<s>
In	O
DOS	B-General_Concept
memory	I-General_Concept
management	I-General_Concept
,	O
conventional	B-Device
memory	I-Device
,	O
also	O
called	O
base	B-Device
memory	I-Device
,	O
is	O
the	O
first	O
640	O
kilobytes	O
of	O
the	O
memory	O
on	O
IBM	B-Device
PC	I-Device
or	O
compatible	O
systems	O
.	O
</s>
<s>
The	O
640	O
KB	O
barrier	O
is	O
an	O
architectural	O
limitation	O
of	O
IBM	O
PC	B-Device
compatible	O
PCs	O
.	O
</s>
<s>
The	O
Intel	B-Device
8088	I-Device
CPU	O
,	O
used	O
in	O
the	O
original	B-Device
IBM	I-Device
PC	I-Device
,	O
was	O
able	O
to	O
address	O
1MB	O
(	O
220bytes	O
)	O
,	O
since	O
the	O
chip	O
offered	O
20	O
address	O
lines	O
.	O
</s>
<s>
In	O
the	O
design	O
of	O
the	O
PC	B-Device
,	O
the	O
memory	O
below	O
640KB	O
was	O
for	O
random-access	B-Architecture
memory	I-Architecture
on	O
the	O
motherboard	O
or	O
on	O
expansion	O
boards	O
,	O
and	O
it	O
was	O
called	O
the	O
conventional	B-Device
memory	I-Device
area	O
.	O
</s>
<s>
The	O
first	O
memory	O
segment	O
(	O
64KB	O
)	O
of	O
the	O
conventional	B-Device
memory	I-Device
area	O
is	O
named	O
lower	O
memory	O
or	O
low	B-Device
memory	I-Device
area	O
.	O
</s>
<s>
The	O
remaining	O
384KB	O
beyond	O
the	O
conventional	B-Device
memory	I-Device
area	O
,	O
called	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
(	O
UMA	B-Device
)	O
,	O
was	O
reserved	O
for	O
system	O
use	O
and	O
optional	O
devices	O
.	O
</s>
<s>
UMA	B-Device
was	O
used	O
for	O
the	O
ROM	B-Operating_System
BIOS	I-Operating_System
,	O
additional	O
read-only	B-Device
memory	I-Device
,	O
BIOS	B-Operating_System
extensions	O
for	O
fixed	O
disk	O
drives	O
and	O
video	O
adapters	O
,	O
video	O
adapter	O
memory	O
,	O
and	O
other	O
memory-mapped	B-Architecture
input	I-Architecture
and	I-Architecture
output	I-Architecture
devices	I-Architecture
.	O
</s>
<s>
The	O
design	O
of	O
the	O
original	B-Device
IBM	I-Device
PC	I-Device
placed	O
the	O
Color	B-Device
Graphics	I-Device
Adapter	I-Device
(	O
CGA	B-Device
)	O
memory	O
map	O
in	O
UMA	B-Device
.	O
</s>
<s>
The	O
need	O
for	O
more	O
RAM	B-Architecture
grew	O
faster	O
than	O
the	O
needs	O
of	O
hardware	O
to	O
utilize	O
the	O
reserved	O
addresses	O
,	O
which	O
resulted	O
in	O
RAM	B-Architecture
eventually	O
being	O
mapped	O
into	O
these	O
unused	O
upper	O
areas	O
to	O
utilize	O
all	O
available	O
addressable	O
space	O
.	O
</s>
<s>
Avoiding	O
such	O
a	O
hole	O
was	O
difficult	O
and	O
ugly	O
and	O
not	O
supported	O
by	O
DOS	B-Device
or	O
most	O
programs	O
that	O
could	O
run	O
on	O
it	O
.	O
</s>
<s>
Later	O
,	O
space	O
between	O
the	O
holes	O
would	O
be	O
used	O
as	O
upper	B-Device
memory	I-Device
blocks	I-Device
