> > The first line interrupt needs to be at a line before line 192 and it
> >should set the LN bit (screen height = 212 lines). The second line
> >interrupt should be at a line beyond line 212 and clear the LN bit
> >(screen height = 192 lines).
>
> Shouldn't the second line interrupt be between 192 and 212?
> Sound more logical to me: the VDP passes 192 and checks for 212 to end the
> screen. But before 212 is reached, you change end of screen to 192, which
> the VDP already passed.
As usual, you are right, Maarten. The first line interrupt should be
before line 191 (set LN) and the second should be between 191 and 211
(clear LN). It seems my memory failed me - again. Sorry.
Another option, which will probably also work, is to put the first
line interrupt between line 191 and 211 (clear LN) and the second line
interrupt beyond line 211 (set LN). You cannot, however, put an line
interrupt beyond line 230 or 240, depending on PAL or NTSC display, but
this is something that can be quickly checked.
> But I wonder if it's just a coincidence that NOP used a repeating pattern
> as the background of their overscanned title screen. Maybe there is some
> nasty sideeffect to the overscan trick that you have to compensate for.
None that I know of. By using the overscan trick the displayed screen
is looped. It is viewing a cilinder from the side; it has no real start
nor end. So what ever picture you want to display, the lines <=255
should match lines =>0 otherwise you will have a non-continuity at the
top of your screen. A repeating pattern, for instance, takes care of
this.
Bye, /\/\ark
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