Re: cbm 8032 motherboard + 4164

From: Segher Boessenkool <>
Date: Thu, 13 Oct 2016 14:29:21 -0500
Message-ID: <>
On Thu, Oct 13, 2016 at 07:03:30PM +0100, smf wrote:
> On 13/10/2016 15:28, Gerrit Heitsch wrote:
> >How much of it is spent in the upper and lower border and the vertical 
> >retrace where no memory access happens (not counting an open border 
> >with sprites)? More than 2 ms?

Yes, and yes.

> >So in order to make sure that the DRAM is refreshed properly, they 
> >just implemented a refresh counter which does 5 cycles per scan line.
> I thought upper/lower border was always fetching $3fff? During the side 
> borders it's fetching sprites, so you can't rely on that for refreshing

It only fetches for active sprites, too.

> Doing 5 refresh cycles per scanline will take 51 scanlines to read all 
> 256 addresses.

This is about 3ms btw, so 2ms DRAMs shouldn't work well at all.

> After 51.2 scanlines the VIC should have read all 256 addresses, just by 
> character fetches. I doubt they thought they'd better put a separate 
> dram refresh in case someone figures out how to mess up the character 
> fetching (duplicating rows/vsp etc).
> There must be another reason.

The vertical blank (not border, *blank*) is almost 2ms already.

They could have changed the VIC-II's memory access patterns.  Either that
would have hurt performance too much, or it was too much work (and/or risk),
or a combination of those, and maybe other less obvious factors.

The normal video matrix accesses are in no way good enough.


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Received on 2016-10-13 20:00:27

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