<s>
The	O
Commodore	O
RAM	B-General_Concept
Expansion	I-General_Concept
Unit	I-General_Concept
(	O
REU	O
)	O
is	O
a	O
range	O
of	O
external	O
RAM	B-Architecture
add-ons	O
.	O
</s>
<s>
At	O
the	O
time	O
of	O
introduction	O
of	O
the	O
Commodore	B-Device
128	I-Device
home	O
computer	O
,	O
two	O
REUs	O
were	O
announced	O
for	O
that	O
model	O
;	O
the	O
1700	O
(	O
128KB	O
)	O
and	O
1750	O
(	O
)	O
REUs	O
.	O
</s>
<s>
By	O
the	O
time	O
of	O
the	O
1985	O
CES	O
show	O
in	O
Chicago	O
,	O
the	O
engineers	O
were	O
able	O
to	O
display	O
a	O
spinning	O
globe	O
of	O
the	O
earth	O
as	O
a	O
demonstration	O
of	O
Direct	B-General_Concept
Memory	I-General_Concept
Access	I-General_Concept
(	O
DMA	B-General_Concept
)	O
by	O
the	O
new	O
REU	O
units	O
.	O
</s>
<s>
The	O
REU	O
hardware	O
was	O
designed	O
by	O
Frank	O
Palia	O
and	O
the	O
dedicated	O
RAM	B-Architecture
Expansion	O
Controller	O
(	O
REC	O
)	O
integrated	O
circuit	O
(	O
IC	O
)	O
was	O
designed	O
by	O
Victor	O
Andrade	O
.	O
</s>
<s>
Fred	O
Bowen	O
and	O
Terry	O
Ryan	O
adapted	O
the	O
C128	O
's	O
KERNAL	B-Operating_System
and	O
BASIC	O
to	O
accommodate	O
the	O
REU	O
natively	O
and	O
Hedley	O
Davis	O
wrote	O
the	O
globe	O
spinning	O
demo	O
which	O
was	O
an	O
impressive	O
display	O
of	O
animation	O
in	O
the	O
mid	O
1980s	O
.	O
</s>
<s>
Although	O
the	O
C128	O
could	O
access	O
more	O
than	O
of	O
RAM	B-Architecture
through	O
bank	B-General_Concept
switching	I-General_Concept
,	O
the	O
memory	O
inside	O
the	O
REU	O
could	O
only	O
be	O
accessed	O
by	O
memory-transfers	O
(	O
STORE/LOAD/SWAP/COMPAREs	O
)	O
between	O
the	O
main	O
memory	O
and	O
the	O
REU	O
memory	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
C128	O
's	O
built-in	O
BASIC	B-Language
7.0	I-Language
had	O
three	O
statements	O
,	O
STASH	O
,	O
FETCH	O
,	O
and	O
SWAP	O
,	O
for	O
storing	O
and	O
retrieving	O
data	O
from	O
the	O
REU	O
.	O
</s>
<s>
The	O
REU	O
model	O
1750	O
can	O
be	O
modified	O
to	O
support	O
up	O
to	O
2MB	O
on	O
memory	O
by	O
installing	O
2	O
additional	O
ICs	O
for	O
bank	B-General_Concept
switch	I-General_Concept
.	O
</s>
<s>
It	O
was	O
possible	O
to	O
check	O
for	O
the	O
presence	O
of	O
a	O
1750	O
by	O
reading	O
memory	B-General_Concept
address	I-General_Concept
DF00	O
's	O
bit	O
4	O
,	O
which	O
was	O
1	O
on	O
a	O
1750	O
,	O
and	O
0	O
on	O
a	O
1700	O
or	O
1764	O
.	O
</s>
<s>
To	O
solve	O
this	O
,	O
programmers	O
wrote	O
to	O
,	O
then	O
read	O
from	O
,	O
the	O
REU	O
's	O
RAM	B-Architecture
to	O
find	O
out	O
the	O
amount	O
of	O
memory	O
installed	O
.	O
</s>
<s>
Due	O
to	O
its	O
high	O
speed	O
relative	O
to	O
Commodore	O
's	O
floppy	O
drives	O
or	O
even	O
the	O
commercially	O
available	O
hard	O
drives	O
,	O
REUs	O
did	O
see	O
support	O
in	O
the	O
Commodore	O
BBS	B-Application
community	O
.	O
</s>
<s>
Programs	O
such	O
as	O
Color64	B-Protocol
required	O
a	O
REU	O
to	O
minimize	O
load	O
times	O
when	O
switching	O
between	O
sections	O
of	O
the	O
BBS	B-Application
.	O
</s>
<s>
The	O
REUs	O
came	O
with	O
software	O
to	O
use	O
the	O
extra	O
memory	O
as	O
a	O
RAM	B-Application
disk	I-Application
,	O
but	O
the	O
RAM	B-Application
disk	I-Application
's	O
compatibility	O
with	O
commercial	O
software	O
varied	O
as	O
some	O
commercial	O
software	O
relied	O
heavily	O
on	O
various	O
quirks	O
of	O
the	O
Commodore	B-Device
1541	I-Device
floppy	O
drive	O
.	O
</s>
<s>
Additionally	O
,	O
many	O
commercial	O
programs	O
simply	O
overwrote	O
the	O
memory	O
space	O
occupied	O
by	O
the	O
RAM	B-Application
disk	I-Application
software	O
.	O
</s>
<s>
The	O
GEOS	B-Device
operating	O
system	O
had	O
built	O
in	O
support	O
for	O
the	O
REU	O
as	O
a	O
RAM	B-Application
disk	I-Application
,	O
as	O
did	O
the	O
C128	O
's	O
version	O
of	O
CP/M	B-Application
,	O
and	O
some	O
disk	O
copy	O
programs	O
used	O
the	O
REU	O
to	O
facilitate	O
high-speed	O
copying	O
with	O
a	O
single	O
disk	O
drive	O
.	O
</s>
<s>
GEOS	B-Device
as	O
well	O
as	O
other	O
programs	O
even	O
used	O
the	O
REU	O
for	O
quick	O
memory	O
transfers	O
within	O
the	O
host	O
machine	O
's	O
main	O
memory	O
by	O
storing	O
a	O
memory	O
block	O
into	O
the	O
REU	O
and	O
then	O
fetching	O
it	O
back	O
to	O
another	O
location	O
.	O
</s>
<s>
Using	O
this	O
method	O
,	O
only	O
the	O
actual	O
data	O
to	O
be	O
transferred	O
needed	O
to	O
travel	O
on	O
the	O
machine	O
's	O
data	O
busunlike	O
the	O
ordinary	O
method	O
,	O
which	O
had	O
the	O
computer	O
's	O
CPU	B-General_Concept
do	O
the	O
transfer	O
,	O
thus	O
spending	O
at	O
least	O
three	O
quarters	O
of	O
the	O
bus	O
capacity	O
on	O
instruction	O
fetches	O
and	O
only	O
one	O
quarter	O
or	O
less	O
on	O
payload	O
data	O
.	O
</s>
