<s>
The	O
sideways	B-General_Concept
address	I-General_Concept
space	I-General_Concept
on	O
the	O
Acorn	B-Device
BBC	I-Device
Microcomputer	O
,	O
Electron	B-Device
and	O
Master-series	B-Device
microcomputer	I-Device
was	O
Acorn	O
's	O
bank	B-General_Concept
switching	I-General_Concept
implementation	O
,	O
providing	O
for	O
permanent	O
system	O
expansion	O
in	O
the	O
days	O
before	O
hard	B-Device
disk	I-Device
drives	I-Device
or	O
even	O
floppy	B-Device
disk	I-Device
drives	I-Device
were	O
commonplace	O
.	O
</s>
<s>
Filing	B-Application
systems	I-Application
,	O
application	O
and	O
utility	O
software	O
,	O
and	O
drivers	O
were	O
made	O
available	O
as	O
sideways	O
ROMs	O
,	O
and	O
extra	O
RAM	B-Architecture
could	O
be	O
fitted	O
via	O
the	O
sideways	B-General_Concept
address	I-General_Concept
space	I-General_Concept
.	O
</s>
<s>
The	O
BBC	B-Device
Micro	I-Device
Advanced	O
User	O
Guide	O
refers	O
to	O
the	O
sideways	B-General_Concept
address	I-General_Concept
space	I-General_Concept
as	O
"	O
paged	O
ROMs	O
"	O
because	O
it	O
predated	O
the	O
use	O
of	O
this	O
address	B-General_Concept
space	I-General_Concept
for	O
RAM	B-Architecture
expansion	O
.	O
</s>
<s>
The	O
BBC	O
B+	O
,	O
B+	O
128	O
and	O
BBC	B-Device
Master	I-Device
all	O
featured	O
sideways	B-General_Concept
RAM	I-General_Concept
as	O
standard	O
.	O
</s>
<s>
The	O
machines	O
used	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	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
The	O
address	B-General_Concept
space	I-General_Concept
was	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
address	I-General_Concept
space	I-General_Concept
(	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>
The	O
sideways	B-General_Concept
address	I-General_Concept
space	I-General_Concept
is	O
a	O
bank-switched	O
(	O
referred	O
to	O
by	O
Acorn	O
as	O
"	O
paged	O
"	O
)	O
address	B-General_Concept
space	I-General_Concept
that	O
allows	O
access	O
to	O
one	O
16	O
KB	O
bank	O
at	O
a	O
time	O
.	O
</s>
<s>
Each	O
bank	O
can	O
be	O
ROM	B-Device
or	O
RAM	B-Architecture
.	O
</s>
<s>
On	O
both	O
the	O
BBC	B-Device
Micro	I-Device
and	O
the	O
BBC	B-Device
Master	I-Device
,	O
there	O
are	O
ROM	B-Device
sockets	O
on	O
the	O
motherboard	O
(	O
four	O
on	O
the	O
BBC	B-Device
Micro	I-Device
)	O
which	O
take	O
sideways	O
ROMs	O
.	O
</s>
<s>
The	O
BBC	B-Device
Micro	I-Device
shipped	O
with	O
a	O
single	O
ROM	B-Device
,	O
containing	O
BBC	B-Language
BASIC	I-Language
;	O
further	O
ROMs	O
can	O
be	O
added	O
to	O
the	O
computer	O
to	O
add	O
software	O
that	O
will	O
remain	O
available	O
at	O
all	O
times	O
.	O
</s>
<s>
The	O
Electron	B-Device
's	O
sideways	B-General_Concept
address	I-General_Concept
space	I-General_Concept
was	O
exposed	O
only	O
by	O
the	O
addition	O
of	O
a	O
Plus	O
1	O
add-on	O
or	O
a	O
third-party	O
equivalent	O
;	O
the	O
Plus	O
1	O
also	O
introduced	O
cartridge	O
slots	O
that	O
were	O
carried	O
over	O
into	O
the	O
BBC	B-Device
Master	I-Device
design	O
as	O
an	O
alternative	O
way	O
to	O
package	O
ROMs	O
.	O
</s>
<s>
Sideways	O
ROMs	O
permitted	O
the	O
addition	O
of	O
new	O
filing	B-Application
systems	I-Application
to	O
the	O
OS	O
(	O
such	O
as	O
the	O
Disc	B-Application
Filing	I-Application
System	I-Application
)	O
and	O
application	O
and	O
utility	O
software	O
.	O
</s>
<s>
Software	O
supplied	O
as	O
ROMs	O
has	O
two	O
main	O
benefits	O
:	O
it	O
loads	O
