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Atari coding BBS
| Re: falcon double vbl bug |
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Posted by: evil
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Jan,08.2013-16:20
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I thought this problem was once and for all gone after the flickery CRT VGA-monitors of the early 1990's finally died out :) Monitors 1995 and forward seemed not to do this signal bounce, same with any TFT or projector I've tried.
Anyways, I figure it must be a matter of measuring the number of VBL interrupts vs Timer-C interrupts.
1. Set the resolution and refresh you will use.
2. Hook into $70 (VBL) and $114 (Timer-C)
3. Count down Timer-C from say 200 (1 second) and abort the test when the counter reaches 0. Meanwhile the test is running, count up a variable on VBL.
4. If the VBL-counter is significantly larger than the refresh-rate you set, it's problably a twin-VBL problem, and you should skip every second VBL interrupt.
The tricky thing I see in this, how do you know which of the VBL interrupts to pass? One certainly do not want to keep the false interrupt which happen in the middle of the screen.
--
Anders Eriksson
ae@dhs.nu
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