[Coco] 1 or 2 meg upgrade

[Aaron Wolfe]

On Fri, Nov 21, 2014 at 10:27 PM, Gene Heskett <gheskett at wdtv.com> wrote:
> 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.
>

When evaluating whether it's worth all the effort required to extend
beyond 512k, one should really consider why more than 512k would be of
any use on a CoCo.
There isn't any software to actually use this extra RAM besides RAM
disks, and RAM disks aren't all that useful in a world where we have
near zero seek time mass storage like the SuperIDE or CoCoSDC
controllers.  OS9 could theoretically use > 512k, but in practice
you'll usually run out of system resources and/or things to do before
that happens.

$0.02
-Aaron