<s>
Flat	B-General_Concept
memory	I-General_Concept
model	I-General_Concept
or	O
linear	O
memory	B-General_Concept
model	O
refers	O
to	O
a	O
memory	B-General_Concept
addressing	I-General_Concept
paradigm	O
in	O
which	O
"	O
memory	B-General_Concept
appears	O
to	O
the	O
program	O
as	O
a	O
single	O
contiguous	O
address	O
space.	O
"	O
</s>
<s>
The	O
CPU	B-Device
can	O
directly	O
(	O
and	O
linearly	O
)	O
address	B-General_Concept
all	O
of	O
the	O
available	O
memory	B-General_Concept
locations	I-General_Concept
without	O
having	O
to	O
resort	O
to	O
any	O
sort	O
of	O
memory	B-General_Concept
segmentation	I-General_Concept
or	O
paging	B-Architecture
schemes	O
.	O
</s>
<s>
Memory	B-General_Concept
management	O
and	O
address	B-General_Concept
translation	O
can	O
still	O
be	O
implemented	O
on	O
top	O
of	O
a	O
flat	B-General_Concept
memory	I-General_Concept
model	I-General_Concept
in	O
order	O
to	O
facilitate	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
functionality	O
,	O
resource	O
protection	O
,	O
multitasking	B-Operating_System
or	O
to	O
increase	O
the	O
memory	B-General_Concept
capacity	O
beyond	O
the	O
limits	O
imposed	O
by	O
the	O
processor	O
's	O
physical	O
address	B-General_Concept
space	I-General_Concept
,	O
but	O
the	O
key	O
feature	O
of	O
a	O
flat	B-General_Concept
memory	I-General_Concept
model	I-General_Concept
is	O
that	O
the	O
entire	O
memory	B-General_Concept
space	O
is	O
linear	O
,	O
sequential	O
and	O
contiguous	O
.	O
</s>
<s>
In	O
a	O
simple	O
controller	O
,	O
or	O
in	O
a	O
single	O
tasking	O
embedded	O
application	O
,	O
where	O
memory	B-General_Concept
management	O
is	O
not	O
needed	O
nor	O
desirable	O
,	O
the	O
flat	B-General_Concept
memory	I-General_Concept
model	I-General_Concept
is	O
the	O
most	O
appropriate	O
,	O
because	O
it	O
provides	O
the	O
simplest	O
interface	O
from	O
the	O
programmer	O
's	O
point	O
of	O
view	O
,	O
with	O
direct	O
access	O
to	O
all	O
memory	B-General_Concept
locations	I-General_Concept
and	O
minimum	O
design	O
complexity	O
.	O
</s>
<s>
In	O
a	O
general	O
purpose	O
computer	O
system	O
,	O
which	O
requires	O
multitasking	B-Operating_System
,	O
resource	O
allocation	O
,	O
and	O
protection	O
,	O
the	O
flat	O
memory	B-General_Concept
system	O
must	O
be	O
augmented	O
by	O
some	O
memory	B-General_Concept
management	O
scheme	O
,	O
which	O
is	O
typically	O
implemented	O
through	O
a	O
combination	O
of	O
dedicated	O
hardware	O
(	O
inside	O
or	O
outside	O
the	O
CPU	B-Device
)	O
and	O
software	O
built	O
into	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
flat	B-General_Concept
memory	I-General_Concept
model	I-General_Concept
(	O
at	O
the	O
physical	O
addressing	O
level	O
)	O
still	O
provides	O
the	O
greatest	O
flexibility	O
for	O
implementing	O
this	O
type	O
of	O
memory	B-General_Concept
management	O
.	O
</s>
<s>
The	O
majority	O
of	O
processor	O
architectures	O
implement	O
a	O
flat	O
memory	B-General_Concept
design	O
,	O
including	O
all	O
early	O
8-bit	O
processors	O
,	O
the	O
Motorola	B-Device
68000	I-Device
