[Coco] COCO FPGA

[gene heskett]

On Thursday, December 22, 2011 12:03:42 PM Tony Cappellini did opine:

> I think the two new coco projects (I forgot the actual names though) are
> quite an accomplishment, and hope to get one myself.
> One limiting factor on these is not having the expansion port. Does
> anyone know if either of these projects intends to add the coco
> expansion port - or better yet- build an MPI right into one of these
> boards?
 
I wasn't aware that was a missing item in the FPGA versions.

However, to those contemplating that addition, let me suggest that the 4 
port mpi should probably be replaced with a multi-slot backplane where the 
IRQ priority is one nibble, and the slot is the other nibble of what is put 
in the bus by the IRQ when reading the status.  This could both expand the 
number of slots to 15, but could serve as an easier way to rate priorities 
than we now have.  We are not now limited to 4 devices capable of 
generating an IRQ pulldown in the OS, only by the direct bit to slot used 
in the representation.

I don't think it is quite that simple though as I can see a potential for 
two IRQs to generate a byte that represents neither single device, and that 
would require calling every service routine until the real one is found.  
But there really does need to be a way to at least double the number of 
usable slots. 4 is not enough.
> 
> Thanks
> 
> --
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Cheers, Gene
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