<s>
Alphanumeric	B-General_Concept
Television	I-General_Concept
Interface	I-General_Concept
Controller	I-General_Concept
(	O
ANTIC	B-General_Concept
)	O
is	O
an	O
LSI	O
ASIC	O
dedicated	O
to	O
generating	O
2D	O
computer	O
graphics	O
to	O
be	O
shown	O
on	O
a	O
television	O
screen	O
or	O
computer	B-Device
display	I-Device
.	O
</s>
<s>
Under	O
the	O
direction	O
of	O
Jay	O
Miner	O
,	O
the	O
chip	O
was	O
designed	O
in	O
1977-1978	O
by	O
Joe	O
Decuir	O
,	O
Francois	O
Michel	O
,	O
and	O
Steve	O
Smith	O
for	O
the	O
Atari	B-Device
8-bit	I-Device
family	I-Device
of	O
home	O
computers	O
first	O
released	O
in	O
1979	O
and	O
was	O
patented	O
by	O
Atari	O
,	O
Inc	O
.	O
in	O
1981	O
.	O
</s>
<s>
ANTIC	B-General_Concept
is	O
also	O
used	O
in	O
the	O
1982	O
Atari	B-General_Concept
5200	I-General_Concept
video	O
game	O
console	O
,	O
which	O
shares	O
most	O
of	O
the	O
same	O
hardware	O
as	O
the	O
8-bit	O
computers	O
.	O
</s>
<s>
For	O
every	O
frame	O
of	O
video	O
,	O
ANTIC	B-General_Concept
reads	O
a	O
program	O
of	O
instructions	B-General_Concept
to	O
define	O
the	O
playfield	O
,	O
or	O
background	O
graphics	O
,	O
then	O
delivers	O
a	O
data	O
stream	O
to	O
the	O
companion	O
CTIA	B-General_Concept
or	I-General_Concept
GTIA	I-General_Concept
chip	O
which	O
adds	O
color	O
and	O
overlays	O
sprites	O
(	O
referred	O
to	O
as	O
"	O
Player/Missile	O
graphics	O
"	O
by	O
Atari	O
)	O
.	O
</s>
<s>
Each	O
ANTIC	B-General_Concept
instruction	O
corresponds	O
to	O
either	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
or	O
one	O
of	O
14	O
graphics	O
modes	O
used	O
for	O
a	O
horizontal	O
band	O
of	O
the	O
display	O
.	O
</s>
<s>
Display	O
list	O
instructions	B-General_Concept
can	O
also	O
enable	O
horizontal	O
and	O
vertical	O
fine	O
scrolling	O
and	O
mark	O
that	O
an	O
interrupt	O
should	O
occur	O
.	O
</s>
<s>
Atari	O
computer	O
magazine	O
Antic	B-General_Concept
was	O
named	O
after	O
the	O
chip	O
.	O
</s>
<s>
The	O
list	O
below	O
describes	O
ANTIC	B-General_Concept
's	O
inherent	O
hardware	O
capabilities	O
meaning	O
the	O
intended	O
functionality	O
of	O
the	O
hardware	O
by	O
itself	O
,	O
not	O
including	O
results	O
achieved	O
by	O
CPU-serviced	O
interrupts	O
or	O
display	O
kernels	O
frequently	O
driving	O
register	O
changes	O
.	O
</s>
<s>
ANTIC	B-General_Concept
uses	O
DMA	B-General_Concept
to	O
read	O
a	O
program	O
called	O
the	O
display	O
list	O
which	O
specifies	O
these	O
playfield	O
features	O
:	O
</s>
<s>
Control	O
the	O
display-oriented	O
Vertical	O
Blank	O
and	O
Display	B-General_Concept
List	I-General_Concept
interrupts	I-General_Concept
,	O
and	O
the	O
Reset	O
key	O
interrupt	O
.	O
</s>
<s>
Non-fixed	O
RAM	B-Architecture
.	O
</s>
<s>
This	O
allows	O
RAM	B-Architecture
for	O
graphics	O
features	O
to	O
be	O
located	O
almost	O
anywhere	O
in	O
the	O
16-bit	O
memory	O
address	O
range	O
.	O
</s>
<s>
C012296	O
NTSC	O
:	O
Used	O
in	O
Atari	O
400	O
,	O
800	O
,	O
and	O
1200XL	B-Device
computers	O
.	O
</s>
<s>
C021697	O
NTSC	O
:	O
Used	O
in	O
Atari	B-Device
600XL	I-Device
,	O
800XL	B-Device
,	O
and	O
XE	O
models	O
.	O
</s>
<s>
C021698	O
PAL/SECAM	O
:	O
Used	O
in	O
Atari	B-Device
XL	I-Device
,	O
and	O
XE	O
models	O
.	O
</s>
<s>
Atari	O
,	O
Inc	O
.	O
intended	O
to	O
combine	O
functions	O
of	O
the	O
ANTIC	B-General_Concept
and	O
GTIA	B-General_Concept
chips	O
in	O
one	O
integrated	O
circuit	O
to	O
reduce	O
production	O
costs	O
of	O
Atari	O
computers	O
and	O
5200	O
consoles	O
.	O
</s>
<s>
Pin	O
Name	O
Pin	O
Number(s )	O
Description	O
A0	O
-	O
A15	O
13	O
,	O
12	O
,	O
11	O
,	O
10	O
,	O
28	O
,	O
27	O
,	O
26	O
,	O
25	O
,	O
24	O
,	O
23	O
,	O
16	O
,	O
22	O
,	O
17	O
,	O
18	O
,	O
19	O
,	O
20	O
Memory	O
Address	O
I/O	O
AN0	O
-	O
AN2	O
2	O
,	O
3	O
,	O
5	O
ANTIC	B-General_Concept
Interface	O
to	O
CTIA/GTIA	B-General_Concept
D0	O
-	O
D7	O
30	O
,	O
31	O
,	O
32	O
,	O
33	O
,	O
40	O
,	O
39	O
,	O
38	O
,	O
37	O
Data	O
Bus	O
I/O	O
FØ0	O
35	O
Fast	O
Phase	O
0	O
Input	O
Clock	O
HALT	O
9	O
Halt	O
Output	O
,	O
Suspends	O
CPU	O
while	O
ANTIC	B-General_Concept
reads	O
memory	O
LP	O
4	O
Light	O
Pen	O
Input	O
NMI	O
7	O
NMI	O
Interrupt	O
Output	O
to	O
CPU	O
RDY	O
15	O
Ready	O
Output	O
.	O
</s>
<s>
The	O
Atari	B-Device
8-bit	I-Device
computers	I-Device
and	O
the	O
Atari	B-General_Concept
5200	I-General_Concept
console	O
map	O
the	O
ANTIC	B-General_Concept
chip	O
to	O
the	O
$D4xxhex	O
page	O
.	O
</s>
<s>
ANTIC	B-General_Concept
provides	O
15	O
Read/Write	O
registers	O
controlling	O
Playfield	O
display	O
parameters	O
,	O
DMA	B-General_Concept
for	O
Player/Missile	O
graphics	O
,	O
fine	O
scrolling	O
,	O
light	O
pen	O
input	O
,	O
and	O
interrupts	O
.	O
</s>
<s>
This	O
problem	O
is	O
solved	O
by	O
Operating	O
System	O
Shadow	O
registers	O
implemented	O
in	O
regular	O
RAM	B-Architecture
as	O
places	O
to	O
store	O
the	O
last	O
value	O
written	O
to	O
registers	O
.	O
</s>
<s>
Operating	O
System	O
Shadow	O
registers	O
are	O
copied	O
from	O
RAM	B-Architecture
to	O
the	O
hardware	O
registers	O
during	O
the	O
vertical	O
blank	O
.	O
</s>
<s>
DMACTL	O
controls	O
ANTIC	B-General_Concept
's	O
DMA	B-General_Concept
behavior	O
for	O
the	O
Playfield	O
and	O
Player-Missile	O
(	O
sprite	O
)	O
graphics	O
.	O
</s>
<s>
Also	O
see	O
Display	O
List	O
DMA	B-General_Concept
bit	O
regarding	O
Playfield	O
display	O
.	O
</s>
<s>
Player/Missile	O
DMA	B-General_Concept
bits	O
values	O
:	O
</s>
<s>
ANTIC	B-General_Concept
's	O
Player/Missile	O
DMA	B-General_Concept
feature	O
reads	O
bytes	O
from	O
memory	O
and	O
delivering	O
data	O
to	O
update	O
CTIA/GTIA	B-General_Concept
'	O
s	O
GRAFP0	O
,	O
GRAFP1	O
,	O
GRAFP2	O
,	O
GRAFP3	O
,	O
and	O
GRAFM	O
graphics	O
pattern	O
registers	O
relieving	O
the	O
CPU	O
from	O
creating	O
Player/Missile	O
(	O
sprite	O
)	O
graphics	O
.	O
</s>
<s>
These	O
bits	O
turn	O
on	O
ANTIC	B-General_Concept
's	O
transmission	O
of	O
Player	O
data	O
and	O
Missile	O
data	O
to	O
CTIA/GTIA	B-General_Concept
.	O
</s>
<s>
CTIA/GTIA	B-General_Concept
must	O
also	O
be	O
configured	O
to	O
receive	O
the	O
data	O
via	O
its	O
GRACTL	O
register	O
in	O
order	O
for	O
Player/Missile	O
DMA	B-General_Concept
to	O
function	O
as	O
expected	O
.	O
</s>
<s>
When	O
Player	O
DMA	B-General_Concept
is	O
enabled	O
,	O
Missile	O
DMA	B-General_Concept
automatically	O
occurs	O
to	O
keep	O
the	O
DMA	B-General_Concept
timing	O
consistent	O
,	O
but	O
the	O
data	O
is	O
not	O
delivered	O
to	O
the	O
Missile	O
's	O
GRAFM	O
register	O
.	O
</s>
<s>
When	O
enabled	O
,	O
Player/Missile	O
DMA	B-General_Concept
occurs	O
on	O
every	O
scan	B-Algorithm
line	I-Algorithm
in	O
the	O
visible	O
display	O
—	O
from	O
scan	B-Algorithm
line	I-Algorithm
8	O
to	O
247	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
Player/Missile	O
data	O
in	O
the	O
memory	O
map	O
(	O
see	O
ANTIC	B-General_Concept
's	O
PMBASE	O
)	O
above	O
and	O
below	O
those	O
scan	B-Algorithm
line	I-Algorithm
counts	O
is	O
unused	O
and	O
undisplayed	O
.	O
</s>
<s>
ANTIC	B-General_Concept
updates	O
its	O
DMA	B-General_Concept
fetch	O
address	O
every	O
other	O
scan	B-Algorithm
line	I-Algorithm
and	O
updates	O
the	O
CTIA/GTIA	B-General_Concept
Player/Missile	O
Graphics	O
pattern	O
registers	O
every	O
scan	B-Algorithm
line	I-Algorithm
,	O
so	O
that	O
each	O
Player/Missile	O
byte	O
pattern	O
is	O
two	O
scan	B-Algorithm
lines	I-Algorithm
tall	O
.	O
</s>
<s>
When	O
Double	O
line	O
resolution	O
is	O
enabled	O
CTIA/GTIA	B-General_Concept
register	O
VDELAY	O
( $	O
D01Chex/53276dec	O
)	O
works	O
by	O
masking	O
updates	O
on	O
even	O
scan	B-Algorithm
lines	I-Algorithm
which	O
results	O
in	O
shifting	O
the	O
bit	O
pattern	O
of	O
individual	O
Players	O
and	O
Missiles	O
down	O
one	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
A	O
DMA	B-General_Concept
fetch	O
and	O
Player/Missile	O
register	O
update	O
occurs	O
on	O
every	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
CTIA/GTIA	B-General_Concept
register	O
VDELAY	O
( $	O
D01Chex/53276dec	O
)	O
which	O
masks	O
updates	O
on	O
even	O
scan	B-Algorithm
lines	I-Algorithm
effectively	O
reduces	O
Single	O
line	O
resolution	O
to	O
Double	O
line	O
resolution	O
.	O
</s>
<s>
ANTIC	B-General_Concept
DMA	B-General_Concept
and	O
Player/Missile	O
pattern	O
register	O
updates	O
occur	O
on	O
each	O
scan	B-Algorithm
line	I-Algorithm
regardless	O
of	O
resolution	O
