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Forums > C64 Coding > Sprite data fetch in sideborder

2014-03-03 22:43

Copyfault

Registered: Dec 2001
Posts: 475

Sprite data fetch in sideborder

I'm playing around with sprites in the sideborder. Usually the ypos for a sprite is one less than the rasterline it should actually show up, e.g. if you want to see a sprite in rasterline $33, you have to put $32 in its ypos-reg.

Now if a sprite has xpos >= $164, the sprite display is turned on in the very same line. For sprite 0, this behaviour can be exploited as the sprite data is read in the cycles directly before. This was done e.g. to get 9 sprites in one rasterline.

If we put any other sprite at this position (xpos >= $164), the display will also start immediatly on the same rasterline (not on the following one). But the sprite data that was fetched looks like a [$ff $00 $ff]-pattern.

I found out that the '$00' in the middle comes from the last byte of the active vic bank (ghostbyte) and that it can be changed accordingly.

Still not clear is the origin of the other $ff-bytes. Are they hardwired just like the $ff-bytes the vic reads as vram on fli-lines? Are they coupled to the adress/data bus somehow?

Maybe someone has already examined this and could explain the behaviour.

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2020-05-18 20:34

chatGPZ

Registered: Dec 2001
Posts: 11354

5 years later i looked at this program and added it to the VICE testbench: https://sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/VIC..

Some questions are open (to me anyway):

- apparently this problem applies to all sprites. (to my surprise) the problem always start at xpos $164. however, the cycles where the sta vicreg,x must be performed is different for each sprite - which are they? just all spaced 3 cycles apart? or 4? a little table would be a great addition to that readme i copypasted together :) it would also be nice to have this proved in testprogram(s) :)
- how does all this relate to sprite-sprite (and perhaps even sprite-background) collisions? i would guess that it works as expected, ie pixels that get displayed will also cause collisions. but is that really what happens? another test for this would nice to have :)

keep it coming :)

2020-05-18 22:16

Copyfault

Registered: Dec 2001
Posts: 475

Quoting Groepaz

[...]- apparently this problem applies to all sprites. (to my surprise) the problem always start at xpos $164. however, the cycles where the sta vicreg,x must be performed is different for each sprite - which are they? just all spaced 3 cycles apart? or 4? a little table would be a great addition to that readme i copypasted together :) it would also be nice to have this proved in testprogram(s) :)

I'd say you need to set the sta abs,x-command two cycles apart, i.e. the last two cycles of that opcode must match with the sprite fetch cycles of the corresponding sprite (with spr0 being an exception, as the sprite fetch cycles have been correctly executed at this pos. of the rasterline).

Quoting Groepaz

- how does all this relate to sprite-sprite (and perhaps even sprite-background) collisions? i would guess that it works as expected, ie pixels that get displayed will also cause collisions. but is that really what happens? another test for this would nice to have :)

keep it coming :)

You're right, this calls for a proper update of the test prog... oh boy /o\

CF

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