(	O
UMBs	O
)	O
.	O
</s>
<s>
To	O
maintain	O
compatibility	O
with	O
older	O
operating	O
systems	O
and	O
applications	O
,	O
the	O
640KB	O
barrier	O
remained	O
part	O
of	O
the	O
PC	B-Device
design	O
even	O
after	O
the	O
8086/8088	O
had	O
been	O
replaced	O
with	O
the	O
Intel	B-General_Concept
80286	I-General_Concept
processor	O
,	O
which	O
could	O
address	O
up	O
to	O
16MB	O
of	O
memory	O
in	O
protected	B-Application
mode	I-Application
.	O
</s>
<s>
The	O
1MB	O
barrier	O
also	O
remained	O
as	O
long	O
as	O
the	O
286	B-General_Concept
was	O
running	O
in	O
real	B-Application
mode	I-Application
,	O
since	O
DOS	B-Device
required	O
real	B-Application
mode	I-Application
which	O
uses	O
the	O
segment	O
and	O
offset	O
registers	O
in	O
an	O
overlapped	O
manner	O
such	O
that	O
addresses	O
with	O
more	O
than	O
20	O
bits	O
are	O
not	O
possible	O
.	O
</s>
<s>
It	O
is	O
still	O
present	O
in	O
IBM	B-Device
PC	I-Device
compatibles	O
today	O
if	O
they	O
are	O
running	O
in	O
real	B-Application
mode	I-Application
such	O
as	O
used	O
by	O
DOS	B-Device
.	O
</s>
<s>
This	O
however	O
is	O
invisible	O
to	O
programs	O
(	O
or	O
even	O
most	O
of	O
the	O
operating	O
system	O
)	O
on	O
newer	O
operating	O
systems	O
(	O
such	O
as	O
Windows	B-Application
,	O
Linux	B-Application
,	O
or	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
)	O
that	O
use	O
virtual	B-Architecture
memory	I-Architecture
,	O
because	O
they	O
have	O
no	O
awareness	O
of	O
physical	O
memory	O
addresses	O
at	O
all	O
.	O
</s>
<s>
Instead	O
they	O
operate	O
within	O
a	O
virtual	O
address	O
space	O
,	O
which	O
is	O
defined	O
independently	O
of	O
available	O
RAM	B-Architecture
addresses	O
.	O
</s>
<s>
Later	O
video	O
cards	O
using	O
the	O
AGP	B-Architecture
(	O
PCI	O
memory	O
space	O
)	O
bus	O
can	O
have	O
256MB	O
memory	O
with	O
1GB	O
aperture	B-General_Concept
size	I-General_Concept
.	O
</s>
<s>
One	O
technique	O
used	O
on	O
early	O
IBM	B-Device
XT	I-Device
computers	O
was	O
to	O
install	O
additional	O
RAM	B-Architecture
into	O
the	O
video	O
memory	O
address	O
range	O
and	O
push	O
the	O
limit	O
up	O
to	O
the	O
start	O
of	O
the	O
Monochrome	B-Device
Display	I-Device
Adapter	I-Device
(	O
MDA	B-Device
)	O
.	O
</s>
<s>
Sometimes	O
software	O
or	O
a	O
custom	O
address	B-Device
decoder	I-Device
was	O
required	O
for	O
this	O
to	O
work	O
.	O
</s>
<s>
This	O
moved	O
the	O
barrier	O
to	O
704KB	O
(	O
with	O
MDA/HGC	O
)	O
or	O
736KB	O
(	O
with	O
CGA	B-Device
)	O
.	O
</s>
<s>
Memory	B-General_Concept
managers	I-General_Concept
on	O
386-based	B-General_Concept
systems	O
(	O
such	O
as	O
QEMM	B-Device
or	O
MEMMAX	O
( +V	O
)	O
in	O
DR-DOS	B-Application