.	O
</s>
<s>
When	O
Double	O
line	O
resolution	O
is	O
in	O
effect	O
the	O
Player/Missile	O
memory	O
can	O
be	O
modified	O
between	O
the	O
redundant	O
DMA	B-General_Concept
fetches	O
thus	O
changing	O
the	O
pattern	O
sent	O
to	O
the	O
GRAF*	O
registers	O
and	O
producing	O
apparent	O
Single	O
line	O
resolution	O
Player/Missiles	O
.	O
</s>
<s>
Display	O
List	O
DMA	B-General_Concept
bit	O
values	O
:	O
</s>
<s>
Playfield	O
display	O
requires	O
that	O
Display	O
List	O
DMA	B-General_Concept
is	O
enabled	O
,	O
and	O
a	O
Playfield	O
width	O
specified	O
.	O
</s>
<s>
The	O
CHACTL	O
Video	O
Inverse	O
and	O
Video	O
Blank	O
bits	O
affect	O
the	O
display	O
of	O
characters	O
in	O
ANTIC	B-General_Concept
Text	O
Modes	O
2	O
and	O
3	O
which	O
have	O
the	O
high	O
bit	O
set	O
(	O
characters	O
$80	O
through	O
$FF	O
)	O
.	O
</s>
<s>
Video	O
Inverse	O
and	O
Video	O
Blank	O
bits	O
work	O
in	O
ANTIC	B-General_Concept
modes	O
2	O
and	O
3	O
,	O
and	O
have	O
no	O
effect	O
on	O
the	O
other	O
text	O
modes	O
4	O
,	O
5	O
,	O
6	O
,	O
and	O
7	O
.	O
</s>
<s>
As	O
this	O
vertically	O
mirrors	O
the	O
glyph	O
data	O
before	O
it	O
is	O
used	O
,	O
the	O
effect	O
appears	O
inconsistent	O
for	O
ANTIC	B-General_Concept
Mode	O
3	O
descenders	O
with	O
glyph	O
bytes	O
6	O
and	O
7	O
appearing	O
at	O
the	O
bottom	O
of	O
the	O
descender	O
area	O
.	O
</s>
<s>
ANTIC	B-General_Concept
begins	O
executing	O
the	O
Display	O
List	O
pointed	O
to	O
by	O
the	O
16-bit	O
address	O
in	O
registers	O
DLISTL/DLISTH	O
(	O
D402-D403hex/54274	O
-54275dec	O
)	O
.	O
</s>
<s>
The	O
address	O
registers	O
are	O
updated	O
during	O
Display	O
List	O
execution	O
by	O
ANTIC	B-General_Concept
's	O
JMP	O
(	O
Jump	O
)	O
and	O
JVB	O
(	O
Jump	O
and	O
wait	O
for	O
Vertical	O
Blank	O
)	O
.	O
</s>
<s>
When	O
the	O
OS	O
Vertical	O
Blank	O
Interrupt	O
is	O
enabled	O
,	O
direct	O
updates	O
to	O
the	O
ANTIC	B-General_Concept
DLIST	O
registers	O
by	O
the	O
CPU	O
or	O
the	O
ANTIC	B-General_Concept
Jump	O
instructions	B-General_Concept
will	O
be	O
overwritten	O
by	O
the	O
OS	O
during	O
the	O
next	O
Vertical	O
Blank	O
by	O
the	O
values	O
in	O
the	O
shadow	O
registers	O
.	O
</s>
<s>
The	O
value	O
range	O
of	O
16	O
color	O
clocks	O
allows	O
ANTIC	B-General_Concept
to	O
shift	O
Mode	O
2	O
Text	O
four	O
characters	O
,	O
and	O
Mode	O
6	O
text	O
two	O
characters	O
before	O
a	O
coarse	O
scroll	O
is	O
needed	O
.	O
</s>
<s>
When	O
Horizontal	O
scrolling	O
is	O
enabled	O
for	O
a	O
mode	O
line	O
ANTIC	B-General_Concept
fetches	O
the	O
next	O
size	O
increment	O
greater	O
than	O
the	O
current	O
screen	O
width	O
to	O
provide	O
the	O
buffer	O
of	O
data	O
subject	O
to	O
the	O
horizontal	O
scrolling	O
control	O
.	O
</s>
<s>
When	O
displaying	O
Narrow	O
width	O
ANTIC	B-General_Concept
fetches	O
the	O
screen	O
RAM	B-Architecture
needed	O
for	O
Normal	O
width	O
.	O
</s>
<s>
Likewise	O
,	O
for	O
Normal	O
width	O
ANTIC	B-General_Concept
fetches	O
the	O
screen	O
RAM	B-Architecture
needed	O
for	O
Wide	O
.	O
</s>
<s>
ANTIC	B-General_Concept
buffers	O
the	O
first	O
few	O
bytes	O
read	O
from	O
screen	O
memory	O
that	O
are	O
sufficient	O
to	O
cover	O
the	O
16	O
color	O
clock	O
range	O
of	O
movement	O
.	O
</s>
<s>
ANTIC	B-General_Concept
Mode	O
F	O
(	O
high-resolution	O
,	O
1/2	O
color	O
clock	O
pixels	O
)	O
can	O
only	O
be	O
scrolled	O
two	O
pixels	O
at	O
a	O
time	O
,	O
because	O
HSCROL	O
specifies	O
color	O
clocks	O
.	O
</s>
<s>
ANTIC	B-General_Concept
Modes	O
using	O
the	O
alternate	O
GTIA	B-General_Concept
color	O
interpretations	O
must	O
be	O
scrolled	O
by	O
an	O
entire	O
GTIA	B-General_Concept
pixel	O
(	O
two	O
color	O
clocks	O
)	O
.	O
</s>
<s>
Odd	O
values	O
of	O
HSCROL	O
will	O
shift	O
the	O
pixel	O
stream	O
into	O
a	O
different	O
state	O
that	O
GTIA	B-General_Concept
will	O
interpret	O
as	O
different	O
colors	O
.	O
</s>
<s>
This	O
register	O
specifies	O
the	O
distance	O
of	O
the	O
Vertical	O
fine	O
scrolling	O
in	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
The	O
lowest	O
4	O
bits	O
are	O
significant	O
,	O
however	O
the	O
Vertical	O
scroll	O
value	O
should	O
range	O
from	O
0	O
to	O
the	O
ANTIC	B-General_Concept
Mode	O
line	O
's	O
scan	B-Algorithm
line	I-Algorithm
height-1	O
.	O
</s>
<s>
Scrolling	O
farther	O
than	O
the	O
ANTIC	B-General_Concept
Mode	O
's	O
number	O
of	O
scan	B-Algorithm
lines	I-Algorithm
results	O
in	O
lines	O
of	O
repeated	O
data	O
upsetting	O
the	O
fine	O
scrolling	O
continuity	O
(	O
though	O
,	O
this	O
can	O
also	O
be	O
used	O
as	O
an	O
exploitable	O
behavior	O
)	O
.	O
</s>
<s>
The	O
Vertical	O
scrolling	O
region	O
is	O
defined	O
by	O
setting	O
the	O
VS	O
bit	O
( $20	O
)	O
on	O
a	O
sequential	O
series	O
of	O
ANTIC	B-General_Concept
Mode	O
Lines	O
in	O
the	O
Display	O
List	O
.	O
</s>
<s>
The	O
Vertical	O
scroll	O
value	O
indicates	O
the	O
scan	B-Algorithm
line	I-Algorithm
number	O
to	O
begin	O
the	O
display	O
in	O
the	O
first	O
Mode	O
line	O
and	O
is	O
also	O
used	O
as	O
the	O
scan	B-Algorithm
line	I-Algorithm
number	O
to	O
end	O
the	O
display	O
in	O
the	O
last	O
Mode	O
line	O
(	O
the	O
line	O
without	O
the	O
VS	O
bit	O
set	O
)	O
.	O
</s>
<s>
Example	O
:	O
Given	O
an	O
eight	O
scan	B-Algorithm
line	I-Algorithm
ANTIC	B-General_Concept
Mode	O
(	O
Text	O
Modes	O
2	O
,	O
4	O
,	O
or	O
6	O
)	O
with	O
the	O
VS	O
bit	O
set	O
on	O
two	O
adjacent	O
Mode	O
lines	O
the	O
scrolling	O
region	O
then	O
consists	O
of	O
three	O
Mode	O
lines	O
—	O
the	O
third	O
line	O
in	O
the	O
scrolling	O
region	O
is	O
the	O
first	O
Mode	O
line	O
encountered	O
without	O
the	O
VS	O
bit	O
set	O
.	O
</s>
<s>
When	O
Vertical	O
scrolling	O
is	O
not	O
set	O
in	O
the	O
Display	O
List	O
the	O
three	O
Modes	O
lines	O
would	O
naturally	O
result	O
in	O
24	O
displayed	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
When	O
the	O
VS	O
bits	O
are	O
set	O
in	O
the	O
Display	O
List	O
Mode	O
instructions	B-General_Concept
as	O
described	O
and	O
the	O
VSCROL	O
value	O
is	O
set	O
to	O
2	O
then	O
the	O
first	O
Mode	O
line	O
in	O
the	O
region	O
begins	O
displaying	O
at	O
scan	B-Algorithm
line	I-Algorithm
2	O
showing	O
scan	B-Algorithm
lines	I-Algorithm
2	O
to	O
7	O
(	O
six	O
scan	B-Algorithm
lines	I-Algorithm
or	O
8-VSCROL	O
)	O
,	O
the	O
second	O
Mode	O
line	O
is	O
displayed	O
completely	O
(	O
eight	O
scan	B-Algorithm
lines	I-Algorithm
)	O
,	O
and	O
the	O
final	O
Mode	O
line	O
ends	O
at	O
scan	B-Algorithm
line	I-Algorithm
2	O
showing	O
scan	B-Algorithm
lines	I-Algorithm
0	O
to	O
2	O
(	O
three	O
scan	B-Algorithm
lines	I-Algorithm
or	O
VSCROL+1	O
)	O
.	O
</s>
<s>
The	O
total	O
scan	B-Algorithm
lines	I-Algorithm
displayed	O
in	O
the	O
scrolling	O
region	O
is	O
then	O
6+8+3	O
=	O
17	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
Since	O
fine	O
scrolling	O
is	O
based	O
on	O
scan	B-Algorithm
lines	I-Algorithm
not	O
pixel	O
height	O
,	O
and	O
scrolling	O
region	O
movement	O
is	O
done	O
simply	O
by	O
skipping	O
scan	B-Algorithm
lines	I-Algorithm
from	O
the	O
first	O
Mode	O
line	O
and	O
adding	O
scan	B-Algorithm
lines	I-Algorithm
to	O
the	O
last	O
Mode	O
line	O
,	O
it	O
is	O
possible	O
to	O
use	O
different	O
graphics	O
Modes	O
within	O
the	O
scrolling	O
region	O
.	O
</s>
<s>
This	O
merely	O
requires	O
additional	O
care	O
when	O
determining	O
how	O
many	O
scan	B-Algorithm
lines	I-Algorithm
can	O
be	O
shifted	O
for	O
the	O
current	O
first	O
and	O
last	O
Mode	O
lines	O
in	O
the	O
scrolling	O
region	O
.	O
</s>
<s>
Blank	O
Mode	O
lines	O
cannot	O
be	O
contained	O
within	O
a	O
scrolling	O
region	O
,	O
because	O
Blank	O
Mode	O
line	O
instructions	B-General_Concept
inherently	O
do	O
not	O
have	O
a	O
Vertical	O
Scroll	O
Modifier	O
bit	O
.	O
</s>
<s>
(	O
Blank	O
Mode	O
line	O
instructions	B-General_Concept
specify	O
the	O
blank	O
line	O
count	O
using	O
the	O
bits	O
7	O
through	O
5	O
that	O
function	O
as	O
LMS	O
,	O
HS	O
,	O
and	O
VS	O
Modifiers	O
for	O
"	O
regular	O
"	O
Playfield	O
Mode	O
instructions	B-General_Concept
.	O
)	O
</s>
<s>
However	O
,	O
a	O
Blank	O
Mode	O
instruction	O
can	O
be	O
used	O
to	O
end	O
a	O
scrolling	O
region	O
and	O
is	O
still	O
subject	O
to	O
the	O
expected	O
scan	B-Algorithm
line	I-Algorithm
height	O
changes	O
per	O
the	O
vertical	O
scroll	O
value	O
.	O
</s>
<s>
ANTIC	B-General_Concept
Modes	O
2	O
,	O
3	O
,	O
4	O
,	O
and	O
5	O
use	O
128	O
characters	O
