From: agodwin@acorn.co.uk (Adrian Godwin)
Date: Thu, 24 Oct 91 18:22:35 +0100
Message-Id: <9110241722.AA00285@snark.acorn.co.uk>
To: broehl@SUNEE.WATERLOO.edu, glove-list@KARAZM.MATH.UH.edu
Subject: Re: Some thoughts on shutter-glasses

> That means we want to do it on every vertical retrace; it would be really
> nice if the VGA card were to generate an interrupt on vertical retrace,
> but it doesn't.  That leaves only two approaches:
>  
>         - polling
>         - using the timer interrupt
>  
> The problem with polling is that you have to pepper your high-level code
> with calls to a routine that checks for vertical retrace and switches the
> screens and the LCD glasses.  That's not too bad, though, since the routine
> is very simple and very fast.  If you miss one swap, it's not the end of
> the world.

This is ridiculous : you're solving the wrong problem, and it's costing you
a fortune in CPU resource to do it. If you need to do something on a vertical
retrace, (and if you can't get out of it by using interlace, say) then 
GENERATE THE INTERRUPT ! You need some hardware to drive the LCD shutter :
drive that hardware from the VSYNC signal and also feed it into an interrupt
input. There are lots of ways you can generate the interrupt - straight onto
the bus, a serial port status line change, the parallel port ACK input, a 
countertimer input .. just choose one !

-adrian