Atari coding BBS
|Re: Screen addr update on Falcon|
Posted by: Orion
Thanks for your answer! ;-)
In fact, I was trying to understand more precisely when (at which scanline) the VIDEL HW fetches the video base address because it seemed to me that it works like on ST: the video base address is fetched once by VBL by the HW, but not at the same scanline.
So I wrote a little program that I sent to a friend who has a Falcon to determine this by experimentation.
The result is that the VIDEL fetches the video base address after the VBL is triggered and before the first pixels are displayed, so in the high border.
If the video base address is updated by a program in this area, the new address will be taken into account by the VIDEL for the current VBL.
On the other hand, if the video base address is updated after the VIDEL has started to display pixels, the new address will only be effective at the next VBL.
I also realized that there is already a thread on DHS about this subject and that Tat gave a similar answer here: https://www.dhs.nu/bbs-coding/index.php?request=5285
So in conclusion, updating vbase high/med/low after the first pixels have been displayed allows to have the same behavior as on ST.
-- Orion / Reps
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