Re: No PETs allowed demo...

From: Michał Pleban <>
Date: Wed, 15 Dec 2010 22:43:22 +0100
Message-ID: <>
Yes, the CRTC register values are set only once. I was actually thinking
of extending the screen beyond 1kB, because that leaves quite little
pixel data.

So I thought of copying/rewriting the screen data while the CRTC draws
them on the screen. But a quick calculation of memory copying speed with
6502 told me that it would rather not be possible.


W dniu 2010-12-15 20:27, Steve Gray pisze:
> It's probably not really a matter of timing, like some C64/VIC graphic tricks. 
> The 6545/6845 CRTC chip is programmable so it's a matter of selecting the proper 
> register values. I don't think you need to change them once set. Basically, you 
> can adjust the top/left margins, number of characters per line, number of lines, 
> as well as the number of pixels high each character is. This is easily done in 
> BASIC. Then you can just print or poke to the screen memory.
> Not having run the demo myself, I'm guessing the character height is set to 2. 
> By creative use of the standard PET character set you could have individual 
> control of 2x2 pixel size "points" in a small area.
> What would be interesting would be to try this method on an 8296, since it can 
> use up to 8K of video ram so you could have a 160x100 pixel graphic mode without 
> any additional hardware. With a replacement character generator ROM you could 
> thoretically have 640x100 or 320x200.
> Steve
> ----- Original Message ----
>> From: Anders Carlsson <>
>> To:
>> Sent: Wed, December 15, 2010 1:07:37 PM
>> Subject: Re: No PETs allowed demo...
>> Cameron Kaiser wrote:
>>> Still, it's a great mode to leverage if someone can port it to 60Hz PETs.
>> In the demo, credits were given to Robin Harbron for testing. Either I am 
>> mixing things up, or he owns a 50 Hz (220V European?) PET. Perhaps testing was 
>> done at an earlier stage when it was a matter of how the CRTC works at all, not 
>> specifically timed for the 4032?
>> My last remaining, working PET is named 4032 on the front but as far as I 
>> remember it contains a 2001N/3000 series motherboard upgraded with Basic V4. 
>> Thus I suppose this demo won't run on my non-CRTC PET.
>> However what are the odds the demo would run on a 8032 using one of the 4032 
>> emulation programs? I suppose the CRTC registers and timing is so critical it 
>> won't work, even if "normal" 40 column software including the (simpler) demo 
>> "Hardcore Petting" released earlier this year runs just as well on a 8032 as the 
>> intended 4032. I've tried it myself.
>> Best regards
>> -- Anders Carlsson
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Received on 2010-12-15 22:00:14

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