[Coco] CoCo3 MMU Register question...

[Richard E. Crislip]

On Mon, 30 Mar 2015 23:04:59 +0200
Tormod Volden <lists.tormod at gmail.com> wrote:

> On Mon, Mar 30, 2015 at 8:53 PM, Zippster wrote:
> > I have a scheme in mind, using a CPLD for address decoding, and
> > high speed latches for the MMU bank registers, that would operate
> > similarly to one set of CoCo3 MMU banks.
> >
> > With 8 banks (0-7), the same register locations in memory as the
> > first instance in the CoCo3 (TR 0). Using each memory location
> > ($FFA0 - $FFA7) to hold an 8-bit value for the memory block to map
> > to (in a latch).
> >
> > Perhaps someone who knows about OS9 can tell me if a simplified MMU
> > scheme like this would be suitable to compile OS9 to use.  This is
> > without a second task register, and no ability to read the MMU
> > registers. Those things could be implemented if necessary, but
> > trying to keep it simple.
> >
> > Any thoughts or suggestions?  Unfortunately I don’t know squat
> > about OS9’s memory usage.
> 
> Hi Ed,
> 
> Bringing OS9 Level 2 to CoCo2 and Dragon makes all sense in the world
> to me! We are many who dream of such an MMU, and of designing one. A
> recent discussion:
> http://archive.worldofdragon.org/phpBB3/viewtopic.php?f=5&t=5234
> 
> For OS9 (or any real OS) you should absolutely have at least two
> tasks. And a way to read out the current task would be nice. I don't
> think reading out the MMU bank registers is needed.
> 
> If this is an internal MMU board it should monitor BS and BA to trap
> interrupt fetches and switch to system mode. The GIME keeps a RAM page
> at FE00 write protected while in user mode (and MMU enabled), and all
> interrupt vectors go here. I suppose also the bank registers are
> locked in user mode.
> 
> It should be possible to make a NItrOS-9 port that runs this MMU and
> otherwise keeps to CoCo2 peripherals. Of course you would not be able
> to run Level 2 programs that depend on the CoCo3's graphics modes.
> 
> In any case, it would be a fantastic platform for FUZIX!
> 
> Take also a look at what the N8VEM guys are cooking. They made their
> own MMU solution, with a different page size IIRC (4 KB instead of
> GIME's 8 KB).
> 
> Best regards,
> Tormod
> 

I'm a little confused. I am not trying to be a wet blanket here so
don't crucify me 8-), but why not just run OS9 on the CoCo3? Or is this
to do something that has never been done before? Again, I wish you the
best, just asking.