[Coco] What would a CoCo successor have to have as a minimum?

Joel Ewy jcewy at swbell.net
Tue Nov 23 16:37:25 EST 2010

On 11/21/2010 04:20 PM, Mark McDougall wrote:
> On 22/11/2010 8:58 AM, Aaron Wolfe wrote:
>> this makes me wonder, should a "coco 4" just use a different or
>> additional processor?
>> since our targets are fpga and emulators, we aren't really tied to
>> processors that exist in silicon.  we could add a 68000 or create some
>> sort of hybrid 6809/68k type of thing.
>> probably a lot of work, but sounds like it would be difficult to see
>> significant improvements to graphics if we don't change something in
>> the architecture.
> My original point was not that we can't improve the graphics, but 
> rather that there needs to be some consideration of the implications, 
> and not just throw around resolutions just because "we can".
> A video controller in HDL is just a bunch of counters. Whether 320x200 
> 4 colours or 1920x1200 32-bit colour is neither here nor there. We 
> just need to be sensible about it.
> ...
> I don't think we need to add another CPU into the mix. An accelerated 
> 6x09 with the above graphics capabilities is adequate for 
> arcade-quality games (after all, some actual arcade games only had 1 
> or 2 6809s in them, and we're talking about a 25MHz 6809!!) I can only 
> imagine what Steve Bjork could do with such a system!

One wouldn't have to do anything wildly different from the 6809/6309, 
but extend the processor model in ways that represent a reasonable 
evolution of the design, and that provide useful functionality.  A wider 
data path to the GIME and RAM, for example.  Or concatenate the index 
registers to be able to point at a 32-bit address, and add an addressing 
mode that bypasses or transparently updates the GIME's DAT.  Stuff like 
that, along with blitters and whatnot would address some of the 
technical limitations of a CoCo made from discrete VLSI chips without 
breaking compatibility with old software.

Maybe the place to start is to decide what enhancements to add, in terms 
of hardware acceleration for sound, graphics and DMA, based on how easy 
they are to implement in existing FPGAs without breaking legacy 
software, keeping in mind that we don't necessarily need to think 
strictly in terms of a separate GIME and CPU if these things are 
integrated into a single FPGA.  Let those enhancements determine what 
kind of spatial and color resolutions would be useful.

But as someone else mentioned, there are more uses for a video display 
than arcade games, so a high resolution desktop mode might be useful 
even if it can't be refreshed quickly.


> Once you throw away the 6809 then you're not talking about a Coco 4 
> any more IMHO. A 68k with enhanced graphics is called an "Amiga".
> Regards,

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