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Old 01 February 2020, 11:16   #3
mcgeezer
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Join Date: Oct 2017
Location: Sunderland, England
Posts: 1,772
So I'm fairly sure in that I'm right about the $CA minterm function being the cause of the problem here.

What is happening is the mask is controlling whether a 1 or 0 is set in the destination bitplane.

From the HRM..."When blitting the car to the background we would want to use a function that, whenever the car mask (fetched with DMA channel A) had a bit set, we would pass through the car data from B, and whenever A did not have a bit set, we would pass through the original background from C. The corresponding function, commonly referred to as the cookie-cut function, is AB+AC, which works out to an LF code value of $CA."

What needs to happen is the car data needs to be OR'd into the original background C.

Not sure if the blitter is capable of this with one go.
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