<s>
In	O
DOS	B-General_Concept
memory	I-General_Concept
management	I-General_Concept
,	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
(	O
UMA	B-Device
)	O
is	O
the	O
memory	B-General_Concept
between	O
the	O
addresses	B-General_Concept
of	O
640KB	O
and	O
1024KB	O
(	O
0xA0000	O
–	O
0xFFFFF	O
)	O
in	O
an	O
IBM	B-Device
PC	I-Device
or	O
compatible	O
.	O
</s>
<s>
IBM	O
reserved	O
the	O
uppermost	O
384KB	O
of	O
the	O
8088	B-Device
CPU	B-General_Concept
's	O
1024KB	O
address	B-General_Concept
space	I-General_Concept
for	O
BIOS	B-Operating_System
ROM	I-Operating_System
,	O
Video	B-Device
BIOS	I-Device
,	O
Option	O
ROMs	O
,	O
video	O
RAM	B-Architecture
,	O
RAM	B-Architecture
on	O
peripherals	O
,	O
memory-mapped	B-Architecture
I/O	I-Architecture
,	O
and	O
obsoleted	O
ROM	B-Device
BASIC	O
.	O
</s>
<s>
However	O
,	O
even	O
with	O
video	O
RAM	B-Architecture
,	O
the	O
ROM	B-Operating_System
BIOS	I-Operating_System
,	O
the	O
Video	B-Device
BIOS	I-Device
,	O
the	O
Option	O
ROMs	O
,	O
and	O
I/O	B-Architecture
ports	I-Architecture
for	O
peripherals	O
,	O
much	O
of	O
this	O
384KB	O
of	O
address	B-General_Concept
space	I-General_Concept
was	O
unused	O
.	O
</s>
<s>
As	O
the	O
640KB	O
memory	B-General_Concept
restriction	O
became	O
ever	O
more	O
of	O
an	O
obstacle	O
,	O
techniques	O
were	O
found	O
to	O
fill	O
the	O
empty	O
areas	O
with	O
RAM	B-Architecture
.	O
</s>
<s>
These	O
areas	O
were	O
referred	O
to	O
as	O
upper	B-Device
memory	I-Device
blocks	I-Device
(	O
UMBs	O
)	O
.	O
</s>
<s>
The	O
next	O
stage	O
in	O
the	O
evolution	O
of	O
DOS	O
was	O
for	O
the	O
operating	O
system	O
to	O
use	O
upper	B-Device
memory	I-Device
blocks	I-Device
(	O
UMBs	O
)	O
and	O
the	O
high	B-Device
memory	I-Device
area	I-Device
(	O
HMA	O
)	O
.	O
</s>
<s>
DRDOS	O
 '	O
built-in	O
memory	B-General_Concept
manager	O
,	O
EMM386.EXE	B-Device
,	O
could	O
perform	O
most	O
of	O
the	O
basic	O
functionality	O
of	O
QEMM	B-Device
and	O
comparable	O
programs	O
.	O
</s>
<s>
The	O
advantage	O
of	O
DRDOS	O
5.0	O
over	O
the	O
combination	O
of	O
an	O
older	O
DOS	O
plus	O
QEMM	B-Device
was	O
that	O
the	O
DRDOS	O
kernel	O
itself	O
and	O
almost	O
all	O
of	O
its	O
data	O
structures	O
could	O
be	O
loaded	O
into	O
high	B-Device
memory	I-Device
.	O
</s>
<s>
This	O
left	O
virtually	O
all	O
the	O
base	B-Device
memory	I-Device
free	O
,	O
allowing	O
configurations	O
with	O
up	O
to	O
620KB	O
out	O
of	O
640KB	O
free	O
.	O
</s>
<s>
Configuration	O
was	O
not	O
automatic	O
-	O
free	O
UMBs	O
had	O
to	O
be	O
identified	O
by	O
hand	O
,	O
manually	O
included	O
in	O
the	O
line	O
that	O
loaded	O
EMM386	B-Device
from	O
CONFIG.SYS	B-Device
,	O
and	O
then	O
drivers	O
and	O
so	O
on	O
had	O
to	O
be	O
