RE: [stella] 262 scanline kernel is rolling

Subject: RE: [stella] 262 scanline kernel is rolling
From: Chris Wilkson <ecwilkso@xxxxxxx>
Date: Sat, 26 Mar 2005 21:17:50 -0500

What I meant was you should turn on VSYNC 3 lines after you turn on VBLANK.
So when are you turning on VSYNC?  Assuming you're numbering your lines so
that the first line of VSYNC is line 0, (that's what most of the docs around
here say to do) then you should turn on VBLANK at the beginning of line 259.
It would then be on for lines 259, 260, and 261.  Then VSYNC comes on at the
beginning of line 0 (= line 262) and turns off at the beginning of line 3.

Is this equivilent to what you meant?


On Sat, 26 Mar 2005, Zach M wrote:

> I just tested a few minutes ago, and the screen does not roll anymore. I
> made the changes Dennis suggested, and also turned on VBLANK at line 261 as
> Chris had recommended. So it is hard to say exactly what the problem was.
> The reason I suspect it had something to do with writing 8 to VBLANK is
> because an earlier version of the kernel did not roll, and there #$42 was
> written to VBLANK (just like in"Our First Kernel"). The "lda #$42" line
> before "sta VBLANK" was inadvertenly removed at some point.
> I might do more experiments with writing 8 to VBLANK if there is enough
> interest.
> >From: "Dennis Debro" <dennis@xxxxxxxxxxxxxxx>
> >Reply-To: stella@xxxxxxxxxxxxxxxxxx
> >To: <stella@xxxxxxxxxxxxxxxxxx>
> >Subject: RE: [stella] 262 scanline kernel is rolling
> >Date: Sat, 26 Mar 2005 14:49:35 -0500
> >
> >Hi there,
> >
> > > Right now you're writing 0 at the start of vertical blank
> > > and 8 at the start of overscan.
> >
> >That is strange. As Thomas said, D1 of VBLANK only controls the TIA
> >At least as documented by the programming manual. So writing 8 to the
> >register is essentially the same as writing 0 (i.e. enabling TIA output).
> >
> >I don't have my hardware setup at the moment to play with this. Was this
> >truly the problem? What happens if you used other values or ignored VBLANK
> >altogether? By the values you were entering this should have been
> >simulated.
> >Are there other bits used by VBLANK that we don't know about?
> >
> >Take care,
> >Dennis
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