> It seems that it's possible that I'm already stepping
> onto a new scannline, between moment the counter expires
> and hitting VSYNC.
This trace log shows the bug that I mentioned in
an other post recently, so you shouldn't count to
much on the scanline counter in this case.
But as it seems there is the possibility, that you
are doing the WSYNC at cycle 75 in some cases. This
is two cycles too late and should result in an extra
scanline.
> Now, what should I do?
>
> -Remove the STA WSYNC again?
> -Try a smaller timer value?
> -Do the LDY #$02 before the STA WSYNC?
> -Any other idea to grant a stable picture?
As it seems the timer loop always expires
after cycle 70 or so. One timer step is only
64 cycles, so if you reduce the timer value
by 1, you should always get a stable picture.
> And why is Z26 not jerking?
Because we are not really sure when the jittering
is supposed to happen and when not. But maybe we
could add a display that constantly shows you the
number of scanlines in the last frame on the screen.
That way you can see for yourself when you are not
generating a stable number of scanlines.
Ciao, Eckhard Stolberg
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