<s>
In	O
computing	O
,	O
the	O
Tube	O
is	O
the	O
expansion	O
interface	O
and	O
architecture	O
of	O
the	B-Device
BBC	I-Device
Microcomputer	I-Device
System	I-Device
which	O
allows	O
the	O
BBC	B-Device
Micro	I-Device
to	O
communicate	O
with	O
a	O
second	O
processor	O
,	O
or	O
coprocessor	O
.	O
</s>
<s>
Under	O
the	O
Tube	O
architecture	O
,	O
the	O
coprocessor	O
runs	O
the	O
application	O
software	O
for	O
the	O
user	O
,	O
whilst	O
the	O
Micro	O
(	O
acting	O
as	O
a	O
host	O
)	O
provides	O
all	O
I/O	B-General_Concept
functions	O
,	O
such	O
as	O
screen	O
display	O
,	O
keyboard	O
and	O
storage	O
devices	O
management	O
.	O
</s>
<s>
A	O
coprocessor	O
unit	O
can	O
be	O
coldplugged	O
into	O
any	O
BBC	B-Device
Micro	I-Device
with	O
a	O
disk	O
interface	O
(	O
whose	O
ROM	B-Device
contained	O
the	O
necessary	O
host	O
software	O
)	O
and	O
used	O
immediately	O
.	O
</s>
<s>
The	O
40-pin	O
IDC	O
"	O
Tube	O
"	O
connector	O
is	O
a	O
simple	O
slave	O
connection	O
to	O
the	O
host	O
processor	O
's	O
main	O
bus	O
,	O
with	O
8	O
data	O
lines	O
,	O
7	O
address	O
lines	O
,	O
and	O
an	O
interrupt	B-Application
input	O
.	O
</s>
<s>
Inside	O
the	O
coprocessor	O
unit	O
a	O
proprietary	O
chip	O
(	O
the	O
Tube	O
ULA	O
,	O
manufactured	O
initially	O
by	O
Ferranti	O
)	O
interfaces	O
and	O
logically	O
isolates	O
the	O
host	O
and	O
coprocessor	O
buses	B-General_Concept
.	O
</s>
<s>
The	O
other	O
active	O
components	O
needed	O
are	O
a	O
microprocessor	B-Architecture
,	O
some	O
RAM	B-Architecture
,	O
a	O
small	O
ROM	B-Device
containing	O
processor	O
specific	O
client	O
code	O
,	O
glue	O
logic	O
such	O
as	O
an	O
address	B-Device
decoder	I-Device
and	O
a	O
power	O
supply	O
.	O
</s>
<s>
The	O
two	O
processors	O
communicate	O
through	O
four	O
pairs	O
of	O
FIFO	B-Operating_System
buffers	O
in	O
the	O
Tube	O
ULA	O
.	O
</s>
<s>
Console	B-Device
input/output	B-General_Concept
,	O
error	B-Error_Name
messages	I-Error_Name
,	O
data	O
transfers	O
and	O
system	B-Operating_System
calls	I-Operating_System
each	O
have	O
their	O
own	O
pair	O
of	O
buffers	O
,	O
one	O
for	O
each	O
direction	O
.	O
</s>
<s>
Each	O
buffer	O
has	O
a	O
control	O
register	O
and	O
status	O
register	O
to	O
monitor	O
its	O
state	O
and	O
configure	O
the	O
raising	O
of	O
interrupts	B-Application
.	O
</s>
<s>
The	O
protocol	O
for	O
the	O
use	O
of	O
these	O
buffers	O
is	O
rigorously	O
specified	O
by	O
Acorn	O
Computers	O
and	O
amounts	O
to	O
interprocess	B-Operating_System
communication	I-Operating_System
by	O
message	B-Architecture
passing	I-Architecture
.	O
</s>
<s>
Most	O
interaction	O
is	O
asynchronous	B-Application
but	O
fast	O
