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PostPosted: Mon Apr 17, 2017 12:19 am 
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I adapted a new notation below, hopefully it's self explanatory. Basically, 24-, * = "24 -, rest *".

PalRendOff1_v3.nes
- 24-, *
- I'm quite sure I got a 24-, 46*, - pattern when the first few times I tried this, but after the 24-, * first appeared it always appeared, over MANY tries (powerons/resets). (I may have been running a wrong test by accident the first few times, who knows.)

PalRendOff2_v3.nes
- 80-, *
- Not really consistent with results from PalRendOff1_v3.

PalRendOn1_v3.nes
- This worked as expected: 24-, *

PalRendOn2_v3.nes
- *
- Certainly not expected, looks like something goes wrong while it tries to read during VBL.

Good news is that the results were consistent across resets/powerons (discounting the first test, but that could've been my mistake). Unfortunately they don't line up with previous results.

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PostPosted: Mon Apr 17, 2017 3:46 pm 
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Well, that's... confusing. I'm not really quite sure what to make of those results.

blargg's framework is able to print the values of registers on the screen, so I'll try to come up with a slightly different test that displays the values actually read from OAM - maybe it'll help figure out what's actually going on.


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PostPosted: Thu Mar 15, 2018 3:11 pm 
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Sorry for reviving a dead topic.

Question. Reading from 2004 and Micro Machines?

why?

And could it be used as a second "sprite zero" style midscreen hit?

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PostPosted: Thu Mar 15, 2018 3:19 pm 
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See - viewtopic.php?p=142001#p142001

If you can guarantee that a specific byte occurs exactly once in OAM, and not as the Y coordinate, and the value is not the $FF "empty" value, then ... I think you might be able to poll reads from $2004 to find out when that sprite is being drawn?

Or maybe not; the rate at which the CPU can poll may well just be simply too slow to catch the exact moment that the right byte shows up in OAM evaluation.


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PostPosted: Thu Mar 15, 2018 3:33 pm 
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What Micro Machines does: viewtopic.php?p=67668#p67668

It's just a simple +1/+0 CPU cycle adjustment based on what value is returned by PPU. I actually did the exact same thing in a demo I did once (before I knew what Micro Machines did...), although I don't think I ever released it in any form.

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PostPosted: Thu Mar 15, 2018 5:06 pm 
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What about Super Cars? It's definitely trying to time a border at the top of the screen via $2004 reads.


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PostPosted: Fri Mar 16, 2018 12:44 am 
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tokumaru wrote:
What about Super Cars? It's definitely trying to time a border at the top of the screen via $2004 reads.

As far as I can tell, it's polling $2004 to wait for the start of pre-render line, then uses a timed loop to go the rest of the way. (They could have polled for sprite 0 hit flag to be cleared instead, had they known about that.)

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PostPosted: Fri Mar 16, 2018 9:09 pm 
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Well that's disappointing.


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