From: Rainer Buchty (rainer_at_buchty.net)
Date: 2008-09-17 18:01:21
On Wed, 17 Sep 2008, firstname.lastname@example.org 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 Message was sent through the cbm-hackers mailing list
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