Date   : Sun, 22 Sep 2002 19:31:05 +0100
From   : Thomas Harte <thomasharte@...>
Subject: Re: rupture technique

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> I thought this was the case (only starts of 
> &3000,&4000,&5800,&6000).
> Can you explain what the rupture technique is 
> please?

I apologise that I have not been keeping up with this thread, but this
is more or less my 'official return' to the Acorn world, having been unfortunately
away for a very long time. 

In any case, I believe the rupture technique is the name for tricking 
the video address counter to reload mid-frame, which is a simple case of
persuading the CRTC to enter and exit vertical sync within a few scanlines,
arbitrarily placed within the display.

I don't know how address wrapping works on a BBC, but if it is anything 
like the Electron, the process is to wrap addresses only when the go past
the top boundary, in which case you wouldn't have to play with the minimum
display address, just put the status line outside of the area [minimum
display address, 0x7fff].

Otherwise, I guess you'd increase the minimum display address while playing
with the CRTC's notion of vsync.

By the way, something which I've always thought would be interesting would 
be to use this technique to program a new start address every line (or 
every other if timing conditions require) in order to make a Wolfenstein
type display on the side. You'd almost certainly not be able to do the
ray cast once per frame with all that going on, but you'd be in the region
of 5-10fps, surely?

-Thomas
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