<s>
The	O
Original	B-Device
Chip	I-Device
Set	I-Device
(	O
OCS	O
)	O
is	O
a	O
chipset	B-Device
used	O
in	O
the	O
earliest	O
Commodore	B-Device
Amiga	I-Device
computers	O
and	O
defined	O
the	O
Amiga	B-Device
's	O
graphics	O
and	O
sound	O
capabilities	O
.	O
</s>
<s>
It	O
was	O
succeeded	O
by	O
the	O
slightly	O
improved	O
Enhanced	B-Device
Chip	I-Device
Set	I-Device
(	O
ECS	B-Device
)	O
and	O
greatly	O
improved	O
Advanced	B-Device
Graphics	I-Device
Architecture	I-Device
(	O
AGA	B-Device
)	O
.	O
</s>
<s>
The	O
original	B-Device
chipset	I-Device
appeared	O
in	O
Amiga	B-Device
models	O
built	O
between	O
1985	O
and	O
1990	O
:	O
the	O
Amiga	B-Device
1000	O
,	O
Amiga	B-Device
2000	O
,	O
Amiga	B-Device
CDTV	O
,	O
and	O
Amiga	B-Device
500	O
.	O
</s>
<s>
The	O
chipset	B-Device
which	O
gave	O
the	O
Amiga	B-Device
its	O
unique	O
graphics	O
features	O
consists	O
of	O
three	O
main	O
"	O
custom	O
"	O
chips	O
;	O
Agnus	O
,	O
Denise	O
,	O
and	O
Paula	O
.	O
</s>
<s>
Both	O
the	O
original	B-Device
chipset	I-Device
and	O
the	O
enhanced	O
chipset	B-Device
were	O
manufactured	O
using	O
NMOS	B-Algorithm
logic	I-Algorithm
technology	O
by	O
Commodore	O
's	O
chip	B-Architecture
manufacturing	I-Architecture
subsidiary	O
,	O
MOS	B-Architecture
Technology	I-Architecture
.	O
</s>
<s>
According	O
to	O
Jay	O
Miner	O
,	O
the	O
OCS	O
chipset	B-Device
was	O
fabricated	O
in	O
5µm	O
manufacturing	O
process	O
while	O
AGA	B-Device
Lisa	O
was	O
implemented	O
in	O
1.5µm	O
process	O
.	O
</s>
<s>
All	O
three	O
custom	O
chips	O
were	O
originally	O
packaged	O
in	O
48-pin	O
DIPs	B-Algorithm
;	O
later	O
versions	O
of	O
Agnus	O
,	O
known	O
as	O
Fat	O
Agnus	O
,	O
were	O
packaged	O
in	O
an	O
84-pin	O
PLCC	B-Algorithm
.	O
</s>
<s>
It	O
controls	O
all	O
access	O
to	O
chip	B-Device
RAM	I-Device
from	O
both	O
the	O
central	O
68000	B-Device
processor	O
and	O
the	O
other	O
custom	O
chips	O
,	O
using	O
a	O
complicated	O
priority	O
system	O
.	O
</s>
<s>
Agnus	O
includes	O
sub-components	O
known	O
as	O
the	O
blitter	B-General_Concept
(	O
fast	O
transfer	O
of	O
data	O
in	O
memory	O
without	O
the	O
intervention	O
of	O
the	O
processor	O
)	O
and	O
the	O
Copper	O
(	O
video-synchronized	O
co-processor	O
)	O
.	O
</s>
<s>
The	O
original	O
Agnus	O
can	O
address	O
of	O
chip	B-Device
RAM	I-Device
.	O
</s>
<s>
Later	O
revisions	O
,	O
dubbed	O
'	O
Fat	O
Agnus	O
 '	O
,	O
added	O
pseudo-fast	O
RAM	O
,	O
which	O
for	O
ECS	B-Device
was	O
changed	O
to	O
1MB	O
(	O
sometimes	O
called	O
'	O
Fatter	O
Agnus	O
 '	O
)	O
and	O
subsequently	O
to	O
2MB	O
chip	B-Device
RAM	I-Device
.	O
</s>
<s>
Without	O
using	O
overscan	O
,	O
the	O
Amiga	B-Device
's	O
graphics	O
display	O
is	O
320	O
or	O
640	O
pixels	B-Algorithm
wide	O
by	O
200	O
(	O
NTSC	O
)	O
or	O
256	O
(	O
PAL	B-Language
)	O
pixels	B-Algorithm
tall	O
.	O
</s>
<s>
Planar	B-Device
bitmap	B-Algorithm
graphics	I-Algorithm
are	O
used	O
,	O
which	O
splits	O
the	O
individual	O
bits	O
per	O
pixel	B-Algorithm
into	O
separate	O
areas	O
of	O
memory	O
,	O
called	O
bitplanes	O
.	O
</s>
<s>
These	O
colors	O
are	O
selected	O
from	O
a	O
palette	B-Data_Structure
of	O
4096	O
colors	O
(	O
four	O
bits	O
per	O
RGB	O
component	O
)	O
.	O
</s>
<s>
Denise	O
also	O
supports	O
eight	O
sprites	O
,	O
single	O
pixel	B-Algorithm
scrolling	O
,	O
and	O
a	O
"	O
dual-playfield	O
"	O
mode	O
.	O
</s>
<s>
Paula	O
is	O
primarily	O
the	O
audio	O
chip	O
,	O
with	O
four	O
independent	O
hardware-mixed	O
8-bit	O
PCM	B-Algorithm
sound	O
channels	O
,	O
each	O
of	O
which	O
supports	O
65	O
volume	O
levels	O
(	O
no	O
sound	O
to	O
maximum	O
volume	O
)	O
and	O
waveform	O
output	O
rates	O
from	O
roughly	O
20	O
samples	O