)	O
could	O
achieve	O
the	O
same	O
effect	O
,	O
adding	O
conventional	B-Device
memory	I-Device
at	O
640KB	O
and	O
moving	O
the	O
barrier	O
to	O
704KB	O
(	O
up	O
to	O
segment	O
B000	O
,	O
the	O
start	O
of	O
MDA/HGC	O
)	O
or	O
736KB	O
(	O
up	O
to	O
segment	O
B800	O
,	O
the	O
start	O
of	O
the	O
CGA	B-Device
)	O
.	O
</s>
<s>
Only	O
CGA	B-Device
could	O
be	O
used	O
in	O
this	O
situation	O
,	O
because	O
Enhanced	B-Device
Graphics	I-Device
Adapter	I-Device
(	O
EGA	B-Device
)	O
video	O
memory	O
was	O
immediately	O
adjacent	O
to	O
the	O
conventional	B-Device
memory	I-Device
area	O
below	O
the	O
640KB	O
line	O
;	O
the	O
same	O
memory	O
area	O
could	O
not	O
be	O
used	O
both	O
for	O
the	O
frame	B-Algorithm
buffer	I-Algorithm
of	O
the	O
video	O
card	O
and	O
for	O
transient	O
programs	O
.	O
</s>
<s>
All	O
Computers	O
 '	O
piggy-back	O
add-on	O
memory	B-General_Concept
management	I-General_Concept
units	I-General_Concept
AllCard	O
for	O
XT	O
-	O
and	O
Chargecard	O
for	O
286/386SX	O
-class	O
computers	O
,	O
as	O
well	O
as	O
MicroWay	O
's	O
ECM	O
(	O
Extended	O
Conventional	B-Device
Memory	I-Device
)	O
add-on-board	O
allowed	O
normal	O
memory	O
to	O
be	O
mapped	O
into	O
the	O
A0000-EFFFF	O
(	O
hex	O
)	O
address	O
range	O
,	O
giving	O
up	O
to	O
952KB	O
for	O
DOS	B-Device
programs	O
.	O
</s>
<s>
Programs	O
such	O
as	O
Lotus	B-Application
1-2-3	I-Application
,	O
which	O
accessed	O
video	O
memory	O
directly	O
,	O
needed	O
to	O
be	O
patched	O
to	O
handle	O
this	O
memory	O
layout	O
.	O
</s>
<s>
It	O
was	O
also	O
possible	O
to	O
use	O
console	O
redirection	O
(	O
either	O
by	O
specifying	O
an	O
alternative	O
console	O
device	O
like	O
AUX	B-Application
:	I-Application
when	O
initially	O
invoking	O
COMMAND.COM	B-Operating_System
or	O
by	O
using	O
CTTY	O
later	O
on	O
)	O
to	O
direct	O
output	O
to	O
and	O
receive	O
input	O
from	O
a	O
dumb	O
terminal	O
or	O
another	O
computer	O
running	O
a	O
terminal	B-Application
emulator	I-Application
.	O
</s>
<s>
Assuming	O
the	O
System	B-Operating_System
BIOS	I-Operating_System
still	O
permitted	O
the	O
machine	O
to	O
boot	O
(	O
which	O
is	O
often	O
the	O
case	O
at	O
least	O
with	O
BIOSes	O
for	O
embedded	O
PCs	O
)	O
,	O
the	O
video	O
card	O
could	O
then	O
be	O
removed	O
completely	O
,	O
and	O
the	O
system	O
could	O
provide	O
a	O
total	O
of	O
960KB	O
of	O
continuous	O
DOS	B-General_Concept
memory	I-General_Concept
for	O
programs	O
to	O
load	O
.	O
</s>
<s>
Similar	O
usage	O
was	O
possible	O
on	O
many	O
DOS	B-Device
-	O
but	O
not	O
IBM-compatible	O
computers	O
with	O
a	O
non-fragmented	O
memory	O
layout	O
,	O
for	O
example	O
SCP	O
S-100	B-Architecture
bus	I-Architecture
