The 6809 is a fun processor to program in assembly and still my favorite 8-bit chip. The 8086/88 was such a let down after it. It would be interesting to have a 64-bit version just to play with for projects.
I've only really heard legends about OS-9. The fact that you could support two users in real time with neither one slowing down on a machine with a bit-banged serial port and a single sided floppy disk set the bar of performance I expect from a processor far too high for decades.
The original of this Microware OS-9 was my first exposure to a "unix like" operating system in the 80s. It was pretty amazing that a mutli-user, multitasking operating system kernel can fit in 8k.
Byte Magazine ran a 2-part article on the design of the 6809<p><a href="http://tlindner.macmess.org/wp-content/uploads/2006/09/Byte_6809_Articles.pdf" rel="nofollow">http://tlindner.macmess.org/wp-content/uploads/2006/09/Byte_...</a>
The first few computers I built were 6909-based and ran OS-9. I implemented a ram disk driver, and a "rom disk" using bank selection to allow > 64k capacity, since at the time I worked for a company that had large quantities of memory chips in the lab :) Good times.
In somewhat the same space, there's FUZIX[1], which supports the Z80, 68k, and 6809.<p>Also, recently, someone ported it for the Raspberry PI pico, which makes it easy to get it up and running with something modern: <a href="https://cowlark.com/2021-02-16-fuzix-pi-pico/" rel="nofollow">https://cowlark.com/2021-02-16-fuzix-pi-pico/</a><p>Edit: Also ported to the 8266: <a href="https://cowlark.com/2021-02-09-esp8266-fuzix/" rel="nofollow">https://cowlark.com/2021-02-09-esp8266-fuzix/</a><p>[1] <a href="http://www.fuzix.org/" rel="nofollow">http://www.fuzix.org/</a>
Here's a video if you'd like to see how its windows look like: <a href="https://www.youtube.com/watch?v=JU8SIC-4Smw" rel="nofollow">https://www.youtube.com/watch?v=JU8SIC-4Smw</a>