<s>
The	O
MC68451	O
is	O
a	O
Motorola	O
(	O
now	O
Freescale	O
)	O
Memory	B-General_Concept
Management	I-General_Concept
Unit	I-General_Concept
(	O
MMU	B-General_Concept
)	O
,	O
which	O
was	O
primarily	O
used	O
in	O
conjunction	O
with	O
the	O
Motorola	O
MC68010	B-Device
microprocessor	B-Architecture
.	O
</s>
<s>
The	O
MC68451	O
supported	O
a	O
16	O
MB	O
address	O
space	O
and	O
provided	O
a	O
MC68000	B-Device
or	O
a	O
MC68010	B-Device
with	O
support	O
for	O
memory	O
management	O
and	O
protection	O
of	O
memory	O
against	O
unauthorized	O
access	O
.	O
</s>
<s>
In	O
combination	O
with	O
a	O
MC68010	B-Device
,	O
the	O
MC68451	O
permitted	O
the	O
realization	O
of	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
With	O
the	O
earlier	O
MC68000	B-Device
,	O
this	O
was	O
not	O
possible	O
due	O
to	O
the	O
way	O
the	O
MC68000	B-Device
treated	O
memory	O
access	O
errors	O
,	O
i.e.	O
</s>
<s>
processor	O
state	O
could	O
not	O
always	O
be	O
properly	O
restored	O
after	O
a	O
page	O
fault	O
;	O
two	O
MC68000s	B-Device
would	O
be	O
required	O
,	O
with	O
the	O
main	O
CPU	O
pausing	O
when	O
it	O
got	O
a	O
memory	O
access	O
error	O
,	O
and	O
the	O
other	O
CPU	O
servicing	O
the	O
page	O
fault	O
.	O
</s>
<s>
The	O
limitation	O
to	O
32	O
segment	O
table	O
entries	O
per	O
MMU	B-General_Concept
made	O
systems	O
based	O
on	O
a	O
MC68010	B-Device
and	O
a	O
MC68451	O
slow	O
,	O
as	O
they	O
often	O
had	O
to	O
modify	O
the	O
segment	O
table	O
due	O
to	O
its	O
small	O
size	O
.	O
</s>
<s>
H	O
.	O
Berthold	O
AG	O
used	O
12	O
MC68451	O
MMUs	O
together	O
with	O
their	O
UNOS	B-Architecture
variant	O
vBertOS	O
.	O
</s>
