<s>
In	O
DOS	B-General_Concept
memory	I-General_Concept
management	I-General_Concept
,	O
extended	B-Device
memory	I-Device
refers	O
to	O
memory	B-General_Concept
above	O
the	O
first	O
megabyte	O
(	O
220	O
bytes	O
)	O
of	O
address	B-General_Concept
space	I-General_Concept
in	O
an	O
IBM	B-Device
PC	I-Device
or	O
compatible	O
with	O
an	O
80286	B-General_Concept
or	O
later	O
processor	B-General_Concept
.	O
</s>
<s>
The	O
term	O
is	O
mainly	O
used	O
under	O
the	O
DOS	B-Device
and	O
Windows	B-Application
operating	I-Application
systems	I-Application
.	O
</s>
<s>
DOS	B-Device
programs	O
,	O
running	O
in	O
real	B-Application
mode	I-Application
or	O
virtual	B-Application
x86	I-Application
mode	I-Application
,	O
cannot	O
directly	O
access	O
this	O
memory	B-General_Concept
,	O
but	O
are	O
able	O
to	O
do	O
so	O
through	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
called	O
the	O
Extended	B-Device
Memory	I-Device
Specification	O
(	O
XMS	O
)	O
.	O
</s>
<s>
This	O
API	B-Application
is	O
implemented	O
by	O
a	O
driver	B-Application
(	O
such	O
as	O
HIMEM.SYS	O
)	O
or	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
which	O
takes	O
care	O
of	O
memory	B-General_Concept
management	I-General_Concept
and	O
copying	O
memory	B-General_Concept
between	O
conventional	B-Device
and	O
extended	B-Device
memory	I-Device
,	O
by	O
temporarily	O
switching	O
the	O
processor	B-General_Concept
into	O
protected	B-Application
mode	I-Application
.	O
</s>
<s>
In	O
this	O
context	O
,	O
the	O
term	O
"	O
extended	B-Device
memory	I-Device
"	O
may	O
refer	O
to	O
either	O
the	O
whole	O
of	O
the	O
extended	B-Device
memory	I-Device
or	O
only	O
the	O
portion	O
available	O
through	O
this	O
API	B-Application
.	O
</s>
<s>
Extended	B-Device
memory	I-Device
can	O
also	O
be	O
accessed	O
directly	O
by	O
DOS	B-Device
programs	O
running	O
in	O
protected	B-Application
mode	I-Application
using	O
VCPI	O
or	O
DPMI	O
,	O
two	O
(	O
different	O
and	O
incompatible	O
)	O
methods	O
of	O
using	O
protected	B-Application
mode	I-Application
under	O
DOS	B-Device
.	O
</s>
<s>
Extended	B-Device
memory	I-Device
should	O
not	O
be	O
confused	O
with	O
expanded	B-Device
memory	I-Device
(	O
EMS	O
)	O
,	O
an	O
earlier	O
method	O
for	O
expanding	O
the	O
IBM	B-Device
PC	I-Device
's	O
memory	B-General_Concept
capacity	O
beyond	O
640	O
kB	O
(	O
655,360	O
bytes	O
)	O
using	O
an	O
expansion	B-Device
card	I-Device
with	O
bank	B-General_Concept
switched	I-General_Concept
memory	B-General_Concept
modules	O
.	O
</s>
<s>
Because	O
of	O
the	O
available	O
support	O
for	O
expanded	B-Device
memory	I-Device
in	O
popular	O
applications	O
,	O
device	B-Application
drivers	I-Application
were	O
developed	O
that	O
emulated	O
expanded	B-Device
memory	I-Device
using	O
extended	B-Device
memory	I-Device
.	O
</s>
<s>
Later	O
two	O
additional	O
methods	O
were	O
developed	O
allowing	O
direct	O
access	O
to	O
a	O
small	O
portion	O
of	O
extended	B-Device
memory	I-Device
from	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
These	O
memory	B-General_Concept
areas	O
are	O
referred	O
to	O
as	O
the	O
high	B-Device
memory	I-Device
area	I-Device
(	O
HMA	O
)	O
and	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
(	O
UMA	B-Device
;	O
also	O
referred	O
to	O
as	O
upper	B-Device
memory	I-Device
blocks	I-Device
or	O
UMBs	O
)	O
.	O
</s>
<s>
On	O
x86-based	O
PCs	O
,	O
extended	B-Device
memory	I-Device
is	O
only	O
available	O
with	O
an	O
Intel	B-General_Concept
80286	I-General_Concept
processor	B-General_Concept
or	O
higher	O
.	O
</s>
<s>
Only	O
these	O
chips	O
can	O
address	O
more	O
than	O
1	O
megabyte	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
The	O
earlier	O
8086/8088	O
processors	O
can	O
make	O
use	O
of	O
more	O
than	O
1	O
MB	O
of	O
RAM	B-Architecture
if	O
one	O
employs	O
special	B-General_Concept
hardware	I-General_Concept
to	O
make	O
selectable	O
parts	O
of	O
it	O
appear	O
at	O
addresses	O
below	O
1	O
MB	O
.	O
</s>
<s>
On	O
a	O
286	B-General_Concept
or	O
better	O
PC	O
equipped	O
with	O
more	O
than	O
640	O
kB	O
of	O
RAM	B-Architecture
,	O
the	O
additional	O
memory	B-General_Concept
would	O
generally	O
be	O
re-mapped	O
above	O
the	O
1	O
MB	O
boundary	O
,	O
since	O
the	O
IBM	B-Device