manually	O
loaded	O
into	O
UMBs	O
from	O
CONFIG.SYS	B-Device
and	O
AUTOEXEC.BAT	O
.	O
</s>
<s>
As	O
it	O
was	O
largely	O
automated	O
by	O
the	O
installation	O
program	O
of	O
QEMM	B-Device
,	O
this	O
program	O
survived	O
on	O
the	O
market	O
;	O
indeed	O
,	O
it	O
worked	O
well	O
with	O
DRDOS	O
 '	O
own	O
HMA	O
and	O
UMB	B-Device
support	O
and	O
went	O
on	O
to	O
be	O
one	O
of	O
the	O
best-selling	O
utilities	O
for	O
the	O
PC	O
.	O
</s>
<s>
Eventually	O
,	O
even	O
more	O
DOS	O
data	O
structures	O
were	O
moved	O
out	O
of	O
conventional	B-Device
memory	I-Device
,	O
allowing	O
up	O
to	O
631KB	O
out	O
of	O
640KB	O
to	O
be	O
left	O
free	O
.	O
</s>
<s>
Starting	O
from	O
version	O
6.0	O
of	O
MS-DOS	O
,	O
Microsoft	O
even	O
included	O
a	O
program	O
called	O
MEMMAKER	O
which	O
was	O
used	O
to	O
automatically	O
optimize	O
conventional	B-Device
memory	I-Device
by	O
moving	O
terminate-and-stay-resident	O
(	O
TSR	O
)	O
programs	O
to	O
the	O
upper	B-Device
memory	I-Device
.	O
</s>
<s>
For	O
a	O
period	O
in	O
the	O
early	O
1990s	O
,	O
manual	O
optimization	O
of	O
the	O
DOS	B-General_Concept
memory	I-General_Concept
map	O
became	O
a	O
highly	O
prized	O
skill	O
,	O
allowing	O
for	O
the	O
largest	O
applications	O
to	O
run	O
on	O
even	O
the	O
most	O
complex	O
PC	O
configurations	O
.	O
</s>
<s>
The	O
technique	O
was	O
to	O
first	O
create	O
as	O
many	O
UMBs	O
as	O
possible	O
,	O
including	O
remapping	O
allocated	O
but	O
unused	O
blocks	O
of	O
memory	B-General_Concept
,	O
such	O
as	O
the	O
monochrome	O
display	O
area	O
on	O
colour	O
machines	O
.	O
</s>
<s>
Then	O
,	O
DOS	O
 '	O
many	O
subcomponents	O
had	O
to	O
be	O
loaded	O
into	O
these	O
UMBs	O
in	O
the	O
correct	O
sequence	O
to	O
use	O
the	O
blocks	O
of	O
memory	B-General_Concept
as	O
efficiently	O
as	O
possible	O
.	O
</s>
<s>
Some	O
TSR	O
programs	O
required	O
additional	O
memory	B-General_Concept
while	O
loading	O
,	O
which	O
was	O
freed	O
up	O
again	O
once	O
loading	O
was	O
complete	O
.	O
</s>
<s>
Exceptions	O
were	O
that	O
to	O
successfully	O
cache	O
CD-ROMs	O
,	O
most	O
disk	O
caches	O
had	O
to	O
be	O
loaded	O
after	O
any	O
CD-ROM	O
drivers	O
,	O
and	O
that	O
the	O
modules	O
of	O
most	O
network	O
stacks	O
had	O
to	O
be	O
loaded	O
in	O
a	O
certain	O
sequence	O
,	O
essentially	O
working	O
progressively	O
up	O
through	O
the	O
layers	O
of	O
the	O
OSI	O
model	O
.	O
</s>
<s>
A	O
basic	O
yet	O
effective	O
method	O
used	O
to	O
optimize	O
conventional	B-Device
memory	I-Device
was	O
to	O
load	O
HIMEM.SYS	O
as	O
a	O
device	O
,	O
thereafter	O
loading	O
EMM386.EXE	B-Device
as	O
a	O
device	O
with	O
the	O
"	O
RAM	B-Architecture
AUTO	O
"	O
option	O
which	O
allows	O
access	O
into	O
the	O
UMA	B-Device
