Weiteks are not standard FPUs and they have their own bus and dedicated cache.

Any 387 that works at 40 MHz will be OK, but bear in mind that if it's not rated for 40 MHz you would be overclocking it, which can reduce the lifespan. I won't try commenting on which FPU is best, as there are a lot of people who know far more than me 😀

You do realise it was popular for 386 owners to install Cyrix Fasmath for several years before the DLC even existed, right?
Cyrix is the FPU to use regardless of which CPU you have.

I often pick up IIT 3C87s because they're still kinda, sorta cheap-ish compared to other FPUs of their speed range. I know they're not the fastest but raw FPU speed is all but irrelevant for a 387 in this day and age. 😜
CHIPS SuperMATH and Cyrix FasMaths are by any measure the fastest "real" 387s you can get, but damn do they command ridiculous prices on eBay anymore. ULSI ones are also far from 'slow'.

The bar-none fastest "387" you can get is an Intel RapidCAD, which was just a 486DX with the L1 cache and 486 opcodes stripped out. The RapidCAD-1 chip replaced the CPU while a PLA marked RapidCAD-2 fit into the 387 socket. Don't think they ever made a 40Mhz model though.

Anonymous Coward wrote on 2021-10-15, 02:08:

You do realise it was popular for 386 owners to install Cyrix Fasmath for several years before the DLC even existed, right?
Cyrix is the FPU to use regardless of which CPU you have.

Yes of course the cyrix should be the right fpu for the 386.
Strangely there is no amd 387.
The speech I was making is that the 387DLC is I guess different from the normal 387, and therefore using a 386 + 387DLC could there be problems?

Eep386 wrote on 2021-10-15, 02:43:

I often pick up IIT 3C87s because they're still kinda, sorta cheap-ish compared to other FPUs of their speed range. I know they're not the fastest but raw FPU speed is all but irrelevant for a 387 in this day and age. 😜
CHIPS SuperMATH and Cyrix FasMaths are by any measure the fastest "real" 387s you can get, but damn do they command ridiculous prices on eBay anymore. ULSI ones are also far from 'slow'.

The bar-none fastest "387" you can get is an Intel RapidCAD, which was just a 486DX with the L1 cache and 486 opcodes stripped out. The RapidCAD-1 chip replaced the CPU while a PLA marked RapidCAD-2 fit into the 387 socket. Don't think they ever made a 40Mhz model though.

I have an IIT on the DLC (see signature), and having no fpu on the 386dx (actually even in the 386SX I don't have fpu), ​​I wanted to find one that does not give problems and works at 40MHz (Synchronous with the CPU).
I have an acquaintance who has problems with FPU 387, I do not remember the exact models but I would not like to buy something that does not fit.
Maybe it would be better to avoid those DLC posts, or maybe they are perfectly compatible.
RapidCAD would be great, but it is quite rare and I think expensive too, and for the moment I have to try to buy only low-cost.

CPU Galaxy did a review (https://www.youtube.com/watch?v=-WCxrtuo8IA) about 386 coprocessors. Cyrix are indeed the fastest coprocessors. But like said before take anyone you have as you probably won't notice the difference if you don't have a very special purpose for them (like Autocad).

Here is one of the graphs:

Even in AutoCAD the CPU speed is more important then the FPU. There are only slight speed differences between them overall (tested regen and hide with large DWG file).

Indeed, I heard about this. The 386 doesn't need a 387 that badly. Unlike 286/287 and before.

In AutoSketch on an older architecture, the performance CPU vs FPU is more dramatic.

https://www.youtube.com/watch?v=ItA-_D2QkTk
https://www.youtube.com/watch?v=SGCUErENKBA
https://www.youtube.com/watch?v=G-h4XNuZe6I

The 8086+8087 are more like equal twins the way they communicate with each others.
In later generations, it's more like master and servant.

I mean. In AutoCAD it's greater performance boost going from 386DX-40 and fastest FPU to 486DLC-40 with slowest FPU. If you don't have a FPU and use FPU emulator, it's slow slow slow - just can't compare it to the real one.

Ah, I see. Makes sense to me. Speaking of FPU/NPU emulators, the "Franke 387" emulator was very sophisticated. Or so I heard.
In Germany, there was a lot of advertising for that FPU emulator back in the late 80s/early 90s.
http://icfs.de/franke387.html

Jo22 wrote on 2021-10-16, 11:32:

Ah, I see. Makes sense to me. Speaking of FPU/NPU emulators, the "Franke 387" emulator was very sophisticated. Or so I heard.
In Germany, there was a lot of advertising for that FPU emulator back in the late 80s/early 90s.
http://icfs.de/franke387.html

Thanks this is very interesting.
I used emulators in the early 90's, first in a 286 waiting for the 287, then on a 386DX-33 for about 5 years, recently I found other x87 emulator software, but I didn't use them because I didn't need them.
Lately I've been rebooting older PCs for an exhibition show, and then I'm reinstalling the period software and some of these excel type (but others too) work better with the coprocessor.

You're welcome. ^^
Before I got real FPUs, I used the humble EMUL87 on a 286/386.

Jo22 wrote on 2021-10-17, 08:54:

You're welcome. ^^
Before I got real FPUs, I used the humble EMUL87 on a 286/386.

Yes the one I used was called EM87 if I remember correctly, it was fine on 286 and 386 (DX included), while it didn't work on the 486SX, and in fact it was, I tried it on a 486SX but it didn't work.

PC@LIVE wrote on 2021-10-17, 09:24:

Jo22 wrote on 2021-10-17, 08:54:

You're welcome. ^^
Before I got real FPUs, I used the humble EMUL87 on a 286/386.

Yes the one I used was called EM87 if I remember correctly, it was fine on 286 and 386 (DX included), while it didn't work on the 486SX, and in fact it was, I tried it on a 486SX but it didn't work.

I remember that one, too! ^^
Btw, there's also 88x87 that runs on XTs.
It uses a different approach to link into programs looking for floating point routines.

And then there's also another math coprocessor emulator for 386 systems called Q387.
Kixs mentioned it a few years ago in this thread over here: Re: x87 co-processor emulator? A real thing or totally fake?

Edit: And here's a chart, too. For a single cycle the real FPU takes, those 8087 emulators with 16-Bit code take several dozen.
The 386 ones with 32-Bit assembly code might be much faster, though.
https://kb.iu.edu/d/aano

That being said, I think that at least sometimes, using EMUL87 is a few times faster than using no FPU emulator, at all.
That's at least how it looked like when I ran my copy of STS+ on a 286 with/without EMUL87.

Last, but not least.. :)

https://www.vogonswiki.com/index.php/File:Coproc.txt

Jo22 wrote on 2021-10-16, 05:16:

Indeed, I heard about this. The 386 doesn't need a 387 that badly. Unlike 286/287 and before. […]
Show full quote

Indeed, I heard about this. The 386 doesn't need a 387 that badly. Unlike 286/287 and before.

In AutoSketch on an older architecture, the performance CPU vs FPU is more dramatic.

https://www.youtube.com/watch?v=ItA-_D2QkTk
https://www.youtube.com/watch?v=SGCUErENKBA
https://www.youtube.com/watch?v=G-h4XNuZe6I

The 8086+8087 are more like equal twins the way they communicate with each others.
In later generations, it's more like master and servant.

What? 286s don't need a fpu unless a program is dealing with a lot of math calculations, like the MS Flight Simulator, a spreadsheet program like Quatro Pro a CAD program, are needed. Exactly the same as a 368DX/SX.

Early 386s used 287 as their math co-processor because the 387 didn't even exist.