in	O
the	O
character	O
set	O
and	O
require	O
the	O
CHBASE	O
value	O
begin	O
on	O
a	O
1K	O
boundary	O
.	O
</s>
<s>
ANTIC	B-General_Concept
Modes	O
6	O
and	O
7	O
use	O
64	O
characters	O
,	O
so	O
the	O
CHBASE	O
value	O
must	O
begin	O
on	O
a	O
512	O
byte	O
boundary	O
.	O
</s>
<s>
A	O
write	O
to	O
this	O
register	O
halts	O
the	O
6502	O
program	O
through	O
the	O
end	O
of	O
the	O
current	O
scanline	B-Algorithm
.	O
</s>
<s>
This	O
behavior	O
is	O
commonly	O
used	O
during	O
Display	B-General_Concept
List	I-General_Concept
Interrupts	I-General_Concept
to	O
produce	O
clean	O
transitions/changes	O
from	O
one	O
scanline	B-Algorithm
to	O
the	O
next	O
.	O
</s>
<s>
This	O
register	O
tracks	O
the	O
scan	B-Algorithm
line	I-Algorithm
currently	O
being	O
generated	O
.	O
</s>
<s>
The	O
value	O
returned	O
is	O
the	O
actual	O
scan	B-Algorithm
line	I-Algorithm
divided	O
by	O
2	O
.	O
</s>
<s>
This	O
value	O
is	O
useful	O
during	O
Display	B-General_Concept
List	I-General_Concept
Interrupts	I-General_Concept
to	O
identify	O
the	O
vertical	O
screen	O
position	O
.	O
</s>
<s>
If	O
NMIEN	O
's	O
DLI	O
bit	O
is	O
set	O
when	O
ANTIC	B-General_Concept
encounters	O
a	O
Display	O
List	O
instruction	O
with	O
the	O
DLI	O
modifier	O
bit	O
set	O
,	O
then	O
ANTIC	B-General_Concept
triggers	O
the	O
DLI	O
on	O
the	O
last	O
scan	B-Algorithm
line	I-Algorithm
of	O
that	O
Display	O
List	O
instruction	O
mode	O
line	O
.	O
</s>
<s>
The	O
last	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
routine	O
on	O
the	O
screen	O
points	O
VDSLST	O
back	O
to	O
the	O
address	O
of	O
the	O
first	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
.	O
</s>
<s>
When	O
NMIEN	O
's	O
VBI	O
bit	O
is	O
set	O
,	O
ANTIC	B-General_Concept
will	O
signals	O
a	O
Vertical	O
Blank	O
Interrupt	O
at	O
the	O
end	O
of	O
processing	O
the	O
JVB	O
(	O
Jump	O
vertical	O
blank	O
)	O
at	O
the	O
end	O
of	O
the	O
Display	O
List	O
.	O
</s>
<s>
The	O
display	O
list	O
is	O
the	O
list	O
of	O
instructions	B-General_Concept
directing	O
ANTIC	B-General_Concept
how	O
to	O
generate	O
the	O
display	O
.	O
</s>
<s>
The	O
kinds	O
of	O
graphics	O
in	O
the	O
output	O
(	O
text	O
vs	O
addressable	O
pixels	O
)	O
is	O
determined	O
by	O
the	O
instructions	B-General_Concept
in	O
the	O
Display	O
List	O
.	O
</s>
<s>
The	O
display	O
List	O
and	O
the	O
display	O
data	O
are	O
written	O
into	O
RAM	B-Architecture
by	O
the	O
CPU	O
.	O
</s>
<s>
ANTIC	B-General_Concept
reads	O
the	O
display	O
List	O
instructions	B-General_Concept
,	O
screen	O
memory	O
,	O
and	O
character	O
set	O
information	O
from	O
RAM	B-Architecture
using	O
a	O
technique	O
known	O
as	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
(	O
DMA	B-General_Concept
)	O
.	O
</s>
<s>
A	O
BASIC	O
or	O
6502	O
machine	O
language	O
program	O
's	O
job	O
is	O
to	O
initialize	O
the	O
display	O
—	O
set	O
up	O
the	O
display	O
List	O
instructions	B-General_Concept
,	O
organize	O
screen	O
memory	O
(	O
and	O
character	O
set	O
if	O
applicable	O
)	O
,	O
and	O
then	O
direct	O
ANTIC	B-General_Concept
to	O
start	O
the	O
display	O
.	O
</s>
<s>
After	O
this	O
,	O
ANTIC	B-General_Concept
automatically	O
takes	O
care	O
of	O
generating	O
the	O
screen	O
display	O
.	O
</s>
<s>
This	O
allows	O
the	O
Atari	B-Device
8-bit	I-Device
computers	I-Device
to	O
produce	O
complex	O
,	O
mixed-mode	O
displays	O
without	O
direct	O
CPU	O
intervention	O
.	O
</s>
<s>
ANTIC	B-General_Concept
processes	O
the	O
instructions	B-General_Concept
in	O
the	O
display	O
List	O
,	O
reads	O
the	O
screen	O
memory	O
(	O
and	O
character	O
set	O
data	O
if	O
applicable	O
)	O
,	O
translates	O
this	O
information	O
into	O
a	O
real-time	O
stream	O
of	O
graphics	O
data	O
,	O
and	O
sends	O
this	O
data	O
stream	O
to	O
the	O
CTIA/GTIA	B-General_Concept
chip	I-General_Concept
which	O
applies	O
the	O
color	O
to	O
the	O
graphics	O
pixels	O
and	O
outputs	O
the	O
video	O
.	O
</s>
<s>
The	O
more	O
advanced	O
version	O
,	O
GTIA	B-General_Concept
,	O
adds	O
three	O
alternative	O
color	O
interpretations	O
for	O
each	O
ANTIC	B-General_Concept
graphics	O
mode	O
providing	O
a	O
total	O
of	O
56	O
(	O
14	O
times	O
four	O
)	O
graphics	O
modes	O
.	O
</s>
<s>
However	O
,	O
only	O
the	O
ANTIC	B-General_Concept
graphics	O
modes	O
based	O
on	O
half-color	O
clock	O
pixels	O
are	O
capable	O
of	O
expressing	O
the	O
complete	O
color	O
palette	O
provided	O
by	O
the	O
new	O
color	O
interpretations	O
,	O
and	O
of	O
those	O
modes	O
the	O
ones	O
convenient	O
for	O
use	O
are	O
ANTIC	B-General_Concept
modes	O
2	O
(	O
OS	O
Graphics	O
mode	O
0	O
text	O
)	O
and	O
ANTIC	B-General_Concept
mode	O
F	O
(	O
OS	O
Graphics	O
mode	O
8	O
)	O
.	O
</s>
<s>
Thus	O
the	O
reasonable	O
number	O
of	O
unique	O
graphics	O
modes	O
available	O
using	O
the	O
inherent	O
hardware	O
capabilities	O
of	O
ANTIC	B-General_Concept
+	O
CTIA/GTIA	B-General_Concept
is	O
20	O
–	O
14	O
ANTIC	B-General_Concept
modes	O
+	O
3	O
additional	O
color	O
interpretations	O
each	O
for	O
ANTIC	B-General_Concept
modes	O
2	O
and	O
F	O
.	O
</s>
<s>
The	O
Atari	O
display	O
is	O
built	O
as	O
a	O
series	O
of	O
ANTIC	B-General_Concept
instructions	B-General_Concept
each	O
describing	O
a	O
line	O
of	O
text	O
or	O
graphics	O
mode	O
progressively	O
from	O
the	O
top	O
to	O
the	O
bottom	O
of	O
the	O
screen	O
until	O
the	O
desired	O
display	O
area	O
is	O
filled	O
.	O
</s>
<s>
In	O
effect	O
,	O
the	O
screen	O
is	O
a	O
vertical	O
stack	O
of	O
ANTIC	B-General_Concept
mode	O
instructions	B-General_Concept
.	O
</s>
<s>
Different	O
modes	O
occupy	O
different	O
numbers	O
of	O
scan	B-Algorithm
lines	I-Algorithm
and	O
use	O
different	O
amounts	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
Designing	O
a	O
display	O
requires	O
counting	O
the	O
scan	B-Algorithm
lines	I-Algorithm
of	O
each	O
Mode	O
instruction	O
and	O
tracking	O
the	O
memory	O
for	O
each	O
Mode	O
line	O
to	O
prevent	O
overrunning	O
ANTIC	B-General_Concept
or	O
video	O
standard	O
limitations	O
.	O
</s>
<s>
Mixing	O
multiple	O
graphics	O
modes	O
in	O
one	O
display	O
is	O
done	O
simply	O
by	O
providing	O
different	O
character	O
or	O
map	O
mode	O
instructions	B-General_Concept
for	O
the	O
desired	O
position	O
on	O
screen	O
.	O
</s>
<s>
This	O
split	O
screen	O
feature	O
is	O
accomplished	O
using	O
the	O
inherent	O
capabilities	O
of	O
ANTIC	B-General_Concept
and	O
its	O
Display	O
List	O
without	O
any	O
CPU	O
interrupts	O
or	O
custom	O
manipulation	O
of	O
the	O
video	O
hardware	O
.	O
</s>
<s>
ANTIC	B-General_Concept
has	O
four	O
types	O
of	O
instructions	B-General_Concept
:	O
</s>
<s>
Map	O
and	O
Character	O
mode	O
instructions	B-General_Concept
have	O
additional	O
modifiers	O
that	O
can	O
be	O
enabled	O
with	O
the	O
instruction	O
:	O
</s>
<s>
ANTIC	B-General_Concept
begins	O
executing	O
the	O
Display	O
List	O
pointed	O
to	O
by	O
the	O
16-bit	O
address	O
in	O
registers	O
DLISTL/DLISTH	O
( $	O
D402-	O
$	O
D403hex/54274	O
-54275dec	O
)	O
.	O
</s>
<s>
The	O
address	O
registers	O
are	O
incremented	O
by	O
ANTIC	B-General_Concept
automatically	O
as	O
each	O
instruction	O
is	O
executed	O
.	O
</s>
<s>
ANTIC	B-General_Concept
can	O
only	O
update	O
the	O
lowest	O
10	O
bits	O
of	O
the	O
address	O
during	O
the	O
automatic	O
increment	O
limiting	O
the	O
display	O
list	O
to	O
begin	O
and	O
end	O
within	O
a	O
1K	O
address	O
range	O
.	O
</s>
<s>
The	O
registers	O
are	O
also	O
updated	O
during	O
Display	O
List	O
execution	O
by	O
ANTIC	B-General_Concept
's	O
JMP	O
(	O
Jump	O
)	O
and	O
JVB	O
(	O
Jump	O
and	O
wait	O
for	O
Vertical	O
Blank	O
)	O
.	O
</s>
<s>
These	O
instructions	B-General_Concept
load	O
the	O
full	O
16-bits	O
of	O
address	O
,	O
so	O
can	O
be	O
used	O
to	O
circumvent	O
the	O
1K	O
Display	O
List	O
limitation	O
.	O
</s>
<s>
When	O
the	O
OS	O
VBI	O
routine	O
is	O
enabled	O
direct	O
updates	O
to	O
the	O
ANTIC	B-General_Concept
DLIST	O
registers	O
by	O
the	O
CPU	O
or	O
the	O
ANTIC	B-General_Concept
Jump	O
instructions	B-General_Concept
will	O
be	O
overwritten	O
by	O
the	O
OS	O
during	O
the	O
next	O
Vertical	O
Blank	O
.	O
</s>
<s>
Bits	O
7:4	O
are	O
modifiers	O
for	O
Playfield	O
Mode	O
instructions	B-General_Concept
in	O
bits	O
3:0	O
.	O
</s>
<s>
Values	O
,	O
and	O
are	O
special	O
instructions	B-General_Concept
.	O
</s>
<s>
Bit	O
7	O
-	O
-	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
