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Old 14 February 2016, 09:11   #202
Toni Wilen
WinUAE developer
Join Date: Aug 2001
Location: Hämeenlinna/Finland
Age: 45
Posts: 23,792
Originally Posted by chaos View Post
I'm debugging an ECS demo (SushiBoyz by Ghostown) on minimig.

In the part with the face and '3D' light, the demo sets no. of bitplanes = 7, and the HAM bit is enabled in BPLCON0 (BPLCON0 = $7A00).

From what I gathered, this should mean the following:
- Agnus 'thinks' this is a four-bitplane mode
- Denise 'thinks' this is a 6-bitplane mode (EHB)
- since HAM bit is enabled, this overrides EHB mode

So this is a HAM mode with 6 bitplanes, but Agnus only does fetches for 4, is my thinking correct?
Yes. From Denise point of view it is normal HAM6 but because Agnus does not update BPLDAT5 and 6, they can be modified with CPU or copper, without Agnus overwriting it.

Annoyingly Denise ignores any BPLDATx registers that are disabled in BPLCON0. This trick makes it sort of possible. (Same works without HAM bit and is also used in few demos)
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