I'm no expert, but from a programming and feature perspective, the 6809 was a far superior processor to the6502.  I am not sure about overall performance however.  The 6809 has 2 16 bit stack pointers (user and supervisor) compared to the 8 bit stack pointer of the 6502.  The 6809 has a much larger instruction set than the 6502.  The 6809 has 3 16 bit registers (on top of the 2 16 bit stack pointers) one that can be used as 2 8 bit registers.    It was developed 3 years after the 6502.  Like I said, I'm no expert.  Maybe it had more features but less efficient.  I was just thinking it would be awesome to be able to program C64 hardware with a 6809.  Anyone who has ever programmed a 6809 and a 6502 will know what i'm talking about.

I mean this was just a thought experiment.  It would require writing an entire C64 kernel from scratch.  I mean if you had access to the original assembler source for the C64 you could port it, but it would be quite a lot of work and in the end just have a C64 that is easier to do assembly level programming on (for some).  I know there are some people who think in 6502 assembly so they would probably not agree.

It's a very interesting thought experiment :-)

you'll have to be careful about the memory map and probably some control signals, particularly the IRQ.

The rest is just software :-D

@Yann Guidon / YGDES , Commodore did build a PET with a 6809 in it.  

I didn't know about that... But I concede that Z80 and 6502 were significantly cheaper, which explains their wide adoption.

Now, that cost doesn't matter anymore and we can consider better alternatives ;-)

Nope. It's a rebuild of the original 6800 that's a little shinier and a little faster, but it's still a 6800 from what I've been told. I'll double check on Wiki tho.

(this, and the preceding comment was in reply to Yann, below.)

I sit corrected. I was under a mistaken impression based on a description that a local friend gave me of the 6809, and had not bothered to verify. The '09 is indeed its own design, and a successor to the 6800 with its own different architecture, and not just a 'tidied up' 6800 as I had been led to believe. I apologize.

I don't think it's impossible. Just incredibly painful because what's worse than a 80's 8-bitter ? 2 of them ! But this should NOT stop you from trying, on the contrary :-D I want to see all the hysteric C64  fanboys cringe :-P

That would actually be a step /backwards/. The 6809 is a slightly enhanced 6800 CPU. The 6502 is, more or less, what the 6800 design team created as an intended successor to the 6800, and the 6510 is nothing more thana 6502 with six bits (eight, internally) of on-chip I/O. Look up the story of the 6502 on Wiki... it's actually kind of entertaining.

you must be kidding about the 6809 :-D https://defining-computers.blogspot.com/2018/12/68hc11-is-not-modified-6809-and-what-if.html

See my comments above, while I go wash the egg off my face, please! (I initially forgot the reply button.)

you're forgiven, for you have seen the Light of True Wisdom and Knowledge. Now go and spread the good news ! :-P
PS: Big Endian sucks anyway :-P