Re: cbm 8032 motherboard + 4164

From: Gerrit Heitsch <>
Date: Fri, 14 Oct 2016 09:27:22 +0200
Message-ID: <>
On 10/14/2016 09:18 AM, smf wrote:
> On 13/10/2016 21:07, Gerrit Heitsch wrote:
>> Even if you couldn't disable display DMA, drawing the upper and lower
>> border would take too much time. Reading $3FFF means it will refresh
>> row $FF, but not the other 255 rows...
> The $3fff fetch could have had the 8 bit cycle in it. Then they could
> have done something else with those extra 5 cycles per line.

No, why? No matter how you do it, you have to cycle through all 256 row 
addresses, meaning in this case from $3FFF to $3F00 every 4 ms max. Each 
of them will cost you one memory cycle. The 5 cycles per scanline was a 
compromise that produced an even enough pattern and didn't get in the 
way with display and sprites.

You cannnot run the refresh cycles only in the upper and lower border, 
the screen part takes more than 4ms to display which will take you out 
of spec for the DRAM.


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Received on 2016-10-14 08:01:25

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