Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!ncar!asuvax!mcdphx!teroach.UUCP!dbk
From: dbk@teroach.UUCP (Dave Kinzer)
Newsgroups: comp.sys.amiga
Subject: Re: Whats wrong with self Modifying Code?
Summary: Lots, but that's not the point here.
Keywords: Cache, of course
Message-ID: <13203@mcdphx.phx.mcd.mot.com>
Date: 15 Jul 90 02:57:05 GMT
References: <6180@helios.ee.lbl.gov> <31727@cup.portal.com>
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Reply-To: dbk@teroach.UUCP (Dave Kinzer)
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Organization: Motorola Microcomputer Division, Tempe, Az.
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In article <31727@cup.portal.com> Sullivan@cup.portal.com (sullivan - segall) writes:
>The 68040 snoops the address bus for writes to cached memory, and updates
>the cache it is sees any.  (I.e: you shouldn't ever have to flush the 
>cache on any processor >= 68040.)  

BZZZZZZZZZZZZZZT!   Sorry, but we have some lovely parting gifts...

   That the 68040 snoops anything it sees on the bus is true, the problem is
that all the blitter (and possibly other DMA in different configurations)
activity is hidden from the processor (allowing the processor do to other 
things.)

   The correct answer is that anytime, and that means ANYTIME, the memory
is loaded with instructions, the cache needs to be flushed.  This should
be a system call since the application program usually should know nothing
about the processor executing its code, with the system call handling the
individual system requirements.  I tried to find one in my 1.2 docs (I
thought I remembered one), but failed.  For 2.0, I expect "It's in 
there."  I left enough hints here that I wanted it.

   More years ago than I care to remember, I had an instructor point out
that all we had ever written was self modifying code.  Now this instructor
drilled us through the many courses we had taken that this was a definate
no no, and I had never stooped so low as to do it, so I was taken aback
by this statement.  What do you mean, self modifying?  None of our
programs would have passed!  The point of view he was taking was that
of the operating system, and that when our program was read into system
memory, the system's code was modified.  This subtile point was never lost
on me.  The system, of course, flushed the cache after loading (if so
equiped.)  You should too.


            * * *   Imminent use of deathnet predicted.   * * *             //
Dave Kinzer  (602)897-3085  asuvax!mcdphx!teroach!dbk  Opinions are mine. \X/