instantaneously	O
(	O
if	O
delivered	O
as	O
language	O
or	O
service	O
ROMs	O
)	O
,	O
and	O
it	O
requires	O
very	O
little	O
RAM	B-Architecture
to	O
operate	O
(	O
and	O
may	O
use	O
the	O
dedicated	O
paged	O
ROM	B-Device
area	O
of	O
RAM	B-Architecture
that	O
normal	O
software	O
keeps	O
clear	O
of	O
)	O
.	O
</s>
<s>
This	O
allowed	O
for	O
application	O
software	O
to	O
have	O
more	O
working	O
space	O
than	O
would	O
normally	O
be	O
possible	O
,	O
and	O
for	O
utility	O
software	O
such	O
as	O
debuggers	B-Application
to	O
operate	O
on	O
software	O
held	O
in	O
RAM	B-Architecture
.	O
</s>
<s>
The	O
ROM	B-Device
filing	O
system	O
also	O
allowed	O
software	O
to	O
reside	O
in	O
ROMs	O
as	O
files	O
that	O
would	O
be	O
loaded	O
in	O
a	O
similar	O
way	O
to	O
cassette	O
programs	O
.	O
</s>
<s>
Such	O
loading	O
was	O
not	O
instantaneous	O
since	O
it	O
involved	O
transferring	O
the	O
files	O
into	O
RAM	B-Architecture
,	O
but	O
was	O
nevertheless	O
used	O
by	O
Acorn	O
to	O
deliver	O
some	O
cartridge-based	O
software	O
such	O
as	O
games	O
and	O
utilities	O
,	O
ostensibly	O
due	O
to	O
the	O
ability	O
to	O
redeploy	O
cassette-based	O
software	O
in	O
another	O
medium	O
without	O
needing	O
to	O
make	O
significant	O
changes	O
to	O
the	O
software	O
.	O
</s>
<s>
These	O
include	O
language	O
and	O
service	O
entry	O
points	O
,	O
ROM	B-Device
type	O
code	O
,	O
version	O
number	O
and	O
a	O
pointer	O
to	O
the	O
copyright	O
information	O
.	O
</s>
<s>
During	O
operation	O
the	O
OS	O
talks	O
to	O
valid	O
ROMs	O
by	O
jumping	O
to	O
the	O
two	O
entry	O
points	O
with	O
a	O
specific	O
value	O
of	O
the	O
accumulator	B-General_Concept
set	O
.	O
</s>
<s>
This	O
provides	O
a	O
clean	O
API	B-Application
for	O
expanding	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
negotiating	O
bank	B-General_Concept
switching	I-General_Concept
and	O
RAM	B-Architecture
sharing	O
.	O
</s>
<s>
ROMs	O
have	O
two	O
entry	O
points	O
:	O
the	O
service	O
entry	O
point	O
provides	O
the	O
API	B-Application
access	O
to	O
the	O
ROM	B-Device
,	O
and	O
the	O
language	O
entry	O
point	O
is	O
the	O
starting	O
point	O
for	O
application	O
software	O
contained	O
in	O
the	O
ROM	B-Device
.	O
</s>
<s>
"	O
Language	O
"	O
ROMs	O
are	O
ROMs	O
that	O
provide	O
application	O
software	O
,	O
and	O
gain	O
their	O
name	O
from	O
the	O
fact	O
that	O
the	O
BBC	B-Language
BASIC	I-Language
language	O
is	O
supplied	O
as	O
the	O
default	O
ROM	B-Device
in	O
bank	O
15	O
.	O
</s>
<s>
The	O
BBC	B-Device
Micro	I-Device
and	O
Electron	B-Device
require	O
one	O
language	O
ROM	B-Device
be	O
present	O
at	O
POST	B-Device
to	O
provide	O
the	O
computer	O
with	O
a	O
user	O
interface	O
,	O
else	O
the	O
OS	O
will	O
report	O
"	O
Language	O
?	O
"	O
</s>
<s>
The	O
version	O
of	O
Acorn	B-Operating_System
MOS	I-Operating_System
on	O
the	O
BBC	B-Device
Master	I-Device
has	O
a	O
built-in	O
command	O
line	O
and	O
will	O
present	O
this	O
if	O
no	O
default	O
language	O
ROM	B-Device
is	O
configured	O
.	O
</s>
<s>
In	O
addition	O
to	O
ROM	B-Device
,	O
banks	O
of	O
RAM	B-Architecture
could	O
be	O
added	O
to	O
the	O
computer	O
via	O
the	O
sideways	B-General_Concept
address	I-General_Concept
space	I-General_Concept
.	O
</s>
<s>
These	O
could	O
either	O
be	O
used	O
to	O
load	O
and	O
use	O
ROM	B-Device
images	O
from	O
disk	O
or	O
as	O
extra	O
workspace	O
for	O
machine	O
code	O
programs	O
.	O
</s>
<s>
The	O
BBC	B-Device
Model	I-Device
