Atari XL User Manual

Player-Missile Graphics 181
The values for the two bits are the same as those
used in the player width registers.
It is possible to use the four missiles as a fifth
player by careful use of the missile horizontal
position registers. If bit 4 of the priority register
GPRIOR is set to 1, the missiles will all appear the
same colour - that indicated in display colour
register number 3.
PRIORITIES
With a possible four players and four missiles
moving around the screen, and passing over
graphics displays, some rules are necessary for
deciding what should be shown where two players
overlap, or where a player or missile overlaps the
other graphics. Another GTIA register is used to
controls the relative priorities of players, missiles,
and graphics of different colours. This register is
called GPRIOR, and is at address 623. The four
lowest bits of this register (bits 0 to 3) are used to
select among four different orders of precedence for
the colours. These are shown in the table overleaf.
Notice that the priorities are given in terms of
colour registers. The players and missiles are not
shown separately as the colour registers are
shared, and it is the colour register used which
determines the priority of a graphics item. Points
to note are that colour register 4 is always used for
the background colour of a graphics display, and
colour register 3, which is not used in most .
graphics modes, may be used as the colour register
for a fifth player ifbit 4 of GPRIOR is set to 1.
If you try setting more than one of the four priority
controlling bits of GPRIOR at the same time, you
will get very strange results, as the order of
precedence will not then be clearly defined.

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