O
.	O
</s>
<s>
It	O
was	O
made	O
widely	O
popular	O
by	O
computer	O
games	O
like	O
Doom	B-Application
.	O
</s>
<s>
Initial	O
versions	O
of	O
DOS/4G	B-Application
had	O
trouble	O
with	O
secondary	O
DMA	O
channels	O
on	O
the	O
ISA	B-Architecture
bus	I-Architecture
,	O
which	O
prevented	O
16-bit	O
devices	O
like	O
Gravis	B-Application
Ultrasound	I-Application
series	O
from	O
normally	O
functioning	O
;	O
Gravis	O
even	O
had	O
to	O
develop	O
PREPGAME	O
,	O
a	O
patch	O
utility	O
which	O
updated	O
the	O
game	O
executable	O
with	O
a	O
new	O
version	O
1.97	O
to	O
fix	O
the	O
incompatibility	O
.	O
</s>
<s>
In	O
case	O
of	O
problems	O
,	O
DOS/4G	B-Application
or	O
DOS/4GW	B-Application
can	O
be	O
replaced	O
with	O
the	O
newer	O
and	O
free	O
DOS/32	B-Device
;	O
a	O
patch	O
utility	O
can	O
even	O
replace	O
DOS/4G	B-Application
code	O
embedded	O
inside	O
a	O
compiled	O
executable	O
file	O
.	O
</s>
