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Demos Amiga Demoscene Archive Forum / Coding / Undocumented priority mode of the bplcon2 register

 

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dissident
Member
#1 - Posted: 4 Jun 2009 19:51
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Some time ago checked the copperlists of Kefrens Megademo VII parts and I found out a very interessting thing. In the second part with the chinese dragons and the chessboard there was a write of $003f to BPLCON2. That means in binary code %0000000000111111. All the priority bits were set. But this mode is not documented in the HRM and only works on OCS machines.
What happens?
Well, a scrolltext in bitplane 5 moves on a background picture with 4 bitplanes. A colourcycle effect runs through the scrolltext. Thus we have a screen with a depth of 5 bitplanes lores. Normally we have to change colour registers 16-31 each scan line for the colourcycle effect. But not in this case. You only change colour 16 and you don't have to change the rest of the colour registers. I tried the copper move command with different bit combinations, the effect only works with the value $3f. On an AGA machine only colour 16 is changed and the rest of the colours do not.
Can someone explain this phenomenon?
Kalms
Member
#2 - Posted: 4 Jun 2009 20:22
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I can't explain it but it is briefly described by Toni Wilen in a thread over at EAB: http://eab.abime.net/showthread.php?t=19676
dissident
Member
#3 - Posted: 7 Jun 2009 09:35
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Hy Kalms,

many thanks for your quick reply. Your hint is very useful for me.

dissident

 

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