.	O
</s>
<s>
When	O
Mode	O
bits	O
are	O
all	O
zero	O
ANTIC	B-General_Concept
performs	O
no	O
playfield	O
DMA	B-General_Concept
and	O
displays	O
one	O
or	O
more	O
blank	O
scan	O
line(s )	O
of	O
the	O
background	O
color	O
.	O
</s>
<s>
For	O
normal	O
color	O
interpretation	O
modes	O
the	O
"	O
background	O
"	O
is	O
color	O
register	O
COLBK	O
(	O
D01Ahex/53274dec	O
)	O
OS	O
Shadow	O
COLOR4	O
(	O
02C8hex/712dec	O
)	O
,	O
though	O
this	O
differs	O
for	O
the	O
GTIA	B-General_Concept
color	O
interpretation	O
modes	O
.	O
</s>
<s>
See	O
discussion	O
of	O
COLBK	O
on	O
the	O
GTIA	B-General_Concept
page	I-General_Concept
for	O
more	O
information	O
.	O
</s>
<s>
The	O
number	O
of	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
are	O
specified	O
by	O
the	O
value	O
of	O
Bits	O
4	O
through	O
Bits	O
6	O
allowing	O
a	O
range	O
from	O
0	O
through	O
7	O
.	O
</s>
<s>
ANTIC	B-General_Concept
adds	O
one	O
to	O
this	O
value	O
and	O
displays	O
the	O
resulting	O
number	O
of	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
ANTIC	B-General_Concept
Instruction	O
Scan	B-Algorithm
Lines	I-Algorithm
00hex/0dec	O
1	O
blank	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
10hex/16dec	O
2	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
20hex/32dec	O
3	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
30hex/48dec	O
4	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
40hex/64dec	O
5	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
50hex/80dec	O
6	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
60hex/96dec	O
7	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
70hex/112dec	O
8	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
The	O
Atari	O
Operating	O
System	O
creates	O
all	O
its	O
graphics	O
modes	O
beginning	O
with	O
three	O
70hex/112dec	O
instructions	B-General_Concept
(	O
or	O
24	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
)	O
for	O
this	O
purpose	O
.	O
</s>
<s>
Blank	O
Mode	O
lines	O
cannot	O
be	O
contained	O
within	O
a	O
scrolling	O
region	O
,	O
because	O
Blank	O
Mode	O
line	O
instructions	B-General_Concept
inherently	O
do	O
not	O
have	O
Fine	O
Scrolling	O
Modifier	O
bits	O
.	O
</s>
<s>
However	O
,	O
a	O
Blank	O
Mode	O
instruction	O
can	O
be	O
used	O
to	O
end	O
a	O
vertical	O
scrolling	O
region	O
and	O
is	O
still	O
subject	O
to	O
the	O
expected	O
scan	B-Algorithm
line	I-Algorithm
height	O
changes	O
per	O
the	O
vertical	O
scroll	O
value	O
.	O
</s>
<s>
The	O
DLI	O
modifier	O
bit	O
is	O
available	O
for	O
blank	O
line	O
instructions	B-General_Concept
.	O
</s>
<s>
When	O
the	O
DLI	O
bit	O
is	O
enabled	O
a	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
will	O
be	O
triggered	O
during	O
the	O
last	O
blank	O
scan	B-Algorithm
line	I-Algorithm
of	O
the	O
instruction	O
.	O
</s>
<s>
Since	O
playfield	O
DMA	B-General_Concept
does	O
not	O
occur	O
during	O
blank	O
lines	O
(	O
only	O
Display	O
list	O
DMA	B-General_Concept
and	O
Player/Missile	O
DMA	B-General_Concept
(	O
if	O
enabled	O
)	O
occurs	O
)	O
the	O
blank	O
lines	O
can	O
be	O
used	O
where	O
Playfield	O
graphics	O
will	O
not	O
be	O
needed	O
giving	O
more	O
time	O
for	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
routines	O
changing	O
color	O
registers	O
or	O
Player/Missile	O
positions	O
.	O
</s>
<s>
When	O
Mode	O
value	O
is	O
1	O
ANTIC	B-General_Concept
executes	O
a	O
Jump	O
in	O
its	O
program	O
(	O
the	O
Display	O
List	O
)	O
.	O
</s>
<s>
The	O
next	O
two	O
bytes	O
in	O
the	O
Display	O
List	O
are	O
loaded	O
into	O
ANTIC	B-General_Concept
registers	O
DLISTL/DLISTH	O
( $	O
D402-	O
$	O
D403hex/54274	O
-54275dec	O
)	O
changing	O
the	O
execution	O
point	O
of	O
the	O
next	O
instruction	O
.	O
</s>
<s>
When	O
the	O
OS	O
VBI	O
routine	O
is	O
enabled	O
direct	O
updates	O
to	O
the	O
ANTIC	B-General_Concept
DLIST	O
registers	O
by	O
the	O
CPU	O
or	O
the	O
ANTIC	B-General_Concept
Jump	O
instructions	B-General_Concept
will	O
be	O
overwritten	O
by	O
the	O
OS	O
during	O
the	O
next	O
Vertical	O
Blank	O
.	O
</s>
<s>
This	O
implies	O
that	O
an	O
ANTIC	B-General_Concept
JVB	O
(	O
Jump	O
and	O
wait	O
for	O
Vertical	O
Blank	O
)	O
instruction	O
that	O
specifies	O
an	O
invalid	O
address	O
(	O
or	O
a	O
valid	O
address	O
that	O
differs	O
from	O
the	O
shadow	O
SDLST	O
value	O
)	O
has	O
no	O
real	O
effect	O
on	O
the	O
display	O
.	O
</s>
<s>
Of	O
course	O
,	O
to	O
give	O
ANTIC	B-General_Concept
control	O
over	O
the	O
start	O
of	O
the	O
next	O
Display	O
List	O
the	O
OS	O
Vertical	O
Blank	O
interrupt	O
must	O
be	O
disabled	O
to	O
prevent	O
it	O
overwriting	O
ANTIC	B-General_Concept
hardware	O
registers	O
with	O
the	O
shadow	O
register	O
values	O
.	O
</s>
<s>
The	O
following	O
is	O
the	O
list	O
of	O
ANTIC	B-General_Concept
graphics	O
mode	O
instructions	B-General_Concept
.	O
</s>
<s>
Bits	O
7:4	O
are	O
modifiers	O
for	O
Playfield	O
Mode	O
instructions	B-General_Concept
in	O
bits	O
3:0	O
.	O
</s>
<s>
Bit	O
7	O
-	O
$	O
80hex/128dec	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
.	O
</s>
<s>
When	O
horizontal	O
scrolling	O
is	O
enabled	O
ANTIC	B-General_Concept
retrieves	O
more	O
screen	O
memory	O
bytes	O
than	O
displayed	O
in	O
order	O
to	O
show	O
partially	O
scrolled	O
display	O
bytes	O
at	O
the	O
beginning	O
and	O
end	O
of	O
the	O
line	O
.	O
</s>
<s>
If	O
the	O
display	O
is	O
in	O
Narrow	O
mode	O
ANTIC	B-General_Concept
retrieves	O
the	O
number	O
of	O
screen	O
bytes	O
necessary	O
for	O
Normal	O
mode	O
.	O
</s>
<s>
If	O
the	O
display	O
is	O
in	O
Normal	O
mode	O
ANTIC	B-General_Concept
retrieves	O
the	O
number	O
of	O
bytes	O
needed	O
for	O
Wide	O
screen	O
mode	O
.	O
</s>
<s>
In	O
Wide	O
screen	O
mode	O
ANTIC	B-General_Concept
does	O
not	O
retrieve	O
any	O
additional	O
screen	O
bytes	O
.	O
</s>
<s>
The	O
Vertical	O
scrolling	O
region	O
in	O
a	O
display	O
is	O
defined	O
by	O
setting	O
the	O
VS	O
bit	O
( $20	O
)	O
on	O
a	O
sequential	O
series	O
of	O
ANTIC	B-General_Concept
Mode	O
Lines	O
in	O
the	O
Display	O
List	O
.	O
</s>
<s>
The	O
LMS	O
option	O
loads	O
ANTIC	B-General_Concept
's	O
screen	O
memory	O
scan	O
address	O
with	O
the	O
16-bit	O
value	O
in	O
the	O
two	O
bytes	O
following	O
the	O
instruction	O
.	O
</s>
<s>
The	O
LMS	O
option	O
must	O
appear	O
in	O
the	O
first	O
Playfield	O
Mode	O
Instruction	O
line	O
(	O
not	O
Blank	O
Line	O
)	O
in	O
the	O
Display	O
List	O
to	O
initialize	O
ANTIC	B-General_Concept
to	O
begin	O
reading	O
at	O
the	O
start	O
of	O
screen	O
memory	O
.	O
</s>
<s>
ANTIC	B-General_Concept
will	O
automatically	O
increment	O
its	O
LMS	O
address	O
at	O
the	O
end	O
of	O
each	O
mode	O
line	O
in	O
preparation	O
for	O
the	O
next	O
line	O
.	O
</s>
<s>
As	O
long	O
as	O
screen	O
memory	O
does	O
not	O
cross	O
over	O
a	O
4K	O
boundary	O
the	O
LMS	O
modifier	O
is	O
not	O
needed	O
on	O
subsequent	O
Playfield	O
Mode	O
Instructions	B-General_Concept
.	O
</s>
<s>
(	O
Mode	O
F	O
displays	O
include	O
high	O
res	O
320	O
pixel	O
lines	O
,	O
and	O
the	O
GTIA	B-General_Concept
display	O
modes	O
.	O
)	O
</s>
<s>
Combining	O
the	O
Map	O
and	O
Character	O
Mode	O
instructions	B-General_Concept
with	O
the	O
LMS	O
bit	O
makes	O
it	O
possible	O
to	O
set	O
the	O
screen	O
memory	O
address	O
freely	O
within	O
the	O
64K	O
address	O
space	O
independently	O
for	O
each	O
display	O
line	O
.	O
</s>
<s>
Bit7	O
:	O
position	O
value	O
,	O
directs	O
ANTIC	B-General_Concept
to	O
invoke	O
a	O
CPU-serviced	O
interrupt	O
on	O
the	O
last	O
scan	B-Algorithm
line	I-Algorithm
of	O
the	O
Playfield	O
Mode	O
.	O
</s>
<s>
If	O
ANTIC	B-General_Concept
register	O
NMIEN	O
(	O
hex/54286dec	O
)	O
has	O
Bit7	O
(	O
)	O
set	O
then	O
on	O
the	O
last	O
scanline	B-Algorithm
of	O
the	O
Mode	O
line	O
an	O
interrupt	O
routine	O
will	O
be	O
triggered	O
which	O
is	O
vectored	O
through	O
address	O
VDSLST	O
(	O
200hex/512dec	O
)	O
.	O
</s>
<s>
As	O
long	O
as	O
the	O
DLI	O
routine	O
exits	O
before	O
the	O
start	O
of	O
the	O
next	O
DLI	O
routine	O
the	O
DLI	O
routine	O
can	O
continue	O
updating	O
values	O
across	O
multiple	O
scanlines	B-Algorithm
.	O
</s>
<s>
The	O
ANTIC	B-General_Concept
chip	O
allows	O
a	O
variety	O
of	O
different	O
Playfield	O
modes	O
and	O
widths	O
.	O
</s>
<s>
ANTIC	B-General_Concept
Playfield	O
modes	O
are	O
exposed	O
to	O
users	O
through	O
Atari	O
BASIC	O
via	O
the	O
"	O
GRAPHICS	O
"	O
command	O
,	O
and	O
to	O
some	O
