<s>
In	O
x86	B-Operating_System
computing	O
,	O
unreal	B-Device
mode	I-Device
,	O
also	O
big	B-Device
real	I-Device
mode	I-Device
,	O
huge	B-Device
real	I-Device
mode	I-Device
,	O
flat	B-Device
real	I-Device
mode	I-Device
,	O
or	O
voodoo	O
mode	O
is	O
a	O
variant	O
of	O
real	B-Application
mode	I-Application
,	O
in	O
which	O
one	O
or	O
more	O
segment	B-Operating_System
descriptors	I-Operating_System
has	O
been	O
loaded	O
with	O
non-standard	O
values	O
,	O
like	O
32-bit	O
limits	O
allowing	O
access	O
to	O
the	O
entire	O
memory	O
.	O
</s>
<s>
Contrary	O
to	O
its	O
name	O
,	O
it	O
is	O
not	O
a	O
separate	O
addressing	O
mode	O
that	O
the	O
x86	B-Operating_System
processors	O
can	O
operate	O
in	O
.	O
</s>
<s>
It	O
is	O
used	O
in	O
the	O
80286	B-General_Concept
and	O
later	O
x86	B-Operating_System
processors	O
.	O
</s>
<s>
For	O
efficiency	O
reasons	O
,	O
the	O
80286	B-General_Concept
and	O
all	O
later	O
x86	B-Operating_System
processors	O
use	O
the	O
base	O
address	O
,	O
size	O
and	O
other	O
attributes	O
stored	O
in	O
their	O
internal	O
segment	B-Operating_System
descriptor	I-Operating_System
cache	O
whenever	O
computing	O
effective	O
memory	O
addresses	O
,	O
even	O
in	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
Therefore	O
,	O
a	O
modification	O
of	O
the	O
internal	O
segment	B-Operating_System
descriptor	I-Operating_System
allows	O
altering	O
some	O
properties	O
of	O
segments	O
in	O
real	B-Application
mode	I-Application
,	O
like	O
the	O
size	O
of	O
addressable	O
memory	O
.	O
</s>
<s>
HIMEM.SYS	O
uses	O
this	O
feature	O
to	O
address	O
extended	B-Device
memory	I-Device
,	O
unless	O
DOS	O
is	O
switched	O
to	O
run	O
in	O
a	O
virtual	B-Application
8086	I-Application
mode	I-Application
that	O
is	O
incompatible	O
with	O
unreal	B-Device
mode	I-Device
.	O
</s>
<s>
One	O
of	O
the	O
very	O
few	O
games	O
—	O
if	O
not	O
the	O
only	O
one	O
—	O
that	O
used	O
unreal	B-Device
mode	I-Device
was	O
Ultima	B-Application
VII	I-Application
.	O
</s>
<s>
Unreal	B-Device
mode	I-Device
is	O
used	O
by	O
BIOS	B-Operating_System
code	O
as	O
this	O
is	O
the	O
initial	O
mode	O
of	O
modern	O
Intel	O
processors	O
.	O
</s>
<s>
Furthermore	O
,	O
the	O
System	B-Architecture
Management	I-Architecture
Mode	I-Architecture
(	O
SMM	O
)	O
in	O
Intel	B-General_Concept
386SL	I-General_Concept
and	O
later	O
processors	O
places	O
the	O
processor	O
in	O
huge	B-Device
real	I-Device
mode	I-Device
.	O
</s>
<s>
A	O
program	O
in	O
unreal	B-Device
mode	I-Device
can	O
call	O
16-bit	O
code	O
programmed	O
for	O
real	B-Application
mode	I-Application
(	O
BIOS	B-Operating_System
,	O
DOS	O
kernel	O
and	O
drivers	O
)	O
without	O
any	O
thunking	B-Application
.	O
</s>
<s>
This	O
makes	O
an	O
unreal	B-Device
mode	I-Device
driver	O
simpler	O
than	O
a	O
DPMI	O
driver	O
.	O
</s>
<s>
However	O
unreal	B-Device
mode	I-Device
is	O
incompatible	O
with	O
protected	B-Application
mode	I-Application
operating	O
systems	O
such	O
as	O
Windows	O
3.x/9x/NT	O
and	O
OS/2	O
.	O
</s>
<s>
Big	B-Device
real	I-Device
mode	I-Device
has	O
a	O
1MiB	O
code	O
segment	O
and	O
a	O
4GiB	O
data	O
segment	O
.	O
</s>
<s>
Some	O
boot	B-Application
loaders	I-Application
(	O
such	O
as	O
LILO	B-Application
)	O
use	O
the	O
unreal	B-Device
mode	I-Device
to	O
access	O
up	O
to	O
4GiB	O
of	O
memory	O
