Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!usc!rutgers!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Two problems with home-build '020 accelarator. Help needed.
Message-ID: <22740@cbmvax.commodore.com>
Date: 27 Jun 91 15:57:08 GMT
References: <1991Jun19.192747.6888@rulway.LeidenUniv.nl> <22708@cbmvax.commodore.com> <1991Jun27.084032.28883@rulway.LeidenUniv.nl>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 31

In article <1991Jun27.084032.28883@rulway.LeidenUniv.nl> breemen@rulcvx.LeidenUniv.nl (E. van Breemen) writes:
>In article <22708@cbmvax.commodore.com> daveh@cbmvax.commodore.com (Dave Haynie) writes:
>>In article <1991Jun19.192747.6888@rulway.LeidenUniv.nl> breemen@rulcvx.LeidenUniv.nl (E. van Breemen) writes:

>>everything themselves.  I never built one without the MMU, so I'm a little
>>unclear on that part of the coprocessor protocol.

>We have decoded the FPU completely. The problem is that if you address an non
>existing MMU, the M68020 will wait for DSACK forever (the FPU won't respond to
>it) and the machine 'hangs'.

But if there's no MMU (or other coprocessor in any particular coprocessor slot),
you're supposed to get an F-line exception for that instruction, rather than 
an infinite wait for a non-existant device.  I'm pretty sure SetCPU works just
fine in this respect for CSA and Ronin/Imtronics 68020 boards.

Wiping a fine layer of dust off of my 68020 book, I find that you need to
generate a bus error for any coprocessor access cycle that doesn't match with
an existing coprocessor.  In other words, when you see a coprocessor 0 cycle
(for MMU), you should be generating a BERR*, which will then cause the 68020
to take an F-line exception.  Done thusly, everything becomes shiny and clear.

>We have been able to patch the driver software by adding some nop's and
>everything works just fine.

Oh, a software timing problem.  One of my favorite kinds of bugs.

-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
	"This is my mistake.  Let me make it good." -R.E.M.