Re: [stella] MultiSpriteDemo update (source+binary)

Subject: Re: [stella] MultiSpriteDemo update (source+binary)
From: emooney@xxxxxxxxxxxxxxxx (Erik Mooney)
Date: Mon, 07 Apr 1997 12:04:37 GMT
>>It's tricky, but I think I've figured it out.
>Thanks... do you think it is faster or slower than the more common,
>SBC-based routine?

I'm pretty sure it is faster.  The new routine uses between 59 and 73
cycles plus the JSR and RTS.  If the check for out-of-range values is
removed and the routine is made a macro, it will fit into one scanline if
needed.  The SBC routine uses at least 7 cycles per iteration (INX, SBC
#15, BPL), which can take at least 70 cycles, and the initialization and
cleanup (shifting the fine coordinate to the upper 4 bits and putting
everything together into one register) would be about 30, I think, for a
range of 30 to 100.  The average is about the same, but I like the new one
because it's more consistent.

>>Obviously, the whole thing would be 12 cycles faster as a macro instead of
>>as a subroutine if you can spare the space and only use it a few times.
>Not really needed if it's called during VBLANK, when there are cycles to lose...

True, but it can be used during screen-display if needed... maybe you're
short on RAM and can only store the standard X coordinate of objects
instead of both the standard X and FC_X. (with a lot of objects, this could
be important)

and combining messages:

>hmmm... you'll have to experiment on the real 2600, as things might be
>different on emulators.
>I wonder why are you asking this :) ...maybe you want to use all the
>available cycles during a scanline, without even doing the STA WSYNC?

I had a routine that needed to rewrite both GRP registers each line while
updating all three PF registers twice per scanline (non-repeating,
non-reflected playfield) and checking the ball's Y coordinate and writing
to ENABL if necessary (all this is possible - see Millipede), and I needed
to know exactly how many instructions I could do before needing to STA
WSYNC.  It turns out that you must STA WSYNC no later than cycle 71 to
catch the end of the current line on a real 2600, while PC Atari allows a
STA WSYNC as late as cycle 74.

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