Atari XL User Manual

176
The Atari XL Handbook
As you have seen, the data in the GRAPH register
controls the appearance of the player. As the GTIA
builds up the video pictures, scanning across the
screen from left to right, it is constantly checking
the HPOS registers to see if there are players to be
swi tched on. When the GTIA finds it has reached
the specified horizontal position for a player,the
data in the appropriate GRAPH register is used to
generate the next part of the horizontal line
instead of the normal data in the screen memory.
This process is repeated as each line of the display
is scanned: as the GTIA reaches a player's HPOS
setting, it takes data from the GRAPH register for
that player to generate the next part of the line.
Because the display is built up a line at a time, if
we could change the contents of the GRAPH
register between lines, we could create much more
interesting shapes for the players.
It would be impossible to modify the registers
quickly enough in BASIC, but it can be done. The
Atari has another special chip, the ANTIC, which
is dedicated to feeding the GTIA registers wi th the
right information at the right time. All we have to
do
is set up a table in memory of the data for each
line of a player, and tell the ANTIC chip to use this
data to feed the GRAPH register of the GTIA. The
next program uses this technique to turn Player 0
into a sunflower.
10 REM SUNFLOWER
20 PMBASE = 54279
30 POCOL = 704
40 POSIZE 53256
50 POHPOS 53248
60 SDMCTL = 559
70 GRACTL = 53277
100 START = INT(PEEK(742)/8)-1
110 FOR X=l TO 25

Atari XL Specifications

General

Release Year	1983
CPU	MOS Technology 6502C
Graphics	ANTIC, GTIA
Sound	POKEY (4 channels)
Storage	Cassette tape
Ports	Cartridge
Operating System	Atari BASIC
CPU Speed	1.79 MHz
RAM	64 KB