Atari 400 Hardware Manual

In order to begin this DMA the main microprocessor must store a
display list of instructions in memory, store data to be displayed in
memory, tell the ANTIC where the display list is (initialize the display
list point'er) and enable the DMA control flags on the ANTIC (DMACTL
register).
In addition to the playfield DMA described above, the ANTIC
chip simultaneously controls another DMA channel. This type of DMA
addresses PLAYER-MISSILE graphics data stored in memory and passes the
graphics data on to the CTIA chip graphics registers. This type of DMA
(if enabled) occurs automatically, interspersed with the playfield DMA
described previously. This PLAYER-MISSILE DMA has no display list or
instructions, and is therefore much simpler than the PLAYFIELD DMA.
In addition to the two types of display DMA, the ANTIC chip also
generates DMA addresses for the refresh of the dynamic memory RAM used
in this system. This is also completely automatic and need be consider-
ed by the programmer only if he is concerned with real-time programming
where an exact count of the computer cycles is important.
Color-luminance: A color-luminance register is used on the CTIA chip
for each Player-Missile and Playfield type. Each color-lum register is
loaded by the microprocessor with a code representing the desired color
and luminance of its corresponding Player-Missile or Playfield type. As
the serial data passes through the CTIA chip it is "impressed" with the
color and luminance values contained in these registers, before being sent
to the TV display. In areas of the screen where there are no objects the
background color (COLBK) is displayed. The CTIA also does collision
detection (to be described later).
Priority: When moving objects, such as players and missiles,
overlap on the TV screen (with each other or with Playfield) a decision
must be made as to which object shows in front of the other. Objects
which appear to pass in front of other ·s are said to have Priority over
them. Priority is assigned to all objects by the CTIA chip before the
serial data from each object is combined with the other objects and sent
to the TV screen.
The priority of objects can be controlled by the microprocessor by
writing into the control register PRIOR. The functions of the bits in
this register are given in the table in the PRIOR register description in
section III.
Players and Missiles: The players and missiles are small objects
which can be moved quickly in the horizontal direction by changing their
position registers. They are called players and missiles because they
were originally designed to be used in games for objects such as airplanes
and bullets. However, there are many other possible applications for
them. The four player-missile color registers, in conjunction with the
four playfield color registers and the background color register, make
it possible to display 9 different colors at the same time.
II.3

Atari 400 Specifications

General

Manufacturer	Atari
Model	400
Type	Home Computer
Release Year	1979
CPU	MOS Technology 6502
CPU Speed	1.79 MHz
ROM	10 KB
Operating System	Atari OS
Keyboard	Membrane keyboard
RAM	8 KB (expandable to 48 KB)
Graphics	ANTIC and GTIA chips
Sound	4 channels
Display	RF output for connection to TV
Storage	Optional cassette tape drive or floppy disk drive
Ports	Cartridge
Display Resolution	320x192 (16 colors)