[53260] in Cypherpunks
Re: Video retraces as a source of entropy...
daemon@ATHENA.MIT.EDU (Deranged Mutant)
Thu Apr 4 21:49:46 1996
From: "Deranged Mutant" <WlkngOwl@UNiX.asb.com>
To: Laszlo Vecsey <master@internexus.net>
Date: Thu, 4 Apr 1996 21:15:34 +0000
Cc: cypherpunks@toad.com
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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