systems	O
equipped	O
with	O
their	O
8086	B-General_Concept
CPU	O
card	O
CP-200B	O
and	O
up	O
to	O
sixteen	O
SCP110A	O
memory	O
cards	O
(	O
with	O
64KB	O
RAM	B-Architecture
on	O
each	O
of	O
them	O
)	O
for	O
a	O
total	O
of	O
up	O
to	O
1024	O
KB	O
(	O
without	O
video	O
card	O
,	O
but	O
utilizing	O
console	O
redirection	O
,	O
and	O
after	O
mapping	O
out	O
the	O
boot/BIOS	O
ROM	O
)	O
,	O
the	O
Victor	O
9000/Sirius	O
1	O
which	O
supported	O
up	O
to	O
896KB	O
,	O
or	O
the	O
Apricot	O
PC	B-Device
with	O
more	O
continuous	O
DOS	B-General_Concept
memory	I-General_Concept
to	O
be	O
used	O
under	O
its	O
custom	O
version	O
of	O
MS-DOS	O
.	O
</s>
<s>
Most	O
standard	O
programs	O
written	O
for	O
DOS	B-Device
did	O
not	O
necessarily	O
need	O
640KB	O
or	O
more	O
of	O
memory	O
.	O
</s>
<s>
Instead	O
,	O
driver	O
software	O
and	O
utilities	O
referred	O
to	O
as	O
terminate-and-stay-resident	O
programs	O
(	O
TSRs	O
)	O
could	O
be	O
used	O
in	O
addition	O
to	O
the	O
standard	O
DOS	B-Device
software	O
.	O
</s>
<s>
These	O
drivers	O
and	O
utilities	O
typically	O
permanently	O
used	O
some	O
conventional	B-Device
memory	I-Device
,	O
reducing	O
the	O
total	O
available	O
for	O
standard	O
DOS	B-Device
programs	O
.	O
</s>
<s>
Some	O
very	O
common	O
DOS	B-Device
drivers	O
and	O
TSRs	O
using	O
conventional	B-Device
memory	I-Device
included	O
:	O
</s>
<s>
Needed	O
in	O
addition	O
to	O
above	O
SCSI	B-Architecture
drivers	O
for	O
access	O
to	O
a	O
SCSI	B-Architecture
CDROM	O
device	O
.	O
</s>
<s>
SBCONFIG.EXE	O
-	O
support	O
for	O
Sound	B-Device
Blaster	I-Device
16	I-Device
audio	O
device	O
;	O
a	O
differently-named	O
driver	O
was	O
used	O
for	O
various	O
other	O
sound	O
cards	O
,	O
also	O
occupying	O
conventional	B-Device
memory	I-Device
.	O
</s>
<s>
SMARTDRV.EXE	B-Application
-	O
install	O
drive	O
cache	O
to	O
speed	O
up	O
disk	O
reads	O
and	O
writes	O
;	O
although	O
it	O
could	O
allocate	O
several	O
megabytes	O
of	O
memory	O
beyond	O
640kb	O
for	O
the	O
drive	O
caching	O
,	O
it	O
still	O
needed	O
a	O
small	O
portion	O
of	O
conventional	B-Device
memory	I-Device
to	O
function	O
.	O
</s>
<s>
But	O
in	O
many	O
cases	O
a	O
choice	O
had	O
to	O
be	O
made	O
by	O
the	O
computer	O
user	O
,	O
to	O
decide	O
whether	O
to	O
be	O
able	O
to	O
run	O
certain	O
standard	O
DOS	B-Device
programs	O
or	O
have	O
all	O
their	O
favorite	O
drivers	O
and	O
TSRs	O
loaded	O
.	O
</s>
<s>
Loading	O
the	O
entire	O
list	O
shown	O
above	O
is	O
likely	O
either	O
impractical	O
or	O
impossible	O
,	O
if	O
the	O
user	O
also	O
wants	O
to	O
run	O
a	O
standard	O
DOS	B-Device
program	O
as	O
well	O
.	O
</s>
<s>
For	O
drivers	O
that	O
