Atari 800

HSCROL (Horizontal Scr oll Register)(D404): This address writes data
into the Horizontal Scroll Register. Only playfield is scrolled, not
players and missiles.
not used
D3 D2 D1 DO
0 to 15 color
clock right shifts
The display is shifted to the right by the number of color clocks
specified by HSCROL for each display list instruction that contains a 1 in
its HSCROL Flag bit (bit 4 of instruction byte).
When horizontal scrolling is . enabled, more bytes of data are needed.
For a ~arrow playfield (see DMACTL bits 1 and 0) there should be the same
number
of bytes per line as for standard playfield with no scrolling.
Similarly, for standard playfield use the same number of bytes as for the
wide playfield. For wide playfield, there is no change in the number of
bytes and background color is shifted in.
VSCROL (Vertical Scroll Register)(D405): This address writes data into
the Vertical Scroll Register.
not used
D2 D1 DO
8 line display modes
not used
D3 D2
D1 DO
16 line display modes
The display is scrolled upward by the number of lines specified i n
the VSCROL register for each display list instruction that contains a 1 in
its VSCROL Flag bit (bit 5 of instruction byte). The scrolled area will
terminate with the first instruction having a zero in bit 5. (see sect i on
II for more details).
PRIOR (Priority)(D01B): This address writes data into the Priority
Control Register.
D7
D6 D5 D4 D3 D2
D1 DO
D7-D6 = 0 D5
Multiple Color Player Enable.
This bit causes the logical "or" function of the bits of
the colors of Player 0 with Player 1, and also of Player 2
with Player 3. This permits overlapping the position of 2
players with a choice of 3 colors in the overlapped region.
III. 7