OMU – “One Man Unix”
You don't need an MMU nor tons of RAM to do multitasking, just a timer interrupt.   It's a lot easier if you require position independent code rather than doing fixups.  The 6809 is a beautiful instruction set that makes position independent re-entrant code easy.<p>Of course an MMU and/or lots of RAM does make multitasking a lot easier.<p>OS-9 is an awesome UNIX-like for the 6809 that can do it in 64K or less.