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Old 31 December 2013, 07:09   #1
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Strange bitmap behavior


last weekend, I was debugging an effect in WinUAE that did not show some bitplanes correctly. In short:
  • 5 bitplanes display (1, 3, 5 for background, 2 and 4 pointing to the same animation bitplane with a different modulo)
  • Areas where BPL 5 is set are rendered with a single color
Surprising details on top:
  • Works on my real A1200 (bottom right in the screenshot)
  • Works when using 6 bitplanes, i.e. EHB (which was my fix to have a WinUAE-compatible version; luckily, I could just re-use the animation bitplane without any visible difference, as the animated shape is supposed to be black, anyway)
  • Works when I leave out some effects before this effect — however, I was not able to find a way to overcome this by reinitializing everything in the copperlist; nor did I find something suspicious in the debugger's custom chip register dump compared to the working version of the effect
Probably I am doing something strange/wrong to cause this effect, but I would really like to know what it is. Also, I cannot quite explain how the difference between the 5 and 6 bitplane modes is possible.

My questions are:
  • How is the behavior on a real A500 (or other ECS machine)?
  • What's the cause of this?
For testing, find a test executable attached (start with "bpltest 5" for 5 bitplanes, "bpltest 6" for 6 bitplanes).

Thanks and cheers,


Edit: tested with 2.7.0 beta 16, A500 config
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Last edited by losso; 31 December 2013 at 07:25.
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Old 31 December 2013, 09:03   #2
Toni Wilen
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Join Date: Aug 2001
Location: Hämeenlinna/Finland
Age: 44
Posts: 23,575
Without downloading and testing anything, my crystal ball says you trigger OCS/ECS undocumented hardware feature: when BPLCON2 has invalid value (too large), bitplane count is 5, other planes are ignored when bitplane 5 has bits set.

This is used in game SWIV for score information.

EDIT: this one http://eab.abime.net/showpost.php?p=206345&postcount=5

Last edited by Toni Wilen; 31 December 2013 at 13:28.
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Old 31 December 2013, 13:42   #3
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Join Date: Oct 2013
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Aaah, thanks! BPLCON2 is indeed set to $7F. Why did I miss this when comparing the register dump… hmpf.

Well, the upside is that my "fix" should work an a real A500, too Will try to convince a friend to undust his machine for the New Year's warmup to verify…

Happy new year, by the way, and cheers to everyone at EAB!
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