could	O
not	O
be	O
unloaded	O
,	O
later	O
versions	O
of	O
DOS	B-Device
included	O
a	O
startup	O
menu	O
capability	O
to	O
allow	O
the	O
computer	O
user	O
to	O
select	O
various	O
groups	O
of	O
drivers	O
and	O
TSRs	O
to	O
load	O
before	O
running	O
certain	O
high-memory-usage	O
standard	O
DOS	B-Device
programs	O
.	O
</s>
<s>
As	O
DOS	B-Device
applications	O
grew	O
larger	O
and	O
more	O
complex	O
in	O
the	O
late	O
1980s	O
and	O
early	O
1990s	O
,	O
it	O
became	O
common	O
practice	O
to	O
free	O
up	O
conventional	B-Device
memory	I-Device
by	O
moving	O
the	O
device	O
drivers	O
and	O
TSR	O
programs	O
into	O
upper	B-Device
memory	I-Device
blocks	I-Device
(	O
UMBs	O
)	O
in	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
(	O
UMA	B-Device
)	O
at	O
boot	O
,	O
in	O
order	O
to	O
maximize	O
the	O
conventional	B-Device
memory	I-Device
available	O
for	O
applications	O
.	O
</s>
<s>
This	O
feature	O
was	O
first	O
provided	O
by	O
third-party	O
products	O
such	O
as	O
QEMM	B-Device
,	O
before	O
being	O
built	O
into	O
DR	O
DOS	B-Device
5.0	O
in	O
1990	O
then	O
MS-DOS	O
5.0	O
in	O
1991	O
.	O
</s>
<s>
Most	O
users	O
used	O
the	O
accompanying	O
driver	O
provided	O
in	O
MS-DOS	O
5	O
,	O
but	O
third-party	O
products	O
from	O
companies	O
such	O
as	O
QEMM	B-Device
also	O
proved	O
popular	O
.	O
</s>
<s>
At	O
startup	O
,	O
drivers	O
could	O
be	O
loaded	O
high	O
using	O
the	O
"	O
DEVICEHIGH	B-Device
=	O
"	O
directive	O
,	O
while	O
TSRs	O
could	O
be	O
loaded	O
high	O
using	O
the	O
"	O
LOADHIGH	O
"	O
,	O
"	O
LH	O
"	O
or	O
"	O
HILOAD	O
"	O
directives	O
.	O
</s>
<s>
If	O
the	O
operation	O
failed	O
,	O
the	O
driver	O
or	O
TSR	O
would	O
automatically	O
load	O
into	O
the	O
regular	O
conventional	B-Device
memory	I-Device
instead	O
.	O
</s>
<s>
CONFIG.SYS	B-Device
,	O
loading	O
ANSI.SYS	O
into	O
UMBs	O
,	O
no	O
EMS	B-Device
support	O
enabled	O
:	O
</s>
<s>
AUTOEXEC.BAT	O
,	O
loading	O
MOUSE	O
,	O
DOSKEY	B-Device
,	O
and	O
SMARTDRV	B-Application
into	O
UMBs	O
if	O
possible	O
:	O
</s>
<s>
The	O
ability	O
of	O
DOS	B-Device
versions	O
5.0	O
and	O
later	O
to	O
move	O
their	O
own	O
system	O
core	O
code	O
into	O
the	O
high	B-Device
memory	I-Device
area	I-Device
(	O
HMA	O
)	O
through	O
the	O
DOS	B-Device
=	O
HIGH	O
command	O
gave	O
another	O
boost	O
to	O
free	O
memory	O
.	O
</s>
<s>
Hardware	O
expansion	O
boards	O
could	O
use	O
any	O
of	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
for	O
ROM	B-Device
addressing	O
,	O
so	O
the	O
upper	B-Device
memory	I-Device
blocks	I-Device
were	O
of	O
variable	O
size	O
and	O
in	O
different	O
locations	O
for	O
each	O
computer	O
,	O
depending	O
on	O
the	O
hardware	O
installed	O
