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From: cth_co@tekno.chalmers.se (Christer Olsson. MedNet, G|teborgs Universitet)
Newsgroups: comp.binaries.ibm.pc.d
Subject: Re: Experience with bargain 80287
Message-ID: <10141@tekno.chalmers.se>
Date: 8 Aug 90 16:44:47 GMT
References: <12085@sdcc6.ucsd.edu> <8405@b11.ingr.com>
Organization: Chalmers Univ. of Technology, Gothenburg, Sweden
Lines: 29

In article <8405@b11.ingr.com>, lhughes@b11.ingr.com (Lawrence Hughes) writes:
> AND NOW FOR SOMETHING COMPLETELY DIFFERENT (or at least I haven't heard anyone
> mention this yet): I also have a JDR 386/25 MB that has both 387 and 287 FPP
> sockets - the manual says you need a 25MHz FPP (!), but if you check with JDR,
> they confirm the info in the Intel books that indicate you CAN do this if you
> run the 287 socket with a lower speed clock (AIN'T no such thing as a 25MHz
> 287...). I tried my 10 MHz AMD 80c287 in this socket, and by golly, not only
> did it work, but all timing tests showed about a 1.5 factor improvement over
> the 286/287 combo... basically on FP intensive stuff, typical timings were:

Most 386 mothercards with both 387 and 287 sockets runs the 387 syncronous
with the 386 (at same speed) and the 287 asyncronous (slower speed). 8Mhz 
are very common and the timings below seems to showing the 386 / 287
running with the 287 at 8Mhz. 

> 
> 	System			Time (sec)
> 
> 	10 MHz 286 alone	10.0
> 	25 MHz 386 alone	 4.5 
> 	286 / 287		 3.2	(287 actually running at 6.67 MHz?)
> 	386 / 287		 2.4

If the 386-mothercard are made with CHIPS & TECH chipsset, it's maybe possible 
to change the 287 clock source beetwen 8Mhz bussclock, 1/2 cpu-clock or 1/3
cpu-clock. On my old 386, I can set FPP-speed to 1/2 cpu-clock (20Mhz) which 
gaves me a FPP-speed at 10Mhz. 1/3 (20/3 = 6.67) are also possible and even
8Mhz from the bussclock. If I run the CPU-clock at 16Mhz, it's possible to
run the FPP at slowy 5.33 Mhz :-)