other	O
languages	O
,	O
via	O
similar	O
system	O
calls	O
.	O
</s>
<s>
The	O
later	O
version	O
of	O
the	O
OS	O
used	O
in	O
the	O
Atari	B-Device
8-bit	I-Device
XL/XE	O
computers	O
added	O
support	O
for	O
most	O
of	O
these	O
"	O
missing	O
"	O
graphics	O
modes	O
.	O
</s>
<s>
The	O
ANTIC	B-General_Concept
chip	O
uses	O
a	O
Display	O
List	O
and	O
other	O
settings	O
to	O
create	O
these	O
modes	O
.	O
</s>
<s>
Any	O
graphics	O
mode	O
in	O
the	O
default	O
GTIA	B-General_Concept
color	O
interpretation	O
can	O
be	O
freely	O
mixed	O
without	O
CPU	O
intervention	O
by	O
changing	O
instructions	B-General_Concept
in	O
the	O
Display	O
List	O
.	O
</s>
<s>
The	O
ANTIC	B-General_Concept
screen	O
geometry	O
is	O
not	O
fixed	O
.	O
</s>
<s>
A	O
hardware	O
register	O
can	O
direct	O
ANTIC	B-General_Concept
to	O
display	O
narrow	O
playfield	O
(	O
128	O
color	O
clocks/256	O
hi-res	O
pixels	O
wide	O
)	O
,	O
normal	O
width	O
playfield	O
(	O
160	O
color	O
clocks/320	O
hi-res	O
pixels	O
wide	O
)	O
,	O
and	O
wide	O
,	O
overscan	O
playfield	O
(	O
192	O
color	O
clocks/384	O
hi-res	O
pixels	O
wide	O
)	O
.	O
</s>
<s>
While	O
the	O
Operating	O
System	O
's	O
default	O
height	O
for	O
graphics	O
modes	O
is	O
192	O
scan	B-Algorithm
lines	I-Algorithm
Antic	B-General_Concept
can	O
display	O
vertical	O
overscan	O
up	O
to	O
240	O
TV	O
scan	B-Algorithm
lines	I-Algorithm
tall	O
by	O
creating	O
a	O
custom	O
Display	O
List	O
.	O
</s>
<s>
Mode	O
F	O
produces	O
colors	O
through	O
NTSC	O
artifacting	O
and	O
works	O
in	O
a	O
similar	O
manner	O
to	O
the	O
Apple	B-Device
II	I-Device
's	O
graphics	O
modes	O
.	O
</s>
<s>
The	O
exact	O
colors	O
produced	O
this	O
way	O
vary	O
with	O
different	O
Atari	B-Device
8-bit	I-Device
models	O
and	O
also	O
between	O
the	O
CTIA	B-General_Concept
and	I-General_Concept
GTIA	I-General_Concept
chips	O
,	O
thus	O
undesirable	O
artifact	O
colors	O
may	O
result	O
from	O
running	O
software	O
that	O
uses	O
Mode	O
F	O
on	O
a	O
machine	O
other	O
than	O
the	O
one	O
it	O
was	O
developed	O
for	O
.	O
</s>
<s>
Mode	O
F	O
was	O
not	O
widely	O
used	O
by	O
software	O
due	O
to	O
its	O
high	O
memory	O
requirements	O
(	O
8k	O
)	O
and	O
inconsistent	O
artifact	O
colors	O
that	O
did	O
not	O
appear	O
the	O
same	O
on	O
every	O
model	O
of	O
Atari	B-Device
8-bit	I-Device
computer	I-Device
.	O
</s>
<s>
Games	O
that	O
utilize	O
it	O
include	O
Sierra	O
graphical	O
adventures	O
such	O
as	O
Troll	O
's	O
Tale	O
and	O
Wizard	O
and	O
the	O
Princess	O
,	O
Lode	B-Application
Runner	I-Application
,	O
and	O
the	O
Ultima	O
series	O
.	O
</s>
<s>
Antic	B-General_Concept
Text	O
Mode	O
OS	O
mode	O
Characters	O
(	O
or	O
Bytes	O
)	O
Per	O
Mode	O
Line	O
(	O
narrow/normal/wide	O
)	O
TV	O
Scan	B-Algorithm
Lines	I-Algorithm
per	O
Mode	O
Line	O
Colors	O
Colors	O
per	O
Character	O
Matrix	O
Characters	O
in	O
Font	O
Matrix	O
Pixel	O
Size	O
(	O
Color	O
Clocks	O
x	O
Scan	B-Algorithm
Lines	I-Algorithm
)	O
Bits	O
Per	O
Matrix	O
Pixel	O
Matrix	O
Map	O
(	O
Color	O
Clocks	O
x	O
Scan	B-Algorithm
Lines	I-Algorithm
)	O
Matrix	O
Map	O
(	O
Pixels	O
x	O
Pixels	O
)	O
Notes	O
2	O
0	O
32/40/48	O
8	O
1.5	O
1	O
128	O
1/2	O
x	O
1	O
1	O
4	O
x	O
8	O
8	O
x	O
8	O
High-res	O
pixels	O
.	O
</s>
<s>
Lowercase	O
characters	O
are	O
displayed	O
2	O
scan	B-Algorithm
lines	I-Algorithm
lower	O
allowing	O
descenders	O
.	O
</s>
<s>
5	O
13	O
(	O
XL	O
OS	O
)	O
32/40/48	O
16	O
5	O
4	O
128	O
1	O
x	O
2	O
2	O
4	O
x	O
16	O
4	O
x	O
8	O
Color	O
same	O
as	O
above	O
Antic	B-General_Concept
Mode	O
4	O
.	O
</s>
<s>
7	O
2	O
16/20/24	O
16	O
5	O
1	O
64	O
1	O
x	O
2	O
1	O
8	O
x	O
16	O
8	O
x	O
8	O
Color	O
same	O
as	O
above	O
Antic	B-General_Concept
Mode	O
6	O
.	O
</s>
<s>
Standard	O
:	O
ANTIC	B-General_Concept
Mode	O
2	O
.	O
</s>
<s>
The	O
character	O
set	O
uses	O
128	O
characters	O
and	O
requires	O
1024	O
bytes	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
Descenders	O
:	O
ANTIC	B-General_Concept
Mode	O
3	O
.	O
</s>
<s>
Each	O
mode	O
line	O
is	O
10	O
scanlines	B-Algorithm
tall	O
,	O
though	O
the	O
character	O
glyph	O
is	O
still	O
8	O
bytes	O
.	O
</s>
<s>
Characters	O
hex/0dec	O
to	O
hex/95dec	O
display	O
in	O
the	O
top	O
8	O
scan	B-Algorithm
lines	I-Algorithm
with	O
two	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
added	O
at	O
the	O
bottom	O
.	O
</s>
<s>
Characters	O
hex/96dec	O
to	O
hex/127dec	O
begin	O
with	O
two	O
blank	O
scan	B-Algorithm
lines	I-Algorithm
at	O
the	O
top	O
,	O
and	O
then	O
display	O
the	O
glyph	O
bytes	O
in	O
the	O
bottom	O
8	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
The	O
first	O
two	O
bytes	O
in	O
the	O
glyph	O
are	O
moved	O
to	O
the	O
bottom	O
two	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
The	O
character	O
set	O
uses	O
128	O
characters	O
and	O
requires	O
1024	O
bytes	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
Single	O
Color	O
:	O
ANTIC	B-General_Concept
Modes	O
6	O
and	O
7	O
.	O
</s>
<s>
Since	O
the	O
character	O
value	O
is	O
reduced	O
to	O
six	O
bits	O
the	O
character	O
set	O
uses	O
64	O
characters	O
and	O
requires	O
512	O
bytes	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
Multi-Color	O
:	O
ANTIC	B-General_Concept
Modes	O
4	O
and	O
5	O
.	O
</s>
<s>
The	O
character	O
set	O
uses	O
128	O
characters	O
and	O
requires	O
1024	O
bytes	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
GTIA	B-General_Concept
modes	O
are	O
Antic	B-General_Concept
Mode	O
F	O
displays	O
with	O
an	O
alternate	O
color	O
interpretation	O
option	O
enabled	O
via	O
GTIA	B-General_Concept
register	O
PRIOR	O
( $	O
D01Bhex/53275dec	O
)	O
shadowed	O
by	O
GPRIOR	O
( $	O
026Fhex/623dec	O
)	O
.	O
</s>
<s>
The	O
full	O
color	O
expression	O
of	O
these	O
GTIA	B-General_Concept
modes	O
can	O
also	O
be	O
engaged	O
in	O
Antic	B-General_Concept
text	O
modes	O
2	O
and	O
3	O
,	O
though	O
these	O
will	O
also	O
require	O
a	O
custom	O
character	O
set	O
to	O
achieve	O
practical	O
use	O
of	O
the	O
colors	O
.	O
</s>
<s>
See	O
GTIA	B-General_Concept
for	O
more	O
information	O
.	O
</s>
<s>
Antic	B-General_Concept
Map	O
Mode	O
OS	O
Mode	O
Pixels	O
Per	O
Mode	O
Line	O
(	O
narrow/normal/wide	O
)	O
TV	O
Scan	B-Algorithm
Lines	I-Algorithm
per	O
Mode	O
Line	O
Bytes	O
per	O
Mode	O
Line	O
(	O
narrow/normal/wide	O
)	O
Bits	O
Per	O
Pixel	O
Colors	O
Color	O
Clocks	O
per	O
Pixel	O
Notes	O
F	O
9	O
64/80/96	O
1	O
32/40/48	O
4	O
16*	O
2	O
16	O
shades	O
of	O
the	O
background	O
color	O
.	O
</s>
<s>
Most	O
computers	O
including	O
the	O
Atari	B-Device
8-bits	I-Device
accomplish	O
coarse	O
scrolling	O
as	O
described	O
above	O
—	O
common	O
examples	O
are	O
viewing	O
a	O
long	O
BASIC	O
program	O
listing	O
or	O
directory	O
of	O
files	O
.	O
</s>
<s>
However	O
,	O
ANTIC	B-General_Concept
provides	O
direct	O
hardware	O
support	O
for	O
vertical	O
and	O
horizontal	O
,	O
coarse	O
and	O
fine	O
scrolling	O
taking	O
the	O
concept	O
of	O
a	O
"	O
view	O
port	O
moving	O
over	O
data	O
"	O
from	O
a	O
metaphorical	O
illusion	O
to	O
literal	O
implementation	O
.	O
</s>
<s>
These	O
ANTIC	B-General_Concept
features	O
deliver	O
rapid	O
,	O
smooth	O
,	O
full	O
screen	O
movement	O
requiring	O
negligible	O
CPU	O
time	O
and	O
so	O
free	O
the	O
CPU	O
for	O
other	O
work	O
.	O
</s>
<s>
All	O
ANTIC	B-General_Concept
display	O
modes	O
,	O
Text	O
and	O
Map	O
modes	O
,	O
can	O
exercise	O
the	O
hardware	O
scrolling	O
features	O
.	O
</s>
<s>
As	O
it	O
generates	O
the	O
display	O
,	O
ANTIC	B-General_Concept
automatically	O
increments	O
its	O
memory	O
scan	O
pointer	O
from	O
Mode	O
line	O
to	O
Mode	O
line	O
to	O
display	O
memory	O
contiguously	O
.	O
</s>
<s>
Since	O
ANTIC	B-General_Concept
's	O
memory	O
scan	O
counter	O
cannot	O
automatically	O
increment	O
over	O
a	O
4K	O
boundary	O
the	O
limit	O
of	O
vertical	O
scrolling	O
in	O
this	O
simple	O
manner	O
is	O
up	O
to	O
4K	O
of	O
movement	O
.	O
</s>
<s>
This	O
problem	O
is	O
solved	O
by	O
the	O
inherent	O
capabilities	O
of	O
the	O
ANTIC	B-General_Concept
Display	O
List	O
.	O
</s>
<s>
In	O
this	O
situation	O
the	O
Display	O
List	O
would	O
add	O
Mode	O
instructions	B-General_Concept
using	O
LMS	O
modifiers	O
on	O
the	O
first	O
mode	O
line	O
of	O
the	O
screen	O
sections	O
that	O
should	O
not	O
move	O
which	O
effectively	O
locks	O
these	O
parts	O
of	O
the	O
display	O
while	O
the	O
LMS	O
modifier	O
address	O
for	O
the	O
scrolling	O
section	O
