1Finding the right system architecture
The first step when thinking about designing a single-board (or modular, doesn't matter actually) retro computer system is of course to think about the appropriate operating system. Disconsidering the Intel world, there's basically two options: simple BASIC (or similar, maybe assembler or C) or "real" operating system, meaning disk operating system.
I went for "operating system" option, because BASIC is all too limited for the envisaged especially "nice" system.
Through time there popped up several alternative operating systems on 6809-based systems, each of which has its own advantages - and disadvantages. There's FLEX and there's OS-9 and there's NitrOS9. There's no CP/M and no DOS.
I think I'll end up with a more or less original OS-9, although I'll give NitrOS9 a try. We'll see, I think this will be challenging.
Anyhow, a "real" OS requires a bit more of an environment than everything, ROM, RAM and periphery cramped into 64KB. With the 6809, there's two options:
First, the ingenious 6883, which makes most sense together with the 6847 video display controller. It's a "synchronous address multiplexer" that enables extending of the address space to 96K, meaning 64K or more of free RAM. It is designed for use with the 6847, so it makes most sense to me. I am not interested - at this stage - in sophisticated video output. That's why I'll hard-wire text-only mode with green-or-amber selection.
Second, there's a real MMU available, which even supports multiple instances, and shamelessly extends the addressable space to 2M. Dynamic RAM, DMA components and interrupt management (chips available in the family as well) can add up to a real machine, including hard disk and networking. Things get complex, but it would represent a real computer being able to run a multi-user multi-tasking operating system. The real challenge.
My formula: 6809+6883+6847 = OS-9 Level-I
If the winter is long :-) I'll checkout the formula 6809E+(2x?)6829+6850+6821+6844+6854+6845+6828+6840+... = OS-9 Level-2. Would be a hell of a retro machine! I don't know of anything alike ever built.