[Coco] 1 or 2 meg upgrade

[Gene Heskett]

On Friday 21 November 2014 15:43:31 Nick Marentes did opine
And Gene did reply:
> On 22/11/2014 6:35 AM, Josh Harper via Coco wrote:
> >   hi guys
> > 
> > I was just wondering how hard it would be to copy the 1 or 2 meg
> > memory upgrade boards from disto   as id love to have one of these
> > myself thanks
> 
> Would be nice to see Cloud9 release a 1/2Mb version of the triad.
> 
> Nick

The Triad is a 1/2 meg board, easy.  But to go to 2 megs you'll need to 
design a circuit that will add 2 more addressing lines to drive the memory 
mapping of the additional memory in 8k blocks.  One could carry that to 
its logical conclusion at 8 megs, which has been done but while it was 
described in the Rainbow, was not to my knowledge ever a commercially 
offered product.  One of the disto complexities was also the widening of 
the refresh counter, needed to deal with the square architecture of 99% of 
the memory sticks out there.  For 8 megs, the addressing IIRC expands to a 
full 24 bits, needing a 12 bit refresh driver, and thats 4 bits wider than 
the coco's built in dram refresh counter.

Cheers, Gene Heskett
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