is	O
updated	O
.	O
</s>
<s>
When	O
the	O
display	O
is	O
managed	O
at	O
this	O
level	O
of	O
detail	O
ANTIC	B-General_Concept
's	O
4K	O
screen	O
RAM	B-Architecture
boundary	O
can	O
be	O
easily	O
circumvented	O
.	O
</s>
<s>
One	O
other	O
limit	O
to	O
be	O
aware	O
of	O
is	O
that	O
the	O
two	O
byte	O
LMS	O
modifier	O
address	O
must	O
not	O
be	O
updated	O
when	O
ANTIC	B-General_Concept
is	O
processing	O
the	O
LMS	O
.	O
</s>
<s>
If	O
ANTIC	B-General_Concept
reads	O
the	O
LMS	O
address	O
while	O
it	O
is	O
partially	O
updated	O
ANTIC	B-General_Concept
will	O
display	O
the	O
incorrect	O
section	O
of	O
memory	O
for	O
that	O
Mode	O
line	O
.	O
</s>
<s>
Programs	O
solve	O
this	O
in	O
a	O
number	O
of	O
ways	O
—	O
performing	O
updates	O
during	O
the	O
Vertical	O
blank	O
,	O
monitoring	O
the	O
VCOUNT	O
before	O
updates	O
,	O
or	O
doing	O
the	O
updates	O
in	O
Display	B-General_Concept
List	I-General_Concept
Interrupts	I-General_Concept
executed	O
during	O
sections	O
of	O
the	O
display	O
away	O
from	O
the	O
scrolling	O
region	O
.	O
</s>
<s>
Vertical	O
Fine	O
Scrolling	O
allows	O
single	O
scan	B-Algorithm
line	I-Algorithm
movement	O
of	O
the	O
scrolling	O
region	O
for	O
Text	O
and	O
Map	O
modes	O
which	O
are	O
greater	O
than	O
one	O
scan	B-Algorithm
line	I-Algorithm
tall	O
.	O
</s>
<s>
Vertical	O
Fine	O
Scrolling	O
for	O
Map	O
modes	O
only	O
one	O
scan	B-Algorithm
line	I-Algorithm
tall	O
is	O
logically	O
not	O
practical	O
.	O
</s>
<s>
The	O
effect	O
of	O
vertical	O
"	O
fine	O
"	O
scrolling	O
for	O
single	O
scan	B-Algorithm
line	I-Algorithm
Map	O
modes	O
can	O
be	O
performed	O
using	O
the	O
easier	O
coarse	O
scrolling	O
method	O
.	O
</s>
<s>
This	O
is	O
done	O
by	O
setting	O
the	O
Vertical	O
Scroll	O
(	O
VS	O
)	O
modifier	O
bit	O
in	O
sequential	O
Display	O
List	O
instructions	B-General_Concept
.	O
</s>
<s>
The	O
first	O
Mode	O
line	O
instruction	O
without	O
the	O
VS	O
bit	O
set	O
becomes	O
the	O
end	O
of	O
the	O
scrolling	O
region	O
and	O
is	O
used	O
as	O
a	O
buffer	O
line	O
to	O
supply	O
the	O
new	O
scan	B-Algorithm
lines	I-Algorithm
to	O
scroll	O
up	O
into	O
the	O
bottom	O
of	O
the	O
scrolling	O
region	O
.	O
</s>
<s>
ANTIC	B-General_Concept
's	O
process	O
of	O
vertical	O
fine	O
scrolling	O
works	O
by	O
simply	O
directing	O
ANTIC	B-General_Concept
to	O
skip	O
a	O
number	O
of	O
scan	B-Algorithm
lines	I-Algorithm
for	O
the	O
first	O
mode	O
line	O
of	O
the	O
scrolling	O
region	O
.	O
</s>
<s>
The	O
last	O
line	O
of	O
the	O
region	O
supplies	O
the	O
bottom	O
border	O
of	O
the	O
scrolling	O
region	O
displaying	O
the	O
number	O
of	O
scan	B-Algorithm
lines	I-Algorithm
that	O
were	O
removed	O
from	O
the	O
first	O
line	O
.	O
</s>
<s>
This	O
number	O
of	O
scan	B-Algorithm
lines	I-Algorithm
is	O
controlled	O
with	O
the	O
VSCROL	O
register	O
.	O
</s>
<s>
The	O
value	O
ranges	O
from	O
0	O
to	O
the	O
ANTIC	B-General_Concept
Mode	O
line	O
's	O
scan	B-Algorithm
line	I-Algorithm
height-1	O
.	O
</s>
<s>
The	O
maximum	O
value	O
range	O
covers	O
fine	O
scrolling	O
16	O
scan	B-Algorithm
lines	I-Algorithm
from	O
0	O
to	O
15	O
.	O
</s>
<s>
While	O
horizontal	O
scrolling	O
is	O
expected	O
to	O
present	O
the	O
illusion	O
of	O
a	O
view	O
port	O
moving	O
left	O
and	O
right	O
across	O
a	O
wide	O
panoramic	O
scene	O
made	O
of	O
screen	O
memory	O
,	O
ANTIC	B-General_Concept
's	O
automatic	O
memory	O
scan	O
increment	O
conflicts	O
with	O
this	O
idea	O
that	O
the	O
rows	O
of	O
screen	O
data	O
is	O
wider	O
than	O
the	O
display	O
.	O
</s>
<s>
The	O
other	O
benefit	O
of	O
this	O
arrangement	O
is	O
that	O
it	O
is	O
easy	O
to	O
manage	O
screen	O
memory	O
around	O
ANTIC	B-General_Concept
's	O
4K	O
screen	O
memory	O
boundary	O
limit	O
.	O
</s>
<s>
ANTIC	B-General_Concept
Modes	O
using	O
the	O
alternate	O
GTIA	B-General_Concept
color	O
interpretations	O
must	O
be	O
scrolled	O
by	O
an	O
entire	O
GTIA	B-General_Concept
pixel	O
(	O
two	O
color	O
clocks	O
)	O
.	O
</s>
<s>
Only	O
even	O
values	O
should	O
be	O
used	O
to	O
ensure	O
correct	O
GTIA	B-General_Concept
pixel	O
scrolling	O
.	O
</s>
<s>
Odd	O
values	O
of	O
HSCROL	O
will	O
shift	O
the	O
GTIA	B-General_Concept
pixel	O
stream	O
into	O
a	O
different	O
state	O
that	O
will	O
be	O
interpreted	O
as	O
different	O
colors	O
.	O
</s>
<s>
The	O
next	O
step	O
is	O
setting	O
the	O
Horizontal	O
Scroll	O
(	O
HS	O
)	O
modifier	O
bit	O
in	O
all	O
Display	O
List	O
Mode	O
line	O
instructions	B-General_Concept
for	O
the	O
scrolling	O
region	O
.	O
</s>
<s>
The	O
color	O
clock	O
information	O
used	O
is	O
buffered	O
from	O
the	O
beginning	O
of	O
the	O
mode	O
line	O
(	O
ANTIC	B-General_Concept
's	O
current	O
memory	O
scan	O
pointer	O
)	O
using	O
enough	O
screen	O
memory	O
bytes	O
to	O
satisfy	O
the	O
16	O
color	O
clocks	O
.	O
</s>
<s>
Horizontal	O
scrolling	O
simply	O
directs	O
ANTIC	B-General_Concept
how	O
many	O
color	O
clocks	O
it	O
can	O
output	O
from	O
the	O
buffered	O
16	O
color	O
clocks	O
starting	O
at	O
the	O
last	O
(	O
right	O
most	O
)	O
color	O
clock	O
of	O
the	O
buffered	O
memory	O
.	O
</s>
<s>
ANTIC	B-General_Concept
's	O
16	O
color	O
clock	O
range	O
permits	O
fine	O
scrolling	O
multiple	O
Text	O
Mode	O
characters	O
or	O
more	O
than	O
one	O
byte	O
worth	O
of	O
Map	O
mode	O
pixels	O
.	O
</s>
<s>
An	O
interesting	O
use	O
of	O
ANTIC	B-General_Concept
's	O
DMA	B-General_Concept
memory	O
scan	O
behavior	O
permits	O
a	O
strategy	O
for	O
apparently	O
long	O
horizontal	O
scrolling	O
distances	O
,	O
but	O
uses	O
a	O
fraction	O
of	O
the	O
real	O
memory	O
required	O
.	O
</s>
<s>
As	O
discussed	O
in	O
Horizontal	O
Coarse	O
Scrolling	O
ANTIC	B-General_Concept
's	O
automatic	O
memory	O
scan	O
increment	O
from	O
Mode	O
line	O
to	O
Mode	O
line	O
conflicts	O
with	O
the	O
idea	O
that	O
the	O
rows	O
of	O
screen	O
data	O
are	O
wider	O
than	O
the	O
display	O
.	O
</s>
<s>
So	O
,	O
if	O
this	O
scrolling	O
process	O
involves	O
a	O
full	O
screen	O
Mode	O
2	O
or	O
Mode	O
4	O
text	O
display	O
(	O
rather	O
worst	O
case	O
)	O
and	O
uses	O
the	O
entire	O
4K	O
capability	O
of	O
ANTIC	B-General_Concept
's	O
automatic	O
screen	O
memory	O
addressing	O
,	O
then	O
the	O
horizontal	O
scrolling	O
can	O
continue	O
in	O
this	O
manner	O
for	O
about	O
3,000	O
steps	O
;	O
equivalent	O
to	O
75	O
full	O
screens	O
.	O
</s>
<s>
When	O
fine	O
scrolling	O
is	O
enabled	O
in	O
the	O
Display	O
List	O
ANTIC	B-General_Concept
retrieves	O
more	O
information	O
than	O
the	O
Mode	O
line	O
requires	O
for	O
display	O
.	O
</s>
<s>
ANTIC	B-General_Concept
includes	O
a	O
feature	O
to	O
trigger	O
a	O
CPU-serviced	O
interrupt	O
at	O
specific	O
vertical	O
positions	O
on	O
the	O
screen	O
.	O
</s>
<s>
This	O
is	O
built	O
into	O
the	O
ANTIC	B-General_Concept
Display	O
List	O
which	O
directs	O
ANTIC	B-General_Concept
to	O
launch	O
the	O
interrupt	O
during	O
the	O
last	O
scan	B-Algorithm
line	I-Algorithm
of	O
the	O
current	O
Mode	O
instruction	O
,	O
and	O
so	O
is	O
called	O
"	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
"	O
.	O
</s>
<s>
Other	O
platforms	O
call	O
this	O
activity	O
,	O
"	O
Raster	B-General_Concept
interrupts	I-General_Concept
"	O
,	O
or	O
"	O
Horizontal	B-General_Concept
blank	I-General_Concept
interrupts	I-General_Concept
"	O
.	O
</s>
<s>
Since	O
the	O
DLI	O
is	O
6502	O
machine	O
code	O
executed	O
by	O
the	O
CPU	O
,	O
any	O
kind	O
of	O
processing	O
work	O
is	O
possible	O
provided	O
it	O
is	O
short	O
enough	O
to	O
not	O
conflict	O
with	O
other	O
activity	O
and	O
does	O
not	O
overrun	O
a	O
subsequent	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
.	O
</s>
<s>
A	O
Display	O
List	O
with	O
a	O
Mode	O
instruction	O
that	O
includes	O
the	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
(	O
DLI	O
)	O
modifier	O
bit	O
set	O
.	O
</s>
<s>
ANTIC	B-General_Concept
's	O
NMIEN	O
register	O
has	O
the	O
DLI	O
bit	O
set	O
.	O
</s>
<s>
The	O
Operating	O
System	O
vector	O
for	O
the	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
(	O
VDSLST	O
,	O
$	O
0200hex/512dec	O
)	O
points	O
to	O
the	O
starting	O
address	O
of	O
the	O
interrupt	O
routine	O
.	O
</s>
<s>
When	O
updating	O
display	O
control	O
values	O
care	O
must	O
be	O
taken	O
to	O
ensure	O
ANTIC	B-General_Concept
is	O
not	O
currently	O
using	O
them	O
.	O
</s>
<s>
If	O
ANTIC	B-General_Concept
is	O
processing	O
the	O
Display	O
List	O
containing	O
