Atari 800

Unlike players and missiles, there are no horizontal position registers
for playfield. Each player can only have one byte of display per line.
Playfield, on the other hand, may require up to 48 bytes per line because
it can fill the entire width of the screen.
There are three different playfield widths: narrow (128 color
clocks), standard (160 color clocks), and wide (192 color clocks).
The width is selected by storing into DMACTL. The advantage of a narrower
width is that less RAM is required and fewer machine cycles are stolen for
DMA. The OS graphics modes use the standard screen width.
Display List: The display list is a sequence of display instructions
stored in memory. These instructions are either one (1) byte or three (3)
bytes long. The display list can be considered a display program, and the
Display List Counter that fetches these instructions can be thought of as
a display program counter. (10 bit counter plus 6 bit base register.)
The display list counter can be initialized by writing to DLISTH and
DLISTL.
(or OS shadow registers SDLSTH and SDLSTL). Once initialized
this counter value is used to address the display list, fetch the instruc-
tion, display one (1) to sixteen (16) lines of data on the TV screen,
increment the Display List Counter, fetch the next display instruction,
and so on automatically without microprocessor control (see DLISTL and
DLISTH).
DLISTL and DLISTH should be altered only during vertical blank
or when DMA is disabled (see DMACTL).
Each instruction defines the type (alpha character or memory map) and
the resolution (size of bits on screen) and the location of data in memory
to be displayed for a group (1 to 16) of lines. Each group of lines is
called a display block.
THE DISPLAY LIST CANNOT CROSS A 1K BYTE MEMORY BOUNDARY UNLESS
A JUMP INSTRUCTION IS USED.
\
Counter
DISPLAY LIST COUNTER
II.7
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