Path: utzoo!attcan!uunet!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.m68k
Subject: Re: mc68040 I/D coherency
Message-ID: <12558@cbmvax.commodore.com>
Date: 11 Jun 90 21:29:27 GMT
References: <41463@brunix.UUCP> <12055@cbmvax.commodore.com> <12131@cbmvax.commodore.com>
Reply-To: daveh@cbmvax (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 34

In article <12131@cbmvax.commodore.com> bryce@cbmvax (Bryce Nesbitt) writes:
>In article <> valentin@cbmvax (Valentin Pepelea) writes:
>>'They' chose not to support this practice at all. Writing self modifying
>>code (Ahem - self distructing code) is not just difficult, it's darn well
>>impossible on most systems.

>Darn well common.  Consider loading code from disk.  Or worse
>yet, a relocating loader.  The case is identical.

That's not self-modifying code.  The code, as loaded, is of course data-space
information.  The loader performs the modifications, and then at some point
this data-space magically is converted to instruction-space.  The only thing
such a transform can affect is a copyback data cache.  

The problem with self modifying code, or something similar, is going from 
I-space to D-space and then immediately back to I-space.  It's quite easy for
such a case to wind up with stale I-space code.  This problem isn't only 
seen with self-modifying code, but it's most likely to occur with self 
modifying code, since the I->D->I transform must take place without flushing
the I cache for a problem to occur.

Some system simplify the problem by flushing caches when they change from 
Supervisor to User space.  That will, for example, solve the copyback problem
mentioned in the first case automatically for systems like UNIX in which the
loader is always a Kernel routine.  Other systems may have to explicitly
flush the data cache or possibly use only write-through caching.

>{X o} .     Bryce Nesbitt, Commodore-Amiga, Inc.


-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
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	"I have been given the freedom to do as I see fit" -REM