by	O
loading	O
device	O
drivers	O
as	O
devicehigh	O
.	O
</s>
<s>
This	O
method	O
effectively	O
loads	O
the	O
fundamental	O
memory	B-General_Concept
managers	O
into	O
conventional	B-Device
memory	I-Device
,	O
and	O
thereafter	O
everything	O
else	O
into	O
the	O
UMA	B-Device
.	O
</s>
<s>
Conventional	B-Device
memory	I-Device
glutton	O
programs	O
such	O
as	O
MSCDEX	O
could	O
also	O
be	O
loaded	O
into	O
the	O
UMA	B-Device
in	O
a	O
similar	O
fashion	O
,	O
hence	O
freeing	O
up	O
a	O
large	O
amount	O
of	O
conventional	B-Device
memory	I-Device
.	O
</s>
<s>
The	O
increasing	O
popularity	O
of	O
Windows	B-Application
3.0	I-Application
made	O
the	O
necessity	O
of	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
less	O
relevant	O
,	O
as	O
Windows	O
applications	O
were	O
not	O
directly	O
affected	O
by	O
DOS	O
 '	O
base	B-Device
memory	I-Device
limits	O
,	O
but	O
DOS	O
programs	O
running	O
under	O
Windows	O
(	O
with	O
Windows	O
itself	O
acting	O
as	O
a	O
multitasking	O
manager	O
)	O
were	O
still	O
thus	O
constrained	O
.	O
</s>
<s>
With	O
the	O
release	O
of	O
Windows	O
95	O
,	O
it	O
became	O
less	O
relevant	O
still	O
,	O
as	O
this	O
version	O
of	O
Windows	O
provides	O
much	O
of	O
the	O
functionality	O
of	O
the	O
DOS	O
device	O
drivers	O
to	O
DOS	O
applications	O
running	O
under	O
Windows	O
,	O
such	O
as	O
CD	O
,	O
network	O
and	O
sound	O
support	O
;	O
the	O
memory	B-General_Concept
map	O
of	O
Windows95	O
DOS	O
boxes	O
was	O
automatically	O
optimised	O
.	O
</s>
<s>
Specifically	O
,	O
programs	O
that	O
tried	O
to	O
directly	O
switch	O
from	O
real	B-Application
mode	I-Application
to	O
protected	O
mode	O
would	O
not	O
work	O
as	O
this	O
was	O
not	O
allowed	O
in	O
the	O
virtual	B-Application
8086	I-Application
mode	I-Application
it	O
was	O
running	O
in	O
.	O
</s>
<s>
Also	O
,	O
programs	O
that	O
tried	O
making	O
the	O
switch	O
using	O
the	O
Virtual	O
Control	O
Program	O
Interface	O
(	O
VCPI	O
)	O
API	B-Application
(	O
which	O
was	O
introduced	O
to	O
allow	O
DOS	O
programs	O
that	O
needed	O
protected	O
mode	O
to	O
enter	O
it	O
from	O
the	O
virtual	B-Application
8086	I-Application
mode	I-Application
set	O
up	O
by	O
a	O
memory	B-General_Concept
manager	O
,	O
as	O
described	O
above	O
)	O
did	O
n't	O
work	O
in	O
Windows95	O
.	O
</s>
<s>
Only	O
the	O
DOS	O
Protected	O
Mode	O
Interface	O
(	O
DPMI	O
)	O
API	B-Application
for	O
switching	O
to	O
protected	O
mode	O
was	O
supported	O
.	O
</s>
<s>
Upper	B-Device
memory	I-Device
blocks	I-Device
can	O
be	O
created	O
by	O
mapping	O
extended	B-Device
memory	I-Device
into	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
when	O
running	O
in	O
virtual	B-Application
8086	I-Application
mode	I-Application