per	O
second	O
to	O
almost	O
29,000	O
samples	O
per	O
second	O
.	O
</s>
<s>
Paula	O
also	O
handles	O
interrupts	B-Application
and	O
various	O
I/O	B-General_Concept
functions	O
including	O
the	O
floppy	B-Device
disk	I-Device
drive	I-Device
,	O
the	O
serial	B-Protocol
port	I-Protocol
,	O
and	O
analog	B-Device
joysticks	I-Device
.	O
</s>
<s>
There	O
are	O
many	O
similarities	O
both	O
in	O
overall	O
functionality	O
and	O
in	O
the	O
division	O
of	O
functionality	O
into	O
the	O
three	O
component	O
chips	O
between	O
the	O
OCS	O
chipset	B-Device
and	O
the	O
much	O
earlier	O
and	O
simpler	O
chipset	B-Device
of	O
the	O
Atari	B-Device
8-bit	I-Device
family	I-Device
of	O
home	O
computers	O
,	O
consisting	O
of	O
the	O
ANTIC	B-General_Concept
,	O
GTIA	B-General_Concept
and	O
POKEY	B-Device
chips	O
;	O
both	O
chipsets	B-Device
were	O
conceptually	O
designed	O
by	O
Jay	O
Miner	O
,	O
which	O
explains	O
the	O
similarity	O
.	O
</s>
<s>
The	O
Agnus	O
chip	O
is	O
in	O
overall	O
control	O
of	O
the	O
entire	O
chipset	B-Device
's	O
operation	O
.	O
</s>
<s>
This	O
includes	O
access	O
to	O
the	O
built-in	O
RAM	B-Architecture
,	O
known	O
as	O
chip	B-Device
RAM	I-Device
because	O
the	O
chipset	B-Device
has	O
access	O
to	O
it	O
.	O
</s>
<s>
Both	O
the	O
central	O
68000	B-Device
processor	O
and	O
other	O
members	O
of	O
the	O
chipset	B-Device
have	O
to	O
arbitrate	O
for	O
access	O
to	O
chip	B-Device
RAM	I-Device
via	O
Agnus	O
.	O
</s>
<s>
In	O
computing	O
architecture	O
terms	O
,	O
this	O
is	O
Direct	B-General_Concept
Memory	I-General_Concept
Access	I-General_Concept
(	O
DMA	B-General_Concept
)	O
,	O
where	O
Agnus	O
is	O
the	O
DMA	B-General_Concept
Controller	I-General_Concept
(	O
DMAC	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
bitplane	O
data	O
fetches	O
are	O
prioritized	O
over	O
blitter	B-General_Concept
transfers	O
as	O
the	O
immediate	O
display	O
of	O
frame	O
buffer	O
data	O
is	O
considered	O
more	O
important	O
than	O
the	O
processing	O
of	O
memory	O
by	O
the	O
blitter	B-General_Concept
.	O
</s>
<s>
Agnus	O
also	O
attempts	O
to	O
order	O
accesses	O
in	O
such	O
a	O
way	O
so	O
as	O
to	O
overlap	O
CPU	B-General_Concept
bus	O
cycles	O
with	O
DMA	B-General_Concept
cycles	O
.	O
</s>
<s>
As	O
the	O
original	O
68000	B-Device
processor	O
in	O
Amigas	B-Device
tended	O
only	O
to	O
access	O
memory	O
on	O
every	O
second	O
available	O
memory	O
cycle	O
,	O
Agnus	O
operates	O
a	O
system	O
where	O
"	O
odd	O
"	O
memory	O
access	O
cycles	O
are	O
allocated	O
first	O
and	O
as	O
needed	O
to	O
time-critical	O
custom	O
chip	O
DMA	B-General_Concept
while	O
any	O
remaining	O
cycles	O
are	O
available	O
to	O
the	O
CPU	B-General_Concept
,	O
thus	O
the	O
CPU	B-General_Concept
does	O
not	O
generally	O
get	O
locked	O
out	O
of	O
memory	O
access	O
and	O
does	O
not	O
appear	O
to	O
slow	O
down	O
.	O
</s>
<s>
However	O
,	O
non-time-critical	O
custom	O
chip	O
access	O
,	O
such	O
as	O
blitter	B-General_Concept
transfers	O
,	O
can	O
use	O
up	O
any	O
spare	O
odd	O
or	O
even	O
cycles	O
and	O
,	O
if	O
the	O
"	O
BLITHOG	O
"	O
(	O
blitter	B-General_Concept
hog	O
)	O
flag	O
is	O
set	O
,	O
Agnus	O
can	O
lock	O
out	O
the	O
even	O
cycles	O
from	O
the	O
CPU	B-General_Concept
in	O
deference	O
to	O
the	O
blitter	B-General_Concept
.	O
</s>
<s>
This	O
is	O
equivalent	O
to	O
two	O
low	O
resolution	O
(	O
140ns	O
)	O
pixels	B-Algorithm
or	O
four	O
high	O
resolution	O
(	O
70ns	O
)	O
pixels	B-Algorithm
.	O
</s>
<s>
The	O
blitter	B-General_Concept
is	O
a	O
sub-component	O
of	O
Agnus	O
.	O
</s>
<s>
"	O
Blit	B-Algorithm
"	O
is	O
shorthand	O
for	O
"	O
block	O
image	O
transfer	O
"	O
or	O
bit	B-Algorithm
