From: Rainer Buchty (rainer_at_buchty.net)
Date: 2008-09-17 18:01:21
On Wed, 17 Sep 2008, ruud.baltissen@apg.nl 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
www.buchty.net/ensoniq 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
command.
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.
Rainer
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