Re: GO64 tech stuff?

From: Nicolas Welte (
Date: 2000-08-25 13:02:08

Ojala Pasi 'Albert' wrote:
> > Guenther Walter from Austria experimented with the TEST bit in $d030 of the
> > VIC-IIe, and found that while this bit is activated, the internal raster
> > counter is incremented once per machine cycle!
> Yes, I found this out also with a test program someone sent me
> (was it Steve Judd, maybe?). Quite an interesting feature, which
> I think actually IS related to testing the video output.. Waiting
> for 20ms is much too much time for chip testing.

I don't think so. One of the test modes features is to update the raster
counter much faster than usual, this is useful for chip testing. That it
can be used to change the video timing is just another unwanted side
effect. Probably it was never meant to be enabled and disabled in a period
of less than a video frame, because after one frame you lose the vsync
signal and obviously several other things inside the VIC-IIe stop working
properly, as can be seen in the grey colored video output.

I'm still very interested what else happens when the test bit is set. It's
possible that RAM refresh gets disabled, and memory contents are lost after
a few seconds. A program to test this should run from an Eprom cartridge.
Does anybody know for how long a program can run from RAM with enabled test
bit? Will it crash sometime or is it possible to disable the test bit after
several seconds and everything is normal again? I never tested that ...

> You can create linecrunch effects much easier with this :-)

Now this get's to deep into VIC programming for me, but I'm sure it's
useless for that. The test bit acts on the real raster counter, that is
also used to generate the sync signals. As soon as you use it, you affect
the video timing directly. Using this for graphical effects is not a good
idea ...

As with all registers that affect video timing directly, it should be
handled with care. From the very beginning I feared that another wave of
"works on the author's display only" programs will appear, just as we saw
it with all those new C128 VDC video modes in the eighties :-( 

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