[Coco] CoCo 3 memory expansion questions

Neil Cherry ncherry at linuxha.com
Sun Feb 3 13:03:10 EST 2019

On 2/3/19 10:38 AM, Frank Swygert wrote:

> I don't think you can use ANY memory expansion at all. The Sardis ST2900
> appears to be made from the Motorola 6809 spec sheet, much the same as the
> CoCo 1 and 2. The CoCo 3 came out in 1987, and uses a custom memory
> management/graphics chip (the GIME) to expand memory beyond 64K. Motorola
> had a spec sheet out for a 6809 compatible MMU, but as far as I know it

Yup found that out after I took a look at more schmatics.

> was never actually produced. You'd have to create such a unit and
> interface it with the circuitry to use more than 64K as anything but a RAM
> disk, which you seem to have according to your web site (about the only
> one I could find with any ST2900 info!). There were some memory bank
> switching schemes for the CoCo 1 and 2 you could probably implement, but I
> don't know if they were ever useful under OS-9. Could probably write some
> kind of control routine to switch data in and out of banked memory, I
> don't know, way beyond my programming skills back in the day (non-existant
> now!!).
> The link to the interesting site:
> http://www.ushomeautomation.com/Projects/Sardis-ST2900/Sardis-ST2900.html

Hehe, that's mine :-) I'll have some more up there. I need to get a working
OS9 system so I can start reading my disks. I also have to get some free
time. I'm also talking to David Weins and another gentleman.

Actually there is a very cool 'hack' for the SWTPC which uses a pair
of 74189s (64bit RAM). It allows each process to have it's own memory.
Unfortunately I'm not sure if OS9 supported DAT ram. This gave the
6809 access to 1M in 8K pages.

Guess I'll be using the ST2900 with Level I. I will still need to get
the CoCo 3 up and running with Level II. I need to demonstrate that
to folks. I have few non-believers. Which I had the GIMIX Ghost. That
had a lot of IO on it. But any I've seen on EBay are really expensive
(or sold as parts and even more expensive).

Linux Home Automation         Neil Cherry       ncherry at linuxha.com
http://www.linuxha.com/                         Main site
http://linuxha.blogspot.com/                    My HA Blog
Author of:    	Linux Smart Homes For Dummies

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