an	O
Instruction	O
with	O
the	O
DLI	O
modifier	O
,	O
then	O
an	O
interrupt	O
could	O
be	O
dispatched	O
while	O
changing	O
the	O
VDSLST	O
address	O
,	O
potentially	O
crashing	O
the	O
system	O
.	O
</s>
<s>
If	O
a	O
Display	O
List	O
uses	O
multiple	O
interrupts	O
,	O
and	O
interrupts	O
are	O
enabled	O
in	O
NMIEN	O
after	O
ANTIC	B-General_Concept
has	O
already	O
passed	O
Mode	O
Instructions	B-General_Concept
with	O
DLI	O
modifiers	O
,	O
then	O
the	O
Display	B-General_Concept
List	I-General_Concept
Interrupts	I-General_Concept
will	O
be	O
started	O
out	O
of	O
order	O
,	O
causing	O
unexpected	O
results	O
.	O
</s>
<s>
A	O
reasonable	O
sequence	O
of	O
events	O
to	O
install	O
a	O
Display	O
List	O
using	O
Display	B-General_Concept
List	I-General_Concept
Interrupts	I-General_Concept
:	O
</s>
<s>
Turn	O
off	O
ANTIC	B-General_Concept
Display	B-General_Concept
List	I-General_Concept
Interrupts	I-General_Concept
by	O
clearing	O
bit	O
7	O
(	O
)	O
of	O
NMIEN	O
by	O
setting	O
the	O
register	O
to	O
value	O
which	O
allows	O
the	O
Vertical	O
Blank	O
Interrupt	O
to	O
continue	O
running	O
.	O
</s>
<s>
Turn	O
off	O
ANTIC	B-General_Concept
Display	O
List	O
DMA	B-General_Concept
.	O
</s>
<s>
Turn	O
off	O
the	O
DMA	B-General_Concept
in	O
the	O
Shadow	O
register	O
first	O
(	O
SDMCTL	O
)	O
,	O
and	O
then	O
if	O
desired	O
the	O
same	O
can	O
be	O
done	O
to	O
the	O
hardware	O
register	O
(	O
DMACTL	O
)	O
.	O
</s>
<s>
Set	O
the	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
address	O
VDSLST	O
(	O
hex/512dec	O
)	O
to	O
the	O
starting	O
address	O
of	O
the	O
interrupt	O
routine	O
.	O
</s>
<s>
Monitor	O
ANTIC	B-General_Concept
's	O
VCOUNT	O
for	O
changes	O
indicating	O
the	O
next	O
frame	O
has	O
started	O
.	O
</s>
<s>
Turn	O
on	O
ANTIC	B-General_Concept
Display	B-General_Concept
List	I-General_Concept
Interrupts	I-General_Concept
by	O
setting	O
bit	O
7	O
(	O
)	O
of	O
NMIEN	O
.	O
</s>
<s>
Turn	O
on	O
ANTIC	B-General_Concept
Display	O
List	O
DMA	B-General_Concept
by	O
updating	O
the	O
Shadow	O
register	O
(	O
SDMCTL	O
)	O
.	O
</s>
<s>
A	O
program	O
tightly	O
integrated	O
with	O
the	O
display	O
cycle	O
and	O
aware	O
of	O
the	O
electron	O
beam	O
position	O
relative	O
to	O
the	O
current	O
display	O
can	O
bypass	O
turning	O
off	O
Display	O
List	O
DMA	B-General_Concept
.	O
</s>
<s>
The	O
last	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
routine	O
on	O
the	O
screen	O
points	O
VDSLST	O
back	O
to	O
the	O
address	O
of	O
the	O
first	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
.	O
</s>
<s>
If	O
a	O
value	O
changed	O
by	O
a	O
DLI	O
has	O
an	O
Operating	O
System	O
Shadow	O
register	O
and	O
the	O
Vertical	O
Blank	O
Interrupts	O
are	O
enabled	O
,	O
then	O
the	O
value	O
in	O
effect	O
before	O
(	O
above	O
)	O
the	O
screen	O
position	O
of	O
the	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
is	O
the	O
value	O
of	O
the	O
Shadow	O
,	O
and	O
the	O
value	O
after	O
(	O
below	O
)	O
the	O
screen	O
position	O
is	O
the	O
value	O
from	O
the	O
DLI	O
(	O
and	O
any	O
other	O
DLI	O
that	O
later	O
changes	O
it	O
.	O
)	O
</s>
<s>
Since	O
the	O
DLI	O
begins	O
executing	O
while	O
the	O
electron	O
beam	O
is	O
in	O
a	O
visible	O
portion	O
of	O
the	O
screen	O
,	O
the	O
transition	O
from	O
the	O
previous	O
background	O
color	O
to	O
the	O
new	O
color	O
written	O
by	O
the	O
DLI	O
will	O
be	O
visible	O
somewhere	O
on	O
the	O
current	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
(	O
The	O
exact	O
location	O
varies	O
based	O
on	O
DMA	B-General_Concept
requirements	O
for	O
the	O
display	O
mode	O
and	O
Player/Missile	O
graphics	O
.	O
)	O
</s>
<s>
To	O
produce	O
a	O
clean	O
transition	O
from	O
scan	B-Algorithm
line	I-Algorithm
to	O
scan	B-Algorithm
line	I-Algorithm
the	O
code	O
should	O
first	O
write	O
to	O
WSYNC	O
which	O
halts	O
the	O
DLI	O
execution	O
until	O
the	O
end	O
of	O
the	O
scan	B-Algorithm
line	I-Algorithm
,	O
and	O
then	O
write	O
to	O
the	O
desired	O
registers	O
.	O
</s>
<s>
It	O
is	O
safe	O
for	O
a	O
DLI	O
to	O
run	O
for	O
several	O
,	O
or	O
many	O
scan	B-Algorithm
lines	I-Algorithm
as	O
long	O
as	O
it	O
exits	O
before	O
the	O
next	O
DLI	O
or	O
the	O
Vertical	O
Blank	O
Interrupt	O
.	O
</s>
<s>
A	O
trivial	O
example	O
of	O
a	O
Display	B-General_Concept
List	I-General_Concept
Interrupt	I-General_Concept
that	O
changes	O
the	O
background	O
color	O
:	O
</s>
<s>
Most	O
Player/Missile	O
graphics	O
functionality	O
is	O
in	O
the	O
domain	O
of	O
the	O
CTIA/GTIA	B-General_Concept
chip	I-General_Concept
.	O
</s>
<s>
CTIA/GTIA	B-General_Concept
controls	O
position	O
,	O
size	O
,	O
color	O
,	O
priority	O
,	O
and	O
collision	O
detection	O
for	O
Player/Missile	O
graphics	O
objects	O
,	O
but	O
its	O
actual	O
shape	O
rendering	O
implementation	O
goes	O
no	O
farther	O
than	O
a	O
single	O
byte	O
pattern	O
per	O
object	O
presented	O
uniformly	O
on	O
all	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
(	O
The	O
patterns	O
are	O
set	O
in	O
CTIA/GTIA	B-General_Concept
'	O
s	O
GRAF*	O
graphics	O
pattern	O
registers	O
.	O
)	O
</s>
<s>
In	O
the	O
end	O
,	O
by	O
itself	O
CTIA/GTIA	B-General_Concept
generates	O
Player/Missile	O
objects	O
as	O
only	O
vertical	O
stripe	O
patterns	O
on	O
the	O
screen	O
.	O
</s>
<s>
ANTIC	B-General_Concept
's	O
design	O
includes	O
DMA	B-General_Concept
capability	O
so	O
that	O
it	O
may	O
read	O
its	O
Display	O
List	O
and	O
Playfield	O
graphics	O
data	O
.	O
</s>
<s>
This	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
capability	O
is	O
leveraged	O
to	O
read	O
data	O
on	O
behalf	O
of	O
CTIA/GTIA	B-General_Concept
and	O
update	O
the	O
GRAF*	O
graphics	O
pattern	O
registers	O
as	O
each	O
display	O
scan	B-Algorithm
line	I-Algorithm
is	O
generated	O
.	O
</s>
<s>
In	O
this	O
way	O
ANTIC	B-General_Concept
provides	O
Player/Missile	O
objects	O
with	O
patterns	O
that	O
vary	O
over	O
the	O
height	O
of	O
the	O
screen	O
and	O
so	O
appear	O
as	O
traditional	O
"	O
sprite	O
"	O
graphics	O
.	O
</s>
<s>
Bits	O
2	O
and	O
3	O
of	O
ANTIC	B-General_Concept
's	O
DMACTL	O
register	O
(	O
shadowed	O
by	O
SDMCTL	O
)	O
control	O
ANTIC	B-General_Concept
's	O
retrieval	O
of	O
Player/Missile	O
data	O
to	O
update	O
CTIA/GTIA	B-General_Concept
graphics	O
pattern	O
registers	O
.	O
</s>
<s>
DMACTL	O
Player/Missile	O
DMA	B-General_Concept
control	O
bits	O
values	O
(	O
bits	O
 [ 3:2 ] 	O
)	O
:	O
</s>
<s>
Missile	O
DMA	B-General_Concept
can	O
be	O
enabled	O
without	O
player	O
DMA	B-General_Concept
.	O
</s>
<s>
However	O
,	O
when	O
Player	O
DMA	B-General_Concept
is	O
enabled	O
,	O
Missile	O
DMA	B-General_Concept
automatically	O
occurs	O
to	O
keep	O
the	O
DMA	B-General_Concept
timing	O
consistent	O
,	O
but	O
the	O
data	O
is	O
not	O
delivered	O
to	O
the	O
Missile	O
's	O
GRAFM	O
register	O
.	O
</s>
<s>
ANTIC	B-General_Concept
retrieves	O
data	O
and	O
updates	O
CTIA/GTIA	B-General_Concept
every	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
The	O
DMACTL	O
resolution	O
settings	O
affect	O
how	O
often	O
ANTIC	B-General_Concept
increments	O
its	O
internal	O
DMA	B-General_Concept
pointer	O
for	O
fetching	O
new	O
data	O
.	O
</s>
<s>
Single	O
Line	O
resolution	O
Player/Missile	O
graphics	O
increments	O
the	O
DMA	B-General_Concept
address	O
every	O
scan	B-Algorithm
line	I-Algorithm
,	O
and	O
Double	O
Line	O
resolution	O
increments	O
the	O
DMA	B-General_Concept
address	O
with	O
every	O
even	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
New	O
data	O
is	O
fetched	O
by	O
DMA	B-General_Concept
on	O
even	O
scan	B-Algorithm
lines	I-Algorithm
,	O
but	O
an	O
update	O
to	O
CTIA/GTIA	B-General_Concept
Player/Missile	O
Graphics	O
pattern	O
registers	O
still	O
occurs	O
on	O
every	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
Consequently	O
,	O
each	O
Player/Missile	O
byte	O
pattern	O
is	O
two	O
scan	B-Algorithm
lines	I-Algorithm
tall	O
.	O
</s>
<s>
When	O
Double	O
line	O
resolution	O
is	O
enabled	O
CTIA/GTIA	B-General_Concept
register	O
VDELAY	O
(	O
D01Chex/53276dec	O
)	O
can	O
be	O
used	O
to	O
mask	O
ANTIC	B-General_Concept
's	O
updates	O
on	O
even	O
scan	B-Algorithm
lines	I-Algorithm
shifting	O
the	O
bit	O
pattern	O
of	O
individual	O
Players	O
and	O
Missiles	O
down	O
one	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
A	O
new	O
DMA	B-General_Concept
fetch	O
and	O
Player/Missile	O
