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Barbarossa
Registered: May 2007
Posts: 31 |
Cycle count in upperborder
Hi guys,
When trying to put together some timing scheme I noticed something I cannot explain.
When I singlestep trace my rasterinterrupt (I use CCS64) which triggers at raster $000, on the first line everything behaves fine. But on the second line ($001) and furtherdown, the cycle counter jumps from 56-58 to 8-10 as if sprites were active.
Sprites are active on the screen however only from Y=$36 to Y=$76.
What I am missing here? There cannot be bad lines in the border, and I am certain there are no sprites active. Why is the VIC stealing my cycles?
Some insight will be greatly appreciated.
Thanks.
John
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Scout
Registered: Dec 2002
Posts: 1568 |
Quote:
Why is the VIC stealing my cycles?
*Insert random seal picture here*
(Sorry for the thread hijack ;-) |
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Ninja
Registered: Jan 2002
Posts: 407 |
One possible answer:
Sprite y position is 8 bit only. So, if you already set the sprite y to $36 before, VIC will display the sprites also beginning with $136 (PAL reaches this line) for the next 21 lines, which will also cover the top of the screen. So, actually this should happen on line 0 as well(can you recheck? could also be an emulator problem, you never know...). |
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Barbarossa
Registered: May 2007
Posts: 31 |
Ninja,
Learning all the time... It looks to me your answer might be the right one, so I did a test. I moved my sprites to $06c. Obviously there can never be a $16c rasterline so in theory the delay should be gone. And... it was.
One question remains, why is it not there on rasterline $000? Maybe $000 is a special case because you cannot put a sprite on that line as well (I think it is not possible, since sprite Y = raster + 1).
Also it seems weird (allthough logical) that a sprite can steals cycles twice a frame.
John
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Barbarossa
Registered: May 2007
Posts: 31 |
So the delay is not there on $000, in both cases. |
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Ninja
Registered: Jan 2002
Posts: 407 |
Have you checked on a real c64? It might as well be an emulator bug. Can't tell, I never checked this. But if a sprite started before $0, I would assume it will be fetched also in 0. If not, abuse it, give it a fancy name and make a demo part out of it :) |
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Barbarossa
Registered: May 2007
Posts: 31 |
I have not checked it on a real C64.
Maybe the users of Vice can confirm if this behaviour does or doesn't occur on that emulator? |
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assiduous
Account closed
Registered: Jun 2007
Posts: 343 |
better check in Hoxs64 if you want to be on a safe side and cannot check on a C64. |
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Ninja
Registered: Jan 2002
Posts: 407 |
Naaah, nothing besides a real C64 can serve as a reference (by defintion, so to say ;)). Maybe someone else is willing to check this... |
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Barbarossa
Registered: May 2007
Posts: 31 |
I can check it, but you know how does things go. Digging up the C64 out of a closet and plugging it on takes some time :-) And then I have to write some program to check, because there is not such thing as a cycle monitor on the real thing.
It could be a bug, however I doubt it. I have seen most things work like a charme on CCS64. |
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Ninja
Registered: Jan 2002
Posts: 407 |
CCS is good, no doubt. Still "most things" is way not enough for a reference, especially as this issue qualifies as a corner-case. I think you get my point :) |
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