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Question about "Disable screen DMA"


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Just getting back to A8 coding for the first time in years and need a little refresher. Just ran the sample program that comes with WUDSN and while I know what's going on, I don't get why it works.

 

The reason is that the first two lines switch off screen DMA (load a zero into mem location 559). According to Mapping, this turns off Antic. But does it? The rest of the code changes the background colour on each line but never actually turns screen DMA back on.

What's powering the background if Antic's turned off?

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Basically, the DMACTL (and its SDMCTL shadow) turns on/off ANTIC's feeding of interpreted data from main memory for playfield, missiles, as well as specify how the P/M data is scanned (single/double line res)...

 

With this turned off, the GTIA is still on. Since the ANTIC is no longer feeding data to the GTIA, that data has to come from directly fiddling the GTIA registers, which the demo program is indeed changing COLUBK, and since the GTIA is providing the WSYNC, it can easily change value, wait for next line, change the next one, and so on. 

 

-Thom

Edited by tschak909
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