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2005-09-12 18:15

Devia

Registered: Oct 2004
Posts: 401

Bad Line Condition

I'm relatively new to C64 coding so bear with me if I sound like a complete n00b :)

..so I was going through Christian Bauer's VIC doc to learn some trix and I stumbled upon the following definition:

"A Bad Line Condition is given at any arbitrary clock cycle, if at the negative edge of ø0 at the beginning of the cycle RASTER >= $30 and RASTER <= $f7 and the lower three bits of RASTER are equal to YSCROLL and if the DEN bit was set during an arbitrary cycle of raster line $30."

Im my attempts to code some really fancy effect >8oD ..I needed to locate a Bad Line and referred to the above mentioned text. When the result was not what I expected, I did some tests:

Now.. the default value of $d011 is $1b. I put an uppercase 'A' at $0400 to verify that the top of the 'A' is adjacent with the top border. I then put a sprite at Y=$33 to verify that the first visible line is $32.

My question now is: How can Christian Bauer's definition be true if the first Bad Line is at $32 (low 3 bits of $d012=%010) when $d011=$1b (low 3 bits of $d011=%011)?

2005-09-12 19:32

Cybernator

Registered: Jun 2002
Posts: 154

Not sure if I understand your question correctly, but you seem to be confused by sprite positioning. Recall that sprites appear one rasterline lower than their actual Y coord. E.g. if you set $d001 to $33, the sprite will appear at rasterline $34. When $d011 is set to $1b, the first badline (and first "visible" line) indeed appears at rasterline $33.

If I misunderstood, sorry for boring to death. ;)

2005-09-12 19:53

Devia

Registered: Oct 2004
Posts: 401

You explanation makes PERFECT sense!!! Thank you!

Completely forgot about the sprite y-pos thing...

Suddently a lot of my routines weird behaviour is totally clear to me ;-)

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