Date   : Mon, 29 Jan 1996 13:40:15 GMT
From   : David Alan Gilbert <gilbertd@...>
Subject: Re: Vertical blank interrupt

> Tom Seddon <T.W.Seddon@...> wrote:
> 
> ;I am aware that the vertical blank interrupt occurs 50 times a second, but
> ;I am unsure as to when it occurs. Does it occur when the visible display
> ;area -- that is, 256 scanlines or (display character rows)*(scan lines per
> ;character row) -- has all been traced, or does it occur when all 312
> ;scanlines (ie both visible and invisible) have been traced? Or is it
> ;rather more weird than this? 
> 
> I didn't find out either, so I guessed at the value by trial and error
> until the Elite screen looked right :)

I found Rocket Raid was pretty critical of this; if you screw it up you find
that the ship disappears as you fly close to the top of the screen.
I've just had a quick look at my code; its a long time since I did it, but as
far as I can figure I do the sync at the bottom just after the last
displayed line is scanned out and take it off about a horizontal scan line or
so later.

Dave