blit	I-Algorithm
.	O
</s>
<s>
The	O
blitter	B-General_Concept
is	O
a	O
highly	O
parallel	O
memory	O
transfer	O
and	O
logic	O
operation	O
unit	O
.	O
</s>
<s>
The	O
blitter	B-General_Concept
allows	O
rapid	O
copying	O
of	O
video	O
memory	O
,	O
meaning	O
that	O
the	O
CPU	B-General_Concept
can	O
be	O
freed	O
for	O
other	O
tasks	O
.	O
</s>
<s>
The	O
blitter	B-General_Concept
was	O
primarily	O
used	O
for	O
drawing	O
and	O
redrawing	O
graphics	O
images	O
on	O
the	O
screen	O
,	O
called	O
"	O
bobs	O
"	O
,	O
short	O
for	O
"	O
blitter	B-General_Concept
objects	O
"	O
.	O
</s>
<s>
The	O
blitter	B-General_Concept
's	O
block	O
copying	O
mode	O
takes	O
zero	O
to	O
three	O
data	O
sources	O
in	O
memory	O
,	O
called	O
A	O
,	O
B	O
and	O
C	O
,	O
performs	O
a	O
programmable	O
boolean	O
function	O
on	O
the	O
data	O
sources	O
and	O
writes	O
the	O
result	O
to	O
a	O
destination	O
area	O
,	O
D	O
.	O
Any	O
of	O
these	O
four	O
areas	O
can	O
overlap	O
.	O
</s>
<s>
The	O
blitter	B-General_Concept
runs	O
either	O
from	O
the	O
start	O
of	O
the	O
block	O
to	O
the	O
end	O
,	O
known	O
as	O
"	O
ascending	O
"	O
mode	O
,	O
or	O
in	O
reverse	O
,	O
"	O
descending	O
"	O
mode	O
.	O
</s>
<s>
Blocks	O
are	O
"	O
rectangular	O
"	O
;	O
they	O
have	O
a	O
"	O
width	O
"	O
in	O
multiples	O
of	O
16	B-Device
bits	I-Device
,	O
a	O
height	O
measured	O
in	O
"	O
lines	O
"	O
,	O
and	O
a	O
"	O
stride	O
"	O
distance	O
to	O
move	O
from	O
the	O
end	O
of	O
one	O
line	O
to	O
the	O
next	O
.	O
</s>
<s>
This	O
allows	O
the	O
blitter	B-General_Concept
to	O
operate	O
on	O
any	O
video	O
resolution	O
up	O
to	O
1,024	O
×	O
1,024	O
pixels	B-Algorithm
.	O
</s>
<s>
The	O
copy	O
automatically	O
performs	O
a	O
per-pixel	O
logical	O
operation	O
.	O
</s>
<s>
This	O
is	O
most	O
commonly	O
used	O
to	O
do	O
direct	O
copies	O
(	O
D	O
=	O
A	O
)	O
,	O
or	O
apply	O
a	O
pixel	B-Algorithm
mask	O
around	O
blitted	O
objects	O
(	O
D	O
=	O
(	O
C	O
AND	O
B	O
)	O
OR	O
A	O
)	O
.	O
</s>
<s>
The	O
copy	O
can	O
also	O
barrel	O
shift	O
each	O
line	O
by	O
0	O
to	O
15	O
pixels	B-Algorithm
.	O
</s>
<s>
This	O
allows	O
the	O
blitter	B-General_Concept
to	O
draw	O
at	O
pixel	B-Algorithm
offsets	O
that	O
are	O
not	O
exactly	O
multiples	O
of	O
16	O
.	O
</s>
<s>
These	O
functions	O
allow	O
the	O
Amiga	B-Device
to	O
move	O
GUI	O
windows	O
around	O
the	O
screen	O
rapidly	O
as	O
each	O
is	O
represented	O
in	O
graphical	O
memory	O
space	O
as	O
a	O
rectangular	O
block	O
of	O
memory	O
which	O
may	O
be	O
shifted	O
to	O
any	O
required	O
screen	O
memory	O
location	O
at	O
will	O
.	O
</s>
<s>
The	O
blitter	B-General_Concept
's	O
line	O
mode	O
draws	O
single-pixel	O
thick	O
lines	O
using	O
Bresenham	B-Algorithm
's	I-Algorithm
line	I-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
It	O
can	O
also	O
apply	O
a	O
16-bit	B-Device
repeating	O
pattern	O
to	O
the	O
line	O
.	O
</s>
<s>
The	O
blitter	B-General_Concept
's	O
filling	O
mode	O
is	O
used	O
to	O
fill	O
per-line	O
horizontal	O
spans	O
.	O
</s>
<s>
On	O
each	O
span	O
,	O
it	O
reads	O
each	O
pixel	B-Algorithm
in	O
turn	O
from	O
right	O
to	O
left	O
.	O
</s>
<s>
Whenever	O
it	O
reads	O
a	O
set	O
pixel	B-Algorithm
,	O
it	O
toggles	O
filling	O
mode	O
on	O
or	O
off	O
.	O
</s>
<s>
When	O
filling	O
mode	O
is	O
on	O
,	O
it	O
sets	O
every	O
pixel	B-Algorithm
until	O
filling	O
mode	O
is	O
turned	O
off	O
or	O
the	O
line	O
ends	O
.	O
</s>
<s>
Together	O
,	O
these	O
modes	O
allow	O
the	O
blitter	B-General_Concept
to	O
draw	O
individual	O
flat-shaded	O
polygons	O
.	O
</s>
<s>
Later	O
Amigas	B-Device
tended	O
to	O
use	O
a	O
combination	O
of	O
a	O
faster	O
CPU	B-General_Concept
and	O
blitter	B-General_Concept
