because the rightmost limit remains ($D8) and only one word is fetched at this limit.
However, horizontal blanking actually limits the displayable video to 368 low-resolution
pixels (23 words). These numbers are the same both for NTSC and for PAL. In addition, it
should be noted that using a data-fetch start earlier than ($38) will disable some sprites.
Table 3-14: Maximum Allowable Horizontal Screen Video
LoRes HiRes
DDFSTRT (standard) $0038 $003C
DDFSTOP (standard) $00D0 $00d4
DDFSTRT (hw limits) $0018 $0018
DDFSTOP (hw limits) $00D8 $00D8
max words fetched 25 49
max display pixels 368 (low res)
MOVING (SCROLLING) PLAYFIELDS
If you want a background display that moves, you can design a playfield larger than the
display window and scroll it. If you are using dual playfields, you can scroll them
separately.
In vertical scrolling, the playfield appears to move smoothly up or down on the screen. All
you need do for vertical scrolling is progressively increase or decrease the starting address
for the bit-plane pointers by the size of a horizontal line in the playfield. This has the
effect of showing a lower or higher part of the picture each field time.
In horizontal scrolling the playfield appears to move from right-to-left or left-to-right on
the screen. Horizontal scrolling works differently from vertical scrolling you must arrange
to fetch one more word of data for each display line and delay the display of this data.
For either type of scrolling, resetting of pointers or data-fetch registers can be handled by
the Copper during the vertical blanking interval.
VERTICAL SCROLLING
You can scroll a playfield upward or downward in the window. Each time you display the
playfield, the bit-plane pointers start at a progressively higher or lower place in the big
picture in memory. As the value of the pointer increases, more of the lower part of the
picture is shown and the picture appears to scroll upward. As the value of the pointer
decreases, more of the upper part
- Playfield Hardware 75 -
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