[Coco] MPI Questions.

[Bruce W. Calkins]

The high addresses are control registers for various SAM, VGA and CPU functions which in the CoCo 1 & 2 were allotted twice as many spaces as were used.  The other half of the space is ghosted, often acting like the actual locations, but not consistently.  The GIME combined the SAM and VGA functions and added several more.  The space was then fully decoded, and the MPI needed to have it's addressing adjusted to match.  Most of the incompatibilities are found under OS-9 and many users do indeed get away with running a CoCo 3 enabled MPI with a CoCo 1 or 2 without seeing any problems.  Indeed some people have run an unupgraded MPI on a CoCo 3 without finding a problem.  BUT, the potential for a problem in the differing address decoding does exist.

 

In the older MPIs the concept of piggybacking the PAL/GAL chips with an enabling switch was bandied about.  The newer MPI with it's trace cuts and added connections for the upgrade was not as easy to set up for dual use, but I'm sure it can be done, should someone care to put their mind to it.

 

Bruce W.



==========================================


  ----- Original Message ----- 
  From: Sean 


  Interesting, I've never had a problem with mine on my CoCo2.

  On Fri, Aug 12, 2011 at 3:41 PM, Bruce W. Calkins
  <brucewcalkins at charter.net> wrote:
  > No!
  > The CoCo 3 uses some high addresses differently than the CoCo 1 & 2. A upgraded MPI often has issues with the CoCo 1 & 2. There are a very few that have a switch to select between the modes.
  > Some programs that don't mess with the high memory might appear to work OK, but you are still at risk for some problems.
  >
  > Bruce W.
  >
  > ======================================
  > ----- Original Message -----
  > From: Mark Ormond
  >
  >
  > I just got a MPI that has been upgraded for use with a coco3. Is it still compatible with a 1 and 2?
  >
  >
  > Later,
  > dabone
  >
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