register	O
update	O
occurs	O
on	O
every	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
CTIA/GTIA	B-General_Concept
register	O
VDELAY	O
(	O
D01Chex/53276dec	O
)	O
masking	O
the	O
updates	O
on	O
even	O
scan	B-Algorithm
lines	I-Algorithm
reduces	O
Single	O
line	O
Player/Missile	O
resolution	O
to	O
appear	O
to	O
be	O
Double	O
line	O
resolution	O
,	O
though	O
the	O
data	O
is	O
shifted	O
one	O
scan	B-Algorithm
line	I-Algorithm
lower	O
.	O
</s>
<s>
CTIA/GTIA	B-General_Concept
must	O
also	O
be	O
specifically	O
enabled	O
to	O
receive	O
the	O
Player/Missile	O
graphics	O
data	O
via	O
DMA	B-General_Concept
to	O
correctly	O
present	O
the	O
objects	O
on	O
screen	O
.	O
</s>
<s>
See	O
CTIA/GTIA	B-General_Concept
'	O
s	O
GRACTL	O
register	O
for	O
more	O
information	O
.	O
</s>
<s>
The	O
location	O
of	O
memory	O
read	O
by	O
ANTIC	B-General_Concept
for	O
Player/Missile	O
graphics	O
is	O
controlled	O
by	O
register	O
PMBASE	O
.	O
</s>
<s>
Double	O
line	O
resolution	O
(	O
1	O
byte	O
per	O
2	O
scanlines	B-Algorithm
,	O
128	O
bytes	O
each	O
object	O
)	O
relative	O
to	O
PMBASE	O
x	O
hex/256dec	O
:	O
</s>
<s>
Single	O
line	O
resolution	O
(	O
1	O
byte	O
per	O
scanline	B-Algorithm
,	O
256	O
bytes	O
each	O
object	O
)	O
relative	O
to	O
PMBASE	O
x	O
hex/256dec	O
:	O
</s>
<s>
ANTIC	B-General_Concept
does	O
not	O
use	O
the	O
first	O
and	O
last	O
8	O
scan	B-Algorithm
lines	I-Algorithm
worth	O
of	O
data	O
of	O
Player/Missile	O
memory	O
in	O
the	O
memory	O
map	O
.	O
</s>
<s>
The	O
vertical	O
extent	O
of	O
the	O
entire	O
Playfield	O
display	O
varies	O
between	O
0	O
and	O
240	O
scanlines	B-Algorithm
-	O
this	O
depends	O
on	O
the	O
number	O
of	O
lines	O
ANTIC	B-General_Concept
is	O
programmed	O
to	O
display	O
according	O
to	O
the	O
Display	O
List	O
.	O
</s>
<s>
ANTIC	B-General_Concept
begins	O
generating	O
scan	B-Algorithm
lines	I-Algorithm
at	O
TV	O
scan	B-Algorithm
line	I-Algorithm
8	O
continuing	O
to	O
line	O
247	O
for	O
a	O
total	O
of	O
240	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
The	O
GTIA	B-General_Concept
Map	O
modes	O
must	O
be	O
horizontally	O
scrolled	O
in	O
steps	O
the	O
size	O
of	O
an	O
entire	O
pixel	O
(	O
2	O
color	O
clocks	O
)	O
,	O
instead	O
of	O
one	O
color	O
clock	O
.	O
</s>
<s>
Vertical	O
fine	O
scrolling	O
moves	O
the	O
scrolling	O
region	O
the	O
number	O
of	O
scan	B-Algorithm
lines	I-Algorithm
in	O
a	O
single	O
Mode	O
line	O
of	O
the	O
scrolling	O
region	O
,	O
up	O
to	O
a	O
maximum	O
of	O
16	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
Although	O
ANTIC	B-General_Concept
's	O
Display	O
List	O
program	O
counter	O
is	O
16-bit	O
,	O
only	O
the	O
lower	O
10	O
bits	O
change	O
during	O
normal	O
(	O
i.e.	O
</s>
<s>
The	O
Memory	O
Scan	O
Register	O
,	O
a	O
register	O
addressing	O
the	O
data	O
stored	O
in	O
the	O
screen	O
memory	O
,	O
is	O
16-bit	O
,	O
but	O
only	O
the	O
lower	O
12	O
bits	O
change	O
when	O
ANTIC	B-General_Concept
is	O
sequentially	O
scanning	O
the	O
video	O
memory	O
.	O
</s>
<s>
ANTIC	B-General_Concept
's	O
graphic	O
modes	O
E	O
and	O
F	O
require	O
more	O
than	O
7.5K	O
of	O
screen	O
memory	O
for	O
a	O
full	O
screen	O
display	O
.	O
</s>
<s>
The	O
character	O
set	O
can	O
be	O
located	O
anywhere	O
in	O
the	O
memory	O
(	O
see	O
CHBASE	O
)	O
,	O
but	O
,	O
depending	O
on	O
the	O
ANTIC	B-General_Concept
Text	O
Mode	O
,	O
the	O
starting	O
address	O
must	O
align	O
to	O
a	O
512-byte	O
or	O
a	O
1K	O
boundary	O
.	O
</s>
<s>
Also	O
,	O
some	O
of	O
ANTIC	B-General_Concept
's	O
activities	O
have	O
critical	O
timing	O
.	O
</s>
<s>
One	O
method	O
of	O
using	O
the	O
enhanced	O
GTIA	B-General_Concept
color	O
modes	O
without	O
the	O
large	O
memory	O
requirements	O
of	O
ANTIC	B-General_Concept
mode	O
F	O
is	O
to	O
direct	O
ANTIC	B-General_Concept
to	O
repeat	O
each	O
line	O
of	O
data	O
one	O
or	O
more	O
times	O
by	O
using	O
Mode	O
line	O
instructions	B-General_Concept
with	O
the	O
LMS	O
modifier	O
to	O
redisplay	O
the	O
same	O
screen	O
data	O
on	O
multiple	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
This	O
produces	O
a	O
lower	O
vertical	O
resolution	O
where	O
each	O
pixel	O
is	O
2	O
or	O
more	O
scan	B-Algorithm
lines	I-Algorithm
tall	O
,	O
but	O
has	O
higher	O
DMA	B-General_Concept
overhead	O
due	O
to	O
ANTIC	B-General_Concept
needing	O
to	O
re-read	O
the	O
same	O
bytes	O
from	O
screen	O
memory	O
.	O
</s>
<s>
Rather	O
than	O
using	O
repeated	O
LMS	O
modifiers	O
,	O
a	O
quirk	O
of	O
vertical	O
scrolling	O
can	O
be	O
exploited	O
to	O
create	O
a	O
four	O
scan-line	B-Algorithm
mode	O
that	O
displays	O
pixels	O
using	O
the	O
hi-res	O
or	O
GTIA	B-General_Concept
color	O
interpretation	O
modes	O
.	O
</s>
<s>
Vertical	O
Scrolling	O
would	O
ordinarily	O
not	O
be	O
considered	O
useful	O
for	O
ANTIC	B-General_Concept
Mode	O
F	O
which	O
is	O
one	O
scan	B-Algorithm
line	I-Algorithm
tall	O
.	O
</s>
<s>
However	O
,	O
if	O
Vertical	O
Scrolling	O
is	O
enabled	O
for	O
the	O
Mode	O
F	O
Display	O
List	O
instruction	O
,	O
and	O
the	O
VSCROL	O
register	O
is	O
set	O
to	O
13	O
,	O
then	O
ANTIC	B-General_Concept
's	O
line	O
counter	O
will	O
count	O
from	O
13	O
to	O
0	O
(	O
four	O
scan	B-Algorithm
lines	I-Algorithm
—	O
13	O
,	O
14	O
,	O
15	O
,	O
0	O
)	O
.	O
</s>
<s>
In	O
doing	O
so	O
ANTIC	B-General_Concept
outputs	O
the	O
same	O
line	O
of	O
graphics	O
for	O
each	O
scan	B-Algorithm
line	I-Algorithm
from	O
its	O
internal	O
buffer	O
without	O
re-reading	O
the	O
same	O
screen	O
memory	O
again	O
.	O
</s>
<s>
The	O
end	O
of	O
the	O
vertical	O
scrolling	O
region	O
needs	O
to	O
have	O
VSCROL	O
reset	O
to	O
3	O
allowing	O
ANTIC	B-General_Concept
to	O
count	O
from	O
0	O
to	O
3	O
(	O
four	O
scan	B-Algorithm
lines	I-Algorithm
)	O
.	O
</s>
<s>
The	O
end	O
result	O
is	O
a	O
Mode	O
F	O
display	O
where	O
the	O
lines	O
are	O
four	O
scan	B-Algorithm
lines	I-Algorithm
tall	O
rather	O
than	O
one	O
,	O
but	O
with	O
the	O
DMA	B-General_Concept
overhead	O
of	O
only	O
one	O
screen	O
memory	O
data	O
read	O
occurring	O
on	O
the	O
first	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
ANTIC	B-General_Concept
Map	O
Modes	O
8	O
and	O
9	O
will	O
be	O
corrupted	O
when	O
immediately	O
following	O
a	O
horizontally	O
scrolled	O
line	O
of	O
a	O
different	O
mode	O
using	O
normal	O
or	O
wide	O
playfield	O
width	O
.	O
</s>
<s>
The	O
problem	O
appears	O
for	O
certain	O
HSCROL	O
values	O
causing	O
ANTIC	B-General_Concept
to	O
output	O
pixels	O
incorrectly	O
and	O
misalign	O
the	O
Mode	O
8-9	O
line	O
.	O
</s>
<s>
Mode	O
2	O
,	O
3	O
,	O
4	O
,	O
5	O
,	O
D	O
,	O
E	O
,	O
or	O
F	O
and	O
HSCROL	O
is	O
or	O
:	O
then	O
corruption	O
resolves	O
itself	O
within	O
two	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
Mode	O
2	O
,	O
3	O
,	O
4	O
,	O
5	O
,	O
D	O
,	O
E	O
,	O
or	O
F	O
:	O
then	O
corruption	O
resolves	O
itself	O
within	O
three	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
Mode	O
6	O
,	O
7	O
,	O
A	O
,	O
B	O
,	O
or	O
C	O
:	O
then	O
corruption	O
resolves	O
itself	O
within	O
two	O
scan	B-Algorithm
lines	I-Algorithm
.	O
</s>
<s>
A	O
display	O
list	O
should	O
not	O
be	O
built	O
with	O
a	O
hi-res	O
(	O
Mode	O
F	O
)	O
instruction	O
in	O
scan	B-Algorithm
line	I-Algorithm
247	O
—	O
the	O
last	O
displayable	O
scan	B-Algorithm
line	I-Algorithm
.	O
</s>
<s>
If	O
the	O
DMACTL	O
Playfield	O
Width	O
bits	O
are	O
not	O
00	O
then	O
ANTIC	B-General_Concept
will	O
not	O
generate	O
the	O
vertical	O
sync	O
properly	O
potentially	O
causing	O
display	O
distortion	O
.	O
</s>
<s>
The	O
Horizontal	O
Fine	O
Scrolling	O
display	O
list	O
modifier	O
causes	O
ANTIC	B-General_Concept
to	O
fetch	O
extra	O
screen	O
memory	O
corresponding	O
to	O
the	O
next	O
larger	O
Playfield	O
width	O
which	O
provides	O
the	O
data	O
needed	O
to	O
scroll	O
into	O
the	O
display	O
.	O
</s>
<s>
Since	O
there	O
is	O
no	O
Playfield	O
width	O
larger	O
than	O
the	O
Wide	O
Playfield	O
,	O
ANTIC	B-General_Concept
has	O
no	O
extra	O
data	O
for	O
fine	O
scrolling	O
.	O
</s>
<s>
ANTIC	B-General_Concept
presents	O
fake	O
data	O
for	O
the	O
area	O
emptied	O
by	O
the	O
shifted	O
screen	O
—	O
on	O
the	O
left	O
side	O
of	O
the	O
display	O
the	O
background	O
color	O
will	O
be	O
shifted	O
in	O
and	O
on	O
the	O
right	O
side	O
a	O
few	O
color	O
clocks	O
of	O
random	O
data	O
will	O
appear	O
at	O
the	O
end	O
of	O
the	O
scrolling	O
range	O
.	O
</s>
