Also Used in the Tandy/Radio Shack color computer

The 6809 wasn't based upon the 6800 it was a from-the-ground-up new design. The key features were a variety of addressing modes two seperate index registers and two seperate stack pointers. I`m not aware of any built-in memory. The addressing modes which included relative addressing for pretty much every situation you`d need addressing made programming for the 6809 extremely elegant. You could write position independent code with ease. While other chips already had the market by the time the `09 came out it grew a sizable fan base that resulted in operating systems like OS-9 being developed for it.