I suggest you run the "Intel387sx Math CoProcessor Utilities" and see what it says. If you google that, you will find it....
also this manual may be of some help: https://archive.org/details/intel-387sx-users-guide

Horun wrote on 2025-05-17, 19:04:

I suggest you run the "Intel387sx Math CoProcessor Utilities" and see what it says. If you google that, you will find it....
also this manual may be of some help: https://archive.org/details/intel-387sx-users-guide

Will try the software utility. Thanks.

in asynchronous mode: In this case,
the ratio of the frequency of NUMCLK2 to the frequency of CPUCLK2 must lie within the range 10:16
to 14:10
I used a sx-33 system with a 25MHz FPU
I think you are trying to get your 387sx-25 to work too fast

the original 387sx has an old core - it doesn't work at 33 Mhz.

If you can, use a 387SL - it has a new core - they work at 33 Mhz.
I noticed that the parts marked 16-25 Mhz also have a new core as do the SL parts

Good point ! at 25/40 (o.625) it is at the extreme low limit of 10/16 which theoretically should work, but due to age of stuff it just may be a bit flaky running that tight.

I think the fpu frequency can only be one degree different from the cpu. It's the same with the Motorola mc 68881/2 fpu

In theory it should work in asynchronous mode at 25Mhz, at the limit but. IN synchronous, it does not even boot every time. 40Mhz is simply not stable.

NSSI strangely reports at 20Mhz, I'm sure that crystal is 50Mhz for fpu. Maybe it is a little lower, that's why it is not working. Maybe I should try a 60Mhz crystal to make it work at 30Mhz if it can handle.

My fpu is old one I think.

Ulsi & Cyrix/Reveal 387sx FPUs work at 40Mhz... not sure if any others do. There is an Ulsi on Ebay at the moment for a good price...

A year or two ago I got 172.73 in Landmark on an ALI1217 board, Ti486sxl2-66 and a Cyrix/Reveal FPU at 33Mhz... performance is 90% the chipset