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From: johnl@esegue.segue.boston.ma.us (John R. Levine)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Detecting an 80486
Message-ID: <9008261311.AA12147@esegue.segue.boston.ma.us>
Date: 26 Aug 90 17:11:27 GMT
References: < <3839@altos86.Altos.COM> <1990Aug21.223404.19523@ddsw1.MCS.COM>
Sender: Postmaster@esegue.segue.boston.ma.us
Organization: Segue Software, Cambridge MA
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In-Reply-To: <2664@dataio.Data-IO.COM>

In article <2664@dataio.Data-IO.COM> you write:
>Are there any differences between a 387 and a 487?

Not many.  The instruction sets seem to be the same.  There are some minor
changes in the state after a reset.  There is one new status bit that
controls whether exceptions cause an internal interrupt 16 as Intel
recommends or wiggle an external pin to be fed back through the external
interrupt controller for PC compatibility.  This seems hard to test, since a
system designer is free to wire up the external interrupt any way he wants.

>Are FWAITs *never* necessary with a 487?

Funny you should ask.  In general, no, but the 387 doesn't need them either.
Despite what the 387 manual says, all memory accesses to the 387 are fully
interlocked with the 386, and loading and storing 387 data never causes a
race condition.  You'll notice that no 386 C compiler ever emits an FWAIT.
(I confirmed this with an Intel designer in Santa Clara, who said it was he
who told all 387 compiler writers to dispense with the waits.  It was a
surprise to the 387 support people in Oregon.)  The only time an FWAIT is
useful is to force a pending floating point interrupt, both on the 387 and
the 486.

Regards,
John Levine, johnl@esegue.segue.boston.ma.us, {spdcc|ima|world}!esegue!johnl