.	O
</s>
<s>
Some	O
windows	B-Application
of	O
upper	B-Device
memory	I-Device
could	O
be	O
large	O
and	O
others	O
small	O
.	O
</s>
<s>
Loading	O
drivers	O
and	O
TSRs	O
high	O
would	O
pick	O
a	O
block	O
and	O
try	O
to	O
fit	O
the	O
program	O
into	O
it	O
,	O
until	O
a	O
block	O
was	O
found	O
where	O
it	O
fit	O
,	O
or	O
it	O
would	O
go	O
into	O
conventional	B-Device
memory	I-Device
.	O
</s>
<s>
An	O
unusual	O
aspect	O
of	O
drivers	O
and	O
TSRs	O
is	O
that	O
they	O
would	O
use	O
different	O
amounts	O
of	O
conventional	O
and/or	O
upper	B-Device
memory	I-Device
,	O
based	O
on	O
the	O
order	O
they	O
were	O
loaded	O
.	O
</s>
<s>
Later	O
versions	O
of	O
DOS	B-Device
allowed	O
the	O
use	O
of	O
a	O
specific	O
load	O
address	O
for	O
a	O
driver	O
or	O
TSR	O
,	O
to	O
fit	O
drivers/TSRs	O
more	O
tightly	O
together	O
.	O
</s>
<s>
In	O
MS-DOS	O
6.0	O
,	O
Microsoft	O
introduced	O
MEMMAKER	O
,	O
which	O
automated	O
this	O
process	O
of	O
block	O
matching	O
,	O
matching	O
the	O
functionality	O
third-party	O
memory	B-General_Concept
managers	I-General_Concept
offered	O
.	O
</s>
<s>
This	O
automatic	O
optimization	O
often	O
still	O
did	O
not	O
provide	O
the	O
same	O
result	O
as	O
doing	O
it	O
by	O
hand	O
,	O
in	O
the	O
sense	O
of	O
providing	O
the	O
greatest	O
free	O
conventional	B-Device
memory	I-Device
.	O
</s>
<s>
Also	O
in	O
some	O
cases	O
third-party	O
companies	O
wrote	O
special	O
multi-function	O
drivers	O
that	O
would	O
combine	O
the	O
capabilities	O
of	O
several	O
standard	O
DOS	B-Device
drivers	O
and	O
TSRs	O
into	O
a	O
single	O
very	O
compact	O
program	O
that	O
used	O
just	O
a	O
few	O
kilobytes	O
of	O
memory	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
functions	O
of	O
mouse	O
driver	O
,	O
CD-ROM	O
driver	O
,	O
ANSI	O
support	O
,	O
DOSKEY	B-Device
command	O
recall	O
,	O
and	O
disk	O
caching	O
would	O
all	O
be	O
combined	O
together	O
in	O
one	O
program	O
,	O
consuming	O
just	O
1	O
-	O
2	O
kilobytes	O
of	O
conventional	B-Device
memory	I-Device
for	O
normal	O
driver/interrupt	O
access	O
,	O
and	O
storing	O
the	O
rest	O
of	O
the	O
multi-function	O
program	O
code	O
in	O
EMS	B-Device
or	O
XMS	B-Device
memory	I-Device
.	O
</s>
<s>
The	O
barrier	O
was	O
only	O
overcome	O
with	O
the	O
arrival	O
of	O
DOS	B-Device
extenders	I-Device
,	O
which	O
allowed	O
DOS	B-Device
applications	O
to	O
run	O
in	O
16-bit	O
or	O
32-bit	O
protected	B-Application
mode	I-Application
,	O
but	O
these	O
were	O
not	O
very	O
widely	O
used	O
outside	O
of	O
computer	O
gaming	O
.	O
</s>
<s>
With	O
a	O
32-bit	O
DOS	B-Device
extender	I-Device
,	O
a	O
game	O
could	O
benefit	O
from	O
a	O
32-bit	O
flat	O
address	O