for	O
many	O
operations	O
.	O
</s>
<s>
The	O
Copper	O
is	O
a	O
programmable	O
finite-state	B-Architecture
machine	I-Architecture
that	O
executes	O
a	O
programmed	O
instruction	O
stream	O
,	O
synchronized	O
with	O
the	O
video	O
hardware	O
.	O
</s>
<s>
The	O
Copper	O
runs	O
a	O
program	O
called	O
the	O
Copper	O
list	O
in	O
parallel	O
with	O
the	O
main	O
CPU	B-General_Concept
.	O
</s>
<s>
Most	O
commonly	O
it	O
is	O
used	O
to	O
control	O
video	O
output	O
,	O
but	O
it	O
can	O
write	O
to	O
most	O
of	O
the	O
chipset	B-Device
registers	O
and	O
thus	O
can	O
be	O
used	O
to	O
initiate	O
blits	B-Algorithm
,	O
set	O
audio	O
registers	O
,	O
or	O
interrupt	B-Application
the	O
CPU	B-General_Concept
.	O
</s>
<s>
The	O
MOVE	O
instruction	O
writes	O
a	O
16-bit	B-Device
value	O
into	O
one	O
of	O
the	O
chipset	B-Device
's	O
hardware	O
registers	O
and	O
is	O
also	O
used	O
to	O
strobe	O
a	O
new	O
address	O
into	O
the	O
Copper	O
's	O
instruction	O
pointer	O
.	O
</s>
<s>
It	O
can	O
also	O
wait	O
for	O
a	O
blitter	B-General_Concept
operation	O
to	O
finish	O
.	O
</s>
<s>
This	O
allows	O
the	O
Amiga	B-Device
to	O
change	O
video	O
configuration	O
,	O
including	O
resolution	O
,	O
between	O
scanlines	B-Algorithm
.	O
</s>
<s>
This	O
allows	O
the	O
Amiga	B-Device
to	O
display	O
different	O
horizontal	O
resolutions	O
,	O
different	O
color	O
depths	O
,	O
and	O
entirely	O
different	O
frame	O
buffers	O
on	O
the	O
same	O
screen	O
.	O
</s>
<s>
The	O
AmigaOS	B-Application
graphical	O
user	O
interface	O
allows	O
two	O
or	O
more	O
programs	O
to	O
operate	O
at	O
different	O
resolutions	O
in	O
different	O
buffers	O
,	O
while	O
all	O
are	O
visible	O
on	O
the	O
screen	O
simultaneously	O
.	O
</s>
<s>
The	O
Copper	O
can	O
also	O
change	O
color	B-Data_Structure
registers	I-Data_Structure
mid-frame	O
,	O
creating	O
the	O
"	O
raster	O
bars	O
"	O
effect	O
seen	O
commonly	O
in	O
Amiga	B-Device
games	O
.	O
</s>
<s>
The	O
Copper	O
can	O
go	O
further	O
than	O
this	O
and	O
change	O
the	O
background	O
color	O
often	O
enough	O
to	O
make	O
a	O
blocky	O
graphics	O
display	O
without	O
using	O
any	O
bitmap	B-Algorithm
graphics	I-Algorithm
at	O
all	O
.	O
</s>
<s>
The	O
Copper	O
allows	O
"	O
re-use	O
"	O
of	O
sprites	O
;	O
after	O
a	O
sprite	O
has	O
been	O
drawn	O
at	O
its	O
programmed	O
location	O
,	O
the	O
Copper	O
can	O
then	O
immediately	O
move	O
it	O
to	O
a	O
new	O
location	O
and	O
it	O
will	O
be	O
drawn	O
again	O
,	O
even	O
on	O
the	O
same	O
scanline	B-Algorithm
.	O
</s>
<s>
The	O
Copper	O
can	O
trigger	O
an	O
interrupt	B-Application
when	O
the	O
video	O
beam	O
reaches	O
a	O
precise	O
location	O
on	O
the	O
display	O
.	O
</s>
<s>
This	O
is	O
useful	O
for	O
synchronizing	O
the	O
CPU	B-General_Concept
to	O
the	O
video	O
beam	O
.	O
</s>
<s>
The	O
Copper	O
can	O
also	O
be	O
used	O
to	O
program	O
and	O
operate	O
the	O
blitter	B-General_Concept
.	O
</s>
<s>
This	O
allows	O
blitter	B-General_Concept
operation	O
and	O
control	O
to	O
proceed	O
independently	O
of	O
,	O
and	O
concurrently	O
with	O
,	O
the	O
CPU	B-General_Concept
.	O
</s>
<s>
The	O
Copper	O
can	O
be	O
used	O
to	O
produce	O
"	O
sliced	O
HAM	O
"	O
,	O
or	O
S-HAM	O
,	O
this	O
consists	O
of	O
building	O
a	O
Copper	O
list	O
that	O
switches	O
the	O
palette	B-Data_Structure
on	O
every	O
scanline	B-Algorithm
,	O
improving	O
the	O
choice	O
of	O
base	O
colors	O
in	O
Hold-And-Modify	O
mode	O
graphics	O
.	O
</s>
<s>
Under	O
normal	O
circumstances	O
,	O
the	O
Amiga	B-Device
generates	O
its	O
own	O
video	O
timings	O
,	O
but	O
Agnus	O
also	O
supports	O
synchronising	O
the	O
system	O
to	O
an	O
external	O
signal	O
so	O
as	O
to	O
achieve	O
genlocking	O
