Re: $DD00

From: Stephen Judd (judd_at_merle.acns.nwu.edu)
Date: 1998-10-26 22:35:29

> > This last part is very stupid.  Of course it switches VIC banks, as the
> > two bitmaps are in two separate banks.  What I meant is that the bitmaps
> > are in banks 1 and 2, and I see no way for $DD00 to accidentally point
> > to bank 0 or 3 for a single frame.  My thinking is that the garbage might
> > be $3FFF or some such.
> 
> How wide is the garbage?  Only 8 pixels?  If yes, then it could be that
> the $DD00 output lines on the CIA do not change exactly simultaneously
> (1->2 could become 1->3->2 or 1->0->2), or they change while the video
> chip is reading the data, changing the A14 and A15 during an "address
> valid" phase.  I haven't experimented with this; I've always switched
> video banks somewhere in the screen border.

I should have explained in more detail.

The interrupt, which does the $DD00 switching, is in the lower border (line
250 or so).  It switches between bitmaps in banks 1 and 2 to get the
interlaced pic.

Most of the time, the picture would display fine.  But at random intervals,
an entire screen of garbage would appear, for just one frame -- the entire
screen is full of garbage for that one frame, though.  Also, the interrupt 
had a habit of "splitting", that is, occur a few lines lower on the screen.  
How can you get a raster interrupt to occur on different lines?

Anyways, I should obviously get ahold of the code and send it over.  But
out of respect to your (and my!) studies, perhaps I will wait until
January :).

-Steve
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