[Coco] CoCo 3 memory expansion questions

Frank Swygert farna at amc-mag.com
Sun Feb 3 10:38:01 EST 2019

From: Neil Cherry <ncherry at linuxha.com>

I've ordered the Boomerang board (with the buffer board) and I'm trying to
understand what I'm hooking up (for better documentation). Does anyone have
a Coco 3 schematic with the 2 memory connections? I'm trying to understand
what is connected to what. I think I can use the same board on my Sardis
ST2900 (a partial clone of the CoCo.


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
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

The link to the interesting site:

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