space	O
and	O
the	O
full	O
32-bit	O
instruction	O
set	O
without	O
the	O
66h/67h	O
operand/address	O
override	O
prefixes	O
.	O
</s>
<s>
32-bit	O
DOS	B-Device
extenders	I-Device
required	O
compiler	O
support	O
(	O
32-bit	O
compilers	O
)	O
while	O
XMS	O
and	O
EMS	B-Device
worked	O
with	O
an	O
old	O
compiler	O
targeting	O
16-bit	O
real-mode	B-Application
DOS	B-Device
applications	O
.	O
</s>
<s>
The	O
two	O
most	O
common	O
specifications	O
for	O
DOS	B-Device
extenders	I-Device
were	O
VCPI	O
-	O
and	O
later	O
DPMI-compatible	O
with	O
Windows	B-Application
3.x	O
.	O
</s>
<s>
The	O
most	O
notable	O
DPMI-compliant	O
DOS	B-Device
extender	I-Device
may	O
be	O
DOS/4GW	B-Application
,	O
shipping	O
with	O
Watcom	B-Application
.	O
</s>
<s>
It	O
was	O
very	O
common	O
in	O
games	O
for	O
DOS	B-Device
.	O
</s>
<s>
Such	O
a	O
game	O
would	O
consist	O
of	O
either	O
a	O
DOS/4GW	B-Application
32-bit	O
kernel	O
,	O
or	O
a	O
stub	O
which	O
loaded	O
a	O
DOS/4GW	B-Application
kernel	O
located	O
in	O
the	O
path	O
or	O
in	O
the	O
same	O
directory	O
and	O
a	O
32-bit	O
"	O
linear	O
executable	O
"	O
.	O
</s>
<s>
Utilities	O
are	O
available	O
which	O
can	O
strip	O
DOS/4GW	B-Application
out	O
of	O
such	O
a	O
program	O
and	O
allow	O
the	O
user	O
to	O
experiment	O
with	O
any	O
of	O
the	O
several	O
,	O
and	O
perhaps	O
improved	O
,	O
DOS/4GW	B-Application
clones	O
.	O
</s>
<s>
Prior	O
to	O
DOS	B-Device
extenders	I-Device
,	O
if	O
a	O
user	O
installed	O
additional	O
memory	O
and	O
wished	O
to	O
use	O
it	O
under	O
DOS	B-Device
,	O
they	O
would	O
first	O
have	O
to	O
install	O
and	O
configure	O
drivers	O
to	O
support	O
either	O
expanded	B-Device
memory	I-Device
specification	O
(	O
EMS	B-Device
)	O
or	O
extended	B-Device
memory	I-Device
specification	O
(	O
XMS	O
)	O
and	O
run	O
programs	O
supporting	O
one	O
of	O
these	O
specifications	O
.	O
</s>
<s>
EMS	B-Device
was	O
a	O
specification	O
available	O
on	O
all	O
PCs	O
,	O
including	O
those	O
based	O
on	O
the	O
Intel	B-General_Concept
8086	I-General_Concept
and	O
Intel	B-Device
8088	I-Device
,	O
which	O
allowed	O
add-on	O
hardware	O
to	O
page	O
small	O
chunks	O
of	O
memory	O
in	O
and	O
out	O
(	O
bank	B-General_Concept
switching	I-General_Concept
)	O
of	O
the	O
"	O
real	B-Application
mode	I-Application
"	O
addressing	O
space	O
(	O
0x0400	O
–	O
0xFFFF	O
)	O
.	O
</s>
<s>
This	O
allowed	O
16-bit	O
real-mode	B-Application
DOS	B-Device
programs	O
to	O
access	O
several	O
megabytes	O
of	O
RAM	B-Architecture
through	O
a	O
hole	O
in	O
real	O
memory	O
,	O
typically	O
(	O
0xE000	O
–	O
0xEFFF	O
)	O
.	O
</s>
<s>
This	O
is	O
very	O
similar	O
to	O
modern	O