with	O
external	O
video	O
hardware	O
.	O
</s>
<s>
There	O
is	O
also	O
a	O
1-bit	O
output	O
on	O
this	O
connector	O
that	O
indicates	O
whether	O
the	O
Amiga	B-Device
is	O
outputting	O
background	O
color	O
or	O
not	O
,	O
permitting	O
easy	O
overlaying	O
of	O
Amiga	B-Device
video	O
onto	O
external	O
video	O
.	O
</s>
<s>
This	O
made	O
the	O
Amiga	B-Device
particularly	O
attractive	O
as	O
a	O
character	O
generator	O
for	O
titling	O
videos	O
and	O
broadcast	O
work	O
,	O
as	O
it	O
avoided	O
the	O
use	O
and	O
expense	O
of	O
AB	O
roll	O
and	O
chromakey	O
units	O
that	O
would	O
be	O
required	O
without	O
the	O
genlock	O
support	O
.	O
</s>
<s>
The	O
support	O
of	O
overscan	O
,	O
interlacing	O
and	O
genlocking	O
capabilities	O
,	O
and	O
the	O
fact	O
that	O
the	O
display	O
timing	O
was	O
very	O
close	O
to	O
broadcast	O
standards	O
(	O
NTSC	O
or	O
PAL	B-Language
)	O
,	O
made	O
the	O
Amiga	B-Device
the	O
first	O
ideal	O
computer	O
for	O
video	O
purposes	O
,	O
and	O
indeed	O
,	O
it	O
was	O
used	O
in	O
many	O
studios	O
for	O
digitizing	O
video	O
data	O
(	O
sometimes	O
called	O
frame-grabbing	O
)	O
,	O
subtitling	O
and	O
interactive	O
video	O
news	O
.	O
</s>
<s>
Denise	O
is	O
programmed	O
to	O
fetch	O
planar	B-Device
video	O
data	O
from	O
one	O
to	O
five	O
bitplanes	O
and	O
translate	O
that	O
into	O
a	O
color	B-Data_Structure
lookup	I-Data_Structure
.	O
</s>
<s>
This	O
allows	O
for	O
very	O
economical	O
use	O
of	O
RAM	B-Architecture
,	O
and	O
balancing	O
of	O
CPU	B-General_Concept
processing	O
speed	O
vs	O
graphical	O
sophistication	O
when	O
executing	O
from	O
Chip	B-Device
RAM	I-Device
(	O
as	O
modes	O
beyond	O
4bpp	O
in	O
lores	O
,	O
or	O
2bpp	O
in	O
hires	O
,	O
use	O
extra	O
DMA	B-General_Concept
channels	O
that	O
can	O
slow	O
or	O
temporarily	O
halt	O
the	O
CPU	B-General_Concept
in	O
addition	O
to	O
the	O
usual	O
non-conflicting	O
channels	O
)	O
.	O
</s>
<s>
In	O
Extra-HalfBrite	O
(	O
EHB	O
)	O
,	O
if	O
a	O
pixel	B-Algorithm
is	O
set	O
on	O
the	O
sixth	O
bitplane	O
,	O
the	O
brightness	O
of	O
the	O
regular	O
32	O
color	O
pixel	B-Algorithm
is	O
halved	O
.	O
</s>
<s>
Early	O
versions	O
of	O
the	O
Amiga	B-Device
1000	O
sold	O
in	O
the	O
United	O
States	O
did	O
not	O
have	O
the	O
Extra-HalfBrite	O
mode	O
.	O
</s>
<s>
In	O
Hold-and-Modify	O
(	O
HAM	O
)	O
mode	O
,	O
each	O
6-bit	O
pixel	B-Algorithm
is	O
interpreted	O
as	O
two	O
control	O
bits	O
and	O
four	O
data	O
bits	O
.	O
</s>
<s>
With	O
one	O
of	O
the	O
"	O
modify	O
"	O
s	O
,	O
the	O
red	O
,	O
green	O
or	O
blue	O
component	O
of	O
the	O
previous	O
pixel	B-Algorithm
is	O
modified	O
to	O
the	O
data	O
value	O
,	O
and	O
the	O
other	O
two	O
components	O
are	O
held	O
from	O
the	O
previous	O
pixel	B-Algorithm
.	O
</s>
<s>
This	O
allows	O
all	O
4096	O
colors	O
on	O
screen	O
at	O
once	O
and	O
is	O
an	O
example	O
of	O
lossy	B-Algorithm
image	I-Algorithm
compression	I-Algorithm
in	O
hardware	O
.	O
</s>
<s>
There	O
are	O
two	O
horizontal	O
graphics	O
resolutions	O
,	O
"	O
lowres	O
"	O
with	O
140ns	O
pixels	B-Algorithm
and	O
"	O
hires	O
"	O
with	O
70ns	O
pixels	B-Algorithm
,	O
with	O
a	O
default	O
of	O
320	O
or	O
640	O
horizontal	O
pixels	B-Algorithm
wide	O
without	O
using	O
overscan	O
.	O
</s>
<s>
As	O
the	O
pixel	B-Algorithm
output	O
is	O
regulated	O
by	O
the	O
main	O
system	O
clock	O
,	O
which	O
is	O
based	O
directly	O
on	O
the	O
NTSC	O
colorburst	O
clock	O
,	O
these	O
sizes	O
very	O
nearly	O
fill	O
the	O
width	O
of	O
a	O
standard	O
television	O
with	O
only	O
a	O
thin	O
"	O
underscan	O
"	O
border	O
between	O
the	O
graphics	O
and	O
the	O
screen	O
border	O
when	O
compared	O
to	O
many	O
other	O
contemporary	O
home	O
