Re: C64 + 6809

From: Rainer Buchty (
Date: 2008-09-17 18:01:21

On Wed, 17 Sep 2008, wrote:

> - are there 6809 experts amongst us?
> - beside the well known "because it can be done", is there another
>   reason we benefit from a 6809 in a C64?

A nice way of tweaking an 6809E into a system with a remote bus master 
(like we would need for the C64 where VIC is the true bus master) can be 
e.g. found within the older Ensoniq synths (have a look at for schematics) where the sound chip is 
generating the master timing and is the true bus master.

Where could we benefit: the 6809 is, compared to the 6502, vastly 
superior with respect to addressing modes (2 accumulators, 16-bit index 
registers, predecrement/postincrement for byte and word access), number 
of stacks (2, S and U), stack position and size freely placeable and not 
fixed to one page, likewise direct page access (comparable to zeropage 
addressing) can be moved to any page using the DP register. Plus 
additional stuff like fast interrupt (more or less like on the 6502) vs. 
regular IRQ (saving the complete state to the stack), a dedicated MUL 

If you go for a 68B09E, you could even bring it up to 2MHz and (e.g. 
when accessing slow peripherals) slow it down again to 1MHz (see the 
"arbiter" in the Ensoniq schematics).

So I'd say there's quite some gain in addition to the pure hacking 
factor; plus, it should be easy to port 6502 code to 6809 using a simple 
table-based translator.


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