[Coco] Re: [Color Computer] 68881/68882 & the CoCo

Mike Pepe lamune at doki-doki.net
Wed Feb 8 17:19:00 EST 2006

Gene Heskett wrote:
> On Monday 06 February 2006 22:07, Mike Pepe wrote:
>> I've got a couple of Motorola FPU's in my stash here.
>> In looking at the manuals, it seems that interfacing the FPU to the
>> CoCo is pretty straightforward. It takes 32 addresses, so it would be
>> a good candidate for the MPI and switching the whole $FF40-$FF5F SCS
>> address range.
> In most coco setups, there isn't 32 contiguous addresses free between 
> the floppy at $ff40, and the gime table ghost at $FF90+ what with 
> floppy controlers with extras, an HD interface, a WP-RS and a dual 
> rs232 pack.  With all of the cards installed in my mpi, I think I have 
> 7 open addresses in that whole range.  I'd suggest an extra decoder 
> latch at some open address, 1 byte wide, and 4 other addresses to 
> service the 32 bit i/o it would need.  The first two addresses would be 
> used to set an address into the chip for the data register to be loaded 
> or read.  5 bits of the first latch would be decoded to give the FPU 
> its one of 32 address it needs, then the data would be written or read 
> by writing or reading the 4 sequential addresses of that 32 bit wide 
> register latch, a nice operastion for the 6309 and its ldq/stq 
> commands.  And the first latches last 3 bits used to initiate one of 8 
> possible operations by the FPU.  Is that enough?  You'd need read/write 
> & maybe a start/finish bit, & I don't know what else.
> Since the mpi itself uses $FF7F for its front panel, maybe the control 
> register could be addressed at $FF7E, and the 4 bytes wide for data 
> could then be from $FF78-7B, although I can't recall if mine has 
> anything there or not.
> Humm, I think I now see what you meant, by using the mpi, the floppy 
> controllers $20 wide decoding could be reused.  Humm.  Intrigueing 
> thought at that if the SCS signal hasn't been shared and is used by all 
> cards plugged in.  I'm not sure mine all meet that spec. 
> OTOH, we have a pretty good math and trig library set for os9 programing 
> use, so I'm not sure the effort will pay off at the end of the day.  
> Fun to do no doubt though, just to prove it can be done.
>> Anyone ever do this before? I'm sure it's been done.
> Not that I've heard about Mike, but then I'm out here in the hollers of 
> WV, so maybe it has been done.

Gene, sounds like you're on my wavelength.

I'm not sure how practical this is, but the bus interface seems 
straightforward, though completely emulating the 68020 co-processor 
protocol would be difficult-- but probably not necessary.

I'm just curious to see if it can be done. It may prove to be an 
interesting experiment to see how much faster the FPU is than the math 


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