computers	O
,	O
for	O
an	O
appearance	O
closer	O
to	O
a	O
games	O
console	O
but	O
with	O
finer	O
detail	O
.	O
</s>
<s>
On	O
top	O
of	O
this	O
,	O
Denise	O
supports	O
reasonably	O
extensive	O
overscan	O
;	O
technically	O
modes	O
with	O
enough	O
data	O
for	O
up	O
to	O
400	O
or	O
800	O
pixels	B-Algorithm
( +25%	O
)	O
may	O
be	O
specified	O
,	O
although	O
this	O
is	O
only	O
actually	O
useful	O
for	O
scrolling	O
and	O
special	O
effects	O
that	O
involve	O
partial	O
display	O
of	O
large	O
graphics	O
,	O
as	O
a	O
separate	O
hardware	O
limit	O
is	O
met	O
at	O
368	O
(	O
or	O
736	O
)	O
pixels	B-Algorithm
,	O
which	O
is	O
the	O
maximum	O
that	O
will	O
fit	O
between	O
the	O
end	O
of	O
one	O
blanking	O
period	O
and	O
the	O
start	O
of	O
the	O
next	O
-	O
although	O
it	O
is	O
unlikely	O
that	O
even	O
this	O
many	O
pixels	B-Algorithm
will	O
be	O
visible	O
on	O
any	O
display	O
other	O
than	O
a	O
dedicated	O
monitor	O
that	O
allows	O
adjustment	O
of	O
horizontal	O
scan	O
width	O
,	O
as	O
much	O
of	O
the	O
image	O
will	O
,	O
by	O
design	O
,	O
disappear	O
seamlessly	O
behind	O
the	O
screen	O
bezel	O
(	O
or	O
,	O
on	O
LCDs	O
,	O
be	O
cropped	O
off	O
at	O
the	O
edge	O
of	O
the	O
panel	O
)	O
.	O
</s>
<s>
Because	O
of	O
the	O
highly	O
regular	O
structure	O
of	O
the	O
Amiga	B-Device
's	O
timing	O
in	O
relation	O
to	O
scanlines	B-Algorithm
and	O
allocation	O
of	O
DMA	B-General_Concept
resources	O
to	O
various	O
uses	O
besides	O
normal	O
"	O
playfield	O
"	O
graphics	O
,	O
increased	O
horizontal	O
resolution	O
is	O
also	O
a	O
tradeoff	O
between	O
number	O
of	O
pixels	B-Algorithm
and	O
how	O
many	O
hardware	O
sprites	O
are	O
available	O
,	O
as	O
increasing	O
the	O
DMA	B-General_Concept
slots	O
dedicated	O
to	O
playfield	O
video	O
ends	O
up	O
stealing	O
some	O
(	O
from	O
1	O
to	O
7	O
of	O
the	O
total	O
8	O
)	O
the	O
sprite	O
engine	O
..	O
Vertical	O
resolution	O
,	O
without	O
overscan	O
,	O
is	O
200	O
pixels	B-Algorithm
for	O
a	O
60Hz	O
NTSC	O
Amiga	B-Device
or	O
256	O
for	O
a	O
50Hz	O
PAL	B-Language
Amiga	B-Device
.	O
</s>
<s>
This	O
can	O
be	O
doubled	O
using	O
an	O
interlaced	O
display	O
,	O
and	O
,	O
as	O
with	O
horizontal	O
resolution	O
,	O
increased	O
using	O
overscan	O
,	O
to	O
a	O
maximum	O
of	O
241	O
(	O
or	O
483	O
)	O
for	O
NTSC	O
,	O
and	O
283	O
(	O
or	O
567	O
)	O
for	O
PAL	B-Language
(	O
interlaced	O
modes	O
gaining	O
one	O
extra	O
line	O
as	O
the	O
maximum	O
is	O
determined	O
by	O
how	O
many	O
lines	O
are	O
taken	O
from	O
the	O
available	O
total	O
by	O
blanking	O
and	O
sync	O
,	O
and	O
the	O
total	O
scanlines	B-Algorithm
in	O
non-interlaced	O
modes	O
are	O
half	O
the	O
original	O
,	O
broadcast-spec	O
odd-numbered	O
interlaced	O
counts	O
,	O
rounded	O
down	O
)	O
.	O
</s>
<s>
Starting	O
with	O
ECS	B-Device
,	O
Denise	O
was	O
upgraded	O
to	O
support	O
"	O
Productivity	O
"	O
mode	O
which	O
allowed	O
for	O
640x400	O
non-interlaced	O
albeit	O
with	O
only	O
4	O
colors	O
.	O
</s>
<s>
Denise	O
can	O
composite	O
up	O
to	O
eight	O
16-pixel-wide	O
sprites	O
per	O
scan	B-Algorithm
line	I-Algorithm
(	O
in	O
automatic	O
mode	O
)	O
on	O
top	O
,	O
underneath	O
,	O
or	O
between	O
playfields	O
,	O
and	O
detect	O
collisions	O
between	O
sprites	O
and	O
the	O
playfields	O
or	O
between	O
sprites	O
.	O
</s>
<s>
Using	O
Copper	O
or	O
CPU	B-General_Concept
register	O
manipulations	O
,	O
each	O
sprite	O
channel	O
can	O
be	O
reused	O
multiple	O
times	O
in	O
a	O
single	O
frame	O
to	O
increase	O
the	O
total	O
sprites	O
per	O
frame	O
.	O
</s>
<s>
Sprite	O
position	O
registers	O
may	O
also	O
be	O
changed	O
during	O
a	O
scanline	B-Algorithm
,	O
increasing	O
the	O
total	O
number	O