B	I-Device
is	O
hard-wired	O
to	O
prevent	O
writing	O
to	O
the	O
sideways	O
area	O
,	O
so	O
a	O
write	O
signal	O
needs	O
to	O
be	O
collected	O
from	O
somewhere	O
.	O
</s>
<s>
The	O
methods	O
vary	O
,	O
but	O
the	O
two	O
most	O
common	O
methods	O
are	O
removing	O
chips	O
from	O
the	O
board	O
and	O
placing	O
them	O
into	O
an	O
expansion	O
board	O
that	O
occupies	O
the	O
chips	O
 '	O
original	O
sockets	O
,	O
and	O
fitting	O
a	O
RAM	B-Architecture
module	O
in	O
a	O
ROM	B-Device
socket	O
with	O
a	O
flying	O
lead	O
connected	O
to	O
a	O
write	O
signal	O
elsewhere	O
on	O
the	O
motherboard	O
.	O
</s>
<s>
The	O
64KB	O
model	O
B+	O
had	O
12KB	O
of	O
"	O
special	O
"	O
sideways	B-General_Concept
RAM	I-General_Concept
.	O
</s>
<s>
This	O
used	O
the	O
sideways	O
address	O
but	O
was	O
selected	O
by	O
the	O
high	O
bit	O
of	O
the	O
ROM	B-Device
select	O
register	O
and	O
could	O
not	O
be	O
used	O
to	O
load	O
ROM	B-Device
images	O
.	O
</s>
<s>
The	O
128KB	O
model	O
B+	O
had	O
an	O
expansion	O
board	O
with	O
64KB	O
of	O
"	O
regular	O
"	O
sideways	B-General_Concept
RAM	I-General_Concept
in	O
addition	O
to	O
the	O
12KB	O
of	O
"	O
special	O
"	O
sideways	B-General_Concept
RAM	I-General_Concept
on	O
the	O
main	O
board	O
.	O
</s>
<s>
The	O
BBC	B-Device
Master	I-Device
came	O
with	O
64KB	O
of	O
regular	O
sideways	B-General_Concept
RAM	I-General_Concept
,	O
and	O
could	O
be	O
configured	O
with	O
motherboard	O
links	O
as	O
to	O
which	O
banks	O
were	O
ROM	B-Device
and	O
which	O
were	O
RAM	B-Architecture
.	O
</s>
<s>
In	O
addition	O
it	O
had	O
4KB	O
of	O
"	O
special	O
"	O
sideways	B-General_Concept
RAM	I-General_Concept
and	O
8KB	O
of	O
RAM	B-Architecture
paged	O
over	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Unlike	O
on	O
the	O
B+	O
where	O
the	O
"	O
special	O
"	O
sideways	B-General_Concept
RAM	I-General_Concept
had	O
been	O
available	O
for	O
user	O
applications	O
these	O
memory	O
areas	O
on	O
the	O
Master	O
were	O
used	O
as	O
operating	B-General_Concept
system	I-General_Concept
and	O
filing	O
system	O
workspace	O
.	O
</s>
<s>
The	O
cartridge	O
port	O
wiring	O
differs	O
between	O
the	O
Electron	B-Device
and	O
Master	O
128	O
with	O
regard	O
to	O
certain	O
RAM-related	O
signals	O
.	O
</s>
<s>
The	O
Master	O
's	O
slots	O
replace	O
the	O
READY	O
signal	O
with	O
a	O
more	O
general	O
R/W	O
signal	O
,	O
preserving	O
the	O
CSRW	O
(	O
chip	O
select	O
,	O
read/write	O
)	O
signal	O
only	O
for	O
certain	O
addresses	O
in	O
pages	O
FC	O
,	O
FD	O
and	O
FE	O
,	O
whereas	O
CSRW	O
corresponds	O
to	O
the	O
CPU	B-Device
read/write	O
line	O
in	O
the	O
Electron	B-Device
Plus	O
1	O
cartridge	O
interface	O
.	O
</s>
<s>
Both	O
systems	O
support	O
RAM	B-Architecture
cartridges	O
,	O
however	O
.	O
</s>
<s>
Acorn	B-Operating_System
MOS	I-Operating_System
supports	O
up	O
to	O
16	O
sideways	O
banks	O
.	O
</s>
<s>
Certain	O
boards	O
,	O
such	O
as	O
the	O
Watford	O
Electronics	O
Sidewise	O
board	O
,	O
also	O
provided	O
the	O
option	O
of	O
permanent	O
,	O
battery	O
backed-up	O
RAM	B-Architecture
.	O
</s>
<s>
This	O
allows	O
for	O
developer	O
testing	O
of	O
new	O
sideways	O
ROM	B-Device
software	O
without	O
burning	O
an	O
EPROM	B-General_Concept
for	O
each	O
attempt	O
.	O
</s>
<s>
A	O
write-protect	O
switch	O
could	O
be	O
used	O
to	O
prevent	O
the	O
contents	O
of	O
sideways	B-General_Concept
RAM	I-General_Concept
from	O
being	O
modified	O
.	O
</s>
