<s>
Shadow	B-General_Concept
RAM	I-General_Concept
,	O
on	O
the	O
Acorn	B-Device
BBC	I-Device
Micro	I-Device
,	O
Master-series	B-Device
and	O
Acorn	B-Device
Electron	I-Device
microcomputers	O
is	O
the	O
name	O
given	O
to	O
a	O
special	O
framebuffer	B-Algorithm
implementation	O
to	O
free	O
up	O
main	O
memory	O
for	O
use	O
by	O
program	O
code	O
and	O
data	O
.	O
</s>
<s>
Some	O
implementations	O
of	O
shadow	B-General_Concept
RAM	I-General_Concept
also	O
permit	O
double-buffered	O
graphics	O
.	O
</s>
<s>
The	O
BBC	B-Device
Micro	I-Device
,	O
Master-series	B-Device
and	O
Electron	O
machines	O
use	O
the	O
8-bit	O
6502	B-General_Concept
and	O
65C102	O
processors	B-Device
with	O
a	O
16-bit	O
address	O
space	O
.	O
</s>
<s>
This	O
address	O
space	O
is	O
split	O
into	O
32	O
KB	O
RAM	B-Architecture
(	O
0x0000	O
to	O
0x7FFF	O
)	O
,	O
16	O
KB	O
sideways	B-General_Concept
"	O
paged	B-General_Concept
"	O
address	O
space	O
(	O
0x8000	O
to	O
0xBFFF	O
)	O
and	O
16	O
KB	O
operating	B-General_Concept
system	I-General_Concept
space	O
(	O
0xC000	O
to	O
0xFFFF	O
)	O
.	O
</s>
<s>
Thus	O
,	O
screen	O
memory	O
can	O
therefore	O
occupy	O
a	O
considerable	O
amount	O
of	O
the	O
available	O
directly-addressed	O
32	O
KB	O
RAM	B-Architecture
.	O
</s>
<s>
Shadow	B-General_Concept
RAM	I-General_Concept
is	O
a	O
block	O
of	O
RAM	B-Architecture
that	O
can	O
be	O
considered	O
to	O
reside	O
in	O
parallel	O
to	O
the	O
normal	O
memory	O
map	O
and	O
is	O
accessed	O
by	O
the	O
system	O
only	O
under	O
certain	O
conditions	O
.	O
</s>
<s>
When	O
shadow	B-General_Concept
RAM	I-General_Concept
is	O
enabled	O
,	O
the	O
memory	O
region	O
normally	O
used	O
for	O
screen	O
memory	O
becomes	O
available	O
for	O
BASIC	B-Language
program	O
use	O
and	O
for	O
applications	O
employing	O
officially	O
documented	O
operating	B-General_Concept
system	I-General_Concept
interfaces	O
.	O
</s>
<s>
Given	O
the	O
maximum	O
requirement	O
of	O
20	O
KB	O
for	O
screen	O
memory	O
with	O
the	O
systems	O
concerned	O
,	O
the	O
amount	O
of	O
shadow	B-General_Concept
RAM	I-General_Concept
provided	O
is	O
typically	O
20	O
KB	O
.	O
</s>
<s>
Shadow	B-General_Concept
RAM	I-General_Concept
was	O
fitted	O
as	O
standard	O
on	O
the	O
BBC	B-Device
Micro	I-Device
Model	O
B+	O
and	O
on	O
the	O
BBC	B-Device
Master	I-Device
series	I-Device
,	O
but	O
was	O
an	O
optional	O
feature	O
provided	O
by	O
third-party	O
expansions	O
on	O
earlier	O
BBC	B-Device
Micro	I-Device
systems	O
and	O
the	O
Acorn	B-Device
Electron	I-Device
.	O
</s>
<s>
The	O
Aries-B20	O
product	O
,	O
initially	O
sold	O
by	O
Cambridge	O
Computer	O
Consultants	O
,	O
offered	O
20	O
KB	O
shadow	B-General_Concept
RAM	I-General_Concept
for	O
the	O
BBC	B-Device
Model	I-Device
B	I-Device
,	O
transparently	O
diverting	O
