68020/68881 Anyone?

[Brian D. Botton]

In article <13 at bagend.UUCP> jan at bagend.UUCP (Jan Isley) writes:
>No, I have not tried this yet.  But, I am getting one.  Who knows, maybe
>it will work.  If it does not work, *and* there is a potential to sell a

  Please let us know either way.

>preliminary questions come to mind for the gurus out there:
>
>  Are there any *known* reasons why an 020/881 would present a problem 
>  hardware wise?  I think this would be the easy part?
>
>  Any 020/881 fatal code in the software?

  I have given this a little bit of thought, after all, if I was going to
make a daughter board for access to video ram why, not include the 020/881
combination?  From a hardware stand point, there is no reason this cannot
be done.  It might take a little work to get it to fit, but as Jan stated,
this kind of thing has been done before.  The problem is the kernel.  On
the surface, there are two problems:

1.	I have not verified this, but the exception stack is probably
	different between an 010 and an 020.  It is different between
	the 030, 010 and 000.  The kernel would have to know how to
	deal with the slight differences.  I think this would be fairly
	easy if you had source.

2.	When there is a context switch you need to save the reigesters in
	the 68881, unless you want to run under the assumptiont that only
	one process can use the 68881 at a time, and that might not even
	work correctly all the time.  Again, this would be easy to fix if
	you had source.

  Personally, I'de love to have the 020/881, and I was even contemplating
reverse engineering part of the kernel to make it work, but it is far down
on my list of things to do.  Also, if it can be done, I won't charge $20k
to do it, ;-).

  At one time I saw a Unix-PC catalog that listed a floating point accelerator.
Does anyone know what this was?  The C compiler does have options for a 68881.
When I tried it I got a message saying I needed the 020 binary for ccom.  Does
this mean the FPA was on the verg of release or just that the compiler is
universial in nature?

Brian
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