of	O
sprites	O
on	O
a	O
single	O
scanline	B-Algorithm
.	O
</s>
<s>
However	O
,	O
the	O
sprite	O
data	O
,	O
or	O
shape	O
,	O
is	O
only	O
fetched	O
a	O
single	O
time	O
per	O
scanline	B-Algorithm
and	O
ca	O
n't	O
change	O
.	O
</s>
<s>
The	O
first	O
Amiga	B-Device
game	O
to	O
utilize	O
the	O
sprite	O
re-position	O
registers	O
during	O
a	O
scanline	B-Algorithm
was	O
Hybris	B-Application
released	O
in	O
1988	O
.	O
</s>
<s>
Denise	O
has	O
a	O
number	O
of	O
bit	O
plane	O
registers	O
which	O
hold	O
16	B-Device
bits	I-Device
of	O
data	O
each	O
,	O
enough	O
to	O
draw	O
16	O
pixels	B-Algorithm
.	O
</s>
<s>
When	O
Agnus	O
issues	O
a	O
write	O
to	O
register	O
1	O
,	O
all	O
registers	O
are	O
then	O
transferred	O
into	O
separate	O
shift	O
registers	O
from	O
which	O
pixels	B-Algorithm
are	O
generated	O
(	O
at	O
the	O
same	O
time	O
new	O
values	O
are	O
loaded	O
from	O
DRAM	O
)	O
.	O
</s>
<s>
The	O
Paula	O
chip	O
,	O
designed	O
by	O
Glenn	O
Keller	O
,	O
from	O
MOS	B-Architecture
Technology	I-Architecture
,	O
is	O
the	O
interrupt	B-Architecture
controller	I-Architecture
,	O
but	O
also	O
includes	O
logic	O
for	O
audio	O
playback	O
,	O
floppy	B-Device
disk	I-Device
drive	I-Device
control	O
,	O
serial	B-Protocol
port	I-Protocol
input/output	B-General_Concept
and	O
mouse/joystick	O
buttons	O
two	O
and	O
three	O
signals	O
.	O
</s>
<s>
The	O
logic	O
remained	O
functionally	O
identical	O
across	O
all	O
Amiga	B-Device
models	O
from	O
Commodore	O
.	O
</s>
<s>
Paula	O
has	O
four	O
DMA-driven	O
8-bit	O
PCM	B-Algorithm
sound	O
channels	O
.	O
</s>
<s>
The	O
only	O
supported	O
hardware	O
sample	O
format	O
is	O
signed	O
linear	O
8-bit	O
two	B-General_Concept
's	I-General_Concept
complement	I-General_Concept
.	O
</s>
<s>
Internally	O
,	O
the	O
audio	O
hardware	O
is	O
implemented	O
by	O
four	O
state	B-Architecture
machines	I-Architecture
,	O
each	O
having	O
eight	O
different	O
states	O
.	O
</s>
<s>
It	O
is	O
rarely	O
used	O
on	O
the	O
Amiga	B-Device
due	O
to	O
both	O
frequency	O
and	O
volume	O
being	O
controllable	O
in	O
better	O
ways	O
,	O
but	O
could	O
be	O
used	O
to	O
achieve	O
different	O
kinds	O
of	O
tremolo	O
and	O
vibrato	O
,	O
and	O
even	O
rudimentary	O
FM	B-Protocol
synthesis	O
effects	O
.	O
</s>
<s>
Most	O
often	O
,	O
DMA-driven	O
audio	O
is	O
used	O
.	O
</s>
<s>
As	O
explained	O
in	O
the	O
discussion	O
of	O
Agnus	O
,	O
memory	O
access	O
is	O
prioritized	O
and	O
one	O
DMA	B-General_Concept
slot	O
per	O
scan	B-Algorithm
line	I-Algorithm
is	O
available	O
for	O
each	O
of	O
the	O
four	O
sound	O
channels	O
.	O
</s>
<s>
On	O
a	O
regular	O
NTSC	O
or	O
PAL	B-Language
display	O
,	O
DMA	B-General_Concept
audio	O
playback	O
is	O
limited	O
to	O
a	O
maximum	O
output	O
rate	O
of	O
28,867	O
values	O
per	O
channel	O
(	O
PAL	B-Language
:	O
28837	O
)	O
per	O
second	O
totaling	O
57674	O
(	O
PAL	B-Language
:	O
57734	O
)	O
values	O
per	O
second	O
on	O
each	O
stereo	O
output	O
.	O
</s>
<s>
This	O
rate	O
can	O
be	O
increased	O
with	O
the	O
ECS	B-Device
and	O
AGA	B-Device
chipsets	B-Device
by	O
using	O
a	O
video	O
mode	O
with	O
higher	O
horizontal	B-General_Concept
scan	I-General_Concept
rate	I-General_Concept
.	O
</s>
<s>
Alternately	O
,	O
Paula	O
may	O
signal	O
the	O
CPU	B-General_Concept
to	O
load	O
a	O
new	O
sample	O
into	O
any	O
of	O
the	O
four	O
audio	O
output	O
buffers	O
by	O
generating	O
an	O
interrupt	B-Application
when	O
a	O
new	O
sample	O
is	O
needed	O
.	O
</s>
<s>
The	O
Amiga	B-Device
contains	O
an	O
analog	O
low-pass	B-Algorithm
filter	I-Algorithm
(	O
reconstruction	O
filter	O
)	O
which	O
is	O
external	O
to	O
Paula	O
.	O
</s>
<s>
The	O
filter	O
is	O
a	O
12dB/oct	O