block	O
transfers	O
are	O
synchronous	O
and	O
consist	O
of	O
the	O
host	O
blindly	O
running	O
a	O
simple	O
fetch-store	O
loop	O
,	O
which	O
defines	O
the	O
transfer	O
rate	O
.	O
</s>
<s>
Most	O
commonly	O
seen	O
is	O
a	O
MOS	B-General_Concept
Technology	I-General_Concept
6502	I-General_Concept
processor	O
which	O
allows	O
unmodified	O
BBC	B-Device
Micro	I-Device
programs	O
to	O
run	O
faster	O
and	O
with	O
more	O
memory	O
,	O
as	O
long	O
as	O
they	O
use	O
the	O
API	O
for	O
all	O
I/O	B-General_Concept
.	O
</s>
<s>
There	O
is	O
also	O
a	O
Zilog	B-General_Concept
Z80	I-General_Concept
processor	O
to	O
run	O
CP/M	B-Application
and	O
a	O
National	B-Device
Semiconductor	I-Device
32016	I-Device
processor	O
running	O
Panos	B-Operating_System
(	O
and	O
unofficially	O
a	O
UNIX	O
variant	O
)	O
.	O
</s>
<s>
These	O
coprocessors	O
form	O
the	O
basis	O
of	O
the	O
Acorn	B-Device
Business	I-Device
Computer	I-Device
series	O
,	O
the	O
higher	O
end	O
machines	O
being	O
repackaged	O
BBC	B-Device
Micros	I-Device
with	O
a	O
coprocessor	O
attached	O
via	O
the	O
Tube	O
.	O
</s>
<s>
An	O
internal	O
6502	B-General_Concept
processor	I-General_Concept
can	O
be	O
fitted	O
,	O
or	O
an	O
Intel	B-Device
80186	I-Device
based	O
system	O
for	O
DOS	B-Device
compatibility	O
(	O
although	O
in	O
practice	O
this	O
is	O
limited	O
)	O
.	O
</s>
<s>
The	O
Tube	O
has	O
also	O
been	O
used	O
during	O
the	O
initial	O
development	O
of	O
the	O
ARM	B-Architecture
processor	I-Architecture
.	O
</s>
<s>
An	O
evaluation	O
board	O
was	O
developed	O
that	O
again	O
uses	O
the	O
BBC	B-Device
Micro	I-Device
as	O
a	O
host	O
system	O
for	O
I/O	B-General_Concept
operations	I-General_Concept
.	O
</s>
<s>
Acorn	O
had	O
strongly	O
discouraged	O
BBC	B-Device
Micro	I-Device
programmers	O
from	O
directly	O
accessing	O
system	O
memory	O
and	O
hardware	O
,	O
favouring	O
official	O
API	O
calls	O
.	O
</s>
<s>
This	O
is	O
ostensibly	O
to	O
ensure	O
applications	O
can	O
be	O
seamlessly	O
moved	O
to	O
the	O
Tube	O
6502	B-General_Concept
coprocessor	O
,	O
since	O
direct	O
access	O
from	O
there	O
is	O
impossible	O
.	O
</s>
<s>
When	O
a	O
program	O
calls	O
one	O
of	O
the	O
MOS	B-Operating_System
entry	O
points	O
,	O
a	O
replacement	O
subroutine	O
in	O
the	O
coprocessor	O
's	O
ROM	B-Device
passes	O
a	O
corresponding	O
message	O
to	O
the	O
host	O
which	O
carries	O
out	O
the	O
operation	O
and	O
passes	O
back	O
the	O
result	O
.	O
</s>
<s>
Other	O
CPU	O
models	O
use	O
a	O
custom	O
API	O
,	O
which	O
is	O
typically	O
an	O
orthogonal	O
translation	O
of	O
the	O
6502	B-General_Concept
API	O
into	O
a	O
native	O
format	O
.	O
</s>
