[Coco] Proprietary, nonstandard memory expansion for the CoCo

[Juan Castro]

This is a hack my (long-defunct) company did back in the day and I would
eventually like to replicate it in a real CoCo someday.

It's a 256 KB memory expansion for the CoCo... 1 or 2. Basically, you
select, with a poke to some $FFxx, which of up to eight 32 KB banks will
play the part of the "upper 32 KB" in a 64 KIB CoCo. (Which you then would
select over the ROM in the usual way.) They raised one pin - SELECT pin? -
of each of the "native" upper 32 KB memory chips out of the socket, wired
them to one of the outputs of a 74LS138 and 6 of the other 7 ones to the
extra 192 KB in the expansion*. It worked pretty well, and although it was
compatible with precisely nothing, it was used in our in-house-developed
business systems. Which did sell, believe it or not. Not really being an
electronics person, I can't tell you what other chips were there, but would
it be too difficult to concoct some schematic based on that description?

Being a total newbie in OS-9, I wonder how easy it would be to make
NitrOS-9 use that memory.

Thoughts?

* Yeah, that could theoretically make it a 288 KB memory expansion.

Juan