Re: DRAM refresh

From: Segher Boessenkool <>
Date: Thu, 13 Oct 2016 01:19:25 -0500
Message-ID: <>
On Thu, Oct 13, 2016 at 08:45:12AM +0300, Marko Mäkelä wrote:
> On Wed, Oct 12, 2016 at 10:49:09PM +0100, smf wrote:
> >The apple 2 apparently doesn't have dram refresh, it uses the video 
> >fetches to refresh the memory. I'm not sure why the c64 is different.
> I think that the memory refresh access cycles (5 per scan line) come 
> into rescue when the video output is blanked, no sprites are enabled, 
> and the CPU is executing in a tight loop, covering only a few addresses 
> for a longer period of time. In this configuration, the VIC-II would be 
> accessing $3fff all the time, except for the 5 refresh cycles per scan 
> line. This is also what happens when the vertical border or the vertical 
> blanking is being generated.

The original VIC-II (the 6566) used SRAM.  To make the DRAM versions it
probably was a lot easier to add this simple counter than to change all
timing stuff to make sure all rows were accessed often enough (also
during "border time", etc. -- the DRAMs need to be refreshed a whole
bunch of times per frame).


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Received on 2016-10-13 07:00:02

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