[Coco] Above 512k upgrades

[jdaggett at gate.net]

On 13 Aug 2007 at 17:38, Gary Coulbourne wrote:

> 
> On Aug 13, 2007, at 5:26 PM, Chester A Patterson wrote:
> 
> > Does anyone know where one could purchase (paypal/plastic) a 1MB or 
> > more RAM upgrade kit for Coco3?
> 
> How do these actually work?  I understand the MMU for the standard 
> 512k, but I must admit to being somewhat baffled with the larger 
> ones.  Do they provide an additional page table for mapping, or 
> something of the kind?  Thanks!
> 
> Peace,
> Gary
> 
************************

While the standard MMU will only access 512K RAM, there are methods to 
extend this. All are essentially paging 512K blocks of memory. The GIME 
chip has an unallocated registers at $FF96, $FF98, and $FF9B. One can 
use on of these bytes to act as a page register to select up to 256 banks of 
512K of RAM. 

The external board would have to do some address decode for what ever 
you wish. I believe the common used address is $FF9B. 

james