<s>
Data	B-Operating_System
diffusion	I-Operating_System
machine	I-Operating_System
is	O
a	O
historical	O
virtual	B-Operating_System
shared	I-Operating_System
memory	I-Operating_System
architecture	O
where	O
data	O
is	O
free	O
to	O
migrate	O
through	O
the	O
machine	O
.	O
</s>
<s>
Shared	B-Operating_System
memory	I-Operating_System
machines	O
are	O
convenient	O
for	O
programming	O
but	O
do	O
not	O
scale	O
beyond	O
tens	O
of	O
processors	O
.	O
</s>
<s>
The	O
Data	B-Operating_System
Diffusion	I-Operating_System
Machine	I-Operating_System
(	O
DDM	O
)	O
overcomes	O
this	O
problem	O
by	O
providing	O
a	O
virtual	B-Architecture
memory	I-Architecture
abstraction	O
on	O
top	O
of	O
a	O
distributed	B-Operating_System
memory	I-Operating_System
machine	O
.	O
</s>
<s>
A	O
DDM	O
appears	O
to	O
the	O
user	O
as	O
a	O
conventional	O
shared	B-Operating_System
memory	I-Operating_System
machine	O
but	O
is	O
implemented	O
using	O
a	O
distributed	B-Operating_System
memory	I-Operating_System
architecture	O
.	O
</s>
<s>
Data	B-Operating_System
diffusion	I-Operating_System
machines	I-Operating_System
were	O
under	O
active	O
research	O
in	O
the	O
late	O
1980s	O
and	O
early	O
1990s	O
,	O
but	O
the	O
research	O
has	O
ceased	O
since	O
then	O
.	O
</s>
