Atari 800

There are a total of four players and four missiles. The four
missiles may be grouped together and used as a 5th player. These
objects are positioned horizontally by 8 horizontal position registers
(HPOS (X)). These registers may be reloaded at any time by the proces-
sor, allowing an object to be replicated many times across a horizontal
TV line.
The shape of a player-missile is determined by the data in its
graphics register (GRAF (X)). Players have independent 8 bit graphics
registers. The four missiles have 2 bit registers (located within one
address). These registers may also be reloaded at any time by the
processor, although they are usually changed during horizontal blank
time. The data in each graphics register is placed on the display
whenever the horizontal sync counter equals the corresponding horizon-
tal position register. The same data will be displayed every line unless
the graphic registers are reloaded with new data.
The player-missile graphic registers may be reloaded by the micro-
processor (GRAF (X)), or automatically from memory with direct memory
access (DMA) (see figure II.3). The programmer must place the object
graphics in memory, write the player-missile base address (PMBASE), and
enable player-missile DMA (DMACTL, GRACTL). The transfer of object
graphics from memory to display is then fully automatic.
PMBASE specifies the most significant byte (MSB) of the address of
the player-missile graphics. The location of the graphics for each
object is determined by adding an offset to PMBASE *256 (decimal). The
bytes between the base address and the missile data are not used by
Antic, so they are available to the programmer.
Only the five most significant bits of PMBASE are used with
single-line resolution and the six most significant bits are used
with two-line resolution. This means that the location of the graphics
in memory is restricted to certain page boundaries. Two-line resolu-
tion means that each byte of data is repeated for two lines. (see
DMACTL, bit 4). 640 (decimal) bytes (5X128) are required for two-line
resolution and 1280 bytes (Sx256) for one-line resolution.
Each byte in the player graphics area represents eight pixels which
are to be displayed on the corresponding line(s) of the TV screen. A
1
indicates that the player's color-lum is to be displayed in that pixel.
The graphics may be anything, not just rectangles like the ones in figure
II.3. The player graphics may fill the entire height of the screen or
they may be only a couple of lines high if the rest of the display data is
all O's. Each byte in the mis$ile display also r epresents eight pixels,
two pixels for each missile. Each pixel may be 1, 2, or 4 color clocks,
and is determined by the SIZE registers.
Plavfield: Playfield is always generated by DMA. There are four
playfields, each identified by its own color-lum register and collision
detection. Playfield is generated by two different DMA techniques:
memory map and character. Both methods provide lists of instructions in
memory, independent of the player-missile generation.
rr.s