Someone else mentioned the black lines on the left of the screen (my
mailer crashed while replying and I lost the original message).. It does
seem to happen when either player is repositioned. It's quite noticeable
in Berzerk, and *very* obvious in Missile Command on a non-black
background.
If I'm right here, Missile Command repositions (probably doesn't rewrite
GRP1, though - only one pixel per scanline is active for each enemy
missile) one sprite every other scanline (hence the "comb" on the left)
to display the missiles, and activates the appropriate number of copies.
Each group of parallel missiles (maximum 2 groups at once, and maximum
three missiles in one group) is also drawn using player 1 on alternating
frames, leaving player 0 to draw the cursor.. this also explains why
there is no flicker when two enemy missiles overlap precisely. It also
appears to use the ball and missile graphics to display the explosions,
hence the limit of three explosions simultaneously.
Hmm.. anyone want to try writing a flicker-free Missile Command using
both players and both missiles for the enemy missiles (allowing four
independent flicker-free missiles), the ball for the cursor, and some
other trick for explosions.. maybe square explosions using the playfield?
:)
Apologies to those who've already figured this out, but putting it into
words helps clarify my understanding of it. Also, I don't know why the
topmost line of the "comb" moves vertically.. anyone know?
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