Path: utzoo!attcan!uunet!cbmvax!valentin
From: valentin@cbmvax.commodore.com (Valentin Pepelea)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Whats wrong with self Modifying Code?
Keywords: religion, gurus, whats up?
Message-ID: <13104@cbmvax.commodore.com>
Date: 7 Jul 90 00:08:49 GMT
References: <1990Jul6.201328.24660@csmil.umich.edu> <1990Jul6.201743.24777@csmil.umich.edu>
Reply-To: valentin@cbmvax (Valentin Pepelea)
Organization: Commodore, West Chester, PA
Lines: 34

In article <1990Jul6.201743.24777@csmil.umich.edu>
chymes@fribourg.csmil.umich.edu (Charles Hymes) writes:
>
>Oh great gurus, enlighten this poor soul who hath not programmed in
>Holy Assembler sence the loly 6502 was mighty.

Image you execute some code located at address $2000. While the CPU fetches
the instructions at address $2000, the cache retains those bytes. Next time
the CPU wants to execute the code located at address $2000, the instruction
bytes are already in the cache, so no external memory cycles have to be
performed. Fine.

Now you modify the instruction byte at address $2000. Next time the CPU wants
to execute at address $2000, it thinks it already has the bytes it wants in
the instruction cache, so it executes the wrong piece of code.

"But when I write to location $2000, will I not modify the contents of the
 cache?"

Yes, but you will modify the contents of the data cache, not the instruction
cache. Therefore, after modifying a piece of code, you must clear the
instruction cache.

Similarly, you must always clear the caches after a DMA transfer. (Direct
Memory Access) That's when an external device, such as a disk controller, moves
data into your motherboard ram. An alternate solution to clearing the caches
is to declare regions of memory as non-cacheable.

Valentin
-- 
The Goddess of democracy? "The tyrants     Name:    Valentin Pepelea
may distroy a statue,  but they cannot     Phone:   (215) 431-9327
kill a god."                               UseNet:  cbmvax!valentin@uunet.uu.net
             - Ancient Chinese Proverb     Claimer: I not Commodore spokesman be