Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!agate!web-3b.berkeley.edu!labc-3dc
From: labc-3dc@web-3b.berkeley.edu (Andy McFadden)
Newsgroups: comp.sys.apple
Subject: Re: Page flipping
Summary: On a //+...
Message-ID: <21767@agate.BERKELEY.EDU>
Date: 17 Mar 89 04:53:22 GMT
References: <371@athertn.Atherton.COM> <1494@umbio.MIAMI.EDU>
Sender: usenet@agate.BERKELEY.EDU
Organization: University of California, Berkeley
Lines: 32

In article <1494@umbio.MIAMI.EDU> dnelson@umbio.MIAMI.EDU (Dru Nelson) writes:
>[somebody else writes: ]
>> [...]       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

It's been a while, but I believe the general idea was to look at some of the
I/O addresses (e.g., $c050)... apparently they reflect the contents of the
screen currently being scanned.

>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.

Yes... but it can be CPU intensive, and it's hard to use.  Somebody mentioned
the article in Lancaster's "Enhancing your Apple //" about a hardware mod to
make the //+ signal VBL interrupts.  I installed that a while back (a long
while back...), and it works just fine except that it sends the logical
complement of the one on the //e.

>Dru Nelson                    UUCP: ....!uunet!gould!umbio!dnelson

-- 
fadden@cory.berkeley.edu (Andy McFadden)
labc-3dc@widow.berkeley.edu