.	O
</s>
<s>
This	O
is	O
similar	O
to	O
how	O
expanded	B-Device
memory	I-Device
can	O
be	O
emulated	O
using	O
extended	B-Device
memory	I-Device
so	O
this	O
method	O
of	O
providing	O
upper	B-Device
memory	I-Device
blocks	I-Device
is	O
usually	O
provided	O
by	O
the	O
expanded	B-Device
memory	I-Device
manager	O
(	O
for	O
example	O
EMM386	B-Device
)	O
.	O
</s>
<s>
The	O
application	B-Application
programming	I-Application
interface	I-Application
for	O
managing	O
the	O
upper	B-Device
memory	I-Device
blocks	I-Device
is	O
specified	O
in	O
the	O
eXtended	B-Device
Memory	I-Device
Specification	O
.	O
</s>
<s>
On	O
many	O
systems	O
including	O
modern	O
ones	O
it	O
is	O
possible	O
to	O
use	O
memory	B-General_Concept
reserved	O
for	O
shadowing	O
expansion	O
card	O
ROM	B-Device
as	O
upper	B-Device
memory	I-Device
.	O
</s>
<s>
Many	O
chipsets	O
reserve	O
up	O
to	O
384KB	O
RAM	B-Architecture
for	O
this	O
purpose	O
and	O
since	O
this	O
RAM	B-Architecture
is	O
generally	O
unused	O
it	O
may	O
be	O
used	O
as	O
real	B-Application
mode	I-Application
upper	B-Device
memory	I-Device
with	O
a	O
custom	O
device	O
driver	O
,	O
such	O
as	O
UMBPCI	O
.	O
</s>
<s>
On	O
IBM	B-Device
XT	I-Device
computers	O
,	O
it	O
was	O
possible	O
to	O
add	O
more	O
memory	B-General_Concept
to	O
the	O
motherboard	O
and	O
use	O
a	O
custom	O
address	B-Device
decoder	I-Device
PROM	B-General_Concept
to	O
make	O
it	O
appear	O
in	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
.	O
</s>
<s>
As	O
with	O
the	O
386-based	O
upper	B-Device
memory	I-Device
described	O
above	O
,	O
the	O
extra	O
RAM	B-Architecture
could	O
be	O
used	O
to	O
load	O
TSR	O
files	O
,	O
or	O
as	O
a	O
RAM	B-Application
disk	I-Application
.	O
</s>
<s>
The	O
AllCard	O
,	O
an	O
add-on	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
for	O
XT-class	O
computers	O
,	O
allowed	O
normal	O
memory	B-General_Concept
to	O
be	O
mapped	O
into	O
the	O
0xA0000-EFFFF	O
address	B-General_Concept
range	I-General_Concept
,	O
giving	O
up	O
to	O
952KB	O
for	O
DOS	O
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	B-General_Concept
directly	O
,	O
needed	O
to	O
be	O
patched	O
to	O
handle	O
this	O
memory	B-General_Concept
layout	O
.	O
</s>
<s>
This	O
usage	O
of	O
the	O
upper	B-Device
memory	I-Device
area	I-Device
is	O
different	O
from	O
using	O
upper	B-Device
memory	I-Device
blocks	I-Device
,	O
which	O
was	O
used	O
to	O
free	O
conventional	B-Device
memory	I-Device
by	O
moving	O
device	O
drivers	O
and	O
TSRs	O
into	O
the	O
upper	O
384KB	O
of	O
the	O
1MB	O
address	B-General_Concept
space	I-General_Concept
,	O
but	O
left	O
the	O
amount	O
of	O
addressable	O
memory	B-General_Concept
(	O
640KB	O
)	O
intact	O
.	O
</s>
