[Coco] MC6829 Memory Management Unit

[gene heskett]

On 10/10/22 00:20, Walter ZAMBOTTI via Coco wrote:
> 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
Huh? level 2 had neither of those limits. I have the 2 meg kit from Tony 
Disto in my coco3, and I can recall having
more than 2 tasks going many times. I had two monitors, and could have a 
compile going in the text screen, while
I had two different editors working on pieces of rzsz. A 4th task might 
have been printform doing a printout of the
compile as the c compiler compiled the program.  That was all in the 
half meg I hadn't used for a ramdisk for the
compiler to use rather than scratchpad files.  The only limitation was 
the time sharing as the more processes i had
going, the slower each got.  Then Boisy started breaking up level 2 into 
more ever more modules, each of which
demanded  its $27 bytes of vtab space using up the vtab space, so today 
I can't even boot the os9boot I was using
all those years ago. There appears to be zero includes or assembler 
directives documenting it. And now my machine
has died, I suspect from old dried out caps.

Today I can't do that, and my complaints about it at the time 20+ years 
ago were ignored.
>
> 6829     2048MB     2K           4
>
>
> The smaller 2K block size makes that quite attractive.
>
Not when it takes 8 of these to do the full 2 megs. That also expands 
the table well beyond the $FF80 to $ff9F it now uses.

> 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
>>
>


Cheers, Gene Heskett.
-- 
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