Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ncar!umigw!umbio!dnelson
From: dnelson@umbio.MIAMI.EDU (Dru Nelson)
Newsgroups: comp.sys.apple
Subject: Re: Page flipping
Message-ID: <1494@umbio.MIAMI.EDU>
Date: 16 Mar 89 20:11:42 GMT
References: <371@athertn.Atherton.COM>
Lines: 26


 
>        Page flipping helps, but on the ][+ it can cause the undesirable split
> effect when you flip while the raster beam is in the vertical middle of your
> character.  Only on the ][e (and presumable ][c, ][gs) can you do page-
> flipping right, since it provides you with information on when the electron
> gun is doing a vertical blank (going back to start at the top again).

There is a way to do it on all ]['s.  Bob Bishop once wrote an article
in Softalk about changing video modes during screen refresh.  He
monitored a location in I/O memory for a certain byte or sequence of
bytes.  These could also be used for page flipping timing.  The ][c
and IIgs are the only ones that give interupts (I believe).  The 
article was titled the "Hi-Lo Split" and was mainly concerned about
mixing text and hires or lo-res or any combination.  It gave an
excellent description (from a programmers point of view) of how this
information could be used and how the video operated.





-- 
Dru Nelson                    UUCP: ....!uunet!gould!umbio!dnelson
Miami, Florida                 MCI: dnelson
                          Internet: dnelson%umbio@umigw.miami.edu