Re: [stella] Sprites source code

Subject: Re: [stella] Sprites source code
From: emooney1@xxxxxxxx (Erik K Mooney)
Date: Sat, 15 Mar 1997 19:11:12 EST
>It still sort of "trips over" that 15 there, but it's accurate enough
>for most purposes.  There certainly isn't much danger of erratic jerking
>around, it'll just snag on every 15 pixels.  In real life, the left
>side of the screen is 0 and the right side of the screen is 152 (160-8).
>Here, the left side of the screen is 8 (eight), and the right side of
>the screen... uh, I'll figure it out later.

Why is the right side of the screen 152 and not 160?  I was experimenting
with sprite moving, and, on PC Atari, whenever RESP0 happened at 144 or
later, the sprite would get displayed on the left side of the screen
instead of the right (couldn't tell if it was dropping down a scanline or
not - I suspect it was) .. if I limited its X-coordinate to 143, it
worked fine.  The right side of the screen being 152 instead of 160 would
explain what I saw, but why can't we use those last eight pixels for
sprites? Playfield gets displayed fine in those columns (haven't tried
missile/ball.)

Also, in PC Atari, both real-time and in the debugger) there seemed to be
a two-cycle (six pixel) delay after the STA RESP0 completed until it took
effect.. as in, I began the STA RESP0 at cycle 22, it completed at cycle
25, which should be the 75th pixel after WSYNC and 7th from the left edge
of the screen... but PC Atari displayed it as "RESP0=12" (six more than
where it should be, because PC Atari counts from zero) both onscreen and
in the debugger.  Is there always a delay after a write to RESxx?  And if
so, is this delay the reason why you can't use the last few pixels of the
screen?

One other question - Is HMP0 reset after writing to RESP0?  When I wrote
HMP0 before RESP0, the sprite jumped at 15-pixel intervals.. if RESP0 was
written before HMP0, it moved smoothly.

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