From: "Deranged Mutant" <WlkngOwl@UNiX.asb.com>
Date: Fri, 5 Apr 1996 19:38:57 +0800
To: Laszlo Vecsey <master@internexus.net>
Subject: Re: Video retraces as a source of entropy...
Message-ID: <199604050236.VAA01556@unix.asb.com>
MIME-Version: 1.0
Content-Type: text/plain


On  4 Apr 96 at 5:58, Laszlo Vecsey wrote:

[..]
> > In pseudo-C:
> > 
> > int retrace(void) { // test for video retrace
> > #ifdef __MSDOS__
> >   return (port[0x3da] & 8); // Some VGA, maybe EGA cards
> > #else
> >   // your OS here
> > #endif
> > }

> >   [..]
> >    x = 0;
> >   while (!retrace()) x++;

> As far as I know, while(retrace()) will loop until vertical retrace 
> begins, and then you call while(!retrace()) and that will loop until 
> vertical retrace is over. (Or it may be the other way around).

Sloppy pseudo-C code.  The code I've been experimenting with waits 
until it's no longer in a vertical retrace (if one is still active 
since the last sample) and then collects the sample when the next 
vertical retrace occurs.

[..]
> What if the screen is filled with different colors, or shapes. On some
> monitors you can actually see the size of the screen changing, warping out
> a little.. maybe the time for retrace will be different when painting
> screens with different data. 

I haven't done a lot of tests yet.... and even then, it's probably 
very system specific.


 
Rob. 

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