<s>
Atari	B-Device
MMU	I-Device
is	O
a	O
custom	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
chip	O
for	O
the	O
Atari	B-Device
8-bit	I-Device
computers	I-Device
.	O
</s>
<s>
It	O
enables	O
access	O
to	O
the	O
hardware	O
registers	O
on	O
ANTIC	B-General_Concept
,	O
GTIA	B-General_Concept
,	O
POKEY	B-Device
and	O
6520	O
PIA	O
.	O
</s>
<s>
The	O
later	O
XL/XE	O
MMU	O
(	O
C061618	O
)	O
also	O
selects	O
OS	B-General_Concept
ROM	B-Device
,	O
Atari	B-Language
BASIC	I-Language
ROM	B-Device
,	O
self-test	O
ROM	B-Device
and	O
LEDs	O
in	O
the	O
1200XL	B-Device
.	O
</s>
<s>
On	O
the	O
128K	O
130XE	B-Device
the	O
EMMU	B-Device
chip	O
handles	O
similar	O
functionality	O
.	O
</s>
<s>
The	O
user	O
cannot	O
directly	O
manipulate	O
the	O
Atari	B-Device
MMU	I-Device
,	O
but	O
selects	O
the	O
various	O
ROMS	O
and	O
memory	O
via	O
the	O
memory-mapped	O
hardware	O
register	O
known	O
as	O
PORTB	O
(	O
5401710	O
or	O
D30116	O
)	O
.	O
</s>
<s>
No	O
synchronization	O
is	O
required	O
as	O
the	O
OS	B-General_Concept
handles	O
the	O
access	O
.	O
</s>
<s>
Description	O
0	O
Controls	O
OS	B-General_Concept
ROM	B-Device
region	O
.	O
</s>
<s>
Disabling	O
OS	B-General_Concept
ROM	B-Device
enables	O
RAM	O
instead	O
.	O
</s>
<s>
1	O
Controls	O
built-in	O
BASIC	B-Language
.	O
</s>
<s>
2	O
Controls	O
LED	O
#1	O
in	O
the	O
1200XL	B-Device
.	O
</s>
<s>
Extended	O
RAM	O
bank	O
selection	O
in	O
the	O
130XE	B-Device
.	O
</s>
<s>
3	O
Controls	O
LED	O
#2	O
in	O
the	O
1200XL	B-Device
.	O
</s>
<s>
Extended	O
RAM	O
bank	O
selection	O
in	O
the	O
130XE	B-Device
.	O
</s>
<s>
4	O
CPU	O
extended	O
RAM	O
bit	O
for	O
130XE	B-Device
only	O
.	O
</s>
<s>
5	O
ANTIC	B-General_Concept
extended	O
RAM	O
bit	O
for	O
130XE	B-Device
only	O
.	O
</s>
<s>
6	O
Missile	O
Command	O
enable	O
bit	O
for	O
XEGS	O
only	O
,	O
but	O
only	O
if	O
BASIC	B-Language
is	O
not	O
enabled	O
.	O
</s>
<s>
7	O
Controls	O
the	O
self	O
test	O
ROM	B-Device
region	O
.	O
</s>
<s>
Note	O
:	O
The	O
1200XL	B-Device
does	O
not	O
have	O
BASIC	B-Language
built-in	O
.	O
</s>
