[Coco] MC6829 Memory Management Unit

[Joel Rees]

Yeah, call it a more thorough version of what the GIME does.

Several reasons it wasn't adopted widely –

(1) Timing was tight. You needed a 68B09 just to run at 1MHz unless you
used glue logic parts that would not be available until a couple of years
after Motorola published the design. (If these are not from the original
Motorola masks the timings may be less restrictive.)

That said, my memory is that you could gang 8 6829s together to have 32
tasks live in memory, waiting for the CPU.

(2) It really needed more signals from the CPU to get the fine level of
control you wanted if you were going that far. Those extra signals would
have required extra pins on the CPU package.

(3) Which pointed to wanting the thing integrated into the CPU package,
which begs the question of market. We can second-guess Motorola about that
now, but it would have required an existing player ready to dedicate two
years marketing a micro as a mini. And Motorola would have needed to be
ready to evolve the 6809 into the market for the 68000. That's a lot of
vision requirements.

I could go on. None of that is relevant now. It could definitely be a fun
project.


2022年10月10日(月) 13:19 Walter ZAMBOTTI via Coco <coco at maltedmedia.com>:

>  From what I can tell from the my brief reading of the manual the device
> works in a similar way to the GIME MMU except
>
> it supports up to 2 megabytes,  2K block sizes and 4 tasks.
>
>            Max Mem  Block Size Tasks
>
> GIME       512MB     8K           2
>
> 6829     2048MB     2K           4
>
>
> The smaller 2K block size makes that quite attractive.
>
> However since OS9 is written for 8x8k pages the MMU register structure
> would need to be enlarged to cater for 32x2k, if it is not already
> reserved.
>
>
> On 7/10/22 20:02, Mathew Boytim via Coco wrote:
> >   Wow that's an interesting story.  I have no idea about the app note
> though.
> > Thanks,
> > Matt
> >      On Friday, October 7, 2022 at 06:28:24 AM EDT, Carlos Camacho via
> Coco <coco at maltedmedia.com> wrote:
> >
> >   Long ago, I remember seeing a Motorola application note with the MC6829
> > Memory Management Unit. I looked for it on eBay and other sources but
> > couldn't find it. I was told that Motorola never released it.
> >
> > Fast forward to two weeks ago when someone posted a European seller on
> eBay
> > was selling them. Probably counterfeit Chinese goods I thought. I reached
> > out to the seller and he explained that he obtained them from a German
> > factory that was liquidating their IC inventory. That sounded promising
> so
> > I bought his entire stock of MC6829 Memory Management Unit.
> >
> > It's on its way to me. I'll need to find that app note and build the
> > circuit board to confirm they are indeed the real deal. Any other advice?
> >
> > Cheers,
> >
> > Carlos Camacho
> > CoCo Store
> >
>
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