Butterworth	O
low-pass	B-Algorithm
filter	I-Algorithm
at	O
approximately	O
3.3kHz	O
.	O
</s>
<s>
Models	O
released	O
before	O
Amiga	B-Application
1200	I-Application
also	O
have	O
a	O
static	O
"	O
tone	O
knob	O
"	O
type	O
low-pass	B-Algorithm
filter	I-Algorithm
that	O
is	O
enabled	O
regardless	O
of	O
the	O
optional	O
"	O
LED	O
filter	O
"	O
.	O
</s>
<s>
This	O
filter	O
is	O
a	O
6dB/oct	O
low-pass	B-Algorithm
filter	I-Algorithm
with	O
cutoff	O
frequency	O
at	O
4.5	O
or	O
5kHz	O
.	O
</s>
<s>
This	O
is	O
achieved	O
by	O
playing	O
the	O
high	O
byte	O
of	O
a	O
16-bit	B-Device
sample	O
at	O
maximum	O
volume	O
,	O
and	O
the	O
low	O
byte	O
at	O
minimum	O
volume	O
(	O
both	O
ranges	O
overlap	O
,	O
so	O
the	O
low	O
byte	O
needs	O
to	O
be	O
shifted	O
right	O
two	O
bits	O
)	O
.	O
</s>
<s>
The	O
bit	O
shift	O
operation	O
requires	O
a	O
small	O
amount	O
of	O
CPU	B-General_Concept
or	O
blitter	B-General_Concept
overhead	O
,	O
whereas	O
conventional	O
8-bit	O
playback	O
is	O
almost	O
entirely	O
DMA	B-General_Concept
driven	O
.	O
</s>
<s>
This	O
technique	O
was	O
incorporated	O
into	O
the	O
retargetable	O
audio	O
subsystem	O
AHI	B-Library
,	O
allowing	O
compatible	O
applications	O
to	O
use	O
this	O
mode	O
transparently	O
.	O
</s>
<s>
The	O
floppy	B-Device
controller	O
is	O
unusually	O
flexible	O
.	O
</s>
<s>
It	O
can	O
read	O
and	O
write	O
raw	O
bit	O
sequences	O
directly	O
from	O
and	O
to	O
the	O
disk	O
via	O
DMA	B-General_Concept
or	O
programmed	B-General_Concept
I/O	I-General_Concept
at	O
500	O
(	O
double	B-Device
density	I-Device
)	O
or	O
250kbit/s	O
(	O
single	B-Device
density	I-Device
or	O
GCR	B-Device
)	O
.	O
</s>
<s>
MFM	B-Protocol
or	O
GCR	B-Device
were	O
the	O
two	O
most	O
commonly	O
used	O
formats	O
though	O
in	O
theory	O
any	O
run-length	B-Protocol
limited	I-Protocol
code	I-Protocol
could	O
be	O
used	O
.	O
</s>
<s>
It	O
also	O
provides	O
a	O
number	O
of	O
convenient	O
features	O
,	O
such	O
as	O
sync-on-word	O
(	O
in	O
MFM	B-Protocol
coding	O
,	O
$4489	O
is	O
usually	O
used	O
as	O
the	O
sync	O
word	O
)	O
.	O
</s>
<s>
MFM	O
encoding/decoding	O
is	O
usually	O
done	O
with	O
the	O
blitter	B-General_Concept
—	O
one	O
pass	O
for	O
decode	O
,	O
three	O
passes	O
for	O
encode	O
.	O
</s>
<s>
Normally	O
the	O
entire	O
track	O
is	O
read	O
or	O
written	O
in	O
one	O
shot	O
,	O
rather	O
than	O
sector-by-sector	O
;	O
this	O
made	O
it	O
possible	O
to	O
get	O
rid	O
of	O
most	O
of	O
the	O
inter-sector	O
gaps	O
that	O
most	O
floppy	B-Device
disk	I-Device
formats	O
need	O
to	O
safely	O
prevent	O
the	O
"	O
bleeding	O
"	O
of	O
a	O
written	O
sector	O
into	O
the	O
previously-existing	O
header	O
of	O
the	O
next	O
sector	O
due	O
to	O
speed	O
variations	O
of	O
the	O
drive	O
.	O
</s>
<s>
This	O
way	O
,	O
for	O
the	O
native	O
Amiga	B-Device
disk	O
format	O
,	O
the	O
raw	O
storage	O
capacity	O
of	O
3.5inch	O
DD	O
disks	O
was	O
increased	O
from	O
the	O
typical	O
720KB	O
to	O
880KB	O
,	O
although	O
the	O
less-than-ideal	O
file	B-Application
system	I-Application
of	O
the	O
earlier	O
Amiga	B-Device
models	O
reduced	O
this	O
again	O
to	O
approximately	O
830KB	O
of	O
actual	O
payload	O
data	O
.	O
</s>
<s>
The	O
Amiga	B-Device
3000	I-Device
introduced	O
a	O
special	O
,	O
dual-speed	O
floppy	B-Device
drive	I-Device
that	O
also	O
allowed	O
to	O
use	O
high	O
density	O
disks	O
with	O
double	O
capacity	O
without	O
any	O
change	O
to	O
Paula	O
's	O
floppy	B-Device
controller	O
.	O
</s>
<s>
The	O
serial	B-Protocol
port	I-Protocol
is	O
rudimentary	O
,	O
using	O
programmed	B-General_Concept
input/output	I-General_Concept
only	O
and	O
lacking	O
a	O
FIFO	B-Operating_System
buffer	O
.	O
</s>
