I recently had the chance to buy a Stellation Two "THE MILL" 6809 card on ebay (the collectors "sancta santorum"!) and install it in my Apple II+ .
The "The Mill" card from "Stellation Two" company is a co-processor card that uses a 1 MHz 6809E and enables execution of Motorola 6809 cpu code in Apple II computer series while sharing the bus, memory, peripherals and clock with the 6502 cpu so that both processors are kept running at the same time, the 6809e running at 100% speed and the 6502 let working in the 6809E dead cycles for the remaining 20% time.
The card was sold in 1982 at a retail price of $369 and included a "Pascal Speedup Kit" disk and a demo disk with some software to show the 6502/6809 interaction capabilities.
This should be the picture of the original package
The most interesting feature of this card was that it offered the option to buy an OS/9 kit to let OS/9 operating system run in the Apple II.
OS/9, created by Microware Systems Corporation was a rather advanced operating system for those times and it's still popular in today's embedded systems.
The OS/9 version offered by Stellation Two is known as "Level One" and its main feature is that it's a realtime OS that allows execution of multiple programs at the same time, something "stellar" for an Apple II.
The kit consisted of the OS/9 operating system on Apple II formatted 140K floppy disks and a daughter board to be installed piggiback to the "The Mill" card.
Unfortunately "The Mill" cards equipped with a daughterboard are extremely rare; as far as I know, just one was sold on ebay in the last few years and it was part of a combo whose price skyrocketed to $1525.
I simply could't afford it and the daughter board does'nt look original!
The appetite for OS/9 was too much so I started digging for those faint, rare, scattered pieces of information about the inner working of the original daughter board; I had to proceed by elimination because much of the available information is wrong, unproven or incomplete.
After a coulple of months of real obsession I had enough knowledge to attempt to build a (functional) replica of the original daughter board.
I spent 3 days designing, programming and pcb etching and here is the result
My The Mill card now runs OS/9 !
It's not been a trivial task because of the lack of information and also due to the fact that a lot of OS/9 disk images that are published on the internet are broken.
Now I have a fully functional OS/9 machine that I even tested in multitasking with a mating Mountain Computer CPS card, a real "taste" of the mainframes of the '80s.
The "The Mill" card with OS/9 is fully compatible with 80 Col. Videx cards, some parallel printer cards, serial and clock cards and an assembler and editor for 6809 opcodes is available from the content of one disk image that apparently comes originally from Microware and that can be found oline.
I initially experienced Apple II freezing (lock-up) while running OS/9 BASIC09 demos. I later discovered that my clock generator IC on the Apple II motherboard, a 74S175 location B1, was faulty.
It was probably going out of specs with the rising of temperature inside the Apple II case, while not showing any sign of malfunctioning during 6502 only cpu operation.
Probably the tight sharing of the bus by the two processors requires a good stable bus clock.