Multiple FPUs in LC's?

[Elfen]

I have a LC III and an LC II, and as you know, they have an empty FPU socket on them. It would be nice to put in an FPU in them, right? You know it.

But I also have a couple of LC Ethernet Cards with an empty FPU Socket on them as well. Actually, of the 3 cards I got, 2 have a FPU in them (68882 @ 16MHz) and one has an empty socket. Its obvious that they were made for the LC.

This got me thinking - the 68K can support multiple FPUs. So can I put in an FPU in the logic board and another FPU in the ethernet card? Though the CPU can support it, will the software support it? Will there be any improvement?

What do you think?

 

[johnklos]

Hey, Fernando,

You'd have to change the coprocessor ID bits for one of the two sockets to put one of the FPUs at a different address. Then, you'd need to compile your own software which calls the alternate ID. I've never heard of any Mac software that already does that.

 

[Bunsen]

IMO you'd have to get into some pretty low level system software hacks.  It might be as "simple" as writing an INIT, or it might not.  The first thing to do would be to find out how applications call the existing FPU I guess.

 

[Elfen]

It would be great to have a FPU-Round-Robin Init that can access several FPUs when needed on a Mac when one is busy another is selected, Bunsen. But as JohnKlos stated, a minimum amount of hardware modification on the board. I have always wondered how FPUs are mapped in the 68K's memory and then accessed. But that would great if it were that easy.

Edit: There are some Nubus Cards with multiple DSP chips for sound and/or video. They are processed by the program in question (like Photoshop) and an extension. So if it can be done on that level for DSPs, why not an FPU?

 

[Bunsen]

Well, it depends whether you're happy to have only your custom code use the second FPU, or if you want it to be available to all apps.  Adobe took the time to write plug-ins for their programs to access the DSPs, and they don't provide access for any other programs.

And those DSP cards were constructed in hardware, and had appropriate firmware, to make each DSP accessible to software.

 

[Bunsen]

I have always wondered how FPUs are mapped in the 68K's memory and then accessed.

I don't think they are memory mapped.  The CPU has some kind of hook that recognises when it's being fed FPU code, and shunts it off via it's built-in hardware co-processor interface.

The '030 has the capacity to address >1 FPU by this method, but it's never been implemented to the best of my knowledge, at least not on the Mac.

 

[Elfen]

7. 7 FPUs can be accessed by the 68K to '030 "simultaneously" according to the 68K and '020/'030 tech manuals I have. It's not exactly simultaneously but as data is processed, it can send math to one FPU and more math to another FPU to solve and wait for their results.

And you're right, it was never done on a Mac. High end graphics workstations, yes but I'm not sure of which ones. And I'm sure NASA has a few things with multiple FPUs in their collection for space and planetary exploration simulations.

A Mac Plus or SE with an FPU would be interesting. At least it would end some of those strange but rare "FPU Not Present" System error on those systems.

 

[olePigeon]

I'm willing to bet some of the Symbolics stuff would take advantage of multiple FPUs.  Maybe Mathematica or other scientific software?