VIC-II DRAM refresh

From: Marko Mäkelä <>
Date: Thu, 13 Oct 2016 22:07:11 +0300
Message-ID: <20161013190711.6smsr36wu4emrwum@x220>
On Thu, Oct 13, 2016 at 07:03:30PM +0100, smf wrote:
>I thought upper/lower border was always fetching $3fff?

Yes, for the 40 cycles that would within the horizontal borders.  
Similarly, if the screen is blanked by clearing bit 4 of $d011, each 
raster line would be like that.

>During the side borders it's fetching sprites, so you can't rely on 
>that for refreshing

The 5 refresh cycles are at the left border, immediately followed by the 
40 columns. The VIC-Artcle.gz at
(published 20 years ago) includes some timing diagrams.

>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.

Remember that the C64 KERNAL blanks the screen during tape access. I 
think that it is possible that the memory accesses of the KERNAL tape 
routines would not guarantee sufficient DRAM refresh.


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