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Re: Vertical Blank on Apple ][+

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From what I gather the vertical blank can be determined without extra
hardware on the Apple II and Apple II+.  The folklore story mentions a tight
loop and another 17 cycle loop.

> To synchronize with the video, Burrell had me fill the Apple II's frame
> buffer so the low bit was on most of the time, but set off at the end of
> the last scan line. I wrote a routine to sit in a tight loop, reading the
> latch. When the low bit changed, we would know the vertical blanking
> interval had just begun.

> The Apple II wasn't fast enough to keep up with its own frame buffer. A
> new byte of video data flew by every microsecond, which was only one
> processor cycle. It took at least 10 processor cycles to iterate through a
> loop, so we ran the risk of never seeing the low bit change. Burrell had a
> way around this, though - if the loop time was relatively prime to the
> display frequency, it eventually had to slip into place. I wrote a 17
> microsecond loop that fit the bill, and we were delighted to see it work
> perfectly.

I wrote routine with a 7 cycle loop that works on at least two emulators.  I
don't have real hardware to test this on.

http://hoop-la.ca/apple2/2015/vbl/

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Vertical Blank on Apple ][+ Norman Davie <ecompservices@gmail.com> - 2016-08-22 15:26 -0700
  Re: Vertical Blank on Apple ][+ John Brooks <jbrooks@blueshiftinc.com> - 2016-08-22 16:47 -0700
  Re: Vertical Blank on Apple ][+ roger.shimada@gmail.com - 2016-08-22 17:48 -0700
    Re: Vertical Blank on Apple ][+ mmphosis <mmphosis@macgui.com> - 2016-08-23 02:30 +0000
      Re: Vertical Blank on Apple ][+ Michael J. Mahon <mjmahon@aol.com> - 2016-08-22 22:06 -0500
      Re: Vertical Blank on Apple ][+ "Mark D. Overholser" <marko@markoverholser.com> - 2016-08-23 07:57 -0700
      Re: Vertical Blank on Apple ][+ roger.shimada@gmail.com - 2016-08-23 17:55 -0700
        Re: Vertical Blank on Apple ][+ Michael J. Mahon <mjmahon@aol.com> - 2016-08-23 21:52 -0500
          Re: Vertical Blank on Apple ][+ John Brooks <jbrooks@blueshiftinc.com> - 2016-08-23 20:44 -0700
            Re: Vertical Blank on Apple ][+ Mark Lemmert <mark.lemmert@gmail.com> - 2016-08-23 22:49 -0700
              Re: Vertical Blank on Apple ][+ zellyn@gmail.com - 2016-08-24 07:15 -0700
                Re: Vertical Blank on Apple ][+ Mark Lemmert <mark.lemmert@gmail.com> - 2016-08-24 19:59 -0700
              Re: Vertical Blank on Apple ][+ Michael J. Mahon <mjmahon@aol.com> - 2016-08-24 11:57 -0500
                Re: Vertical Blank on Apple ][+ Mark Lemmert <mark.lemmert@gmail.com> - 2016-08-24 20:00 -0700
                Re: Vertical Blank on Apple ][+ "Mark D. Overholser" <marko@markoverholser.com> - 2016-08-24 23:32 -0700
                Re: Vertical Blank on Apple ][+ Michael J. Mahon <mjmahon@aol.com> - 2016-08-25 20:51 -0500
                Re: Vertical Blank on Apple ][+ Michael J. Mahon <mjmahon@aol.com> - 2016-08-25 20:51 -0500
                Re: Vertical Blank on Apple ][+ gids.rs@sasktel.net - 2016-08-25 21:41 -0700
                Re: Vertical Blank on Apple ][+ mmphosis <mmphosis@macgui.com> - 2016-08-26 18:52 +0000
                Re: Vertical Blank on Apple ][+ Mark Lemmert <mark.lemmert@gmail.com> - 2016-08-26 06:47 -0700
            Re: Vertical Blank on Apple ][+ roger.shimada@gmail.com - 2016-08-28 08:33 -0700
  Re: Vertical Blank on Apple ][+ olivier.zardini@itn-group.eu - 2016-08-25 09:18 -0700
    Re: Vertical Blank on Apple ][+ Michael Pohoreski <michael.pohoreski@gmail.com> - 2016-08-28 14:58 -0700
      Re: Vertical Blank on Apple ][+ Norman Davie <ecompservices@gmail.com> - 2016-08-28 16:19 -0700
        Re: Vertical Blank on Apple ][+ Dave Kay <thavith@gmail.com> - 2023-03-16 03:35 -0700
          Re: Vertical Blank on Apple ][+ Michael Pohoreski <michael.pohoreski@gmail.com> - 2023-03-21 08:59 -0700
            Re: Vertical Blank on Apple ][+ I am Rob <gids.rs@sasktel.net> - 2023-03-21 15:32 -0700
              Re: Vertical Blank on Apple ][+ mmphosis <mmphosis@macgui.com> - 2023-03-23 14:33 +0000

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