non-framebuffer	O
accesses	O
to	O
the	O
shadow	B-General_Concept
RAM	I-General_Concept
for	O
addresses	O
in	O
the	O
20	O
KB	O
video	O
memory	O
region	O
.	O
</s>
<s>
In	O
systems	O
based	O
on	O
the	O
BBC	O
Model	O
B+	O
,	O
like	O
the	O
Acorn	B-Device
Cambridge	I-Device
Workstation	I-Device
,	O
a	O
programmable	O
array	O
logic	O
(	O
PAL	O
)	O
chip	O
controls	O
access	O
to	O
memory	O
and	O
exposes	O
the	O
screen	O
memory	O
to	O
the	O
CPU	B-Device
by	O
redirecting	O
memory	O
accesses	O
to	O
the	O
appropriate	O
memory	O
locations	O
.	O
</s>
<s>
When	O
shadow	O
mode	O
is	O
enabled	O
,	O
the	O
PAL	O
chip	O
monitors	O
the	O
addresses	O
of	O
instructions	O
fetched	O
by	O
the	O
CPU	B-Device
,	O
and	O
where	O
such	O
instructions	O
have	O
been	O
fetched	O
from	O
a	O
range	O
of	O
memory	O
from	O
0xA000	O
to	O
0xAFFF	O
or	O
from	O
0xC000	O
to	O
0xDFFF	O
,	O
they	O
are	O
considered	O
to	O
be	O
part	O
of	O
the	O
VDU	O
drivers	O
that	O
may	O
access	O
the	O
screen	O
memory	O
.	O
</s>
<s>
All	O
other	O
RAM	B-Architecture
accesses	O
while	O
in	O
shadow	O
mode	O
are	O
directed	O
towards	O
a	O
contiguous	O
region	O
of	O
RAM	B-Architecture
from	O
0x0000	O
to	O
0x7FFF	O
(	O
and	O
potentially	O
up	O
to	O
0xAFFF	O
,	O
if	O
exposed	O
)	O
that	O
does	O
not	O
host	O
the	O
screen	O
memory	O
.	O
</s>
<s>
Meanwhile	O
,	O
in	O
non-shadow	O
or	O
"	O
normal	O
"	O
mode	O
,	O
this	O
same	O
contiguous	O
region	O
does	O
host	O
the	O
screen	O
memory	O
,	O
as	O
is	O
the	O
convention	O
in	O
the	O
traditional	O
BBC	B-Device
Micro	I-Device
architecture	O
,	O
and	O
the	O
shadow	O
memory	O
is	O
not	O
accessible	O
.	O
</s>
<s>
Unlike	O
the	O
expansion	O
boards	O
for	O
earlier	O
systems	O
,	O
the	O
BBC	B-Device
Master	I-Device
implementation	O
of	O
shadow	B-General_Concept
RAM	I-General_Concept
permits	O
the	O
selection	O
of	O
either	O
shadow	O
memory	O
or	O
main	O
memory	O
for	O
use	O
as	O
screen	O
memory	O
,	O
this	O
being	O
done	O
via	O
the	O
Access	O
Control	O
Register	O
.	O
</s>
<s>
Acorn	O
provided	O
a	O
demonstration	O
program	O
in	O
BASIC	B-Language
showing	O
scrolling	O
cloud	O
animation	O
with	O
and	O
without	O
double	O
buffering	O
.	O
</s>
<s>
The	O
video	O
game	O
Firetrack	B-Application
would	O
also	O
use	O
double	O
buffering	O
if	O
shadow	B-General_Concept
RAM	I-General_Concept
was	O
present	O
.	O
</s>
<s>
On	O
the	O
BBC	B-Device
Master	I-Device
(	O
and	O
also	O
the	O
BBC	O
Model	O
B+	O
)	O
,	O
shadow	B-General_Concept
RAM	I-General_Concept
is	O
activated	O
by	O
setting	O
the	O
most	O
significant	O
bit	O
of	O
the	O
memory	O
mode	O
number	O
.	O
</s>
<s>
For	O
example	O
,	O
to	O
use	O
mode	O
1	O
with	O
shadow	B-General_Concept
RAM	I-General_Concept
enabled	O
,	O
mode	O
129	O
(	O
128	O
combined	O
with	O
1	O
)	O
is	O
selected	O
.	O
</s>
