Atari XL

Player-Missile Graphics
177
120 READ F: POKE START*2048 + 1024
+ 120 + X, F
130 NEXT X
150 POKE POCOL, 28
160 POKE POSIZE, 1
170 POKE PMBASE, START *8
180 POKE SDMCTL, 58
190 POKE GRACTL, 2
200 FOR X=20 TO 180
210 POKE POHPOS, X
220 FOR T=1 TO 10: NEXT T
230 NEXT X
500 DATA 30, 42, 42, 93, 93, 93,
93, 42, 42, 3D, 8, 8, 8, 8, 8,
8, 8, 8, 8, 8, 8, 107, 42, 42,
28
Three new control registers are used by this
program: PMBASE (54279), SDMCTL (559) and
GRACTL (53277).
PMBASE is a control register in the ANTIC chip,
telling it where in memory to find the data tables
for the players and missiles. SDMCTL controls the
memory access of the ANTIC, and GRACTL is a
GTIA register, again controlling memory access.
The GRACTL (graphics control) register is used as
follows:
Bit 1 is set to 1 to use memory tables for player
graphics.
Bi t 0 is set to 1 to use memory tables for missile
graphics.
The other bits ofGRACTL have no meaning.
SDMCTL is more complicated: