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The Atari 5200 is essentially a console version of the Atari 8-bit
Computers (400/800, XL, XE, XEGS). The functions of the system are
divided up between four major IC chips:
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The CPU in the 5200 is a modified 6502 processor. The only
difference is that Atari's version of this CPU has some extra
hardware on board that allows the ANTIC chip to take over the bus to
do Direct Memory Access (DMA).
POKEY's main job is to generate sound and to perform a variety of
other miscellaneous functions. The POKEY chip has four separate
channels, and the pitch, volume and distortion values of each of
these channels can be controlled individually. POKEY is also used to
read the position of each of the joysticks, and scans the keypad on
each controller for key presses. Its other functions include the
random number generator, IRQ handling, and 3 high-speed counters.
POKEY also has a serial communications port that is connected to the
5200's expansion port.
ANTIC can be thought of as the 5200's graphics co-processor. ANTIC
has direct access to RAM where it reads a special program called the
display list. The display list tells ANTIC exactly how to draw the
display and then sends information to the GTIA which generates the
actual video signals. The 5200 supports 17 seperate video modes;
some are character modes, others graphic. Each mode has a different
combination of vertical size, horizontal size, and number of colors.
Using the display list, these modes can be mixed freely on a single
screen, so for example a screen could have a couple lines of text
mode at the top, a block of hi-res graphics in the middle, and more
text at the bottom. The location in memory that the display date
comes from can easily be controlled through ANTIC. The display data
can be almost anywhere in memory, and it is even possible for the
data for each line to come from totally different places in memory.
This allows horizontal and vertical scrolling to be implemented very
easily. ANTIC is also responsible for controlling Non-Maskable
Interrupt to the processor.
The GTIA's main responsibility is to generate the video signals to
the TV and to handle sprites. The GTIA is where the actual colors
are put into the video signal. The 5200 has a palette of 256 colors.
Most video modes only allow four colors, but a facility in the ANTIC
chip allows the values of these colors to be changed on each line of
the screen, so it is possible to have all 256 colors on screen at
once. The 5200 has four player and four missile sprites. Player
sprite are eight pixels wide and either 128 or 256 pixels high,
missiles are two pixels wide and either 128 or 256 pixels high. Each
sprite can have its on color, independent from the normal screen
colors. The GTIA chip can detect collisions between players,
missiles, and the playfield. This chip is also responsible for
reading joystick trigger buttons, and controlling which controller
is being read by POKEY.