Posted By
Gaia
on 2005-10-20
07:15:40
|  Re: Writing to the borders of the screen
A bit late on this one, but here it goes anyway.
Crown: method 3) will not work on the plus/4. Unlike on the C64, the counters used for the DMA always count until 40 no matter if you delayed the DMA or not. This can happen, as possibly an existing counter has been applied for the DMA. It is already counting - when the TED is not in an idle state of course -, disregarding the fact if it's a DMA or a "normal" screen line. The actual character position comes to mind as best candidate. The only way to force the DMA to latch with a different value is to write to the horizontal count register ($FF1E). By the way, the place where the DMA counter is set to 0 seems to have a weird connection to the vertical sub counter (lowest 3 bits of $FF1F).
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