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

[Gene Heskett]

On Wednesday 08 February 2006 17:19, Mike Pepe wrote:
>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
>library.

Well, with all the setup, loads and readbacks being done at the coco's 
bus's leasurely rate, I'm not sure it would be that much faster.  If 
the FPU was running at its spec sheet clock speed, and not loafing 
along at the coco's speed, it might be faster. I suspect one would have 
to go ahead and do it before that question could be answered with any 
authority though.

One things for sure, it would be an interesting experiment. :)

Just recall the 6309's integer divide (16 into 32, with 16 bit 
remainders and dividends) operation is 39 cycles worst case, and its 
wide mul operations is even faster, 29 cycles worst case IIRC.  Unforch 
I don't think any of the math libraries have been optimized for that.  
If I ever get curious, I'll do the trig library again, but stop it at 
the assembly code stage and see if I can optimize it for the 6309, I 
sense there might be a considerable speedup possible by doing that.

>-Mike
>
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-- 
Cheers, Gene
People having trouble with vz bouncing email to me should add the word
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Copyright 2006 by Maurice Eugene Heskett, all rights reserved.