<s>
Another	O
significant	O
difference	O
between	O
the	O
Master	O
implementation	O
of	O
shadow	B-General_Concept
RAM	I-General_Concept
and	O
previous	O
implementations	O
also	O
offering	O
32KB	O
of	O
shadow	B-General_Concept
RAM	I-General_Concept
is	O
the	O
allocation	O
of	O
the	O
extra	O
12KB	O
beyond	O
the	O
20KB	O
shadowing	O
the	O
screen	O
memory	O
.	O
</s>
<s>
Instead	O
of	O
this	O
memory	O
occupying	O
a	O
single	O
region	O
from	O
0x8000	O
to	O
0xAFFF	O
in	O
sideways	B-General_Concept
RAM	I-General_Concept
space	O
,	O
as	O
it	O
does	O
with	O
the	O
Aries-B32	O
product	O
and	O
the	O
BBC	O
Model	O
B+	O
,	O
it	O
instead	O
occupies	O
two	O
regions	O
in	O
the	O
Master	O
as	O
"	O
private	O
RAM	B-Architecture
"	O
:	O
a	O
4KB	O
region	O
from	O
0x8000	O
to	O
0x8FFF	O
holding	O
function	O
key	O
definitions	O
,	O
workspace	O
for	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
and	O
character	O
and	O
font	O
definitions	O
;	O
an	O
8KB	O
region	O
from	O
0xC000	O
to	O
0xDFFF	O
holding	O
paged	B-General_Concept
(	O
sideways	B-General_Concept
)	O
ROM	O
and	O
operating	B-General_Concept
system	I-General_Concept
workspace	O
.	O
</s>
<s>
Thus	O
,	O
the	O
Master	O
was	O
able	O
to	O
support	O
character	O
set	O
redefinition	O
and	O
to	O
allocate	O
memory	O
to	O
filing	O
systems	O
without	O
the	O
amount	O
of	O
available	O
user	O
RAM	B-Architecture
being	O
reduced	O
.	O
</s>
<s>
Subsequent	O
products	O
augmented	O
the	O
shadow	B-General_Concept
RAM	I-General_Concept
with	O
additional	O
RAM	B-Architecture
that	O
could	O
be	O
used	O
for	O
other	O
purposes	O
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
Aries-B32	O
product	O
permitted	O
shadow/sideways	O
RAM	B-Architecture
combinations	O
of	O
20	O
KB/12	O
KB	O
and	O
16KB/16KB	O
,	O
or	O
the	O
use	O
of	O
the	O
32	O
KB	O
RAM	B-Architecture
as	O
two	O
sideways	B-General_Concept
RAM	I-General_Concept
banks	O
.	O
</s>
<s>
The	O
Slogger	O
Master	O
RAM	B-Architecture
Board	O
offered	O
a	O
32K	O
RAM	B-Architecture
solution	O
for	O
the	O
Acorn	B-Device
Electron	I-Device
alongside	O
a	O
"	O
turbo	O
mode	O
"	O
enhancement	O
.	O
</s>
<s>
A	O
dispute	O
arose	O
between	O
the	O
designers	O
of	O
the	O
Aries-B20	O
shadow	B-General_Concept
RAM	I-General_Concept
board	O
(	O
Aries	O
Computers	O
Limited	O
)	O
and	O
two	O
other	O
companies	O
offering	O
similar	O
products	O
,	O
Raven	O
Micro	O
Products	O
and	O
Watford	O
Electronics	O
,	O
over	O
the	O
alleged	O
infringement	O
of	O
patent	O
GB2137382A	O
describing	O
techniques	O
employed	O
in	O
the	O
design	O
of	O
the	O
Aries-B20	O
board	O
.	O
</s>
<s>
The	O
products	O
involved	O
were	O
the	O
Raven	O
Micro	O
Products	O
(	O
Raven-20	O
)	O
and	O
Watford	O
Electronics	O
(	O
32K	O
RAM	B-Architecture
Expansion	O
Board	O
)	O
.	O
</s>
