FULL
SCREEN
AREA
~--------------------------------------~o
----------~--------------------+__128
VSTOP
of
DISPLAY
WINDOW occurs in
this region.
(NTSC)------11--262
Figure
3-22:
Display Window Vertical Stopping Position
To
set the display window stopping position, write HSTOP into bits 0 through 7 and
VSTOP into bits 8 through
15
of DIWSTOP.
MAXIMUM
DISPLAY
WINDOW
SIZE
The
maximum size of a playfield display
is
determined by the maximum number of lines
and the maximum number of columns. Vertically, the restrictions are simple. No
data
can be displayed
in
the vertical blanking area, which ranges from line 0 through line
19
(20 lines total). This leaves
242
lines of displayable screen video (interlaced mode dou-
bles this to 484).
Horizontally, the situation
is
similar. Strictly speaking, the hardware sets a rightmost
limit to DDFSTOP of ($D8) and a leftmost limit to DDFSTRT of ($18). This gives a
maximum of
25
words fetched
in
low-resolution mode. In high-resolution mode the max-
imum here is
49
words, because the rightmost limit remains ($D8) and only one word is
fetched
at
this limit. However, horizontal blanking actually limits the displayable video
to
376 low-resolution pixels (23.5 words). In addition,
it
should be noted
that
using a
data-fetch
start
earlier than ($38) will disable some sprites.
72 Playfield Hardware
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