.	O
</s>
<s>
To	O
put	O
an	O
80386	B-General_Concept
or	O
higher	O
microprocessor	O
into	O
unreal	B-Device
mode	I-Device
,	O
a	O
program	O
must	O
first	O
enter	O
protected	B-Application
mode	I-Application
,	O
find	O
or	O
create	O
a	O
flat	O
descriptor	O
in	O
the	O
GDT	B-General_Concept
or	O
LDT	B-General_Concept
,	O
load	O
some	O
of	O
the	O
data	O
segment	O
registers	O
with	O
the	O
respective	O
protected	B-Application
mode	I-Application
"	O
selector	O
"	O
,	O
and	O
then	O
switch	O
back	O
to	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
After	O
returning	O
to	O
real	B-Application
mode	I-Application
,	O
the	O
processor	O
will	O
continue	O
using	O
the	O
cached	O
descriptors	O
as	O
established	O
in	O
protected	B-Application
mode	I-Application
,	O
thus	O
allowing	O
access	O
to	O
4	O
GiB	O
of	O
extended	B-Device
memory	I-Device
from	O
real	B-Application
mode	I-Application
.	O
</s>
<s>
The	O
80286	B-General_Concept
microprocessor	I-General_Concept
can	O
be	O
put	O
into	O
unreal	B-Device
mode	I-Device
only	O
with	O
help	O
of	O
the	O
undocumented	O
instruction	O
LOADALL	B-Device
to	O
modify	O
the	O
hidden	O
segment	O
base	O
registers	O
to	O
point	O
to	O
the	O
source	O
or	O
target	O
memory	O
location	O
above	O
1	O
MiB	O
.	O
</s>
<s>
Starting	O
with	O
the	O
80386	B-General_Concept
,	O
real	B-Application
mode	I-Application
programs	O
can	O
use	O
the	O
32	O
bit	O
registers	O
with	O
the	O
Address	O
Size	O
Override	O
Prefix	O
.	O
</s>
<s>
In	O
normal	O
real	B-Application
mode	I-Application
,	O
a	O
fault	O
occurs	O
if	O
EBX	O
exceeds	O
0xFFFF	O
.	O
</s>
<s>
In	O
unreal	B-Device
mode	I-Device
,	O
the	O
access	O
is	O
allowed	O
.	O
</s>
<s>
As	O
described	O
above	O
,	O
unreal	B-Device
mode	I-Device
usually	O
involves	O
using	O
one	O
or	O
more	O
data	O
selectors	O
to	O
address	O
data	O
in	O
memory	O
more	O
efficiently	O
.	O
</s>
<s>
This	O
has	O
been	O
common	O
practice	O
and	O
often	O
referred	O
to	O
as	O
"	O
flat	B-Device
real	I-Device
mode	I-Device
"	O
or	O
"	O
big	B-Device
real	I-Device
mode	I-Device
"	O
.	O
</s>
<s>
The	O
"	O
huge	B-Device
real	I-Device
mode	I-Device
"	O
is	O
a	O
variant	O
where	O
also	O
the	O
code	O
segment	O
is	O
extended	O
to	O
4	O
GiB	O
.	O
</s>
<s>
A	O
special	O
case	O
of	O
unreal	B-Device
mode	I-Device
is	O
attained	O
by	O
,	O
in	O
addition	O
,	O
loading	O
the	O
code	O
selector	O
(	O
CS	O
)	O
from	O
a	O
descriptor	O
having	O
the	O
32-bit	O
attribute	O
(	O
"	O
D	O
"	O
bit	O
)	O
set	O
to	O
one	O
.	O
</s>
<s>
Since	O
real	B-Application
mode	I-Application
interrupts	O
do	O
not	O
automatically	O
preserve	O
the	O
high	O
sixteen	O
bits	O
of	O
the	O
extended	O
instruction	O
pointer	O
(	O
EIP	O
)	O
,	O
the	O
code	O
needs	O
to	O
either	O
fit	O
in	O
the	O
low	O
portion	O
of	O
code	O
segment	O
or	O
be	O
able	O
to	O
preserve	O
and	O
restore	O
the	O
high	O
bits	O
of	O
EIP	O
in	O
some	O
way	O
.	O
</s>
<s>
In	O
addition	O
,	O
if	O
BIOS	B-Operating_System
or	O
DOS	O
interrupt	O
functions	O
that	O
use	O
16-bit	O
code	O
are	O
to	O
be	O
called	O
,	O
mode	O
switching	O
is	O
required	O
to	O
switch	O
the	O
"	O
D	O
"	O
bit	O
back	O
and	O
forth	O
.	O
</s>
<s>
Some	O
emulators	O
do	O
not	O
support	O
emulation	O
of	O
programs	O
running	O
in	O
this	O
variant	O
of	O
unreal	B-Device
mode	I-Device
.	O
</s>