PC	I-Device
architecture	O
reserves	O
addresses	O
between	O
640	O
kB	O
and	O
1	O
MB	O
for	O
system	O
ROM	O
and	O
peripherals	O
.	O
</s>
<s>
Extended	B-Device
memory	I-Device
is	O
not	O
accessible	O
in	O
real	B-Application
mode	I-Application
(	O
except	O
for	O
a	O
small	O
portion	O
called	O
the	O
high	B-Device
memory	I-Device
area	I-Device
)	O
.	O
</s>
<s>
Only	O
applications	O
executing	O
in	O
protected	B-Application
mode	I-Application
can	O
use	O
extended	B-Device
memory	I-Device
directly	O
.	O
</s>
<s>
A	O
supervising	O
protected-mode	B-Application
operating	B-General_Concept
system	I-General_Concept
such	O
as	O
Microsoft	B-Application
Windows	I-Application
manages	O
application	O
programs	O
 '	O
access	O
to	O
memory	B-General_Concept
.	O
</s>
<s>
The	O
processor	B-General_Concept
makes	O
this	O
memory	B-General_Concept
available	O
through	O
the	O
Global	B-General_Concept
Descriptor	I-General_Concept
Table	I-General_Concept
(	O
GDT	O
)	O
and	O
one	O
or	O
more	O
Local	B-General_Concept
Descriptor	I-General_Concept
Tables	I-General_Concept
(	O
LDTs	O
)	O
.	O
</s>
<s>
The	O
memory	B-General_Concept
is	O
"	O
protected	O
"	O
in	O
the	O
sense	O
that	O
memory	B-General_Concept
segments	O
assigned	O
a	O
local	O
descriptor	O
cannot	O
be	O
accessed	O
by	O
another	O
program	O
because	O
that	O
program	O
uses	O
a	O
different	O
LDT	O
,	O
and	O
memory	B-General_Concept
segments	O
assigned	O
a	O
global	O
descriptor	O
can	O
have	O
their	O
access	O
rights	O
restricted	O
,	O
causing	O
a	O
processor	B-General_Concept
exception	B-General_Concept
(	O
e.g.	O
,	O
a	O
general	B-Operating_System
protection	I-Operating_System
fault	I-Operating_System
or	O
GPF	O
)	O
on	O
violation	O
.	O
</s>
<s>
This	O
prevents	O
programs	O
running	O
in	O
protected	B-Application
mode	I-Application
from	O
interfering	O
with	O
each	O
other	O
's	O
memory	B-General_Concept
.	O
</s>
<s>
A	O
protected-mode	B-Application
operating	B-General_Concept
system	I-General_Concept
such	O
as	O
Microsoft	B-Application
Windows	I-Application
can	O
also	O
run	O
real-mode	B-Application
programs	O
and	O
provide	O
expanded	B-Device
memory	I-Device
to	O
them	O
.	O
</s>
<s>
The	O
DOS	B-Device
Protected	B-Application
Mode	I-Application
Interface	O
(	O
DPMI	O
)	O
is	O
Microsoft	O
's	O
prescribed	O
method	O
for	O
an	O
DOS	B-Device
program	O
to	O
access	O
extended	B-Device
memory	I-Device
under	O
a	O
multitasking	B-Operating_System
environment	O
.	O
</s>
<s>
The	O
Extended	B-Device
Memory	I-Device
Specification	O
(	O
XMS	O
)	O
is	O
the	O
specification	O
describing	O
the	O
use	O
of	O
IBM	B-Device
PC	I-Device
extended	B-Device
memory	I-Device
in	O
real	B-Application
mode	I-Application
for	O
storing	O
data	O
(	O
but	O
not	O
for	O
running	O
executable	O
code	O
in	O
it	O
)	O
.	O
</s>
<s>
Memory	B-General_Concept
is	O
made	O
available	O
by	O
extended	B-Device
memory	I-Device
manager	O
(	O
XMM	O
)	O
software	O
such	O
as	O
HIMEM.SYS	O
.	O
</s>
<s>
XMS	O
version	O
2.0	O
,	O
released	O
in	O
July	O
1988	O
,	O
allowed	O
for	O
up	O
to	O
64	O
MB	O
of	O
memory	B-General_Concept
,	O
with	O
XMS	O
version	O
3.0	O
this	O
increased	O
to	O
4	O
GB	O
(	O
232	O
bytes	O
)	O
.	O
</s>
<s>
To	O
differentiate	O
between	O
the	O
possibly	O
different	O
amount	O
of	O
memory	B-General_Concept
that	O
might	O
be	O
available	O
to	O
applications	O
,	O
depending	O
on	O
which	O
version	O
of	O
the	O
specification	O
they	O
were	O
developed	O
to	O
,	O
the	O
latter	O
may	O
be	O
referred	O
to	O
as	O
super	B-Device
extended	I-Device
memory	I-Device
(	O
SXMS	B-Device
)	O
.	O
</s>
<s>
The	O
extended	B-Device
memory	I-Device
manager	O
is	O
also	O
responsible	O
for	O
managing	O
allocations	O
in	O
the	O
high	B-Device
memory	I-Device
area	I-Device
(	O
HMA	O
)	O
and	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
(	O
UMA	B-Device
;	O
also	O
referred	O
to	O
as	O
upper	B-Device
memory	I-Device
blocks	I-Device
or	O
UMBs	O
)	O
.	O
</s>
<s>
In	O
practice	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
will	O
be	O
provided	O
by	O
the	O
expanded	B-Device
memory	I-Device
manager	O
(	O
EMM	O
)	O
,	O
after	O
which	O
DOS	B-Device
will	O
try	O
to	O
allocate	O
them	O
all	O
and	O
manage	O
them	O
itself	O
.	O
</s>