series	I-Device
,	O
etc	O
.	O
</s>
<s>
One	O
exception	O
was	O
the	O
original	O
8086	B-General_Concept
,	O
Intel	O
's	O
first	O
16-bit	O
microprocessor	O
,	O
which	O
implemented	O
a	O
crude	O
segmented	B-General_Concept
memory	I-General_Concept
model	O
which	O
allowed	O
access	O
to	O
more	O
than	O
64	O
KiB	O
of	O
memory	B-General_Concept
without	O
the	O
cost	O
of	O
extending	O
all	O
addresses	O
to	O
more	O
than	O
16-bits	O
.	O
</s>
<s>
Most	O
modern	O
memory	B-General_Concept
models	O
fall	O
into	O
one	O
of	O
three	O
categories	O
:	O
</s>
<s>
Maximum	O
execution	O
speed	O
,	O
Simply	O
there	O
is	O
n't	O
any	O
latency	O
due	O
to	O
CPU	B-Device
or	O
logic	O
.	O
</s>
<s>
Not	O
suitable	O
for	O
general	O
computing	O
or	O
multitasking	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
unless	O
enhanced	O
with	O
additional	O
memory	B-General_Concept
management	O
hardware/software	O
;	O
but	O
this	O
is	O
almost	O
always	O
the	O
case	O
in	O
modern	O
CISC	B-Architecture
processors	I-Architecture
,	O
which	O
implement	O
advanced	O
memory	B-General_Concept
management	O
and	O
protection	O
technology	O
over	O
a	O
flat	B-General_Concept
memory	I-General_Concept
model	I-General_Concept
.	O
</s>
<s>
Linux	B-Application
e.g.	O
</s>
<s>
uses	O
a	O
flat	B-General_Concept
memory	I-General_Concept
model	I-General_Concept
,	O
see	O
x86	B-Device
memory	I-Device
segmentation	I-Device
#Practices	O
.	O
</s>
<s>
This	O
memory	B-General_Concept
model	O
is	O
required	O
when	O
using	O
Physical	B-General_Concept
Address	I-General_Concept
Extension	I-General_Concept
(	O
PAE	O
)	O
in	O
Pentium	B-Device
Pro	I-Device
and	O
later	O
x86	B-Operating_System
CPUs	B-Device
to	O
support	O
36-bit	O
physical	O
addresses	O
to	O
address	B-General_Concept
more	O
than	O
4GB	O
of	O
physical	O
memory	B-General_Concept
.	O
</s>
<s>
Implemented	O
in	O
the	O
original	O
Intel	B-General_Concept
8086	I-General_Concept
,	O
8088	B-Device
,	O
80186	B-Device
,	O
80286	B-General_Concept
,	O
and	O
supported	O
by	O
80386	B-General_Concept
and	O
all	O
subsequent	O
x86	B-Operating_System
machines	O
through	O
to	O
present	O
day	O
Pentium	B-General_Concept
and	O
Core	B-Device
2	I-Device
processors	O
.	O
</s>
<s>
This	O
memory	B-General_Concept
model	O
has	O
remained	O
ever	O
since	O
in	O
the	O
x86	B-Operating_System
machines	O
,	O
which	O
now	O
provide	O
multi-mode	O
operation	O
and	O
rarely	O
operate	O
in	O
the	O
compatible	O
segmented	O
mode	O
.	O
</s>
<s>
See	O
x86	B-Device
memory	I-Device
segmentation	I-Device
for	O
details	O
.	O
</s>
<s>
Within	O
the	O
x86	B-Operating_System
architectures	I-Operating_System
,	O
when	O
operating	O
in	O
the	O
real	B-Application
mode	I-Application
(	O
or	O
emulation	O
)	O
,	O
physical	O
address	B-General_Concept
is	O
computed	O
as	O
:	O
</s>
<s>
(	O
I.e.	O
,	O
the	O
16-bit	O
segment	B-General_Concept
register	I-General_Concept
is	O
shifted	O
left	O
by	O
4	O
bits	O
and	O
added	O
to	O
a	O
16-bit	O
offset	O
,	O
resulting	O
in	O
a	O
20-bit	O
address	B-General_Concept
.	O
)	O
</s>