paged	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
However	O
,	O
in	O
a	O
virtual	B-Architecture
memory	I-Architecture
system	O
,	O
the	O
operating	O
system	O
handles	O
all	O
paging	B-Architecture
operations	O
,	O
while	O
paging	B-Architecture
was	O
explicit	O
with	O
EMS	B-Device
.	O
</s>
<s>
XMS	O
provided	O
a	O
basic	O
protocol	O
which	O
allowed	O
a	O
16-bit	O
DOS	B-Device
programs	O
to	O
load	O
chunks	O
of	O
80286	B-General_Concept
or	O
80386	B-General_Concept
extended	B-Device
memory	I-Device
in	O
low	B-Device
memory	I-Device
(	O
address	O
0x0400-0xFFFF	O
)	O
.	O
</s>
<s>
A	O
typical	O
XMS	O
driver	O
had	O
to	O
switch	O
to	O
protected	B-Application
mode	I-Application
in	O
order	O
to	O
load	O
this	O
memory	O
.	O
</s>
<s>
The	O
problem	O
with	O
this	O
approach	O
is	O
that	O
while	O
in	O
286	B-Application
protected	I-Application
mode	I-Application
,	O
direct	O
DOS	B-Device
calls	O
could	O
not	O
be	O
made	O
.	O
</s>
<s>
The	O
workaround	O
was	O
to	O
implement	O
a	O
callback	O
mechanism	O
,	O
requiring	O
a	O
reset	O
of	O
the	O
286	B-General_Concept
.	O
</s>
<s>
On	O
the	O
286	B-General_Concept
,	O
this	O
was	O
a	O
major	O
problem	O
.	O
</s>
<s>
The	O
Intel	B-General_Concept
80386	I-General_Concept
,	O
which	O
introduced	O
"	O
virtual	B-Application
8086	I-Application
mode	I-Application
"	O
,	O
allowed	O
the	O
guest	O
kernel	O
to	O
emulate	O
the	O
8086	B-General_Concept
and	O
run	O
the	O
host	O
operating	O
system	O
without	O
having	O
to	O
actually	O
force	O
the	O
processor	O
back	O
into	O
"	O
real	B-Application
mode	I-Application
"	O
.	O
</s>
<s>
HIMEM.SYS	O
2.03	O
and	O
higher	O
used	O
unreal	B-Device
mode	I-Device
on	O
the	O
80386	B-General_Concept
and	O
higher	O
CPUs	O
while	O
HIMEM.SYS	O
2.06	O
and	O
higher	O
used	O
LOADALL	B-Device
to	O
change	O
undocumented	O
internal	O
registers	O
on	O
the	O
80286	B-General_Concept
,	O
significantly	O
improving	O
interrupt	O
latency	O
by	O
avoiding	O
repeated	O
real	O
mode/protected	O
mode	O
switches	O
.	O
</s>
<s>
Windows	B-Application
installs	O
its	O
own	O
version	O
of	O
HIMEM.SYS	O
on	O
DOS3.3	O
and	O
higher	O
.	O
</s>
<s>
Windows	B-Application
HIMEM.SYS	O
launches	O
32-bit	O
protected	B-Application
mode	I-Application
XMS	O
(	O
n	O
)	O
.0	O
services	O
provider	O
for	O
the	O
Windows	B-Application
Virtual	O
Machine	O
Manager	O
,	O
which	O
then	O
provides	O
XMS	O
(	O
n-1	O
)	O
.0	O
services	O
to	O
DOS	B-Device
boxes	O
and	O
the	O
16-bit	O
Windows	B-Application
machine	O
(	O
e.g.	O
</s>
<s>
DOS7	O
HIMEM.SYS	O
is	O
XMS3.0	O
but	O
running	O
'	O
MEM	O
 '	O
command	O
in	O
a	O
Windows95	O
DOS	B-Device
window	O
shows	O
XMS2.0	O
information	O
)	O
.	O
</s>
