I still expect the timing to get *very* close, because you want to
position all objects with single line precision. It might not be
possible without some compromises.
<<
I had no idea until now how tricky this is going to be. With the Y register
already used throughout for the scanline counter and the X register used to
index the playfield graphics, it's going to be hard to do all the writes
bunched up close together. That's a write to GRP0, GRP1, (on one of the two
scanline pairs) and the horizontal motion and size registers for M0 and M1
that (depending on the Y position) could have to happen on the same scanline
as the player writes.
Anyway this probably means going back to a solid line on the safezone for
one thing. That's probably inevitable.
I also have to worry about putting in any conditional logic into the NOP
area before the PF2 rewrite...
Note that only the pedestrians move across the entire screen. Well, not
necessarily the ENTIRE screen. I can bound them in by 2 playfield pixels on
the left or right maximum if necessary, just as long as I start them out
behind the safezone. The cars are bounded between the two safezone (PF0)
areas. So that gains me some time, doesn't it? If the sprites extend to
within 2 playfield pixels of the edge, what is the safe cycle ranges to
update? How much of a window on the right side of the line? If bounded by
PF0 (cars) how much more time to I gain on the left and on the right??
In the end I might be able to do something really sneaky...
I could have different versions of the kernel that update at different spots
depending on the X position of the objects in order to avoid stomping on the
sprite while it's being drawn. If I evaluate the screen as two halves and
have two different kernels depending on whether the sprite is on the left or
right half, then that's 16 possible combinations if done for all 4 objects.
The game is simple enough that even if I went that far it might still fit
into 4K.
Do any other games do this kind of thing? I'm sure I can't be the first to
think of this.
Also, Piero, is this tearing phenomenon what happened with the player's
shots in Oystron? If that's the case, I can really sympathize now ;)
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